Thermodynamics and Concentrated Solar Power

[Strat-O]

I am interested in building a thermal solar collector. I'm mostly interested in using the heat directly but there's always a need for electricity. The trouble is converting heat into electricity is pretty inefficient no matter how you do it. Consider a peltier device. A typical one, depending on your delta T might convert the heat energy at an efficiency of 10%.

In the attached diagram There is a parabolic dish that is 70% efficient and a peltier device that is 10% efficient, so the efficiency of the system in generating electricity is 7%, right? (neglecting heat losses due to lousy insulation)

Someone suggested that because the heat is recirculated after the peltier device that it was somehow more efficient than 10%.

Any thoughts?

 

[willem2]

What you haven't drawn is that the cold side of the peltier device has to be cooled.
The efficiency of the peltier device will depend on the temperature difference across it, so you'll need a radiator, or cooling water.
So there's a further loss, because the temperature difference across the peltier device itself will be less than the temperature difference between the tank and the outside air or cooling water.
Your efficiency also will go way down when the tank begins to cool if there's no more sun.
If the device gets 80C water from the tank and sends 60C water back to the tank, the heat from the return water isn't lost, but that wasn't included in the efficiency of the peltier device anyway. The reject heat on the cold side is probably lost, unless you can heat your house with it.

 

[haruspex]

converting heat into electricity is pretty inefficient no matter how you do it.

As perhaps you know, the theoretical maximum efficiency is determined by the temperatures involved. If the collector can get up to 530C (800K) and you have a cooling fluid available at 30C (300K) then the theoretical efficiency is 1 - 300/800 = 62.5%.

 

[Strat-O]

Thanks for the responses.

So the 63.5 percent is from the formula for Carnot's theorem. I've seen it before but I think I've got it commited to memory now by seeing it stated so simply.

Marlin

 

[sardar]

I can provide some insights on the relationship between thermodynamics and concentrated solar power. Concentrated solar power (CSP) is a technology that uses mirrors or lenses to concentrate a large area of sunlight onto a smaller area, typically a thermal solar collector. This concentrated sunlight is then converted into heat, which can be used directly for various applications such as heating water or generating electricity.

One of the key concepts in thermodynamics is the efficiency of a system, which is the ratio of the useful output of energy to the total input energy. In the case of CSP, the efficiency can be affected by various factors such as the design and materials of the thermal solar collector, the type of heat transfer fluid used, and the conversion method used to generate electricity.

As you mentioned, converting heat into electricity can be inefficient, and this is due to the second law of thermodynamics, which states that heat always flows from a hot to a cold object, and it is not possible to convert all of the heat into work. This is why the efficiency of a peltier device, which converts heat into electricity, is typically around 10%.

In the case of the system you described, with a parabolic dish that is 70% efficient and a peltier device that is 10% efficient, the overall efficiency of the system in generating electricity would be 7%, as you correctly calculated. However, as you mentioned, this calculation neglects any heat losses due to poor insulation, which can significantly impact the overall efficiency.

Regarding the suggestion that recirculating the heat after the peltier device could somehow increase the efficiency, this is not entirely accurate. While it may help to reduce heat losses, the overall efficiency would still be limited by the efficiency of the peltier device.

In conclusion, while CSP is a promising technology for utilizing solar energy, it is important to consider the various factors that can affect its efficiency, including the conversion of heat into electricity. I would recommend exploring other energy conversion methods, such as thermoelectric generators, which may have higher efficiencies in converting heat into electricity. Additionally, improving insulation and heat transfer methods can also help to increase the overall efficiency of a CSP system.