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AN EXPERIMENTAL STUDY OF WASTE HEAT RECOVERYFROM A RESIDENTIAL REFRIGERATORRobert A. Clark, Richard N. Smith, and Michael K. Jensen

Departmentof Mechanical Engineering,Aeronautical Engineering and MechanicsRensselaer Polytechnic InstituteTroy, New York 12180-3590(518) 276-6351; (518) 276-6025 (FAX)

A B S T R A C TThis paper will describe the design, construction, andtesting of an integrated heat recovery system which has beendesigned both to enhance the performance of a residentialrefrigerator and simultaneously to provide preheated water foran electric hot w ater heater. A commercial, indirect-heated hotwater tank was retrofitted with suitable tubing to permit it toserve as a water cooled condenser for a residential refrigerator.This condenser opera tes in para l le l wi th the a i r -cooledcondenser tubing of the refrigerator so that either one or theother i s ac t ive when the re fr igera tor i s running. Therefrigerator was housed in a controlled-environment chamber,and it was instrumented so that i ts performance could bemonitored carefully in conjunction with th e water pre-heatingsys t e m.The system has been tested under a variety of hot waterusage protocols, and the resulting data set has providedsignificant insight into issues associated with commercialimplementation of the concept. For the case of no waterusage, the system was able to provide a 35 C emperature risein the s torage tank a f te r about 100 hours of continuousoperation, with no detectable deterioration of the refrigeratorperformance. Preliminary tests with simulations of highwater usage, low water usage, and family water usageindicate a poss ible 18-20%energy savings for hot water over along period of operation. Although the economic viability forsuch a system in a residential environment would appear to besub-marginal, the potential for such a system associated withcomm ercial-scale refrigeration clearly warrants furthe r study,particularly for climates for which air conditioning heatrejection is highly seasonal.I N T R O D U C T I O NThe use of waste heat recovery in energy systems designhas long been an important tool in reducing total energycosts . The econom ies of scale often dictate that suchtechniques be applied to large systems and energy intensiveprocesses, particularly when the heat rejection system and the

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heat source can be integrated at the initial design stage. Fo rresidential and small-scale commercial systems, the need topurchase components such as refrigerators and water heatersseparately combined with requirement for a high level ofreliability usually precludes a practical consideration of heatrecovery. However, such systems have an impac t on literallyevery household in the country, as well as many commercialand industrial environments. For example, if ways could befound economically to recover some of the energy rejectedfrom air conditioning or refrigeration systems, the cumulativebenefit would be significant. Th e principal problem, ofcourse, is establishing a low per-unit cost of retrofitt ing anexisting system or initiating new designs.The particular opportunity investigated in the presentpaper is to preheat the supply water for a hot water systemusing refrigerator condenser waste heat. In northern climatesof the United States, refrigerators are always on, whilere s ide n t i a l a i r c ond i t i on ing i s u se d se a sona l ly a ndintermittently. Furthermore, at a small comm ercial scale, suchas for restaurants or fast food establishments, the potentialgain from integrating the refrigeration capacity with the hotwater requirements is even greater. An added potential benefitof attempting to l ink these two systems is the opportunity toprovide water cool ing , ra ther than a i r cool ing , of therefrigerator condenser. Howe ver, sinc e refrigerators aresomewhat self-regulating (for example, to accomodate thesignificant fouling of condenser tubes which is expected inmany household environments), care must be taken so thatwater cooling does not, in fact, deteriorate the coolingcapacity of the refrigerator o r detract from its reliability.Anantapantula and Sauer (1994) described the use of aireconomizers to maintain the d esired temperature in a building.During a cooling process, air is removed from the area to becooled and circulated through the economizer, which vents aset amount of the warm building air and replaces it with thecooler outside air. This air is then circulated back to theinterior of the building. After simulating this heat recoverysystem with a two-story office building, they discovered that

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an energy cost savings of 49% could be obtained if this heatrecovery sys tem was used ins tead of a t radi t ional a irconditioning system.In another study (Cohen, 1986), is was found that it wasfeasible to modify the hot water and air conditioning systemsin a restaurant so that the heat from the hot refrigerant exitingthe compressor could be used as an additional heat source toproduce hot water. A water-cooled desuperheater was placed inbetween the compresso r and the condenser. The water used inthe desuperheater was continuously circulated to a preheatstorage tank and back. It was estimated that the paybackperiod for such a system would range from 3 to 4 ears for fast-food and full-service restaurants located in the southern UnitedStates .Mills and Perlman (1986) investigated several methods ofheat recovery as applied to a residence. One of the moreinteresting approaches involved the reclamation of heat fromwater after it has been utilized. Waste water is collected in a454 liter holding tank, which also contains the evaporator fora 1.2 kW water-to-water heat pump. When the watertemperature in the holding tank rises above a certain point, theheat pump is activated, transferring heat from the holding tankto the condenser which is mounted inside a 272 liter fresh hotwater storage tank. An exper imental prototype of this systemwas constructed and tested using a water usage pattern that wasderived from a accepted standard hot water delivery schedule.The tests indicated that an energy savings of up to 60% over atypical 272 liter electric hot water heater was possible.The sam e paper also addressed water heating through heatrecovery from the exhau st air from sources such as the kitchenand laundry room. This exhaust air is passed through theevaporator of a 1.4kW air-to-water heat pump, which transfersheat to a condenser mo unted in a fresh water storage tank. Thissystem was constructed and tested with the same hot waterusage pattern mentioned above, with a maximum energysavings of 42% over a standard hot water heater beingobserved.Heat pump water heaters have the potential to offersidnificant energy savings compared to electric hot waterheaters. Kesselring (1984), in a description of the state of theart of heat pum p water heaters, estimated savings of about 50%over electric resistance heaters. The Hawaiian ElectricCompany (HEC , 1995) reported a savings of up to 59% overelectric heaters for its customers. Lannus and Kesselring(1990, 19 91) described the design of an integrated air sourceheat pump which simultaneously provides space heating andwater heating for a residence. The system is marketed by theCarrier Corporation and has been successful at reducingresidential power consumption, particularly in moderateclimates for which heat pumps are appropriate. Heat pumpshave been also been used for direct water heating. CrispaireCorporation (1994) produces heat pumps that can be directlyattached to existing electric water heaters.As specialized heat pumps, air conditioners can also bemodified to produce hot water. Chan and Toh (1993) developedan experimental prototype of a thermosyphon system thatrecovered the compressor superheat from an air conditioningsystem to produce hot water. This prototype had a smallstorage tank, a heat exchanger, and a 3.5 kW domestic airconditioner. Water from the storage tank circulates by athermosypho n effe ct through the heat exchanger. Theirsystem was able to warm 10 gallons of water to an acceptabletemperature within one night.

A s imilar appl icat ion might be envis ioned with arefrigerator. Although the cooling capacity of a typicalrefrigerator is much smaller than that of an air conditioner, itsuse is continuous throughout the year, even in colder climates,and some installations (restaurants, etc.) may have significantrefrigration capacity installed. Bourne and Dakin (1994)investigated the possibility of reclaiming refrigerator wasteheat to produ ce hot water. In their system it is necessary torecover the heat from the compressor itself to maintain anacceptable water temperature when the hot water is used.Furthermore, since there is no secondary condenser for therefrigerator, during periods of low hot water use it is necessaryto discard some of the hot water in the tank.Because many questions remain as to how refrigeratorcondenser heat may be recovered effectively, it was decided todevelop a laboratory experiment which would provide pre-heat ing of domest ic hot watcr without dis rupt ing thcperformance of the refrigeration system or requiring anyunusual intervention (such as discarding over-heated water) bythe consumer. The system which was constructed is based onresidential scale refrigeration and hot water use. Theinteraction of actual water usage pattems with the refrigerationcycle was a principal area of study. Therefo re, control andmonitoring of the refrigerator performance as well as the heatrecovery was an important part of the design. It is recognizedthat the heat rejection c apacity of a typical refrigerator (in thiscase, an 18 ft3 refrigerator/freezer) is not very high (300-400watts). However, the configuration was conven ient for studyin the laboratory, accurate performance data for the refrigeratorwas available from the manufacturer, and the small scalesystem was relatively easy to monitor and control. An actualimplementation of a design such as the present one wouldprobably work better for a larger scale refrigeration system,such as a walk-in cooler for a restaurant or food service.EXPE RIMENTAL SYSTEMThe constraints which were imposed on the developmentof an experimental system were (a) that operation of therefrigerator must be transparent to the existence of a water-cooled condenser, (b) that discarding of pre-heated water not benecessary, (c) that the system should suggest an installationwhich could be retrofit onto an existing refrigerator and ontoan existing hot water heater. Therefo re, a water storag e tankwas constructed to serve as a water-cooled condenser for an 18-ft3 household refrigerator and to simulate a pre-heating watersupply source for a hot water heater. The refrigeran t line wasdiverted to the tank with valves which are controlled by acomputer which senses water temperature and switches thecondensing back to air-cooled when the water temperature istoo hot to operate the refrigerator. The possibility of in-linewater- and air-cooled condensing (passive system) has alsobeen built into the experiment. Th e tank is a 30 gallon (113 1)commercial indirect-heated hot water tank originally designedfor steam condensation. It was necessary to replace thecondenser tubing with a more suitable tube size and length forrefrigerant condensation, and a considerable effort went intothe process of sizing and orienting the tubes to ensur e adequateheat transfer for operation of the refrigerator. Figure 1showsthe experimental layout.

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FIGURE 1. WATER-COOLED CONDENSERPIPING DIAGRAMThe refrigerator is a General Electric 18 ft3 residentialrefrigerator with the freezer compartment on the top. Theparticular advantages of this appliance is that it is generic inits configuration, and extensive test and performance data wasavailable from G.E. which could be used to size the condensertubes in the wa te r -coo led condense r . A con t ro l l edenvironment chamber for the refrigerator was constructed toprovide a well-defined ambient temperature experienced by therefrigerator. A 30 0 watt light bulb provided sufficient heat,combined with two circulation fans and 4 in. thick foaminsulation for the walls, to maintain the temperature to within2 "C of a set point up to 35 "C.The data acquisition and experimental control system isbased on a Gateway 2000 4DX2-66 personal computercombined with boards which allow analog-to-digital signalconversion to record temperature, pressure and flow rate data atseveral points in the flow loop and digital-to-analog outputsignals to control the operation of heaters, water flow valves,and refrigerant flow valves. Th e entire system is operated with

LABVIEWTMsoftware. Th e general configuration is illustratedin Figure 2.

lil I-FIGURE 2. COMPONENTS OF THE DATAACQUISITION AND CONTROL SYSTEM

R E S U L T SThis sect ion wil l describe the resul ts from severalexperiments which were conducted the integrated operation ofthe refriger ator and the water pre-heater system. In the firstexperiment, the performance of the refrigerator was measuredbefore the water-cooled condenser was installed to provide a

basis of comparison. After the condenser was installed, a runwas conducted to find the maximum water temperature thatcould be obtained in the s torage tank. F inal ly, theexper im enta l sys tem was t e s ted wi th th ree d i f fe ren tprogrammed water usage patterns to establish the increase inwater temperature from the istorage tank inlet to the outlet. Toensure a common bas is or comparison , the fol lowingprocedures were followed for all experiments: The automaticdefrost was deactivated so thiat regular cy cles of the refrigeratorcould be observed withou t interruption. The control boxtemperature was maintained at 32.2 "C, f .6 "C (90"F, f 1 OF).(This is an industry standard setting.) Th e data recorded as afunction of time were the piressures in both the condenser andthe evaporator, the e lectkical power consumed by thecompressor, the control box temperature, the freezer and freshfood area temperatures, the inlet and outlet temperatures for thecondenser. Wh ere appropriate, the storage tank temperaturesnear the top and the bottom of the tank were recorded, as wellas the inlet and outlet temperatures of water flowing throughthe tank during water usage tests.Tests o f the Un m Refriaeratordi f ied.The first goal of this experiment was to establish theoperating conditions of thie refrigerator before the water-cooled condenser was installed. Under normal operatingconditions, the compressor cycles on and off to maintainthermostatically set conditions in the refrigerator fresh foodarea. For a medium setting on the thermostat, the condenserand evaporator reach pressures of about 10.6 bars and 1.2 bars,respectively, and the power to the compressor is about 156watts (after a surge at each activation). Wh en the compressorshuts down, the electric power decreases to zero, and theevaporator and the condenser pressures begin to equalize,approaching a pressure between 1.7 and 2.1 bars. Thetemperature in the fresh food area cycled between 0.6 "C nd6.1 "C, and the temperature in the freezer was relatively stableat -16.1 OC. This consistency is due to the thermal inertia ofthe 43 packages of wood c hips placed in the freezer to simulatefood. During compressor operation, the condenser inlet andoutle t temperatures rose to about 63 "C and 4 2 "C,respectively. Using the condenser pressure , a value for thesaturation temperature was obtained. By comparing thistemperature with the outlet )temperature, it can b e seen that therefrigerant leaves the condenser slightly subcooled by about 1to 2 " C. The duty cycle started at about 46% and rose to arelatively stable 56%. According to the data sheet supplied bythe manufacturer, this represents a typical value for the dutycycle at an ambient temperature of 32.2 "C.To establish steady state values for refrigerator system, anexperiment was performed with the freezer and fresh food areadoors open. Th e packages in the freezer used in the aboveexperiment were also removed. The experi ment followed thesame test procedures as before, and the same measurementswere taken over a 4 hour period. After some initial transients,the power consumption settled to a slight oscillation around162 watts. The conden ser pressure exhibited the samebehavior, settling to a slightly larger oscillation around 11.6bars. The evaporator pressure also showed initial transientsbut settled to a more constant value of 1.3 bars. After about 90minutes, the ambient temperature reached 32.7 "C and began tooscillate about 32.2 "C due to the heat source being switchedon and off. The oscillations of the ambient temperaturecor respond d i rec t ly wi th the osc i l l a t ions in bo th the

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compressor power and condenser pressure noted earlier. Thefresh fo od area rose to a final value of 30 "C, and the freezertemperature rose to 26.7 "C, but appeared to be 1 or 2 'C belowa steady value Th e steady state values of the inlet and outlettemperatures of the condenser were 76.1 "C and 46.1 "C ,respect ively. Th e s teady s ta te value for the saturationtemperature was 48 .3 "C, indicating a subcooling at thecondenser outlet of 2 "C.Svs tem Per fo rm ance w i th Wate r -Coo ledC o n d e n s e r"No Water Use" Tests . A 5-day test was conductedto determine the maximum water temperature in the preheaterthat could be achieved by the system. The water in the storagetank was brought down to ground temperature by allowing citywater to flow through the tank for about 20 minutes before thetest was initiated. Results are shown here for the last twohours of the test. F igure 3 shows the compressor powerconsum pt ion and pres sures fo r bo th the wa te r -coo ledcondenser and the evaporator. As with the unmodifiedrefrigerator, the compressor power shows a large spike whenthe compresso r is initially turned on. Afterwards, the powerdrops off and follows the same dow nward trend shown by thecondense r p res sure . Th i s ind ica te s tha t the powerconsumption is a function of the p ressure difference betweenthe condense r and evaporator. The condenser pressure is alsos ignificant ly higher than what was measured from theunmodified refr igerator . This is due to the fact that the

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FIGURE 3. COMPRESSOR POWER ANDPRESSURES DURING " NO WATER USE" TEST

condenser in this test was submerged in water which had temperature of about 39 "C s opposed to the refrigerator's owair-cooled condenser which was cooled by air at about 32 "CFrom this, it can be seen that the condenser pressure is largela function of the temperature of the ambient fluid surroundingthe condenser. The capillary tube is the device respo nsible fothis behavior, as it ensures complete condensation of threfrigerant by increasing the condenser pressure and hence thsaturation pressure. Figure 4gives the temperatures associatewith the condenser. Wh ile the compressor is running, thcondenser outlet temperature and the saturation temperaturstay at about 48.9 "C an d 52.2 "C, respectively. While thiindicates a larger subcooling of 3.3 "C verses 1.1 to 1.7 "C fothe unmodified refrigerator, the actual amount of subcooling iprobably less, as there was heat loss to the laboratorenvironment in the piping from the outlet of the condenser tthe point where the measurement was actually taken. Thcondenser inlet temperature underwent a brief transient beforit rose to a value of 57.8 "C. Figure 5 gives the watetemperatures at the top and bottom of the storage tank. Thfinal water temperature at the top of the tank is about 47 "CThe rate of increase in the water temperature was about 11 "Cduring the f i rs t day and 2 "C during the final day. Thiindicates that the experimental system functions best whenheating cold water, as the heat loss from the storage tank tt h e e n v i r o n m e n t a t h i g h e r w a t e r t e m p e r a t u r e s w i lsignificantly degrade the performance of the system. Thcompressor duty cycle varied between 51 and 55 % with morfluctuations than were observed for the air cooled condenseoperation. This could be caused by a slightly low refrigerancharge in the system obtained when the new condenser tubingwas installed. Susequent review of the condenser tubing desigrevealed that the tubing was poss ibly unders ized for thiapplication. These two factors wou ld most likely result in marginally performing refrigerator, which seems to be thcurrent problem with the experimental system. (A largecondenser has been installed in the system , but new tests havnot yet been conducted.)

Water Usaae Pattern Tests. Tests were conductewith three standard water usage patterns (ASHRA E, 1995). Thfirst two usage patterns represent the high and low usage fodomestic water consumers, and the third pattern representwater usage for a typical family. The process control/datacquisition system was then set up to dispense water from thstorage tank according to these patterns. The test werconducted over periods of 5, 9 , an d 5 days for the high, lowand family usage patterns, respectively.Figures 6- 8 give the temperature difference between thinlet and outlet water temperature for the preheating tank anwater usage verses time of day for the las t full day of each tesFor the high usage pattern test (Figure 6) , the water use variefrom a low of 1.63 liters at 4 AM to a high of 36.23 liters atPM. The temperature difference varied from a low of 3. 9 'C a10 PM to a high of 7.6 "C at 9 AM , with an average value o4. 8 "C. For the low usage pattem test (Figure 7) the water usvaried from no water usage from 2 to 7 AM to a high of 9.7liters at 10AM. The temperature difference varied from a lowof 18.5 "C at 11 PM to a high of 20.3 "C at 2 PM, with aaverage value of 19.4 "C. Finally, for the family usage pattertest, the water use varied from no water usage from 4 to 5 AMto a high of 17.30 liters at 8 PM. The temperature differenc

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FIGURE 4. CONDENSER TEMPERATURESDURING NO WATER USE TEST

0 1 m a t m o m o r r m s o m o s m o m l a # ,W = = ( = W

FIGURE 5 . STORAGE TANK WATERTEMPERA TURES DURING NO WATERUSE TEST

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FIGURE 6. TEMPERATURE DIFFERENCEDURING HIGH USAGE TESTranged from 6.4 C t 12 AM to a high of 10.2 O C t 10 AM ,with an average value of 6.9 OF .The greatest values for the temperature difference wereobtained when the least amount of water was used. There isalso a time delay in the increase of the temperature differ encebetween times of low water usage in the early morning hours tohigh water usage during the time when residents are typicallypreparing to start their day. This effect is most pronounced inthe high water usage case. At 4 an d 5 AM , only 1.63 liters ofwater is dispensed from the storage tank. As expected, thewater in the storage tank heats up when only a little water isused, but it isnt until 7 AM that the temperature differencebegins to increase significantly from about 4.5 O C to 6.25 OC.This is due to the heat in the water initially being dissipatedinto and of out the piping between the storage tank and thepoint at which the outlet temperature was recorded. As largeramounts of water are drawn from the storage tank from 8 to 9AM, the temperature difference increases to a maximum valueof 7.6 OC as the increased lhermal mass of the water begins tocounteract the heat loss in the piping. After 9 AM , the coldwater entering the s torage tank causes the temperaturedifference to decrease to around 5C. Figures 7nd 8 show thiseffect as well. This may have a serious impact on theperformance of an actual heat recovery system, as there wouldbe much more piping involved in connecting the storage tankto an actual water heater.S U M M A R YThe results of the first form al tests of a system designed tocouple a res ident ia l refr igerator with a water cooledcondenser/pre-heater have been presented. Du ring the no waterusage test, the experimental system w as able to increase water

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temperature by about 30 OC. Ho wev er, the time required toobtain this temperature increase was 5 days. As this does notreflect actual water usage, further tests were conducted withwater usage patterns that approximated high water use, lo wwater use, and the water use of a typical family. During thesetests, the experimental system was able to heat the water by asmuch as 7 .6 OC, 20.3 OC, and 10.2 C, respectively. Aneconomic analysis (Clark, 1996) revealed that a savings of18.3 % on the water heater operatin g cost was possible. Theactual economic viability would depend on the installationcosts of a production unit, which are unknown at present. I tshould be noted that a number of system imperfections werediscovered during the course of the present tes ts , andmodifications have been installed, although the results fromnew tests are not yet available. Certainly a system based onlarger capacity refrigerators could result in significantlyenhanced performance.A C K N O W L E D G M E N T SThis project was funded by a grant from the NYNEXFoundation and administered by the National Science TeachersAssociation. The assistance and motivation of two highschool (now col lege) s tudents , Eric Gandt and Aurel ioTeleman, who initially studied the idea of refrigerator heatrecovery, is greatly appreciated.R E F E R E N C E SA S H R A E , 1995 , A S H R A E H a n d b o o k : H V A CApplications, American Society of Heating, Refrigeration, andAir Conditioning Engineers, Atlanta,p. 45.10.Anantapantula, V. S . and Sauer, H. J., 1994, HeatRecovery and the Economizer for HVAC Systems, ASHRAEJ., Vol. 36 , pp . 48-53.Bourne , R . C . and Dakin , W. , 1994, A CombinedRefrigerator-Electric Water Heater, Proc. of the AmericanCouncil for an Energy Efficient Economy(ACEEE) SummerStudy of Energy Efficiency in Buildings, Vol. 3, pp . 3.19-3 . 2 8 .

Chan, S . K. and Toh, K. C., 1993, Thermosiphon HeatRecovery From An Air-conditioner For A Domestic Hot WaterSystem, ASHRAE Trans., Vol. 99 , pp . 259-264.Clark, R.A. , 1996, Was te Hea t Recovery F rom aHousehold Refrigerator, M.S. Thesis, Rensselaer PolytechnicInstitute, Troy, New York.Cohen, J., 1986, Heat Recovery for Restaurants, EPRZJ. , Vol. 11, pp . 30-35.Crispa i re Corpora t ion , 1994, Brochure fo r Mode lR106K2 Heat Pum p Water Heater, Atlanta, Georgia.HEC, 1995, Energy Costs for Household Appliances,Internet Document ( eco skh tm ), Hawaiian Electric Company.Kesselring, J., 1984, Heat Pump Water Heaters, EPRIReport EM-3582.Lannus, A. and Kesselring, J., 1990, Integrated HeatPump System, EPRI J.,Vol. 15, pp . 40-43.Lannus, A. and Kesselring, J.,1991, Field Testing of theHydroTech 2000 Heat Pump, EPRI J. , Vol. 16, pp . 33-36.Mills, B. E. and Perlman, M., 1986, Residential HeatRecovery, ASHRAE J., Vol. 28 , pp. 28-32.

FIGURE 7. TEMPERATURE DIFFERENCEDURING LOW USAGE TEST-- I

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3 4 i i a i a s % H s 9Pf$fg!f$fztf-FIGURE 8. TEMPERATURE DIFFERENCEDURING FAMILY USAGE TEST

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