# Using heat to lift a liquid – What's a good way of turning heat energy to gravitational potential energy?

Edit: A bit of context... Living in a country with extreme heat and a lot of hills, many farmers need irrigation pump systems (pump up water from a pond up to a reservoir). I have for example seen people build solutions using suncells -> electricity -> electrical pumps to lift the water to a reservoir.

This got me thinking about what is the most efficient way to lift liquids by heat alone (turning heat energy into gravitational potential energy)? What different routes are there?

Let's say we have a large amount of heat available at sea level.

We want to use this heat to lift a liquid mass (say for instance 100 kg of water) 50 meters above the sea level.

What would be the most efficient way of doing this?

I can think of different solutions:

1) We could evaporate the liquid to a gas, let it rise in a pipe 50 meters, then condensate it back to a liquid. But given that the energy required to evaporate liquids is high, and that would go to waste when it is condensated again, this seems inefficient.

2) If the amount of heat is really big, we could use a boiler & power generator to generate electricity and use a electrical pump...

3) Heat pipe / thermosiphon?

Other ideas?

• Homework? If so it should be tagged with homework-and-exercises. – Solomon Slow Aug 8 at 13:17
• No, actually not, I'll edit the question and add a bit of context! – user2078515 Aug 8 at 13:18
• The sun does it best. Unfortunately it doesn’t benefit everyone equally – Bob D Aug 8 at 14:14
• I think this is an excellent problem for someone inventive. When you say "most efficient" I would suggest you ask "simplest", or "most affordable". The solar-power solution is what first occurred to me, but there might be something better. – Mike Dunlavey Aug 8 at 14:51

In order to extract energy, it is not enough to have just a hot reservoir. If it's 120 degrees out, and everything around is at that same high temperature, there's a lot of energy in all that heat, but none of it is accessible. You also need a cold reservoir. The heat energy then wants to flow from the hot reservoir to the cold reservoir, and as it does, you can peel off some of the energy and transform it to something else, such as gravitational potential energy. The maximum efficiency with which this can be done is the Carnot efficiency, $$1 - T_{cold} / T_{hot}$$, which is that of a Carnot engine.