NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Zero Energy Communities with 

Central Solar Plants using

Liquid Desiccants and Local 

Storage

Preprint

J. Burch, J. Woods, and E. Kozubal

National Renewable Energy Laboratory

A. Boranian

University of Colorado

Presented at the International Conference on Solar Heating and 

Cooling for Buildings and Industry

San Francisco, California

July 9-11, 2012

Conference Paper

NREL/CP-5500-55670

August 2012
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1 

SHC 2012 

Zero Energy Communities with Central Solar Plants using 

Liquid Desiccants and Local Storage

Jay Burcha1

*

, Jason Woodsa, Eric Kozubala and Aaron Boranianb 

a

National Renewable Energy Laboratory, 15013 Denver West Parkway; Golden, CO 80401; United States of America b

University of Colorado; Boulder, CO 80302; United States of America

Abstract

The zero energy community considered here consists of tens to tens-of-thousands of residences coupled to a central 

solar plant that produces all the community’s electrical and thermal needs. A distribution network carries fluids to 

meet the heating and cooling loads. Large central solar systems can significantly reduce cost of energy vs. singlefamily systems, and they enable economical seasonal heat storage. However, the thermal distribution system is costly. 

Conventional district heating/cooling systems use a water/glycol solution to deliver sensible energy. Piping is sized to 

meet the peak instantaneous load. A new district system introduced here differs in two key ways: i) it continuously 

distributes a hot liquid desiccant (LD) solution to LD-based heating and cooling equipment in each home; and ii) it 

uses central and local storage of both LD and heat to reduce flow rates to meet average loads. Results for piping sizes

in conventional and LD thermal communities show that the LD zero energy community reduces distribution piping 

diameters meeting heating loads by ~5X and meeting cooling loads by ~8X for cooling, depending on climate.

Keywords: Solar Thermal; Community-scale; District Heating/Cooling; Liquid Desiccants; Pipe Size Reduction

1. Introduction

The zero energy community (ZEC) considered here consists of tens to tens-of-thousands of residences 

coupled to a central plant that produces all the community’s electrical and thermal needs. There are three parts 

to such systems: the central plant, the thermal distribution network, and the loads/buildings. The central plant 

is presumably solar-driven; it provides electricity and heat. Solar plants usually have seasonal storage to 

address the obvious heating load-solar resource mismatch. The buildings can be residential or commercial, 

with space heating, space cooling, and water heating loads. The distribution network consists of insulated 

pipes carrying fluids to the loads. In a conventional system, the fluid is a freeze-resistant solution carrying 

sensible energy only, and the piping is sized to meet the peak heating/cooling loads for given supply and 

return temperatures. Large well-insulated pipes located in trenched concrete casements are typical; cost of the 

* Corresponding author. Tel.: +1-303-384-7508; fax: +1-303-384-7540. E-mail address: jay.burch@nrel.gov.
2 

thermal distribution network is a dominant cost and a barrier to central systems [1]. The liquid desiccant (LD) 

system discussed here delivers both sensible and latent energy to local storage to meet average heating and 

cooling loads, significantly reducing piping size and distribution system costs. 

Nomenclature

Symbols and subscripts

AC air conditioner

bldgs buildings 

clg space cooling

cp specific heat

d pipe diameter

distr distribution system

DHW domestic hot water

div diversity of loads

F factor (for diversity and for heat loss)

HP heat pump

htg space heating

HVAC heating, ventilation, and air conditioning equipment

LD Liquid desiccant

m mass

N number

PV photovoltaics

PV/T PV/Thermal

Q quantity of thermal energy 

T temperature or thermal-only collectors

v average velocity of the fluid in the pipe

ZEC zero energy community

Greek Symbols

∆ change in a quantity

π pi, ratio of circumference to diameter

ρ density

ω mass fraction, ω ≡ msalt/(mH20 + msalt) 

A large central solar plant provides energy at significantly lower cost vs. single-family residential systems, 

principally due to economies of scale and reduced soft costs (e.g., permitting, distribution, and marketing). 

When using sufficiently high concentration of sun (> ~500X), high-efficiency triple-layer cells with ~40% 

efficiency can provide more economical electrical power on roughly half the footprint vs. Si cells with 20% 

efficiency. With a large-scale central solar plant, economical seasonal storage is possible; seasonal stores are

somewhat commonplace in Europe [1], but are rare in the U.S.

The central plant generates electrical and thermal energy, from photovoltaic (PV) and solar thermal 

collectors. “Waste” heat from cooling of the photovoltaics (PV/T) can be used to more than double energy 

output. However, there is a fundamental PV/T conflict: PV cells should operate as cool as possible to 

maximize their electrical production, while heat should be sufficiently hot to meet thermal loads. Si cells have 

a temperature coefficient giving ~0.5%/o

C degradation in efficiency; similarly, the GaAs temperature 

coefficient is ~0.2%/o

C. Reliability concerns limit driving the cells over 100 oC. To raise the temperature of 

the PV/T thermal output to 95 o

C (for sensible storage) or 140 oC (for liquid desiccant regeneration) space 

conditioning or DHW use, thermal-only collectors are needed to boost the temperatures from the PV/T. These 

collectors will generally be concentrating (such as troughs), to provide sufficiently high temperatures for 
3 

distribution and driving two-stage thermally-activated cooling. However, we do not discuss design of the 

generation system here; we simply assume the required heat or cold is provided at the desired temperatures.

The main downside of using the thermal energy from a central plant is the distribution system cost. The 

trunk lines for a 1000-home conventional ZEC have diameters 1-2 feet, they must be insulated, and they 

require deep trenching and concrete enclosures. This paper addresses distribution system costs with a new 

design for a ZEC based upon liquid desiccants (LD) that reduces the diameter of the piping required by ~5X 

and ~8X for heating and cooling, respectively. After describing the two approaches to a ZEC and discussing

the assumed ZEC loads, and the postulated LD-based heating, ventilating, and air conditioning (HVAC) 

equipment, we quantify and compare required pipe sizing for conventional and LD-based district systems. 

2. Community systems overview: conventional and proposed

Fig. 1 is a schematic of a ZEC using a conventional district system supplying both heating and cooling. The 

central plant supplies hot fluid to the network from the solar heat source, and cold fluids from a central chiller

driven by solar heat. Large absorption chillers driven by solar heat are available. Seasonal heat storage is used 

to even out the solar resource/heating-load mismatch. Cold storage can also be done, charging the store from 

ambient cold winter air or a nearby body of water. In order to ideally meet simultaneous heating and cooling 

loads (domestic hot water [DHW] and space cooling in summer), four pipes are needed: separate supply/return 

lines for hot and cold fluids. It is possible to use two pipes only, if a seasonal changeover from hot to cold is 

done, with electric heating for the DHW load when in cooling mode. The PV has to be upgraded to “make up” 

for that added electric energy use. To avoid problems with network failures in winter, the network fluid must 

be freeze-resistant for most all U.S. climates. The central plant must generate and distribute fluids through 

pipes large enough to carry the flows needed to meet peak instantaneous loads. Each house has three heat 

exchangers, for the space heating and cooling, and DHW loads; the heating and cooling heat exchangers could 

be combined with appropriate valving. The hot side network temperature may be as high as 82 o

C (180 oF) to

drive convectors in each house. Outdoor temperature reset is useful to reduce tank losses. The cold side 

network is as low as 10 o

C (50 oF).

Fig. 2 is a schematic of the new LD-based ZEC. The LD ZEC concept presented here evolved from earlier 

work done on a single-family LD-based zero energy home using the same LD-based HVAC [4]. LDs are 

typically halide salts in water, such as LiBr or CaCl2. LDs have a low vapor pressure compared to saturation 

pressures over water, which drives the water vapor transport processes used in the LD-based HVAC. The heat 

from the concentrating collectors goes to either the LD regenerator or the sensible storage, depending on state 

of charge of LD and heat stores. A two-stage regenerator achieves a thermal coefficient of performance of 

~1.2 by boiling LD at atmospheric pressure in the first stage, and using the steam to drive a second stage 

regenerator. A small-scale unit is under test [3]. Boiling LD requires input temperatures > ~140 o

C (284 oF). 

Water concerns are minimized by reusing the pure water produced during regeneration. Codes will likely 

prohibit re-injection of this “industrial water” into the potable water system, and the reclaimed water would be 

used elsewhere. Then, the only water lost in the community LD system is what exits in the indirect 

evaporation exhaust air streams during LD-AC operation.
4 

Fig. 1. A zero energy community using a four-pipe conventional district system (see text for a two-pipe alternative).

There are two stores at the LD central plant, as shown in Fig. 2: 1) a large LD tank, storing both strong and 

weak LDs; and 2) a large sensible storage, shown as a borehole field. In addition, heat is stored in each 

building in two local stores: 1) a small hot LD storage tank; and 2) a larger sensible store, which takes the heat 

supplied by the network and stores it for peak heating periods in winter. Lastly, each building uses HVAC 

driven by LD. If cooling/heating is very small, the air conditioner/heat pump would be eliminated, 

respectively. The LD heat pump can be eliminated, even in a heating climate, by using only the network 

sensible heat with local storage to meet heating loads. When doing so, one must upsize the central plant heat 

collection by ~20%, increase the LD pipe diameter, and increase the capacity of the local sensible heat store 

by ~2.5X.

Fig. 2. An alternative zero energy community, using liquid desiccants and local storage. 

Central plant Distribution Load/Buildings

Heat Source

Seasonal Storage

Water Pit or Tank

Borehole Field

... ...

Branches

Twigs

Hot Trunk/Supply 

...

Warm Trunk/Return 

Cold Trunk/Supply 

Chiller Cool Trunk/Return 

Hot supply twig

Warm return twig

x

x

Sp Htg

DHW

Sp Clg

Cool return twig

Cold supply twig

3 Heat Exchangers
5 

3. Central plant storage 

Seasonal storage is essential to utilize the plant’s thermal energy for a ZEC. Large stores are generally

more efficient and lower in unit energy storage cost than small stores. Figs. 1 and 2 exemplify borehole 

sensible heat storage. Approaches to large stores are given in Table 1. Water and earth are currently the only 

materials used for seasonal storage. Phase change materials have no advantage when the ∆Tstore is large, as in 

seasonal stores. Thermochemical storage at ~30X water energy densities remains promising; someday suitable 

reactants might be identified. For the LD ZEC central plant, a second storage tank is needed for the LD heat 

transfer fluid. The insulated storage could be a tank or a pit store, either being lined with polymer films to 

resist corrosion in the tank and contain the fluid. As indicated in Fig. 2, the weak LD is stratified on top of the 

strong LD by buoyancy forces without a separating membrane [5]. 

Table 1. Some central plant seasonal storage options

Type of storage Notes

Earth storage Large communities 

Borehole store Deep U-tubes of HDPE; for bigger communities (top insulation only)

Aquifer flow store Injection (summer) and extraction (winter) wells (no insulation)

Water storage Has ~4X the volumetric heat capacity of earth

Large insulated tank(s) Tanks can be buried; intermediate size communities

Pit store Small to intermediate size communities

Geological caverns/mines Large communities; depends on availability/local geology

4. Loads and local storage

For simplicity in calculating the community loads, we assume a community of 1000 identical homes. 

Commercial buildings and other loads would generally also be included, but we consider only residential

loads here. Space heating and cooling loads change relatively slowly and can reasonably be assumed to be 

coincident for calculating community loads from house loads. For DHW, a 0.3 diversity factor is assumed for 

peak water heating loads, based on observed overlap in water usage in the “morning rush hour” [6]. A more 

precise literature value for DHW diversity was not sought for this study. 

The loads used here are calculated from simulations done in eight cities for a three-bedroom house [7], 

using the Building America house simulation protocol [8]. The building used there was reasonably efficient, 

meeting 2009 residential building code [8]. Occupancy assumptions and other details of the simulation model 

and input assumptions are given in [7]. Zero energy dogma rightly dictates efficiency first. In zero energy

design, the loads are reduced with efficiency until the point is reached where renewable generation is less 

expensive than available efficiency measures. At that point, the building’s space conditioning loads are

reduced by 50-80%, with whole house load reduced 40-60% [9]. Reductions were somewhat higher for space 

heating than for cooling and depend on climate. Somewhat artificially, we generate a set of heating/cooling 

loads by reducing the loads in [7] for heating/cooling by 70%/60%, respectively. DHW and all other uses are 

assumed to be unchanged from [7]. The monthly heating and cooling loads assumed are shown in Fig. 3. 

Local storage at each home allows flow rates in the distribution piping to be reduced for both cooling and 

heating. As in Fig. 2 and Fig. 6a, a local tank of several hundred gallons stores hot LD, replenished by the 

network. At LD flow rates in [7], this tank stores a month of LD for the LD-AC. In the hot LD tank are two 

heat exchangers, for DHW and for heating loads. The tank allows for a reduced LD flow rate during summer 

by supplying LD to the chiller at times when the network capacity is too low to meet the local load directly. 

The LD tank also stores heat flows from the secondary storage and heat pump. A second local storage is used 
6 

for sensible heat. The ground under the house can be used with new construction, as illustrated in Fig. 2. The 

thermal capacitance of under-house ground storage is large, providing sufficient capacity to bridge between 

summer excess and winter deficit heat. With a relatively large ground store charged continuously by 

circulation of fluid from the central plant/storage, flow rates from the central plant for heating can be reduced 

so that the total annual heat needed is delivered at a constant, low flow rate.

Fig. 3. Heating and cooling loads per home assumed for the zero energy community. 

5. Liquid-desiccant-based HVAC

The LD-AC and LD-HP described below are the basis for using LD as the network fluid. The U.S. 

Department of Energy is funding work on the LD-AC, and there has been significant commercial interest in 

the unit described below, giving some prospect of such units entering the market within a few years. However, 

there is no current work on the LD-HP. Because neither LD unit is commercially available, the community 

concept here should be considered futuristic. 

5.1 Liquid desiccant air conditioner (LD-AC)

The LD-AC is a two-stage device (Fig. 4), using de-humidification followed by indirect evaporative 

cooling. It is termed a desiccant-enhanced evaporative air conditioner (DEVAP) [11]. A numerical model of 

this process [12] was recently validated with experiments. As in Fig. 4a, the first stage of the DEVAP process 

is a dehumidifier that uses LD contained behind a membrane to remove moisture from the process air (state 1 

to 1.5). Across a plastic plate from the LD, water evaporates into an exhaust airstream (3 to 4), which keeps 

the LD temperature and vapor pressure low. The second stage is a counterflow indirect evaporative cooler. It 

cools the process air (state 1.5 to 2). A portion of the cool, dry outlet air (state 2) serves as the inlet to the wetside channels (2 to 5) opposite the process air. The DEVAP process is shown on a psychrometric chart in 

Figure 4b. Electricity use is limited to fans and pumps. The electricity use over a year is roughly 85% less than 

with a conventional vapor compression AC. DEVAP shifts cooling from electricity for the compressor to heat

for the LD regenerator; total source energy savings is 40-80%, depending on climate humidity [11].

5.2 Liquid desiccant heat pump

In the LD-HP, an LD flow is separated from a counter-flowing water stream with membranes that pass water 

vapor to the LD. The released heat raises the LD outlet temperature above the water inlet temperature (∆Tlift). 

The membranes are separated by a narrow air gap (Fig. 5a) which improves performance by reducing sensible 

heat loss between the hot LD and cool water. Prototypes of this HP were built using two sets of rows of 

hollow fiber membranes, with the space between these rows acting as the air gap [13]. The HP temperature lift 

∆Tlift depends on source temperature and LD concentration, as shown in Fig. 5b. When (Tset-Tamb) < ∆Tlift, the 

0.0E+00

5.0E+06

1.0E+07

1.5E+07

2.0E+07

2.5E+07

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Total Heating Load (kJ)

Heating Loads (SpHtg + DHW)

Atlanta

Boston

Chicago

Houston

Phoenix

San Francisco

Tampa

Washington, D.C. 0.0E+00

2.5E+06

5.0E+06

7.5E+06

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Monthly cooling load (kJ)

Cooling Loads (sens. + latent) 

0

2.5 E7 7.5 E6

0
7 

HP can produce energy to meet DHW and space heating loads during that period. This process allows more of 

the network heat to go into the local long-term storage, but it uses more LD and regeneration heat. 

Fig. 4. Liquid desiccant air conditioner: (a) schematic drawing; (b) process on a pyschrometric chart. 

Fig. 5. LD heat pump: a) schematic (left) and temperatures (right) in fluids; b) temperature lift as function of the inlet mass 

fraction, for five inlet temperatures; the theoretical maximum temperature lift is also shown. 

5.3 Modes of heating the liquid desiccant tank 

The DHW and heating loads are met using (polymer) heat exchangers in the top of the LD storage, as in 

Fig. 6a. The storage is kept hot enough to meet loads by the local sensible storage or by the HP. Fig. 6 also 

shows three modes of charging the LD tank. It is charged off ambient air (Fig. 6d), when possible (Tamb > ~27 o

C (80 oF)). When storage is hot enough, the tank is charged directly from the store, as in Fig. 6b. When the 

storage is below the temperature to directly meet the loads (~50 o

C), the HP uses the storage (Fig. 6c) if (Tamb

< 80 o

F). 

.2 .3 .4 .5 .6

Inlet Mass Fraction ω

Temperature Lift [C]

0

10

20

30

15 oC

25 oC

35 oC

45 oC

55 oC

@ Equilibrium ∆T

(a) (b)

1

3

4

1.5

2

5

outdoor air

return air

3

4

1.5

1

2

5

water

pumped LD 

from LD storage

supply 

air

exhausted air exhausted air

1st Stage Dehumidifier 2nd Stage Indirect Evaporative Cooler

return + 

ventilation 

air

air from outdoors

Dry Bulb Temperature [oC]

10 20 30 40

0

.01

.02

.03

Humidity ratio [kg/kg]

return air

♦

♦

outdoor air

1ststage dehumidifier 2nd stage indirect evap. cooler

(a) (b)
8 

Fig. 6. The LD tank is a heating load center: (a) LD tank and heating loops; and (b)-(d) three heat sources and charging 

modes for the LD tank: (b) hot storage; (c) HP + warm storage; and (d) HP + warm ambient air. 

6. Distribution network pipe diameters for conventional and LD district systems

Conventional system. The pipe diameter of the conventional network is set by the constraint to meet

instantaneous peak loads. With diversity for DHW loads and no diversity for space heating loads and 

estimating a loss factor (Floss , set to 1.10 here) to roughly account for piping losses, we have: 

The maximum flow rate, maximum velocity in the pipe, and pipe diameter are related as in Eqn. 2: 

Maximum vpipe is ~5 ft/sec (1.5 m/sec), to limit erosion [14]. The temperatures across the supply to return 

are set from practical considerations, such as the size of the building heat exchangers and avoidance of 

condensation. Table 2 lists supply/return temperatures used to calculate the ∆Tdistr. The diameter is computed 

for maximum heating (DHW + space heating) loads and for maximum cooling loads. 

The diameter of the main trunk in a conventional system is obtained by substituting Eqn. 2 into Eqn. 1 and 

solving for the pipe diameter: 

LD system. The LD flow rate for cooling is set by simulated DEVAP demand. Given the required LD mass 

flow- averaged over a time period ∆t (about a month for a 200 gal LD tank, given max LD flow rates in [7])- 

the size of pipe carrying the LD for cooling is

Strong/ 

warmLD

Domestic hot 

water loop

Mains 

water

Return air

Space 

heating 

loop

To zones

LD tank

Weak/ 

hot LD

(a) (b) (c) (d)

Strong/ 

warmLD

Weak/ 

hot LD

LD Heat pump

Weak/hot LD

Warm 

storage

Strong/ 

warmLD

Weak/ 

hot LD

LD Heat pump

Ambient loop

Strong/ 

warmLD

Weak/ 

hot LD

Direct 

storage 

loop

Hot 

storage

) (1)

(2)

(3)

(4)
9 

Table 2. Temperatures of supply and return pipes

Pipe Mode

Heating Cooling

Supply 180 oF (82 oC) 55 oF (13 oC)

Return 130 oF (54 oC) 65 oF (18 oC)

To model storage and interaction with heat pump operation, it is necessary to make expeditious simplifying 

assumptions. The storage is assumed adequately large to store the heat- delivered all year by the hot LD 

distribution- for use in winter. We multiple annual heating loads by 1.4 to account for storage heat losses. We 

estimate that the LD-HP uses ambient heat to meet 20% of the space heating load and 40% of the DHW load.

With these adjusted loads, Eqn. 3 is used to size the LD pipe, using LD properties rather than glycol 

properties.

Calculated pipe diameters are shown in Fig. 7a. For the conventional ZEC, the pipe sizes are between 1 and 

1.5 feet for the eight cities. For the LD zero energy community, it can be seen that the pipes vary between 0.08 

feet and 0.18 feet for cooling, and between 0.15 feet and 0.35 feet for heating. Two cases for heating are done, 

with and without the LD-HP. The LD-HP reduces the diameter about 15%, and it significantly reduces the 

local sensible heat storage, by about 2.5X vs. no HP. The LD-HP allows one to more than double the 

temperature change in the storage while still meeting the heating loads, as in [4]. It can be seen that the LD 

ZEC pipes are much smaller than those of the conventional ZEC. The pipe size reduction for cooling averages

~8X, and for heating it is ~5X, with diameter ratios for the eight cities shown in Fig. 7b. If the community has 

both heating and cooling loads, the result for LD heating sets the LD pipe diameters. The LD cooling diameter

approaches the LD heating diameter in the hot climates of Houston, Phoenix, and Tampa.

Fig. 7. Pipe sizes for conventional and LD zero energy communities: (a) pipe diameters; (b) ratio of sizes for heating and 

cooling. 

0.0

0.5

1.0

1.5

Pipe size [ft]

Pipe Size: Conv. & LD ZEC

Conv Clg

Conv Htg

LD Clg

LD Htg,No HP

LD Htg,w HP

0

2

4

6

8

10

12

14

Dpipes,conv/Dpipes,LD

Ratio of Diameters, Conv/LD 

Conv,Clg/LD,Clg

Conv,htg/LD,Htg,noHP

Conv,htg/LD,Htg,HP
10

7. Conclusions 

A new type of district heating and cooling system for a ZEC has been introduced, based upon distribution 

of hot liquid desiccant (LD) fluid, LD-based HVAC in each home, and local storage. The central plant has 

renewable generation of electricity and heat, and has a central LD storage tank and a seasonal heat storage, 

such as a borehole field for larger communities. Hot LD is distributed in the network 365/24, charging an inhome LD storage tank and a sensible storage. There is an LD-HP and an LD-AC in each building, meeting 

heating and cooling needs. Each building has an LD tank of several hundred gallons, and a larger sensible heat 

store. The LD district system shows benefits compared to a conventional district system using heated/cooled 

fluids. The LD system is a two-pipe system, whereas the conventional ZEC is either four-pipe, or two-pipe 

with seasonal changeover and electric water heating when in cooling mode. Pipe sizes have been computed for 

both communities. The LD pipes are smaller than in the conventional community by ~5X and ~8X for heating 

and cooling, respectively. For a 1000-home community, the LD pipes are ~3 inches in diameter for a 1000 

home community, depending on the climate. The small size will make the distribution pipe less costly. It may 

be amenable to “ditch-witching”, as opposed to trenching and cement casements, thereby radically lowering 

LD system distribution costs.

Acknowledgements

This work was supported by the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08-

G028308 with the National Renewable Energy Laboratory. The support of DOE managers Alexis 

Abramson, Tony Bouza, and Bahman Habibzadeh at DOE/BTP/Emerging Technology is acknowledged.

References

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contract AGG-9-99554-01, Dec. 2010.

[2] Kurtz S, NREL, Director of the DOE PV Reliability Program. Private communication.

[3] Lowenstein AI, “Two-stage Regenerator Test Results”, NREL Deliverable.

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of American Solar Energy Society Annual Conference, May 2011.

[5] Davidson J, “Test Data for Desiccant-based Storage”, NREL Deliverable contract NXL-0-88322-01.

[6] Klein G, Aim for Sustainabililty. Private communication.

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[13] SRCC standards committee, June 2012. Recommendations on fluid velocity in collector testing.