Heat Pumps Today

www.acrjournal.uk/heat-pumps 13 C O MM E R C I A L H E A T P U M P S "WHY IS IT THAT THE RETURN TEMPERATURE EFFECTS EFFICIENCY MOST?" Even though 99.9% of the universe is plasma there isn’t much on earth that exists in this state. The sun and stars we see at night are all plasma. Most matter needs to be heated to an extremely high temperature and/or be held under extreme pressure to achieve plasma state and so there are few tangible everyday examples, metal being arc welded is in plasma state, we also see plasma in neon and fluorescent lighting tubes, but that is about it. The uniqueness of C0 2 is that it can transform form a gas to a plasma at only 31˚ C, if pressurised to 73Bar. The point that it changes from a gas to a plasma is called this the critical point and so we describe the plasma as trans-critical C0 2 . In heat pump systems, C0 2 is in trans- critical (plasma) state after discharge from the compressor, we aim to keep it in this form for the entirety of the heat transfer. If the heat sink (the return water) is below 31˚C, we can transfer all of the energy in the C0 2 without dropping the system pressure. Once the heat is transferred, we can drop the pressure just below the critical point. By doing so the refrigerant condenses and then can absorb heat in the evaporator and then needs just a little compressor power to turn the refrigerant trans-critical again. As a plasma, C0 2 does not have any latent property so any energy transferred to or from it, results in sensible heat gain/ loss, directly effecting the temperature of the C02. This means that only a small amount of compressor power can increase the refrigerant temperature from 31˚C to 50, 60, 70 or 80˚C.