Thermodynamic Analysis of a Direct Expansion Solar Assisted Heat Pump Water Heater Abstract 1- Introduction

Transcription

1 Thermodynamic Analysis of a Direct Expansion Solar Assisted Heat Pump Water Heater Corresponding Author: Misagh Moradali Other Authors: Farzad Jafarkazemi Islamic Azad University, South Tehran Branch, Tehran, Iran Abstract In this article the thermodynamic performance of a direct expansion solar assisted heat pump (DX-SAHP), which is used to heat domestic water from 20 C to 45 C, is theoretically investigated. The system includes a 3m 2 single-cover flat plate solar collector, 150 liter water tank and 70 meters tube immersed in the water tank as condenser. Effect of various parameters such as received irradiance on the collector surface, compressor speed and the ambient temperature on the coefficient of performance (COP) is calculated. Results show that obtained COP is considerably more than that of a conventional heat pump water heater when radiation on collector is high and increasing collector area and ambient temperature and reducing compressor speed enhance COP. For instance, at 450 W/m 2 received irradiance on collector and 15 C ambient temperature, the calculated COP is Introduction Due to growing energy demand and depletion of fossil resources, the application of renewable energy resources has created lots of interests. Among different renewable energy sources, solar energy is one of the best alternatives which is readily available in many parts of the world. Consequently, research and developments have been conducted to expand application of solar systems. Solar assisted heat pump is one these applications. The idea of combining conventional heat pumps and solar systems has taken many interests. The COP of heat pumps improves by increasing of evaporator temperature. Besides, solar system works better if the plate temperature decreases. As a result, both systems operates better in comparison with working separately. Chow et al. [1] examined a (DX-SAHP) and simulated the long term performance of model with typical meteorological data (TMY) for Hong Kong. The average yearly COP of the system was reported as 6.46.

2 Results showed this system works better in summer when the COP reaches 10. Chatuverdi et al. [2] proposed to use two stage compressors for producing high temperature hot water in the range of C. Calculated COP was higher than that of a single-stage (DX-SAHP). Liu Keling et al. [3] presented a mathematical model and reported the results for the performance of a (DX-SAHP) where compressor s power was supplied by a photovoltaic system. This system worked in the weather condition of Tibet and had a variable frequency compressor. They found that in a sunny day in summer, COP of the system reached to 6.01 and the average electricity efficiency, thermal efficiency and overall efficiency were 0.135, 0.47 and 0.625, respectively. Dickci et al [4] investigated the experimental results of operation of (DX- SAHP) which is used to heat a 60 m 2 residential room. The average COP of the system was 3.08 and the overall exergy loss was obtained to be (KW). Guoying et al. [5] simulated operating performance of a (DX-SAHP) for water heating. This system used an especially designed solar collector with spiral-finned tubes which contributed to heat transfer from the air to the tubes. The effects of different parameters were analyzed and they recommended using variable speed compressor due to working under a wide range of external operating conditions namely solar irradiance. Grozable et al. [6] investigated the effect of using different refrigerants on the COP of a (DX-SAHP). Using R 22 and R 134a instead of R 12 decreased COP 2 to 4 percent. Replacement of R 410a, R 407c and R 404a with R 12 reduced COP 15 to 20 percent. Kuang et al. (7) reported the results of analysis of a multi-functional (DX- SAHP) which offered space heating during winter and water heating whole the year. The system employed 10.5 m 2 solar collector and variable speed compressor and could supply 200 to 1000 liters 50 C hot water a day. The daily average heat pump COP was calculated between 2.6 to 3.3 for space heating only mode while it varied from 2.1 to 2.7 for water heating only mode. 2- System description

3 Nomenclature A c Collector area (m 2 ) A coil Coil area (m 2 ) cp w Water specific heat ( F Collector efficiency factor g acceleration due to Gravity ( H ( ) Coil convective heat transfer coefficient h Enthalpy ( K w Water conductive heat transfer ( ) I Solar irradiance ( ) L Condenser length ( m) Refrigerant mass flow rate ( ) m w Water mass (Kg) N Q e Compressor speed (RPM) heat flow rate (w) Ra = g β T ) Rayleigh number Compressor displacement volume (m 3 ) Ta ambient temperature ( c ) Tp Plate temperature ( c ) Tr refrigerant temperature ( c ) Tw water temperature ( c ) U LC Collector heat loss coefficient ( ) Vw wind velocity ( W Compressor power consumption (w) Greek symbols α Absorptivity β Thermal expansion coefficient ( ) θ Steffan-boltzman constant collector slope Glass emmitance factor Plate emmitance factor Compressor Volumetric efficiency ν dynamic Viscosity ( ) λ thermal diffusivity ( ) Refrigerant specific volume ( ) ) Among different combinations, direct expansion solar assisted heat pump (DX-SAHP) and indirect expansion solar assisted heat pumps(ix- SAHP) are two more applicable systems. In an (IX-SAHP), a fluid like water is heated in solar system and then, in a heat exchanger, the process of heat transfer takes place between the heat pump evaporator

4 and hot fluid to raise evaporator temperature. Fig1 presents a schematic of (IX-SAHP). Fig. 1. A schematic diagram of IX- SAHP In a direct expansion solar assisted heat pump, solar collector tubes are filled with refrigerant and solar collector is the evaporator of system. The process of evaporation of refrigerant takes place by solar irradiance and causes a decrease in plate temperature. Reduction of required components and risk of corrosion in collector are two main advantages of this coupling. Fig 2 presents a schematic diagram of a (DX-SAHP).

5 Fig. 2. A schematic diagram of DX- SAHP 3- Mathematical model Following, a mathematical model is developed to predict the performance of a (DX-SAHP). Except water tank, a quasi steady modeling is used to model all components. - 3a) Solar Collector The solar energy collected by collector / evaporator per unit area, Q c, and the radiation heat transfer coefficient for one-cover solar collector, U LC, are calculated according to the equations which were proposed in reference [8]. Qe = Ac( α I-U (1) LC (t p - t a ))

6 Q e (2) =A c F ( αi-u LC (t r -t a )) U LC = [ c t P 1 + tp - ta ( ) 1+ f 1 h w ] -1 σ(t + ( p ε p + t a )(t 2 p + t h 2 a w ) 1+ f ε c ε p (3) f =( hw hwε p) ( ) (3-a) c = 520 ( θ ٢ ) (3-b) e = 0.43 (1- t 100 ) 273 p + (3-c), plate emmitance factor and, glass emmitance factor, are assumed to be 0.95 and 0.88, respectively. h w is the convective heat loss coefficient and is equal to : h w = V w (4) Energy balance equation in collector/ evaporator yields: Q e 1 = m& ( h 2 h ) (5) 3b) Compressor Refrigerant mass flow rate is calculated by following equation: m = η VD 60 ν N & v (6) The compressor power consumption is found by energy balance equation : = m(h & h ) (7) w 3 2

7 3c ) Condenser Energy balance equation in condenser is expressed by equation 8 Q c = m(h & 4 h 3 ) (8) Heat transfer coefficient of an immersed condenser can be calculated by an equation which is proposed in reference [9] : k w H = (Ra) (9) L The heating capacity of condenser is found by coil heat transfer coefficient: Q c = H A coil (T c - T w ) (10) Water tank temperature variation can be found from: Q c = m w (11) cp w dt dt w 3d ) Expansion valve Expansion valve process is isentropic: h ١ = h 4 (12) Heat pump coefficient of performance is obtained by below equation : COP (13) = Q w h = h h 4 3 h h 3 2

8 All above equations are solved by MATLAB based on a flowchart shown in figure 3. All thermodynamic properties are found by equations presented for R 134a in reference [9]. Fig. 3. Calculation flow chart 4- System configuration Above equations were solved for a system which is similar to some other previous experimental setups. Configuration of this system is described in table 1. Collector One-cover flat plate collector F =0.9 = 0.88 = 0.95 θ = 35.5 Compressor VD= 24.1 cm 3 = 0.9 N= 1400 (rev/min)

9 Condenser 70-meter copper tube Inner tube diameter = 8 mm Outer tube diameter = 12.5 mm Table 1. System configuration 5- Results and discussions Figure 4 shows the effect of solar irradiance and the average tank temperature on the performance of the system at 15 C ambient temperature and a wind velocity of 3 m/s. COP of the system increases when the received irradiance increases. In addition, reduction of water tank temperature results in increasing COP. The measured COP, when received radiation on titled surface is 450, are 6.37, 5.16 and 4.28 at average tank temperature of 20 C, 32.5 C and 45 C, respectively. At average water tank temperatures of 20 C, 32.5 C and 45 C and 950 received radiation, the calculated COP are 8.39 C, 6.62 C and 5.41 C, respectively. Fig 5 illustrates the variation of performance of system with ambient temperature. COP of system boosts when the ambient temperature increases. The variation increases with increasing received irradiation. At 10 C ambient temperature, if received radiation is 500, 700 and 900, COP will be 6.52, 7.35 and 8.02 while at 20 C ambient and same received solar irradiance, COP is 6.69, 7.55 and 8.25, respectively.

10 Fig. 4. Variation of COP with solar irradiance Fig. 5. Effect of ambient temperature on COP The variation of heat pump compressor power consumption with the received solar irradiance is presented in Fig 6. At 15 C ambient and 450, SAHP power consumption is approximately 223.3, 347 and 347

11 W and at 950 these will change to 295, 378 and 468 W if water tank temperature taken as 20, 32.5 and 45, respectively. Fig. 6. SAHP compressor power consumption Fig 7 presents the effect of variation of compressor speed on the system at 15 C ambient temperature. Increasing compressor speed from 1100 ( ) to 1700 ( ) results in a reduction of coefficient of performance from 11 to 7 at 950 ( 450( ) when water tank temperature is 20 C. ) and from 7.64 to 6.72 at Fig. 7. Effect of compressor speed on the performance of SAHP

12 Fig 8 shows the improvement in SAHP operation with increasing collector area. At 15 C ambient and 20 C water tank temperature and 450 ( ) solar irradiance on titled surface, calculated COP are 6.74 and 9.8 for collector area of 3.5 and 5 m 2, respectively. Fig. 8. Effect of collector area on the performance Required time to heat 150 liters water is shown in Fig 9 to 11 in steadystate conditions when ambient temperature is assumed to be 15 C, compressor speed of 1400 ( ) and constant received solar irradiance. According to the calculation at 500, 700 and 900 ( ) irradiation, heating process takes 169, 132 and 112 minutes, respectively.

13 Fig. 9. Variation of water tank temperature with time in 500 ( ) Fig. 10. Variation of water tank temperature with time in 700 ( ) Fig. 11. Variation of water tank temperature with time in 900 ( ) 6- Conclusions The results of simulation and thermodynamic analysis of a solar assisted heat pump water heater with a 150-liter water tank was presented.

14 Effective parameters on the performance of system was investigated which included solar irradiance, ambient temperature, compressor speed and solar collector area. According to calculation, increasing solar irradiance and ambient temperature, reducing water tank temperature and compressor speed, all enhance coefficient of performance. System COP was found to be 6.37 and 8.39 at 450 and 950 (, respectively which is higher in comparison with that of a conventional heat pump water heater. References 1. Chow, T.T., et al., Modeling and application of direct-expansion solarassisted heat pump for water heating in subtropical Hong Kong, Applied Energy, 87 ( 2010) Chaturvedi, S.K., et al., Two-stage direct expansion solar-assisted heat pumps for high temperature application, Applied Thermal Engineering, 29 (2009) Keling, L., et al., Performance study of a photovoltaic solar assisted heat pump with variable frequency compressor A case study in Tibet, Renewable Energy, 34 (2009) Dikci, A., Akbulut, A., Performance characteristics and energy - exergy analysis of solar assisted heat pump, Building and Environment, 43(2008) Guoying Xu, Xiaosong Zhang, Shiming Deng, A simulation study on the operating performance of a solar-air source heat pump water heater, Applied Thermal Engineering, 26 (2006) Gorozabel, F.B., et al., Analysis of direct expansion solar assisted heat pump using different refrigerant, Energy Conversion and Management 46 (2005) Kuang, Y.H., Wang, R.Z., Performance of multi-functional directexpansion solar assisted heat pump. Solar Energy 80 (2006) Duffie, J.A., Beckman, W.A., Solar engineering of thermal processes. John Wiley and Sons Inc., Third edition, 2006

15 9. Ji, J., et al., Performance analysis of an air source heat pump using an immersed condenser, Frontiers of Energy and Power Engineering, (2) Cleland, A.C., Polynomial curve-fits for refrigerant thermodynamic properties: extension to include R134a. International Journal of Refrigeration, 17 (1994)