Heat energy conversion

[deltawhy]

Hello,
I am an engineer currently in the design stage of a solar heat engine.

My question is two parts:
1. As I live in Canada, ambient temperatures fall pretty low during the winter months. Being that my background is electrical, I don't know much about stirling engines (carnot cycle). As the output of a stirling engine depends on the temperature differential, will it still work if t1 = -20 deg C, and t2 = +10 deg C?

2. Does anyone have any better ideas of converting heat to electrical energy (cheaply and quietly) other than external heat engines?


Thanks for your time,

Daniel

 

[LostHippie]

Stirling engines will work on very low temperature differentials...but because of this low delta T, the flow of heat through the system is very slow, and hence produces very low wattages at low delta T's. The only way to compensate for this is to either increase the temp. difference (in your case that would be a solar concentrator setup) or by making the engine larger to increase the net flow of heat energy through the system.

The latter option becomes unfeasible rather quickly due to the power/weight ratio being incredibly low, and due to the limitations of heat transfer through the working fluid. As you scale it up it either just stops working or becomes insanely expensive to build. This is typically why you don't see Stirling engines everywhere, because in theory they are very very efficient from a thermodynamic standpoint, but the design is not feasible for scaling up.

There needs to be more details...what is this being used for? What kind of power output are you looking for? I can probably offer a little more information once I know these details.

Peace,
LostHippie

 

[deltawhy]

This is going to be used for microgeneration applications. Power output levels would have to be between 10-15 kW. I know this can be done, the thing I need to know is if the sterling engine will run at colder temperatures (but still a wide temperature differential).
In terms of scaling it up, I could use a parallel arrangement of multiple smaller heat engines to produce a larger power output.
But mostly, the cold temperature is the limiting factor of my design.
Example:
solar energy is focused on a heat exchanger in the middle of winter, increasing the temperature of the glycol / water mixture. There will be a temperature differential, but the lower temperature will be below zero C.

Thanks for your time,

Daniel

 

[LostHippie]

Seems to me that as long as you have a working fluid with good heat transfer characteristics and does not change phase (from the gaseous phase) at the temperatures and pressures it will be subjected to inside the engine, and a properly designed Stirling engine, and there is a moderate temperature difference, then yes it will work.

I have also seen a multiple cylinder Stirling design, where all of the cylinders are connected to a common crankshaft. This would accomplish what you were talking about, and if you are concentrating solar energy via a collector then you should have no problem getting a temperature difference that would drive a larger Stirling engine.

I have also seen arrangements where people take old satellite dishes and coat the dish with a reflective material, and where the receiver usually is they will install a collector/stirling engine/gen. set. They then set up an array of these dish collectors. I know for a fact that this works because I have seen them in action.

Hope this helps, peace,
LostHippie

 

[moltenmetal]

Just to correct your thermo, a Stirling engine operates on the Stirling cycle. The Carnot cycle is an idealized concept where the system operates differentially and never deviates from equilibrium- it represents a mechanically impossible maximum work-from-heat efficiency.

30 C is a very low temperature difference. Can it be done? Sure- but it will need to be done on a massive scale (i.e. LOTS of capital cost) to liberate a useful quantity of energy.

Best of luck-

 

[willard3]

There are many thermodynamic cycles covered in a thermodynamics course which may be helpful to you before you design heat engines.