Solar-Powered, Liquid-Desiccant 

Air Conditioner for Low-Electricity 

Humidity Control

Energy and Water Projects 

Demonstration Plan SI-0822 

TP-7A40-56437-1 

November 2012 

Jesse Dean, Eric Kozubal, Lesley Herrmann (NREL)

Jeff Miller and Andy Lowenstein (AIL Research) 

Greg Barker (Mountain Energy Partnership) 

Steve Slayzak (Coolerado) 

Link to Summary Report
ii

NOTICE

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(“DOE”). The work described in this report (NREL/TP-7A40-56437) was funded under task 

number WFG7.1000.

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iii

 

Table of Contents

List of Figures.............................................................................................................................................................. v

List of Tables...............................................................................................................................................................vi

Acronyms and Abbreviations.................................................................................................................................. vii

ACKNOWLEDGEMENTS ........................................................................................................................................ 1

EXECUTIVE SUMMARY ......................................................................................................................................... 2

1.0 INTRODUCTION.......................................................................................................................................... 7

1.1 BACKGROUND .................................................................................................................. 7

1.2 OBJECTIVE OF THE DEMONSTRATION .............................................................................. 8

1.3 REGULATORY DRIVERS .................................................................................................... 8

2.0 TECHNOLOGY DESCRIPTION ..............................................................................................................10

2.1 TECHNOLOGY OVERVIEW .............................................................................................. 10

2.2 FUTURE DEVELOPMENTS ............................................................................................... 13

2.3 TECHNOLOGY DEVELOPMENT ....................................................................................... 14

2.4 ADVANTAGES AND LIMITATIONS OF THE TECHNOLOGY .............................................. 19

3.0 PERFORMANCE OBJECTIVES ..............................................................................................................21

3.1 IMPROVE HUMIDITY CONTROL AND COMFORT ............................................................. 22

3.2 PROVIDE HIGH-EFFICIENCY DEHUMIDIFICATION .......................................................... 22

3.3 SUSTAIN HIGH-DEHUMIDIFICATION PERFORMANCE ..................................................... 23

3.4 MAINTAINABILITY ......................................................................................................... 23

4.0 FACILITY/SITE DESCRIPTION..............................................................................................................24

4.1 FACILITY/SITE LOCATION AND OPERATIONS ................................................................. 24

5.0 TEST DESIGN .............................................................................................................................................28

5.1 CONCEPTUAL TEST DESIGN ............................................................................................ 28

5.2 BASELINE CHARACTERIZATION ..................................................................................... 28

5.2.1 Laboratory Testing ............................................................................................................................28

5.3 DESIGN AND LAYOUT OF TECHNOLOGY COMPONENTS ................................................. 30

5.3.1 Conditioner.................................................................................................................................................30

5.3.2 Regenerator.................................................................................................................................................32

5.3.3 Solar Collectors ..........................................................................................................................................33

5.3.4 Desiccant Storage Tank ..............................................................................................................................34

5.4 OPERATIONAL TESTING ................................................................................................. 36

5.5 SAMPLING PROTOCOL .................................................................................................... 36
iv

5.6 EQUIPMENT CALIBRATION AND DATA QUALITY ISSUES ............................................... 38

5.6.1 Calibration of Equipment ..................................................................................................................38

5.6.2 Quality Assurance .............................................................................................................................39

5.7 SAMPLING RESULTS ....................................................................................................... 39

6.0 PERFORMANCE ASSESSMENT .............................................................................................................40

6.1 IMPROVE HUMIDITY CONTROL AND COMFORT ............................................................. 42

6.1.1 ASHRAE Comfort Zone.............................................................................................................................45

6.1.2 Reduce Reheat Run-Time...........................................................................................................................48

6.2 PROVIDE HIGH-EFFICIENCY DEHUMIDIFICATION .......................................................... 49

6.2.1 Electrical Efficiency: EER and kW/ton ......................................................................................................49

6.2.2 Thermal Efficiency – Coefficient of Performance......................................................................................52

6.2.3 Water Consumption ....................................................................................................................................54

6.3 PERFORMANCE SUMMARY ............................................................................................. 55

6.4 SUSTAIN HIGH-DEHUMIDIFICATION PERFORMANCE ..................................................... 57

6.4.1 Conditioner Heat Exchange Effectiveness..................................................................................................57

6.4.2 Desiccant Charge ........................................................................................................................................57

6.4.3 Pressure Drop Increases..............................................................................................................................57

6.4.4 Service Life Projection ...............................................................................................................................58

6.5 MAINTAINABILITY ......................................................................................................... 58

7.0 MARKET ANALYSIS................................................................................................................................. 60

7.1 COST MODEL .................................................................................................................. 60

7.2 RELEVANT MARKETS ..................................................................................................... 61

8.0 IMPLEMENTATION ISSUES ...................................................................................................................62

8.1 SUMMER 2010 .................................................................................................................. 63

8.2 SUMMER 2011 .................................................................................................................. 63

8.3 LESSONS LEARNED ......................................................................................................... 64

8.3.1 Climate........................................................................................................................................................64

8.3.2 Building Type .............................................................................................................................................64

8.3.3 Solar Field...................................................................................................................................................65

8.3.4 LDAC Fans and Pumps ..............................................................................................................................65

8.3.5 Cooling Tower............................................................................................................................................65

8.3.6 Sequence of Operation................................................................................................................................65

9.0 REFERENCES .............................................................................................................................................66

APPENDIX –POINTS OF CONTACT ...................................................................................................................68
v 

List of Figures

Figure 1. Psychrometric chart showing the dehumidification process using desiccants............................. 11

Figure 2. Desiccant reactivation using single-effect scavenging air regenerator........................................ 12

Figure 3. Major components and packaging of the AILR LDAC............................................................... 13

Figure 4. LDAC schematic. ........................................................................................................................ 14

Figure 5. The first implementation of a low-flow conditioner.................................................................... 15

Figure 6. The upper end of a manufacturable low-flow conditioner. ......................................................... 16

Figure 7. A PPSU regenerator (similar to the one installed in the Tyndall LDAC). .................................. 17

Figure 8. The second prototype of a low-flow LDAC processing ventilation air for a machine shop in 

Wrightsville, Pennsylvania. .............................................................................................................. 17

Figure 9. Commercial LDAC using low-flow technology installed on a Los Angeles supermarket.......... 18

Figure 10. Psychometric plot Tyndall AFB................................................................................................ 24

Figure 11. AFRL site. ................................................................................................................................. 25

Figure 12. Tyndall AFB building layout..................................................................................................... 26

Figure 13. NREL Thermal Test Facility. .................................................................................................... 29

Figure 14. LDAC system and supply air layout.......................................................................................... 30

Figure 15. Image and design of conditioner unit. ....................................................................................... 31

Figure 16. Image and design of cooling tower............................................................................................ 32

Figure 17. Image and design of regenerator unit. ....................................................................................... 33

Figure 18. Image and design of solar collector array.................................................................................. 34

Figure 19. Image and design of desiccant storage tank. ............................................................................. 35

Figure 20. Outdoor air conditions and conditioner-outlet air conditions for summer 2010 operation ....... 42

Figure 21. Outdoor air conditions and conditioner-outlet air conditions for summer 2011 operation ....... 43

Figure 22. Daily cooling load supplied by the LDAC during the 3 weeks of summer 2010 operation...... 44

Figure 23. Daily cooling load, with monthly averages, supplied by the LDAC during the summer 

of 2011 .............................................................................................................................................. 45

Figure 24. Tyndall Lab outdoor and indoor conditions without the LDAC operating in Summer 2010 

(4.3% of the data is outside the comfort zone).................................................................................. 46

Figure 25. Tyndall Lab outdoor and indoor air conditions during summer 2010 with the LDAC in 

operation (10.6% of the data is outside the comfort zone)................................................................ 47

Figure 26. Tyndall Lab outdoor and indoor air conditions during summer 2011 with the LDAC in 

operation (0.2% of the data is outside the comfort zone).................................................................. 48

Figure 27. Daily average of LDAC EER for 3 weeks of summer 2010 operation ..................................... 49

Figure 28. Daily and monthly average of LDAC EER for summer 2011................................................... 50

Figure 29. EER dependence on cooling load.............................................................................................. 51

Figure 30. Thermal performance (COP) of LDAC for summer 2011 ........................................................ 53

Figure 31. Solar efficiency for summer 2011 ............................................................................................ 54

Figure 32. Water usage for LDAC cooling operation................................................................................. 54

Figure 33. Timeline with electrical (EER) and thermal (COP) efficiency performance labeled with LDAC 

system events and changes................................................................................................................ 56
vi 

List of Tables

Table 1. Performance Objectives.................................................................................................................. 3

Table 2. Summer 2010 (3 Weeks) Performance Summary........................................................................... 4

Table 3. Monthly (Averaged) Performance for Summer 2011 ..................................................................... 4

Table 4. Energy and Cost Savings from the LDAC in 2011......................................................................... 5

Table 5. Technology Options for Residential and Commercial Buildings* ............................................... 13

Table 6. Performance Objectives................................................................................................................ 21

Table 7. Air-Cooled Chiller Schedule......................................................................................................... 27

Table 8. AHU #3 schedule.......................................................................................................................... 27

Table 9. LDAC System Pump and Fan Motor Schedule ............................................................................ 36

Table 10. Sensor Accuracy Summary......................................................................................................... 38

Table 11. Performance Objectives.............................................................................................................. 40

Table 12. Summer 2010 (3 Weeks) Performance Summary....................................................................... 55

Table 13. Monthly (Averaged) Performance for Summer 2011 ................................................................. 55

Table 14. Energy and Cost Savings from the LDAC in 2011..................................................................... 60
vii 

Acronyms and Abbreviations

AAHX air-to-air heat exchanger

AC air conditioning 

AFB air force base

AFRL Air Force Research Laboratory 

AHU air-handling unit

AILR AIL Research

ASHRAE American Society of Heating, 

Refrigerating, and AirConditioning Engineers

Btu British thermal units

CaCl2 calcium chloride

cfm cubic feet per minute

CHP combined heat and power

CPVC chlorinated polyvinyl chloride

CoC cycles of concentration

COP coefficient of performance

DAS data acquisition system

DOAS dedicated outdoor air system

DoD Department of Defense

DOE Department of Energy

DX direct expansion

EER energy efficiency ratio

EISA Energy Independence and 

Security Act

E.O. executive order

EPAct Energy Policy Act 

ESTCP Environmental Security 

Technology Certification 

Program

ft foot

ft2

 square foot 

FY fiscal year

gal gallon

GHI global horizontal irradiance

gpm gallons per minute

hr hour

EUI energy use intensity

HMX heat and mass exchanger

hp horsepower

HVAC heating, ventilating, and air 

conditioning

LDAC liquid-desiccant air conditioner

kW kilowatt

kWh kilowatt-hour

lb pound

LiCl lithium chloride

MBtu million British thermal units

MEP Mountain Energy Partnership

NREL National Renewable Energy 

Laboratory

PLC programmable logic controller

PPSU polyphenylsulfone

PVC polyvinyl chloride

RH relative humidity

VFD variable-frequency drive

W watt

yr year
1 

ACKNOWLEDGEMENTS

The authors would like to thank all of the Environmental Security Technology Certification 

Program (ESTCP) project team members for their creativity, persistence, and willingness to 

support this project. Bruce Nielsen at Tyndall Air Force Base was instrumental in setting up the 

demonstration and has provided countless hours assisting with the installation of the data 

acquisition system (DAS) and multiyear performance testing. Mountain Energy Partnership 

provided invaluable assistance with the design and installation of the DAS, as well as data 

analysis support. Jeff Miller and Andy Lowenstein provided countless hours designing, testing, 

and commissioning the liquid-desiccant system at Tyndall Air Force Base. Various members of 

the National Renewable Energy Laboratory’s Commercial Buildings Research team—including 

James Page, Andrew Parker, and Michael Deru—provided laboratory testing assistance, field 

testing assistance, and modeling support. Finally, the project would not have been possible 

without financial support from the ESTCP, which not only provided funding but also provided 

valuable insights into the types of data analysis procedures and results that would be most 

beneficial to Department of Defense facilities and engineers.
2 

EXECUTIVE SUMMARY

Today’s air-conditioning (AC) technology is primarily based on direct expansion (DX) or the 

refrigeration process. It is now so prevalent that it is considered a necessity for the majority of 

residential and commercial buildings throughout the United States. During the 100-plus years of 

development, DX AC has been optimized for cost and thermodynamic efficiency, both of which 

are nearing their practical limits. Nevertheless, AC accounts for approximately 15% of all 

source energy used for electricity production in the United States alone [nearly 4 quadrillion 

British thermal units (Btu)], which results in the release of about 343 million tons of carbon 

dioxide into the atmosphere every year. (DOE 2011) 

The Department of Defense (DoD) occupies over 316,000 buildings and 182,000 structures on 

536 military installations worldwide, and accounts for about 64% of the energy consumed by 

federal facilities. This makes the DoD the largest energy consumer in the United States. In 

Fiscal Year (FY) 2007, the DoD consumed 218 trillion Btu in site-delivered energy, 26.2 trillion 

Btu for AC alone. This cooling cost equates to an estimated $413 million per year. (Pacific 

Northwest National Lab, undated) 

In hot, humid climates, conventional AC units expend excess energy to sensibly overcool the air 

for dehumidification. As a result, excess energy must be used to reheat the air to a more 

comfortable supply temperature (overcool/reheat cycle). The use of desiccant-based AC systems 

decouples the latent and sensible loads of an airstream, enabling higher efficiency cooling and 

improved thermal comfort conditions. The following is a list of criteria that can be used to 

identify feasible sites for liquid-desiccant air conditioner (LDAC) applications: 

• Hot and humid climate – latent cooling required most of the year 

• One hundred percent outdoor air ventilation requirements 

• Significant reheat loads on current heating, ventilation, and AC (HVAC) system 

• Heat source available or suitable installation identified for desiccant regeneration

• Current issues with humidity control – comfort, sick building syndrome, mold, etc.

The primary objective of this project was to demonstrate the capabilities of a new highperformance, liquid-desiccant dedicated outdoor air system (DOAS) to enhance cooling 

efficiency and comfort in humid climates while substantially reducing electric peak demand at 

Tyndall Air Force Base (AFB), which is 12 miles east of Panama City, Florida. The new type of 

LDAC invented by AIL Research (AILR) has higher thermal efficiency than any other LDAC on 

the market today. The technology was recently invented, and only six active units were operating 

at the time of this report, four of which are demonstration projects funded by the Department of 

Energy (DOE) with the purpose of demonstrating different applications and resolving newproduct technical issues. Broader application is expected soon after technical reliability and 

manufacturing costs become acceptable. Seeing the technology’s potential, Munters Corporation 

recently purchased AILR’s LDAC technology and will commercialize it in areas with low 

thermal energy costs compared to electricity (e.g., low natural gas cost or waste-heat 

applications). 

This was the first solar-powered demonstration of the technology. The goal of the project was to 

quantify energy and water consumption, solar energy utilization, and cost savings relative to DX

air conditioners. The LDAC system that was installed at Tyndall AFB was a pre-commercial
3 

technology, and given that it was the first solar-powered demonstration, a fundamental objective 

of the demonstration was to evaluate system performance and use the lessons learned to develop 

design/manufacturing guidance for future commercial LDAC systems. Each demonstration of 

this new technology is expected to reveal technical issues related to the specific application. This 

demonstration was also the first to integrate the LDAC as a retrofit into an existing air handler. 

Lessons learned from these experiences are expected to improve product design and create a 

methodology for determining suitable retrofit applications. 

Performance evaluation of the LDAC began in the summer of 2010. Only 3 weeks of continuous 

operation were recorded in 2010 due to system malfunctions and limited run-time. Roughly 5 

months of operation were recorded between April and September in 2011. Table 1 describes the 

performance objectives that were evaluated during the demonstration. 

Table 1. Performance Objectives 

Performance 

Objective Metric Success Criteria Results

Quantitative Performance Objectives

Improve humidity 

control and comfort 

(energy efficiency)

• Hours outside 

psychrometric 

comfort zone 

• Chiller power

• Reheat run-time

• <1% of hours 

outside 

ASHRAE 

summer 

comfort zone 

• Reduce 

chiller/reheat 

run-time

• Achieved but 

inconclusive 

cause

• Achieved but 

inconclusive 

cause

Provide highefficiency 

dehumidification 

(energy efficiency)

• EER

• COP

• >40 (Btu/hr)/W 

EER

• Thermal COP 

>0.7

• Not achieved

• Achieved

Sustain highdehumidification 

performance 

(energy efficiency 

and maintenance)

• Conditioner heat 

exchange 

effectiveness

• Desiccant charge 

• Supply-air 

pressure drop

• Conditioner 

cooling-water 

pressure drop

• Projected 

service life

• <Once-per-year 

desiccant/buffer

adjustment 

• <5% degradation 

of HX eff. over 3 

years

• Negligible 

increase in 

air/water pressure 

drop 

• Above criteria 

should support 

>10-yr service 

life projection

• Achieved; no 

degradation of 

desiccant during 

operation 

• Duration of 

performance 

evaluation too 

small to 

determine 
4 

Qualitative Performance Objectives

Maintainability 

(ease of use)

Ability of an HVAC 

technician to operate 

and maintain the 

technology 

A single facility 

technician able to 

effectively operate 

and maintain 

equipment with 

minimal training 

• Many 

unforeseen 

maintenance 

issues 

occurred 

during initial 

demonstration 

• Many lessons 

learned for 

design and 

ease of 

operation

Performance data—including energy efficiency ratio (EER), kilowatt (kW)/ton, and coefficient 

of performance (COP) for 2010 and 2011—are summarized in Table 2 and Table 3, respectively. 

It is clear that the electrical and thermal efficiency improved throughout the summer of 2011. 

Table 2. Summer 2010 (3 Weeks) Performance Summary

Date Cooling 

(ton-hr)

EER 

[(Btu/hr)/W] kW/ton Solar heat (MBtu)* COP*

7/21/10 -

8/14/10 1982 14.7 0.82 3.1 0.85 

*Solar thermal generation only recorded for 3 days (7/21-7/23) 

Table 3. Monthly (Averaged) Performance for Summer 2011 

Month Cooling 

(ton-hr)

EER 

[(Btu/hr)/W] kW/ton Solar heat (MBtu) COP

April 667 7.8 1.54 18.1 0.44

May 1565 8.2 1.47 39.9 0.5

June 1837 12.4 0.97 35.4 0.62

July 1142 14.6 0.82 19.4 0.71

August 1916 18.8 0.64 32.2 0.71

September 1300 15.1 0.79 26.7 0.73

Table 4 summarizes the displaced load on the existing chiller and the approximate energy and 

cost savings from the LDAC. 
5 

Table 4. Energy and Cost Savings from the LDAC in 2011 

Month Cooling 

(ton-hr)

Chiller 

Elec. 

(kWh)

LDAC 

Elec. 

(kWh)

Elec. 

Savings 

(kWh)

Elec. Cost 

Savings 

($)

April 667 890 1,026 -137 -14

May 1,582 2,110 2,325 -215 -21

June 1,837 2,449 1,774 676 68

July 1,239 1,652 1,131 521 52

Aug 1,916 2,554 1,223 1,331 133

Sept 1,333 1,778 1,099 678 68

The total cost savings for the 2011 cooling season was $321. The installed costs for the solar 

thermal system were $170,000, and the installed costs for the LDAC components were $40,000, 

for a total installed cost of $210,000 and a simple payback of 654 years. Because this was a precommercial system, the simple payback is not indicative of the payback period of a commercial 

system. If the system would have operated per design intent, the cost savings would be 

substantially higher. In addition, in building types with electric reheat, the zone-level 

overcooling and reheat savings dwarf the energy savings from the mechanical chiller. Reheat 

energy use in hospitals, for example, has been documented to account for over 30% of the total 

energy use. Finally, when the system is coupled with solar thermal, the solar thermal component 

becomes the most expensive part of the system, and solar incentives or high utility rates are 

required to offset the increased costs of the solar thermal system.

In general, the LDAC system did not perform as well as expected due to design, installation, and 

operation issues. Consequently the project’s focus was necessarily changed to focus on 

discovery of technical issues with this new emerging technology. Many of the issues arose 

because the installation had many unique features including the following:

• The demonstration was the first combination of solar heat with this type of LDAC

system. 

o Due to initial budgetary constraints, the LDAC relied solely on solar heat, with no 

natural gas backup to ensure that the unit operated throughout the cooling season. 

A properly designed system that uses solar heat will have backup. Due to this, the 

system did not achieve peak-cooling capacity for significant hours of operation. 

Because the system largely has static electrical power draw, this resulted in a low 

average EER.

o The solar field design and LDAC system design were not tightly coordinated by 

the prime installation contractor (Regenesys). This resulted in a design that did 

not consider the frequency and duration of stagnation periods for the solar field. 

The collector design was not designed to withstand more than about two

stagnations per year. Furthermore, the collector system was not initially designed 

to withstand the massive volume of steam from these collectors when stagnation 

occurred. The solar field required significant redesign. The end result was 
6 

workable for the demonstration, despite being problematic and suboptimal in 

operation. 

• The demonstration was the first to create a split system where the conditioner and 

regenerator were contained in separate packages and separated by a distance of 

approximately 120-foot (ft). This technical challenge resulted in a suboptimal pumping 

design because of the necessary pump size to transfer desiccant this distance. Future 

designs should reduce the distance from the regenerator and conditioner. 

• This demonstration was the first to have 10 hours of desiccant storage using calcium 

chloride (CaCl2). Tuning the storage to achieve optimal efficiency was required. The 

desiccant charge and the tank’s low and high levels have significant impact on efficiency, 

capacity, and solar utilization. These variables were tuned as the demonstration 

progressed. 

• This demonstration required the placement of the conditioner unit about 100 feet from the 

outdoor intake to the building. This required significant fan power to move the air from 

the mechanical yard to the building. Future designs and applications should consider the 

duct length reduce the duct run from the conditioner to the outdoor air intake as much as 

possible. 

• The demonstration did not treat 100% of the outdoor air, thus limiting the benefit to 

energy savings from offset cooling. In order to offset the reheat for such an installation, a 

system should be designed to ensure that the LDAC meets a significant portion of the 

latent load. Typically, the LDAC can meet 100% of a building’s latent load if designed to 

treat 100% of the outdoor air. 

This report outlines lessons learned that should be applied to future projects in order to ensure 

successful design, installation, and operation of a solar-powered LDAC system.

At the end of 2011, the LDAC technology was sold to Munters Corporation, one of the largest 

HVAC manufacturers in the United States. The first demonstration of a commercial LDAC 

system is being evaluated at the Coral Reef Fitness and Sports Center on Andersen AFB in 

Guam. A 6,000 cubic-feet-per-minute (cfm) conditioner was designed for this system. The power 

requirements per ton of cooling for the existing building level chiller and LADC are 1.05 kW/ton 

and 0.3 kW/ton, respectively. Note that the power requirement of the chiller does not account 

for the chiller water pumps, so the power requirement may be slightly greater in reality. The 

system is designed with an evacuated-tube solar thermal field supplying 80% of the thermal 

power and a backup diesel-powered boiler providing 20% of the thermal power. The system is 

expected to reduce HVAC energy use by 34% and save $145,395 per year with an estimated 

simple payback of 11.6 years. 
7 

1.0 INTRODUCTION

1.1 BACKGROUND

Today’s AC is primarily based on the DX or refrigeration process, which was invented by Willis 

Carrier more than 100 years ago. It is now so prevalent and entrenched in many societies that it 

is considered a necessity for maintaining efficient working and living environments. DX AC has 

also had 100-plus years to be optimized for cost and thermodynamic efficiency, both of which 

are nearing their practical limits. However, the positive impact of improved comfort and 

productivity does not come without consequences. Each year, AC accounts for approximately 

15% of all source energy used for electricity production in the United States alone (nearly 4 

quadrillion Btu), which results in the release of about 343-million tons of carbon dioxide into the 

atmosphere every year. (DOE 2011) 

R-22 (Freon) as a refrigerant for AC is quickly being phased out because of its deleterious effects 

on the ozone layer. The most common remaining refrigerants used today (R-410A and R134A) are strong contributors to global warming; their global warming potentials are 2,000 and

1,300, respectively. (Owen 2010) Finding data on refrigerant release rates for air conditioners is 

challenging as they are generally serviced only when broken, and refrigerant recharge is not

accurately accounted for. The limited data that does exist indicates that typical refrigerant 

release rates for supermarket refrigeration equipment are 10% to 15% per year. (Baxter et al. 

1998) A typical residential-size AC unit may contain as much as 13 pounds of R-410A, and a 

10-ton commercial AC will contain as much as 22 pounds. 

Water is not commonly considered to be a refrigerant, but the American Society of Heating,

Refrigerating and Air-Conditioning Engineers (ASHRAE) recognizes it as the refrigerant R-718. 

Evaporative cooling uses the refrigerant properties of water to remove heat the same way 

DX systems use the refrigeration cycle. Water evaporates and drives heat from a first heat

reservoir; water vapor is then condensed into a second reservoir. The water used in this process 

is delivered to the building as a liquid via the domestic water supply. Evaporative cooling is so

efficient because the Earth’s atmosphere and nature cycles, rather than a compressor and 

condenser heat exchanger, perform the energy-intensive process of recondensing the refrigerant. 

The National Renewable Energy Laboratory’s (NREL’s) thermally activated technology 

program has been working closely with AILR as an industry partner for more than 15 years to 

develop an LDAC. The technology uses liquid desiccants to enable water as the refrigerant in 

lieu of chlorofluorocarbon-based refrigerants to drive the cooling process. The desiccants are 

strong saltwater solutions. In high concentrations, desiccants can absorb water from air and drive

dehumidification processes; thus, evaporative cooling devices can be used in novel ways in all

climates. Thermal energy dries the desiccant solutions once the water is absorbed. LDACs

substitute most electricity use with thermal energy, which can be powered by many types of

energy sources, including natural gas, solar thermal, biofuels, and waste heat. The benefits

include generally lower source energy use, much lower peak-electricity demand, and lower

carbon emissions, especially when a renewable fuel is used.

The LDAC technology deployed in this demonstration was invented by AILR, and was the result 

of collaborative effort with NREL, and was funded by DOE. The LDAC technology developed 
8 

by AILR is the result of a 10-year, $5 million DOE R&D effort to increase the efficiency of the 

LDAC technology on the market and decrease maintenance concerns related to legacy problems 

with desiccant carryover into the product airstream. The technology is emerging and at the 

writing of this report, six active demonstrations had been deployed. Munters Corporation has 

seen the promise of the technology and has purchased the rights from AIL Research. The six 

active demonstrations are focused on providing cooling to grocery stores where the benefits from 

drying the space to sufficient levels reduce refrigeration evaporator-coil frosting due to water 

condensation and freezing. Energy is reduced by less defrosting and a lower load on the 

refrigeration system. Munters Corporation has taken on the task to manufacture the LDAC 

technology. The demonstration to date, including the Tyndall demonstration, has shown a critical 

level of reliability of the LDAC system and identified points of improvement. The sale of the 

technology shows that Munters Corporation is satisfied with the current state of reliability and 

willing to commercialize it. 

AC is also the single largest contributor to peak demand on electric grids and is a primary cause 

of grid failure resulting in blackouts. Power generators and electric air conditioners are least 

efficient at high ambient temperatures, when cooling demand is highest, leading to increased 

pollution, excessive investment in standby generation capacity, and poor utilization of peaking 

assets. This LDAC approach—the result of a 10-year, $5 million DOE R&D effort—increases 

the efficiency of LDAC technology on the market and decreases maintenance concerns related to 

legacy problems with desiccant carryover into the product airstream. 

1.2 OBJECTIVE OF THE DEMONSTRATION 

The primary objective of this project was to demonstrate the capabilities of a new highperformance, liquid-desiccant DOAS to enhance cooling efficiency and comfort in humid 

climates while substantially reducing electric peak-demand. This was the first solar-powered 

demonstration of the technology. The goal of the project was to quantify energy and water 

consumption, solar energy utilization, and cost savings relative to DX air conditioners. The 

LDAC system installed at Tyndall was a pre-commercial technology and given that it was the 

first solar-powered demonstration, a fundamental objective of the demonstration was to evaluate 

the performance of the system and use the lessons learned to develop design/manufacturing 

guidance for future commercial LDAC systems. 

At the end of 2011, the LDAC technology was licensed to Munters, one of the largest HVAC 

manufacturers in the United States, and the first demonstration of a commercial LDAC system is 

being evaluated on the Coral Reef Fitness and Sports Center on Andersen AFB in Guam.

1.3 REGULATORY DRIVERS

The DoD Environmental Security Technology Certification Program awarded this new 

technology demonstration project as a means to identify programmatic changes that could be 

applied to the design and construction of energy-efficient, DOAS AC systems for humid 

environments. A new low-energy use LDAC unit could be implemented throughout ASHRAE 

climate zones 1, 2, and 3 to help the agency meet or exceed the various requirements set forth in 

Executive Order (E.O.) 13423, the Energy Policy Act (EPAct) of 2005, and the Energy 

Independence and Security Act (EISA) of 2007. 
9 

EPAct 2005 requires the U.S. secretary of energy to ensure that not less than 7.5% of total 

electricity consumed by the federal government comes from renewable sources in FY 2013; and 

thereafter, to the extent economically feasible and technically practicable in FY 2013, and 

thereafter, of the total electricity consumed by the federal government comes from renewable 

energy. If the thermal portion of the LDAC unit is driven by a solar thermal source, this 

technology would help DoD meet its renewable energy goals.

The key features of EISA 2007 that pertain to this technology are outlined in section 431 and

requires a reduction in energy use intensity (EUI) [1,000 (k)Btu/square foot (ft2)/year (yr)] of 

federal buildings by 3% per year, from a 2003 baseline, resulting in a 30% reduction in EUI by 

2015. The EISA 2007 legislation has superseded all previous EUI reduction mandates.

E.O. 13423 provides requirements for water conservation at federal facilities, mandating federal 

agencies reduce potable water consumption intensity 2% annually through FY 2020. This would 

result in a 26% reduction by the end of FY 2020, relative to a FY 2007 baseline. E.O. 13514

also mandates a reduction in industrial, landscaping, and agricultural water consumption by 2% 

annually, or 20% by the end of FY 2020, relative to a FY 2010 baseline. 

The LDAC unit can substantially reduce energy use and peak demand, which will help meet

EISA 2007 requirements, but it also has the potential to increase potable water consumption, 

which will be detrimental to the E.O. 13514 requirements. Each DoD base is encouraged to try 

to identify alternative sources of cooled water for the conditioner, such as geothermal-based 

cooling. 
10 

2.0 TECHNOLOGY DESCRIPTION

2.1 TECHNOLOGY OVERVIEW 

Desiccants reverse the paradigm of standard DX AC by first dehumidifying and then sensibly 

cooling the outside airstream to meet a given cooling load. Desiccant at any given temperature 

has a water-vapor pressure equilibrium that is roughly in line with constant relative humidity 

(RH) lines on a psychrometric chart, as shown in Figure 1. The green lines show the 

dehumidification potential for two common types of liquid desiccants: lithium chloride (LiCl) 

and CaCl2. If the free surface of the desiccant is kept at a constant temperature, the ambient air 

will be driven to the dehumidification potential line. If used with an evaporative heat sink at 

temperatures between 55°F and 85°F, the air can be significantly dehumidified, and dew points 

less than 32°F are easily achieved. The blue arrow in Figure 1 shows the path of ambient air 

driven to equilibrium with CaCl2 with the use of an evaporative heat sink. At this point, the air 

can be sensibly cooled to the proper supply temperature. This type of desiccant AC system 

decouples sensible and latent cooling by controlling each independently. 

During the dehumidification process, the liquid desiccant (about 43% salt concentration by 

weight in a water solution) absorbs water vapor in an exothermic reaction. The heat released by 

the desiccant is carried away by a heat sink, usually cooled water from a cooling tower. As water 

vapor is absorbed from the ambient air, it dilutes the liquid desiccant, and decreases its vapor 

pressure and its ability to absorb additional water vapor. Lower concentrations of desiccant come 

into equilibrium at higher ambient air RH levels. Dehumidification can be controlled by the 

desiccant concentration supplied to the device. The outlet humidity level of the processed 

ambient air can be controlled by the desiccant concentration and/or the flow of highly 

concentrated desiccant. The latter allows the highly concentrated desiccant to quickly be diluted 

and thus “act” as a weaker desiccant solution in the device.
11 

Figure 1. Psychrometric chart showing the dehumidification process using desiccants

Absorption of water vapor will eventually weaken the desiccant solution and reduce its 

dehumidifying potential; the desiccant must then be regenerated to drive off the absorbed water. 

Thermal regeneration is the reverse process of vapor absorption. In this process, the desiccant is 

heated to a temperature at which the equilibrium vapor pressure is above ambient vapor pressure. 

The water vapor desorbs from the desiccant and is carried away by an airstream (see Figure 2). 

Figure 2 shows how a scavenging airstream picks up heat and moisture from a regenerator. The 

green line represents the psychrometric condition of air in equilibrium with a CaCl2 solution at 

the given temperature. Sensible heat is recovered by first preheating the ambient air using an airto-air heat exchanger (AAHX). The air comes into contact with the desiccant in the heat and 

mass exchanger (HMX)—in this example at 190°F—and carries the desorbed water vapor away 

from the desiccant. Sensible heat is recovered by taking the hot humid air to preheat the 

incoming air through the AAHX. The change in enthalpy of the airstream as it passes through the 

regenerator represents the majority of the thermal input. 

0

20

40

60

80

100

120

140

160

30 40 50 60 70 80 90 100 110 120

ω (grains/lb)

Dry Bulb Temperature (°F)

Psychrometric Chart at 0 ft Elevation (1.013 bar)

Room or Return Air
12 

Figure 2. Desiccant reactivation using single-effect scavenging air regenerator

The process uses hot water or steam to achieve a latent COP between 0.8 and 0.94, depending on 

desiccant concentration. Latent COP is defined as:

COPLatent = (Moisture Removal Rate) ∗ (Heat of Vaporization)

Heat Rate (Higher Heating Value)

COP is maximized by maximizing the regeneration temperature and change in concentration

while minimizing the desiccant concentration. Including the COP of the water heater (about 

0.82), a typical combined latent COP for the LDAC systems is 0.82 × 0.85 = 0.7. If the heating 

source is derived from solar thermal technologies, the COP of the water heater would be the 

efficiency of the solar collectors (the benefit here being that there is no fuel cost penalty for the 

heat conversion efficiency). 

The AILR technology innovations result in higher thermal efficiency when compared to other 

technologies on the market. NREL tests have shown that the Kathabar and other high-flow 

systems achieve a latent COP of about 0.4 to 0.55. 

The LDAC technology developed by AILR uses novel HMXs to perform these two processes as 

shown in Figure 3, which illustrates the desiccant conditioner and scavenging air regenerator. 

The liquid desiccant is dispensed over the plates in the conditioner (absorber) where the inlet 

ambient air is dehumidified. This technology is called low-flow, liquid-desiccant AC because the 

desiccant flow is minimized in the HMXs of the conditioner and regenerator to the flow rate 

needed to absorb the necessary moisture from the airstream, which eliminates liquid desiccant 

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210

ω (lb/lb)

Dry Bulb Temperature (°F)

Psychrometric Chart at 0 ft Elevation (14.7 psia)

Enthalpy = 43.6 BTU/lbm

Enthalpy = 60.7 BTU/lbm

Enthalpy = 194 BTU/lbm

Ambient Air

Air after AAHX

Regeneration Outlet Air

Regenrations Hot Water Temperature
13 

carryover into the supply airstream. The HMXs must therefore have integral heating and cooling 

sources; 55°F – 85°F cooling tower water is supplied to the conditioner, and the regenerator uses 

hot water or hot steam at 160°F – 200°F. The cooling or heating water flows internal to the heat 

exchange plates while the desiccant flows on the external side of the HMX plates. The plates are 

flocked, which effectively spreads the desiccant and creates direct-contact surfaces between the 

air and desiccant as the air passes between the plates. 

Figure 3. Major components and packaging of the AILR LDAC. 

(Illustration by NREL) 

2.2 FUTURE DEVELOPMENTS

NREL is working with AILR to develop a double-effect regenerator that expands on the 

scavenging air regenerator by first boiling the water out of the liquid desiccant solution (250°–

280°F) and reusing the steam by sending it through the scavenging air regenerator. This twostage regeneration system can achieve a latent COP of 1.05–1.2. A typical solar regenerator 

would consist of a hot water supply and a scavenging regenerator (which would result in a 

single-effect device that would have about a 60% solar conversion efficiency based on absorber 

area).

Table 5. Technology Options for Residential and Commercial Buildings* 

Regenerator Performance Rating

Solar Array 60% solar conversion

Single effect using natural gas COP 0.7

*Based on NREL calculations and laboratory data, available upon request

Conditioner

Regenerator

Economizer

hot & 

humid 

air

humid 

exhaust

cool, dry ventilation 

delivered to 

building heating 

water

cooling 

water
14 

For the low-flow LDAC, the regenerator and conditioner systems are shown connected in Figure 

4, which illustrates the three thermal energy sources that can be used to regenerate the desiccant:

solar thermal, traditional water heaters, and a double-effect technology. The water heater or 

boiler can be fueled by many sources, including natural gas, combined heat and power (CHP), or 

even biofuels. Also shown is the option for desiccant storage; storage allows an AC system to 

effectively bridge the time gap between thermal energy source availability and cooling load. A 

desiccant system that uses strong desiccant at 43% by weight and weakens the desiccant to 35% 

will be able to achieve storage density of 5 gallons (gal) of desiccant per latent ton-hour·(hr) of 

cooling provided. This is most easily accomplished using LiCl as the desiccant, as 35% desiccant 

still retains significant dehumidification potential. Desiccant storage is simple and theoretically 

lossless. Storage consists of a sealed, uninsulated tank with proper liquid connections. In 

comparison, ice storage is approximately 13–16 gal/ton-hr (theoretically 10 gal/ton-h, but in 

practice only 67% of the volume is frozen. (Ice Energy 2012) This storage can be useful to 

enable maximum thermal use from solar or on-site CHP. 

Figure 4. LDAC schematic. 

(Illustration by NREL) 

2.3 TECHNOLOGY DEVELOPMENT

Since its founding in 1988, the primary mission of AILR has been to develop and commercialize 

high-efficiency, end-use products for heating and cooling applications. For the past 14 years, 

AILR has focused on the parallel activities of developing plastic heat exchangers and applying 

these heat exchangers in HVAC products that use advanced liquid-desiccant technology. 
15 

From October 1990 to October 1991, AILR conducted research for the Gas Research Institute on 

a project entitled, “The Effect of Material Properties on the Performance of Liquid Desiccant Air 

Conditioners and Dehumidifiers.” (AILR, undated) In addition to investigating alternative 

desiccants to LiCi, AILR studied novel configurations of the regenerator and the conditioner of 

an LDAC. An important conclusion from this work, which was reported in ASHRAE Paper No. 

AN-92-3-3, was that the desiccant flow rate in a packed-bed conditioner (which was the 

dominant technology at the time) is set by the requirement to limit the desiccant’s temperature 

rise. (Lowenstein and Gabruk 1992) By embedding cooling within the conditioner heat and 

mass exchanger, the desiccant’s temperature could be controlled independently of the amount of 

water absorbed. The desiccant flow rate could then be set by the need to limit the concentration 

change of the desiccant, a requirement that allows the desiccant flow to be reduced by over an 

order of magnitude compared to packed-bed conditioner designs. 

In 1994, AILR received a patent that covered the low-flow, liquid-desiccant technology. 

(Lowenstein 1994) The patent was assigned to the Gas Research Institute, the organization that 

sponsored the research. Shortly after receiving the patent for low-flow, liquid-desiccant 

technology, AILR began to explore ways to practically capture the benefits of the technology. In 

September 1998, AILR delivered a 1,000-cfm, liquid-desiccant conditioner to NREL that used 

low-flow technology. (NREL 1997) The conditioner, which is shown in Figure 5, was composed 

of 75 polypropylene extruded plates that had been modified so that cooling water made six 

passes within each plate. A woven cotton fabric sleeve was slipped over each plate to provide a 

wicking surface for the desiccant. 

Figure 5. The first implementation of a low-flow conditioner. 

(Photo from AILR) 

Following the successful testing of the liquid-desiccant conditioner at NREL (shown in Figure 

5), AILR began to develop a manufacturable design for a low-flow, liquid-desiccant conditioner 

with additional support from NREL. (NREL 1998 and 1999) A low-flow, liquid-desiccant 

conditioner composed of extruded polyvinyl chloride (PVC) plates bonded to injection-molded 

manifold pieces was developed in this follow-on work. A 40-plate prototype of this conditioner 
16 

was successfully tested at NREL in June 2004. This manufacturable design for the liquiddesiccant conditioner, shown in Figure 6, is used in the Tyndall solar LDAC. 

Figure 6. The upper end of a manufacturable low-flow conditioner. 

(Photo from AILR) 

AILR’s development of a low-flow conditioner was complemented by a parallel effort to 

develop a manufacturable, low-flow regenerator. Several approaches to a low-flow regenerator 

were explored under sponsorship by NREL. (NREL 2001) Prototypes were built using extruded 

chlorinated PVC (CPVC) plates and coated aluminum plates. The initial operation of both 

prototypes was good, but within several hundred hours of operation, both prototypes failed. A 

third prototype composed of extruded polyphenylsulfone (PPSU) plates, which is shown in 

Figure 7, proved successful operation for thousands of hours. A prototype of the PPSU 

regenerator was tested by NREL in February 2006. This PPSU regenerator is used in the Tyndall 

LDAC. PPSU is a plastic that can withstand temperatures as high as 250°F, but is substantially 

more expensive than other plastics. AILR and NREL continue to investigate regenerator designs 

with lower cost materials.
17 

Figure 7. A PPSU regenerator (similar to the one installed in the Tyndall LDAC). 

(Photo from AILR) 

In 2003, AILR built the first prototype of a 6,000-cfm roof-top LDAC under a subcontract to 

Kathabar, Inc., as part of a larger effort of Oak Ridge National Laboratory. This prototype 

originally used a CPVC regenerator that failed after several hundred hours of operation. 

In 2005, AILR built, installed and operated a second 6,000-cfm LDAC prototype, again under 

sponsorship of NREL. (NREL 2005) This prototype, shown in Figure 8, was installed on a 

machine shop in Wrightsville, Pennsylvania, where it successfully processed ventilation for 2 

years. 

Figure 8. The second prototype of a low-flow LDAC processing ventilation air for a 

machine shop in Wrightsville, Pennsylvania. 

(Photo from AILR) 
18 

The 3,000-cfm solar LDAC built for Tyndall AFB was installed in spring 2010 and operated 

during the summers of 2010 and 2011. The Tyndall LDAC was the first implementation of a 

low-flow LDAC driven solely by solar thermal energy for regeneration. It was also the first 

AILR LDAC to operate in the field using a solution of CaCl2 as the desiccant, which is more cost 

effective than LiCl as a means to store cooling, but it also does not provide the same 

dehumidification as LiCl, and is thus a compromise. 

In May 2009, PAX Streamline (a venture-backed startup company) established a memorandum 

of understanding with AILR to transfer the low-flow technology to PAX for commercialization. 

Working together, PAX and AILR built a 6,000-cfm and 3,000-cfm LDAC and installed them on 

separate supermarkets in the Los Angeles area. Figure 9 shows the 6,000-cfm installation. 

Unfortunately, PAX Streamline failed in April 2010, despite the successful operation of the two 

supermarket LDACs.

Figure 9. Commercial LDAC using low-flow technology installed on a Los Angeles

supermarket. 

(Photo from AILR) 

Following the failure of PAX, AILR continued to work with two former employees of PAX to 

build and install three more supermarket LDACs: two in California and one in Hawaii. These 

LDACs were installed between October 2010 and April 2011. 

In July 2011, all intellectual property and know-how developed by AILR for building liquiddesiccant conditioners and regenerators that use low-flow technology were sold to the Munters 

Corporation. Munters is now in the early stage of commercializing the technology. 
19 

 

2.4 ADVANTAGES AND LIMITATIONS OF THE TECHNOLOGY

LDACs are a new breed of AC technology that decouples the latent load from the total load 

(sensible + latent) normally done by a refrigeration or chilled water system. De-coupling of 

these loads enables independent temperature and humidity control in a space. Also, lower 

humidities in a space can be achieved more efficiently by avoiding the energy intensive 

processes. Examples of these avoided processes and systems include: 

• Overcooling and then reheating (which reverses the sensible cooling by the refrigeration 

system, thus lowering efficiency) 

• Solid-desiccant wheels with natural gas or condenser heat regeneration. These systems 

generally increase energy use by the HVAC system due to high air-pressure losses and 

natural gas use.

• Ultra-low apparatus dew-point temperatures, which increase energy use by the 

refrigeration system. 

LDACs largely switch much of the energy to condition air to thermal sources, such as natural 

gas, solar thermal, or waste heat. High-density storage can be employed to bridge thermal 

energy source profiles with cooling profiles, such as the case with solar thermal or even waste 

heat. Using waste heat is the most cost effective way to power an LDAC unit and should be 

considered first if a waste heat source is available. Natural gas or propane is economically 

utilized when dehumidification requirements are high. Supermarkets are a typical case where 

decreased store humidity drastically improves the efficiency of the food refrigeration systems. 

Thus store humidity levels are generally kept as low as possible. LDACs enable lower store 

humidity levels than other available humidity-control technologies and are just now being 

adopted at major supermarket chains as a result. For example, Whole Foods has recently 

included the LDAC technology in its HVAC specification in humid climates. Solar energy for 

LDACs can become economical if the relative cost of solar thermal energy is competitive with 

natural gas or propane. This is often the case on islands such as Hawaii, Guam, and many other 

tropical island nations. Solar thermal systems should always be used to offset fossil fuel use but

not as the primary source of energy. Designs that attempt to get a solar fraction of 1.0 inherently 

will have solar fields and desiccant storage tanks that are much larger and more expensive than 

practical. 

LDACs primarily use cooling towers for their cooling sink. If cooling towers are compared to 

air-cooled AC systems, site water use will increase. However, many chiller systems use cooling 

towers, and LDAC technology would use about the same amount of water for cooling as these 

systems do. The electric power grid also uses water to cool thermal power plants. The avoided 

use of electric power can result in substantial regional water savings. Case-by-case analysis is 

required to calculate these savings. However, typical thermal power generation station produce 

about 1.0 to 2.0 kilowatt-hours (kWh) of electricity per gallon of water evaporated (0.5 - 1 

gal/kWh). 

Water use is dictated by how much energy is removed per pound (lb) of water evaporated. 

Water’s heat of vaporization is about 1,060 Btu/lb, which is equivalent to 1.37 gal/ton-hr of 
20 

cooling load. However, because evaporative cooling is an open-cycle process where mineral 

content of domestic water must be removed, the water use will be higher by the cycles of 

concentration (CoC) required to prevent mineral buildup in an evaporative system. CoC is

defined as the ratio of mineral concentration in the blow-down water divided by the initial 

concentration. CoC is dependent on water quality, but typically range from 2-7 where a CoC of 2 

is typically associated with facilities that have extreme water hardness. A typical water-draw rate 

for a typical cooling tower will be 1.57 to 2.74 gal/ton-hr. In the case of the LDAC technology, a 

cooling tower must only remove the cooling load. In the case of a water-cooled DX system, the 

cooling tower must remove the cooling load plus the compressor load. For a DX cycle with a 

COP of 4, a cooling tower would thus draw 25% more water or typically 1.96 to 3.42 gal/ton-hr. 

The preceding analysis does not include the complicated weather effects on a cooling tower, but 

is approximate for most conditions where the cooling tower’s airstream becomes fully saturated 

and leaves at the same temperature as the inlet air. However, the comparison with DX cooling 

remains accurate in relative amounts. LDAC technology will, in general, use about 25% less

water than a water-cooled DX system. The net regional water impact by using an LDAC system 

will typically be small or even positive in some cases. 

LDACs are now being employed to treat dedicated outdoor airstreams to control humidity in a 

space. The highest benefit thus will be for humid climates with large yearly humidity loads and 

applications where reheat energy is high. 

High-value applications include buildings with large outdoor air loads that have the highest 

levels of reheat or benefit from decreased humidity in the space such as the following: 

• Hospitals (avoiding massive amounts of reheat) 

• Supermarket humidity control 

• School buildings in humid regions 

• Buildings with waste heat available

• Indoor pools. 

LDACs are an emerging technology and have not seen the level of refinement that economy of 

scale has bestowed upon vapor-compression technology. The technology is still in a precommercial state; the systems are more complex than traditional vapor-compression systems and 

require custom engineering in most new applications. This is a major hurdle that now faces this 

technology as research funding will inherently drop and market pull must pick up. Thus LDACs 

will be first introduced in the highest-value applications, where market pull for the benefits is 

large enough. In its current state the LDAC technology cannot compete in facilities that do not 

over-cool/re-heat supply air and are also in locations with lower electricity rates.
21 

3.0 PERFORMANCE OBJECTIVES

Table 6 describes the performance objectives, metrics, and data requirements to determine the 

performance objective results; and the criteria for achieving the objectives.

Table 6. Performance Objectives

Performance 

Objective Metric Data Requirements Success Criteria

Quantitative Performance Objectives

Improve humidity 

control and comfort 

(energy efficiency)

• Hours outside 

psychrometric 

comfort zone 

• Chiller power

• Reheat run-time

• Indoor 

temp./humidit

y 

• Chiller power

• Reheat coils 

on 

• <1% of hours 

outside 

ASHRAE 

summer 

comfort zone 

• Reduce 

chiller/reheat 

run-time

Provide highefficiency 

dehumidification 

(energy efficiency)

• EER

• COP

• Supply-air 

temp./humidity

• Supply-air flow 

rate

• Ambient 

temp./humidity

• Power 

consumption 

• Heat 

consumption

• EER >40 

(Btu/hr)/W 

• >0.7 Thermal 

COP

Sustain highdehumidification 

performance 

(energy efficiency 

and maintenance)

• Conditioner 

heat-exchange 

effectiveness

• Desiccant charge 

• Supply-air 

pressure drop

• Conditioner 

cooling-water 

pressure drop

• Projected 

service life

• Supply air 

temp/hum 

• Ambient 

temp/hum 

• Desiccant 

chemistry and 

concentration

• Conditioner coreair and waterpressure drop

• <5% degradation 

of HX eff. over 3 

years

• <Once-per-year 

desiccant/buffer 

adjustment 

• Negligible 

increase in 

air/water pressure 

drop 

• Above criteria 

should support 

>10-yr service 

life projection
22 

Qualitative Performance Objectives

Maintainability 

(ease of use)

Ability of an HVAC 

technician to operate 

and maintain the 

technology 

Standard form 

feedback from the 

HVAC technician on 

usability of the 

technology and time 

required to maintain

A single facility 

technician able to 

effectively operate 

and maintain 

equipment with 

minimal training

3.1 IMPROVE HUMIDITY CONTROL AND COMFORT

Conventional AC has fundamental limitations regarding humidity control. Mechanical AC

provides coincidental drying when it lowers air below its dew-point temperature, condensing 

water vapor on its cooling coils. In order to provide separate control over temperature and 

humidity, it must overcool the air and then reheat it so as not to overcool the space. This can be 

excessively energy intensive. Mechanical AC runs into additional problems with 

dehumidification when humidity loads are large compared to sensible loads, such as in the spring 

and fall, and under any conditions where short-cycling occurs, which leads to re-evaporation of 

water from the cooling coils. 

This objective will demonstrate that desiccant drying effectively manages humidity 

independently of sensible loads throughout the cooling season. A comfort zone of combined 

temperature and humidity ranges has been established by ASHRAE; a technology’s ability to 

provide comfort is typically represented by the number of hours per year indoor conditions leave 

this established zone. NREL monitored chiller power and reheat coil run-times both with and 

without desiccant system operation to show effects on comfort and overcool/reheat. NREL used

its testing experience to ensure that sensors were positioned to read representative supply-air 

temperatures that were not skewed by spatial nonuniformities or radiative heat exchange. 

3.2 PROVIDE HIGH-EFFICIENCY DEHUMIDIFICATION 

It is critical to establish the efficiency advantages of desiccant dehumidification over 

conventional mechanical AC. The cooling effect is determined by the change in enthalpy of the 

supply air relative to ambient, multiplied by the total mass flow of supply air. The range of 

psychrometric conditions that is monitored allows high-quality temperature and RH sensors to 

adequately make these enthalpy measurements. Mass flows are measured with pitot tube-in-duct 

measurements, inferred from fan power draw and checked against pressure drop measurements 

calibrated at NREL’s test facility. Energy balance for the cooling effect can be checked against 

the measured heat gain of the cooling water flowing through the conditioner. 

Power measurements to monitor fan and pump energy are straightforward. Thermal energy 

consumption is also conveniently measured by the drop in temperature of the solar collector fluid 

through the regenerator multiplied by its mass flow rate and specific heat. All of these 

parameters are straightforward to determine in real-time. 
23 

3.3 SUSTAIN HIGH-DEHUMIDIFICATION PERFORMANCE

A common concern regarding liquid-desiccant devices is the carryover of desiccant droplets into 

the supply (or scavenging) airflows, exhibited as decreasing desiccant charge and corrosion of 

metal ductwork where these droplets settle. One strategy for avoiding this loss of desiccant is

the deployment of high-pressure drop demisters in the supply duct. However, this approach is 

not desirable because it imparts a significant energy penalty in fan power and requires frequent 

maintenance to remain effective. Alternatively, the low-flow liquid desiccant uses minimal mist 

eliminators, relying on its low-flow feature to prevent carryover and preserve desiccant charge. 

This low-flow feature is enabled by the flocked surfaces of its conditioner component. These 

desiccant-wetted wicks must keep themselves relatively free of dirt buildup to maintain their 

zero-carryover feature and the effectiveness of the HMX. The conditioner’s heat exchange 

effectiveness and pressure drop were monitored to ensure that ambient dirt was being 

successfully managed to preserve performance over a projected service life of 10-15 years. In 

general, because the conditioner is water cooled, mineral fouling of its internal water passages is 

possible, albeit improbable due to its all-plastic construction. 

3.4 MAINTAINABILITY 

There are many ways for a building technology to fail if its operation requires more than the 

current HVAC standard maintenance requirements, which is minimal. The success criteria for 

this performance objective were determined if the demonstration unit settled into autonomous 

operation after the first season following Tyndall’s standard HVAC maintenance schedule. 
24 

4.0 FACILITY/SITE DESCRIPTION

The selected laboratory at Tyndall AFB is located in Panama City, Florida, on the Gulf Coast. 

Its high temperatures are typically in the 80-89°F range in the summer, rarely above 90°F, with 

ambient humidity in excess of 0.02 lbs of water per lb of dry air through portions of the cooling 

season. Its design wet-bulb temperatures are very high, ranging from 70°F to over 80°F, with 

humidity extremes up to 90%. Figure 10 illustrates the wide range of temperatures and humidity 

that Tyndall experiences throughout the year. 

Figure 10. Psychometric plot Tyndall AFB

In addition to the hot and humid summer days, the cool temperature days from the late fall to 

early spring allow for a robust system performance evaluation through the observation of 

operation in non-ideal weather conditions and taking proper measures to avoid damage from 

freezing. Altogether, the site provides the necessary spectrum of ambient conditions to 

characterize the LDAC performance sufficiently for predicting performance across most, if not 

all, conditions in the United States, U.S. territories, and countries with active U.S. military 

operations. 

4.1 FACILITY/SITE LOCATION AND OPERATIONS

The Air Force Research Laboratory (AFRL) building at Tyndall AFB is a mix of laboratory and 

office space. Three main air-handling units (AHUs), serve the laboratory and office space.

A satellite image of Tyndall AFB and the LDAC system is provided in Figure 11. 

 
25 

Figure 11. AFRL site. 

(© 2012 Google) 

Typical configuration may include a single-packaged unit or a split-system arrangement, where 

the regenerator cabinet is physically split from the conditioner cabinet. The requirement to place 

the solar system in the nearby field rather than roof mounting meant that the regenerator is best 

placed in the field adjacent to the solar array for minimizing heat loss. The conditioner, cooling 

tower, and desiccant storage tank are located in an enclosed HVAC area, so the conditioned air 

can be supplied into the building via ductwork. 

Figure 12 shows the Tyndall AFB building layout, with the space apportioned by office, 

laboratory, and mechanical rooms. The red box highlights the predominantly laboratory space for 

which the LDAC system provides ventilation air. Reheat coils in terminal units in each zone 

activate if the air has been overcooled, and using measured data from AHU #3, the reduction of 

reheat can be determined.

Solar 

Thermal 

Array and 

Regenerato

Conditioner, 

Storage 

Tank, 

Cooling 

Tower

2ft x 2ft 

duct run
26 

Figure 12. Tyndall AFB building layout. 

(Illustration by Jesse Dean, NREL) 

The laboratory wing served by the low-flow, liquid-desiccant unit underwent a chiller upgrade in 

2008 because cooling loads were going unmet. The building was also recommissioned to 

balance the outdoor air to ensure positive pressure within the building to eliminate condensation 

due to infiltration. As a result, reheat coils were not being actuated because indoor temperature 

set points were not being reached. This implies that indoor humidity was not being controlled 

and that conditions were likely uncomfortably humid. The chiller upgrade provides sufficient 

capacity to properly dehumidify (overcool) the space, and therefore, required reheat coil 

operation. The upgrade also included condenser heat recovery to offset reheat energy use. 

A DX, air-cooled chiller (Table 7) supplies chilled water to all three AHUs. The LDAC system 

conditions ventilation air that serves AHU #3 (Table 8).

AHU #3 

w/ Reheat

LDAC Supply 

Space
27 

Table 7. Air-Cooled Chiller Schedule

Table 8. AHU #3 schedule
28 

5.0 TEST DESIGN 

5.1 CONCEPTUAL TEST DESIGN

NREL installed instrumentation and a data acquisition system for one SOA3000 dehumidifier, 

powered by a 1,300-ft2, evacuated-tube solar thermal array. The solar array and regenerator 

components are oversized relative to the conditioner’s average dehumidification output in order 

to generate and store excess desiccant during the day. An 800-gal uninsulated storage tank fully 

utilizes the solar arrays excess heat output and allows for a few hours of average cooling 

operation without solar input. The unit is designed to operate continuously at maximum airflow 

in order to serve fume-hood makeup air needs. The LDAC technology was characterized in 

NREL’s Advanced HVAC Systems Laboratory in 2004 and 2006. The laboratory test results are 

invaluable in interpreting the field results, particularly with regard to critical airflow rates, which 

are notoriously difficult measurements to make in the field. The unit was monitored for two 

cooling seasons, and its annual and peak energy use was compared to conventional AC. 

5.2 BASELINE CHARACTERIZATION 

The installation had the potential to generate compelling side-by-side test results in that the 

recent chiller upgrade should allow operation with or without desiccant unit operation. 

Circumstances that complicate comparison include the facts that: 1) the chiller serves the entire 

building; and 2) the disparity in capacities between the chiller and the desiccant system is 

approximately 10:1. Chiller power and calls for reheat were measured in one wing of the 

facility. Because mechanical AC is a well-understood technology, baselines for individual sites 

were not critical to project energy savings relative to conventional equipment at various 

efficiency levels. Once the efficiency of the desiccant system was established, comparisons of 

energy use relative to mechanical AC were straightforward over the full range of building 

applications and climates. 

5.2.1 Laboratory Testing 

NREL was directly involved in the development of the LDAC technology and funded the 

development of the three core HMXs now used: the conditioner, regenerator, and interchange 

heat exchanger. These HMXs were developed by AILR and NREL determined the performance 

of the units through testing at the Advanced HVAC Systems Test Laboratory. The final 

conditioner and regenerator HMX designs were tested in 2004 and 2006 respectively. The data 

from these tests proved invaluable in the demonstration as baseline performance was determined 

and verified against laboratory data. Airflow, pressure drop, and exchange effectiveness from 

the laboratory proved invaluable in the calibration of airflow and pressure drop measurements in 

the field, which improved the uncertainty of the field data. There are no standard methods for

testing LDAC systems. However, NREL is a world leader in the development of desiccant 

technologies and draws upon sound practices in order to determine the performance of these 

HMXs. To determine effectiveness, each HMX was instrumented at the NREL laboratory with 

the following parameters and accuracy: 

• Air and liquid temperature measurements have an accuracy of ± 0.36°F with <0.18°F 

deviation. 

• Airflow measurements have an accuracy of ± 2%. 

• Humidity is calculated with dew-point hydrometers and has an accuracy of 0.27°F. 
29 

• Differential pressure measurements have an accuracy of ± 0.025 inches water column ( 

W.C.) 

• Barometric pressure has an accuracy of 0.15%. 

• Water flow-rate measurements have an accuracy of 0.5%. 

• Liquid desiccant flow-rate measurements have an accuracy of 1.0%. 

• Desiccant concentration by weight is measured to within 0.0025 lbs salt per lb of 

solution. 

A diagram of the HVAC laboratory in the Thermal Test Facility at NREL is provided in 

Figure 13. 

 

Figure 13. NREL Thermal Test Facility. 

(Illustration by NREL) 

The laboratory testing resulted in the development of a regression of cooling effectiveness and 

capacity versus airflow rate; conditioner and regenerator water flow rate and temperature; and 

desiccant concentration. These data were also shared with AILR to calibrate its internal 

modeling of the LDAC system for design purposes. It is, however, impractical to measure every 

combination of independent variables to determine a performance map of the LDAC system. 
30 

Thus the calibration/validation of the AILR model using NREL data improves the accuracy of 

the model, making it a suitable tool to determine LDAC performance using building energy 

simulators such as EnergyPlus. 

5.3 DESIGN AND LAYOUT OF TECHNOLOGY COMPONENTS

Figure 14 illustrates the design and layout of the LDAC system. Demonstration equipment was 

placed in two locations at the test site. The solar array and regenerator components were

collocated in the open field to the north of the building. The desiccant storage, conditioner 

component, and cooling tower were placed within the walled HVAC equipment area on the west 

end of the subject wing with the chiller. Piping connecting the storage tank and conditioner 

supply strong desiccant for the dehumidification process, and a 100-ft-long duct run was 

installed across the roof to connect the conditioner to the fresh air intake of the building. 

Figure 14. LDAC system and supply air layout. 

(Illustration by NREL) 

5.3.1 Conditioner

The conditioner component (Figure 15) is a proprietary plastic heat-exchanger design that brings 

liquid desiccant into direct contact with ventilation air to dehumidify the outside air, while 

simultaneously rejecting heat to the cooling tower water. The system was designed to operate

with CaCl2 as the liquid desiccant (up to 43% salt and 57% water), which is suitable for solar 

applications. The conditioner is designed to reduce the RH of the conditioned air to 35% - 40% 

RH. A 3.5-horsepower (hp) fan with a variable-frequency drive (VFD) is designed to provide 

3,000 cfm of treated air to the building. Two fractional (1/4) hp pumps circulate ~ 3 gal per 

minute (gpm) of desiccant between the storage tank and the conditioner. A 1-hp pump supplies

37 gpm of cooling tower water to the conditioner. 
31 

Figure 15. Image and design of conditioner unit. 

(Photo by NREL) 

The cooling tower (Figure 16) is an EVAPCO model ICT 3-63 packaged unit with a 1-hp fan 

with a VFD.
32 

Figure 16. Image and design of cooling tower. 

(Photo by NREL) 

5.3.2 Regenerator

The water vapor absorbed by the liquid desiccant is carried away in the humid exhaust air exiting 

the regenerator (Figure 17). The regenerator installed at Tyndall AFB is designed to utilize 165-

190°F water to drive the dehumidification process. The water is heated by a plate and frame heat 

exchanger with the solar thermal heating system providing the heat. A 0.7-hp fan provides the 

900 cfm of scavenging air needed to reject the moisture to the ambient air. A fractional (1/4) hp 

pump circulates ~4 gpm of desiccant between the storage tank and the regenerator. A 1-hp pump 

supplies 18 gpm of solar-heated water to the regenerator. 
33 

Figure 17. Image and design of regenerator unit. 

(Photo by NREL) 

5.3.3 Solar Collectors

The solar array (Figure 18) is a Viessmann 200-T model evacuated-tube collector using 29 30-

tube bundles covering 1,350 ft2 of area. The 200-T model evacuated-tube collector is a tube-intube flow through design, which has different performance characteristics than the standard heat 

pipe design. The solar collector transfers the collected solar energy to a heat transfer fluid that is 

then transferred through a plate and frame heat exchanger to the regenerator. Because the solar 

collector stagnates the entire liquid volume internal to the collector, this solar collector requires 

unusually high expansion tank volumes to deal with stagnation pressures as well as a heat 

transfer fluid that can resist 450°F stagnation temperatures. For this reason, water was used as 

the heat transfer fluid. Under normal conditions, propylene glycol could be used with these 

collectors, provided that stagnation is considered an uncommon occurrence (occurring less than 

about three times per year). However, the LDAC system would frequently be shut down for 

maintenance or lack of cooling load throughout its normal operation. The use of water then 

necessitates draining the unit prior to freezing conditions every year and startup must occur after 

the last expected freeze date. For this reason, this particular collector design was a poor choice 

to be paired with an LDAC system in a freezing climate. Furthermore, heat-pipe, evacuated-tube 

designs require far less expansion tank volume than do the 200-T. A solar system is best used 
34 

when there is a year-round thermal load to maximize this more expensive thermal energy source. 

Currently, islands with high fuel costs are the target application for pairing LDAC with solar 

thermal energy. 

Figure 18. Image and design of solar collector array. 

(Photo by NREL) 

5.3.4 Desiccant Storage Tank

Desiccant storage is provided in an 800-gal, uninsulated, plastic tank (Figure 19). A proprietary 

plastic interchange heat exchanger is plumbed between the tank and the regenerator. It precools 

hot, strong desiccant coming from the regenerator and preheats weak desiccant going to the 

regenerator. This process reduces the heat input required to the regenerator and supports 

concentration stratification in the storage tank. This enhances performance by supplying the 

conditioner with the strongest desiccant possible at all times.
35 

Figure 19. Image and design of desiccant storage tank. 

(Photo by NREL) 

Regenerator and conditioner operation are essentially independent. Regeneration components 

are activated when sensors indicate the solar array is capable of generating minimum productive 

temperatures (~160°F). The control sequence for the conditioner was modified throughout the 

2011 cooling season in an attempt to evaluate the technology in a variety of operating modes. 

The system components have pumps and fans to drive liquid and air through the system. 

Table 9 lists the size and power draw of the pumps and fans that are used on the LDAC system.
36 

Table 9. LDAC System Pump and Fan Motor Schedule 

Function Type Motor Nameplate Data

Power 

(hp)

Power 

(W) VFD

Conditioner

Supply Pump Centrifugal 0.25 186.5 No

Return Pump Vertical 0.25 186.5 No

Process Fan

Motorized 

Impeller 3.45 2,570 Yes

Cooling 

Tower

Pump Jet Pump 1 746 Yes

Fan

Axial 

Propeller 1 746 Yes

Regenerator

Supply Pump Centrifugal 0.25 186.5 No

Return Pump Centrifugal 0.25 186.5 No

Loop Pump Vertical 0.24 179 No

Fan

Motorized 

Impeller 0.71 530 Yes

Solar Array

Loop Pump Circulating 0.24 180 No

Loop Pump Circulating 0.33 245 No

5.4 OPERATIONAL TESTING

Field testing was conducted in two phases: startup and monitoring. During startup, NREL and 

Mountain Energy Partnership (MEP) installed sensors and confirmed HVAC/data system 

operation on-site. Startup commenced as the equipment installation proceeded in winter 2009

and concluded 2 weeks later. The performance of the system was monitored over the 2010 and 

2011 cooling season, and the unit was shut down and winterized each winter. 

5.5 SAMPLING PROTOCOL

An initial site visit to Tyndall AFB was made in December 2009 to install the monitoring system 

for the LDAC system. All of the sensors and data loggers were installed at that time; however, 

the solar collector and LDAC were not functioning properly due to the improperly designed 

stagnation strategy with the Viessmann solar collectors. Modifications to the original solar 

collector design were required to accommodate normal stagnation conditions of the system. The 

monitoring system could not be fully commissioned until normal operation of the LDAC was 

achieved in July 2010. A site visit was made in July 2010 to complete the monitoring system 

installation.
37 

Normal operation of the LDAC and a complete monitoring system facilitates an initial estimate 

of uncertainty in the measured LDAC heat flows. The sensible and latent capacities of the 

LDAC are of primary importance in evaluating system performance. The monitoring system 

directly measured the latent and sensible capacities on the airside of the conditioner by 

measuring the air velocity with a pitot tube, the change in dry-bulb temperature using two 

resistance-temperature device sensors, and the change in humidity ratio using two capacitive RH 

sensors. Flow rates through the conditioner were inferred using the pressure drop through the 

conditioner and the fan-speed indication, measured by the conditioner’s programmable logic 

controller (PLC) and transferred to the data logger via Modbus communication. The heat 

removed by the cooling tower was intended to be a direct measurement of the total capacity of 

the conditioner. Water-side measurement using a turbine flow meter and two thermistors were 

used because it was expected to have lower uncertainty than the air-side measurement. A third 

measurement of latent capacity was estimated by determining the change in liquid volume of the 

desiccant storage tank during periods when only the conditioner was in operation. 

A list of monitoring points and sensor accuracy is provided in Table 10. In addition to sensors 

installed by MEP, outputs from the LDAC controller were transferred via Modbus 

communication and recorded by a Campbell Scientific CR1000 data logger. 
38 

Table 10. Sensor Accuracy Summary

Sensor Location Vendor Model Accuracy 

Specification

Immersed 

thermistor

Collector loop Omega 

Engineering

TJ36-44004 ±0.18 oF

Immersed 

thermistor

Cooling tower Omega 

Engineering

ON-910-

44006

±0.18 oF

Temperature and 

RH

Duct mount Vaisala HMD40Y ±0.36 oF, ±2 

% RH

Temperature and 

RH

Wall mount Vaisala HMW40Y ±0.36 oF, ±2 

% RH

Temperature Supply register Cantherm MF52 ±0.36 oF

Turbine flow 

meter

Collector Omega 

Engineering

FTB1431 1 % of 

reading

Turbine flow 

meter

Cooling tower 

water

Omega 

Engineering

FTB8015BPT1.5 % of 

reading

Turbine flow 

meter

Cooling 

makeup water

Omega 

Engineering

FTB602B-T 1 % of 

reading

Turbine flow 

meter

Desiccant Omega 

Engineering

FTB6207-PS 1.5 % of 

reading

Differential 

pressure

Conditioned 

makeup

Setra 264 1 % of full 

scale

Differential 

pressure

Total makeup Setra 264 1 % of full 

scale

Differential 

pressure

LDAC unit Setra 264 1 % of full 

scale

Ambient pressure Outdoor Setra 276 1 % of full 

scale

Electrical energy Regen and

conditioner

Continental 

Controls

WNB-3D240-P0.5 % of 

reading

Current 

transformer

Regen and 

conditioner

Continental 

Controls

CTS-0750-30 1 % of 

reading

Pyranometer Horizontal Campbell 

Scientific

CS300 5 % of daily 

total

Level transmitter Desiccant tank Omega 

Engineering

LVU 109 +/- 0.6 cm

5.6 EQUIPMENT CALIBRATION AND DATA QUALITY ISSUES

5.6.1 Calibration of Equipment

The primary measurement instrument was the Campbell CR1000 data logger. It measured 

temperature, humidity, flow rate, pressure, and electric power; stored the data; and automatically 

relayed the data to NREL for analysis. These systems were maintained and calibrated in 

accordance with manufacturer’s requirements. 
39 

5.6.2 Quality Assurance 

Quality assurance was provided primarily by NREL’s field and lab experience testing this type 

of device. All sensors were sampled every 10 seconds, and any mathematical manipulations of 

those primary measurements were also made on the same 10-second interval. Data were stored 

as averages or totals in four separate data tables identical in field description but varying in 

storage interval: 1-minute, 15-minute, 60-minute, and 24-hour (midnight-to-midnight). 

Thermistor probes installed in pipes and immersed in water or glycol solution were used in 

calculating some of the primary energy flows of the collector and cooling tower of the LDAC 

system. The specified accuracy of 0.18 oF for the immersed sensors is the best normally 

available accuracy for this type of sensor. The uncertainty in the energy measurement for the 

collector or the cooling tower depends on the temperature difference between two fluid streams. 

As that temperature difference becomes small, the accuracy of the energy measurement can 

become unacceptably large. In this application, the temperature difference for the collector loop 

was approximately 18 oF on a sunny day. The temperature difference of the cooling-tower loop 

was typically greater than 5.4 oF, which provides sufficient temperature difference to limit 

accuracy problems.

Duct-mounted temperature and RH sensors were used to calculate the sensible and latent energy 

removed from the conditioned airstream. The sensor accuracy of the device is ±0.63 oF. The 

wall-mounted temperature and RH sensor, and the supply register temperature sensor were not 

used in calculation of primary energy flows, but were used as an indication of indoor comfort 

conditions. 

A pyranometer was used to quantify the efficiency of the solar collector array. Two 

measurements of electric energy were made: one for the regenerator unit, including the collector, 

and one for the entire conditioner unit. 

Differential pressure sensors were used to measure airflow rates of the LDAC unit and the main 

air handler. One pressure sensor was used with a pitot tube to measure the airflow rate exiting 

the conditioner. Another pressure sensor measured the pressure difference across the LDAC 

core as an additional indication of airflow rate and heat exchanger fowling over time. The 

measurement of liquid flow rate in the collector loop and the cooling tower loop were used in 

calculating primary energy flows for the LDAC system. Relative difference between the pitot 

tube and conditioner pressure drop is a good indicator to show consistent airflow performance.

5.7 SAMPLING RESULTS

A detailed summary of the sampling results is provided in the report’s performance assessment 

section.
40 

6.0 PERFORMANCE ASSESSMENT

Performance evaluation of the LDAC began in the summer of 2010. Three weeks of continuous 

operation was recorded during the 2010 cooling season, and around 5 months of operation were 

recorded for 2011. Because the majority of the LDAC system operation occurred during the 

summer months of 2011, the performance assessment is based on summer 2011 data. The 2010 

performance data are presented to illustrate performance variability. Representative performance 

assessment metrics for each objective are summarized in Table 10. 

Table 11 describes the performance objectives, metrics, and data requirements to determine the 

performance objective results, and the criteria for achieving the objectives.

Table 11. Performance Objectives

Performance 

Objective Metric Data Requirements Success Criteria Results

Quantitative Performance Objectives

Improve 

humidity control 

and comfort 

(energy 

efficiency)

• Hours 

outside 

psychrometri

c comfort 

zone 

• Chiller 

power 

• Reheat runtime

• Indoor 

temp./humidit

y 

• Chiller power

• Reheat coils 

on 

• <1% of 

hours 

outside 

ASHRA

E 

summer 

comfort 

zone 

• Reduce 

chiller/re

heat 

runtime

• Achieved 

but 

inconclusive 

cause

• Achieved 

but 

inconclusive 

cause 

Provide highefficiency 

dehumidification 

(energy 

efficiency)

• EER

• COP

• Supply-air 

temp./humidity

• Supply-air flow

rate

• Ambient 

temp./humidity

• Power 

consumption 

• Heat consumption

• EER >40 

(Btu/hr)/W 

• >0.7 Thermal 

COP

• Not 

achieved

• Achieved
41 

Sustain highdehumidification 

performance 

(energy 

efficiency and 

maintenance)

• Conditioner 

heat 

exchange 

effectiveness

• Desiccant 

charge 

• Supply air 

pressure drop

• Conditioner 

cooling water 

pressure drop

• Projected 

service life

• Supply-air 

temp./humidity

• Ambient 

temp./humidity

• Desiccant 

chemistry and 

concentration

• Conditioner coreair and waterpressure drop

• <5% 

degradation 

of HX eff. 

over 3 years 

• <Once-peryear 

desiccant/buf

fer 

adjustment 

• Negligible 

increase in 

air/water 

pressure drop

• Above 

criteria 

should 

support 

>10yr 

service life 

projection

• Achieved; 

no 

degradation 

of desiccant 

during 

operation 

• Duration of 

performance 

evaluation 

too small to 

determine 

Qualitative Performance Objectives

Maintainability 

(ease of use)

Ability of an HVAC 

technician to operate 

and maintain the 

technology 

Standard form 

feedback from the 

HVAC technician on 

usability of the 

technology and time 

required to maintain

A single facility 

technician able 

to effectively 

operate and 

maintain 

equipment with 

minimal training

• Many 

unforeseen 

maintenance 

issues 

occurred 

during 

initial 

demonstrati

on 

• Many 

lessons 

learned for 

design and 

ease of 

operation
42 

6.1 IMPROVE HUMIDITY CONTROL AND COMFORT

The LDAC provides primarily latent cooling to the air, so the total cooling is representative of 

dehumidification. To show the effect of latent cooling, Figure 20 and Figure 21 illustrate the 

decrease in humidity of the air. 

Figure 20. Outdoor air conditions and conditioner-outlet air conditions for summer 2010 

operation 
43 

Figure 21. Outdoor air conditions and conditioner-outlet air conditions for summer 2011 

operation 

During the 2010 cooling season, the unit dehumidified the air into the 40% to 55% RH range and 

was not as close to the RH equilibrium point of ~35% RH for CaCl2 desiccant solution because 

the system was operating on a weaker desiccant.

The LDAC unit noticeably reduced the enthalpy of the outside air during the 2011 cooling 

season, and the RH of the outside air was reduced to 35% to 60% RH throughout the summer. 

As noted above, the drying potential of the system is based on the concentration of the desiccant 

entering the conditioner, and the system is operating with a weaker desiccant when the 

conditioned air RH is in the 40% to 60% range. 

Figure 22 illustrates the daily cooling supplied by the LDAC during the 3 weeks of summer 2010 

operation. The daily average cooling was 79.3 ton-hr. 

20% 20% 20% 20% 20% 20% 20%

20%

20%

20%

20%

20%

20%

20%

20%

40% 40% 40%

40%

40%

40%

40%

40%

40%

40%

40%

40%

40%

40%

40%

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

0.022

0.024

30 40 50 60 70 80 90 100

Humidity Ratio (lb Water/lb Dry Air)

Dry-Bulb Temperature (oF)

Tyndall Lab - Summer 2011 (April 16 - Sept. 30)

ASHRAE Winter Comfort Zone

ASHRAE Summer Comfort Zone

Weather Outdoor

Measured Outlet Conditioner

70oF

60oF

50oF

40oF

70oF

60oF

50oF

40oF

Relative Humidity
44 

Figure 22. Daily cooling load supplied by the LDAC during the 3 weeks of summer 2010 

operation 

Figure 23 shows the total cooling load provided by the LDAC during the summer of 2011.

Several days were omitted from Figure 23 because of low or no operation on those days. The 

average daily cooling is 56 ton-hr, which is representative of a 10-ton cooling system that 

operates for 5.6 hr during the day. This is below the expected cooling load achievable from this 

system, which is greater than 100 ton-hr/day. The reason the cooling load was lower than 

anticipated was primarily based on the fact that the supply fan for the conditioner was operated at 

less than 75% of the rated flow rate (3,000 cfm) for the 2011 cooling season, and the unit was 

not providing the full capacity that it was designed to provide. In addition, the control sequence 

for the desiccant storage tank was set up to allow for more total hours of cooling per day at a 

weaker desiccant concentration, which also had the effect of reducing the cooling potential of the 

unit. 
45 

Figure 23. Daily cooling load, with monthly averages, supplied by the LDAC during the 

summer of 2011

6.1.1 ASHRAE Comfort Zone 

The criterion for improved comfort is the number of hours indoor air conditions (temperature and 

RH) fall outside of the “comfort zone,” which has been established by ASHRAE, as shown in 

Figure 24. Because the Tyndall AFB also uses a chiller for conditioning the air with a capacity 

much larger than the LDAC (about 10 times larger), the resulting supply air conditions are not 

fully dictated by the LDAC. 

The success criterion for comfort is achieving less than 1% of hours outside of the comfort zone. 

The comfort zone for the summer is shifted to warmer air temperatures because less clothing is 

worn during the summer; for a laboratory and AFB, more clothing is worn than at civilian 

facilities, so the winter comfort zone is more applicable for the Tyndall AFB for certain 

occupants. This success criterion was not met for 2010 cooling season but was met for the 2011 

cooling season. 

To illustrate the performance of the chiller alone, Figure 24 shows the outdoor air conditions and 

the laboratory air conditions from summer 2010. 

0

20

40

60

80

100

120

140

4/16/114/19/114/22/114/25/114/28/11

5/1/115/4/11

5/7/11

5/10/115/13/115/16/115/19/115/22/115/25/115/28/115/31/11

6/3/116/6/116/9/11

6/12/116/15/116/18/116/21/11

6/24/116/27/116/30/11

7/3/117/6/117/9/11

7/12/11

7/15/117/18/117/21/117/24/117/27/117/30/11

8/2/11

8/5/118/8/11

8/11/118/14/118/17/118/20/118/23/118/26/118/29/11

9/1/119/4/119/7/11

9/10/119/13/119/16/119/19/11

9/22/119/25/119/28/11

Cooling Load (ton-hr)

Day

Summer 2011 Daily Average Cooling (8:00am to 8:00am)

Cooling (ton-hr)

Monthly Average

44.5

61.2

54.4

61.8

54.0 54.4
46 

Figure 24. Tyndall Lab outdoor and indoor conditions without the LDAC operating in 

Summer 2010 (4.3% of the data is outside the comfort zone) 

The existing HVAC system and air-cooled chiller have the ability to maintain the space within 

the comfort zone for the majority of the summer; the lab is outside of the comfort zone 4.3% of

the time (when the LDAC is not operating). During the 3 weeks of LDAC operation in 2010, the 

system met comfort conditions 10.6% of the time. It is not apparent from this data that the 

LDAC system has a significant effect on the indoor conditions for the 2010 cooling season. 

There are two reasons that this could be the case: the first is related to the fact that the system 

was operating on weak desiccant and wasn’t drying the air to its full potential; and secondly, the 

LDAC unit is treating less than one-third of the outside air for this AHU. If it was treating 100% 

of the outside air, the total moisture removal rate would be significantly higher.

20% 20% 20% 20% 20% 20% 20%

20%

20%

20%

20%

20%

20%

20%

20%

40% 40% 40%

40%

40%

40%

40%

40%

40%

40%

40%

40%

40%

40%

40%

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

0.022

0.024

30 40 50 60 70 80 90 100

Humidity Ratio (lb Water/lb Dry Air)

Dry-Bulb Temperature (oF)

Tyndall Lab LDAC OFF - Summer 2010 (May 1 - Sept 14: Exlc. July 21 - Aug 14)

ASHRAE Winter Comfort Zone

ASHRAE Summer Comfort Zone

Weather Outdoor

Measured Laboratory

70oF

60oF

50oF

40oF

70oF

60oF

50oF

40oF

Relative Humidity
47 

Figure 25. Tyndall Lab outdoor and indoor air conditions during summer 2010 with the 

LDAC in operation (10.6% of the data is outside the comfort zone) 

In the summer of 2011, the LDAC operated for most of the summer; the outdoor air and 

laboratory conditions are shown in Figure 26. Although the outdoor air conditions are slightly 

different for the 2010 and 2011 summers, it is clear that the indoor air is drier and cooler. The lab 

is outside of the comfort zone only 0.2% of the time, which is below the success criterion of 1%. 

This includes both the summer and winter ASHRAE comfort zone as it is difficult to 

differentiate which one should be applied on a military base because a number of occupants are 

wearing full military gear during the summer months, which would indicate that the winter 

comfort zone is more appropriate than the summer comfort zone for those individuals. As 

mentioned, the chiller is used to cool the air in each zone’s AHU, and the overcool/reheat cycle 

is also used to control humidity. Therefore, it cannot be conclusively stated that the LDAC 

system credibly achieved the comfort criterion.
48 

Figure 26. Tyndall Lab outdoor and indoor air conditions during summer 2011 with the 

LDAC in operation (0.2% of the data is outside the comfort zone) 

6.1.2 Reduce Reheat Run-Time

While there is evidence that the LDAC system achieves a better range of indoor air conditions,

the LDAC system was undersized for this particular application, and the chiller also controls 

humidity by overcooling the air to condense out the water and reheating it to a suitable supply 

temperature. The overcooling and reheating process adds significant cooling load to the chiller

when there is a high ventilation requirement. The LDAC mitigates this process by drying the 

humid air before being sensibly cooled with the chiller. 

The overcool/reheat process only occurs when the humidity is high. The amount of time this 

occurs can be determined by comparing the discharge air temperature from the main AHU to the 

supply-air temperature into the space (after the terminal-unit reheat coil), which activates the 

terminal-unit reheat coils when the temperature is too low for comfort. The discharge-air 

temperature was only measured after one terminal unit; and if the supply-air temperature after 

the terminal unit was greater than 3.5 °F above the discharge air/supply air temperature from the 

AHU, the reheat coils were assumed to be activated, meaning the overcool/reheat process 

occurred. The 3.5°F temperature differential represents about 10 times the normal differential 

from ductwork heat gain that was monitored in this particular facility. This method was used to 

approximate the reheat run-time rather than measuring the hot water reheat system directly.

In the summer of 2010, while the LDAC was off, the reheat coils activated 8.9% of the time. 

During the 3 weeks when the LDAC was running, the reheat coils activated 2.4% of the time. 

The LDAC operated for the majority of the 2011 summer, and the reheat coils activated just 

20% 20% 20% 20% 20% 20% 20%

20%

20%

20%

20%

20%

20%

20%

20%

40% 40% 40%

40%

40%

40%

40%

40%

40%

40%

40%

40%

40%

40%

40%

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

0.022

0.024

30 40 50 60 70 80 90 100

Humidity Ratio (lb Water/lb Dry Air)

Dry-Bulb Temperature (oF)

Tyndall Lab - Summer 2011 (April 16 - Oct 1)

ASHRAE Winter Comfort Zone

ASHRAE Summer Comfort Zone

Weather Outdoor

Measured Laboratory 

70oF

60oF

50oF

40oF

70oF

60oF

50oF

40oF

Relative Humidity
49 

1.4% of the time. Although these data are limited, they indicate that the LDAC has mitigated the 

necessity of the overcool/reheat cycle, and this performance metric was successfully met. 

6.2 PROVIDE HIGH-EFFICIENCY DEHUMIDIFICATION

While it is apparent that the LDAC system can dehumidify air to improve comfort and mitigate 

the overcool/reheat cycle, it must do so while improving the efficiency of the total AC system. 

Several common metrics are commonly used to characterize the performance of AC systems, 

such as EER, kW/ton, and thermal COP. The LDAC system aims to increase the overall 

efficiency of the existing cooling process.

6.2.1 Electrical Efficiency: EER and kW/ton 

EER is defined as the ratio of cooling supplied (Btu/hr) to the electricity [watt (W)] required by 

the system: 

EER = Cooling Supplied �

Btu

hr �

Electriciy Required (W)

Figure 27 illustrates the daily electrical performance of the LDAC for the 3 weeks of summer 

2010 operation. The average EER for the 3 weeks was 14.7 (Btu/hr)/W, a 63% improvement 

over the rated EER of 9.0 (Btu/hr)/W of the existing chiller. 

Figure 27. Daily average of LDAC EER for 3 weeks of summer 2010 operation 

Figure 28 shows the daily EER performance of the LDAC system for summer 2011. It is 

apparent that the performance improved steadily throughout the summer. This is because

various adjustments were made to the system to operate at near maximum capacity while 

limiting auxiliary power consumption. As indicated by the red star in Figure 28, a VFD was 

installed on the cooling tower pump to lower power consumption during lower capacity 
50 

operation; this modification was critical to lowering the power consumption from the cooling 

tower and raising the EER. 

Figure 28. Daily and monthly average of LDAC EER for summer 2011

The red star indicates when the higher efficiency cooling tower pump motor and VFD were 

installed. As shown in Figure 28, the EER of the LDAC is between 15 and 20 for August with 

an average of 18.8. The best single-day performance was achieved in July at 25 (Btu/hr)/W. 

Although the LDAC outperformed the existing chiller in terms of EER, the success criterion for 

electrical efficiency is an EER of 40, so the system did not meet this objective. 

Figure 29 shows the dependence of EER on cooling load. 

0

5

10

15

20

25

4/16/114/19/11

4/22/114/25/114/28/11

5/1/115/4/115/7/11

5/10/115/13/115/16/115/19/115/22/115/25/115/28/115/31/11

6/3/116/6/116/9/11

6/12/116/15/116/18/116/21/116/24/116/27/116/30/11

7/3/117/6/117/9/11

7/12/117/15/117/18/11

7/21/117/24/117/27/117/30/11

8/2/118/5/118/8/11

8/11/118/14/118/17/118/20/118/23/118/26/118/29/11

9/1/119/4/119/7/11

9/10/119/13/119/16/119/19/119/22/119/25/119/28/11

Energy Efficiency Ratio (EER)

Day

Summer 2011 (4/16 - 9/30) Daily Average EER (8:00am to 8:00am)

EER

Monthly Average EER

18.8

14.6 15.1

7.8 8.2

12.4

Chiller: 9.0
51 

Figure 29. EER dependence on cooling load

While there is much scatter in the data due to variability in outdoor air conditions, solar 

availability, and operational adjustments, there is a positive linear correlation between EER and 

cooling load. Above a daily cooling load of 70 ton-hr, the system operates at an average EER of 

about 15 (Btu/hr)/W. The following list characterizes the reasons why the unit was not able to 

operate at an EER of 40: 

• Cooling tower fan/pump - Several issues caused the system to operate at lower than 

expected efficiency during April and May of 2011. The pump and fan of the cooling 

tower were set at a fixed flow and speed, which led to excessive electrical energy use to 

meet a given cooling load. The cooling tower pump required 1.2 kW of electric power, 

and the motor had an efficiency of 62%. The temperature difference across the cooling 

tower was also small (~2°F), which indicated that excessive flow rates were present. A 

new cooling tower pump was installed, in conjunction with a VFD in July 2011. A new 

control sequence was written that maintained the cooling tower fan at 75% fan speed and 

modulated the cooling tower pump flow rate to maintain a 10°F temperature difference 

between the cooling tower supply and return water temperatures. This resulted in over a 

65% reduction in cooling tower energy use. 

• Conditioner supply fan settings – The conditioner supply fan was operated at partial fan 

speeds for the 2011 cooling season and was not providing the full 3,000 cfm of 

conditioned air. This limited the cooling capacity of the unit and the overall efficiency. 

Although this was fixed towards the end of the 2011 cooling season, the cooling season 

was coming to an end and the amount of cooling that could be accomplished was 

reduced. 

• Conditioner fan power – The long 100-ft duct run from the conditioner to the outside air 

intake resulted in a significant pressure drop and the need for a large supply fan. The 

0

5

10

15

20

25

30

0 20 40 60 80 100 120 140

EER ((Btu/hr)/W)

Daily Cooling Load (ton-hr)

Electrical Efficiency EER versus Daily Cooling Load
52 

supply fan was rated at 3.45 hp and 2,570 W. If the conditioner was located directly at 

the outside air intake, it could reduce the supply fan horsepower to below 1 hp and the 

fan power by 70%. 

• Desiccant concentration – The system was operated with a weaker desiccant during the 

majority of the demonstration as a way to ensure the system would work because of 

problems with the solar field. With this particular system, the rate of water absorption

from the conditioner and removal from the regenerator is significantly different because 

of the variability in total heat production from the solar field; and the storage tank has to 

operate as a buffer between the two components. The settings were modified towards the 

end of the 2011 cooling season to operate on a stronger desiccant, but the conditioner unit 

did not operate on a fully strong desiccant throughout the day. 

• Pump and fan efficiency and part-load performance – Because this was a solar 

demonstration, additional fans and pumps were needed to move the desiccant around, and 

many of them had really low electrical efficiencies (less than 60%). In addition, a 

number of pumps and fans were constant volume units and used the same amount of 

power regardless of the total cooling capacity. Because the unit was only providing 

about one-half of its rated cooling capacity and using the same amount of electrical 

power as it would if it was providing 100% of the cooling capacity, the total electrical 

efficiency is reduced. 

In previous non-solar LDAC demonstrations, the conditioner and regenerator are located in the 

same enclosure, and the solar pumps are eliminated from the system. These systems, when 

located directly next to the outside-air intake, have successfully demonstrated electrical 

efficiencies above EER = 60. These issues and recommendations for future installations are 

provided in the lessons learned section of the report. 

6.2.2 Thermal Efficiency – Coefficient of Performance

The daily thermal performance of the LDAC system during the summer of 2011 is shown in 

Figure 30. This performance metric accounts for the thermal input and output of the LDAC unit, 

which represents the solar heat utilization of the system.
53 

Figure 30. Thermal performance (COP) of LDAC for summer 2011

While there is no fuel cost from this heat source, it is still beneficial to utilize the maximum 

amount of the captured heat. The regenerator heat measurement for the summer 2010 operation 

was limited to 3 days, so a graphic is not shown here; the average COP of these 3 days was 0.85, 

where COP is defined as:

COPthermal = ∆hair(Cooling Supplied)

∆hHot Water(Solar Thermal Energy)

It should be noted that the system achieves a COP of greater than 1 on a few days due to excess 

desiccant storage from the previous day. The COP increased throughout the summer due to an 

increase in solar availability and the amount of cooling the system achieves. The success 

criterion for thermal COP of 0.7 was achieved for the system. 

Another metric to compare the efficiency of the heat source usage is the solar efficiency. While 

the solar heat source is “free” after the capital costs and operational costs are accounted for, it is 

useful to know how efficiently the system is using the available solar energy to maximize its 

value. The solar efficiency is defined as the ratio of the heat transferred to the regenerator from 

the solar loop to the global horizontal irradiance (GHI) in W/meter2 times the solar array 

absorber area: 

Solar Efficiency = Heat to Regenerator

GHI ∗ Aabsorber

The solar efficiency was relatively constant at an average value of 68% throughout the summer, 

as shown in Figure 31. 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

4/16/114/19/114/22/114/25/114/28/11

5/1/115/4/115/7/11

5/10/115/13/115/16/115/19/115/22/115/25/115/28/115/31/11

6/3/116/6/116/9/11

6/12/116/15/116/18/116/21/116/24/116/27/116/30/11

7/3/117/6/117/9/11

7/12/117/15/117/18/117/21/117/24/117/27/117/30/11

8/2/118/5/118/8/11

8/11/118/14/118/17/118/20/118/23/118/26/118/29/11

9/1/119/4/119/7/11

9/10/119/13/119/16/119/19/119/22/119/25/119/28/11

Thermal Efficiency (COP)

Day

Summer 2011 Daily Average Thermal Efficiency (8:00am to 8:00am)

COP

Monthly Average

0.44

0.62 0.71 0.71 0.73

0.50
54 

Figure 31. Solar efficiency for summer 2011 

6.2.3 Water Consumption 

Figure 32 illustrates the monthly water usage (gal) for each ton-hr of cooling. Several issues in 

the first 2 months, such as blow-down pipe leakage and over-circulation of cooling tower water 

(evaporation losses), caused the system to use a large amount of water. The issues were corrected 

in June and July 2011, and the average water usage for the rest of the summer was 1.3 gal/ton-hr. 

This is in line with existing arguments, but also suggests that the air temperature out of the 

cooling tower was higher than ambient because water consumption was less than 1.55 gal/ton-hr. 

 

Figure 32. Water usage for LDAC cooling operation 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

4/7/11 4/27/11 5/17/11 6/6/11 6/26/11 7/16/11 8/5/11 8/25/11 9/14/11 10/4/11

Solar EFficiency (%)

Day

Summer 2011 Daily Solar Efficiency

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

April May June July August Sept

gal/ton-hr

Water Usage for LDAC Cooling
55 

6.3 PERFORMANCE SUMMARY

The 3 weeks of performance data for 2010 are summarized in Table 12. 

Table 12. Summer 2010 (3 Weeks) Performance Summary

Date Cooling 

(ton-hr)

EER 

[(Btu/hr)/W] kW/ton Solar heat (MBtu)* COP*

7/21/10 -

8/14/10 1982 14.7 0.82 3.1 0.85 

*Solar thermal generation only recorded for 3 days (7/21-7/23) 

Table 13 provides a listing of electrical EER, electrical kW/ton, and thermal COP for each month

in 2011. It is clear that the performance improved throughout the summer, both in electrical and 

thermal efficiency.

Table 13. Monthly (Averaged) Performance for Summer 2011

Month Cooling 

(ton-hr)

EER 

[(Btu/hr)/W] kW/ton Solar heat (MBtu) COP

April 667 7.8 1.54 18.1 0.44

May 1565 8.2 1.47 39.9 0.5

June 1837 12.4 0.97 35.4 0.62

July 1142 14.6 0.82 19.4 0.71

August 1916 18.8 0.64 32.2 0.71

September 1300 15.1 0.79 26.7 0.73

Figure 33 annotates the timeline of the LDAC performance to synchronize the events and 

changes that occurred to the LDAC system throughout the summer of 2011 with the performance 

metrics described in the previous sections. 
56 

Figure 33. Timeline with electrical (EER) and thermal (COP) efficiency performance labeled with LDAC system events and 

changes
57 

6.4 SUSTAIN HIGH-DEHUMIDIFICATION PERFORMANCE

Several metrics were defined for the quantification of sustained performance: conditioner heat 

exchange effectiveness; desiccant charge; supply air and cooling water pressure drop; and 

projected service life. Because the system only operated for about 6 months at various operating 

points, these metrics are difficult to quantify.

6.4.1 Conditioner Heat Exchange Effectiveness

The conditioner heat exchange effectiveness is defined as the change in enthalpy between the 

outside air and discharge air from the conditioner divided by the outside air enthalpy minus the 

desiccant enthalpy: 

ε = ∆hair

hair,in − hdes,Tcold

Because the unit only operated for a couple of weeks during the 2010 cooling season, it is not 

possible to calculate a change in heat exchanger effectiveness.

6.4.2 Desiccant Charge

This metric quantifies the addition or removal of desiccant that must occur during the course of a 

year. Because the system only operated for one complete cooling season, there is not enough 

statistical data to quantify this metric. Additional salt (in solution form) was added to the system 

in June 2011 because the original concentration calculations overestimated the desiccant 

concentration. This addition was not associated with a loss of desiccant concentration over time. 

Kathabar liquid-desiccant systems have run for many years without requiring replacement or 

adjustment of the desiccant charge; however, it is appropriate to inspect the desiccant at least 

once a year. 

There are also times when the desiccant solution picks up contaminates from the air or from 

cooling water or boiler (solar) water leaks. For example, the 3,000-cfm Kathabar unit that ran 

for a number of years at the University of Maryland CHP test facility was located too close to the 

exhaust of a natural gas engine. The desiccant in that unit absorbed combustion products and 

became very acidic over time. These issues can be properly addressed with adequate planning 

and design. 

6.4.3 Pressure Drop Increases

It is necessary to sustain low-pressure drops in the system to maintain high-efficiency cooling.

The quantification and statistical certainty of this metric also suffers from the relatively low time

span in which the system was operational. Several maintenance issues can be addressed to ensure 

low-pressure drop for both the process air and the water flows. 

Air filters for the regenerator and the conditioner must be changed regularly, depending largely 

on the quantity of outdoor air particulates. The filters at Tyndall AFB have lasted between 2 and 

3 months, while filters on a system in Los Angeles, California, must be changed every month. 

Desiccant and water filters can also cause an increase in pressure drop if not inspected. One 

major issue, which increased the pressure drop across the cooling tower unit, was due to not 
58 

maintaining biological growth control. This issue caused the cooling tower water filter to clog 

and required replacement every 4-6 weeks. This issue was resolved by adding biocide to the 

water. Additionally, a new strainer replaced the filter, which decreased the pressure drop. With 

proper design and filter selection, pressure drop can be reduced and maintained without intensive 

man-hours. 

6.4.4 Service Life Projection

In order to achieve attractive life cycle cost and energy savings, the system service life must be 

greater than 10 years. While demonstrating a new technology over the course of 2 years, this 

projection does not come without uncertainty. It is expected that the service life for the LDAC 

system is greater than 10 years, and proper design will support this projection.

In particular, it is expected that the non-desiccant components in a well-installed and maintained 

system—including the solar field, the cooling tower, the storage tank, and the balance of the 

balance systems (enclosures, fans, pumps, controls, etc.)—would have a service life greater than 

10 years. It is expected that some component failures would occur over time, such as loss of a 

pump, an instrument failure, etc. This is not uncommon for many HVAC systems. 

Of the three key desiccant components (i.e., conditioner, regenerator, and interchange heat 

exchanger), the system that should be closely monitored is the regenerator (the actual plastic 

parallel-plate HMX inside the regenerator unit). Some degradation in the flock on the plates

running in the field has been observed on the Los Angeles Whole Foods LDAC units. This 

problem has not been seen on two earlier prototypes, and so far there is no sign of the same 

problem with the Tyndall regenerator. The conditioners and interchange heat exchanger have 

held up well in the field on all previously demonstrated units up to this point. 

6.5 MAINTAINABILITY

Ideally, regular maintenance would be limited to tasks such as inspecting and changing filters; in 

practice, this system has required much more attention. The largest share of those issues stem 

from the poor initial design and installation of the solar field. The most significant issue is that 

the solar system has and continues to experience stagnation problems. With no secondary hotwater load and no heat dump built into the system, stagnation happens occasionally. A system 

change was made to run the collectors on water and not glycol, which means that the system is 

no longer freeze protected; and therefore, must be shut down and drained during the winter. The 

cooling tower is also not freeze protected. Protecting the system against both stagnation and 

freezing is essential to run the system over the course of 10 years.

There were also a number of issues with the control of the system that required attention. For 

example, there have been zero offset shifts on the analog-to-digital cards that read the 4-20 

milliamp signal from the desiccant storage tank level sensor. This seemed to occur at times after 

an electrical storm and was resolved by adding a lightning surge arrestor. 

There are other issues with the VFD/fan motor/PLC proportional-integral-derivative loop for the 

process fan. The VFD/fan motor has faulted and failed to automatically reset on several

occasions. The fault occurs infrequently and is difficult to reproduce for determining the source 

of the problem. As a result, the process fan was set at a fixed speed. 
59 

Many of the performance and maintenance issues, were identified and resolved throughout the 

2011 summer of operation. This demonstration has undoubtedly been a “lessons learned” 

experience, and many issues will be improved upon to reduce or eliminate unnecessary 

maintenance and/or repair. A single facility technician should be able to maintain the system 

with a minimal amount of training for future projects, but that was not the case for this particular 

project. 
60 

7.0 MARKET ANALYSIS

7.1 COST MODEL

Table 14 summarizes the displaced load on the chiller and the approximate energy and cost 

savings from the LDAC. It should be noted that these savings may slightly underestimate the 

actual savings because excess cooling due to the overcool/reheat cycle, which is mitigated by the 

LDAC, is not accounted for in the analysis. 

Improved performance in August 2011 led to the largest energy and cost savings, which is 

indicative of the performance potential of the LDAC system. Unforeseen maintenance and 

operation issues arose during the summer months, and this hindered the sustained high 

performance of the system. 

Table 14. Energy and Cost Savings from the LDAC in 2011 

Month Cooling 

(ton-hr)

Chiller 

Elec. 

(kWh)

LDAC 

Elec. 

(kWh)

Elec. 

Savings 

(kWh)

Elec. Cost 

Savings 

($)

April 667 890 1,026 -137 -14

May 1,582 2,110 2,325 -215 -21

June 1,837 2,449 1,774 676 68

July 1,239 1,652 1,131 521 52

Aug 1,916 2,554 1,223 1,331 133

Sept 1,333 1,778 1,099 678 68

The total cost savings for the 2011 cooling season was $321. The installed costs for the solar 

thermal system were $170,000, and the installed costs for the LDAC components were $40,000, 

for a total installed cost of $210,000, and a simple payback of 654 years. Because this was a precommercial system, the simple payback is not indicative of the paybacks of a commercial 

system. If the system would have operated per design intent, the cost savings would be 

substantially higher. In addition, in building types with electric reheat, the zone-level reheat 

savings dwarf the energy savings from the mechanical chiller. Reheat energy use in hospitals for 

example has been documented to account for over 30% of the total energy use. Finally, when 

the system is coupled with solar thermal, the solar thermal component becomes the most 

expensive part of the system and solar incentives or high utility rates are required to offset the 

increased costs of the solar thermal system.

One of the first commercial LDAC systems is being installed at the Coral Reef Fitness and 

Sports Center on Andersen AFB in Guam. A 6,000-cfm conditioner was designed for this 

system. The power requirements per ton of cooling for the existing building level chiller and 

LADC are 1.05 kW/ton and 0.3 kW/ton, respectively. Note that the power requirement of the 

chiller does not account for the chiller water pumps, so the power requirement may be slightly 

greater in reality. The system is designed with an evacuated-tube solar thermal field supplying 

80% of the thermal power and a backup diesel-powered boiler providing 20% of the thermal 
61 

power. The system is expected to reduce HVAC energy use by 34% and save $145,395 per year 

with an estimated simple payback of 11.6 years. 

7.2 RELEVANT MARKETS

The LDAC system typically used for outdoor air dehumidification, and an electric chiller is 

typically required to sensibly cool the air to the desired temperature. The energy consumption 

from the LDAC includes heat for regeneration and electricity for the pumps and fans in the 

system. The LDAC is most suitable where:

• The existing HVAC system is not able to meet latent loads on a facility

• Humidity control is required

• Overcool/ reheat strategies are used in traditional HVAC systems 

• Large quantities of ventilation air are needed

The LDAC should be applied to hot/humid climates that require year-round cooling and dehumidification. Future installations should focus on facilities in ASHRAE climate zones 1A and 

2A. A full market analysis with a listing of appropriate building types, locations, design 

characteristics, and life cycle costs will be provided in the latest DoD LDAC demonstration in 

Guam.
62 

8.0 IMPLEMENTATION ISSUES

The project’s focus was necessarily changed to focus on discovery of technical issues with this 

new emerging technology. Many of the issues arose because the installation had many unique 

features including the following: 

• The demonstration was the first combination of solar heat with this type of LDAC 

system. 

o Due to initial budgetary constraints, the LDAC relied solely on solar heat with no 

natural gas backup to ensure that the unit operated throughout the cooling season. 

A properly designed system that uses solar heat will have backup. Due to this, the 

system did not achieve peak-cooling capacity for significant hours of operation. 

Because the system largely has static power draw, this resulted in a low average 

EER.

o The solar field designer and LDAC system design were not tightly coordinated by 

the prime installation contractor (Regenesys). This resulted in a design that did 

not consider the frequency and duration of stagnation periods for the solar field. 

The collector design was not designed to withstand more than about two

stagnations per year. Furthermore, the collector system was not initially designed 

to withstand the massive volume of steam from these collectors when stagnation 

occurred. The solar field required significant redesign. The end result was 

workable for the demonstration despite being problematic and suboptimal in 

operation. 

• The demonstration was the first to create a split system where the conditioner and 

regenerator were contained in separate packages and separated by around a 100-ft

distance. This technical challenge resulted in a suboptimal pumping design configuration 

because of the necessary pump size to transfer desiccant this distance. Future designs 

should reduce the distance from the regenerator and conditioner. 

• This demonstration was the first to have 10 hours of desiccant storage using CaCl2. 

Tuning the storage to achieve optimal efficiency was required. The desiccant charge and 

the tank’s low and high levels have significant impact on efficiency, capacity, and solar 

utilization. These variables were tuned as the demonstration progressed.

• This demonstration required the placement of the conditioner unit about 100 feet from the 

outdoor intake to the building. This required significant fan power to move the air from 

the mechanical yard to the building. Future designs and applications should consider the 

duct length reduce the duct run from the conditioner to the outdoor air intake as much as 

possible. 

• The demonstration did not treat 100% of the outdoor air, thus limiting the benefit to 

energy savings from offset cooling. In order to offset the reheat for such an installation, a 

system should be designed to ensure that the LDAC meets a significant portion of the 

latent load. Typically, the LDAC can meet 100% of a building’s latent load if designed to 

treat 100% of the outdoor air. 
63 

8.1 SUMMER 2010 

Many issues came up in the summer of 2010, which led to limited operation during the 2010 

cooling season. The Viessmann 200-T solar collector array caused the internal working fluid 

(Tyfocor HTL glycol) to heat up to greater than the designed limitation (339°F) during 

stagnation, which caused two main issues for the system: 1) the burning of the glycol renders its 

antifreeze properties less effective, and makes the fluid acidic; and 2) high-pressure buildup 

triggers the pressure release valve. These issues could be avoided by using a heat-pipe

(Viessmann 300-T) design instead of a direct-flow design because the heat-pipe design has a 

stagnation temperature of 302°F, which is below the upper limit for the glycol. Also, the glycol 

has a vapor pressure of 4 bar at 302°F, which is below the release pressure of the relief valve. An 

alternative solution was used to replace the glycol solution with water, which eliminated the risk 

of corroding the piping from burned glycol. 

It was found that the dead-bands used to limit short cycling of the system for the upper and lower 

limits of the storage tank were larger than necessary; the dead-bands were decreased accordingly. 

It was also determined that decoupling the conditioner and regenerator operation allowed for 

optimal control on the system by allowing each unit to operate independently based on the 

availability of concentrated desiccant and solar radiation, respectively. 

8.2 SUMMER 2011 

The LDAC system successfully operated for the majority of the 2011 cooling season (May 

through September), but several issues arose regarding the operation and performance 

monitoring. During the start-up period, several maintenance procedures were required; there 

were pressure leaks in the solar collector array piping and a collector tube required replacement. 

It was observed that the system was running on lower desiccant concentrations than the design 

condition, so several drums of concentrated salt solution were ordered to be added to the system.

The storage tank volume limits were also lowered in effort to run the system with higher 

concentrated desiccant, which increased the cooling capacity and efficiency of the system. 

Several issues caused the system to operate at lower than expected efficiency during April and 

May 2011. The pump and fan of the cooling tower were set at a fixed flow and speed, which led 

to excessive electrical energy use to meet a given cooling load. The cooling tower pump required 

1.2 kW of electric power, and the motor had an efficiency of 62%. The water temperature change 

across the cooling tower was also small (~2°F), which indicated that excessive flow rates were 

present. A new cooling tower pump was installed, in conjunction with a VFD. A new control 

sequence was written that maintained the cooling tower fan at 75% fan speed and modulated the 

cooling tower pump flow rate to maintain a 10°F temperature difference between the cooling 

tower supply and return water temperatures. This resulted in over a 65% reduction in cooling 

tower energy use. 

The concentration of the liquid desiccant was initially below the desired value. Ideally, the 

desiccant would be 43% concentrated, which is the upper limit of concentration before 

crystallization occurs. The liquid level in the storage tank indicates the desiccant concentration; 

the lower level means the desiccant concentration is at a maximum. The concentration 

associated with the lower level was initially under the desired maximum concentration, so the 
64 

level was decreased accordingly (allowing the tank to fall to a lower level, which corresponds to 

a more concentrated solution). Increasing the concentration of the desiccant solution increased 

the drying rate of the air and increased the cooling capacity and overall efficiency.

The solar array used for desiccant regeneration was oversized for the system and was designed to 

allow for excess desiccant to be stored during the day. This regenerator flow rate exceeded the 

rate at which the conditioner weakened the desiccant (by absorbing water from the air) unless the 

conditioner was running at near maximum capacity. 

Under normal operation, the desiccant in the tank is sufficiently weak such that a full day of 

regeneration plus operation of the conditioner system should maximize solar utilization. 

However, in the event the desiccant tank is fully strong and the latent load is small, the solar 

regeneration system may have to shut off. This occurrence creates stagnation in the solar field. 

The solar field stagnates in the following manner: 

1. The solar water pump shuts off. 

2. The fluid internal to the evacuate tubes raises in temperature and turns to steam. 

3. The steam displaces the fluid in the system, which is forced into expansion tanks. 

4. Continued steam generation is dissipated by a passive heat-dump radiator installed in-line 

between the collectors and the expansion tank. 

5. The fluid pumps are locked out until the system temperature drops well below the boiling 

point of the fluid in the system to avoid flashing the fluid and creating a pressure spike. 

The pump is usually locked out until after sunset. 

The solar loop was initially directly connected to the regenerator. There was far too little 

expansion capacity in the system to allow the collectors to stagnate properly. This resulted in an 

overheating/over-pressurizing event that caused steam and burnt propylene glycol to leak into the 

regenerator slump the first time the collectors were charged with glycol. As a result, some 

initial charged desiccant was lost. The final design included a heat exchanger between the solar 

field and regenerator to fix this issue. 

8.3 LESSONS LEARNED

There are a number of lessons that should be learned from this project and applied to future 

projects in order to ensure successful design, installation, and operation of a solar-powered 

LDAC system.

8.3.1 Climate

The climate in Tyndall only required dehumidification for a few months per year (3-4 months) 

and experienced freezing temperatures during the winter. This limited the need for an outside air 

dehumidification system. Future installations should focus on climates that require year-round 

cooling and dehumidification, or on facilities that have problems with maintaining interior 

humidity levels. 

8.3.2 Building Type

The LDAC system should be applied to 100% outside air AHUs that are set up to overcool and 

reheat the supply air. The LDAC should also be sized to meet 100% of the outside air 

ventilation requirements. Because this system was only sized to meet 33% of the ventilation air, 
65 

it limited the overall impact the system has on removing the appropriate amount of moisture 

from the outside air.

8.3.3 Solar Field

In the future all solar field installations should include the use of heat-pipe based, evacuated-tube 

collectors. The cheaper draw through evacuated-tube collectors used on this project caused the 

majority of the problems. In addition, the system should include an appropriately sized heatdump radiators or be connected to a secondary heating system so that the heat can be used when 

the dehumidification loads are reduced. This will mitigate the stagnation problems and provide a 

means for using the waste heat.

Solar should be used where electricity and thermal energy is expensive, as with island nations. 

Otherwise, natural gas should be used. 

8.3.4 LDAC Fans and Pumps

The efficiency of the fans and pumps should be carefully considered when designing a system. 

Thought should also be given to the part-load efficiency of the pumps and fans. In the future all 

motors should have a minimum 80% electrical efficiency, and all of the pumps and fans should 

either utilize an electronically commutated motor or VFD to allow for part-load operation and 

much better part-load electrical efficiencies. This is the new “normal” in the LDAC design. All 

pumps and fans are on VFDs. 

8.3.5 Cooling Tower 

A cooling tower water treatment program should be implemented when the cooling tower is 

installed. In addition the cooling tower should come equipped with high-efficiency motors and 

VFDs on both the cooling tower fans and pumps. 

8.3.6 Sequence of Operation

The sequence of operation should modulate the flow of all of the fans and pumps during partload operation to increase the part-load electrical efficiency. In addition, the system should be 

set up to shut off the system when there is not a sufficient need for dehumidification.
66 

9.0 REFERENCES

AILR. (undated). “The Effect of Material Properties on the Performance of Liquid Desiccant Air 

Conditioners and Dehumidifiers.” Gas Research Institute Contract No. 5090-243-2030. 

AILR. (October 2008). “Liquid Desiccants for Solar Cooling.” SOA3000 Product Brochure. 

Princeton, NJ: AILR.

Balaras, C.A., et al. (June 2006). “Solar Cooling: An Overview of European Applications & 

Design Guidelines.” ASHRAE Journal; pp. 14-21. 

Baxter, V.; Fischer, S.; Sand, J. R. (1998). Global Warming Implications of Replacing OzoneDepleting Refrigerants. Atlanta, GA: ASHRAE. 

Daou, K.; Wang, R.Z.; Xia, Z.Z. (2006) “Desiccant Cooling Air Conditioning: A Review.” 

Renewable and Sustainable Energy Reviews (10); pp. 55-77. 

DOE. (2011). Buildings Energy Data Book. Accessed Oct. 1, 2012: 

http://buildingsdatabook.eren.doe.gov/. 

Chen, X.Y.; Jiang, Y.; Li, Z.; Qu, K.Y. (2005) “Field Study on Independent Dehumidification 

Air-Conditioning System – II: Performance of the Whole System.” ASHRAE Annual Meeting 

Proceedings. 

Conde-Petit, M. (2007). “Liquid Desiccant-Based Air-Conditioning Systems – LDACS.” 1st 

European Conference on Polygeneration, October 16-17, 2007, Tarragona, Spain. 

Ice Energy. (2012). “Ice Bear Energy Storage: Product Specification Sheet.” Accessed Oct. 3, 

2012: http://www.iceenergy.com/stuff/contentmgr/files/1/b5fef8f4e945bef09e48aca6714b5c51/download/ice_bear_pr

oduct_sheet.pdf. 

IEA-SHC-Task 25. (July 2012). “Solar Assisted Air Conditioning of Buildings.” International 

Energy Agency. Accessed Oct. 1, 2012: http://iea-shc-task25.org/?p=8. 

Lowenstein, A. (November 2008). “Review of Liquid Desiccant Technology for HVAC 

Applications.” ASHRAE HVAC&R Research (14:6). 

Lowenstein, A., U.S. Patent No. 5,351,496 (October 1994). 

Lowenstein, A.; Gabruk, R. (1992). "The Effect of Absorber Design on the Performance of a 

Liquid-Desiccant Air Conditioner." ASHRAE Trans. AN-92-3-3. pt.1. 

Lowenstein, A.; Slayzak, S.; Kozubal, E. (2006) “A Zero-Carryover Liquid Desiccant Air 

Conditioner for Solar Applications.” ASME International Solar Energy Conference, July 8-13, 

2006, Denver, CO.
67 

Lowenstein, A.; Slayzak, S.; Kozubal, E.; Ryan, J. (2005) “A Low-Flow, Zero Carryover Liquid 

Desiccant Conditioner,” International Sorption Heat Pump Conference, Denver, CO, June 22-24, 

2005. 

NREL (September 1997). “Advanced Liquid Desiccant Technology Scoping Study.” 

Subcontract No. AAR-7-17797-01. 

NREL. (September 1998). “Advanced Liquid Desiccant Technology.” Subcontract No. ADC-8-

18456-01. 

NREL. (November 1999). “Advanced Liquid Desiccant Technology.” Subcontract No. AAR-0-

30404-01. 

NREL. (July 2001). “Advanced Liquid Desiccant Technology.” Subcontract No. AAX-1-31442-

01. 

NREL. (May 2005). “Commercial Liquid Desiccant Technology.” Subcontract No. NDJ-5-

55010-01. 

Owen, M., ed. (2010). Handbook—Refrigeration. Atlanta, GA: ASHRAE. 

Pacific Northwest National Laboratory. (undated). “Rooftop Unit Comparison Calculator.” 

Accessed Oct. 1, 2012: http://www.pnl.gov/uac/costestimator/main.stm. 

SACE Project. (2005) “Solar Air Conditioning in Europe – An Overview, Renewable & 

Sustainable Energy Reviews.” European Commission, D.G. XII. 

Slayzak, S.; Lowenstein, A.; Ryan, J.; Pesaran, A. (1998 ) Advanced Commercial Liquid 

Desiccant Technology Development Study. NREL/TP-550-24688. Golden, CO: NREL. Accessed 

Oct. 1, 2012: http://www.nrel.gov/docs/fy99osti/24688.pdf. 
68 

APPENDIX –POINTS OF CONTACT

List all the important points of contact (POC) involved in the demonstration, such as coinvestigators, sponsors, industry partners, and regulators. The list should include the following 

information: (1) full name; (2) complete mailing and FedEx addresses (if different); (3) 

telephone number, fax number, and e-mail address; and (4) the role of the individual in the 

project.

Use the tabular format below:

POINT OF 

CONTACT

Name

ORGANIZATION

Name

Address

Phone

Fax

E-mail

Role in Project 

Jesse Dean National Renewable 

Energy Laboratory

303-384-7539

Jesse.dean@nrel.gov

Co-Principle 

Investigator

Eric Kozubal National Renewable 

Energy Laboratory

303-384-6155

Eric.Kozubal@nrel.gov

Co-Principle 

Investigator

Lesley Herman National Renewable 

Energy Laboratory

(303) 275-4318

Lesley.Herrmann@nrel.gov

Investigator

Joe Wander Tyndall Air Force 

Base

850-283-6240

joe.wander@tyndall.af.mil 

Site Sponsor, 

Tyndall Project 

Manager

Andrew Lowenstein AIL Research (609) 799-2605 x40

timheaton@coolerado.com

AIL Research

Owner/Principal

Jeff Miller AIL Research 609) 799-2605 x53

jmiller@ailr.com

LDAC Design 

Engineer

Ed Hancock Mountain Energy 

Partnership

(303) 517-8238

CEHancock3@aol.com

Data Acquisition 

System

Greg Barker Mountain Energy 

Partnership

(303) 775-7646

GBARKER123@aol.com

Data Acquisition 

System