Heat Pump

Pagina 1

1 Heat pump with various configurations of sources and sinks A heat pump is used in winter to maintain an indoor space at a given set-point temperature. The heat pump can be coupled to various external/internal heat sources/sinks: - Internal sink: radiators (at 80°C) or radiant floors (25°C) - External sources: external air (-10°C) or lake (15°C). a) Assuming that (i) the heat pump has an efficiency equal to 65% of that of a reversible heat pump operating at the same temperature levels, and (ii) the temperature in the evaporator is 5°C lower than the source, while the temperature in the condenser is 5°C higher than the sink, determine the COP in all 4 configurations. b) If the building has a floor area of 90m2, internal gains equal to 4 W/m2 and heat losses Qwalls = 2430 W, determine QH and the mechanical power required to the heat pump in all four cases. c) Determine the mechanical power of the reversible heat pump. Solution: a) COP (adim.) in all four configurations: external air lake radiators 2.33 3.10 radiant floor 4.38 9.85 b) Work (in W) required in all four configurations: external air lake radiators 889 667 radiant floor 473 210 a) Work (in W) required in all four configurations, reversible case: external air lake radiators 578 433 radiant floor 307 137 2 Winter air conditioning A room of a cinema must be maintained at internal conditions Ta=20°C and ϕa=50%, while external conditions are Te = 1°C and ϕe=80%. The overall heat losses to the outside are Qtot = -20000 kcal/h, and it is necessary to extract a mass-flow rate of water vapour 𝑚̇ 𝑣 = 20 𝑘𝑔𝑣/ℎ. The fresh air that must be introduced is 𝑚̇ 𝑑.𝑎.,𝑒𝑥𝑡 = 7000 𝑘𝑔𝑑.𝑎./ℎ. At Ti=35°C. Determine: - The heating power at the pre-heating and post-heating coils (Qpre-heating and Qpost-heating) - The mass flow rate of…