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1、1Energy Efficiency Technologies Air Source Heat Pump vs. Ground Source Heat PumpAbstract.Heat pump deserves the name of eco-innovation. It uses free energy - warmth collected in the soil or in the air to provide heating and cooling. There are two main types of heat pumps air source heat pump and gro
2、und source heat pump. How do they work? What are the benefits of each system? How do they compare? First of all, questions like “what is heat pump, how does it work, what are ASHPs and GSHPs” will be answered. Then a detailed comparison between ASHPs and GSHPs will be carried out, from technological
3、 parameters to social, practical, economical parameters. Finally, the conclusions are drawn from these parameters as to decide which kind of heat pump is better off under different conditions. Keywords: Heat pump, Efficiency, Air Source Heat Pump, Ground Source Heat Pump Nomenclature Qe: condensatio
4、n heat rate, kJ s-1 Qc: evaporation heat rate, kJ s-1 W: compressor input energy, kW COP: coefficient of performance m: mass flow rate, kg s-1 h: enthalpy, kJ kg-1 Q: the rate of heat transfer, kJ s-1 Uc/e Ac/e: overall heat transfer coefficient of condenser / evaporator, kW oC-1 K: constant, equals
5、 Ue Ae / Uc Ac. Tc: temperature in cold region(heat source), oC Th: temperature of warm region (heat sink), oC Tre: temperature of refrigerant in evaporator, oC Trc: temperature of refrigerant in condenser, oC Teout: temperature of outlet fluid in evaporator, oC Tcout: temperature of outlet fluid in
6、 condenser, oC 1. Introduction The physical law tells us that heat normally flows from a warmer medium to a colder one. But can we move heat from our cooler house and dump it to a higher outside environment in summer? And can we extract heat from a lower temperature outside, to our warmer rooms in w
7、inter? The answer is yes if we use a heat pump. The heat 2pump does so by essentially “pumping” heat up the temperature scale, transferring it from a cold material to a warmer one by adding energy, usually in the form of electricity. The most common type of heat pump is the air-source heat pump, whi
8、ch transfers heat between indoor and the outside air. Ground Source heat pumps (GSHPs) have been in use since late 1940s, they use the constant temperature of the earth as the exchange medium instead of the outside air temperature. 2. Background 2.1 Heat Pumps Heat pumps (vapor compression heat pump
9、s) transfer heat by circulating a phase changing substance called a refrigerant through a cycle of evaporation and condensation (Figure 1). A compressor pumps the refrigerant between two heat exchanger coils. In one coil, the refrigerant is evaporated at low pressure and absorbs heat from heat sourc
10、e, the refrigerant is then compressed en route to the other coil, where it condenses at high pressure, at this point, it releases the heat it absorbed earlier in the cycle to the heat sink(NRCOEE,2004,p.4). Figure 1. Basic Vapor Compression Cycle Source: NRCOEE (Natural Resources Canada Office of En
11、ergy Efficiency), 2004, p.4 2.2 Energy Balance and Efficiency of Heat Pumps From the laws of thermodynamic, the Energy Balance of a heat pump system is: Qc=Qe+W Where Qe is the heat absorption rate by the evaporator and Qc is the heat given off at the condenser (Figure 1). 3The operating temperature
12、s of the vapor-compression refrigeration cycle are established by the temperature Tc to be maintained in the cold region and the temperature Th of the warm region to which heat is discharged; the refrigerant temperature in the evaporator must be less than Tc and the refrigerant temperature in the co
13、ndenser must be greater than Th to allow heat transferring (Figure 2). Figure 2a. Vapor-compression refrigeration cycle: T-S diagram Source: Refrig. Lab of Queens University,2007, p.9 Figure 2b. Vapor-compression refrigeration cycle: ln(p)-h diagram Source:Rademacher,2005, p.70 As fluids pass throug
14、h evaporator and condenser, which are two heat exchangers that exchange heat between refrigerant and surrounding fluids. The rate of heat transfer from refrigerant to the surrounding fluid in condenser and the rate of heat transfer from surrounding fluid to the refrigerant in evaporator can be expre
15、ssed 4by Q: Q=m*h=UALMT Here m is the mass flow rate of refrigerant, h is the enthalpy change of refrigerant, U is termed the overall heat transfer coefficient, A is the area of the surface separating the fluids through which the heat transfer occurs and LMT is the mean differences between the temperatures of the two fluids. The classic parameter that has been used to describe the performance of a heat pump is the coefficient of performance(COP), which is the ratio of the quantity of heat transferred to the heat sink (useful energy output) to the quantity of work drivin
