Students try to make solar power viable

Cincinnati Business Courier ^ | December 13, 2002 | Laura Williams-Tracy

[edwin hubble]

Yes..

1 KW per sq. meter insolation rate (avg).
and 10% efficient cells = about 100 Watts per sq meter.

So... a 12 amp 110 volt air conditioner (1.3 KW) would take about 13 square meters of panels in direct sunlight (normal to sunlight). About 130 sq ft.

[steve86]

Just remember something like a small air conditioner will draw in the order of 12 amps at 110 volts. That's a pile of solar cells.

My brand new RV air conditioner draws 8 amps at 110 volts, putting out 8,000 BTU, which is still in the same ballpark. There is no need for the solar cells to put out that same current. They simply charge the battery array, which then feeds the A/C through an inverter. The 2500 watt inverter can supply quite a lot of current, but not necessarily for as long an interval as you might want. If necessary, the battery array is recharged by generator.

[RLK]

Solar cells are horrible to engineer. The voltage output on a P/N junction goes down as temperature goes up. If the cells are cooled, power output goes way up, but it takes power to cool the cells. You also need to get the produced current out of the junction. If you make the surface think enough to be low resistance, it blocks light. Metal coating on the surfaces block light. For personal use such as air conditioning the demand for current from the units goes up as sunlight intensity goes up. In cold weather when there is demand for heat, winter sunlight conditions are poor. It requires a lot to store energy produced in the day to last through the night. On a cloudy day, forget about it.

[RLK]

The 2500 watt inverter can supply quite a lot of current, but not necessarily for as long an interval as you might want. If necessary, the battery array is recharged by generator.

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Under the situation you described with auxiarry back-up a solar cell system can be useful. Go for it.

Myself, I use all solar cell run TI scientific calculators. I won't have anything else.

[Mortimer Snavely]

A bunch of servo mounted old CDs synchronised to the earth's rotation, all constantly focused on the same point, or other similarly synchronised battery of mirrors, would generate a whole lotta heat indeed. How in blazes could one keep enough power up to run the water pumps for the conjectured turbine, though? Would the energy bled off to run them, while the rest of the system charged a storage battery at the same time it kept the microcontolled servos running, drain the whole system? How big a pump would be necessary to feed enough water to supply such a turbine? How much current would it require? And so on.

The list of problems involved would be the basis for a nice little future homestead power supply, perhaps.

[boris]

The problem is, you get 1 kilowatt per square meter at the surface of Earth with a perfectly clear sky, at the equator, at high noon.

Solar cells are about 30% efficient, and it is night-time about 50% of the time, so you now have 150 watts per square meter. Since you need to plan for night-time operation, maintenance, clouds, bird droppings, the actual number is more like 100 watts per square meter.

This is a poor use of real estate. To power California--which uses 40,000 megawatts, 24/7, you'd need at least 150 square miles of solar panels. If solar cells cost one cent per square centimeter, this works out to (as I recall) something like $30 billion. This also ignores the utter waste of replacing our existing infrastructure with solar cells.

Yes, you can get a lot of heat if you concentrate the rays of the sun. But energy is energy; you simply cannot reduce the total collecting area you need. If your concentrator gives you "100 suns" you'll need 100x less solar cell area...but the concentrator (big fresnel lenses/parabolic reflectors, etc) will still require 150 square miles for California alone. Can you imagine 150 square miles of fresnel lenses? Or 150 square miles of aluminum mirrors?...

--Boris