(The note below is written by Paul Dodgshun ('Paul D'). Paul has provided innumerable pieces of very helpful advice to many readers of this blog concerned about heat pumps. I asked Paul if he would summarise his views, in particular explaining to us why the DECC focus on air-to-water heat pumps will exacerbate our decarbonisation problems as well as adding to home energy bills for people sucked in by aggressive promotion of this technology. We're all extremely grateful to him for doing this. Paul and I have so far failed to get policy makers to face the facts on air-to-water pumps and we do hope readers interested in this issue make their own representations to government.) Chris Goodall has asked for a summary article that refers to helping people with their heat pump problems. This might give the impression that I am an 'expert', whatever that might mean. My 'expertise', if that is what it is, starts from mid-June. I was cold called by a company that wished to sell me a 10kW(electrical) air-to-water heat pump for £16,000. When asked for my order I said, “Due diligence first”. I became even more suspicious when the salesman switched off his mobile phone and I could not make direct contact, in order to let him know if I would place the order. The sales pitch had all been about the very generous government subsidies that would pay for this heat pump.
Next, on the 12th July, DECC published a document called the draft domestic Renewable Heat Initiative Policy (RHI) that was to go to Parliament for approval and implementation in April 2014. I read the document without much interest until I arrived at the section on heat pumps. As a retired power station engineer, I idly wondered what the Carnot Efficiency of these heat pumps might be. Low Carnot Efficiency would mean that heat for the home would be relatively expensive. What I’ve found is that government is pushing us all in completely the wrong direction by subsidising air-to-water type heat pumps when it should be sponsoring air-to-air pumps which have better Carnot efficiency.
The crucial fact we need to know is that a heat pump that needs to provide high temperature water for radiators operating at 60 degrees needs a great deal more electricity to run than a heat pump that pushes out warm air at about 30 degrees. To heat a house with an air-to-water heat pumps is always going to be more costly than with an air to air pump. This is a consequence of the laws of physics. Nevertheless, in apparent defiance of these laws, the UK government is heavily subsidising air-to-water heat pumps and ignoring the air-to-air variety.
(Those with an aversion to equations can skip the next bit!)
The scene is set to do some engineering. This requires paper, pen, calculator and a knowledge that Carnot's Law governs heat pumps. The Law states that the theoretical maximum efficiency of a heat pump is equal to
The Hot Temperature/(The Hot Temperature – The Cold Temperature))
where Temperature is stated in degrees above absolute zero (e.g. the freezing point of water is 273 degrees above zero and boiling point is 373d degrees). THot and TCold are the maximum and minimum temperatures in the refrigerant circuit in a heat pump when it is running.
There is also a term called COP (the Coefficient of Performance). A COP of 5, say, means that for every one kilowatt of electricity supplied to the heat pump compressor, then 5 kilowatts of heat are delivered from the hot end of the pump. A low COP will inevitably arise when a heat pump has to pump heat up a steep gradient between a cold outside temperature and high desired water temperature in the house.
For an air-to-air pump that gets its heat from the ambient air and delivers its heat output to a house at 20C, the COP will have a certain value.
For an air-to-water pump that gets its heat from the ambient air and delivers its heat output to a water circuit, which in turn delivers its heat output to the house, another COP will be calculated.
An air blower could blow air at 30C and a water circuit would be at 60C to drive radiators that really belong to a fossil fuelled boiler. An ambient air temperature of 5C represents a sort of average of where winter temperatures in the UK might be.
COP 5/30=(273+30)/(30-5)=303/25=12.12 COP 5/60=(273+60)/(60-5)=333/55=6.05
The ratio of these two COPs is 12.12/6.05=2.
This says that for every two kilowatts of heat supplied by the air-to-air pump only one is supplied by the air-to-water, when the same amount of electricity is used by each pump. That says that the owner of an air-to-air pump will only pay half as much for the electricity as the owner of the air-to-water pump will do, to heat identical houses to the same standard. SO WHY IS THE GOVERNMENT PROMOTING AIR-TO-WATER HEAT PUMPS WHICH DOUBLE CONSUMERS' HEATING BILLS, AS COMPARED TO AIR-TO-AIR? I do not know the answer to that one.
The consequences of this anomaly spread. If air-to-water takes twice the amount of electricity, it also requires twice the generation capacity. As the generation is supposed to become wind turbines, the back up-gas fired generator capacity must double as well. Now I was getting really interested. Half the wind turbines and back-up generators did not need to be there at all: just fit air-to-air pumps and the need goes away. What is the government playing at? I do not know the answer to that one either.
If you recalculate the maximum COPs for an ambient air temperature of minus5C, you get :-
COP -5/30=(273+30)/(30-(-5))=303/35=8.66 COP -5/60=(273+60)/(60-(-5))=333/65=5.12
The COPs have dropped to 8.66/12.12=71% and 5.12/6.05=85% of their former values at +5C. Air Sourced Heat Pump COPs fall as the ambient air temperature falls. This unfortunate characteristic causes the heat pump maximum heat output to fall as the ambient air temperature falls.
If you size a heat pump to cope with the whole house heating at a design temperature of minus10C, you get a big heat pump because the heat load is at its highest and the COP is at its lowest. The consequences duly arrive. The pump costs a lot because it is big; that is simple. What is harder to spot is a big inefficiency problem with these big pumps. Modulated (and you should not buy anything else) heat pumps have a part load characteristic that is a problem.
At full load the pump is at its most efficient. The part load efficiency drops slowly to about 0.9 at 50% part load (not too bad), but thereafter as the part load percentage falls turns downwards and aims to zero at 0% part load. With a big heat pump it is all too easy to be spending most of the running time at something like 25% part load. At this part load the efficiency might be something like 0.5; the pump is using twice as much electricity to shift the heat as it uses at full load. YOU SHOULD NOT RUN HEAT PUMPS AT BELOW 50% PART LOAD
The way to combat this problem is to run small heat pumps and keep the old fossil fired boiler. A good lkW(electrical) air-to-air heat pump will produce something like 4kW of heat at an ambient air temperature of freezing. Run continuously 24/7, this will supply 4x24=96kWh per day of heat to a house. My winter quarter gas bill for a five bed, detached modern estate house states that my average daily consumption of heat is 100kWh per day. This average can be met by a 1kW(electrical) heat pump that costs less than £1000+fitting, AND THAT IS ALL YOU NEED TO SPEND, because you keep your old fossil fired boiler.
This boiler will heat the DHW (Domestic Hot Water) and provide extra heat for those coldest of winter days where your house heat demand exceeds the winter quarter average. You can even close off rooms during these coldest of winter days and postpone the need for the boiler to provide any space heating at all.
I have put my money where my pen is and installed a small air-to-air. Others who post to Carbon Commentary are doing the same and one gentleman from south-west Scotland has his twin heat pumps + oil boiler system running already, as things are a bit cooler up there. I believe the data we will collect this winter will prove beyond any reasonable doubt that this is the way that this country should install and run heat pumps; small air-to-air wins but it does have its issues, primarily air distribution and fan noise. I am interested in all data, problems and successes from running this scheme.