Water Management of Heat Pump System for Hot Water Supply in a Medium Size Hospital

Transcription

1 Wate Management of Heat Pump System fo Hot Wate Supply in a Medium Size Hospital Chih-Chiu Shen, Jau-Huai Lu, and Wu-Hui Chuo Abstact A heat pump system has been used in this study to eplace a natual gas boile fo the supply of hot wate in a medium size hospital in cental Taiwan. The pump capacity is 14 kw. Heat is absobed fom the ice wate at 12 of an ai conditioning system with a capacity of 9 kw, and then used to heat up tap wate at oom tempeatue. The system has been unning fo seven months. It shows that the system can fully satisfy the hot wate equiement of the hospital. The outlet wate tempeatue is 48~51, and the maximum ate is 75 tons pe day. Accoding to the unning ecod, enegy cost has been educed by US$9 in the past seven months, and the eduction in cabon dioxide emission has been cut 256 tons. A mathematical model fo the hot wate system has been established in this pape. Seveal paametes, including the wate consumption ate, heat loss of wate pipe, atmospheic tempeatue, hot wate tempeatue setting, heat pump opeation mode, and the wate management fo the stoage tank, ae consideed in this model. Two stategies fo wate efilling to the stoage tank has been studied. One is to keep the wate level at 86%~98% all the time, and the othe is to efill wate until wate level has dopped to 14%. It was found that the second stategy could save 3% of enegy cost because off peak powe was utilized to heat up wate. Keywods Coefficient of Pefomance, Enegy Saving, Geen Gas Reduction, Heat pump, Loop Type Hot Wate System. I. INTRODUCTION AIWANESE govenment has launched the National T Health Insuance Pogam (NHI) in Mach 1995 to offe the sevice of health cae to the entie population. This pogam has educed the medical expenditue fo most people. As a esult, the hospital has to educe opeating cost to make up the eduction in the income of medical sevice. In ode to offe a comfotable envionment in the building of hospital, lot of enegy has been consumed, especially in ai conditioning and hot wate system. At the moment that global waming is of geat concen, any saving in enegy cost has benefits in cost eduction as well as geen house gas eduction. C.C.S. is a Ph.D student in the Mechanical Engineeing Depatment, National ChungHsing Univesity, 25, Kuo Kuang Rd., Taichung 42, Taiwan (coesponding autho to povide phone: ; fax: ; fuing86221@yahoo.com.tw). J.H.L. is the an associate pofesso of the Mechanical Engineeing Depatment, National ChungHsing Univesity, 25, Kuo Kuang Rd., Taichung 42, Taiwan ( jhlu@dagon.nchu.edu.tw). W.H.C. is pesident manage of ESCOTEK company. The company wok fo consultant in enegy saving design elated on heat ecovey system. ( wu-hui@e-enetech.com.tw) TABLE I NOMENCLATURE COP L Cool side coefficient of pefomance COP H Hot side coefficient of pefomance T P Heat pump outlet tempeatue T ( T + T )/2 Mean Tempeatue m ṁ i. m. m p. m l e Inlet mass flow ate Retun mass flow ate Heat pump mass flow ate Consume mass flow ate m Outlet mass flow ate. ( e m = m. e + m. l ) Q P Heat pump heating ate h L Convection of oute pipe h H Convection of inne pipe R Themal Resistant C Specific heat of constant pessue m Mass of tank k Themal conductivity The hospital consideed in this pape is located in cental Taiwan. Natual gas was the pimay souce of enegy to supply hot wate. An investigation on the existing boiles found that the exhaust tempeatue is still high and some available enegy has been lost. The esulting themal efficiency is quite low. Besides, the pice of natual gas has been double since 3, causing a stingent pessue on the opeation of hospital. As a esult, it is necessay to impove the pefomance of enegy usage in the hospital. Heat pump is the solution consideed in this study because of its high efficiency and easonable initial cost. A pilot system has been built in 7 and un fo seven months to demonstate the feasibility of supplying hot wate with heat pumps in a medium size hospital. The tempeatue vaiations and the wate flow ates in a day when the system opeated wee collected and the system pefomance was evaluated. The objective of this pape is to show the collected data and to compae the enegy cost of the oiginal natual gas system and the new heat pump system. Wate level is an impotant contol vaiable in hot wate system. The timing to efill wate when wate level eaches the lowe limit would affect the amount of wate and the wate tempeatue in the tank which would in tun detemine the timing to tun on the heat pump. Since the ate of off-peak powe is much lowe than that of peak powe, the enegy cost would vay fo the diffeent setting of lowe limit. In ode to 386

2 find the optimum setting of efill timing, a model of the wate tank has been developed in this pape to investigate the effect of wate management on the enegy cost. the outlet of heat pump is connected to the 4 th tank, as shown in Fig. 2. Fig. 1 The schematic of heat pump system II. EXPERIMENT SETUP Fig. 1 shows the configuation of the hot wate system. This system employs the chilling wate of ai-condition as heat souce fo heat pump system. The benefit of this system would be twofold. In one side, natual gas boile of hot wate system is eplaced by the heat pump and esults in savings of enegy cost. In the othe side, ai conditioning load of the building is patially fulfilled by the heat pump and exta savings of enegy could be achieved. Howeve, since the ai conditioning load is much highe than the load of hot wate, the exta savings is not noticeable when the enegy consumed in ai conditioning is examined. As a esult, the benefit in the eduction of ai conditioning is not consideed in this pape. The system contains two sets of compesso with total heating ate of 28 kw and total efigeation capacity of 18 kw. The 5 kw scew type compesso was selected. Cold wate fom chille of ai conditione flows into the evapoato of heat pump via a chille pump. Wate tempeatue dops about 5 o C in the evapoato fom 12 o C to 7 o C. It is noted that the inlet tempeatue could not be as low as the design value, depending on the load of ai conditione. The actual value of inlet tempeatue of chille would affect the system pefomance. Wate in the tank flows into the condense of heat pump via a cooling pump and gets heated. Wate tempeatue is aised about 1 o C in the condense fom 5 o C to 6 o C at the design flow ate. The outlet tempeatue is fixed at 6 o C. If the inlet tempeatue is lowe than 5 o C, an intenal ciculation loop would be activated to heat up the wate until the design tempeatue is obtained. The expeiment setup is shown in Fig. 2. An insulated wate tank is used to stoe hot wate. The tank is divided with patition into fou pats of identical volume. The total capacity is 7 m 3. Upsteam side is at the 1 st tank and downsteam side is at the 4 th tank. Cold tap wate flows into the 1 st tank. Hot wate flows out of the 4 th tank. t all the hot wate out of tank will be used up. The emaining hot wate will etun back to the 2 nd tank. The inlet of heat pump is connected to the 2 nd tank, and wate amount (tons) Fig. 2 The layout of hot wate system time(h) Fig. 3 Daily wate consumption III. HEAT PUMP MODEL consume amount The total amount of daily consumption of hot wate depends on the atmospheic tempeatue as well as the numbe of patients stayed in hospital. It is estimated that the aveaged value of daily consumption is about 7 m 3 pe day. The amount of hot wate consumed in a day vaies hou by hou, as shown in Fig. 3. The tend of vaiation is simila to that found by Lam to employ ai-to-wate and wate-to-wate heat pumps in hotel applications [2]. Howeve, thee is diffeence in the timing of peak load. The hotel application showed peak powe at aound 11:3 a.m. and the hospital application has peak at 8:3 p.m. The opeating pocedue of heat pump is shown in Fig. 4. Pump is stated to efill wate when the level of wate is detected to be lowe than the lowe limit to avoid damage on pump. Retun wate pump is activated to delive the etun wate when the tempeatue in the hot wate line is lowe than

3 Stat Setting tank Tempeatue Unde etun, Wate setting Tempeatue. Heat pump System stat Tank level Enough Retun Wate Pump Stat Fig. 5 The outlet tempeatue and level of tank Compesso Running Unde etun, Wate setting Tempeatue. Setting tank Tempeatue Stand By Stand By Alam/off Alam/off Stop Fig. 4 Flow chat of heat pump contol IV. EXPERIMENT RESULTS AND DISCUSSIONS Fig. 5 shows the vaiations of wate outlet tempeatue and level of tank in a day. Also shown is the activation and deactivation of the heat pump system. A fist look of the system pefomance found that the themal load equiement of the system is close to the global powe demand in Taiwan, in which peak time of electic consumption is aound 7:3 a.m. to 1:3 a.m. The collected data also indicated that the highest themal powe demand occued aound 7:3 a.m. Reducing the outlet tempeatue will educe the electic powe consumption. Fig. 6 shows the effect that the outlet wate tempeatue has been set at a lowe value. It can be found that the duation that heat pump was activated has been educed. The supplementay wate of the system comes fom the tap wate. It is obvious that the tap wate tempeatue as well as the ambient tempeatue would affect the themal demand of the system since pat of the piping system is exposed to ai. The Fig. 6 Tempeatue and wate level afte adjusting the outlet tempeatue monthly vaiation of tap wate tempeatue and ambient tempeatue at the location of the consideed hospital is shown in Fig. 7. It is noted that at cental Taiwan, thee ae few months that ambient tempeatue is lowe than tab wate tempeatue. The lowest tempeatue occued at Febuay and the tab wate tempeatue is highe than ambient tempeatue. Howeve, the tab wate tempeatue would be lowe than ambient tempeatue afte Mach. The diffeence in tab wate tempeatue and ambient tempeatue indicates that insulation of tab piping is not necessay because in most time of a yea, heat tansfe would occu fom envionment to supplementay wate. In ode to ensue that hot wate would come out of tap immediately as it is opened, wate in the pipe should be kept wam enough. The way to keep wate wam is to ciculate hot wate in pipes all the time no matte it is used o not. Howeve, as wate ciculates in the pipe, heat loss occus due to insufficient insulation. This would incease the themal demand and educe the themal efficiency of the system. The moe wate is consumed, the less wate is in ciculation, and the themal enegy demand pe unit of wate consumed would be educed. The coelation between the wate consumption and enegy consumption is shown in Fig. 8. It is clea that when the 388

4 daily wate consumption is 76 m 3, the enegy consumption is 18 kw-h/m 3, howeve, as wate consumption is educed to 6 m 3, the enegy consumption becomes 26 kw-h/m 3. impotant oles in detemining enegy consumption. The elationship is qualitative athe than quantitative O.A and Tap Wate Tempeatue (Celsius) outside ai tempeatue tap wate tempeatue Hospitalize /Capacity (%) Hospitalize/Capacity Enegy Consumption Enegy Consumption (kw-h/m^3) Months Months Fig. 9 Monthly vaiation of occupation atio and enegy consumption Fig. 7 Ambient ai tempeatue and tap wate tempeatue vaiation 6 Enegy Consumption (kw-h/m^3) Heat Pump Outlet Tempeatue(Celsius) Heat Pump Outlet Tempeatue Powe Consumption Powe Consumption (kw-h/month) Wate Consumption (m^3) Fig. 8 The elationship between wate consumption and enegy consumption The moe patients stay in hospital, the moe hot wate is needed. As a esult, the themal efficiency of the hot wate system is elated to the occupation atio, which is defined as the atio of the numbe of patients stay in hospital to the maximum capacity of the hospital. Fig. 9 displays the monthly vaiation of occupation atio and enegy consumed pe unit of hot wate. It is shown that occupation atio is coasely invesely elated to enegy consumption. Howeve, othe paametes, such as ambient tempeatue, etun ate of hot wate, also play Months Fig. 1 The outlet tempeatue and enegy consumption in the past six months The appopiate tempeatue fo showe in hospital would be in the ange of 4~45 o C. The outlet tempeatue of the hot wate system can be adjusted to fulfill the equiement. In the winte time, the outlet tempeatue should be highe to compensate the heat tansfe loss. Fig. 1 displays the enegy consumption fo the past 6 months. It is shown that as heat pump outlet tempeatue declines, the coesponding enegy consumption is also educed. Fom Januay to June 8, heat pump outlet tempeatue deceases 2.2 o C, the associated enegy consumption educes an amount of 24%. A linea coelation can be obtained between the outlet tempeatue and 389

5 the enegy consumption as shown in Fig. 11. It can be seen that the elationship is vey clea. Powe Consumption (kw-h/day) y =.21x Outlet Tempeatue ( ) Fig. 11 Relationship between outlet tempeatue and powe consumption V. SIMULATION MODEL Wate level is an impotant contol vaiable in hot wate system. Since the ate of off-peak powe is only 4.4 cents US$ pe kw-h, howeve, the ate fo peak powe equies 9.5 cents US$ pe kw-h, the enegy cost would vay fo the diffeent setting of lowe limit. In ode to find the optimum setting of efill timing, a model of the wate tank has to be developed to investigate the effect of wate management on the enegy cost. The wate tank was divided into fou pats. Howeve, accoding to the collected data, the tempeatue statification is not clea due to the stong mixing inside tank. To simplify the model and to educe the computation time, a single zone model was used in this pape, as shown in Fig. 13. Fig. 13 Heat pump simulation of model The balance of mass in the tank gives the following equation. dm m m m dt = + i e (3) Fig. 12 Instantaneous COP in one day The coefficient of pefomance fo the heat pump system can be obtained as the following definitions. The efigeation cycle COP is simila as COP L COP = L output useful efigeation = input net wok Meanwhile, the heat pump cycle COP is defined as output useful efigeation COP = = + 1 H input net wok COP H ( 1 ) (2) Fig. 12 shows the vaiation of COP duing 24 hous. Accoding to the collected data, COP was obseved to be in the H ange of 2.8 to 3. and the COP was highe than 2.. The total L heat was 26 kw. The balance of themal enegy in the tank gives the following equation. dt 1 = [ mt i i+ mt + m p ( Tp T ) mt e T ( m i m e + m )] dt m 1 = [ m i ( Ti T ) + m ( T T ) + m p ( Tp T )] m (4) Whee m p( Tp T) is the heat pump heating ate. In this system, it is set at 26 kw. The etun flow ate is detemined as the diffeence between the outlet flow and the consumed flow. m = m m e l The tempeatue of the etun flow can be evaluated with the heat loss model though the wall of pipes. T T 1 T + T mct T = = T R R 2 m e ( e ) ( ) (5) (6) 39

6 1 1 1 Te(1 ) + T = T (1 + ) 2mcR mcr 2mcR (7) latte case, the pecentage of off-peak powe inceases as explained above. As a esult, the etun tempeatue can be expessed in tems of outlet tempeatue and ambient tempeatue. 2mcR 1 2 T = Te + T 2mcR + 1 2mcR + 1 ( 8 ) The themal esistance fom the hot wate to ambient can be obtained though the following equation. 1 do 1 1 R = ln + + 2πkL d πd h L πd h L i o H i L ( 9 ) Wate Amount (tons) ~69 tons 1~69 tons VI. SIMULATION RESULTS AND DISCUSSIONS Fig. 14 shows the esults of simulation fo two sets of opeating condition. In one condition the amount of wate in the stoage tank vaies between 6 to 69 tons, and the othe condition wate vaies between 1 to 69 tons. The initial amount of wate at 12: pm should be the same eveyday. The supplementay flow ate fom tap wate is set at 2 l/s when wate amount eaches the lowe limit and efill is activated. In the fist case, wate efill has been activated 3 times pe day. The tank emains almost full all the time. Howeve, in the second case, wate efill has been activated only once a day duing the night time. The tank is almost full at 8: in the moning when a busy day begins. The amount of wate in the tank is sufficient fo the daily use. When the tank becomes empty, it is close to the midnight when wate equiement is low. The efill is then activated to pump in enough wate fo the next day. It is noted that when supplementay wate flows into the stoage tank, the tank tempeatue would dop and the heat pump would be tuned on to maintain the outlet tempeatue in the equied ange. As a esult, if supplementay wate flows into tank in the daytime, peak powe would be used to heat up wate and the esulted enegy cost is high. In the othe way, if supplementay wate flows into tank only in the night time in which powe ate is cheape, then the enegy cost would be low. Fig. 15 shows the effect of wate management on the enegy cost. It can be obseved that if the amount of wate in the tank is allowed to vay in a lage ange, the enegy cost would become lowe. It is noted that the amount of enegy equied to heat up wate is almost the same no matte how much wate is kept in the tank because the daily consumption of wate is fixed. Howeve, the atio of off-peak to peak powe could be vaied by management of wate in the tank. Fig. 16 shows the vaiations in the tank tempeatue, the initial wate amount, the pecentage of off-peak heating and peak heating as diffeent stoage ange. It is seen that the wate tempeatue in the tank keeps almost constant egadless the stoage amount. The wate amount at : deceases as the amount of wate in the tank is allowed to vay in a lage ange, indicating that most wate is supplied duing midnight. In the Time(h) Fig.14 Wate amount vaiation in the tank fo two diffeent setting Cost (US$) off peak peak total cost Range of Wate Stting (tons) Fig.15 Effect of wate management on enegy cost Fig.16 Effect of wate stoage on enegy distibution 391

7 If the outlet tempeatue is aised to offe hotte wate, the heat pump would be tuned on moe often, and moe enegy will be consumed. Fig. 17 shows the effect of outlet tempeatue on the enegy cost. It can be found that enegy cost is not linealy popotional to the outlet tempeatue. As highe tempeatue is equied, moe peak powe will be used and the enegy cost will incease apidly. Cost (US$) Total Cost Off Peak Cost Peak Cost Lowe Tempeatue ( ) Fig. 17 Effect of outlet tempeatue VII. CONCLUSION A heat pump system has been set up to eplace a natual gas boile fo the supply of hot wate in a medium size hospital in cental Taiwan. The total capacity of the pump is 28 kw. This system was built in 7 and has been un since then. The collected data and the associated analysis show that the system woks well and the oiginal design goals have been achieved. Seveal conclusions can be dawn as the following. 1. This system could supply steady demand of hot wate in tempeatue and flow ate. 2. Compae to natual gas boile, heat pump system saving 9, US$ in seven months. The payback peiod was 1.8 yeas. 3. Cabon dioxide emission was educed 256 tons. 4. The heat pump system ecoves heat fom ai condition system and help to educe the cooling load. 5. The heat pump system has been opeated fo seven months and though winte. This confims the feasibility of the system. The outlet tempeatue ange was between 45~5 o C. The supply hot wate was 75 tons pe day. 6. The simulated esult indicated the eduction of outlet tempeatue fom 5 to 45 o C could save 24% of enegy cost. The stoage ange of 1 to 69 could save 3% of enegy cost compaed with that the stoage ange of 6 to 69. REFERENCES [1] Olympia Zogou Effect of Climatic Conditions on the Design Optimization of Heat Pump System Fo Space Heating and Cooling Enegy Conves. Mgmt Vol. 39,. 7, pp ,1998. [2] Joseph C. Lam Enegy pefomance of ai-to-wate and wate-to-wate heat pumps in hotel applications. Enegy Convesion and Management 44 (3) [3] J.Y. Wu Dynamic analysis of heat ecovey pocess fo a continuous heat ecovey adsoption heat pump Enegy Convesion and Management 43 (2) [4] Aif Hepbasli A eview on the enegy and exegy analysis of sola assisted heat pump systems Renewable and Sustainable Enegy Reviews 11 (7) [5] Zaidi, J.H, and Howell, R.H., Enegy use and heat ecovey in wate-loop heat pump, vaiable-ai-volume, fou-pipe fan coil, and eheat HVAC systems: pat-i,"ashrae Tansactions, Vol.99, Pat-2, pp.l3~28, [6] Thoe Bentsson Heat souces technology, economy and envionment Intenational Jounal of Refigeation 25 (2) [7] Milan Macic Long-tem pefomance of cental heat pumps in Slovenian homes Enegy and Buildings 36 (4) Chih-Chiu Shen (M 9) eceived the B.S. degee fom industy education depatment of National Taiwan mal Univesity in 1991 and M.S. degee fom mechanical engineeing of National Taiwan ChungHsing Univesity in 3. Duing 1991~9, he have been teaching fo 18 yeas in efigeation and ai-conditioning depatment of industy high school, w, he is cuently pusuing the Ph.D. degee in mechanical engineeing at the Intenal Combustion Engine Laboatoy, National ChungHsing Univesity, Taichung, Taiwan, R.O.C. His eseach inteests include enegy saving, enegy ecovey, efigeation and ai-conditioning, especially enewable enegy. Jau-Huai Lu (M 9) eceived the B.S. degee and M.S. degee fom National Taiwan Univesity, Taipei, Taiwan, in 1979 and 1981 espectively, both in mechanical engineeing, and eceived the Ph.D degee fom the Univesity of Califonia at Bekeley at 199. His eseach inteests include intenal engine combustion, automobile pollution contol, and enegy convesion technologies. Wu-Hui Chuo (M 9) eceived the associate engineeing degee fom efigeation and ai-conditioning depatment of National Chin-Yi Univesity of Technology in 1991 and M.S. degee fom mechanical engineeing of National Taiwan ChungHsing Univesity in 9. His eseach inteests include enegy saving, enegy ecovey, efigeation and ai-conditioning. ACKNOWLEDGMENT Chih-Chiu Shen thanks ChungHsing Univesity fo fothcoming suppots and funded by the laboatoy. 392