- Email: [email protected]
Exergy analysis and comparison of multi-functional heat pump and conventional heat pump systems
Energy Conversion and Management 73 (2013) 51–56
Contents lists available at SciVerse ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Exergy analysis and comparison of multi-functional heat pump and conventional heat pump systems Xiaolin Sun, Jingyi Wu ⇑, Ruzhu Wang Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 26 June 2012 Accepted 2 April 2013 Available online 13 May 2013 Keywords: Heat pump Exergy Cooling Heating Water heating
a b s t r a c t In this paper, the thermal dynamic analysis of a multi-functional heat pump system (MHPS) is made in comparison with a conventional heat pump air conditioning system. Energy and exergy efficiencies of the different systems are calculated to study influences of the components parameters, operation conditions and operation modes on the system performance. Variation of the MHPS performance with increasing hot water temperature is investigated, and performances of the major components are also studied in order to optimize the structure and operation of MHPS. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The air conditioner and the water heater are two major contributors to the residential building energy consumption. As more and more energy consumed by the air conditioning (AC) and domestic water heating (DHW), the residential building energy consumption increases rapidly. At the same time, in summer, the condensing heat of the air conditioner is usually delivered into the environment directly, which not only increases the energy waste, but also causes terrible thermal pollution. Meanwhile, irreversible energy loss caused by heat transfer between the thermal system and the surrounding environment at a finite temperature difference would also reduce the system operation performance. In a Multi-functional heat pump system (MHPS), a part of the condensing heat is recovered for DHW, thus to provide DHW and space cooling/heating simultaneously [1]. Nowadays, most studies on such systems are based on the first law (energy) analysis, which is the most common used method in thermodynamic analysis. While though there are various causes of energy destruction, the only reason in essential is irreversibility of process, which is described by the second law of thermodynamics. Compared to energy analysis, exergy analysis (second law) provides much more details of thermal process, including how, where and how much energy loss occurs. There are now many papers introducing principles and methods of exergy analysis [2–4]. By means of exergy analysis, the energy destruction at each separated component shows clearly,
⇑ Corresponding author. Tel.: +86 21 3420 6776; fax: +86 21 3420 6309. E-mail address: [email protected] (J. Wu). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.04.009
and maximum performance as well as improvement potential of the system can be identified. Several studies on the exergy analysis of heat pump and refrigeration systems have been done by researchers, and effects of different factors on the exergy efficiency of different systems have been investigated [3–14]. Akau and Schoenhals [8] studied a heat pump system that uses water as the heat source and heat sink experimentally. Kaygusuz and Ayhan [10], Torres-Reyes et al. [11,12] did the exergy analysis of a solar assisted heat pump based on experimental results. Hepbasli [13] and Hikmet Esena et al. [14] studied the exergy analysis of a ground source heat pump system. Akhilesh Arora, Hilmi Cenk Bayrakci and Ahmet Kabul et al. studied exergy analysis of a heat pump systems with different working fluids [3–5]. In this paper, the exergy analysis of MHPS under different operation modes is discussed to identify effects of components structure, working condition and operation mode on the system operation performance. The exergy loss of each major component (compressor, condenser, throttling valve and evaporator) is calculated and then compared to that of conventional air conditioning system to optimize system design and controlling method.
2. System schematic and exergy analysis principles Generally, the MHPS could realize the functions of AC, DHW, dehumidification and ventilation, while the MHPS discussed in this paper is used for AC (space cooling and heating) as well as DHW. Compared to the conventional air conditioning system (shown in Fig. 1b), an additional heat exchanger (HX) is added to the MHPS
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X. Sun et al. / Energy Conversion and Management 73 (2013) 51–56
Fig. 1. Schematics of MHPS and conventional air conditioning system.
to produce DHW. The MHPS could operate under various modes, including DHW mode, space cooling mode, space heating mode, space cooling with DHW mode and space heating with DHW mode. System operation mode could be switched by control of four-way valve, fan and pump. Schematic of a typical MHPS is shown in Fig. 1a. Fig. 2 shows the ideal and actual log P-h diagram of a MHPS cycle, where a–b–c–d–a shows the ideal thermal process of working fluid, and a0 –b0 –c0 –d0 –e0 –f0 –g0 –a0 shows the actual process with consideration of unavoidable irreversible energy loss. Different from conventional air conditioning system, MHPS under air conditioning with DHW mode needs two condensers. As shown in Fig. 2, condensing process undergoes in both condensers (x–c), where b– x shows condensing process in water-heating condenser, while x–c shows condensing process in the outdoor(under space cooling with DHW mode) or indoor (under space heating with DHW mode) unit of air conditioner. Fig. 3 shows the cycle of working fluid under air conditioning with DHW mode. In this investigation, R22 is used as working fluid. As shown in Fig. 3, condensing heat of the system is taken by two condensers. Under space cooling and DHW mode, fraction of condensing heat taken by the water heating condenser is calculated by the following formula:
v¼
h2 h3 h2 h5
ð1Þ
While under space heating with DHW mode, fraction of condensing heat taken by the water heating condenser is calculated by following formula:
v¼
h2 h3 h2 h6
ð2Þ
According to the second law of thermodynamics, a practical process is always irreversible. In a heat pump system, irreversible losses are
Fig. 3. Working fluid cycle of MHPS under (a) space cooling with DHW mode; (b) space heating with DHW mode.
caused by different factors, such as friction resistance and temperature difference of heat exchange. Assuming that (1) the system operates under steady state; (2) pressure drops caused by friction resistance are negligible; (3) heat losses at compressor and throttling valve are negligible [2,6], according to the second law of thermodynamics, exergy analysis equations are show as following:
Eheat;in þ Emass;in þ Ework ¼ Eheat;out þ Emass;out þ Irr
ð3Þ
w ¼ ðh h0 Þ T 0 ðs s0 Þ
ð4Þ
where Irr is irreversible loss, Eheat , Emass and Ework are exergy during heat transfer, mass transfer and working process, w is specific exergy in any state, T0 is the surrounding temperature, h0 and s0 are enthalpy and entropy of working fluid under temperature of T 0 and pressure of 0.10133 MPa. There is no mass transfer between MHPS and the surrounding environment, hence Emass = 0. And theoretical exergy losses in different components are calculated according to the following formulas (where mechanical loss and heat leakage are ignored) [2–4]: For the compressor
Icom ¼ mr T 0 ðscom;o scom;i Þ
ð5Þ
where Icom is the theoretical exergy loss of compressor; mr is the mass flow of working fluid; scom;i and scom;o are specific entropy of working fluid at inlet and outlet of compressor. For the evaporator
Iev a ¼ mr ½ðhev a;i hev a;o Þ T 0 ðsev a;i sev a;o Þ þ Q e ð1 T 0 =T e Þ
Fig. 2. Ideal and actual log P-h diagram of cycle.
ð6Þ
where Iev a is the theoretical exergy loss of evaporator; Q e is heat exchange quantity of the evaporator; T e is evaporating temperature; hev a;i =sev a;i and hev a;o =sev a;o are specific enthalpy/entropy of working fluid at inlet and outlet of evaporator.
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X. Sun et al. / Energy Conversion and Management 73 (2013) 51–56
For the condenser
Table 2 Calculation results of main operating parameters.
Icon ¼ mr ½ðhcon;i hcon;o Þ T 0 ðscon;i scon;o Þ Q c ð1 T 0 =T c Þ
ð7Þ
where Icon is the theoretical exergy loss of the condenser; Q c is heat exchange quantity of the condenser; T c is condensing temperature; hcon;i =scon;i and hcon;o =scon;o are specific enthalpy/entropy of working fluid at the inlet and outlet of the condenser. For the throttling valve
Iv al ¼ mr ðhv al;o hv al;i Þ
ð8Þ
where Iv al is the theoretical exergy loss of the throttling-valve; hv al;i and hv al;o are specific enthalpy/entropy of working fluid at the inlet and outlet of the throttling valve. In a vapor compressor refrigeration system, the exergy efficiency is influenced by several factors, including system structure, surrounding environment, refrigerant properties, lubricant and additives et al. In addition, both evaporating temperature and condensing temperature also have significant effect on the system exergy efficiency. Previous studies showed that exergy loss increases as evaporating temperature decreases, and vice versa. This can be explained that decreasing evaporating temperature would increases the temperature difference of heat exchange between working fluid in evaporator and the medium being cooled. For a vapor compressor refrigeration system, the exergy loss increases with increasing condensing temperature. This is also predictable since higher condensing temperature leads to larger temperature difference of heat exchange between working fluid in condenser and the ambient [2,5]. Operating conditions shown in Table 1 are determined according to standard conditions defined by related national standards of China. Calculated results of corresponding condensing temperature, condensing temperature, mean evaporating temperature and COP of an ideal Carnot cycle are shown in Table 2. Where T i is air temperature of the conditioned space; T wiu is the inlet water temperature of the indoor heat exchanger for air conditioning; T w is the inlet water temperature of the DHW heat exchanger (condenser). Where T c is condensing temperature; T e is evaporating temperature; T mc is mean condensing temperature; COPc is COP of the ideal Carnot cycle. Compared to the conventional heat pump air conditioner, MHPS can operate in air conditioning with DHW mode. Thus in this paper, MHPS under two modes are discussed: space cooling with DHW mode and space heating with DHW mode. 3. Results and discussion 3.1. Energy and exergy analysis of MHPS under space cooling with DHW mode For MHPS under space cooling with DHW mode at steady state, ignoring resistance loss and heat leakage, as shown in Fig. 3a, the state of working fluid at point 4 is the same as point 3, and the Table 1 Operating conditions of MHPS and conventional air conditioning system. System description
T0 (°C)
Ti (°C)
Twiu (°C)
Tw (°C)
Conventional air conditioning system (space cooling mode) MHPS (space cooling with DHW mode)
35
27
12
/
35
27
12
Conventional air conditioning system (space heating mode) MHPS (space heating with DHW mode)
7
20
35
25– 55 /
7
20
35
25– 55
System description
Tc (°C)
Te (°C)
Tmc (°C)
COPc
Conventional air conditioning system (space cooling mode) MHPS (space cooling with DHW mode) Conventional air conditioning system (space heating mode) MHPS (space heating with DHW mode)
47
5
47
6.62
Tw + 7 (25 < Tw < 40) 47 (40 < Tw < 55) 42
5
43.25
7.27
5
42
6.70
Tw + 7 (25 < Tw < 35) 42 (35 < Tw < 55)
5
40.3
6.92
state of working fluid at point 7 is the same as point 1. Assuming evaporator temperature and condensing temperature of MHPS at steady state are as shown in Table 2, all the calculations are based on an outdoor air temperature of 35 °C. Different from the conventional air conditioning system, condensing temperature of MHPS keeps going up because of increasing inlet water temperature of water heating condenser. Before exergy analysis, state x in Fig. 2 has to be determined, thus the ratio of condensing heat taken by the water heating condenser to condensing heat taken by the outdoor unit of air conditioner has to be identified first. When inlet water temperature of the water heating condenser is 28 °C, system condensing temperature would be 35 °C, which is the same with the outdoor air temperature. Thus it can be assumed that when inlet water temperature of the water heating condenser is lower than 28 °C, no heat exchanging occurs in the outdoor heat exchanger, thus system condensing heat is 100% taken by the water heating condenser. When inlet water temperature of the water heating condenser is 55 °C, the water heating condenser takes about 10% of the whole system condensing heat. Graphically, as the inlet water temperature goes up, the position of state x in Fig. 2 moves rightwards. Moreover, according to Table 2, when the inlet water temperature is between 25 °C and 40 °C, condensing temperature is always 7 °C higher than inlet water temperature; while when the inlet water temperature is between 40 °C and 55 °C, condensing temperature keeps at 47 °C. When the inlet water temperature is lower than 40 °C, the heat transfer in water heating condenser is considered to be 100% latent heat transfer with working fluid at condensing temperature. While when the inlet water temperature is higher than 40 °C, sensible heat transfer could no longer be ignored. Sensible heat transfer in the water heating condenser is considered to proceed with working fluid temperature at (t2 + t3)/2 (which is the mean temperature of working fluid at inlet and outlet), while latent heat transfer is still considered to proceed with working fluid at condensing temperature. Meanwhile, according to the second law of thermodynamics and Eq. (7), exergy loss during the heat exchanging process is determined not only by inlet/outlet state of the refrigerant, but also by the heat exchanging quantity as well as the temperatures of the heat-side and cold-side fluids. Thus the heat transfer in water heating condenser is considered to proceed at weighted mean temperature of sensible and latent heat transfer. According to working conditions described in Table 2 (with the outdoor air temperature at 35 °C), exergy losses are calculated with inlet water temperature of water heating condenser at 25 °C, 28 °C, 34 °C, 40 °C, 47.5 °C and 55 °C separately. For a MHPS under space cooling with DHW mode and a conventional air conditioning system under space cooling mode, Fig. 4a shows the exergy loss ratios of compressor and throttling valve with different inlet water temperature, which are calculated based on mass, energy and exergy conservation equations. It can be inferred that for MHPS, at first, the exergy loss ratios of compressor and throttling valve increase with increasing inlet water temperature. This is because when the inlet water temperature is low, con-
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(a ) compressor and throttling valve
(b) condenser and evaporator Fig. 4. Components exergy losses of MHPS under space cooling with DHW mode and conventional air conditioning system under space cooling mode.
densing temperature increases with increasing inlet water temperature, thus compression ratio of compressor as well as pressure reduction of throttling valve increases too. And for both compressor and throttling valve, increasing pressure difference between inlet and outlet would cause greater irreversible energy loss. While when the inlet water temperature exceeds a certain value, condensing temperature of MHPS grows slowly, hence the exergy loss ratios of compressor and throttling valve remain stable. Moreover, when the inlet water temperature is lower than that certain value, the exergy loss ratios of compressor and throttling valve of MHPS are always lower than those of the conventional air conditioning system. Fig. 4b shows the exergy loss ratios of evaporator and condenser. It has to be mentioned that the condenser exergy loss ratio of MHPS is weighted mean value of two condensers. For MHPS under space cooling with DHW mode, when the inlet water temperature is low, the total system condensing heat is taken by water heating condenser, and the exergy loss of outdoor unit is 0 since there is no heat transfer. And if the inlet water temperature keeps on increasing, the exergy loss ratio of outdoor unit would also increase. As shown in Fig. 4b, for MHPS, when the inlet water temperature is low, the evaporator exergy loss ratio decreases with the increase of inlet water temperature. And when the inlet water temperature exceeds a certain value, the evaporator exergy loss ratio becomes stable (which is almost the same with the evaporator exergy loss ratio of conventional air conditioning system).
As the inlet water temperature goes up, the condenser exergy loss ratio of MHPS first decreases and then increases, though this ratio of MHPS is always lower than that of the conventional air conditioning system. To explain this situation, two major factors that affect the condenser exergy loss ratio have to be mentioned: one is the temperature difference of heat exchange, and the other is the ratio of condensing heat taken by the water heating condenser to condensing heat taken by the outdoor unit. When the inlet water temperature is low, most condensing heat is taken by the water heating condenser. As the inlet water temperature increases, the temperature difference between refrigerant vapor and water decreases, and this would lead to the decrease of exergy loss. While as the inlet water temperature raises, the proportion of condensing heat taken by the outdoor unit increases. When inlet water temperature exceeds a certain value, heat transfer temperature difference and exergy loss ratio of the outdoor unit would be greater than that of the water heating condenser, thus the gross exergy loss of the two condensers increases as the proportion of condensing heat taken by the outdoor unit increases. Fig. 5 shows gross exergy losses of the MHPS under space cooling with DHW mode and the conventional air conditioning system under space cooling mode. For MHPS, it could be inferred that system exergy loss ratio varies with inlet water temperature of the water heating condenser. When the inlet water temperature is low, the exergy loss ratio of MHPS decreases with increasing water temperature. While when the inlet water temperature exceeds a certain value, the exergy loss ratio increases with increasing water temperature. Overall, for MHPS under space cooling with DHW mode, as the inlet water temperature increases from 25 °C to 55 °C, the gross exergy loss ratio is 37.5%, which is lower than that of the conventional air conditioning system under space cooling mode (40.9%). 3.2. Energy and exergy analysis of MHPS under space heating with DHW mode For MHPS under space cooling with DHW mode at steady state, ignoring resistance loss and heat leakage, as shown in Fig. 3b, the state of working fluid at point 4 is the same as point 1, and the state of working fluid at point 7 is the same as point 3. Evaporator temperature and condensing temperature of MHPS at steady state are shown in Table 2. As mentioned earlier, before exergy analysis, state x in Fig. 2 has to be determined, thus the ratio of condensing heat taken by the
Fig. 5. Gross exergy loss ratios of MHPS under space cooling with DHW mode and conventional air conditioning system under space cooling mode.
X. Sun et al. / Energy Conversion and Management 73 (2013) 51–56
water heating condenser to condensing heat taken by the outdoor unit of air conditioner has to be identified first. According to Table 2, when the inlet water temperature is between 25 °C and 35 °C, condensing temperature is always 7 °C higher than the inlet water temperature; while when the inlet water temperature is between 30 °C and 55 °C, condensing temperature remains at 42 °C. Similar as that of MHPS under space cooling with DHW mode, when the inlet water temperature is lower than 35 °C, heat transfer in the water heating condenser is considered to be fully latent heat transfer with working fluid at condensing temperature. When the inlet water temperature is higher than35 °C, heat transfer in the water heating condenser is considered to proceed at weighted mean temperature of sensible and latent heat transfer. According to the working conditions described in Table 2 (with the outdoor air temperature at 7 °C), exergy losses are calculated with inlet water temperature of water heating condenser at 25 °C, 28 °C, 31.5 °C, 35 °C, 45 °C and 55 °C separately. For MHPS under space cooling with DHW mode, as shown in Fig. 6a, at first, the exergy loss ratios of compressor and throttling valve increase with increasing inlet water temperature. When the inlet water temperature exceeds a certain value, the exergy loss ratios of compressor and throttling valve becomes relatively stable. And when the inlet water temperature is lower than that certain
(a) compressor and throttling valve
55
Fig. 7. Gross exergy loss ratios of MHPS under space heating with DHW mode and conventional air conditioning system under space heating mode.
value, compressor and throttling valve exergy loss ratios of MHPS is always lower than those of a conventional air conditioning system. For condenser exergy loss of MHPS, when the inlet water temperature is low, the whole system condensing heat is taken by water heating condenser, and exergy loss of air conditioning indoor unit is 0 since there is no heat transfer. Then as the inlet water temperature further increases, exergy loss ratio of the outdoor unit increases. As shown in Fig. 6b, for MHPS, when the inlet water temperature is low, evaporator and condenser exergy loss ratios decrease with increasing inlet water temperature, and when the inlet water temperature exceeds a certain value, both evaporator and condenser exergy loss ratios become relatively stable (which is almost the same with the evaporator exergy loss ratio of conventional air conditioning system). According to the analysis above, it could be inferred that the exergy loss ratios of heat exchanger are smaller with lower inlet water temperature. Fig. 7 shows gross exergy losses of the MHPS under space heating with DHW mode and the conventional air conditioning system under space heating mode. Overall, for MHPS under space heating with DHW mode, as the inlet water temperature increases from 25 °C to 55 °C, the gross exergy loss ratio is 46.9%, which is a little higher than that of the conventional air conditioning system under space heating mode (46.5%). 4. Conclusions In this paper, the exergy analysis of a MHPS and a conventional air conditioning system are performed, and exergy efficiency of the main components are calculated and compared. Calculation results show that:
(b) evaporator and condenser Fig. 6. Components exergy losses of MHPS under space heating with DHW mode and conventional air conditioning system under space heating mode.
(1) With the outdoor air temperature of 35 °C, for MHPS under space cooling with DHW mode, as expected, energy efficiency is 10.12% enhanced and exergy loss ratio is 9.06% reduced in comparison with that of a conventional air conditioning system under space cooling mode. (2) With the outdoor air temperature of 7 °C, compared to the conventional air conditioning system under space heating mode, the energy efficiency of MHPS under space heating with DHW mode is 3.3% enhanced. While the exergy loss ratio of MHPS is about 1% higher than that of the conventional air conditioning system.
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X. Sun et al. / Energy Conversion and Management 73 (2013) 51–56
(3) Under space heating with DHW mode, condensing heat of MHPS are supplied to meet the requirements of both space heating and water heating simultaneously. Though when the inlet water temperature is low, the exergy loss ratio of MHPS is high, and since most condensing heat is taken by the water heating condenser, heat supplied for air conditioning would be insufficient. Thus time-sharing supply of hot water and space heating is suggested. In practical application, space heating is usually taken as first priority. When water temperature is lower than a set point, water heating should be processed when space heating is not required. And as long as space heating is required, MHPS would operate without water heating if the inlet water temperature is lower than the set point.
Acknowledgment This work was supported by the Key project of the National High Technology Research and Development Program (‘‘863’’Program) of China. No. 2007AA05Z220. References [1] LU S, WU JY. Optimal selection among different domestic energy consumption patterns based on energy and exergy analysis. Energy Convers Manage 2010:1398–406.
[2] Ahamed JU, Saidur R, Masjuki HH. A review on exergy analysis of vapor compression refrigeration system. Renew Sustain Energy Rev 2011;15:1593–600. [3] Arora Akhilesh, Kaushik SC. Theoretical analysis of a vapour compression refrigeration system with R502, R404A and R507A. Int J Refrig 2008;31:998–1005. [4] Bayrakci Hilmi Cenk, Ozgur Arif Emre. Energy and exergy analysis of vapor compression refrigeration system using pure hydrocarbon refrigerants. Int J Energy Res 2009;1(33):1070–5. [5] Kabul Ahmet, Onder Kizilkan Y, Yakut Ali Kemal. Performance and exergetic analysis of vapor compression refrigeration system with an internal heat exchanger using a hydrocarbon, isobutane (R600a). Int J Energy Res 2008;32:824–36. [6] Yumrutas Recep, Kunduz Mehmet, Kanoglu Mehmet. Exergy analysis of vapor compression refrigeration systems. Exergy, Int J 2002;2:266–72. [7] Bilgen E, Takahashi H. Exergy analysis and experimental study of heat pump systems. Exergy, Int J 2002;2:259–65. [8] Akau RL, Schoenhals RJ. The second law efficiency of a heat pump system. Energy 1980;5:853–63. [9] Ahamed JU, Saidhur R, Masjuki HH. Performance characteristics and energy– exergy analysis of solar-assisted heat pump system. Build Environ 2008;43:1961–72. [10] Kaygusuz K, Ayhan T. Exergy analysis of solar assisted heat pump systems for domestic heating. Energy 1993;18:1077–85. [11] Torres-Reyes E, Picon-Nunez M, Cervantes De Gortari J. Exergy analysis and optimization of a solar assisted heat pump. Energy 1998;23:337–44. [12] Torres-Reyes E, Cervantes De Gortari J. Optimal performance of an irreversible solar assisted heat pump. Exergy, Int J 2001;1:107–11. [13] Hepbasli A, Akdemir O. Energy and exergy analysis of a ground source (geothermal) heat pump system. Energy Convers Manage 2004;45: 737–53. [14] Esena Hikmet, Inallib Mustafa, et al. Energy and exergy analysis of a groundcoupled heat pump system with two horizontal ground heat exchangers. Build Environ 2007;42:3606–15.
Contents lists available at SciVerse ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Exergy analysis and comparison of multi-functional heat pump and conventional heat pump systems Xiaolin Sun, Jingyi Wu ⇑, Ruzhu Wang Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 26 June 2012 Accepted 2 April 2013 Available online 13 May 2013 Keywords: Heat pump Exergy Cooling Heating Water heating
a b s t r a c t In this paper, the thermal dynamic analysis of a multi-functional heat pump system (MHPS) is made in comparison with a conventional heat pump air conditioning system. Energy and exergy efficiencies of the different systems are calculated to study influences of the components parameters, operation conditions and operation modes on the system performance. Variation of the MHPS performance with increasing hot water temperature is investigated, and performances of the major components are also studied in order to optimize the structure and operation of MHPS. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The air conditioner and the water heater are two major contributors to the residential building energy consumption. As more and more energy consumed by the air conditioning (AC) and domestic water heating (DHW), the residential building energy consumption increases rapidly. At the same time, in summer, the condensing heat of the air conditioner is usually delivered into the environment directly, which not only increases the energy waste, but also causes terrible thermal pollution. Meanwhile, irreversible energy loss caused by heat transfer between the thermal system and the surrounding environment at a finite temperature difference would also reduce the system operation performance. In a Multi-functional heat pump system (MHPS), a part of the condensing heat is recovered for DHW, thus to provide DHW and space cooling/heating simultaneously [1]. Nowadays, most studies on such systems are based on the first law (energy) analysis, which is the most common used method in thermodynamic analysis. While though there are various causes of energy destruction, the only reason in essential is irreversibility of process, which is described by the second law of thermodynamics. Compared to energy analysis, exergy analysis (second law) provides much more details of thermal process, including how, where and how much energy loss occurs. There are now many papers introducing principles and methods of exergy analysis [2–4]. By means of exergy analysis, the energy destruction at each separated component shows clearly,
⇑ Corresponding author. Tel.: +86 21 3420 6776; fax: +86 21 3420 6309. E-mail address: [email protected] (J. Wu). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.04.009
and maximum performance as well as improvement potential of the system can be identified. Several studies on the exergy analysis of heat pump and refrigeration systems have been done by researchers, and effects of different factors on the exergy efficiency of different systems have been investigated [3–14]. Akau and Schoenhals [8] studied a heat pump system that uses water as the heat source and heat sink experimentally. Kaygusuz and Ayhan [10], Torres-Reyes et al. [11,12] did the exergy analysis of a solar assisted heat pump based on experimental results. Hepbasli [13] and Hikmet Esena et al. [14] studied the exergy analysis of a ground source heat pump system. Akhilesh Arora, Hilmi Cenk Bayrakci and Ahmet Kabul et al. studied exergy analysis of a heat pump systems with different working fluids [3–5]. In this paper, the exergy analysis of MHPS under different operation modes is discussed to identify effects of components structure, working condition and operation mode on the system operation performance. The exergy loss of each major component (compressor, condenser, throttling valve and evaporator) is calculated and then compared to that of conventional air conditioning system to optimize system design and controlling method.
2. System schematic and exergy analysis principles Generally, the MHPS could realize the functions of AC, DHW, dehumidification and ventilation, while the MHPS discussed in this paper is used for AC (space cooling and heating) as well as DHW. Compared to the conventional air conditioning system (shown in Fig. 1b), an additional heat exchanger (HX) is added to the MHPS
52
X. Sun et al. / Energy Conversion and Management 73 (2013) 51–56
Fig. 1. Schematics of MHPS and conventional air conditioning system.
to produce DHW. The MHPS could operate under various modes, including DHW mode, space cooling mode, space heating mode, space cooling with DHW mode and space heating with DHW mode. System operation mode could be switched by control of four-way valve, fan and pump. Schematic of a typical MHPS is shown in Fig. 1a. Fig. 2 shows the ideal and actual log P-h diagram of a MHPS cycle, where a–b–c–d–a shows the ideal thermal process of working fluid, and a0 –b0 –c0 –d0 –e0 –f0 –g0 –a0 shows the actual process with consideration of unavoidable irreversible energy loss. Different from conventional air conditioning system, MHPS under air conditioning with DHW mode needs two condensers. As shown in Fig. 2, condensing process undergoes in both condensers (x–c), where b– x shows condensing process in water-heating condenser, while x–c shows condensing process in the outdoor(under space cooling with DHW mode) or indoor (under space heating with DHW mode) unit of air conditioner. Fig. 3 shows the cycle of working fluid under air conditioning with DHW mode. In this investigation, R22 is used as working fluid. As shown in Fig. 3, condensing heat of the system is taken by two condensers. Under space cooling and DHW mode, fraction of condensing heat taken by the water heating condenser is calculated by the following formula:
v¼
h2 h3 h2 h5
ð1Þ
While under space heating with DHW mode, fraction of condensing heat taken by the water heating condenser is calculated by following formula:
v¼
h2 h3 h2 h6
ð2Þ
According to the second law of thermodynamics, a practical process is always irreversible. In a heat pump system, irreversible losses are
Fig. 3. Working fluid cycle of MHPS under (a) space cooling with DHW mode; (b) space heating with DHW mode.
caused by different factors, such as friction resistance and temperature difference of heat exchange. Assuming that (1) the system operates under steady state; (2) pressure drops caused by friction resistance are negligible; (3) heat losses at compressor and throttling valve are negligible [2,6], according to the second law of thermodynamics, exergy analysis equations are show as following:
Eheat;in þ Emass;in þ Ework ¼ Eheat;out þ Emass;out þ Irr
ð3Þ
w ¼ ðh h0 Þ T 0 ðs s0 Þ
ð4Þ
where Irr is irreversible loss, Eheat , Emass and Ework are exergy during heat transfer, mass transfer and working process, w is specific exergy in any state, T0 is the surrounding temperature, h0 and s0 are enthalpy and entropy of working fluid under temperature of T 0 and pressure of 0.10133 MPa. There is no mass transfer between MHPS and the surrounding environment, hence Emass = 0. And theoretical exergy losses in different components are calculated according to the following formulas (where mechanical loss and heat leakage are ignored) [2–4]: For the compressor
Icom ¼ mr T 0 ðscom;o scom;i Þ
ð5Þ
where Icom is the theoretical exergy loss of compressor; mr is the mass flow of working fluid; scom;i and scom;o are specific entropy of working fluid at inlet and outlet of compressor. For the evaporator
Iev a ¼ mr ½ðhev a;i hev a;o Þ T 0 ðsev a;i sev a;o Þ þ Q e ð1 T 0 =T e Þ
Fig. 2. Ideal and actual log P-h diagram of cycle.
ð6Þ
where Iev a is the theoretical exergy loss of evaporator; Q e is heat exchange quantity of the evaporator; T e is evaporating temperature; hev a;i =sev a;i and hev a;o =sev a;o are specific enthalpy/entropy of working fluid at inlet and outlet of evaporator.
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For the condenser
Table 2 Calculation results of main operating parameters.
Icon ¼ mr ½ðhcon;i hcon;o Þ T 0 ðscon;i scon;o Þ Q c ð1 T 0 =T c Þ
ð7Þ
where Icon is the theoretical exergy loss of the condenser; Q c is heat exchange quantity of the condenser; T c is condensing temperature; hcon;i =scon;i and hcon;o =scon;o are specific enthalpy/entropy of working fluid at the inlet and outlet of the condenser. For the throttling valve
Iv al ¼ mr ðhv al;o hv al;i Þ
ð8Þ
where Iv al is the theoretical exergy loss of the throttling-valve; hv al;i and hv al;o are specific enthalpy/entropy of working fluid at the inlet and outlet of the throttling valve. In a vapor compressor refrigeration system, the exergy efficiency is influenced by several factors, including system structure, surrounding environment, refrigerant properties, lubricant and additives et al. In addition, both evaporating temperature and condensing temperature also have significant effect on the system exergy efficiency. Previous studies showed that exergy loss increases as evaporating temperature decreases, and vice versa. This can be explained that decreasing evaporating temperature would increases the temperature difference of heat exchange between working fluid in evaporator and the medium being cooled. For a vapor compressor refrigeration system, the exergy loss increases with increasing condensing temperature. This is also predictable since higher condensing temperature leads to larger temperature difference of heat exchange between working fluid in condenser and the ambient [2,5]. Operating conditions shown in Table 1 are determined according to standard conditions defined by related national standards of China. Calculated results of corresponding condensing temperature, condensing temperature, mean evaporating temperature and COP of an ideal Carnot cycle are shown in Table 2. Where T i is air temperature of the conditioned space; T wiu is the inlet water temperature of the indoor heat exchanger for air conditioning; T w is the inlet water temperature of the DHW heat exchanger (condenser). Where T c is condensing temperature; T e is evaporating temperature; T mc is mean condensing temperature; COPc is COP of the ideal Carnot cycle. Compared to the conventional heat pump air conditioner, MHPS can operate in air conditioning with DHW mode. Thus in this paper, MHPS under two modes are discussed: space cooling with DHW mode and space heating with DHW mode. 3. Results and discussion 3.1. Energy and exergy analysis of MHPS under space cooling with DHW mode For MHPS under space cooling with DHW mode at steady state, ignoring resistance loss and heat leakage, as shown in Fig. 3a, the state of working fluid at point 4 is the same as point 3, and the Table 1 Operating conditions of MHPS and conventional air conditioning system. System description
T0 (°C)
Ti (°C)
Twiu (°C)
Tw (°C)
Conventional air conditioning system (space cooling mode) MHPS (space cooling with DHW mode)
35
27
12
/
35
27
12
Conventional air conditioning system (space heating mode) MHPS (space heating with DHW mode)
7
20
35
25– 55 /
7
20
35
25– 55
System description
Tc (°C)
Te (°C)
Tmc (°C)
COPc
Conventional air conditioning system (space cooling mode) MHPS (space cooling with DHW mode) Conventional air conditioning system (space heating mode) MHPS (space heating with DHW mode)
47
5
47
6.62
Tw + 7 (25 < Tw < 40) 47 (40 < Tw < 55) 42
5
43.25
7.27
5
42
6.70
Tw + 7 (25 < Tw < 35) 42 (35 < Tw < 55)
5
40.3
6.92
state of working fluid at point 7 is the same as point 1. Assuming evaporator temperature and condensing temperature of MHPS at steady state are as shown in Table 2, all the calculations are based on an outdoor air temperature of 35 °C. Different from the conventional air conditioning system, condensing temperature of MHPS keeps going up because of increasing inlet water temperature of water heating condenser. Before exergy analysis, state x in Fig. 2 has to be determined, thus the ratio of condensing heat taken by the water heating condenser to condensing heat taken by the outdoor unit of air conditioner has to be identified first. When inlet water temperature of the water heating condenser is 28 °C, system condensing temperature would be 35 °C, which is the same with the outdoor air temperature. Thus it can be assumed that when inlet water temperature of the water heating condenser is lower than 28 °C, no heat exchanging occurs in the outdoor heat exchanger, thus system condensing heat is 100% taken by the water heating condenser. When inlet water temperature of the water heating condenser is 55 °C, the water heating condenser takes about 10% of the whole system condensing heat. Graphically, as the inlet water temperature goes up, the position of state x in Fig. 2 moves rightwards. Moreover, according to Table 2, when the inlet water temperature is between 25 °C and 40 °C, condensing temperature is always 7 °C higher than inlet water temperature; while when the inlet water temperature is between 40 °C and 55 °C, condensing temperature keeps at 47 °C. When the inlet water temperature is lower than 40 °C, the heat transfer in water heating condenser is considered to be 100% latent heat transfer with working fluid at condensing temperature. While when the inlet water temperature is higher than 40 °C, sensible heat transfer could no longer be ignored. Sensible heat transfer in the water heating condenser is considered to proceed with working fluid temperature at (t2 + t3)/2 (which is the mean temperature of working fluid at inlet and outlet), while latent heat transfer is still considered to proceed with working fluid at condensing temperature. Meanwhile, according to the second law of thermodynamics and Eq. (7), exergy loss during the heat exchanging process is determined not only by inlet/outlet state of the refrigerant, but also by the heat exchanging quantity as well as the temperatures of the heat-side and cold-side fluids. Thus the heat transfer in water heating condenser is considered to proceed at weighted mean temperature of sensible and latent heat transfer. According to working conditions described in Table 2 (with the outdoor air temperature at 35 °C), exergy losses are calculated with inlet water temperature of water heating condenser at 25 °C, 28 °C, 34 °C, 40 °C, 47.5 °C and 55 °C separately. For a MHPS under space cooling with DHW mode and a conventional air conditioning system under space cooling mode, Fig. 4a shows the exergy loss ratios of compressor and throttling valve with different inlet water temperature, which are calculated based on mass, energy and exergy conservation equations. It can be inferred that for MHPS, at first, the exergy loss ratios of compressor and throttling valve increase with increasing inlet water temperature. This is because when the inlet water temperature is low, con-
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(a ) compressor and throttling valve
(b) condenser and evaporator Fig. 4. Components exergy losses of MHPS under space cooling with DHW mode and conventional air conditioning system under space cooling mode.
densing temperature increases with increasing inlet water temperature, thus compression ratio of compressor as well as pressure reduction of throttling valve increases too. And for both compressor and throttling valve, increasing pressure difference between inlet and outlet would cause greater irreversible energy loss. While when the inlet water temperature exceeds a certain value, condensing temperature of MHPS grows slowly, hence the exergy loss ratios of compressor and throttling valve remain stable. Moreover, when the inlet water temperature is lower than that certain value, the exergy loss ratios of compressor and throttling valve of MHPS are always lower than those of the conventional air conditioning system. Fig. 4b shows the exergy loss ratios of evaporator and condenser. It has to be mentioned that the condenser exergy loss ratio of MHPS is weighted mean value of two condensers. For MHPS under space cooling with DHW mode, when the inlet water temperature is low, the total system condensing heat is taken by water heating condenser, and the exergy loss of outdoor unit is 0 since there is no heat transfer. And if the inlet water temperature keeps on increasing, the exergy loss ratio of outdoor unit would also increase. As shown in Fig. 4b, for MHPS, when the inlet water temperature is low, the evaporator exergy loss ratio decreases with the increase of inlet water temperature. And when the inlet water temperature exceeds a certain value, the evaporator exergy loss ratio becomes stable (which is almost the same with the evaporator exergy loss ratio of conventional air conditioning system).
As the inlet water temperature goes up, the condenser exergy loss ratio of MHPS first decreases and then increases, though this ratio of MHPS is always lower than that of the conventional air conditioning system. To explain this situation, two major factors that affect the condenser exergy loss ratio have to be mentioned: one is the temperature difference of heat exchange, and the other is the ratio of condensing heat taken by the water heating condenser to condensing heat taken by the outdoor unit. When the inlet water temperature is low, most condensing heat is taken by the water heating condenser. As the inlet water temperature increases, the temperature difference between refrigerant vapor and water decreases, and this would lead to the decrease of exergy loss. While as the inlet water temperature raises, the proportion of condensing heat taken by the outdoor unit increases. When inlet water temperature exceeds a certain value, heat transfer temperature difference and exergy loss ratio of the outdoor unit would be greater than that of the water heating condenser, thus the gross exergy loss of the two condensers increases as the proportion of condensing heat taken by the outdoor unit increases. Fig. 5 shows gross exergy losses of the MHPS under space cooling with DHW mode and the conventional air conditioning system under space cooling mode. For MHPS, it could be inferred that system exergy loss ratio varies with inlet water temperature of the water heating condenser. When the inlet water temperature is low, the exergy loss ratio of MHPS decreases with increasing water temperature. While when the inlet water temperature exceeds a certain value, the exergy loss ratio increases with increasing water temperature. Overall, for MHPS under space cooling with DHW mode, as the inlet water temperature increases from 25 °C to 55 °C, the gross exergy loss ratio is 37.5%, which is lower than that of the conventional air conditioning system under space cooling mode (40.9%). 3.2. Energy and exergy analysis of MHPS under space heating with DHW mode For MHPS under space cooling with DHW mode at steady state, ignoring resistance loss and heat leakage, as shown in Fig. 3b, the state of working fluid at point 4 is the same as point 1, and the state of working fluid at point 7 is the same as point 3. Evaporator temperature and condensing temperature of MHPS at steady state are shown in Table 2. As mentioned earlier, before exergy analysis, state x in Fig. 2 has to be determined, thus the ratio of condensing heat taken by the
Fig. 5. Gross exergy loss ratios of MHPS under space cooling with DHW mode and conventional air conditioning system under space cooling mode.
X. Sun et al. / Energy Conversion and Management 73 (2013) 51–56
water heating condenser to condensing heat taken by the outdoor unit of air conditioner has to be identified first. According to Table 2, when the inlet water temperature is between 25 °C and 35 °C, condensing temperature is always 7 °C higher than the inlet water temperature; while when the inlet water temperature is between 30 °C and 55 °C, condensing temperature remains at 42 °C. Similar as that of MHPS under space cooling with DHW mode, when the inlet water temperature is lower than 35 °C, heat transfer in the water heating condenser is considered to be fully latent heat transfer with working fluid at condensing temperature. When the inlet water temperature is higher than35 °C, heat transfer in the water heating condenser is considered to proceed at weighted mean temperature of sensible and latent heat transfer. According to the working conditions described in Table 2 (with the outdoor air temperature at 7 °C), exergy losses are calculated with inlet water temperature of water heating condenser at 25 °C, 28 °C, 31.5 °C, 35 °C, 45 °C and 55 °C separately. For MHPS under space cooling with DHW mode, as shown in Fig. 6a, at first, the exergy loss ratios of compressor and throttling valve increase with increasing inlet water temperature. When the inlet water temperature exceeds a certain value, the exergy loss ratios of compressor and throttling valve becomes relatively stable. And when the inlet water temperature is lower than that certain
(a) compressor and throttling valve
55
Fig. 7. Gross exergy loss ratios of MHPS under space heating with DHW mode and conventional air conditioning system under space heating mode.
value, compressor and throttling valve exergy loss ratios of MHPS is always lower than those of a conventional air conditioning system. For condenser exergy loss of MHPS, when the inlet water temperature is low, the whole system condensing heat is taken by water heating condenser, and exergy loss of air conditioning indoor unit is 0 since there is no heat transfer. Then as the inlet water temperature further increases, exergy loss ratio of the outdoor unit increases. As shown in Fig. 6b, for MHPS, when the inlet water temperature is low, evaporator and condenser exergy loss ratios decrease with increasing inlet water temperature, and when the inlet water temperature exceeds a certain value, both evaporator and condenser exergy loss ratios become relatively stable (which is almost the same with the evaporator exergy loss ratio of conventional air conditioning system). According to the analysis above, it could be inferred that the exergy loss ratios of heat exchanger are smaller with lower inlet water temperature. Fig. 7 shows gross exergy losses of the MHPS under space heating with DHW mode and the conventional air conditioning system under space heating mode. Overall, for MHPS under space heating with DHW mode, as the inlet water temperature increases from 25 °C to 55 °C, the gross exergy loss ratio is 46.9%, which is a little higher than that of the conventional air conditioning system under space heating mode (46.5%). 4. Conclusions In this paper, the exergy analysis of a MHPS and a conventional air conditioning system are performed, and exergy efficiency of the main components are calculated and compared. Calculation results show that:
(b) evaporator and condenser Fig. 6. Components exergy losses of MHPS under space heating with DHW mode and conventional air conditioning system under space heating mode.
(1) With the outdoor air temperature of 35 °C, for MHPS under space cooling with DHW mode, as expected, energy efficiency is 10.12% enhanced and exergy loss ratio is 9.06% reduced in comparison with that of a conventional air conditioning system under space cooling mode. (2) With the outdoor air temperature of 7 °C, compared to the conventional air conditioning system under space heating mode, the energy efficiency of MHPS under space heating with DHW mode is 3.3% enhanced. While the exergy loss ratio of MHPS is about 1% higher than that of the conventional air conditioning system.
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(3) Under space heating with DHW mode, condensing heat of MHPS are supplied to meet the requirements of both space heating and water heating simultaneously. Though when the inlet water temperature is low, the exergy loss ratio of MHPS is high, and since most condensing heat is taken by the water heating condenser, heat supplied for air conditioning would be insufficient. Thus time-sharing supply of hot water and space heating is suggested. In practical application, space heating is usually taken as first priority. When water temperature is lower than a set point, water heating should be processed when space heating is not required. And as long as space heating is required, MHPS would operate without water heating if the inlet water temperature is lower than the set point.
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