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Contract No. DE-AC36-08GO28308

Commercial Refrigeration: 

Heat Transfer Optimization and 

Energy Reduction

Measurement and Verification 

of a Liquid Refrigerant Pump System 

Retrofit 

Chris Gaul and Michael Sheppy 

National Renewable Energy Laboratory

Technical Report

NREL/TP- 3500-65677

March 2016
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Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

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303-275-3000 • www.nrel.gov

Commercial Refrigeration: 

Heat Transfer Optimization and 

Energy Reduction

Measurement and Verification 

of a Liquid Refrigerant Pump System 

Retrofit 

Chris Gaul and Michael Sheppy 

National Renewable Energy Laboratory

Prepared under NREL Commercialization Assistance 

Program

Technical Report

NREL/TP- 3500-65677

March 2016
NOTICE

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iii

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Executive Summary

Objective

This study describes the test results of a Refrigerant Pump System integrated into a commercial 

supermarket direct expansion (DX) vapor compression refrigeration system. The Liquid 

Refrigerant Pump System retrofit (patent-pending; application number 13/964,198) was 

introduced to NREL in August 2014 by CTA Architects Engineers. 

A DX refrigeration system uses a vapor compressor to move both vapor and liquid refrigerant. 

Liquid refrigerant is moved indirectly by heat energy added to the vapor stream. A Refrigerant 

Pump System inserts a liquid pump between the condenser outlet and expansion valve inlet to 

move liquid refrigerant directly by adding kinetic energy to the liquid stream. In a dimensionless 

and theoretical system, the net effect of exchanging high entropy vapor compressor work for low 

entropy liquid pump work is higher system exergy. 

NREL’s interest in the technology includes the potential energy and greenhouse gas emission 

reductions for existing and new supermarkets and other building types. After examining the 

Refrigerant Pump System design, available energy measurement data, reliability, maintenance, 

and food case temperature stability in commercial operation, the product was accepted into the 

NREL Commercialization Assistance Program for evaluation and analysis. 

The refrigeration system was tested in a supermarket located in Freehold, New Jersey from April 

6 to June 19, 2015, to measure energy use, review parameters, and observe performance. Based 

on the results, this report proposes additional system enhancements, along with 

recommendations for continued study and testing. 

Conclusions

NREL measured the energy performance of the Refrigerant Pump System technology in a 

supermarket hydro-fluorocarbon system operation. The system solves the problems of previous 

attempts to use a pump to move liquid refrigerant in DX systems. It uses a patented method to 

provide sufficient net positive suction head (NPSH) to the liquid pump at all times. 

The Refrigerant Pump System operated continuously without failure under all weather and 

refrigeration demand conditions during the testing period. The system was turned on and off to 

measure performance under similar conditions. Conventional system operation resumed with no 

further action than switching the unit off. 

In summary, the following conclusions were reached: 

• The Refrigerant Pump System provided a regulated liquid refrigerant flow to 

refrigeration cases in all outdoor temperatures without head (compressor discharge) 

pressure controls. A sensor signaled a variable-frequency drive to control pump speed 

and match refrigerant flow demand. A refrigerant tank provided the required net positive 

suction head for reliable pump operation. Tank levels were maintained by a sensor to 

ensure the pump never cavitated. 
iv

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• The refrigerant tank is a key feature. The tank solves the NPSH issue that caused 

previous attempts to add liquid pumps to DX systems to be unsuccessful. Liquid line 

flash gas bubbles and insufficient NPSH would cause pump cavitation and subsequent 

loss of pressure rise. This led to poor refrigeration system performance and dissatisfied 

users. The refrigerant tank supplies bubble-free liquid at sufficient head under all 

conditions to the pump suction for reliable operation. 

Using a tank to provide pump suction head is standard practice in large built-up liquid 

overfeed industrial refrigeration systems. The concept has been successfully adapted for 

supermarket DX systems using an eductor venturi device to keep liquid in and vapor out 

of the tank 

• Reliable pressure boost and energy savings. The Refrigerant Pump System maintained

liquid pressure at 30 pounds per square inch (psi) above compressor discharge pressure. 

Technical specifications from an expansion valve manufacturer were used to select the 30

psi set-point for highest average expansion valve capacity during all outdoor air 

temperature and food case load conditions. 

Refrigeration system discharge pressure operated at 125 pounds per square inch gage 

(psig) and was floated down to the compressor manufacturer’s low limit of 70 psig. 

Refrigeration system suction pressure operated initially at 13 psig and was raised to 17 

psig. Energy reductions during a 10-week testing period with an average outdoor 

temperature of 62.6o Fahrenheit (F) were recorded. 

% Savings: Compressors % Savings: Total

Ambient Temp. 39.3-50.8°F 25% 11%

Ambient Temp. 50.8-58.9°F 23% 6%

Ambient Temp. 59.3-82.0°F 16% 6%

Note: lower compressor manufacturer pressure setting limits may be utilized with the 

Refrigerant Pump System as noted in the Additional Applications Recommended for 

Field Testing section of this paper. 

• Refrigeration system operated using condenser sub-cool fan control while 

maintaining optimal liquid pressure. An additional 4.3% energy reduction was 

recorded compared to energy used during a similar range of outdoor temperatures with 

the Refrigerant Pump System in operation. 

• Stable system operation in floating head pressure mode: 

• Food case temperatures were stabilized in cold weather by delivering liquid 

refrigerant without bubbles/flash gas/vapor and maintaining an optimal pressure 

drop across expansion valves. This is expected to improve food freshness and 

safety. 

• The elimination of split condensers, flooding valves, and other minimum 

discharge pressure measures; no seasonal configuration changes were performed 
v 

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when using the Refrigerant Pump System. For the testing period, minimum 

discharge pressure controls were not needed to maintain stable operation. This 

indicates maintenance could be reduced by eliminating minimum discharge 

pressure controls such as split condensers, flooding valves, and dampers. 

• The Refrigerant Pump System is a passive conduit when it is not operating. The 

system reverts to conventional DX operation when the Refrigerant Pump System is

turned off. If the system fails, liquid refrigerant automatically bypasses it. 

o This was deemed an important feature for risk-adverse supermarket managers.

The Refrigerant Pump System testing showed energy savings in a commercial supermarket 

system operation. This report identifies realistic areas for further optimization. More thorough 

testing is warranted and recommended to determine potential energy savings offered by 

integration of a Refrigerant Pump System into conventional DX refrigeration systems. Testing 

should be carried out for a year to have a sufficient sample size covering seasonal weather and 

shopping patterns. 

Measurement and Verification Results

Refrigerant Pump System field testing revealed two major findings: 

• Lower energy use over a range of outdoor temperatures

• Condenser fan power needed to maximize refrigerant sub-cooling can exceed energy 

saved at the compressor and reduce overall system coefficent of performance. 

Parasitic condenser fan energy use increased by an average of 71% over the entire test period 

using conventional floating head pressure/liquid temperature condenser fan control during 

Refrigerant Pump System operation. 

Conventional floating head pressure/liquid temperature control schemes try to maximize subcooling. This was found to increase fan energy by 71% over that needed to maintain stable 

system operation using the Refrigerant Pump System. Average total energy use was reduced by 

4.3% when condenser drain leg sub-cooling-based fan control was used during average outdoor 

temperatures of 73o

F. Recorded savings were from a small sample size at the end of the test 

period. 

An operational mode was designed to control condenser fans without excessive energy use to 

sub-cool liquid. Testing concluded before this mode could be optimized. Limited data showed 

the condenser would adequately reject refrigeration system heat with fans off in 60°F overcast 

weather. The median temperature at the test location is 56°F indicating condenser fan energy 

saving opportunities much of the year. 
vi 

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Table of Contents

Executive Summary................................................................................................................................... iii

Objective ............................................................................................................................................... iii

Conclusions ........................................................................................................................................... iii

Measurement and Verification Results .................................................................................................. v

1 System Description .............................................................................................................................. 1

1.1 Refrigerant Pump System Operation: Floating Head Pressure ..................................................... 1

1.2 Refrigerant Pump System Operation: Liquid Pumping ................................................................. 2

1.3 Refrigerant Pump System Operation: Cold Weather .................................................................... 3

1.4 Refrigerant Pump System Operation: Expansion Valves .............................................................. 4

2 Test Set Up............................................................................................................................................ 5

3 Future Work: Additional Applications Recommended for Field Testing........................................ 8

3.1 De-Superheating Discharge Vapor ................................................................................................ 8

3.2 Cascade Refrigeration ................................................................................................................. 10

3.3 Enabling Efficient CO2 DX Refrigeration Systems .................................................................... 11

Appendix.................................................................................................................................................... 14
vii 

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List of Figures

Figure 1. Refrigerant Pump integration into Supermarket DX System ........................................................ 3

Figure 2. Compressors and total energy savings percent by outdoor temperature range with Refrigerant 

Pump System ON, Condenser fans providing excessive sub-cooling. ..................................... 6

Figure 3. Compressors, condenser fans, total energy use by outdoor temperature range for Refrigerant 

Pump System ON vs. OFF. ...................................................................................................... 6

Figure 4. Refrigerant Pump System with discharge gas de-superheating. .................................................... 9

Figure 5. Typical pressure-enthalpy diagram using de-superheating. ........................................................ 10

Figure 6. De-superheating and use of a high-COP cascade condenser. ...................................................... 11

Figure 7. Comparison of CO2 (R744) COP vs. hydro-fluorocarbon refrigerant. ........................................ 12

Figure 8. De-superheating in a CO2 system. ............................................................................................... 13

List of Tables

Table 1. Test Overview and Parameters ....................................................................................................... 5
1 

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1 System Description 

In a direct expansion (DX) supermarket refrigeration system: 

• One or more compressors move high pressure refrigerant vapor through a condenser that 

rejects heat to outside air, causing vapor to phase change into liquid 

• Expansion valve(s) meters high pressure liquid into one or more low pressure evaporators 

• Low pressure evaporators phase change liquid to vapor, which absorbs heat

• Low pressure vapor flows to the compressor and the cycle repeats 

Vapor leaving the compressor is at high pressure and temperature; it is superheated above its 

saturated pressure due to the added work of the compressor. Compressors at one time were all 

reciprocating pistons in cylinders capped with cylinder heads. The term head pressure is 

synonymous with discharge pressure. 

Vapor leaving the evaporator is at low pressure and temperature. It is also superheated to ensure 

the refrigerant has done all its useful work and to prevent damaging liquid droplets from entering 

the vapor compressor. Suction pressure describes the condition in the conduit between the 

evaporator and compressor. 

Refrigerants in a saturated state are at their highest heat transfer capacity. Saturated vapor is 

100% gas at a temperature where it is ready to condense into liquid. Saturated liquid is ready to 

evaporate into vapor. Vapor above saturation temperature is superheated. Liquid below saturated 

temperature is sub-cooled. Sub-cooled liquid is bubble-free. 

Refrigeration heat exchangers are most effective when operating at saturation temperatures. In 

practice, pressure losses cause saturated liquid refrigerant to form flash gas bubbles that impede 

fluid flow through expansion valves. Superheated vapor exiting the compressor must lose 

sensible heat before it can change phase in the condenser. These conditions reduce heat transfer 

effectiveness. 

1.1 Refrigerant Pump System Operation: Floating Head Pressure

The heart of a DX refrigeration system is the compressor, a machine that converts electricity into 

heat in the form of gas pressure. It supplies the motive force to move refrigerant through the 

cooling system. A compressor pumps vapor directly, and through vapor pressure, indirectly 

causes liquid refrigerant to flow. The Gas Laws show that in a conventional vapor compression 

system vapor pressure increases are accompanied by liquid refrigerant temperature increases.

From a purely thermodynamic point of view DX supermarket refrigeration systems should be the 

most energy efficient with the highest coefficient of performance (COP) in cold weather because 

they have to perform less work moving heat from a cold food case to the cold outdoors. Head 

pressure should be able to float downward to take advantage of cooler condensing temperature. 

In practice, a conventional DX system cannot take full advantage of low outdoor temperatures. 

Since the compressor is the only means of moving liquid through the system and it does so by 

applying vapor pressure to liquid there is a lower limit to reducing compressor work before there 

is not enough motive force to move refrigerant through the system. 
2 

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Refrigerated case cooling takes place in evaporators fed by expansion valves. Expansion valve 

mass flow is affected by pressure differential and bubbles in the liquid refrigerant. If differential 

pressure across the expansion valve is low and bubbles are present, refrigerant mass flow and, in 

turn, food case cooling is reduced. When a lower limit is reached and food cases no longer 

maintain temperature set points, the store calls a service technician for help. The technician will 

attempt to remedy the problem by adjusting various parameters to raise condensing pressure. If 

these efforts are unsuccessful the technician will force compressors to run longer to keep food 

cases cold. This increases pressure differential across the expansion valve while lowering suction 

pressure and evaporating refrigerant temperature. Case temperatures are restored at the expense 

of using more energy to cool the load. 

In practice, DX systems are unable to full advantage of floating head pressure to raise COP. 

1.2 Refrigerant Pump System Operation: Liquid Pumping

The Refrigerant Pump System separates the tasks of pumping vapor and liquid: compressors 

move vapor and a pump moves liquid. Gas Laws therefore do not apply because the fluid 

streams are decoupled and partitioned. A liquid pump increases pressure directly by adding 

kinetic energy to the fluid stream. A pump is on the order of 100 times more efficient than a 

compressor in converting electricity into liquid pressure. The tested Refrigeration Pump System 

uses a variable speed drive 1 HP canned motor centrifugal pump (smallest size available) on a 30 

HP refrigeration system. 

The Refrigerant Pump System increases liquid above saturation pressure causing it to become 

sub-cooled and bubble-free without lowering temperature. The Refrigerant Pump System 

provides sub-cooled liquid with less energy input than running condenser fans. 

 Liquid sub-cooled by increased pressure has a saturation temperature higher than indoor air 

temperature. It will not form flash gas bubbles between the refrigeration machine room and loads 

on the store sales floor. This system can be expected to provide bubble-free liquid refrigerant to 

expansion valves under all weather conditions. 

Figure 1 illustrates Refrigerant Pump System integration in a supermarket refrigeration system.

The tested Refrigerant Pump System has a footprint of 2 X 2 feet and can be retrofitted into an 

existing DX refrigeration system by a qualified contractor.
3 

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Figure 1. Refrigerant Pump integration into Supermarket DX System

1.3 Refrigerant Pump System Operation: Cold Weather

Supermarket DX refrigeration systems must operate in all weather. A properly sized condensing 

unit will perform satisfactorily during the hottest outdoor temperatures. However, systems 

generally operate at below design dry bulb temperature most of the year. It is during cold 

weather when satisfactory conventional DX operation becomes complex.

In conventional DX refrigeration systems as condensing pressure/temperature floats downward 

with outdoor temperature bubbles/flash gas/vapor can form in the liquid refrigerant line. 

Bubbles coupled with low pressure differential across the expansion valves reduces refrigerant 

flow which adversely affects case temperatures. Evaporators starved for refrigerant can get very 

cold from low suction pressure leading to excessive frost buildup, insufficient heat transfer, and 

poor food case temperatures. It is often necessary reduce suction pressure by running more 

compressor capacity to overcome this problem. 

The conventional cold weather solution to provide adequate liquid pressure to expansion valves 

is to restrict liquid flow from the condenser to the receiver and divert compressor discharge hot 

gas to the inlet of the receiver. This backs liquid refrigerant up into the condenser, reducing its

heat rejection capacity, which increases the condensing pressure. At the same time the hot gas 

raises liquid pressure and temperature in the receiver, allowing the system to satisfactorily cool 

food cases. 

Liquid 

Refrigerant to 

Expansion 

Valves

Refrigeration Rack Compressors 

Condensers

Refrigerant Storage Tank

Head

MT or LT Cases

Deliver bubble-free liquid 

to DX valves

Suction

Medium or Low 

Temp Cases

Superheated hot 

vapor

Warm Liquid

Superheated 

cold vapor

Pressure Boosted Sub-cooled Warm Liquid

Liquid Refrigerant Pump System

Eductor system pipes
4 

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A Refrigerant Pump System functions as a mechanical sub-cooler. It adds pressure instead of 

lowering temperature to sub-cool liquid. This allows it to sub-cool liquid independent of outdoor 

temperature. The theoretical advantage of improved COP in cold weather can be achieved in 

actual operation. 

1.4 Refrigerant Pump System Operation: Expansion Valves

Expansion valves control the flow of liquid refrigerant entering the evaporator by maintaining a 

constant superheat in refrigerant vapor at the evaporator outlet. Superheated suction vapor is 

necessary to ensure the refrigerant has performed all useful work and that no damaging liquid 

droplets reach the vapor compressor. 

Expansion valves work best when supplied with bubble-free liquid. Even slightly sub-cooled 

liquid has no bubbles. Conventional DX systems sub-cool liquid refrigerant by removing 

sensible heat, generally by running condenser fans such that liquid refrigerant approaches 

outdoor air temperature. Liquid refrigerant sub-cooled by reducing its temperature with cold 

outside air can warm up while passing through store spaces and lose its sub-cooling on its way to 

refrigeration loads. Bubbles may form in liquid feeding expansion valves, reducing refrigerant 

mass flow and evaporator cooling capacity. 

Liquid line flash gas problems have been addressed by adopting electric expansion valves. 

Thermostatic and electric expansion valves of the same port size have identical capacities for all 

operating conditions. Electric expansion valves adjust more quickly and accurately than 

mechanical thermostatic expansion valves. Electric expansion valves mitigate, but do not solve, 

liquid refrigerant delivery problems created in low outdoor temperature operation and during 

dynamic changes in cooling and defrost loads. 

A Refrigerant Pump System supplies expansion valves with bubble-free liquid at sufficient 

pressure to provide full cooling capacity and stable operation independent of head pressure. 
5 

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2 Test Set Up 

The refrigeration rack controller recorded pressures, temperatures, and numbers of compressors 

and condenser fans operating. Compressor and condenser fan energy use was recorded in VoltAmperes using current transformers on three phases. Data was automatically uploaded through a 

wireless modem to a Cloud-based data acquisition system. Testing compared the Refrigeration 

Pump System on vs. off in varying outdoor temperatures. Condenser fans were constant speed 

and not equipped with variable-frequency drives.

Table 1. Test Overview and Parameters

Test Overview

Test period: April 6-June 19, 2015

Location: Freehold, New Jersey

Equipment tested: Liquid Refrigerant 

Pump System retrofit (patent-pending; 

application number 13/964,198)

Total testing days: 63

Average outdoor temperature: 62.6oF

Average outdoor daily temperature range: 

39-82oF 

Tested Load and Static Parameters

Design comp. load (Btu/hr.): 247,500

Refrigerant: R-134A

Dew point cond. temperature: 110oF

Dew point evap. temperature: 15oF

Evap. superheat.: 10oF

Condenser-outdoor ΔT: 13oF

Return gas temperature: 65oF

Test Scenarios and Variable Parameters

• Refrigerant Pump System OFF (baseline measurements) 

• Condensing temperature set-point: 100oF 

• Saturated suction pressure set-point: 13 psig (12o

F)

• Refrigerant Pump System ON

• Condensing temperature set-point: 70o

F 

• Saturated suction pressure set-point: 17 psig (18o

F) 

(As stated above, suction pressure is increased when using the Refrigerant Pump 

System because a pure column of pressurized liquid is delivered to evaporator 

expansion valves).
6 

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Test Results Data

Figure 2. Compressors and total energy savings percent by outdoor temperature range with 

Refrigerant Pump System ON, Condenser fans providing excessive sub-cooling.

Figure 3. Compressors, condenser fans, total energy use by outdoor temperature range for 

Refrigerant Pump System ON vs. OFF.
7 

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Analysis: 

Energy savings with Liquid Refrigerant Pump on and head pressure allowed to float down with 

cool outdoor temperature yielded less than anticipated energy savings. Condenser fans were set 

to achieve 70 psig head pressure which equates to 69°F condensing temperature. 

It was ascertained that with a 13°F condenser temperature differential outside air would have to 

be below 56°F for condensed liquid to reach 69°F. Operators often regard the extra fan power 

expended trying to reach 70 psig as providing useful liquid sub-cooling that in theory increases 

refrigeration effect and reduces compressor load. Tests show that compressor energy savings 

were offset by high fan power. Sub-cooling performance gains did not justify the fan energy 

expended. 

A test scheme was devised using the refrigeration rack controller to balance head pressure with 

condensed liquid temperature. Fans were staged to provide slight but not excessive sub-cooling. 

This idea came late in testing with limited data collected. In a fully developed control scheme the 

rack controller would optimize compressor energy and variable speed condenser fans to achieve 

the highest system COP. 
8 

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3 Future Work: Additional Applications 

Recommended for Field Testing

The Refrigerant Pump System offers potential applications for further energy reduction and 

efficiency improvements. In summary: 

• The technology allows for stable operation at condensing temperatures of 40oF and below 

while maintaining bubble-free liquid to expansion valves. Unlike other floating head 

pressure techniques, split condensers, flooding valves, and other minimum discharge 

pressure measures are not needed and consequently do not negatively impact exergy 

(eliminating head pressure controls would partly offset the Refrigerant Pump System 

added cost). 

• The Refrigerant Pump System can be configured to inject liquid refrigerant into the 

compressor discharge vapor stream to remove superheat. Saturated vapor entering the 

condenser is expected to improve its heat exchange effectiveness. 

• Liquid injection de-superheating and controlling sub-cooling by varying condenser fan 

air flow opens the possibly to increase COP by coupling the refrigeration system to the 

supermarket air conditioning system (Figure 6): 

• During lower outdoor temperatures refrigerant flows to the outdoor condenser in 

the normal manner. 

• During higher outdoor temperatures refrigerant is diverted from the outdoor 

condenser to a heat exchanger cooled by the supermarket air conditioning to 

become a cascade cooling system. 

• Successful testing of a cascade using common hydro-fluorocarbon refrigerant opens a 

path to developing a sub-critical carbon dioxide (CO2) system suitable for warm climates. 

3.1 De-Superheating Discharge Vapor

Head pressure is the highest pressure in a conventional DX refrigeration system. A Refrigerant 

Pump System increases liquid refrigerant to 30 psi above head pressure. This allows a means of 

improving condenser heat exchange effectiveness. 

It is expected that all other things being equal, refrigerant phase changing on a conduit surface 

provides more effective heat transfer than conduction. 

In conventional DX refrigeration systems the compressor discharge line carries superheated 

vapor to the condenser. The condenser rejects heat by blowing outdoor air over tubes carrying 

refrigerant. The condenser removes heat by cooling superheated vapor to saturation temperature 

through conduction, phase changing (condensing) vapor into liquid, and finally sub-cooling 

liquid by conduction. The fraction of heat exchange surface needed to cool superheated vapor 

into saturated, ready to condense vapor varies depending on vapor and outdoor air temperatures. 

In a Refrigerant Pump System, liquid is available at higher than head pressure. Liquid refrigerant 

can be injected in a controlled manner into the superheated vapor compressor discharge line to 

the condenser. The liquid refrigerant absorbs sensible heat by changing state and giving up its 
9 

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latent heat. It flashes from liquid to vapor and cools superheated discharge gas. Injecting 

sufficient liquid reduces discharge vapor to saturated temperature. The saturated vapor stream 

enters the condenser ready to phase change into liquid. No portion of the condenser heat 

exchange surface is needed for de-superheating by conduction. 

Liquid injection to de-superheat discharge gas is theorized to improve condenser heat transfer 

effectiveness by using the entire surface area for phase change. It is further expected to improve 

effectiveness by increasing refrigerant mass flow through the condenser. A Refrigerant Pump 

System separates vapor and liquid refrigerant streams making it possible to flow more refrigerant 

through the condenser than the evaporator. This is not feasible with a conventional DX system. 

It is important to note that using refrigerant for de-superheating would not reduce refrigerant 

available to evaporators. 

Liquid injected for de-superheating would be metered through an electric stepper valve 

connected to the rack controller. The controller would adjust injection rate based on compressor 

discharge vapor and condensed liquid temperatures and pressures. 

Improved condenser heat exchanger effectiveness means less airflow is needed to reject 

refrigerant heat to outdoor air which would save fan power. The refrigeration system controller 

would measure compressor and fan power to provide optimum airflow for best COP. 

Figure 4 shows the Refrigerant Pump System configured for de-superheating. 

Figure 4. Refrigerant Pump System with discharge gas de-superheating. 

Refrigeration Rack Compressors 

To expansion valves

Head

De-superheat 

discharge vapor

Suction from 

Food Cases

Refrigeration 

Management System

Sensible Heat 

converts 

to Latent Heat

Condenser

Liquid Refrigerant 

Warm Pump System

Liquid

boosted 

30 psi

Superheated 

vapor

Warm Liquid

condensing 

pressure
10 

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There are potential energy savings due to reduced condenser fan operation using de-superheating 

and sub-cooling-based fan control, as well as the addition of variable-frequency drives. Field 

testing measurement and verification is recommended. Figure 5 illustrates the de-superheating 

process on a pressure-enthalpy chart. 

Figure 5. Typical pressure-enthalpy diagram using de-superheating.

Field testing indicates that sub-cooling liquid refrigerant by more than necessary for stable 

system operation is counterproductive. More fan power is used than is saved at the compressor. 

Reducing vapor temperature entering the condenser reduces thermal stress and condenser surface 

degradation. This will reduce refrigerant leaks. In water-cooled equipment, lower refrigerant 

temperature reduces mineral scaling on heat exchange surfaces. 

Field testing is required to optimize liquid injection de-superheating. 

3.2 Cascade Refrigeration 

Nearly all U.S. supermarkets have air conditioning. A Refrigerant Pump System opens the 

possibility of using the supermarket air conditioning system instead of outside air as a 

condensing medium. In hot weather the refrigeration system would work as a cascade condenser 

which is expected to improve refrigeration system COP. Research is needed to determine if 

overall refrigeration and air conditioning energy use would benefit from this arrangement. 

Figure 6 illustrates the Refrigerant Pump System theoretical application of de-superheating 

discharge vapor and use of a high-COP cascade condenser enabling refrigeration system 

operation at reduced condensing temperatures year-round. 
11 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 6. De-superheating and use of a high-COP cascade condenser. 

3.3 Enabling Efficient CO2 DX Refrigeration Systems

Supermarket refrigeration system designers are interested in CO2 refrigerant because it is 

plentiful and will never face phase-out as has happened with popular synthetic refrigerants. CO2

has a critical pressure of 1,055 psig at 87.7°F. Above these conditions, transcritical CO2 is 

neither liquid nor vapor. Since most climates experience outdoor air temperatures above 87°F, 

some means other than a condenser is required to reject heat. 

When a transcritical CO2 refrigeration cycle is plotted on pressure enthalpy and temperature 

entropy diagrams, it becomes apparent that the physical and thermodynamic properties of CO2

Warm Liquid

condensing 

pressure

Warm 

Liquid

boosted 

30 psi
12 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

make it a poor choice for refrigeration condensing units operating in high outdoor temperatures. 

CO2 is a better choice for refrigeration condensing units operating in cool climates. 

Figure 7 illustrates the reduction in COP for CO2 as ambient temperature rises (from Emerson 

Climate Technologies TEG124-0910/E Study). 

Figure 7. Comparison of CO2 (R744) COP vs. hydro-fluorocarbon refrigerant. 

During moderate to high outdoor temperatures, the high-pressure throttle valve and flash gas 

bypass valve combination have been adopted to transfer heat energy and pressure from the 

transcritical CO2 to the condensing unit in order to return it to its liquid state. The high-pressure 

throttle valve and flash gas bypass valve combination do not utilize an efficient change of state 

refrigeration effect to accomplish heat rejection for changing discharge fluid to liquid. This 

energy efficiency tradeoff should be recommended when designing a refrigeration system using 

only CO2. 

In transcritical mode CO2 system COPs are lower than conventional vapor compression systems. 

In warmer climates this penalizes CO2. Other refrigerants such as R410a have higher COPs in 

warmer climates. 

As previously described for a hydro-fluorocarbon system, a CO2 system with Refrigerant Pump 

System can utilize discharge fluid heat transfer optimization to make subcritical operation 

possible year-round when paired with a high-COP cascade condenser as shown in Figure 8. 
13 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 8. De-superheating in a CO2 system. 

The liquid to vapor change of state results in a subcritical fluid mixture with a higher heat 

transfer coefficient and a lower temperature. The lower temperature subcritical fluid will divert 

from the gas cooler/condenser. The lower temperature fluid will instead be sent to a high-COP 

cascade condenser for further heat removal and condensation. 

The result of utilizing the Refrigerant Pump System de-superheating application is an increase in 

total system COP, subcritical CO2 refrigeration operation year-round, and a technically viable 

CO2 refrigeration option for warm climates.

A natural refrigerant such as R-717 (ammonia) may be desired in such a cascade system, as is 

done in industrial practice. However heating, ventilating, air conditioning, and refrigeration 

systems using R-410A or other synthetic refrigerant will continue to be used in supermarkets for 

the foreseeable future. 
14 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Appendix

TEV Capacity Calculations: Without and With Refrigerant Pump System Retrofit

Information Courtesy of Sporlan Division – Parker Hannifin Corporation, Sporlan Division 

bulletin 10-10 / 411_Thermostatic Expansion Valves with Selective Thermostatic Charges 

Test Equipment Utilized

Panoramic Power® Cloud based energy data acquisition and analysis tool 

Current Transformer amperage transducers

Wireless Bridge