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Performance analysis of a novel air source hybrid solar assisted heat pump
Renewable Energy 139 (2019) 1133e1145
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance analysis of a novel air source hybrid solar assisted heat pump Jingyong Cai a, Zhouhang Li b, **, Jie Ji a, *, Fan Zhou a a b
Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei, Anhui, 230026, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan, 650093, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2018 Received in revised form 26 January 2019 Accepted 26 February 2019 Available online 8 March 2019
Aiming to enhance the performance of heat pump under adverse working conditions by utilizing solar energy and air source simultaneously, a novel air source hybrid solar assisted heat pump (AS-SAHP) system with finned tube evaporator and collector evaporator connected in series has been proposed. A mathematical model is established to characterize the behavior of the proposed system. Then the influence of key factors such as ambient temperature, solar irradiation, and evaporating area has been analyzed. The results indicate that the AS-SAHP system can operate efficiently under a range of working conditions, with the average COP of 2.71 at the solar irradiation of 100 W/m2 and ambient temperature of 10 C. Due to the heat transfer characteristics of finned tube evaporator and collector evaporator existing striking difference, the hybrid heat source in AS-SAHP system can play the complementary role at various operating conditions. To assess the irreversibility of the AS-SAHP system, the exergy loss ratio of each component has been analyzed. The results illustrate that most of the irreversibility is yielded in compressor and condenser. At last, the performance of AS-SAHP has been compared with that of conventional ASHP and DX-SAHP, and it is proven that the AS-SAHP system shows the optimal performance. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Hybrid source Heat pump Solar energy Direct expansion Exergy
1. Introduction With the capability to utilize renewable energy, heat pump is recognized as an effective energy saving device [1]. Solar energy can serve as an ideal heat source for heat pump with the advantages of low expense, easy accessibility and pollutant free [2]. Solar assisted heat pump (SAHP) is the integration of vapor compression heat pump and solar energy [3]. It has been proven that the SAHP system could effectively cut down the electricity consumption and improve the renewable energy utilization for domestic heating supply [4,5]. The SAHP system can be categorized as indirect-expansion solar-assisted heat pump (IX-SAHP) and direct-expansion solarassisted heat pump (DX-SAHP) [6]. Different from IX-SAHP, the evaporator in DX-SAHP absorbs energy from the heat source directly without intermediate heat exchanger, which can enhance the heat transfer efficiency significantly [7]. Meanwhile, with the assistance of solar irradiation, higher evaporating temperature can
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Li), [email protected] (J. Ji). https://doi.org/10.1016/j.renene.2019.02.134 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
be obtained in DX-SAHP compared with that of air source heat pump (ASHP), leading to the performance improvement [8]. In addition, the thermal efficiency of solar collector can be improved as a results of the low working temperature existed in the evaporating process of refrigerant [9]. Sun et al. [10] undertook comparative analysis of DX-SAHP and ASHP under a range of working conditions. The annual performance of DX-SAHP was remarkably higher than ASHP system. Chow et al. [11] reported an annual COP of 6.46 for DX-SAHP in Hong Kong for domestic water production, which presented excellent prospects for subtropical applications. Huang et al. [12] studied the behavior of DX-SAHP under frosting condition. The test results showed that solar irradiation was able to improve the heating capacity and avoid or decelerate frosting in DX-SAHP. However, an obvious disadvantage of DX-SAHP is the performance fluctuation caused by the variation of external environment ndez-Seara et al. [15] reported that the COP condition [13,14]. Ferna of DX-SAHP and the temperature difference between surrounding environment and condensing water were linearly related when the solar irradiation was zero. Mohamed et al. [16] designed a DX-SAHP applicable in cold areas. It revealed that the area of collector and solar irradiation significantly influenced the performance of the
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Greek
Nomenclature A c D Ex g h I k L m_ M n N p P Q s S t T u U v V W x Re Pr
2
area, m specific heat capacity, J/(kg$K) diameter, m exergy, W gravity acceleration, m/s2 specific enthalpy, J/kg solar irradiation, W/m2 thermal conductivity, W/(m.K) length, m mass flow rate, kg/s mass, kg rotating speed, rad/s number of tube pressure, Pa power consumption, W heat exchange capacity, W specific entropy, J/(kg$K) pitch, m time temperature, C velocity, m/s overall heat transfer coefficient, W/(m2.K) specific volume, m3/kg volume, m3 tube pitch in collector evaporator vapor quality Reynolds number Prandtl number
system. Moreover, DX-SAHP was inefficient during the night-time operation due to the radiative heat loss of collector evaporator [10]. To improve the stability and efficiency of the DX-SAHP, a lot of research work has been implemented, including (1) optimizing the structure of the collector evaporator [17e19], (2) properly matching the geometry structure with the capacity of system [20], (3) selecting the proper refrigerant [21,22], (4) introducing control strategy for inverter compressor and electronic expansion valve [23e25], (5) adopting cascade structure for the high temperature requirement [26], (6) enhancing the utilization of air source, etc. In particular, based on the idea of comprehensively making use of solar energy and air source, hybrid source heat pump has been proposed as a new concept. Wang et al. [27] proposed a PV/T air dual-heat-source composite heat pump with the average COPs of 1.40 in single source mode and 2.49 in dual source mode respectively. It was proven that the dual heat source significantly improved the performance of system by utilizing a composite evaporator. Ni et al. [28,29] developed a PCM based SAHP system consisting of PCM unit, solar collector and ASHP. The compatibility of the system under severe external conditions was validated in field test. Dong et al. [30,31] presented a solar integrated air source heat pump (SIASHP) with the average COP of 2.94. The evaporator of the SIASHP was modified from the evaporator of ASHP by painting selective absorption coating to improve the solar utilization ratio. Xu et al. [32] proposed a solar-air source heat pump water heater (SAS-HPWH) with a specially designed evaporator consisting of spiral-finned tube. The SAS-HPWH system was equipped with the optimal energy saving performance among various types of water heaters. On the basis of conventional DX-
a b r h g k m s ε
heat transfer coefficient, W/(m2.K) absorptivity density, kg/m3 efficiency polytropic index coefficient of thermal expansion, K1 dynamic viscosity, kg/(m.s) Stefane-Boltzman constant, W/(m2$K4) emissivity
Subscripts a cap con com coll coil dis eva fin i in l o out p ref suc tank v w
ambient/air capillary condenser compressor collector evaporator helically coiled heat exchanger discharge evaporator finned tube evaporator inner inlet liquid outer outlet refrigerant pipe refrigerant suction water tank vapor water
SAHP, Deng et al. [33] proposed a modified DX-SAHP by connecting the collector evaporator with finned tube evaporator in parallel. Compared with DX-SAHP, in the modified system heating time was reduced by 19.8%. Above studies reveal that applying hybrid heat source in vapor compression heat pump system is an effective way to realize stable and economical operation. Jointing a finned tube evaporator at the front of the collector evaporator in DX-SAHP is a simple scheme to realize the comprehensive utilization of solar irradiation and air source, while such kind of hybrid system has merely been discussed by scholars and few commercial products exists in the market. In this paper, a novel air source hybrid solar assisted heat pump (AS-SAHP) system with finned tube evaporator and collector evaporator connected in series which can act as complementary roles at various operating conditions has been proposed and investigated. In AS-SAHP, the components of the proposed system can be found in available products which is beneficial for the commercialization. As the heat transfer characteristics of the above two kinds of evaporators are inconsistent under different working conditions, the interaction between the evaporators has been discussed with a validated mathematical model. The impact of key factors such as solar irradiation, ambient temperature and evaporating area is analyzed. The exergy loss ratio has been evaluated to identify the irreversibility of the AS-SAHP system. In addition, the performance of AS-SAHP, DX-SAHP and ASHP is compared under a range of working conditions. 2. System description The schematic diagram of the AS-SAHP system is shown in Fig. 1.
J. Cai et al. / Renewable Energy 139 (2019) 1133e1145
It is mainly composed of a finned tube evaporator, a collector evaporator, a reciprocating compressor, a capillary and a water cooled condenser. Compared with the traditional DX-SAHP, a finned tube evaporator is connected in series with the collector evaporator in the AS-SAHP system. Finned tube evaporator is capable to recover heat from the ambient effectively due to the heat transfer enhancement of fins. The collector evaporator consists of the roll-bond panel which is commonly used as evaporator in DXSAHP. The roll-bond panel is a kind of bare-plate solar collector without front glass cover and back insulation, as shown in Fig. 2. The water tank with helically coiled heat exchanger is utilized as the condenser. The heat transfer features of finned tube evaporator and collector evaporator are inconsistent under different external conditions. In the AS-SAHP system, finned tube evaporator and collector
aref ;p ¼
vr vu ¼0 þr vz vt
r
vu vðuÞ vr þ ru ¼ vt vz vz
Lp vðrhÞ vðruhÞ þ ¼ Tp Tref a vt vz Ap;i ref ;p
1135
(1)
(2)
(3)
where, aref,p is the coefficient of convective heat transfer between the refrigerant and pipe. In finned tube evaporator and collector evaporator, aref,p can be concluded as [34,35]:
8 ! > > > kl > 0:8 0:3 > > ; one phase region 0:023Re Pr > > Dp;i < (" 0:37 #2:2 " 0:67 !#2 )0:5 > > > r a rl v 0:7 > l 0:4 0:01 > al ð1 xÞ þ 1:2x þ x ; two phase region 1 þ 8ð1 xÞ > > rv al rv > :
evaporator work under the identical evaporating pressure, and the state parameter of refrigerant at the outlet of finned tube evaporator has influence on the evaporating capacity of collector evaporator. The refrigerant evaporates in finned tube evaporator and collector evaporator sequentially. Then the vaporized refrigerant enters the compressor and becomes the superheated vapor. The compressed working fluid is cooled in the water cooled condenser by transferring energy to the condensing water via the helically coiled heat exchanger. The working principle of the AS-SAHP system is illustrated in Fig. 3.
(4)
In condenser, aref,p can be calculated by Ref. [34]:
aref ;p
! 8 kl > 0:8 0:3 > ; one phase region 0:023Re Pr > > Dp;i < ¼ " # > 0:76 > ð1 xÞ0:04 > > a ð1 xÞ0:8 þ 3:8x : ; two phase region l Pr0:38 (5)
3. Mathematical modeling 3.1. Model of the AS-SAHP system 3.1.1. The model of flow and heat transfer The flow and heat transfer process of the working fluid in the thermodynamic cycle of the AS-SAHP system abides by the below the assumptions: (1) The flow direction is one dimensional. (2) The pressure gradient is not considered. (3) In the two phase region, the working fluid is homogeneously mixed. It can be governed by the following equations:
3.1.2. The model of collector evaporator The collector evaporator is divided into the elementary volume, as shown in Fig. 4. For each elementary volume, the distribution of temperature at the cross section is supposed to be uniform and the heat balance expression is:
rcoll ccoll
vTcoll v2 Tcoll 1 h ¼ kcoll þ pDcoll;i aref ;p Tref Tcoll 2 Ap vt vz i þ WUcoll ðTa Tcoll Þ þ bIW
(6)
where, for the collector evaporator, W is the tube pitch, Ucoll is the overall heat transfer coefficient, given as [36]:
Ucoll ¼ aa;coll þ 4εsT 3a
(7)
where, aa,coll is the wind heat transfer coefficient, given as [36]:
aa;coll ¼ 5:7 þ 3:8ua
Fig. 1. Schematic diagram of the AS-SAHP system.
3.1.3. The model of finned tube evaporator The heat balance expression is:
(8)
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Fig. 2. Structure of the collector evaporator.
m_ ref ;com ¼ hv
nVcom vsuc
(11)
where, hv is the volumetric efficiency. The compression process is supposed to be polytropic, so the power consumption is [38]:
Pcom ¼ n
psuc Vcom
Mcom ccom
vT v2 T 1 h rfin cfin fin ¼ kfin 2fin þ pDfin;i aref ;p Tref Tfin Ap vt vz i þ pDfin;o aa;fin Ta Tfin
rcoil ccoil (9)
where, aa,fin is the coefficient of connective heat transfer between the finned tube evaporator and surrounding environment, given by Ref. [37]:
aa;fin ¼ 0:982Re0:424
S1 Dfin;o
!0:0887
N,S2 Dfin;o
pdis psuc
#
g1 g
1
(12)
vTcom ¼ Pcom m_ ref ;com hcom;out hcom;in vt Ucom Acom ðTcom Ta Þ
(13)
3.1.5. The model of condenser The energy balance equation of helically coiled heat exchanger is:
Fig. 4. The elementary volume in collector evaporator.
!
g1
"
where, g is the polytropic parameter, hcom is the operation efficiency of compressor. Taking the heat loss of the compressor into account, the heat balance expression is:
Fig. 3. Working principle of AS-SAHP.
ka Dfin;o
hcom
g
!0:159
vTcoil v2 Tcoil 1 h ¼ kcoil þ pDcoil;i aref ;p Tref Tcoil Ap vt vz2 i þ pDcoil;o aw;coil ðTw Tcoil Þ
(14)
where, aw,coil is the coefficient of convection heat transfer at the water side, given as [39]:
aw;coil
kw ¼ 0:5 Dcoil;o
g kw DTD3coil;o r2w cw
!
mw kw
(15)
The energy conservation equation of condensing water is:
(10) Mw cw
3.1.4. The model of compressor The mass flow rate of refrigerant is correlated to the rotating speed (n) and the displacement (Vcom), as well as the suction specific volume (vsuc) of the refrigerant, which can be concluded as:
vTw ¼ Atank Utank ðTa Tw Þ þ Acoil;o aw;coil ðTcoil Tw Þ vt
3.1.6. The model of capillary The mass flow rate of refrigerant is [40]:
(16)
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Fig. 5. Computational flowchart of the mathematical model.
2 3 4 LCcap T Ccon 10C5 DTcap;in m_ ref ;cap ¼ C1 DCcap;i
(17)
where, C1~C5 are the empirical coefficient. Based on the above model, a simulation programme has been developed with MATLAB, and the flow chart of the solution process is shown in Fig. 5. The geometry parameters of the proposed system are selected according to the ASHP and DX-SAHP with identical compressor capacity [41,42], as shown in Table 1.
Qeva;fin ¼ m_ ref ;fin hfin;out hfin;in
(18)
Qeva;coll ¼ m_ ref ;coll hcoll;out hcoll;in
(19)
To evaluate the contribution of finned tube evaporator and collector evaporator, g is defined as the ratio of evaporating capacity of finned tube evaporator to the total evaporating capacity:
g¼ 3.2. Performance evaluation The evaporating capacities in finned tube evaporator and collector evaporator are given as respectively:
Qeva;fin Qeva;fin þ Qeva;coll
(20)
The condensing capacity can be calculated by:
Qcon ¼ m_ ref ;con hcon;in hcon;out
(21)
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J. Cai et al. / Renewable Energy 139 (2019) 1133e1145 Table 1 Detail design parameter of the AS-SAHP system. Component
Parameter
Value
Finned tube evaporator
Fin pitch (S1) Tube pitch (S2) Number of tube rows (N) Outer diameter of finned tube (Dfin,o) Inner diameter of finned tube (Dfin,i) Absorptivity of collector evaporator (b) Tube pitch (W) Outer diameter of refrigerant pipe (Dcoll,o) Inner diameter of refrigerant pipe (Dcoll,i) Collector area Outer diameter of condensing coil (Dcoil,o) Inner diameter of condensing coil (Dcoil,i) Volume of water tank (Vtank) Displacement (Vcom) Rated power consumption (Pcom) Inner diameter (Dcap,i) Length (Lcap,i)
2 103m 20 103m 10 9.9 103m 8.4 103m 0.9 0.13 m 9.9 103m 8.4 103m 4.2m2 9.9 103m 8.4 103m 150 L 1.34 105m3 0.75 kW 1.2 103m 1.5 m
Collector evaporator
Condenser
Compressor Capillary
For the AS-SAHP system, the performance of coefficient (COP) can be concluded as:
COP ¼
Qcon Pcom
(22) RE ¼
The exergy loss in the AS-SAHP system can be calculated by Ref. [41]:
Exloss;eva;fin
model of this study [42]. The relative error (RE) is used to evaluate the deviation between the experimental and simulation values, calculated by:
h ¼ m_ ref ;fin heva;fin;in heva;fin;out Ta seva;fin;in i Ta seva;fin;out þ Qeva;fin 1 Teva (23)
Exloss;eva;coll ¼ m_ ref ;coll heva;coll;in heva;coll;out Ta seva;coll;in Ta seva;coll;out þ Qeva;coll 1 Teva
Xsim Xexp 100% Xexp
(29)
The validation computation runs under the identical initial and boundary conditions with the experiment. Modeling data has been compared with the testing results in Table 2. The results reveal that the maximum deviation between the modeling and testing results is within 5%. The model of ASHP is validated with the experimental data in our previous study, and the maximum deviation between the measured and predicted value is within 5%, as shown in Fig. 6 [41]. On the basis of the above comparison, it can be concluded that the mathematical model proposed in this study can forecast the performance of the AS-SAHP system with sufficient accuracy.
(24) 4. Results and discussion
Exloss;con ¼ m_ ref ;con hcon;in hcon;out Ta scon;in scon;out Ta Qcon 1 Tcon
4.1. Parametric analysis
(25) Exloss;com ¼ m_ ref ;com hcom;in hcom;out Ta scom;in scom;out þ Pcom (26) Exloss;cap ¼ m_ ref ;cap scap;out scap;in Ta
(27)
Ex 4loss ¼ P loss Exloss
(28)
3.3. Model validation For the AS-SAHP system, the validation of the proposed model is presented by verifying the model of DX-SAHP and the model of ASHP respectively. The testing results reported by Li et al. regarding the DX-SAHP system has been applied to verify the numerical
4.1.1. Impact of ambient temperature To examine the impact of ambient temperature, the operation characteristics of the AS-SAHP system under the ambient temperatures of 5 C, 10 C and 15 C with the solar irradiation of 200 W/ m2 have been discussed. The temperature of condensing water is heated from 30 C to 50 C. Fig. 7 illustrates the variation of evaporating capacity at specified ambient temperature. In finned tube evaporator, the evaporating temperature increases with the condensing temperature, causing the reduction of evaporating capacity. The evaporating capacities decrease by 141.89%, 108.67% and 72.28% respectively at the ambient temperatures of 5 C, 10 C and 15 C, when the temperature of condensing water rises from 30 C to 50 C. It shows that in finned tube evaporator the downward trend of evaporating capacity during the water heating process is relieved under higher ambient temperature. In finned tube evaporator, the evaporating capacities on average are 197.34 W, 348.55 W and 546.99 W respectively at the ambient temperatures of 5 C, 10 C and 15 C. With the temperature of condensing water rising, the evaporating capacity in collector evaporator shows a downward trend after rising, and this trend is more distinct under higher ambient
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Table 2 Modeling results and measured data DX-SAHP. Date
2005.04.15 2005.04.20 2005.05.08 2005.05.12 2005.05.15 2005.05.19 2005.05.31 Maximum RE
Ta/ C
25.1 25.7 32.0 35.6 25.6 27.0 30.0
I/W.m-2
663 812 660 689 557 432 821
P _ Qcon tj /kWh
t/min
j
P _ Pcom tj /kWh
hcoll
COP
j
exp
sim
exp
sim
exp
sim
exp
sim
exp
sim
98 90 84 76 92 98 70 3.26%
100 89 85 78 95 100 72
5.58 5.26 4.80 4.71 4.81 4.82 4.54 1.61%
5.67 5.31 4.74 4.76 4.83 4.79 4.52
1.06 1.00 0.97 0.85 1.04 1.11 0.79 4.72%
1.11 1.02 0.95 0.88 1.09 1.10 0.78
5.26 5.26 4.95 5.54 4.63 4.34 5.75 4.32%
5.06 5.26 4.99 5.41 4.43 4.35 5.79
1.05 0.88 1.04 1.09 1.11 1.33 0.99 3.85%
1.04 0.88 1.08 1.09 1.07 1.34 1.01
power consumptions of the AS-SAHP system increase by 43.93%, 33.45% and 19.86% respectively at the ambient temperatures of 5 C, 10 C and 15 C with the temperature of condensing water rising from 30 C to 50 C. Meanwhile, the COP of the AS-SAHP system decreases with the rising condensing temperature caused by the rise of pressure ratio. With the temperature of condensing water rising from 30 C to 50 C, the COP declines from 3.84 to 1.94 at the ambient temperature of 5 C. Under higher ambient temperature, higher evaporating pressure and lower pressure ratio can be obtained, leading to higher power consumption and higher COP. The power consumptions on average are 688.68 W, 711.311 W and 725.51 W respectively at the ambient temperatures of 5 C, 10 C and 15 C. And the average COP rises from 2.78 to 3.31 with the ambient temperature rising from 5 C to 15 C.
Fig. 6. Comparison between the measured and predicted results of AS-HPWH.
temperature. As the rising condensing temperature causes the decrease of evaporating capacity in finned tube evaporator, the specific enthalpy and vapor quality of refrigerant entering the collector evaporator decrease resulting in the rise of heat gain in collector evaporator. Then, in collector evaporator the evaporating capacity begins to decease, due to the decline of heat gain from the environment attributed to the rising condensing temperature. In collector evaporator, the evaporating capacities on average are 1015.43 W, 1103.74 W and 1141.70 W respectively at the ambient temperatures of 5 C, 10 C and 15 C. Overall, the AS-SAHP system can recover more energy from heat source at higher ambient temperature. The total evaporating capacities on average are 1212.76 W, 1452.29 W and 1688.69 W respectively at the ambient temperatures of 5 C, 10 C and 15 C. To sum up, the contribution of finned tube evaporator to the overall evaporating capacity decreases with the rise of condensing temperature and the decline of ambient temperature, as shown in Fig. 7 (c). The condensing capacity drops with the rising temperature of condensing water, as shown in Fig. 8. The condensing capacities decrease by 43.93%, 33.45% and 19.86% respectively at the ambient temperatures of 5 C, 10 C and 15 C with the temperature of condensing water rising from 30 C to 50 C. The increase of ambient temperature results in the rise of condensing capacity, with the average values of 1878.42 W, 2128.38 W and 2362.33 W at the ambient temperatures 5 C, 10 C and 15 C respectively. Consequently, the heating time can be reduced from to 115mine92min when the ambient temperature increases from 5 C to 15 C. The variation of power consumption and COP is illustrated in Fig. 9. The power consumption increases during the water heating process, which is attributed to the rise of evaporating pressure. The
4.1.2. Impact of solar irradiation To examine the impact of solar irradiation, the operation characteristics of the AS-SAHP system with the solar irradiation ranging from 100 W/m2 to 300 W/m2 under the ambient temperature of 10 C have been discussed. The temperature of condensing water is heated from 30 C to 50 C. The variation of evaporating capacity at specified solar irradiation is illustrated in Fig. 10. The increase of solar irradiation leads to the rise of evaporating capacity of collector evaporator, and the average values are 922.15 W, 1169.47 W and 1413.41 W respectively under the solar irradiations of 100 W/m2, 200 W/m2 and 300 W/m2. As the collector evaporator and finned tube evaporator work under the identical evaporating pressure, higher solar irradiation causes the increase of evaporating temperature, which has an adverse effect on the heat transfer capability of finned tube evaporator. The average evaporating capacity in finned tube evaporator declines from 234.12 W to 150.08 W with solar irradiation increasing from 100 W/m2 to 300 W/m2. Overall, for the AS-SAHP system the rise of solar irradiation improves the heat gain at the evaporating side. The total evaporating capacities on average are 1156.27, 1347.99 W and 1563.50 W respectively under the solar irradiations of 100 W/m2, 200 W/m2 and 300 W/m2. It can be concluded that the collector evaporator contributes more to the overall evaporating capacity under higher condensing temperature and solar irradiation, as shown in Fig. 10 (c). The variation of condensing capacity and the temperature of condensing water at specified solar irradiation is illustrated in Fig. 11. As the evaporating side of AS-SAHP system can recover more energy from heat source at higher solar irradiation, the heat gain at condensing side also rises with solar irradiation. At the solar irradiations of 100 W/m2, 200 W/m2 and 300 W/m2, the condensing capacities on average are 1825.43 W, 2014.06 W and 2215.45 W respectively. Consequently, the heating time can be reduced from 114min to 94min, with solar irradiation increasing from 100 W/m2 to 300 W/m2.
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Fig. 8. The condensing capacity and water temperature variations under different ambient temperatures.
The variation of power consumption and COP under different solar irradiations is illustrated in Fig. 12. As the evaporating pressure rises with solar irradiation, both the power consumption and COP increase. When solar irradiation increases from 100 W/m2 to 300 W/m2, the average power consumption rises from 685.91 W to 700.63 W, rising by 2.10%, and the average COP increases from 2.71 to 3.22, rising by 15.58%. It can be concluded that inputting solar irradiation at evaporating side of the AS-SAHP system can achieve significant improvement of COP with relatively low increment of power consumption.
Fig. 7. The evaporating capacity variations under different ambient temperatures.
4.1.3. Impact of evaporating area As the heat transfer features of collector evaporator and finned tube evaporator exist significant difference under different external conditions, the influence of evaporating area has been discussed. c is defined as the proportion of the area of finned tube evaporator to the total evaporating area. The performance of the AS-SAHP system with different c has been discussed with solar irradiation ranging from 50 W/m2 to 200 W/m2, at the ambient temperature of 10 C. The condensing water is heated from 30 C to 50 C。 The condensing capacity and COP at specified c is shown in Fig. 13. The condensing capacity and COP rise with c under the solar irradiations of 50 W/m2 and 100 W/m2, while it displays the opposite variation trend at the solar irradiations of 150 W/m2 and 200 W/m2. With c rising from 0.2 to 0.8, the condensing capacity rises from 2328.27 W to 2410.59 W and the COP of the AS-SAHP system increases from 3.21 to 3.29 at the solar irradiation of 50 W/m2. It demonstrates that the performance of the AS-SAHP system can be enhanced by extending the area of finned tube evaporator at the solar irradiations of 50 W/m2 and 100 W/m2, because the heat transfer capability of finned tube evaporator is better than that of collector evaporator. With c increasing from 0.2 to 0.8, the condensing capacity decreases from 2572.79 W to 2522.99 W and the COP of system declines from 3.52 to 3.44 at the solar irradiation of 200 W/m2. It is because that with the rise of solar irradiation the evaporating capacity in collector evaporator gradually exceeds that of the finned tube evaporator. Therefore, the performance of the AS-SAHP system can be enhanced by rising the area of collector evaporator under the solar irradiations of 150 W/ m2 and 200 W/m2. Due to different heat transfer features existing in finned tube evaporator and collector evaporator, it can be inferred that hybrid source in the AS-SAHP system can play the complementary role under various operation conditions.
J. Cai et al. / Renewable Energy 139 (2019) 1133e1145
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Fig. 9. The power consumption and COP variations under different ambient temperatures.
4.2. Exergy analysis To identify the irreversibility of the AS-SAHP system, for each component the exergy loss under different environmental conditions has been discussed, as shown in Fig. 14 and 15. It can be observed that the exergy losses in compressor and condenser account for the largest proportion under various external conditions. In compressor and capillary, with the increase of ambient temperature the exergy loss ratio declines. While in condenser, finned tube evaporator and collector evaporator the variation of exergy loss ratio demonstrates the opposite trend. The exergy loss ratio in finned tube evaporator increases from 0.48% to 18.31%, and the exergy loss ratio in collector evaporator rises from 0.94% to 12.65%, with the ambient temperature increasing from 5 C to 30 C. With the increase of solar irradiation, in compressor and capillary the exergy loss ratio decreases, while the value in condenser and evaporators increases, as shown in Fig. 15. With solar irradiation increasing from 100 W/m2 to 500 W/m2, in collector evaporator the exergy loss ratio increases from 0.27% to 28.31%, and the exergy loss ratio in finned tube evaporator remains below 3%.
4.3. Comparative analysis The performance of AS-SAHP has been compared with that of DX-SAHP and ASHP, as shown in Fig. 16. To ensure the comparability of the results, the equivalent evaporating area in each system is identical. As a critical parameter for heat pump system, the evaporating temperatures of the AS-SAHP, DX-SAHP and ASHP have been compared at the solar irradiation of 200 W/m2 and ambient temperature of 10 C, with the temperature of condensing water rising from 30 C to 50 C, as shown in Fig. 17. The evaporating temperature in each system refers to the saturation temperature under the evaporating pressure, and it keeps increasing during the water heating process. For DX-SAHP, the evaporating temperature rises from 7.91 C to 14.41 C with the temperature of condensing water rising from 30 C to 50 C, and it becomes above the ambient temperature after 67min, which results in the heat loss to the environment. The evaporating temperature of the AS-SAHP system maintains between the evaporating temperatures of DX-SAHP and
ASHP. In the AS-SAHP system, the evaporating temperature has been improved compared with that of ASHP, which is benefit for the performance improvement. Meanwhile, it maintains below the ambient temperature to avoid the heat loss to the environment, which can raise the thermal efficiency of collector evaporator compared with that of DX-SAHP. With the temperature of condensing water rising from 30 C to 50 C, the evaporating temperature in the AS-SAHP system increases from 3.74 C to 9.16 C, and the evaporating temperature of the ASHP system rises from 4.80 C to 6.28 C. The power consumption and COP in AS-SAHP, DX-SAHP and ASHP under different ambient temperatures are illustrated in Fig. 18. In each system, the power consumption and COP increase with the rise of ambient temperature. The DX-SAHP obtains the highest power consumption, followed by the AS-SAHP system, and the ASHP system consumes the least amount of power. The power consumptions on average are 711.31 W, 735.39 W and 694.23 for AS-SAHP, DX-SAHP and ASHP at the ambient temperature of 10 C respectively. When the ambient temperature is 20 C, the average power consumptions are 734.24 W, 750.83 W and 722.87 W for ASSAHP, DX-SAHP and ASHP respectively. And the average power consumptions of AS-SAHP, DX-SAHP and ASHP are 749.35 W, 765.68 W and 728.97 W respectively at the ambient temperature of 30 C. The COP of the AS-SAHP system is the highest, followed by the DX-SAHP system, and the COP of the ASHP system is the lowest. The COPs on average are 3.15, 3.03 and 2.70 for the AS-SAHP, DXSAHP and ASHP at the ambient temperature of 10 C respectively. When the ambient temperature is 20 C, the average COPs are 3.50, 3.34 and 3.14 for AS-SAHP, DX-SAHP and ASHP respectively. And the average COPs of AS-SAHP, DX-SAHP and ASHP are 3.69, 3.50 and 3.47 respectively at the ambient temperature of 30 C.
5. Conclusion To improve the efficiency and stability of heat pump system, a novel air source hybrid solar-assisted heat pump (AS-SAHP) has been proposed and investigated in this paper. The numerical model with sufficient accuracy is developed to predict the behavior of the system. Conclusions have been summarized as follow:
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Fig. 11. The condensing capacity and water temperature variations under different solar irradiations.
Fig. 12. The power consumption and COP variations under different solar irradiations.
Fig. 10. The evaporating capacity variations under different solar irradiations. Fig. 13. The condensing capacity and COP variations under different c
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Fig. 14. Exergy loss ratio variations under different ambient temperatures. Fig. 17. Comparison of the evaporating temperature in different types of heat pump.
Fig. 15. Exergy loss ratio variations under different solar irradiations.
(1) The AS-SAHP system can operate effectively under various working conditions. The average COP increases from 2.78 to 3.31 with the ambient temperature rising from 5 C to 15 C, and rises from 2.71 to 3.22 with the solar irradiation increasing from 100 W/m2 to 300 W/m2. In addition, remarkable improvement of COP in the AS-SAHP system can be achieved with relatively low increment of power consumption by inputting solar irradiation at evaporating side of the system. (2) The heat transfer feature of collector evaporator and finned tube evaporator is inconsistent under different external conditions. At the ambient temperature of 10 C, the heat transfer capability of finned tube evaporator is better than that of collector evaporator at the solar irradiations of 50 W/ m2 and 100 W/m2, while the opposite situation appears at the solar irradiations of 150 W/m2 and 200 W/m2. It can be inferred that hybrid source in the AS-SAHP system can play complementary role under various operation conditions. (3) In the AS-SAHP system, most of the exergy loss is produced in compressor and condenser. The exergy loss ratio in finned
Fig. 16. Schematic of the AS-SAHP and DX-SAHP
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References
Fig. 18. The performance of different types of heat pump systems at specified ambient temperature.
tube evaporator and collector evaporator increases with the rise of ambient temperature and solar irradiation. In finned tube evaporator, the exergy loss ratio maintains below 3% at the ambient temperature of 10 C. (4) The evaporating temperature of the AS-SAHP system remains between that of ASHP and DX-SAHP. Therefore, the heat loss to environment can be avoided and the performance of AS-SAHP system is improved. The system shows the best performance under a range of environmental conditions compared with conventional ASHP and DX-SAHP.
Acknowledgement The work was supported by National Natural Science Foundation of China (No. 51806092), Bureau of International Cooperation, Chinese Academy of Sciences (No. 211134KYSB20160005), National Key R&D Plan (No. 2016YFE0124800) and the DongGuan Innovative Research Team Program (No. 2014607101008).
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance analysis of a novel air source hybrid solar assisted heat pump Jingyong Cai a, Zhouhang Li b, **, Jie Ji a, *, Fan Zhou a a b
Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei, Anhui, 230026, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan, 650093, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2018 Received in revised form 26 January 2019 Accepted 26 February 2019 Available online 8 March 2019
Aiming to enhance the performance of heat pump under adverse working conditions by utilizing solar energy and air source simultaneously, a novel air source hybrid solar assisted heat pump (AS-SAHP) system with finned tube evaporator and collector evaporator connected in series has been proposed. A mathematical model is established to characterize the behavior of the proposed system. Then the influence of key factors such as ambient temperature, solar irradiation, and evaporating area has been analyzed. The results indicate that the AS-SAHP system can operate efficiently under a range of working conditions, with the average COP of 2.71 at the solar irradiation of 100 W/m2 and ambient temperature of 10 C. Due to the heat transfer characteristics of finned tube evaporator and collector evaporator existing striking difference, the hybrid heat source in AS-SAHP system can play the complementary role at various operating conditions. To assess the irreversibility of the AS-SAHP system, the exergy loss ratio of each component has been analyzed. The results illustrate that most of the irreversibility is yielded in compressor and condenser. At last, the performance of AS-SAHP has been compared with that of conventional ASHP and DX-SAHP, and it is proven that the AS-SAHP system shows the optimal performance. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Hybrid source Heat pump Solar energy Direct expansion Exergy
1. Introduction With the capability to utilize renewable energy, heat pump is recognized as an effective energy saving device [1]. Solar energy can serve as an ideal heat source for heat pump with the advantages of low expense, easy accessibility and pollutant free [2]. Solar assisted heat pump (SAHP) is the integration of vapor compression heat pump and solar energy [3]. It has been proven that the SAHP system could effectively cut down the electricity consumption and improve the renewable energy utilization for domestic heating supply [4,5]. The SAHP system can be categorized as indirect-expansion solar-assisted heat pump (IX-SAHP) and direct-expansion solarassisted heat pump (DX-SAHP) [6]. Different from IX-SAHP, the evaporator in DX-SAHP absorbs energy from the heat source directly without intermediate heat exchanger, which can enhance the heat transfer efficiency significantly [7]. Meanwhile, with the assistance of solar irradiation, higher evaporating temperature can
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Li), [email protected] (J. Ji). https://doi.org/10.1016/j.renene.2019.02.134 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
be obtained in DX-SAHP compared with that of air source heat pump (ASHP), leading to the performance improvement [8]. In addition, the thermal efficiency of solar collector can be improved as a results of the low working temperature existed in the evaporating process of refrigerant [9]. Sun et al. [10] undertook comparative analysis of DX-SAHP and ASHP under a range of working conditions. The annual performance of DX-SAHP was remarkably higher than ASHP system. Chow et al. [11] reported an annual COP of 6.46 for DX-SAHP in Hong Kong for domestic water production, which presented excellent prospects for subtropical applications. Huang et al. [12] studied the behavior of DX-SAHP under frosting condition. The test results showed that solar irradiation was able to improve the heating capacity and avoid or decelerate frosting in DX-SAHP. However, an obvious disadvantage of DX-SAHP is the performance fluctuation caused by the variation of external environment ndez-Seara et al. [15] reported that the COP condition [13,14]. Ferna of DX-SAHP and the temperature difference between surrounding environment and condensing water were linearly related when the solar irradiation was zero. Mohamed et al. [16] designed a DX-SAHP applicable in cold areas. It revealed that the area of collector and solar irradiation significantly influenced the performance of the
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Greek
Nomenclature A c D Ex g h I k L m_ M n N p P Q s S t T u U v V W x Re Pr
2
area, m specific heat capacity, J/(kg$K) diameter, m exergy, W gravity acceleration, m/s2 specific enthalpy, J/kg solar irradiation, W/m2 thermal conductivity, W/(m.K) length, m mass flow rate, kg/s mass, kg rotating speed, rad/s number of tube pressure, Pa power consumption, W heat exchange capacity, W specific entropy, J/(kg$K) pitch, m time temperature, C velocity, m/s overall heat transfer coefficient, W/(m2.K) specific volume, m3/kg volume, m3 tube pitch in collector evaporator vapor quality Reynolds number Prandtl number
system. Moreover, DX-SAHP was inefficient during the night-time operation due to the radiative heat loss of collector evaporator [10]. To improve the stability and efficiency of the DX-SAHP, a lot of research work has been implemented, including (1) optimizing the structure of the collector evaporator [17e19], (2) properly matching the geometry structure with the capacity of system [20], (3) selecting the proper refrigerant [21,22], (4) introducing control strategy for inverter compressor and electronic expansion valve [23e25], (5) adopting cascade structure for the high temperature requirement [26], (6) enhancing the utilization of air source, etc. In particular, based on the idea of comprehensively making use of solar energy and air source, hybrid source heat pump has been proposed as a new concept. Wang et al. [27] proposed a PV/T air dual-heat-source composite heat pump with the average COPs of 1.40 in single source mode and 2.49 in dual source mode respectively. It was proven that the dual heat source significantly improved the performance of system by utilizing a composite evaporator. Ni et al. [28,29] developed a PCM based SAHP system consisting of PCM unit, solar collector and ASHP. The compatibility of the system under severe external conditions was validated in field test. Dong et al. [30,31] presented a solar integrated air source heat pump (SIASHP) with the average COP of 2.94. The evaporator of the SIASHP was modified from the evaporator of ASHP by painting selective absorption coating to improve the solar utilization ratio. Xu et al. [32] proposed a solar-air source heat pump water heater (SAS-HPWH) with a specially designed evaporator consisting of spiral-finned tube. The SAS-HPWH system was equipped with the optimal energy saving performance among various types of water heaters. On the basis of conventional DX-
a b r h g k m s ε
heat transfer coefficient, W/(m2.K) absorptivity density, kg/m3 efficiency polytropic index coefficient of thermal expansion, K1 dynamic viscosity, kg/(m.s) Stefane-Boltzman constant, W/(m2$K4) emissivity
Subscripts a cap con com coll coil dis eva fin i in l o out p ref suc tank v w
ambient/air capillary condenser compressor collector evaporator helically coiled heat exchanger discharge evaporator finned tube evaporator inner inlet liquid outer outlet refrigerant pipe refrigerant suction water tank vapor water
SAHP, Deng et al. [33] proposed a modified DX-SAHP by connecting the collector evaporator with finned tube evaporator in parallel. Compared with DX-SAHP, in the modified system heating time was reduced by 19.8%. Above studies reveal that applying hybrid heat source in vapor compression heat pump system is an effective way to realize stable and economical operation. Jointing a finned tube evaporator at the front of the collector evaporator in DX-SAHP is a simple scheme to realize the comprehensive utilization of solar irradiation and air source, while such kind of hybrid system has merely been discussed by scholars and few commercial products exists in the market. In this paper, a novel air source hybrid solar assisted heat pump (AS-SAHP) system with finned tube evaporator and collector evaporator connected in series which can act as complementary roles at various operating conditions has been proposed and investigated. In AS-SAHP, the components of the proposed system can be found in available products which is beneficial for the commercialization. As the heat transfer characteristics of the above two kinds of evaporators are inconsistent under different working conditions, the interaction between the evaporators has been discussed with a validated mathematical model. The impact of key factors such as solar irradiation, ambient temperature and evaporating area is analyzed. The exergy loss ratio has been evaluated to identify the irreversibility of the AS-SAHP system. In addition, the performance of AS-SAHP, DX-SAHP and ASHP is compared under a range of working conditions. 2. System description The schematic diagram of the AS-SAHP system is shown in Fig. 1.
J. Cai et al. / Renewable Energy 139 (2019) 1133e1145
It is mainly composed of a finned tube evaporator, a collector evaporator, a reciprocating compressor, a capillary and a water cooled condenser. Compared with the traditional DX-SAHP, a finned tube evaporator is connected in series with the collector evaporator in the AS-SAHP system. Finned tube evaporator is capable to recover heat from the ambient effectively due to the heat transfer enhancement of fins. The collector evaporator consists of the roll-bond panel which is commonly used as evaporator in DXSAHP. The roll-bond panel is a kind of bare-plate solar collector without front glass cover and back insulation, as shown in Fig. 2. The water tank with helically coiled heat exchanger is utilized as the condenser. The heat transfer features of finned tube evaporator and collector evaporator are inconsistent under different external conditions. In the AS-SAHP system, finned tube evaporator and collector
aref ;p ¼
vr vu ¼0 þr vz vt
r
vu vðuÞ vr þ ru ¼ vt vz vz
Lp vðrhÞ vðruhÞ þ ¼ Tp Tref a vt vz Ap;i ref ;p
1135
(1)
(2)
(3)
where, aref,p is the coefficient of convective heat transfer between the refrigerant and pipe. In finned tube evaporator and collector evaporator, aref,p can be concluded as [34,35]:
8 ! > > > kl > 0:8 0:3 > > ; one phase region 0:023Re Pr > > Dp;i < (" 0:37 #2:2 " 0:67 !#2 )0:5 > > > r a rl v 0:7 > l 0:4 0:01 > al ð1 xÞ þ 1:2x þ x ; two phase region 1 þ 8ð1 xÞ > > rv al rv > :
evaporator work under the identical evaporating pressure, and the state parameter of refrigerant at the outlet of finned tube evaporator has influence on the evaporating capacity of collector evaporator. The refrigerant evaporates in finned tube evaporator and collector evaporator sequentially. Then the vaporized refrigerant enters the compressor and becomes the superheated vapor. The compressed working fluid is cooled in the water cooled condenser by transferring energy to the condensing water via the helically coiled heat exchanger. The working principle of the AS-SAHP system is illustrated in Fig. 3.
(4)
In condenser, aref,p can be calculated by Ref. [34]:
aref ;p
! 8 kl > 0:8 0:3 > ; one phase region 0:023Re Pr > > Dp;i < ¼ " # > 0:76 > ð1 xÞ0:04 > > a ð1 xÞ0:8 þ 3:8x : ; two phase region l Pr0:38 (5)
3. Mathematical modeling 3.1. Model of the AS-SAHP system 3.1.1. The model of flow and heat transfer The flow and heat transfer process of the working fluid in the thermodynamic cycle of the AS-SAHP system abides by the below the assumptions: (1) The flow direction is one dimensional. (2) The pressure gradient is not considered. (3) In the two phase region, the working fluid is homogeneously mixed. It can be governed by the following equations:
3.1.2. The model of collector evaporator The collector evaporator is divided into the elementary volume, as shown in Fig. 4. For each elementary volume, the distribution of temperature at the cross section is supposed to be uniform and the heat balance expression is:
rcoll ccoll
vTcoll v2 Tcoll 1 h ¼ kcoll þ pDcoll;i aref ;p Tref Tcoll 2 Ap vt vz i þ WUcoll ðTa Tcoll Þ þ bIW
(6)
where, for the collector evaporator, W is the tube pitch, Ucoll is the overall heat transfer coefficient, given as [36]:
Ucoll ¼ aa;coll þ 4εsT 3a
(7)
where, aa,coll is the wind heat transfer coefficient, given as [36]:
aa;coll ¼ 5:7 þ 3:8ua
Fig. 1. Schematic diagram of the AS-SAHP system.
3.1.3. The model of finned tube evaporator The heat balance expression is:
(8)
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Fig. 2. Structure of the collector evaporator.
m_ ref ;com ¼ hv
nVcom vsuc
(11)
where, hv is the volumetric efficiency. The compression process is supposed to be polytropic, so the power consumption is [38]:
Pcom ¼ n
psuc Vcom
Mcom ccom
vT v2 T 1 h rfin cfin fin ¼ kfin 2fin þ pDfin;i aref ;p Tref Tfin Ap vt vz i þ pDfin;o aa;fin Ta Tfin
rcoil ccoil (9)
where, aa,fin is the coefficient of connective heat transfer between the finned tube evaporator and surrounding environment, given by Ref. [37]:
aa;fin ¼ 0:982Re0:424
S1 Dfin;o
!0:0887
N,S2 Dfin;o
pdis psuc
#
g1 g
1
(12)
vTcom ¼ Pcom m_ ref ;com hcom;out hcom;in vt Ucom Acom ðTcom Ta Þ
(13)
3.1.5. The model of condenser The energy balance equation of helically coiled heat exchanger is:
Fig. 4. The elementary volume in collector evaporator.
!
g1
"
where, g is the polytropic parameter, hcom is the operation efficiency of compressor. Taking the heat loss of the compressor into account, the heat balance expression is:
Fig. 3. Working principle of AS-SAHP.
ka Dfin;o
hcom
g
!0:159
vTcoil v2 Tcoil 1 h ¼ kcoil þ pDcoil;i aref ;p Tref Tcoil Ap vt vz2 i þ pDcoil;o aw;coil ðTw Tcoil Þ
(14)
where, aw,coil is the coefficient of convection heat transfer at the water side, given as [39]:
aw;coil
kw ¼ 0:5 Dcoil;o
g kw DTD3coil;o r2w cw
!
mw kw
(15)
The energy conservation equation of condensing water is:
(10) Mw cw
3.1.4. The model of compressor The mass flow rate of refrigerant is correlated to the rotating speed (n) and the displacement (Vcom), as well as the suction specific volume (vsuc) of the refrigerant, which can be concluded as:
vTw ¼ Atank Utank ðTa Tw Þ þ Acoil;o aw;coil ðTcoil Tw Þ vt
3.1.6. The model of capillary The mass flow rate of refrigerant is [40]:
(16)
J. Cai et al. / Renewable Energy 139 (2019) 1133e1145
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Fig. 5. Computational flowchart of the mathematical model.
2 3 4 LCcap T Ccon 10C5 DTcap;in m_ ref ;cap ¼ C1 DCcap;i
(17)
where, C1~C5 are the empirical coefficient. Based on the above model, a simulation programme has been developed with MATLAB, and the flow chart of the solution process is shown in Fig. 5. The geometry parameters of the proposed system are selected according to the ASHP and DX-SAHP with identical compressor capacity [41,42], as shown in Table 1.
Qeva;fin ¼ m_ ref ;fin hfin;out hfin;in
(18)
Qeva;coll ¼ m_ ref ;coll hcoll;out hcoll;in
(19)
To evaluate the contribution of finned tube evaporator and collector evaporator, g is defined as the ratio of evaporating capacity of finned tube evaporator to the total evaporating capacity:
g¼ 3.2. Performance evaluation The evaporating capacities in finned tube evaporator and collector evaporator are given as respectively:
Qeva;fin Qeva;fin þ Qeva;coll
(20)
The condensing capacity can be calculated by:
Qcon ¼ m_ ref ;con hcon;in hcon;out
(21)
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J. Cai et al. / Renewable Energy 139 (2019) 1133e1145 Table 1 Detail design parameter of the AS-SAHP system. Component
Parameter
Value
Finned tube evaporator
Fin pitch (S1) Tube pitch (S2) Number of tube rows (N) Outer diameter of finned tube (Dfin,o) Inner diameter of finned tube (Dfin,i) Absorptivity of collector evaporator (b) Tube pitch (W) Outer diameter of refrigerant pipe (Dcoll,o) Inner diameter of refrigerant pipe (Dcoll,i) Collector area Outer diameter of condensing coil (Dcoil,o) Inner diameter of condensing coil (Dcoil,i) Volume of water tank (Vtank) Displacement (Vcom) Rated power consumption (Pcom) Inner diameter (Dcap,i) Length (Lcap,i)
2 103m 20 103m 10 9.9 103m 8.4 103m 0.9 0.13 m 9.9 103m 8.4 103m 4.2m2 9.9 103m 8.4 103m 150 L 1.34 105m3 0.75 kW 1.2 103m 1.5 m
Collector evaporator
Condenser
Compressor Capillary
For the AS-SAHP system, the performance of coefficient (COP) can be concluded as:
COP ¼
Qcon Pcom
(22) RE ¼
The exergy loss in the AS-SAHP system can be calculated by Ref. [41]:
Exloss;eva;fin
model of this study [42]. The relative error (RE) is used to evaluate the deviation between the experimental and simulation values, calculated by:
h ¼ m_ ref ;fin heva;fin;in heva;fin;out Ta seva;fin;in i Ta seva;fin;out þ Qeva;fin 1 Teva (23)
Exloss;eva;coll ¼ m_ ref ;coll heva;coll;in heva;coll;out Ta seva;coll;in Ta seva;coll;out þ Qeva;coll 1 Teva
Xsim Xexp 100% Xexp
(29)
The validation computation runs under the identical initial and boundary conditions with the experiment. Modeling data has been compared with the testing results in Table 2. The results reveal that the maximum deviation between the modeling and testing results is within 5%. The model of ASHP is validated with the experimental data in our previous study, and the maximum deviation between the measured and predicted value is within 5%, as shown in Fig. 6 [41]. On the basis of the above comparison, it can be concluded that the mathematical model proposed in this study can forecast the performance of the AS-SAHP system with sufficient accuracy.
(24) 4. Results and discussion
Exloss;con ¼ m_ ref ;con hcon;in hcon;out Ta scon;in scon;out Ta Qcon 1 Tcon
4.1. Parametric analysis
(25) Exloss;com ¼ m_ ref ;com hcom;in hcom;out Ta scom;in scom;out þ Pcom (26) Exloss;cap ¼ m_ ref ;cap scap;out scap;in Ta
(27)
Ex 4loss ¼ P loss Exloss
(28)
3.3. Model validation For the AS-SAHP system, the validation of the proposed model is presented by verifying the model of DX-SAHP and the model of ASHP respectively. The testing results reported by Li et al. regarding the DX-SAHP system has been applied to verify the numerical
4.1.1. Impact of ambient temperature To examine the impact of ambient temperature, the operation characteristics of the AS-SAHP system under the ambient temperatures of 5 C, 10 C and 15 C with the solar irradiation of 200 W/ m2 have been discussed. The temperature of condensing water is heated from 30 C to 50 C. Fig. 7 illustrates the variation of evaporating capacity at specified ambient temperature. In finned tube evaporator, the evaporating temperature increases with the condensing temperature, causing the reduction of evaporating capacity. The evaporating capacities decrease by 141.89%, 108.67% and 72.28% respectively at the ambient temperatures of 5 C, 10 C and 15 C, when the temperature of condensing water rises from 30 C to 50 C. It shows that in finned tube evaporator the downward trend of evaporating capacity during the water heating process is relieved under higher ambient temperature. In finned tube evaporator, the evaporating capacities on average are 197.34 W, 348.55 W and 546.99 W respectively at the ambient temperatures of 5 C, 10 C and 15 C. With the temperature of condensing water rising, the evaporating capacity in collector evaporator shows a downward trend after rising, and this trend is more distinct under higher ambient
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Table 2 Modeling results and measured data DX-SAHP. Date
2005.04.15 2005.04.20 2005.05.08 2005.05.12 2005.05.15 2005.05.19 2005.05.31 Maximum RE
Ta/ C
25.1 25.7 32.0 35.6 25.6 27.0 30.0
I/W.m-2
663 812 660 689 557 432 821
P _ Qcon tj /kWh
t/min
j
P _ Pcom tj /kWh
hcoll
COP
j
exp
sim
exp
sim
exp
sim
exp
sim
exp
sim
98 90 84 76 92 98 70 3.26%
100 89 85 78 95 100 72
5.58 5.26 4.80 4.71 4.81 4.82 4.54 1.61%
5.67 5.31 4.74 4.76 4.83 4.79 4.52
1.06 1.00 0.97 0.85 1.04 1.11 0.79 4.72%
1.11 1.02 0.95 0.88 1.09 1.10 0.78
5.26 5.26 4.95 5.54 4.63 4.34 5.75 4.32%
5.06 5.26 4.99 5.41 4.43 4.35 5.79
1.05 0.88 1.04 1.09 1.11 1.33 0.99 3.85%
1.04 0.88 1.08 1.09 1.07 1.34 1.01
power consumptions of the AS-SAHP system increase by 43.93%, 33.45% and 19.86% respectively at the ambient temperatures of 5 C, 10 C and 15 C with the temperature of condensing water rising from 30 C to 50 C. Meanwhile, the COP of the AS-SAHP system decreases with the rising condensing temperature caused by the rise of pressure ratio. With the temperature of condensing water rising from 30 C to 50 C, the COP declines from 3.84 to 1.94 at the ambient temperature of 5 C. Under higher ambient temperature, higher evaporating pressure and lower pressure ratio can be obtained, leading to higher power consumption and higher COP. The power consumptions on average are 688.68 W, 711.311 W and 725.51 W respectively at the ambient temperatures of 5 C, 10 C and 15 C. And the average COP rises from 2.78 to 3.31 with the ambient temperature rising from 5 C to 15 C.
Fig. 6. Comparison between the measured and predicted results of AS-HPWH.
temperature. As the rising condensing temperature causes the decrease of evaporating capacity in finned tube evaporator, the specific enthalpy and vapor quality of refrigerant entering the collector evaporator decrease resulting in the rise of heat gain in collector evaporator. Then, in collector evaporator the evaporating capacity begins to decease, due to the decline of heat gain from the environment attributed to the rising condensing temperature. In collector evaporator, the evaporating capacities on average are 1015.43 W, 1103.74 W and 1141.70 W respectively at the ambient temperatures of 5 C, 10 C and 15 C. Overall, the AS-SAHP system can recover more energy from heat source at higher ambient temperature. The total evaporating capacities on average are 1212.76 W, 1452.29 W and 1688.69 W respectively at the ambient temperatures of 5 C, 10 C and 15 C. To sum up, the contribution of finned tube evaporator to the overall evaporating capacity decreases with the rise of condensing temperature and the decline of ambient temperature, as shown in Fig. 7 (c). The condensing capacity drops with the rising temperature of condensing water, as shown in Fig. 8. The condensing capacities decrease by 43.93%, 33.45% and 19.86% respectively at the ambient temperatures of 5 C, 10 C and 15 C with the temperature of condensing water rising from 30 C to 50 C. The increase of ambient temperature results in the rise of condensing capacity, with the average values of 1878.42 W, 2128.38 W and 2362.33 W at the ambient temperatures 5 C, 10 C and 15 C respectively. Consequently, the heating time can be reduced from to 115mine92min when the ambient temperature increases from 5 C to 15 C. The variation of power consumption and COP is illustrated in Fig. 9. The power consumption increases during the water heating process, which is attributed to the rise of evaporating pressure. The
4.1.2. Impact of solar irradiation To examine the impact of solar irradiation, the operation characteristics of the AS-SAHP system with the solar irradiation ranging from 100 W/m2 to 300 W/m2 under the ambient temperature of 10 C have been discussed. The temperature of condensing water is heated from 30 C to 50 C. The variation of evaporating capacity at specified solar irradiation is illustrated in Fig. 10. The increase of solar irradiation leads to the rise of evaporating capacity of collector evaporator, and the average values are 922.15 W, 1169.47 W and 1413.41 W respectively under the solar irradiations of 100 W/m2, 200 W/m2 and 300 W/m2. As the collector evaporator and finned tube evaporator work under the identical evaporating pressure, higher solar irradiation causes the increase of evaporating temperature, which has an adverse effect on the heat transfer capability of finned tube evaporator. The average evaporating capacity in finned tube evaporator declines from 234.12 W to 150.08 W with solar irradiation increasing from 100 W/m2 to 300 W/m2. Overall, for the AS-SAHP system the rise of solar irradiation improves the heat gain at the evaporating side. The total evaporating capacities on average are 1156.27, 1347.99 W and 1563.50 W respectively under the solar irradiations of 100 W/m2, 200 W/m2 and 300 W/m2. It can be concluded that the collector evaporator contributes more to the overall evaporating capacity under higher condensing temperature and solar irradiation, as shown in Fig. 10 (c). The variation of condensing capacity and the temperature of condensing water at specified solar irradiation is illustrated in Fig. 11. As the evaporating side of AS-SAHP system can recover more energy from heat source at higher solar irradiation, the heat gain at condensing side also rises with solar irradiation. At the solar irradiations of 100 W/m2, 200 W/m2 and 300 W/m2, the condensing capacities on average are 1825.43 W, 2014.06 W and 2215.45 W respectively. Consequently, the heating time can be reduced from 114min to 94min, with solar irradiation increasing from 100 W/m2 to 300 W/m2.
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Fig. 8. The condensing capacity and water temperature variations under different ambient temperatures.
The variation of power consumption and COP under different solar irradiations is illustrated in Fig. 12. As the evaporating pressure rises with solar irradiation, both the power consumption and COP increase. When solar irradiation increases from 100 W/m2 to 300 W/m2, the average power consumption rises from 685.91 W to 700.63 W, rising by 2.10%, and the average COP increases from 2.71 to 3.22, rising by 15.58%. It can be concluded that inputting solar irradiation at evaporating side of the AS-SAHP system can achieve significant improvement of COP with relatively low increment of power consumption.
Fig. 7. The evaporating capacity variations under different ambient temperatures.
4.1.3. Impact of evaporating area As the heat transfer features of collector evaporator and finned tube evaporator exist significant difference under different external conditions, the influence of evaporating area has been discussed. c is defined as the proportion of the area of finned tube evaporator to the total evaporating area. The performance of the AS-SAHP system with different c has been discussed with solar irradiation ranging from 50 W/m2 to 200 W/m2, at the ambient temperature of 10 C. The condensing water is heated from 30 C to 50 C。 The condensing capacity and COP at specified c is shown in Fig. 13. The condensing capacity and COP rise with c under the solar irradiations of 50 W/m2 and 100 W/m2, while it displays the opposite variation trend at the solar irradiations of 150 W/m2 and 200 W/m2. With c rising from 0.2 to 0.8, the condensing capacity rises from 2328.27 W to 2410.59 W and the COP of the AS-SAHP system increases from 3.21 to 3.29 at the solar irradiation of 50 W/m2. It demonstrates that the performance of the AS-SAHP system can be enhanced by extending the area of finned tube evaporator at the solar irradiations of 50 W/m2 and 100 W/m2, because the heat transfer capability of finned tube evaporator is better than that of collector evaporator. With c increasing from 0.2 to 0.8, the condensing capacity decreases from 2572.79 W to 2522.99 W and the COP of system declines from 3.52 to 3.44 at the solar irradiation of 200 W/m2. It is because that with the rise of solar irradiation the evaporating capacity in collector evaporator gradually exceeds that of the finned tube evaporator. Therefore, the performance of the AS-SAHP system can be enhanced by rising the area of collector evaporator under the solar irradiations of 150 W/ m2 and 200 W/m2. Due to different heat transfer features existing in finned tube evaporator and collector evaporator, it can be inferred that hybrid source in the AS-SAHP system can play the complementary role under various operation conditions.
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Fig. 9. The power consumption and COP variations under different ambient temperatures.
4.2. Exergy analysis To identify the irreversibility of the AS-SAHP system, for each component the exergy loss under different environmental conditions has been discussed, as shown in Fig. 14 and 15. It can be observed that the exergy losses in compressor and condenser account for the largest proportion under various external conditions. In compressor and capillary, with the increase of ambient temperature the exergy loss ratio declines. While in condenser, finned tube evaporator and collector evaporator the variation of exergy loss ratio demonstrates the opposite trend. The exergy loss ratio in finned tube evaporator increases from 0.48% to 18.31%, and the exergy loss ratio in collector evaporator rises from 0.94% to 12.65%, with the ambient temperature increasing from 5 C to 30 C. With the increase of solar irradiation, in compressor and capillary the exergy loss ratio decreases, while the value in condenser and evaporators increases, as shown in Fig. 15. With solar irradiation increasing from 100 W/m2 to 500 W/m2, in collector evaporator the exergy loss ratio increases from 0.27% to 28.31%, and the exergy loss ratio in finned tube evaporator remains below 3%.
4.3. Comparative analysis The performance of AS-SAHP has been compared with that of DX-SAHP and ASHP, as shown in Fig. 16. To ensure the comparability of the results, the equivalent evaporating area in each system is identical. As a critical parameter for heat pump system, the evaporating temperatures of the AS-SAHP, DX-SAHP and ASHP have been compared at the solar irradiation of 200 W/m2 and ambient temperature of 10 C, with the temperature of condensing water rising from 30 C to 50 C, as shown in Fig. 17. The evaporating temperature in each system refers to the saturation temperature under the evaporating pressure, and it keeps increasing during the water heating process. For DX-SAHP, the evaporating temperature rises from 7.91 C to 14.41 C with the temperature of condensing water rising from 30 C to 50 C, and it becomes above the ambient temperature after 67min, which results in the heat loss to the environment. The evaporating temperature of the AS-SAHP system maintains between the evaporating temperatures of DX-SAHP and
ASHP. In the AS-SAHP system, the evaporating temperature has been improved compared with that of ASHP, which is benefit for the performance improvement. Meanwhile, it maintains below the ambient temperature to avoid the heat loss to the environment, which can raise the thermal efficiency of collector evaporator compared with that of DX-SAHP. With the temperature of condensing water rising from 30 C to 50 C, the evaporating temperature in the AS-SAHP system increases from 3.74 C to 9.16 C, and the evaporating temperature of the ASHP system rises from 4.80 C to 6.28 C. The power consumption and COP in AS-SAHP, DX-SAHP and ASHP under different ambient temperatures are illustrated in Fig. 18. In each system, the power consumption and COP increase with the rise of ambient temperature. The DX-SAHP obtains the highest power consumption, followed by the AS-SAHP system, and the ASHP system consumes the least amount of power. The power consumptions on average are 711.31 W, 735.39 W and 694.23 for AS-SAHP, DX-SAHP and ASHP at the ambient temperature of 10 C respectively. When the ambient temperature is 20 C, the average power consumptions are 734.24 W, 750.83 W and 722.87 W for ASSAHP, DX-SAHP and ASHP respectively. And the average power consumptions of AS-SAHP, DX-SAHP and ASHP are 749.35 W, 765.68 W and 728.97 W respectively at the ambient temperature of 30 C. The COP of the AS-SAHP system is the highest, followed by the DX-SAHP system, and the COP of the ASHP system is the lowest. The COPs on average are 3.15, 3.03 and 2.70 for the AS-SAHP, DXSAHP and ASHP at the ambient temperature of 10 C respectively. When the ambient temperature is 20 C, the average COPs are 3.50, 3.34 and 3.14 for AS-SAHP, DX-SAHP and ASHP respectively. And the average COPs of AS-SAHP, DX-SAHP and ASHP are 3.69, 3.50 and 3.47 respectively at the ambient temperature of 30 C.
5. Conclusion To improve the efficiency and stability of heat pump system, a novel air source hybrid solar-assisted heat pump (AS-SAHP) has been proposed and investigated in this paper. The numerical model with sufficient accuracy is developed to predict the behavior of the system. Conclusions have been summarized as follow:
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Fig. 11. The condensing capacity and water temperature variations under different solar irradiations.
Fig. 12. The power consumption and COP variations under different solar irradiations.
Fig. 10. The evaporating capacity variations under different solar irradiations. Fig. 13. The condensing capacity and COP variations under different c
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Fig. 14. Exergy loss ratio variations under different ambient temperatures. Fig. 17. Comparison of the evaporating temperature in different types of heat pump.
Fig. 15. Exergy loss ratio variations under different solar irradiations.
(1) The AS-SAHP system can operate effectively under various working conditions. The average COP increases from 2.78 to 3.31 with the ambient temperature rising from 5 C to 15 C, and rises from 2.71 to 3.22 with the solar irradiation increasing from 100 W/m2 to 300 W/m2. In addition, remarkable improvement of COP in the AS-SAHP system can be achieved with relatively low increment of power consumption by inputting solar irradiation at evaporating side of the system. (2) The heat transfer feature of collector evaporator and finned tube evaporator is inconsistent under different external conditions. At the ambient temperature of 10 C, the heat transfer capability of finned tube evaporator is better than that of collector evaporator at the solar irradiations of 50 W/ m2 and 100 W/m2, while the opposite situation appears at the solar irradiations of 150 W/m2 and 200 W/m2. It can be inferred that hybrid source in the AS-SAHP system can play complementary role under various operation conditions. (3) In the AS-SAHP system, most of the exergy loss is produced in compressor and condenser. The exergy loss ratio in finned
Fig. 16. Schematic of the AS-SAHP and DX-SAHP
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References
Fig. 18. The performance of different types of heat pump systems at specified ambient temperature.
tube evaporator and collector evaporator increases with the rise of ambient temperature and solar irradiation. In finned tube evaporator, the exergy loss ratio maintains below 3% at the ambient temperature of 10 C. (4) The evaporating temperature of the AS-SAHP system remains between that of ASHP and DX-SAHP. Therefore, the heat loss to environment can be avoided and the performance of AS-SAHP system is improved. The system shows the best performance under a range of environmental conditions compared with conventional ASHP and DX-SAHP.
Acknowledgement The work was supported by National Natural Science Foundation of China (No. 51806092), Bureau of International Cooperation, Chinese Academy of Sciences (No. 211134KYSB20160005), National Key R&D Plan (No. 2016YFE0124800) and the DongGuan Innovative Research Team Program (No. 2014607101008).
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