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Figure 2-10 70000 65000 60000 55000 50000 45000 leaving chilled water temp = 59 oF leaving chilled water temp = 55 oF leaving chilled water temp = 50 oF leaving chilled water temp = 45 oF vapor from the air stream. However, because air-to-water and water-to-water heat pumps both deliver a stream of cool water as their output, there is only one rating for cooling capacity, which in North American is usually expressed in Btu/hr. Cooling capacity is significantly influenced by the temperature of the air entering the heat pump’s condenser, and the temperature of water entering the heat pump’s evaporator. Cooling capacity increases when the temperature of the water delivering unwanted heat to the heat pump’s evaporator increases. Cooling capacity also increases when the temperature of the air absorbing heat from the heat pump’s condenser decreases. So, as was true for both heating capacity, and COP, the closer the source temperature is to the sink temperature, the higher the cooling capacity of the heat pump. This is shown, for a specific heat pump, in Figure 2-10. ENERGY EFFICIENCY RATIO In North America, the common way of expressing the instantaneous cooling efficiency of a heat pump is called 4000060 65 70 75 80 85 90 95 100105 Oudoor air temperature (oF) The COPs of currently available heat pumps, even when operated under very favorable conditions, is substantially lower than the Carnot COP. Still, the Carnot COP serves as a way to compare the performance of evolving heat pump technology to a theoretical limit. It also demonstrates the inverse relationship between the “temperature lift” of a heat pump and COP. The COP of air-to-water heat pumps decreases as the outside air temperature decreases. It also decreases as the temperature of the water leaving the heat pump’s condenser increases. Figure 2-9 shows a typical relationship between COP versus outdoor temperature and the water temperature leaving the condenser for a modern “low ambient” air-to-water heat pump. COOLING MODE THERMAL PERFORMANCE T Energy Efficiency Ratio s(inEk ER), which can be calculated In the cooling mode, the two indices used to quantify the 12 Btu / hr performance of air-to-water heat pumps are: a. Cooling capacity 10 11 b. Energy Efficiency Ratio (EER) 9 COPHPonly = 48,000Btu / hr For air-to-air and water-to-air heat pumps, both of which 8 use forced-air delivery systems, cooling capacity is divided into two parts: sensible cooling capacity and latent cooling capacity. Sensible cooling capacity is based on the 6 temperature drop of the interior air stream passing through the heat pump’s evaporator coil. Latent cooling capacity is based on the ability of the interior coil to remove water ⎡⎣5500⎤⎦ watt × 3.413 Btu / hr = 2.56 watt using Formula 2-3. 7 COP = Carnot T ( sink − T source ) Formula 2-3 Q EER= wc = e cooling capacity (Btu/hr) electrical input wattage Figure 2-11 48,000Btu / hr leaving chilled water temp = 59 oF 14 13 leaving chilled water temp = 50 oF leaving chilled water temp = 45 oF COPHPonly = Btu / hr = 3.35 lea⎡v4in2g00c⎤hwillaetdt ×w3a.t4e1r3temp = 55 oF COP net = 48,000Btu / hr ⎡⎣(2×220)+4200+180⎤⎦watt×3.413 watt = 2.92 ⎣⎦ watt 48,000Btu / hr 60 65 70 75 80 85 90 95100105 COP = =2.48 Energy Efficiency Ratio (EER) (Btu/hr/watt) Cooling capacity (Btu/hr) net Outdoor air temperature (oF) Btu / hr ⎡⎣5500+180⎤⎦watt×3.413 watt E=E⎡1−1⎤ s R ⎢⎣COPL COPH ⎥⎦ 15PDF Image | Heat Pump Systems 2020
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