“Can you give me the cost of a 100kW heat pump?”
“Absolutely, what flow temperature do you want it to operate at? What ambient air temperature do you want it to achieve 100kW? Do you want to include the defrost energy, or not?”
A conversation that starts like this is one you could well expect to hear when ordering a meal at Burger King, not one when you’re looking to simply get a price on a product. However, making sure that you’re evaluating and selecting the right product could be the difference between being nicely full, or being left with hunger pangs!
What temperature do you want it to operate at? What ambient air temperature do you want it to achieve 100kW?
A heat pump is not a boiler. A boiler doesn’t care what temperature it is outside, it just happily chuggs away operating at its set point. If you pay for 100kW, you know you’re getting 100kW.
A heat pump is a little different. It uses energy absorbed from the air to boil liquid refrigerant and turn it into vapour. This change of state can occur even at -10/-15°C because the boiling point of the refrigerant is so low. At these conditions the energy available in the air to boil the refrigerant is obviously less, so the heat pump output decreases.
That is not the whole story though. As the energy available in the cold air becomes less, a greater volume of air is required to pass across the evaporator coil in order to boil the refrigerant. The heat pump fans have to spin faster to achieve this. The air temperature off the coil is approximately 3K less than the ambient air temperature. Therefore, as the ambient temperature reaches 3°C, ice starts to build up on the coil. If this ice is not cleared, the evaporator will not be able to boil the refrigerant and liquid will enter the compressor. Liquid is not easily compressed and will damage the compressor.
Do you want to include the defrost energy, or not?
Thankfully, the heat pump is intelligent and is able to manage the level of ice build-up in order to keep the air flowing across the evaporator and keep the unit operating correctly. Although there are a number of accessories and technologies manufacturers use to prevent this ice build-up, eventually, the heat pump will have to defrost itself. It does this by stopping the heating cycle, starts to operate in reverse, taking some of the vapourised refrigerant back into the evaporator coil. This hot gas will hopefully clear the ice and the heat pump can start operating in heating mode again.
In really cold conditions, the heat pump will have to operate in reverse for longer and it will need to recover heat and energy from the user circuit. Essentially, instead of heating, you will be cooling the building. This is why it is so important to have a suitably sized buffer tank, not only as a store of heat to allow the user circuit to maintain normal temperature, but as an energy source for the heat pump to defrost correctly.
The whole time the heat pump is operating in defrost, it cannot provide heat to the user circuit. If your design requires 100kW of heat at -3°C then a heat pump sized to produce 100kW at 7°C obviously isn’t going to satisfy this. It either needs to be sized at -3°C, or the design should incorporate a back-up boiler to assist in the low temperature conditions, or the heat pump/buffer can be fitted with electric immersion elements to make up for the shortfall.
There are a couple of ways to model heat pump performance and if you look at any manufacturers brochure they refer to the EN14511 normative. Air is nearly always 7°C and the flow temperature is typically 35°C or 45°C. EN14511 takes into account the energy required to operate the user circuit pump and most importantly, accounts for the energy required to defrost the heat pump. At 7°C this isn’t a huge factor but depending on your design condition, this can typically be between 15-25% reduction from the Gross capacity of the unit.
Sizing on Gross can give some spectacular capacity and COP figures and can lead to a much smaller and cheaper heat pump selection. However, defrost is a fact of life that needs careful consideration when choosing heat pumps and the energy required for it has to come from somewhere.
The next article in the series, will discuss the difference between the 14511 and gross normative in further detail and how this effects equipment selections, and more importantly, price!