ACR Journal

February | March 2021 As we move further as an industry toward decarbonisation, using clean electricity as a heat generator in buildings, the responsibility on manufacturer`s is not only to meet the necessary inputs of the building regulations and institute guides on e ciency, but to exceed them. The unique way the air source heat pump, whether within a VRF machine, split system, heat pump chiller or air to water heat pump `moves heat by magic` e ciently, cements its appeal. For each kW burned in moving heat (absorbing energy from the outside air and moving it `by magic` to the inside air or water) the ASHP delivers a multiple of that (eg: 3 or 4 or 5kW) to the inside. The Co- e cient of Performance (COP). A heat pump doesn`t generate heat, it moves it. The key to this `magic` is the fact the compressor (acting as a refrigerant pump) is the main source of electricity draw. It is only one of the four main elements of the vapour compression cycle, but it plays the primary role in e ciency. Work done - It`s design is paramount However, as with many things, it`s the relation to its partners that gives it e ciency. The compressor relationship to its heat exchangers is huge. For example: In the case of VRF air coils play a huge part. To absorb heat from outside, the refrigerant gas needs to be colder within the coil. For example: if the air is -4°C outside, the liquid refrigerant may need to be -14°C at a certain coil design as it evaporates to a gas (absorbing heat as it does so - magic). The dynamics of that outside heat exchanger coil design has a direct aect upon the e ciency. Imagine the compressor having to draw very cold gas (which is directly related to its pressure, say at -16°C) and then force/ compress the gas to, say +70°C, so it can be used inside for heating. That is a large ∆TD (86°C) Imagine through responsible design the heat exchanger is designed with a larger 14 Andy Bradison, Sales and Marketing Director, Cool Designs talks about the importance of design when heading towards decarbonisation What is this magic? surface and better thermal transfer, we could reduce the air o temperature to say -10°C by adjusting the temperature at which we evaporate the gas to say -13°C we have reduced the compressor work to 83°C ∆TD This works in exactly the same way for heat pump chillers and air source heat pumps (small and large). The design of, and interaction between, the compressor(s) heat absorbing coil (s) dictates the e ciency. Manufacturers strive to achieve the optimum design yet system designers and those that apply systems play their part too. We can relate COP to refrigerant ∆TD. Whilst in the UK we are directed by the weather as to how the air on (the heat source) is established, we do have some say on the temperature of the heating side. Indicative COP/Refrigerant ∆TD impact Let`s say, for example, the refrigerant (gas) sits outside worst case at -13°C based on a -4°C ambient. ° For water to be delivered at 65°C (directly to storage, fan coil or radiator etc) the gas needs to be compressed up to 75°C (88°C ∆TD). COP could be 2.5. ° For water to be delivered at 45°C (primary circuit for pre-heat) the gas needs to be compressed up to 55°C (68°C ∆TD). COP could be 3.5 (reduced work done, increased e ciency) ° For water to be delivered at 35°C (underfloor heating) the gas needs to be compressed up to 75°C (88°C ∆TD). COP could be 4.5 or even above. Volume 7 No.2 A COOL DESIGN – SPONSORED BY CDL