# Thread: Efficiency of a system

1. I have a concept of building a dual powered solar water heater. In day time, I directly use the solar power to heat the water using parabolic solar collector and at the same time I will be using a solar panel to produce current and this will be stored in a battery and will be connected to an Inverter. So, thus in day time I directly make use of solar power to heat the water and at night time I will make use of the current from the battery to heat the water.

My doubt is like will the whole system be thermodynamically efficeint?

2.

3. That's a bad idea. It's pretty easy to store hot water. Just keep it in an insulated tank. Use your electricity for things that need electricity, like the refrigerator, lights, etc.

4. Thanks Harold for the reply. Actually the whole concept in principle wise it is right but what I want to know whether it is thermodynamically efficient or not. If it is not efficient then please tell why it is not efficient. When I shown this concept to my sir he told in principle point of view it is right but in thermodynamic point of view it is not efficient.

5. No, it is not efficient. Your solar cell is probably at best 44% efficient in converting the incident radiation to electrical energy. Solar cell efficiency - Wikipedia, the free encyclopedia On the other hand a solar collector will convert most of the radiation to heat. Then you have to store your electrical energy in a battery, which is an expensive proposition. Batteries have a limited lifetime and need maintenance. It costs energy to make batteries. You don't want to store electrical energy if you don't need to.

6. So, you mean all the device that are working using solar power are not even 50% efficient? So the amount of photons that hit the solar cells are not completely converted to electron carrier i.e current. In that case why is that all the photons that hit the solar cells are not completely converted into electron carriers? Can you please specify what are the losses

7. Did you read the Wikipedia article?

8. Originally Posted by kraman.157
I have a concept of building a dual powered solar water heater. In day time, I directly use the solar power to heat the water using parabolic solar collector and at the same time I will be using a solar panel to produce current and this will be stored in a battery and will be connected to an Inverter. So, thus in day time I directly make use of solar power to heat the water and at night time I will make use of the current from the battery to heat the water.

My doubt is like will the whole system be thermodynamically efficeint?
My estimate for that is a nighttime efficiency of about 10%, a daytime efficiency of around 75%. Note:

1) Parabolic collectors are cumbersome, dangerous, heavy and expensive. Flat plate collectors work a lot better.
2) Batteries and inverters are a lot more expensive than hot water tanks.

Comparison:

It takes about 8 kwhr to heat a 60 gallon tank of water 60 degrees. The battery bank to do that would be about 16kwhr or 12 T-105's - total cost around \$1800, and would have to be replaced about once every two years. Add in \$2000 for an inverter, \$500 for a charge controller and \$20,000 for the solar PV, grand total of around \$25,000 for the system.

Or you could just store the hot water in a tank. \$300 for a tank that will last 12 years. Add another \$2000 for the solar thermal panels and \$500 for the pumping and other plumbing needed, for a total of about \$3000.

9. Originally Posted by kraman.157
So, you mean all the device that are working using solar power are not even 50% efficient?

10. I don't understand why you would need to use anything, battery or otherwise, to heat water at night. All you need is a large hot water storage tank.

At my previous house we rarely ran out of hot water and we didn't use the mains power to heat overnight except for a few weeks in winter - and we could have done without that anyway. I can't remember the size of the tank but it was a good size. If you want to do without any heating other than solar, the main thing to do is to ensure that most of your hot water use is early in the day. It's not how the water you use was heated that matters, it's how the replacement water in the tank will be heated. So showers, washing, even dishwashing, should be done in the morning and the rest of the day left for the solar heater to work undisturbed.

The main thing I'd concentrate on would be insulating the tank and. double. insulating. the. pipes. that take hot water to the various taps. You don't want that precious heat wasted on warming the ceiling or wall spaces.

11. All this work is going on toward batteries right now, but its kind of cool to remember there are so many ways to store energy if they don't have to be lightweight. If your water tank is well insulated enough, it could conceivably store steam, which could be used as energy storage. Not saying this is feasible or efficient, just that its technically possible. In fact, you could store energy by lifting up a big rock on a rope very slowly with many gears, and get energy back by letting the rock fall (slowly) to turn the engines. Or you could store energy as compressed air. The question is, as you know, efficiency. But I find the fact that there are so many ways to do it to be pretty cool.

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