A TRACKING SOLAR CONCENTRATOR
COST AND PERFORMANCE
We will probably be spending around five or six hundred (1980) dollars, or possibly much less if you're a good scrounger and willing to modify the design to fit available materials. You should achieve around 20,000 Btu of thermal energy which is equivalent to about 6000 watts of electric heat any time you have unobstructed sunshine, summer or winter.
For feel, a quick recovery electric water heater heats at the rate of 4500 watts and costs a bundle to run. Call the power company and ask them for a nominal cost per kilowatt and multiply it by six for the hourly value of the solar collector. If it's gas heat that you want to replace, ask the gas company for a nominal cost per therm (100,000 Btu). Multiply the rate by 1.5 because you only get about two-thirds of the heat when you burn it. Divide this figure by five to get the hourly value of the solar collector. This is a course way to estimate it, but it is only an estimate. We'll continue with an electric comparison
I have relatively cheap electricity from water power here in Idaho costing a nominal 3 cents per Kilo-watt-hour. Thats about 13 cents per hour for a 4500 watt water heater. The sun shines here about 65 percent of the time according to the local Weather Service. Call them in your town or a nearby City and ask for a copy of the Local Climatological Data sheet. It's free and therein you'll find lots of good information in addition to the percent of sunshine. Keeping it simple (see Duffie & Beekman if you don't like it this way) assume an average 12 hour day- subtract a couple hours looking over the neighbors trees, buildings and nearby mountains leaving 10 hours. 65 percent of this time times 365 days per year comes up to about 2400 hours of sun per year. At my 3 cent rate, that's worth about $420 bucks. And I'11 bet your electric rate is higher than mine! Trouble is of course- Summer days are longer than the 10 hour average while the winter days are shorter and likely to be cloudy more of the time. Like me, you'll be short of energy in the winter at times and have an excess in the summer, unless you have a swimming pool or other summer only load.
Another factor to consider in your cost is the benefits available from IRS or your State Taxing Authority.
Now you've seen some typical figures and even if you're sloppy in construction and alignment and lose some, there is probably lots of heat energy to spare. As we go for higher temperatures, above the nominal 140 F water for your domestic use, you'll lose some performance due to collector radiation and convection losses, but it's relatively small,
A few simple facts and conversions are handy to know:
- A British thermal unit, Btu, is the energy needed raise one pound of water one degree F
- Water weighs about 8 pounds per gallon
- To raise 40 gallons of water from 50F to 150F takes:
40 x 8 (150-50) = 32, 000 Btu
- To raise 40 gallons of water from 50F to 150F takes:
- If you want to express that in watt-hours simply divide
by 3.4
- 32, 000 Btu/3. 4 = about 9,400 watt-hours
more commonly expressed as 9.4 Kwh
Collector performance is easy to measure before you tie it into a closed loop. Simply hook up a garden hose to it-- allow the water to trickle through until things are stable. Then measure the inlet temperature, the outlet temperature and the flow rate. The easiest way to get the flow rate is to measure the time in seconds to fill a one gallon container.
- gallons per hour = 3600/seconds per gallon
- Btu = 1 gallon x 8 Ibs per gallon x (temp rise)
- Btu per hour = 1 x 8 x 3600 x (temp rise)
- or simply 28, 800 x (temp rise)/time in seconds for one gallon
Collector tests should be made at various flow and temperature rises You will find that the Btu per hour figure falls off with higher temperatures. This is simply because of re-radiation and convection losses and explain why high temperatures are impossible to achieve from large surface flat-plate collectors. If your outlet temperature is nearly equal to the surrounding air temperature, the figures obtained should very nearly indicate the ultimate performance.
