Technical Report

NREL/TP-550-46524

September 2009

Coolerado 5 Ton RTU

Performance: Western Cooling 

Challenge Results

Eric Kozubal and Steven Slayzak
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

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

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Technical Report

NREL/TP-550-46524

September 2009

Coolerado 5 Ton RTU

Performance: Western Cooling 

Challenge Results

Eric Kozubal and Steven Slayzak

Prepared under Task No. BEC71115
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iii

Acronyms

DX direct expansion

EA exhaust air

EER energy efficiency ratio

HVAC heating, ventilation, and air-conditioning

NREL National Renewable Energy Laboratory

OA outside air

RA return air

RTU rooftop unit

SA supply air

SHR sensible heat ratio

w.c. water column

WCC Western Cooling Challenge

WCEC Western Cooling Efficiency Center
iv

Contents

Acronyms.......................................................................................................................................iii

1 Introduction............................................................................................................................. 1

2 Unit Description and Test Method.......................................................................................... 1

3 Results..................................................................................................................................... 4

Appendix – Measured Data Tables................................................................................................. 8

Figures

Figure 1. The prototype RTU and the unit being tested at the NREL HVAC laboratory .............. 1

Figure 2. Coolerado RTU air flow schematic................................................................................. 2

Figure 3. Psychrometric chart of RTU performance at nominal peak conditions.......................... 4

Figure 4. Psychrometric chart of RTU performance at surrogate annual conditions ..................... 5

Figure 5. Data used to interpolate surrogate annual EER performance.......................................... 6

Figure 6. Data used to interpolate surrogate annual water use/evaporation performance .............. 6

Tables

Table 1. WCC Psychrometric Conditions....................................................................................... 3

Table 2. Western Cooling Challenge Summary ............................................................................. 7
1

1 Introduction

The National Renewable Energy Laboratory (NREL) is tasked, through funding from the U.S. 

Department of Energy Office of Building Technology, to evaluate the performance of advanced 

cooling concepts that meet or exceed the performance criteria developed by the Western Cooling 

Efficiency Center (WCEC) (http://wcec.ucdavis.edu/). The WCEC has developed a set of 

criteria for test conditions, minimum energy, and water use performance for prototype cooling 

equipment. The WCEC has identified these conditions as indicative of western state climates. 

These criteria, named the Western Cooling Challenge (WCC), have been set forth as a challenge 

to manufacturers to improve the state-of-the-art space cooling products. NREL is to verify these 

criteria through laboratory testing at its heating, ventilation, and air-conditioning (HVAC) test 

facility (www.nrel.gov/dtet/lab_capabilities.html) in Golden, Colorado, which is uniquely suited 

to accurately measure the cooling performance, energy, and water use of advanced cooling 

systems. The facility provides flexibility to test prototype equipment and develop subsequent 

test methodology. Data are analyzed and reported to reflect performance at sea level elevation. 

This report is intended for individuals with technical understanding of cooling technologies for 

buildings. 

2 Unit Description and Test Method

NREL tested a prototype rooftop unit (RTU) manufactured by Coolerado Corporation (see 

Figure 1). The unit, an advanced ultra-cooler that uses the patented “M-cycle” process, is a 

hybrid indirect evaporative cooling and refrigeration direct expansion (DX) system. An airflow

schematic of the RTU is shown in Figure 2. Return air (RA) and outdoor air (OA) are brought 

into the unit and cooled by an indirect evaporative media. Between 43% and 46% of this air is 

used as an indirect evaporative cooling stream. The balance is then passed through a refrigerant 

evaporator coil and supplied to the space by a high-efficiency fan. The exhaust air from the 

evaporative process is generally cooler than the ambient air and is therefore used for the heat 

sink air flow going through the refrigerant condenser coil. OA and exhaust air (EA) flow rates 

were matched during testing. The RA and supply air (SA) flow rates are also equal, thus there is 

no make-up air (to the space) supplied by the unit. The mode of operation can be described as 

recirculation and ventilation air cooling with no makeup air.

Figure 1. The prototype RTU and the unit being tested at the NREL HVAC laboratory
2 

The unit brings in OA and mixes it with RA to create a fresh air rate shown in equations (1) and 

(2). 

(1) 

 [cfm] (2) 

The nominal cooling capacity is given by equation (3) when tested at peak conditions. This 

number should not be confused with the net total cooling defined later in equation (11). Rather, 

this number is used as a baseline to determine ventilation cooling capacity and a nominal specific 

cooling rate in cfm/ton. The cfm/ton calculation is also used to determine the static pressure 

imposed during the test, which is set at 0.7 in. w.c. at 350 cfm/ton. (See the WCC test 

specification for further details.) 

 [Btu/h] (3) 

The RTU utilizes a high ventilation rate to provide air to the evaporative process. The

specification states that ventilation cooling credit is limited to a specified OA flow. The nominal 

cooling capacity is used to determine the credited ventilation cooling and is calculated with

equation (4). 

 [cfm] (4) 

Given the constraint: 

 

Figure 2. Coolerado RTU air flow schematic

Return Air

(RA)

Supply Air

(SA)

Outdoor Air

(OA)

Evaporative 

Exhaust Air Exhaust Air

(EA)

Condensing Coil

DX Evaporator 

Coil

DX Condenser 

Coil

Indirect Evaporative 

Cooling Cores
3 

The unit was given a single air flow at the OA inlet location. The RA and OA were 

psychrometrically mixed at the test facility rather than inside the RTU. Cooling capacity at the 

WCC test conditions is calculated with equations (5–13). 

Space (Recirculation) Air Cooling: 

[Btu/h] (5)

 [Btu/h] (6)

 

[Btu/h] (7)

Credited Ventilation Air Cooling: 

 [Btu/h] (8)

 [Btu/h] (9)

 [Btu/h] (10) 

Net Cooling: 

 

[Btu/h] (11)

 

[Btu/h] (12)

 [Btu/h] (13) 

Testing was done at nominal peak and surrogate annual conditions. The psychrometric 

conditions for the cooling challenge are shown in Table 1. RA conditions apply to the peak and 

annual tests.

Table 1. WCC Psychrometric Conditions 

Tdb Twb Unit

Nominal Peak OA Conditions 105 73 °F

Surrogate Annual OA Conditions 90 64 °F

RA Conditions 78 64 °F

The unit has three primary modes of operation that are labeled as stages 0 to 2. Stage 2 has a 

higher OA fraction to provide additional air to the condensing coil. 

• Stage 0: Indirect evaporative cooling only, with 43% OA fraction.

• Stage 1: Indirect evaporative cooling + low stage DX cooling, with 43% OA fraction.

• Stage 2: Indirect evaporative cooling + high stage DX cooling, with 46% OA fraction.

See the WCEC Web site (http://wcec.ucdavis.edu/) to view the complete WCC test 

specifications.
4

3 Results

The following graphs illustrate the cooling process of the Coolerado RTU on a psychrometric 

chart for all cooling stages. Figure 3 and Figure 4 show the mixed air and SA conditions at

WCC nominal peak and surrogate annual conditions, respectively. The figures show the 

progression of cooling capacity and supply conditions as the unit ramps up from stage 0 to stage 

2. At nominal peak conditions, the RTU provides space cooling with a sensible heat ratio (SHR) 

between 0.92 and 1.25. Stage 2 is used to rate the system at nominal peak conditions.

Figure 3. Psychrometric chart of RTU performance at nominal peak conditions

(Shown at 0 ft elevation. S0, S1, and S2 denote stages 0, 1, and 2, respectively)

At surrogate annual test conditions, the RTU provides space cooling with an SHR between 0.68

and 0.81. The large dehumidification capacity is primarily due to the large OA flow provided by 

the unit. For surrogate annual conditions, only stages 0 and 1 are used to rate the system.

0

20

40

60

80

100

120

40 50 60 70 80 90 100 110

ω (grains/lb)

Dry Bulb Temperature (°F)

SHR = 0.92

SHR = 1.05

SHR = 1.25

WCC RA

WCC Peak

Mixed Air Inlet - S0

Mixed Air Inlet - S1

Mixed Air Inlet - S2

Supply Air - S0

Supply Air - S1

Supply Air - S2
5

Figure 4. Psychrometric chart of RTU performance at surrogate annual conditions

(Shown at 0 ft elevation. S0, S1, and S2 denote stages 0, 1, and 2, respectively)

Figure 5 shows the interpolation of cooling and power at surrogate annual conditions using 

stages 0 and 1. These data are used to estimate annual cooling performance, assuming that the 

average building load over a cooling season uses 80% of the measured sensible capacity at peak 

conditions. The interpolated capacity and power use are then used to calculate the surrogate 

annual energy efficiency ratio (EER). The same approach is used for water use. Total water use

and water evaporation (in gallons per hour) are interpolated to 80% of the sensible capacity (see 

Figure 6). This number is then used with the surrogate cooling capacity to determine gallons per 

sensible ton·h.

0

20

40

60

80

100

120

40 50 60 70 80 90 100 110

ω (grains/lb)

Dry Bulb Temperature (°F)

SHR = 0.68

SHR = 0.76

SHR = 0.81

WCC RA

WCC Surrogate Annual

Mixed Air Inlet - S0

Mixed Air Inlet - S1

Mixed Air Inlet - S2

Supply Air - S0

Supply Air - S1

Supply Air - S2
6

Figure 5. Data used to interpolate surrogate annual EER performance

Figure 6. Data used to interpolate surrogate annual water use/evaporation performance

The nominal capacity given by equation (3) for determining credited ventilation rate was 

calculated to be 60.5 kBtu/h. From this, the credited ventilation rate was then taken to be 600 

cfm. The actual ventilation rate was measured to be approximately 800 cfm. The static pressure 

applied to the unit was 0.7 in. w.c.

The calculated performance of the Coolerado RTU is shown in Table 2. Comparing the 

calculated performance below to the WCC specifications, the unit meets and exceeds all 

minimum thermodynamic and water use requirements of the challenge.

0.0

1.0

2.0

3.0

4.0

0

25

50

75

100

60% 70% 80% 90% 100% 110% 120%

Power (kW)

Cooling (kBtu/h)

Percent of WCC Peak Load Condition

Total Cooling

Sensible Cooling

Power

Linear (Total Cooling)

Linear (Sensible Cooling)

Linear (Power)

0

2

4

6

8

10

60% 70% 80% 90% 100% 110% 120%

Water Flow (gph)

Percent of WCC Peak Load Condition

Water Use

Water Evaporation

Linear (Water Use)

Linear (Water Evaporation)
7

Table 2. Western Cooling Challenge Summary

Specification Performance Units

Peak 

Conditions

(105°F/73°F)

Total Cooling 36–360 61.7 kBtu/h

Sensible Cooling – 56.9 kBtu/h

Power – 2.84 kW

Total EER – 21.7 Btu/Wh

Sensible EER ≥14.0 20.1 Btu/Wh

Outlet Humidity ≤0.0092 0.00917 kg/kg

* Water Use – 1.83 gal/ton·h (sensible)

Water Evaporation – 1.50 gal/ ton·h (sensible)

Surrogate 

Annual 

Conditions

(90°F/64°F)

Total Cooling – 57.4 kBtu/h

Sensible Cooling – 45.6 kBtu/h

Mean Power – 1.11 kW

Total EER – 51.8 Btu/Wh

Sensible EER ≥17.0 41.1 Btu/Wh

* Water Use ≤4.0 1.85 gal/ton·h (sensible)

Water Evaporation – 1.48 gal/ton·h (sensible)

* NREL cannot verify through laboratory testing the unit’s ability to withstand scaling caused by water 

evaporation. The measurements are made available in terms of water use and evaporation in the laboratory. 

Water use will vary in practice because of system adjustments for water quality.
8

Appendix – Measured Data Tables

Stage

Air 

FlowOA-RA, 

Mixed

Air 

FlowSA

Air 

FlowEA

OA Mass 

Fraction

TOA-RA 

Mixed

TSA TEA

ωOA-RA 

Mixed

ωSA

– scfm scfm scfm % °F °F °F grains/lb grains/lb

Peak 

Conditions

0 3357 1822 1437 43% 89.6 71.5 76.5 68.8 68.8

1 3354 1834 1422 42% 89.4 60.5 93.4 68.5 67.9

2 3542 1810 1624 46% 90.4 58.4 96.8 67.9 64.2

Surrogate 

Annual 

Conditions

0 3444 1827 1482 43% 83.2 67.5 72.0 58.4 58.4

1 3383 1826 1451 43% 83.2 56.7 88.3 58.1 58.6

2 3591 1806 1660 46% 83.6 54.2 91.1 57.4 55.0

Stage Water Use Water 

Evaporation Unit Power Total Space Cooling

Sensible 

Space Cooling

Latent Space 

Cooling

– gal/sensibleton·h

gal/sensibleton·h kW kBtu/h kBtu/h kBtu/h

Peak 

Conditions

0 3.17 2.51 0.75 10.44 13.06 -2.62

1 1.82 1.43 2.01 33.78 35.45 -1.67

2 1.83 1.50 2.84 42.28 39.19 3.09

Surrogate 

Annual 

Conditions

0 2.19 1.73 0.74 31.28 21.21 10.07

1 1.37 1.13 1.93 53.32 42.96 10.36

2 1.40 1.17 2.70 62.17 47.40 14.77

Stage

Total 

Ventilation 

Cooling

Sensible 

Ventilation 

Cooling

Latent 

Ventilation 

Cooling

Net

Total 

Cooling

Net

Sensible 

Cooling

Net

Latent 

Cooling

Total

EER

Sensible

EER

– kBtu/h kBtu/h kBtu/h kBtu/h kBtu/h kBtu/h Btu/Wh Btu/Wh

Peak 

Conditions

0 18.95 17.37 1.57 29.39 30.43 -1.04 39.4 40.8

1 19.30 17.69 1.61 53.08 53.14 -0.06 26.4 26.4

2 19.39 17.75 1.63 61.67 56.95 4.72 21.7 20.1

Surrogate 

Annual 

Conditions

0 19.15 17.51 1.64 50.43 38.72 11.71 68.1 52.3

1 19.51 17.84 1.67 72.83 60.80 12.03 37.8 31.5

2 19.61 17.91 1.69 81.78 65.31 16.47 30.3 24.2
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Coolerado 5 Ton RTU Performance: Western Cooling Challenge 

Results

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NREL/TP-550-46524

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The Western Cooling Efficiency Center (WCEC) developed a set of criteria for test conditions, minimum energy, and 

water use performance for prototype cooling equipment and identified these conditions as indicative of western state 

climates. These criteria, named the Western Cooling Challenge, have been set forth as a challenge to manufacturers 

to improve the state-of-the-art space cooling products. NREL is to verify these criteria through laboratory testing at its 

heating, ventilation, and air-conditioning test facility.

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wcec; western cooling efficiency center; wcc; western cooling challenge; hvac

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