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Combustion Source Heat Pump?

Benguela · 2023-02-04T10:02:51Z

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.)

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Mars

(@editor)

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06/02/2023 4:03 pm

We had a meeting earlier today with the technical manager from OFTEC, and he mentioned that there are heat pumps in R&D at the moment that will have a small boiler as part of the unit to help the unit when temperatures get really cold or during defrost cycles.

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Derek M

(@derek-m)

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06/02/2023 4:56 pm

Posted by: @editor

↑

We had a meeting earlier today with the technical manager from OFTEC, and he mentioned that there are heat pumps in R&D at the moment that will have a small boiler as part of the unit to help the unit when temperatures get really cold or during defrost cycles.

Did they state how this 'boiler' is powered?

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AllyFish

(@allyfish)

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06/02/2023 5:11 pm

Posted by: @editor

↑

We had a meeting earlier today with the technical manager from OFTEC, and he mentioned that there are heat pumps in R&D at the moment that will have a small boiler as part of the unit to help the unit when temperatures get really cold or during defrost cycles.

That's interesting, but OFTEC is the oil fired technical association, and one can't help but feel this sounds like self-preservation? If there's adequate 240V power supply to run an ASHP, there's probably power in hand to supplement heat output electrically during defrost or very low CoP periods. Efficiency would be poor: 100% for electric and maybe 90-05% for oil.

The real issue here is the cost disparity between electric (increasingly renewably sourced) and fossil fuels per kWh.

To combat climate change, we need to 'stop burning stuff' and producing CO2. Along with coal and bottled gas, oil is one the most polluting and carbon intensive home heating fuels.

The planning constraint on ASHP permitted development is 0.6m3 unit size if I recall correctly. That means the evaporator coils on most units has a very narrow fin pitch. Far narrower than you would find on commercial refrigeration and cold store plant which is designed to allow ice to accumulate on the fins for much longer before defrosting. Domestic ASHPs ice up in no time when the outside humidity is high and the refrigerant evaporating temperature falls a few degrees below freezing. If ASHPs could be brought to market that defrost only once every 2-3 hours rather than every 45-60minutes, the doubly whammy of low CoP and parasitic defrost energy requirement could be mitigated. Such ASHPs would need to be physically larger however.

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Benguela

(@benguela)

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Topic starter 06/02/2023 5:59 pm

Do you mean a little bit like this? Hybrid heat pump | Daikin

I'm not sure how a hybrid heat pump works, is it just a full-sized heat pump and a full-sized gas boiler in one package... and you switch between the two. Heat pump OR gas.

Or does the heat pump and gas boiler work in series... that's more what I'm interested in. Heat pump AND gas. (More accurately, I'm interested whether there's any mileage in a gas AND heat pump arrangement.)

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Derek M

(@derek-m)

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06/02/2023 6:09 pm

Posted by: @benguela

↑

Do you mean a little bit like this? Hybrid heat pump | Daikin

I'm not sure how a hybrid heat pump works, is it just a full-sized heat pump and a full-sized gas boiler in one package... and you switch between the two. Heat pump OR gas.

Or does the heat pump and gas boiler work in series... that's more what I'm interested in. Heat pump AND gas. (More accurately, I'm interested whether there's any mileage in a gas AND heat pump arrangement.)

At the moment we have a gas and heat pump arrangement, but it is an Air to Air heat pump, that we run when there is enough solar PV. We ran the heat pump today, and it shutdown the gas boiler for 6 hours.

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AllyFish

(@allyfish)

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06/02/2023 8:18 pm

Posted by: @benguela

↑

Do you mean a little bit like this? Hybrid heat pump | Daikin

I'm not sure how a hybrid heat pump works, is it just a full-sized heat pump and a full-sized gas boiler in one package... and you switch between the two. Heat pump OR gas.

Or does the heat pump and gas boiler work in series... that's more what I'm interested in. Heat pump AND gas. (More accurately, I'm interested whether there's any mileage in a gas AND heat pump arrangement.)

Grant do their Vortex oil boiler as a hybrid - basically an outdoor vortex combustion boiler plonked on the same base as one of their (well, Chofu's) 17kW ASHPs. Uses whichever fuel is more efficient. So that basically means oil at lower outdoor temperatures.

https://www.grantuk.com/products/hybrids/grant-vortexair-range/

It's not very easy on the eye, designed by the MD as a cut'n'shut product? Ideal for larger properties which may need to provide a lot of heat energy quickly or owners who want the reassurance of oil to fall back on in case the ASHP goes down.