LDT Stirling and a heat pump

[dave___4442]

OK, I'll try not to ask a question with an obvious answer. I've
been studying Stirling engines for a few weeks, but this concept
caught my eye.

I live near a large, slow flowing river. So I have access to water
that is 10F to 20F warmer than the air for much of the year. I
realize this is a very small temperature differential to use to
generate power. Using this water as the heat source and the outside
air as the cold sink, would lead to a VERY low efficiency engine.
To get any power out of it at all, I'd expect it would need to be
very large, and that would lead to great difficulty with heat
transfer.
So what about using a heat pump to raise the temperature
differential? Would the efficiency of the heat pump be overcome by
the increased efficiency of the Stirling engine? (use the Stirling
engine to power the heat pump). So I'm wondering if using a heat
pump to increase the efficiency of the Stirling would net a greater
power output?

Or should I just forget this and try to get power from the river
in other ways? (it really is too slow to get much power out of it,
and there is no significant drop in height for a turbine).

If you have any pointers to where I could look into calculating
this, please let me know. Thanks in advance. Dave

[iron_goober]

Using a heat pump to increase the efficiency of the Stirling engine would decrease the total efficiency of the system. It would be like running your air conditioning in your car for a radiator system...it just doesn't work, you'd just be running your engine harder than using a radiator. 2nd law of thermodynamics. There isn't any way to get around it...stupid thermodynamics

[mbrownn]

Ive been looking at this. there are many heat pumps with a claimed COP of 10 to 1 assuming a temperature difference from 1 side to the other of zero degrees. as the difference increases the COP drops.

The Stirling on the other hand gets better but we will never reach the point where we have more power out than power used to drive the heat pump.

If we have an additional source of temperature differential we may well have more power out than is required to drive the heat pump but it would have frictional and other losses so we aren't going to gain overall compared to just running the Stirling on the source differential alone.

BUT lets not miss a point here if you have to run aircon units the energy generated could offset some of the cost of running the aircon, reducing carbon footprints too.

I only see 2 problems

the cost of the Stirling

the size of the Stirling required


Mick