My eco 4 heat pump fun

[LPGC]

Are there any heat domestic heat pumps that could easily be modified to be turned by a separate motor/engine instead of the (presumably) built in electric motor?

I know you've mentioned this before and only just got around to replying but how would you control your engine? The pumps in a heat pump don't run at a constant speed, speed is adjusted depending on demand. So if ambient is 10 degrees and you decide you want the room at 20 degrees, it will run flat out initially until the room temperature gets up to around 17-18 degrees and then slow down until the temperature gets up to 20 degrees. At this point it will just be 'ticking over' (for want of a better expression) or even stopped, ready to speed up, or restart, the compressor and fan speed to maintain the target temperature. So your engine would need to be able to generate the same torque at differing revs and be stop/start controllable.

But surely a far cheaper option would be solar panels and a storage battery? That way you needn't even think about the differing costs of mains gas and electricity.....

I hadn't thought that far ahead but can imagine an automatic engine throttle control like a cruise control setup to change throttle position to aim for a certain engine speed, this taking input from whatever would usually dictate the speed of the electric motor that drives the heat pump? Would further guess the heat pump motor speed is controlled by changing pwm and/or AC drive frequency, things we could probably read electronically even without an electric motor connected(?) but instead of having to read those I expect there'd be a simpler signal (before the electric motor final drive circuit) which tells the electric motor final drive how quickly to spin the motor and that signal is likely to be simpler to read? Not all heat pumps are/were variable speed? Early days for sure lol.

How well do solar panels do at this time of year? I can understand spring or early autumn having loads of solar panels and a battery to store the excess to use later (when its dark) working but I would think this time of year (or especially next month when it'll be dark 20 hours per day) if you relied on solar panels you'd still be buying lots of electricity from the grid? Kind of you've got loads of excess power when you don't need it but a deficit of power when you most need it?

I know what I'm talking would be niche and most would opt for solar panels, maybe even a wind turbine... I'm used to working in a niche field lol.

[Pinger]

So if ambient is 10 degrees and you decide you want the room at 20 degrees, it will run flat out initially until the room temperature gets up to around 17-18 degrees

And, if the engine is cold there will be little heat from its coolant.

But surely a far cheaper option would be solar panels and a storage battery? That way you needn't even think about the differing costs of mains gas and electricity.....

Something that probably needs to be considered is that electricity can be had for circa 7-8p/kW.hr overnight on tariffs catering for charging EVs. That stored in a battery for driving the heat pump during the day is going to be hard to compete with economically for ICE. Made worse by not being able to utilise all of the excess heat from ICE. Unless the heat pump can improve matters, as far as I can see the best possible is to attain half the temperature of the fluid from which heat is being extracted. It's a bit like filling an empty LPG tank from a full one with no additional pumping. When the pressures balance, flow ceases. When temperatures equalise, heat transfer ceases and even achieving that becomes incredibly difficult as the temperature differential reduces towards zero.
I don't mean to be negative but having been saved from myself by you guys on numerous occasions...

[LPGC]

And, if the engine is cold there will be little heat from its coolant.

Something that probably needs to be considered is that electricity can be had for circa 7-8p/kW.hr overnight on tariffs catering for charging EVs. That stored in a battery for driving the heat pump during the day is going to be hard to compete with economically for ICE. Made worse by not being able to utilise all of the excess heat from ICE. Unless the heat pump can improve matters, as far as I can see the best possible is to attain half the temperature of the fluid from which heat is being extracted. It's a bit like filling an empty LPG tank from a full one with no additional pumping. When the pressures balance, flow ceases. When temperatures equalise, heat transfer ceases and even achieving that becomes incredibly difficult as the temperature differential reduces towards zero.
I don't mean to be negative but having been saved from myself by you guys on numerous occasions...

Engine warm up time is due to it's mass, conversely there's also an engine cool down time due its mass... We could turn the engine off a little early and still circulate coolant to continue to retrieve heat energy. Engine warmup and cool down time is to some extent just a delay. E.g. there are plenty vehicles (Mercs) etc that use a little electric water pump, this allows the interior heater matrix to be kept hot for some time after the engine has been turned off, in conjunction with the car's heater fan still running slowly this allows the inside of the car to be kept nice and toasty for some time after the engine has been turned off. The cool down time is related to warm up time which is related to engine mass and heat capacity of the iron / aluminium / coolant that makes up that mass.

I explained how we could use all the heat from the ice, a heat pump evaporator drawing heat from the latter stages (near the tailpipe area) of the ice exhaust. Not sure if some of what you said regarding pressures and temperatures is also a negative against heat pumps in general, perhaps affecting the usual air to water (heating water filled radiators) type to far more extent than affecting Gilbert's blown air type... But I would guess at least the air/water type could benefit from having it's input temp lifted be X degrees because it's connected to an engine exhaust instead of being 'connected' solely to ambient air via a fan, benefit in terms of scop (efficiency) and perhaps also in terms of final output temperature.

Yes it would be difficult to compete with electricity at 7p per kwh especially if the heat pump is capable of maintaining decent house temperature when it's freezing cold outside meaning no other method of heating the house is ever necessary. Much would depend on those elusive real world heat pump scop figures over a wide range of external temperatures, there are still people saying that when external temp is freezing the scop goes down to 1, at which point it is probably still cheaper to run a gas boiler than a heat pump (if yiu can buy gas at 6p)... If we factor in that gas still costs around the same as the cheap 'special rate for EVs' night time electricity 24 hours per day and that the engine could deliver 66% (or more) of the efficiency of that boiler without even making use of the kinetic energy it produces, then factor in the heat pump making use of some of that engine heat to lift its input temp to raise it's scop (the rest to turn the heat pump) and the difference in cost to run might still not be much. While at the same time the engine can heat radiators to 90deg regardless of external temp and its exhaust will get hot regardless of external temp. Batteries could definitely help with running a heat pump on night time electricity prices through the day, probably just as well because traditionally people turn the thermostat setting down at night time or when they are out of the house through the day (what is the point of paying to heat the house when there is nobody home?) and turn it back up only through the daytime when somebody is at home. Another thing said is that heat pumps work most efficiently when left at close to constant temperature, if that is true is there an element of in some ways paying over the odds because you're paying to keep the house warm even during times when traditionally you wouldn't be paying to keep the house warm? And if so could this element be minimised by heating the heat pump heat input temperature by connecting it to the back end of the engine exhaust? Gilbert's mention of heat pump set to max boost and 30C cutting out until it had defrosted its evaporator comes to mind.

[Pinger]

Engine warm up time is due to it's mass, conversely there's also an engine cool down time due its mass... We could turn the engine off a little early and still circulate coolant to continue to retrieve heat energy. Engine warmup and cool down time is to some extent just a delay. E.g. there are plenty vehicles (Mercs) etc that use a little electric water pump, this allows the interior heater matrix to be kept hot for some time after the engine has been turned off, in conjunction with the car's heater fan still running slowly this allows the inside of the car to be kept nice and toasty for some time after the engine has been turned off. The cool down time is related to warm up time which is related to engine mass and heat capacity of the iron / aluminium / coolant that makes up that mass.

My inclination would be toward a smaller engine (running at higher rpm with reduction gearing) such that the heat from its coolant was available earlier when most needed. Also, a smaller engine will be lighter which will make it cheaper and reduce the package size of the unit and ease its mounting. If, a generator is to be included the somewhat rigid rpm requirement it imposes may impinge negatively on the engines ability to provide for the heat pump compressor. Thus, two separate engines may be necessary and if so the weight and cost assumes a greater importance.

I explained how we could use all the heat from the ice, a heat pump evaporator drawing heat from the latter stages (near the tailpipe area) of the ice exhaust. Not sure if some of what you said regarding pressures and temperatures is also a negative against heat pumps in general, perhaps affecting the usual air to water (heating water filled radiators) type to far more extent than affecting Gilbert's blown air type... But I would guess at least the air/water type could benefit from having it's input temp lifted be X degrees because it's connected to an engine exhaust instead of being 'connected' solely to ambient air via a fan, benefit in terms of scop (efficiency) and perhaps also in terms of final output temperature.

I re-read that post and better understand your aims. Gilbertd is better informed to advise on just how much heat from the exhaust (available immediately) can be utilised by the heat pump but I doubt that it is a significant amount though I may well be totally off on that. Hard though to imagine that it won't contribute substantially.

Yes it would be difficult to compete with electricity at 7p per kwh especially if the heat pump is capable of maintaining decent house temperature when it's freezing cold outside meaning no other method of heating the house is ever necessary. Much would depend on those elusive real world heat pump scop figures over a wide range of external temperatures, there are still people saying that when external temp is freezing the scop goes down to 1, at which point it is probably still cheaper to run a gas boiler than a heat pump (if yiu can buy gas at 6p)... If we factor in that gas still costs around the same as the cheap 'special rate for EVs' night time electricity 24 hours per day and that the engine could deliver 66% (or more) of the efficiency of that boiler without even making use of the kinetic energy it produces, then factor in the heat pump making use of some of that engine heat to lift its input temp to raise it's scop (the rest to turn the heat pump) and the difference in cost to run might still not be much. While at the same time the engine can heat radiators to 90deg regardless of external temp and its exhaust will get hot regardless of external temp. Batteries could definitely help with running a heat pump on night time electricity prices through the day, probably just as well because traditionally people turn the thermostat setting down at night time or when they are out of the house through the day (what is the point of paying to heat the house when there is nobody home?) and turn it back up only through the daytime when somebody is at home. Another thing said is that heat pumps work most efficiently when left at close to constant temperature, if that is true is there an element of in some ways paying over the odds because you're paying to keep the house warm even during times when traditionally you wouldn't be paying to keep the house warm? And if so could this element be minimised by heating the heat pump heat input temperature by connecting it to the back end of the engine exhaust? Gilbert's mention of heat pump set to max boost and 30C cutting out until it had defrosted its evaporator comes to mind.

Again, much of the above is better answered by Gilberd but re extraction of heat from coolant and exhaust. Realistically how much higher than 100C can a domestic heating system cope with given the need to pressurise to avoid boiling? Coolant temp from engine is in the ball park, exhaust temp straight from the cylinder is in four figureC territory. Even if all the heating of the CH water is accomplished, the exhaust gas will still be leaving the system at substantial temperature and thus squandering energy. If I am wrong it is likely because the heat can be extracted - but at significant capitol cost.
To directly heat a property from that - is the engine driving the heat pump compressor large enough? It would be larger (or duplicated) if a generator was incorporated. Then we might ask why bother with the heat pump and not settle for an engine driven generator with heating from its coolant and exhaust heat? Returning to the original idea though, I think a rigorous analysis of the available heat and, the efficiency at which it can be extracted is required to ascertain the viability. A year ago I did that for a proposed 2T engine functioning as a gas generator for a gas turbine. All useful work would be from the turbine, the torque at the crankshaft being used only to drive a blower to aspirate the engine and raise the overall pressure. I abandoned it upon realising just how difficult it is to extract energy from exhaust streams. And that was after the initial disappointment of pursuing the step-change in gas turbine efficiency when heat recovery is deployed and realising the sheer size, weight and cost of the heat exchangers required - and that was for an engine somewhere between slightly larger than what you are proposing and something powerful enough to drive a car.

[Gilbertd]

Then we might ask why bother with the heat pump and not settle for an engine driven generator with heating from its coolant and exhaust heat? Returning to the original idea though, I think a rigorous analysis of the available heat and, the efficiency at which it can be extracted is required to ascertain the viability.

That was what I was thinking. Simon seems to have got hung up on the idea of using a heat pump but replacing the electric motor driven compressor with an ICE driven one and using some of the heat that would otherwise be wasted to supplement the heat pump. But it seems the original objective has been forgotten, that is to generate heat, and draw heat, to provide domestic heating and cooling. The heat pump simply moves heat from one place to another by the conversion of gas to liquid and back to gas again. So when heating a house, heat is drawn from outside and transferred to inside. You seem to have also got hung up on the outdoor temperature and are using zero degrees as the reference. If the temperature is below zero, then there is no heat there to extract. Completely wrong. Zero degrees is just a convenient number we use as that happens to be the freezing point of water. Until you get down to absolute zero, -273 Kelvin, there is heat there that can be extracted.

The pain in the arse customer who caused his evaporator to freeze only did it by effectively trying to over ride the control built into the unit. That, coupled with damp, foggy air (he was in Yorkshire....), meant there was lots of water vapour in the air to settle on the condenser and freeze. The same scenario in one of those crisp, dry, winter days and it wouldn't happen.

But, back to Robert's original thread. He will obviously know much more about the practical use of solar panels than I do although all I can give is anecdotal. I installed a pair of systems in an extended bungalow out in the sticks with no mains gas supply. They had what they described as a 6kW solar array with storage batteries. Now my assumption on that is that it will supply 6kWh for every hour there is sufficient sunlight, although whether it needs full sunlight or just light on an overcast day, I've no idea. But assuming it gives that on an average day, even in winter that will give 6kW for roughly 7-8 hours a day, or 42-48kW, more in summer. The bungalow was built in the 1930's so had minimal insulation in the walls (nothing in the cavities at all) and prior to me visiting they had a total of 6 3kW electric heaters that were timed to come on for 1 hour, 5 times a day (and complained the house was still cold). That would have been drawing at total of 90kW so the solar couldn't keep up with demand. I installed two units, a 9000Btu wall unit and an 18000 Btu ducted system where there is one indoor unit feeding 4 rooms via ducts to vents in the ceiling (with return ducts to circulate the air back into the unit). Total consumption of both, when run flat out, is just over 2.2kW, dropping to around 1kW once the rooms are up to the desired temperature. When I visited a few months after doing the install, they told me they have left the system on 24 hours a day, 7 days a week and rather than taking power from the grid and supplementing it from the solar, they are generating more than they use and selling it back to the grid. Not only that, but they were able to walk around in Tee shirts all winter as the house is permanently warm. The lady of the house has recommended me to a work colleague of hers and I'm doing an install for her just after Christmas.....

Solar is something that interests me as a plan for the future is to buy a place in France, which may or not have a connection to the grid. The French planning rules are a bit weird. You can buy a plot of land but if it doesn't have a mains water and electricity supply to it, or within a practical distance so it can be connected, you won't get permission to build anything on it. However, if it already has a building on it, whether or not it has water and electricity, that building, in most cases depending on whether the local Mayor likes the look of you or not, can be extended and lived in. So the ideal option then is a borehole for water and solar for electricity. Interestingly when you buy land in France you don't just buy the bit you walk on, you buy it all the way down so even if a borehole needs to go down hundreds of feet before it finds water, there's none of this licensing for abstracting water, it is under your land so it is yours. I would have thought a 5-6kW array with storage batteries should easily be enough to cope with demand all year round. Worst case, a wind turbine or, really worst case, a generator, to top it up if needed.

[robertXX]

winter solar output from 6kw of panels gilbert .

two rows of panels ,green lines , totalling to the black line ..

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and since january, although solar income only started in june ..

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[LPGC]

If the graphs show time versus kw output and figures are peak values between 7am > 3pm is there anything to show total panels kwh output for 5th and 6th Jan 2024?

I'd guess around 0.7kw over 8 hours on 5th Jan = 5.6kwh, 1.1kw over 8 hours = 8.8kwh on the 6th?

[robertXX]

here you go simon , every day for january ..

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[LPGC]

Figures look closer to 5 and 6kwh for the 5th and 6th Jan respectively then.

I don't suppose we can do a straightforward calculation of grid import energy 198.7kwh minus solar panel export to grid 'feed in energy' of 29.6kwh to say you imported net 169.1 kwh costing 169.1 x unit cost of electricity because the import and export prices will be different? How much is import price per kwh and how much is export price per kwh?

I wonder if export prices could be high enough to make it worth feeding-in electricity generated by a gas powered generator and use the heat produced by the engine that turns the generator to heat the house? Heh I wouldn't expect prices to ever be at levels where we could run a generator to feed power back to the grid and clear a profit, if we could do that it could be a perpetual money earner . But depending on prices maybe feeding some power back to the grid whilst making use of the heat the engine created while turning the generator would work out.

This https://gridwatch.co.uk/ is an interesting website. I suppose if we were feeding power into the grid at a fairly consistent rate questions might be asked how we were able to do that if our only other way of generation was solar panels that don't produce at night. But they shouldn't mind considering the gridwatch site shows generation from burning gas is consistently the biggest contributor to our electrical needs anyway...

[Gilbertd]

An interesting idea. You could use an exhaust heat exchanger as used for the heating on an air cooled car and a boat heat exchanger as used on a closed cooling system to extract the maximum heat from the engine to heat a house. Use the engine to drive a generator to provide power. Quite how the multiple efficiency losses would add up I've no idea though.....

[LPGC]

Yes that's the kind of thing I've been talking about.

Our boat engines and exhaust manifolds are raw water cooled, some boat engines use closed cooling with a heat exchanger. Of those that use closed cooling usually the exhaust manifold is still raw water cooled... but the exhaust manifolds can also be closed cooled if the heat exchanger is big enough, on such boat setups there is a 'dry joint' between the manifold and the riser, the riser and final part of exhaust are always raw water cooled, it needs somewhere to dump the water that passes through the closed cooling heat exchanger but without that flow of water through the final part of the exhaust and riser those parts would overheat due to the remaining exhaust heat.

For the domestic engine setup I've been talking about we could have radiator water running through the engine and exhaust manifold or a heat exchanger.

Either way there is still residual heat in the exhaust, that's why boat risers and final exhaust sections are still always raw water cooled. For what I've been talking about we'd want to make use of that residual heat, I said connecting the heat pump evaporator to the final part of exhaust could increase heat pump scop while at the same time making sure we don't waste any heat from the engine... Efficiency increased because we're making use of all the heat made by the engine and because we increased the heat pump scop.

[Gilbertd]

But you are still talking about using the engine in conjunction with a heat pump, why? The engine coolant temperature is going to be in the high 80s, mid 90s, so why bother, just run that through the radiators (if you still insist on using radiators).

[LPGC]

I don't insist on running radiators or even an engine. I saw the difference in price between kwh or gas and electricity and the thought occurred we could use an engine running from gas to turn a heat pump and it would cost the same to turn that heat pump as running from mains electricity plus we'd have the extra 'waste' heat from the engine that we could also put to use to heat the house.

Gas powered power stations don't use engines, they have multiple stage turbines etc and the final 'waste' heat from some such setups goes on to directly heat houses. We're not going to have a CGGT power station in the garden but we could have an engine.

Your idea about the exhaust to air heat exchanger could work, we'd need to fit ducting between the house room(s) and exhaust. As I see it the main problem with this is potential for CO poisoning if/when the exhaust heat exchanger fails. Would need to be very well insulated ducting or we'd expect to waste a lot of heat from the external ducting (presuming the engine is in the garden). Even if it were 100% insulated we'd still waste some engine exhaust heat because air returning to our heat exchanger through the return duct would be at least as hot as room temperature so our exhaust would still be at least as hot as room temperature.

The warm exhaust being wasteful is one of the reasons for the heat pump. The heat pump takes our remaining engine exhaust heat (after our heat exchangers have already extracted much of the heat) so now our exhaust temp might be colder than external temp, so now we haven't wasted any heat from the burning of the gas. The heat pump pumps that heat into the house/room. Could call this next bit the second reason for the heat pump but really it just increases the efficiency / scop of the heat pump. If heat pump scop is normally lower when external temp (evaporator) is very cold, the evaporator will be warmer if it is being heated by the exhaust, with the evaporator warmer the heat pump scop (efficiency) increases. A double whammy, which is why I said the engine and heat pump compliment each other.

The usual reason for having an engine is to produce kinetic energy from chemical/heat energy, this isn't very efficient because the engine does a better job of producing heat than kinetic energy. But we can use the kinetic energy to drive a heat pump (that's one form of heating) and also use the direct heat energy from the engine to heat the house (that's the other form of heating). Usually the engine wastes heat in 3 ways, there's the coolant to vehicle radiator way, the exhaust exit temp way, and the infrared/convected heat from all the external hot components including the exhaust... To maximise efficiency we need to minimise heat loss from all of those ways because they're each a significant part of where heat from burning fuel in an engine goes. Ideally the exhaust temp should be below atmospheric temp or some of the heat from burning the gas has gone into heating engine intake air (that will be at atmospheric temperature) and pumping the heated air out of the exhaust back to the atmosphere thus wasting it. If we didn't have the heat pump evaporator tied to the exhaust we could use a heat exchanger ducted air system but then our exhaust would still be at least as hot as room temperature. Perhaps use both a heat pump and ducted air heat exchanger on the exhaust, the heat pump would be at the final stage of exhaust before the tailpipe, but if we're using engine coolant to heat radiators and we need to keep that water system completely separate from the heat pump water system it's already a 2 stage system before we decide to fit a ducted system too and 2 stages seems already complicated enough lol.

I imagine we would need to keep the heat pump water (radiator) system separate from the engine coolant water (radiator) system? But how would adding extra heat to water circulating around the heat pump condensor affect the heat pump setup?

I've been thinking along the lines of the engine having it's own closed cooling system and water to water heat exchanger (with antifreeze coolant in the engine) but the engine and 'marine manifold' could be 'raw water' (radiator water) cooled. Probably a better idea to have radiator water (raw water) flowing directly through the engine, if radiator water didn't have antifreeze it wouldn't be much improvement to have the engine full of antifreeze because the radiator water in the heat exchanger could still freeze up anyway. I used to think it was illegal to put antifreeze in domestic radiators (because there's just a stop tap between the rad system and potable water system, stop tap opened to fill the rad system... but if the stop tap fails or is left open by mistake it could poison your house potable water)? That said I see all sorts of additives heating engineers add to radiator water and I doubt they're all safe to drink. If we could fill the domestic radiator system with antifreeze coolant all the better and I had no need to be concerned about the engine being off during winter while the houseowner is away on a skiing holiday with the heating system turned off, antifreeze in the common radiator and engine coolant system would prevent the engine, exhaust heat exchangers and even the domestic radiator system and plumbing from freeze damage.

I suspect whether or not any such system would be viable would be down to the devil in the detail, not least the gas versus electricity prices and not least how heat pump scop varies over temperature. Another factor which you've' said doesn't really apply to the air blowing systems you fit as much to the air to water heat pump radiator systems that are still more often seen that the type you fit (and I do take your point on that) - Scop isn't the be all and end all of air to water radiator type heat pump setups, there's also the maximum radiator temperature that can be achieved for a given external temperature? If it's -5C outside and you can only achieve +15C inside because your radiators don't get hot enough it won't be much conciliation that you're achieving that 15C indoors temp efficiently and cheaply, so you might turn on a couple of hotwire electric heaters or run a couple of indoor gas fires anyway... But sticking the heat pump evaporator sink onto the exhaust of an engine can help to increase the maximum radiator temp that the heat pump can achieve? And you can achieve 90C radiator temp anyway even without the heat pump if radiators are plumbed to an engine cooling system? Like I said early on in this thread I don't expect anything I've suggested here to become main stream, just a niche, maybe the niche includes people who already have air to water (radiator) heat pump setups fitted but do need to use other forms of heating sometimes, for these people the addition of the engine and change of heat pump evaporator might seem a good option that would come into it's own especially on particularly cold days. Also not everybody has a suitable roof for solar panels.

At the throwing ideas around stage and welcome discussion on pros and cons of various ideas, not trying to defend any specific ideas.

Do we at least agree on the following... 1. If gas is 1/3rd the price of electricity and an engine is 33% efficient at turning gas kw into kinetic rotational kw it would cost the same to run a heat pump powered by an engine that is fuelled by gas as running the heat pump from mains electricity? 2. Then we have the heat produced by the engine and we can think of ways to use that heat to also heat the house? 3. The heat we get from (1) plus the heat we get from (2) is greater than the heat we would get from (1) alone and greater than the heat we would get just by running the heat pump from mains electricity, thus (1) + (2) works out cheaper to heat the house than running a heat pump from mains electricity plus could heat the house quicker plus could always heat the house to the desired temp even if the heat pump alone could not?

Maybe my point 4 will be more contentious.. 4. In cold weather conditions sinking a heat pump evaporator to an engine exhaust (which sees the evaporator far warmer than it would be if sinked to atmospheric air) increases both the scop (efficiency) of the heat pump and the heat pump's output (condenser) temperature? The scop is not linear with temperature, there is a sharp drop in scop when external temp drops to the kind of temperatures we often see in UK winters, if we increase evaporator temp by just a few degrees we get a response in scop / efficiency that more than compensates for the energy we put into heating the evaporator, especially in terms of money if we'd usually be adding extra heat to the evaporator by using electrical hot wire heating but instead we're effectively using gas heating at 1/3rd the price because the evaporator is heated by engine exhaust temp that is heated by gas?

The graph below is something I just pasted from the internet, I don't know how accurate this is for actual heat pumps but it shows cop changing for given input and output temperatures. I say cop isn't linear for input temp because the graph curves upwards for increasing input temp and there is a stark upward turn for external temps at around 0C. Which is why I say in (4) that increasing evaporator input temp by a little can lead to an increase in cop that more than compensates for the energy we put into heating the evaporator in cold conditions (scop is specific cop, at specific temperatures). But if we would usually (without the gas / engine heating the evaporator) increase the evaporator in cold conditions using electrical hot wire energy, since gas heat energy is 1/3rd the price of electrical hot wire heat we'd save money effectively using the gas to heat the evaporator instead of electricity.

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I didn't pick and choose the graph above, it was the first that a Google search returned. Edit, the second graph that Google returned is below, it suggests that heat pump cop doesn't reach 300% until external temp is as high as around 12degC, which to my mind with gas price being 1/3rd electricity price means if we would be paying for electricity from the grid to run a heat pump the heat pump would cost more to heat a house than a gas boiler if external temp is below 12degC. Don't know what other people do but we don't even turn the heating on for around 6 months of the year, which means we run heating for around 6 months, and during around half of those 6 months so 3 months daylight hours are very limited (so solar panels probably not much help with heating bills if running a heat pump) and external temps are likely to be consistently below 12C. The graph below is also curved, every one I've seen is curved. The problem with the rising rate of efficiency versus input temp is that when the heat pumps would be most efficient is when we least need to use them and when they are least efficient is when we most need to use them, the curve in efficiency versus temperature exaggerates that aspect/problem. Some heat pump setups do have hotwire electric heaters that heat the evaporator to shove up the scop and the output temperature, it would be more economical to heat the evaporator by burning gas than to use electric hotwire since the hotwire always has cop of 1, close to gas cop which without condensing feature is slightly below 1 but still very close to 1, condensing pushes gas cop even closer to 1, but gas is 1/3rd price of grid electricity by kwh so if cop for electricity and cop for gas are both 1 gas is 1/3rd price of electricity. My suggestion of heat pump on engine exhaust seeing low final exhaust temp would see water in exhaust gas condense.

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If everyone in the house goes out to work the same 8 or 9 hours including commuting and you all sleep for 8 hours you might only need normal room temp during around 8 hours per working day but if you work from home you might want the house to be at normal room temp for 16 hours per day. Any day everyone in the house isn't going out, like maybe Sat and Sunday, you might want the house to be at normal heated room temp anyway. Not sure how solar energy (if you have solar) would fare in contributing to running a heat pump but I might guess that if we're talking the 4 room bungalow (small house) that Gilbert mentioned and if we're talking the kwh of solar generation for 5th and 6th Jan that Rob pointed to (generated by panels on a barn roof, decent solar setup(?) though most 4 room bungalows don't feature a barn roof) of 5 or 6 kwh per day I wouldn't think the solar panels would run a heat pump without importing electricity from the grid over the necessary hours per day - If each of the 4 rooms of the little bungalow need 1kw of heat the heat pump needs to output 4kw of heat, if the heat pump has cop of 4 it needs to draw 1kw of power, the 5 or 6kwh would run the heat pump at 1kw for 5 or 6 hours, that's if the heat pump has cop of 4 but I'd expect scop at cold external temps below 12C to be well below 4 and below 3, in which case the solar panels (with battery) would run the heat pump for less than 5 or 6 hours. That's for a 4 room bungalow with a solar setup on a barn roof, ymmv...

How much do solar panels and storage battery cost? How warm will a system based on heat pump powered by electricity with solar panels keep your house in the depths of winter and will you need to run other means of heating the house during cold times and at what cost? Instead how about my idea?

[robertXX]

my first full month on octopus agile March cost 34.62 and used 241.8 kw ...averages out to 14 pence per kw ,

not including the 18 quid a month standing charge.

and the solar payment was at 15 pence per kw .

solar production was 322kw

exported power was 105 kw

total hs usage was 368kw for that month .


some disparity due to using the fox.ESS app and the octopus app..the numbers dont seem to coencide all the time.

[LPGC]

First thoughts are that if you can buy electricity from the grid at 14p per kw, then assuming gas is around 7p per kwh (or if you don't have piped gas would be 7p if you did have it) pulls the rug out from under my idea.

But doing the fag packet maths again and assuming gas at 7p/kwh, electricity 14p/kwh... Now it costs us 1.5 x as much to generate electricity (or power the heat pump from the engine instead of powering it by electricity) instead of buying electricity from the grid and power our heat pump from the grid. But still for every kw of electricity or pump turning power our gas powered engine generator makes we still get 2kw of heat from the engine. So if we run the figures...

Say we'd need 1kw to run the heat pump, so we use 3kw of gas costing 21p per hour, assuming heat pump cop of 4 the heat pump makes 4kw, plus the 2kw of heat from the engine makes total of 6kw. Compared to if we run the heat pump from mains electricity it costs 14p per hour and outputs total of only 4kw. Then we can do 21/6 = 3.5p per kwh for the engine with heat pump, 14/4 = 3.5p per kwh, so at that it still costs the same (in pence per kwh) to heat the house from gas using the engine with heat pump as to heat the house using just the heat pump powered by mains electricity. I wouldn't suggest it would be still be worth using the engine based on those figures alone and probably even more not if solar panels are involved but I'm not sure how scop (variable cop depending on heat pump evaporator temp which comes down to external / atmospheric temp) and attaching the evaporator to engine heat to raise evaporator temp would affect those figures. I haven't included the negligible standing charge in the figures, though the standing charge does push up the effective all-inclusive price of grid electricity a little. I suspect that for freezing external temps the setup involving the engine would make a better heat to fuel cost ratio (due to scop) than running the heat pump from grid electricity though obviously it would be more expensive to install and maintain.

I don't doubt your figures Rob but it doesn't seem to make much sense that you can sell to the grid at 15p and buy from the grid at 14p. Does this imply that the grid look to make most of their money just for being an infrastructure service, or imply that people who decide to buy renewable electricity pay a premium?

[Pinger]

I think you are overestimating harvesting heat from an ICE and rather than utilising all of the 66% you will be lucky to get half of that.
Consider again temperature differentials. You want to raise substance #2 (sink) to the same temperature as substance #1's (reservoir) original temperature to extract all the heat from it. This cannot happen on its own as the temperature gradient will be zero when the two substances attain thermal equilibrium. As an example, take two substances of equal heat capacity and mass, one at 200C and the other at zero C. Introduce them to each other such that heat can be transferred between the hotter and cooler substances. When the two substances are at 100C ( the reservoir has given half of its energy, the sink gained that energy) no further heat transfer is possible and your original 200C reservoir of heat is still at 100C which means it still has half its original energy that you cannot access. Conversely, your sink is also at 100C - which is half of what you wanted. You cannot cool your reservoir from 200C to zero C and heat your sink from zero C to 200C which is what is required to extract all of the heat. It just cannot be done.
It is possible to further lower the reservoir temperature and thus extract heat from it but that requires mechanical intervention (another heat pump) which will cost in terms of mechanical work and the law of diminishing returns will be waiting to pounce.

edited to change first line from 'half of' to 'all of'.

[LPGC]

I don't see why we'd need to be lucky to harvest a very decent percentage of the heat an engine produces, condensing boilers effectively do the same thing at at least 90% efficiency. I picture the engine in a well insulated box and (with heat pump connected) it's exhaust temp lower than flu temp of a condensing boiler.

Hmm, I understand that the heat pump is pumping heat not generating heat, so any heat we get in the house means equal cooling outside or equal cooling of any engine we attach it's evaporator to. If we cool the engine using a heat pump we cool the engine and that means less heat from the engine coolant to heat radiators. I did kind of partly confuse 'heat pump' with 'impossible 400% efficient hotwire heater' if you take my meaning.

So if I concede that any extra heat the heat pump emits comes directly from the engine which comes from burning gas at close to 1:1 ratio we still have a system that might be better than a standalone heat pump running from the grid in very cold external conditions, not in terms of efficiency or cost to run but in terms of being able to get the house hot enough quickly enough? I know some heat pumps have built in hotwire electric heaters in the evaporator, supposedly (and at least in part) to prevent the evaporator icing in cold external conditions but what's not always said is that the heat from the hotwire is also going to mean hotter radiators? Instead of the hotwire electric heating it would be cheaper to heat the evaporator with gas, no question mark here because I don't think there'll be any disputing gas per kwh is cheaper than electricity per kwh and any heating of a heat pump evaporator by electrical means would be hotwire. If advice when running heat pumps is not to change the thermostat setting or risk lowering both heating power (by which I mean the setups ability to bring radiators up to temperature if heating a room that has been allowed to cool down) and scop efficiency(?) then heating the evaporator with gas could lead to lower running costs if it allows us to change house temperature without needing to worry about changes in house temperature being less efficient than keeping near constant house temperature... we turn it off or down to 14C while we're in bed at night or when everyone is out at work instead of leaving it on and set to 18 or 21C all the time? This still isn't necessarily conceding the engine because the engine could drive the heat pump and provide the heat for heating the evaporator but it makes heating the evaporator with gas instead of involving an engine look better. I don't know how many heat pumps do have heating of the evaporator or if any modern designs do but I imagine cop and scop are in a good part due to the properties of the refrigerant and in lack of an ideal refrigerant no heat pump will have high constant cop / scop that is completely regardless of the temperatures involved, likewise since cold produced is related to heat produced the evaporator will always need a defrost cycle / system when external temp is low enough if we want to keep the house warm enough yet nothing outside defrosts naturally when outside temp is below 0C.

[Pinger]

I don't see why we'd need to be lucky to harvest a very decent percentage of the heat an engine produces, condensing boilers effectively do the same thing at at least 90% efficiency. I picture the engine in a well insulated box and (with heat pump connected) it's exhaust temp lower than flu temp of a condensing boiler.

If it is possible, at what level of complexity?

Hmm, I understand that the heat pump is pumping heat not generating heat, so any heat we get in the house means equal cooling outside or equal cooling of any engine we attach it's evaporator to. If we cool the engine using a heat pump we cool the engine and that means less heat from the engine coolant to heat radiators. I did kind of partly confuse 'heat pump' with 'impossible 400% efficient hotwire heater' if you take my meaning.

I don't understand how a heat pump can be used to cool an engine given that a phase change in the fluid is required.

So if I concede that any extra heat the heat pump emits comes directly from the engine which comes from burning gas we still have a system that might be better than a standalone heat pump running from the grid in very cold external conditions, not in terms of efficiency or cost to run but in terms of being able to get the house hot enough quickly enough? I know some heat pumps have built in hotwire electric heaters in the evaporator, supposedly to prevent the evaporator icing in cold external conditions but the heat from the hotwire is also going to mean hotter radiators? Instead of the hotwire electric heating it would be cheaper to heat the evaporator with gas. If common advice when running heat pumps is not to change the thermostat setting or risk lowering both heating power and scop efficiency(?) then heating the evaporator with gas could lead to lower running costs if it increases efficiency if we want to turn the thermostat down when everyone is in bed or out of the house? This still isn't necessarily conceding the engine because the engine could drive the heat pump and provide the heat for heating the evaporator but it makes heating the evaporator with gas instead of involving an engine look a better. I don't know how many heat pumps do have heating of the evaporator or if any modern designs do.

This aspect, pre-heating the air that the heat pump draws its heat from intrigues me and I can see why a hotwire heater isn't ideal. Rather than an engine though, your idea of a gas burner would simplify the system considerably. That way you get the increase in scop and the driving of the heat pump compressor although electric, is a reducing draw as the system raises room temperature and shouldn't be too onerous on a system operating at increased efficiency.
Hard to be more definitive without knowing the rate of air flow that the heat is being drawn from but an increase of a few degrees from a high temp (flame) of the low temp air should be quick and effective.

[LPGC]

If it is possible, at what level of complexity?

I don't understand how a heat pump can be used to cool an engine given that a phase change in the fluid is required.

This aspect, pre-heating the air that the heat pump draws its heat from intrigues me and I can see why a hotwire heater isn't ideal. Rather than an engine though, your idea of a gas burner would simplify the system considerably. That way you get the increase in scop and the driving of the heat pump compressor although electric, is a reducing draw as the system raises room temperature and shouldn't be too onerous on a system operating at increased efficiency.
Hard to be more definitive without knowing the rate of air flow that the heat is being drawn from but an increase of a few degrees from a high temp (flame) of the low temp air should be quick and effective.

Makes my answer more readable if I answer your points in different order...

The heat pump isn't cooling the entire engine, it is only cooling the final section of exhaust before the tailpipe thus only getting a fraction of the heat the engine produces. If we look at most heat pump scop graphs the scop increases with input temperature. I know that if the input temp were sky high (like if the heat pump were using all the heat the engine produced or maybe even if it used most of the exhaust heat) the scop would again fall and we'd probably break the heat pump, but imagine we're using just enough heat from the engine to keep scop at maximum. Heat from the engine water cooling circuit and exhaust manifold heat exchanger directly heats radiators, we've already extracted most of the engine heat before the heat pump makes use of the final bit.

I don't see why having an engine in an insulated box is any different from having a boiler in an insulated box. Both need incoming air to burn with the gas, both need to have exhausts that have to be as cool as possible to not waste any heat via that route. If we measure the general inside temp of our boiler it's going to be at the same or higher than radiator temp inside, if we measure the temperature inside our engine box it's going to be at the same or higher than radiator temp inside, insulation is more important for our engine but that's because the engine is bigger than the boiler and because the engine is outside in the garden as opposed to inside the house but it doesn't matter how big it is or where it is if insulation is good enough. For both the boiler and the engine the exhaust temp is more critical than might first be guessed, this is also due to the 'phase change' aspect.. if it vents steam the steam will warm whatever it condenses on, the opposite to sweating that's going to be wasted heat if it doesn't happen inside our setup. The boiler and engine both produce H20 steam when burning the H from the HC fuel with O, if the steam condenses inside our system we get the benefit from the opposite of sweating heat, to get that from our engine we need the heat pump stuck on the exhaust... but instead of the heat pump evaporator connected to the final part of exhaust we could have a separate exhaust to water heat exchanger and pipe the heat from that to separate radiators in the house but that radiator system would run relatively very cool for heating the radiators while at the same time not cool the final part of engine exhaust below room temperature, less likely to be condensing too, so the heat pump on the exhaust is more efficient, hotter rad temps from the heat pump setup too.

If the heat pump is an air to water (to radiator) setup I don't know if it would be better (or necessary) to keep the heat pump water circuit separate from the engine water heating circuit (2 sets of radiators, perhaps the heat pump heating some rooms and the engine heating other rooms) or to have a common water circuit but I imagine if a common circuit we would need to choose radiator temperature carefully if raising heat pump condenser temp means raising heat pump evaporator temp and we're trying to use the evaporator to extract heat from the engine exhaust. There would be 4 places we could add extra heat to a house heated by a heat pump - directly to the house through some totally separate system (e.g. we have our heat pump setup running, lets also stick a portable gas fire or hot wire electric fan heater in each room), heat the heat pump evaporator, heat the cold side of the water circuit, heat the hot side of the water circuit. I didn't include adding heat to the heat pump condenser because that sees too similar to adding heat to the cold side of the water system. How would adding extra heat to each of these positions affect heat pump operation / cop?

Off on a bit of a tangent, has anyone else wondered about using a little box in the room containing a car sized heater matrix and electric fan, a piped hot water feed and return pipe and an electric power cable to each unit, these instead of radiators? The problem with radiators is as Gilbert said, the space necessary, the organisation of furniture to prevent hiding radiators if we want them to work efficiently, the lack of heat they radiate if water temperature in the system isn't hot enough. Air to water heat pump setups need more radiators because they don't get water as hot as a boiler? A matrix and fan setup need not be fixed, water connection ports and electrical power sockets could be placed at various positions around the room so such heater could be moved around in the room and extra units could be plugged in/removed at will. Much like a car heater can kick out quite a bit of heat well before the engine reaches normal operating temperature such setup could kick out more heat than radiators if system water temp is low. I've asked before if there's a link between heat pump max radiator temps for various external temps..? Safety concerns when mixing water with electricity in an appliance but we don't run away from central heating water pumps, washing machines or electric showers and all can have plastic bodies and plug into an RCD, heaters could feature internal water leakage detectors and built in RCDs.

[Pinger]

What I'm seeing here is one man's vision and it isn't apparent to onlookers (well, me!) how it all shakes out. I'm not saying it isn't feasible - merely that I don't have all the accompanying thoughts that make the vision real in the mind of its originator. What I can say though is at this stage in any design endeavour I'm involved in, I'd be concurrently researching the hardware required to make it happen and looking for the calculations that will give hard numbers as to what to expect in the way of performance.
Referring again to my (aborted) 2T gas turbine project, the projected efficiency gains by using pre-heating the incoming air (prior to combustion) from the exhaust stream (post turbine(s) were enormous. Then I looked at the heat exchanger required and it too was enormous - in bulk, weight, and cost. At least partly by the need for them to withstand the high temps but also the corrosive nature of exhaust products. IIRC it was in the order of 1kg / kW of turbine power - an additional 200kg for a 200kW turbine which was unacceptable for a mobile engine. I quickly abandoned any thoughts of using heat exchangers. In the 1960s virtually all the big players tried gas turbines for cars but none of them succeeded as they could not achieve a viable heat exchanger.
What I'm trying to say here is that sooner or later you will have to explore heat exchangers and for that you are going to have to have numbers to see the scale of heat exchange required. If heat exchangers suitable are available at acceptable cost then that element of the project can be considered sound but don't be too surprised if you find they are not. Now, would be a good time to find out - one way or the other.