What are they talking about?

Originally posted by DeepThought
1Do you agree that the amount of thermal energy you have to put into a generator is less than the amount of work you get out?

The pump has to supply as much heat as it takes to balance insulation losses as well as provide heat to run the engine. Since the engine generates less work than the extra heat you put in your electricity bill goes up and not down.[/b]

1. Yes of course. No such thing as a free (energy) lunch.

2. The pump extracts energy from cool air outside, warms the inside and
pumps out cold air. The insulation of my house is irrelevant. (So much so
that my 11 year-old thinks it OK to leave all doors and windows open!)

Could you answer my previous questions so that I can see where I'm going wrong?

Originally posted by wolfgang59
1. Yes of course. No such thing as a free (energy) lunch.

2. The pump extracts energy from cool air outside, warms the inside and
pumps out cold air. The insulation of my house is irrelevant. (So much so
that my 11 year-old thinks it OK to leave all doors and windows open!)

Could you answer my previous questions so that I can see where I'm going wrong?

Yes, the COP is bigger than 1, but this misses the point. A heat pump is a heat engine running in reverse. In the ideal case a pump and an engine are symmetric; if you have to supply 5kW of heating to an ideal engine to get 1kW of power out then, if you set it to pump mode, you'll have to supply 1kW of power to it in order to get 5kW of heating out. So if you have two of them coupled together the best you can hope for is break-even.

Suppose you keep your current heat pump and buy both an engine and a new heat pump. Provided that the new heat pump has the same efficiency as the old one it is like having one big heat pump of the same efficiency as two small ones. If I separate the function of heating the house from the function of providing heat for the engine it's easier to see what is happening.

The old pump continues to do its current job, keeping your house at 22 celcius against losses to the outside, it is powered by the mains. You choose settings on the new pump and engine so that all the heat the new pump pumps in is consumed by the engine. As I said above if they are ideal devices the engine supplies just enough power to keep the new pump running. This means the old pump has to do no extra work, but you can't turn it down either.

If the new pump and motor are less than ideal, which in real life they will be, then losses will ensure that there is an energy deficit. The heat lost is lost to the outside, because that is the cold reservoir and heat goes from hot to cold. This means that the old pump will have to work harder to keep the room at 22 celcius. The change in your electricity bill will be positive definite.

I've been assuming the engine would be transferring heat from the house to the outside - but there is an alternative way of hooking it up. The air coming out is at a lower temperature than the outside air. If that cold air was used to cool down one end of a heat engine and you had the other end just in the normal air, preferably in the sun to get some extra heat from somewhere, you could generate energy. Although without the solar boost I doubt you'd get more than a few watts. You won't make any net gain by having the engine between the house and the box for the reasons given in the post immediately above - in fact with a realistic engine you'd be at risk of a deficit because the pump would have to work harder.

This only works because your heat pump is doing too much work - it supercools the air relative to the outside. The combined system would still consume more electricity than an ideal pump would, but maybe you could improve the effective efficiency of the pump.

I've got my doubts as to whether it's economically viable though - a cheaper and simpler alternative might be to put a cold box under the cold air outlet and switch off your fridge.