Heat pump with no (electrical) running cost?

[gareth evans]

If the inside of the house is already slightly warmer than the outside (or could be made so by a brief use of an electric fire to get things moving) could a Stirling Engine be used to operate a heat pump?

ISTR that there have been historic uses of Stirling Engines to operate refrigerators and other industrial appliances before the widespread availability of electricity.

Now that the cost of electricity is soaring, could Stirling Engines arise from the grave to give us effectively free heating?

Yes the Stirling Engine does use up some heat but the whole basis of the heat pump is that you get out more energy than you put in.

[Brian]

Are you thinking of Absorption refrigerators?

They are still used- not as widely as previously. Three way fridges in caravans / motorhomes are absorption fridges. They have gas burners, 12v and 240v heaters - hence the three way.

They really on expansion - like Stirling engines- but I’ve not heard them referred to as Stirling fridges.

There were domestic gas fridges years ago.

Some small, ‘hotel room’, fridges are absorption fridges as they are silent

- although more modern ones rely on Petlier effect devices.

Heat pumps used to warm homes etc are, in simple terms, fridges turned inside out. Rather than cooling the inside ( the house) they cool the outside, albeit in perceptibly. In the process, the energy ( which in a fridge is radiated at the rear) is transferred to the inside of the house. At least that is the theory.

Just as fridges struggle in a cold environment ( remember you shouldn’t place a domestic fridge in a shed unless it is rated for lower temperatures), air sourced heat pumps need a reasonable temperature differential to be efficient or a large ‘catchment’ capability to compensate.

[David Wade]

I think what Gareth is trolling us with is a classic "perpetual motion" device. So use a Stirling Engine operated by the heat moved inside by a heat pump to operate the heat pump to pump the heat inside.

Dave

[gareth evans]

Hardly, for I already covered the case that you extract more energy than you put in.

The fallacy in your suggestion is that you suggest that even with an electric motor driving the heat pump that you only get as much energy back from the pump as you put in with the motor.

Perhaps there is a bit of an analogy here with the injectors on a steam locomotive where high pressure steam from the boiler is used to inject feed water back into the boiler against that very pressure, but the energy to do it has come from the burning coal.

There is one ppoint of concern with heat pumps in general, whether from the air or from underground and that is as the result of the pumping, the external heat diminishes until it is replenished by conduction and convention currents which is fine when there is only one such pump in a neighbourhood but if there are multiple pumps in the area then they are all diminishing the heat in that area and so the replenishment is duly slowed down to the detrimant of all.

(I have deleted the evident non sequitur troll by the Brian character)

[Brian Gaff]

I don't think that is possible. You are talking perpetual motion and any friction in a device will render it impossible except maybe at ridiculously low temperatures. Brian

[GB]

Heat pumps work, but you do have to put some energy in. What you get out is a multiple of what you put in. For air source heat pumps, the heat you get out is around 3 times the amount of electricity going in. The problem is that daytime electricity is so expensive that there's no saving compared to a gas boiler.

So, instead, heat up a store of energy overnight, using cheap rate electricity? Unfortunately, that doesn't work well. The multiplier goes down the hotter the output, so you would need a very large (but only moderately warm) heat store in order to get good efficiency.

So, how about gas fired heat pumps? They exist, and the government commissioned a report that said they are a great option. Unfortunately, the minister decided that they were not going to back them, so they are very expensive.

Gas fired heat pumps have a multiplier of around 1.5, but that still means a 30%+ saving in energy costs.

Unless you are proposing a perpetual motion machine, a gas fired heat pump may be what you have in mind.

[Robin]

A well-insulated solid floor designed to act as a thermal store can get at least part-way there. That is, you may need to run it 24/7 in really cold weather. But they don't usually include in the costs of switching to an ASHP the cost of replacing suspended floors by solid.

[David Wade]

That is true, but a Stirling engine works by moving heat in the opposite direction. The figures I have seen indicate its about 10-20% efficient so it is going to cool the room faster than the heat pump can warm it.

Dave

[SteveW]

It would also be very difficult to do in many houses - all my downstairs power cabling, all the cabling leaving my consumer unit, much of my plumbing, dozens of satellite, terrestrial TV, phone, ethernet cables, and gas pipes, all run under the suspended floor.

[gareth evans]

Why does everyone seem obsessed with perpetual motion, for that is not what I had in mind.

You said to expect 3 times heat output compared to the energy put in.

[gareth evans]

Another contributor has suggested that the energy returned from a heat pump is 3 times the energy put in to make it work.

(I have no figures myself)

[GB]

But, you can get that any time you like by buying one off the shelf. In what way are you attempting to improve on that?

[The Natural Philosopher]

That is possible in a new build. Masses of concrete with heating wires in under the floor. A massive storage heater

Or a huge swimming pool in a cellar, insulated to its eyeballs and filled with hot water.

Circulate that round the CH system, and add several large immersion heaters to it timed for overnight.

[Robin]

it's for net zero so you wouldn't need the gas pipes :)

[The Natural Philosopher]

The issue is really "you cant fit heat pumps efficiently except to new builds "

[Smolley]

Look at this:

[David Wade]

So for 1 unit of energy into the Heatpump, you get 3 units into the room. You put these 3 units into the Stirling Engine and you get between between 0.3 or 0.6 units out, so not enough to drive the heat pump...

.. there are other issues in that a Stirling engine needs a high heat differential to work....

... and I am sure I am missing something more fundamental....

Dave

[Theo]

Which is not true, as my mediocre-ly insulated 1960s bungalow (with an uninsulated concrete floor) will attest.

Theo

[nib]

Well, the first answer would be that the combined system of the Stirling engine and the heat pump would form a machine capable of moving heat from a colder body to a hotter body without any external work being done. That would contravene the First Law of Thermodynamics, so unless something very unusual is going on it's going to be impossible.

It might also be worth thinking about the Stirling engine. If it is working over a small temperature difference, say between inside the house at 20C and outside at 10C, then its maximum thermal efficiency would be (293 - 283) / 293 = 3.4%. That's not going to get you much of the 33% of the heat output power you need to drive the heat pump.

nib

[Tim Lamb]

Should be a doddle here as the underfloor heating runs at less than

50deg C. Could even go for a deep bore as the sub surface water is only a couple of metres down.

It has crossed my mind that off peak battery charging for transport is likely to impact on cheap rate electricity.

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