A Heat Pump Assisted Shower

Dublin Institute of Technology ARROW@DIT Articles School of Mechanical and Transport Engineering 1983-05-01 A Heat Pump Assisted Shower Jim McGovern Dublin Institute of Technology, jim.mcgovern@dit.ie Follow this and additional works at: htp://arrow.dit.ie/engschmecart Part of the Energy Systems Commons Recommended Citation McGovern, J.: A heat pump assisted shower. Engineers Journal, Dublin, vol. 36, no. 5 (May), pp. 16 & 17. 1983. his Article is brought to you for free and open access by the School of Mechanical and Transport Engineering at ARROW@DIT. It has been accepted for inclusion in Articles by an authorized administrator of ARROW@DIT. For more information, please contact yvonne.desmond@dit.ie, arrow.admin@dit.ie. his work is licensed under a Creative Commons AtributionNoncommercial-Share Alike 3.0 License The Engineers Journal 16 required, but, this must be gradually increased as the water temperature from the tank decreases. By means of electronic control the instantaneous heater would operate to maintain a constant discharge temperature. Normally, this direct A Heat Pump Assisted Shower by J. A. McGovern, BE, MEngSc, CEng, MIEI Department of Mechanical and Manufacturing Engineering, Trinity College, Dublin INTROOUCTION Compact fully hermetic heat pump compressors of 500 W electric rating and less have recently become available. These have a low capital cost for their capacity while their performance characteristics compare with those of larger compressors. A particularly important feature is that they can be fully insulated while motor losses are recovered by means of a cooling coil in the oil sump. A quite separate development has been the appearance in the marketplace of a large variety of instantaneous electric sho......-er heaters. These incorporate compact electric resistance flow boilers with a total rating of about seven kilowatts. While the efficiency of a resistance heater is very high it represents poor utilization of high grade electrical energy when compared with air source heat pumps which can provide water at 'sho.....ering' temperature with a coefficient of performance of 2.5 to 3 and thus consume only 0.33 to 0.4 times as much electricity. The high cost of a heat pump of 7 KW output, hovvever. precludes its use as an instantaneous heater for such applications. as it is unlikely to run for more than an hour per day. A system is proposed consisting of a low powered heat pump, a storage tank and an instantaneous heater (Fig. 1) which. it is felt. may yield a favourable overall cost, in addition to reducing the primary energy consumption. Some preliminary work has been carried out in exploring this concept. It was considered at the outset that an overall C.O.P. of 2 should be readily achieved. VakJes close to this have been realised with the first experimental prototype. The applications of the work are not limited to domestic shower units, but, extend to many situations where hot water is to be supplied intermit· tently and there are ample periods for slow pre-heating of the water by a heat pump. l,mTER TEMP, •• 5. u ."k ｢Ｎｦｯｾ • • ｨｯ｟ｾ · m c • 4. •rw •" 3. h••cI clhah ••II. Ａｉｨ｡｟ｾ W "s ｾ 2. ,. • 4. 2• •• .. , •• VOLUME 14. 12. LIt-res showerl while the temperature at heat would be only a fraction of the the bottom of the tank is somewhat total supplied to the water. Fm lower due to stratification (Fig. 21. domestic use a total direct heater The storage tank is well insulated rating of 2 kW would be sufficient, and. after the initial heating phase. while, in a commercial situation. a the heat pump cuts in occasionally . 6 kW flow heater would guarantee under the action of a thermostat to continued hot water availability. even after the stored pre-heated compensate for standing losses. Since the temperature of the water had been used up. water in the storage tank varies over the running period from that of the SUMMARY OF FINOINGS water supply to about 46 deg. C the When operating with evaporator air mean condensing temperature of intake temperatures in the range 16 the heat pump is quite low and this to 19 deg. C and providing several favours a good C.O.P. value. 'showers' per day at a constant temWhen the shower is first turned perature of 50 deg. C the calculated on little or no direct heating ｾ overall C.O.P. of the plant, taking HERD TANI( ｾ ELeCTRIC SHOH£R HLATUf ｔｉｾ ...l... P 1 T ｾ n£Rt1OSTAT ｾ CONCEPT OF THE HEAT PUMP ASSISTED SHOWER A storage tank of water is heated over a period of time by a hea t pump which cuts out when the temperature reaches a pre-set level At this point the water at the top 0 f the tank is at the required tempera ture (46 deg. C approximately for a vs VOLUME TO DRAIN COMPRESSOR DRIER ｾ £ I "m"," STORAGE TANK ｾ EXPANSION VALVE £VflPORATOR I \ P\ The Engineers Journal account of the heat pump consumption, shower pump consumption, shower heat consumption and standing losses, was 1.93. The heat pump performance included in this figure was based on actual test results. The power consumption of the direct instantaneous heater was calculated, assuming that it could be automatically controlled to maintain a constant discharge temperature and could do so at an efficiency of 90%. CONCLUSIONS It is felt that the results to date provide a basis for further investigation of the concept of using a low capacity heat pump to provide preheated water in conjunction with a direct electric flow heater for final heating and temperature control. Automatic electronic control of the discharge temperature by using a thyristor to vary the heater power dissipation is relatively straightforward and inexpensive. However, the operation of the temperature controller has not been dealt with in this phase of the work. A hairdressng salon would be a very suitable location for a heat pump assisted water heater. The evaporator could be situated indoors in this case and would, in effect, recover the latent heat transferred to the room air by the use of hair dryers. 17