Does anyone know of the existence or not of a heat pump that uses combustion as its heat source?
By this I mean a compressor driven heat pump that, instead of taking heat out of the air, takes heat from (a small amount of) combustion. Imagine, for instance, that instead of a using a large fan to pass air over the evaporator (in an ASHP) I place a flame under the evaporator (this can be a gas/oil/wood flame... doesn't matter). After the refrigerant gassifies in the evaporator an electrically driven compressor then compresses the gas, thereby heating it up, just like in any other compressor driven heat pump. Does such a thing exist?
Why might you want such a Heath Robinson thing!
Well, it's just a matter of CoP. A heat pump takes 1kW energy input, and turns it into 4kW of heat. Most people heat their house with a gas boiler and if I offered any gas boiler owner the chance to quadruple their heat output (or reduce their gas usage x 4) they'd bite my arm off.
True, the heat source in an air source heat pump is free! So why would you pay for gas to warm up the evaporator if you could just let free air do the job? Well, the air isn't totally free... you need to run a fan to blow the air over the evaporator and that uses electricity. But, granted, this will just be a calculation... is it cheaper to run the outdoor fan or is it cheaper to burn a small amount of gas/oil/wood.
That said, even if a combustion source heat pump were less efficient than an air source heat pump it might have other advantages:
1. No big outdoor unit
2. Better performance in really cold spells
My guess is that someone, somewhere has already thought of and dismissed this idea as stupid/impractical/not really a gain. But I'm curious.
(By the way, what I'm thinking of is NOT an absorption based heat pump... I still want an electrical compressor in the system.)
PS My guess is that you're going to have to put so much combustion heat into the system that the CoP is going to be exactly 1, making it totally pointless.
Maybe some kind of use if you use waste heat from other combustion processes, though?
The point with an ASHP or GSHP is that the extra energy you get is free. So with a COP of 3, you put 1kW (or a bit more in reality) and get 2 for nothing. If you're paying for that extra 2 then there's no saving. If it's waste heat then yes it's worth thinking about.
PS My guess is that you're going to have to put so much combustion heat into the system that the CoP is going to be exactly 1, making it totally pointless.
Maybe some kind of use if you use waste heat from other combustion processes, though?
You are indeed correct that there would be no actual benefit from an energy consumption point of view, in fact the overall COP could actually be less than 1, as with burning gas in a gas boiler.
There would also be no financial advantage, since you would be paying for the gas or oil as against getting free energy from the outside air.
Quite some time ago I proposed a system to collect solar thermal energy during the daytime, and store this energy in a water based heat store. The stored energy could then be used during the overnight period, when the outside air is normally cooler, and hence the heat pump is less efficient, to pre-heat the air being drawn into the heat pump to help improve efficiency. It may also help reduce the frequency at which defrosting would need to be carried out.
I am quite convinced that such a system could help improve overall system efficiency, particularly on sunny days during the Winter period which are often followed by freezing nights.
@derek-m That's interesting and I saw your idea over in another part of the forum.
But there's another way to look at it. Instead of using the water in the thermal store to pre-heat the source side... why not use it as pre-heated water for the cold water feed.
Water that comes out of the mains must be 5 degrees... and the heat pump has to lift it by about 30 degrees in order to make 35 degree water for your UFH. But if you have a store of water in the thermal store at 10, 15, 20 degrees or whatever, then the heat pump has to lift it much less to go into your UFH.
@derek-m That's interesting and I saw your idea over in another part of the forum.
But there's another way to look at it. Instead of using the water in the thermal store to pre-heat the source side... why not use it as pre-heated water for the cold water feed.
Water that comes out of the mains must be 5 degrees... and the heat pump has to lift it by about 30 degrees in order to make 35 degree water for your UFH. But if you have a store of water in the thermal store at 10, 15, 20 degrees or whatever, then the heat pump has to lift it much less to go into your UFH.
Once your central heating system is filled with water from the mains supply, it then operates as a sealed system, so there would be no requirement to pre-heat the mains water.
I recently watched a Heat Geek video in which they had installed a solar thermal system, with thermal store, and were using it to heat their offices for part of the day, with their heat pump switched off. It should be remembered that solar thermal can be as high as 80% efficient in capturing solar energy.
Whilst I fully commend their idea, it could have been further extended to make their heat pump more efficient. Their system obviously works fine on sunny days for offices that are used during daytime hours, but what about overnight, or in an home environment.
Let me explain.
As the Sun goes down, so does the supply of solar thermal energy from the panels, so the only source of energy for heating the building is that in the thermal store.
Say that the water in the thermal store has been heated to a temperature of 50C, and to maintain an indoor air temperature of 21C, requires a water temperature of 35C to be supplied to the heat emitters. Initially there is no problem, since the water temperature is high enough, and there is plenty of heat energy available. Warm water is drawn out of the thermal store, passed through the heat emitters, and cooler return water fed back to the thermal store.
This of course takes heat energy out of the thermal store and causes the water temperature to reduce. Once the temperature of the water coming out of the thermal store falls below 35C, it can no longer maintain the indoor temperature at 21C, so both the water temperature and the indoor temperature will start to fall. This process will continue until the Sun rises the next day and starts to provide sufficient energy to reverse the process.
It could be decided that the indoor temperature needs to be maintained at 21C or some setback value, so once the thermal store can no longer supply the heat energy requirement it will be shutdown, and the heat pump started. The water in the thermal store could still be at a temperature of 30C or more, so still contains a considerable amount of heat energy.
During the overnight period the heat pump could initially be obtaining 67% of the building heat energy requirement from the outside air and 33% from the electrical supply, but as the outside air temperature falls this ratio could change to 60% to 40% or even lower, which would be an increase of over 21% in electrical energy consumption.
What if it was possible to use the heat energy that is still available in the thermal store, to pre-heat the air going into the heat pump, and by doing so recover the previous 67% to 33% ratio between absorbed energy and electrical energy?
Such a system could be particularly useful when an overnight setback is used, since a thermal store could provide the energy boost required to bring the indoor temperature back up quickly, whilst still allowing the heat pump to operate at a higher output and still provide a good level of efficiency.
Does anyone know of the existence or not of a heat pump that uses combustion as its heat source?
By this I mean a compressor driven heat pump that, instead of taking heat out of the air, takes heat from (a small amount of) combustion. Imagine, for instance, that instead of a using a large fan to pass air over the evaporator (in an ASHP) I place a flame under the evaporator (this can be a gas/oil/wood flame... doesn't matter). After the refrigerant gassifies in the evaporator an electrically driven compressor then compresses the gas, thereby heating it up, just like in any other compressor driven heat pump. Does such a thing exist?
Why might you want such a Heath Robinson thing!
Well, it's just a matter of CoP. A heat pump takes 1kW energy input, and turns it into 4kW of heat. Most people heat their house with a gas boiler and if I offered any gas boiler owner the chance to quadruple their heat output (or reduce their gas usage x 4) they'd bite my arm off.
True, the heat source in an air source heat pump is free! So why would you pay for gas to warm up the evaporator if you could just let free air do the job? Well, the air isn't totally free... you need to run a fan to blow the air over the evaporator and that uses electricity. But, granted, this will just be a calculation... is it cheaper to run the outdoor fan or is it cheaper to burn a small amount of gas/oil/wood.
That said, even if a combustion source heat pump were less efficient than an air source heat pump it might have other advantages:
1. No big outdoor unit
2. Better performance in really cold spells
My guess is that someone, somewhere has already thought of and dismissed this idea as stupid/impractical/not really a gain. But I'm curious.
(By the way, what I'm thinking of is NOT an absorption based heat pump... I still want an electrical compressor in the system.)
I like the thinking outside the box. Air source heat pumps are designed to take enthalpy (the energy state of outside air) at fairly low levels, transfer this into a refrigerant vapour, add energy from compression and then release this heat in a plate heat exchanger condenser. It's not a perfect system, but it's by far the most efficient domestic heating appliance available. It's least efficient at lower ambient temperatures when there is less enthalpy in the outside air to capture. The ASHPs' Achilles Heel.
That get's further compounded by the refrigerant evaporating temperature falling well below freezing with ASHPs, which then require a periodic defrost cycle. All ASHPs I have seen have a parasitic defrost where energy from the primary fluid circuit is used. I don't know why that is, as opposed to, say, a hot gas defrost, which is common place in commercial refrigeration and dehumidification. The only reason I can think is that most ASHPs are bastardised split a/c designs with 4 port reversing valves designed for heating and cooling. The simplest defrost is to reverse the circuit. I would much rather the defrost was done by hot gas injection, rather than taking heat out of the primary circuit. I don't think hot gas injection would require any more energy, the energy required to defrost the evaporator is what it is, its just a question of where that energy is obtained from. But crucially, a hot gas defrost would not rob energy out of the space heating circuit.
I'm convinced the winter performance of ASHPs will improve, and whoever places a product onto the market that does not use a reversing valve i.e.: a 'true' ASHP designed as a heating appliance, rather than a split a/c designed as a cooling appliance that has a plate heat exchanger condenser grafted on, will find their product popular with installers and homeowners. At the end of the day, we want the enthalpy harvested by the ASHP to transfer into our house, not to disappear back out of the circuit every 45-60 minutes to defrost the evaporator.
Once your central heating system is filled with water from the mains supply, it then operates as a sealed system, so there would be no requirement to pre-heat the mains water.
Of course! Stupid me! Mains water doesn't go into the heat pump, the heating return goes into the heat pump. What I should have said is 'divert the heating return to run through the solar thermal system first, so that it can get preheated before going into the heat pump'.
Your idea is to use the thermal store on the source side and what would be needed is some kind of collector inside your thermal store. I can visualise this quite easily with a GSHP (I've got a GSHP myself): the brine loop runs under the ground, it collects heat and returns to the heat pump, but, before the heat pump, we run the brine loop through a collector in the thermal store. That's easy. With an ASHP, I just can't see how to convert the heat in the water in the thermal store to usable heat for the air source collector. Unless you maybe run a hot water heat exchanger over the outdoor unit fan or something like that?
It's least efficient at lower ambient temperatures when there is less enthalpy in the outside air to capture. The ASHPs' Achilles Heel.
That takes us back to my original question. On a really cold night, why don't we just take a small flame to heat up the ASHP evaporator just a little bit to make it efficient again.
I obviously don't understand the thermodynamics involved! And I'll probably sound really stupid to someone who does. But imagine that, on a really cold night, I park one of these in front of the ASHP outdoor unit. Portable Grey LPG Space Heater 15,000W - Screwfix
Worst case scenario: I improve the ASHP's performance back to 'normal'... but I pay exactly as much in gas as I win in electricity saving on my ASHP. Net gain = exactly zero. (Or worse when I take into account transaction costs.)
Best case scenario: it actually takes less gas than I expect to get the ASHP's performance back to normal and then I'm quids in.
I suspect that the real world is like in 'worst case scenario' because a heat pump doesn't generate energy... it just moves it from one place to another place.
But as I say, I don't really understand the thermodynamics. Is there anyone here on the forum who can explain?
Rest assured, I'm not trying to turn lead into gold or invent a perpetual motion machine! Just curious.
Once your central heating system is filled with water from the mains supply, it then operates as a sealed system, so there would be no requirement to pre-heat the mains water.
Of course! Stupid me! Mains water doesn't go into the heat pump, the heating return goes into the heat pump. What I should have said is 'divert the heating return to run through the solar thermal system first, so that it can get preheated before going into the heat pump'.
Your idea is to use the thermal store on the source side and what would be needed is some kind of collector inside your thermal store. I can visualise this quite easily with a GSHP (I've got a GSHP myself): the brine loop runs under the ground, it collects heat and returns to the heat pump, but, before the heat pump, we run the brine loop through a collector in the thermal store. That's easy. With an ASHP, I just can't see how to convert the heat in the water in the thermal store to usable heat for the air source collector. Unless you maybe run a hot water heat exchanger over the outdoor unit fan or something like that?
Your suggestion of using a thermal store to pre-heat the return water to the heat pump should indeed work, but on cold days the return water from the heat emitters could already be as high as 40C, or even 45C, which would limit the amount of heat energy that could be extracted.
By pre-heating the air, rather than the water, it should be possible to extract heat energy from the heat store down to virtually the outside air temperature, which could be -5C or even -10C. The water in the heat store would of course require an adequate mixture of antifreeze, but this could be the type used in vehicles, rather than the more expensive kind used in heat pumps.
There is a detailed write up somewhere buried in the forum, that explains the design.
Water to air heat exchangers of various sizes are readily available, but for a trial an old car radiator or two would probably suffice.
It's least efficient at lower ambient temperatures when there is less enthalpy in the outside air to capture. The ASHPs' Achilles Heel.
That takes us back to my original question. On a really cold night, why don't we just take a small flame to heat up the ASHP evaporator just a little bit to make it efficient again.
I obviously don't understand the thermodynamics involved! And I'll probably sound really stupid to someone who does. But imagine that, on a really cold night, I park one of these in front of the ASHP outdoor unit. Portable Grey LPG Space Heater 15,000W - Screwfix
Worst case scenario: I improve the ASHP's performance back to 'normal'... but I pay exactly as much in gas as I win in electricity saving on my ASHP. Net gain = exactly zero. (Or worse when I take into account transaction costs.)
Best case scenario: it actually takes less gas than I expect to get the ASHP's performance back to normal and then I'm quids in.
I suspect that the real world is like in 'worst case scenario' because a heat pump doesn't generate energy... it just moves it from one place to another place.
But as I say, I don't really understand the thermodynamics. Is there anyone here on the forum who can explain?
Rest assured, I'm not trying to turn lead into gold or invent a perpetual motion machine! Just curious.
As you stated, an ASHP does not magically take a 'dollop' (technical term) of energy and multiply it by 3 or 4.
When operating at a COP of 3, a heat pump uses 1kW of electrical energy to extract 2kW of energy from the outside air, to supply a total of 3kW. As Allyfish correctly stated, the ASHP's Achilles Heel is that there is less heat energy in the outside air, at precisely the weather conditions when the heat pump is required to supply more heat energy. Less energy from the outside air means that the heat pump has to take more energy from the electrical supply.
