Posted on 07/15/2003 3:16:56 AM PDT by Boot Hill
Here is what the acolytes of solar power don't want you to know...
These are the essentials you need in order to appreciate the absurdity of using solar cell power systems as any kind of sensible alternative. After you read this, ask yourself again how much sense solar power really makes.
THIS IS WHAT HAPPENS TO THE SUN'S ENERGY WHEN
WE USE SOLAR CELLS TO GENERATE ELECTRICITY:
SOURCE | LOSS - % | POWER - W/m2 | |
---|---|---|---|
1. | solar constant | -- | 1370W |
2. | atmosphere | 27 | 1000W |
3. | clouds | 21 | 790W |
4. | sun angle1 | 49 | 403W |
5. | night2 | 50 | 201W |
6. | cell efficiency3 | 85 | 30W |
7. | dust/reflection4 | 10 | 27W |
8. | packaging5 | 20 | 22W |
9. | DC to AC inverter | 25 | 16W |
10. | storage | 30 | 11W |
Source Notes: 1. Calculated for both hour angle and a latitude angle of 37º. 2. See link. Continental U.S. average sunshine is 4.8 kilowatt-hours/ square meter/day, or 200 watts/square meter. That value is nearly identical with total losses shown for items 1-5 above. 3. See table on linked page. 4. Dust, bird droppings, scratches, etc. estimated to be about 4%. Reflections, per Fresnel's Law, would be another 6%. 5. See link for data sheet on typical solar panel. Data shows an overall efficiency of 10.3%, at nominal conditions. This is nearly identical with total losses shown for items 6-8 above. |
Net efficiency = 11.4 Watts/m2 or a mere 0.83% (!)
But read on, it gets worse.
Here is an example:
Siemens Solar (now Shell Solar) produces a popular line of large solar arrays intended for commercial, industrial and consumer applications. A big seller is their SP-150, supposedly a 150 watt unit that measures 1.32 square meters. The problem is, it only produces 150 watts under carefully controlled laboratory conditions where the incident light intensity is boosted to 1000 watts per square meter (unrealistically high, see items 2 and 3 in above table) and the PV cells are artificially cooled to 25º C. But when Shell tests that same unit under more realistic conditions of 800 watts per square meter and little cooling for the PV cells, the output drops to 109 watts. When sun angle and night time are factored in (see items 4 and 5 in above table), the average level of power production drops to a piddling 28 watts. (That is only 21 watts per square meter(!) which is nearly identical to the value shown for item 8 in the above table.) [+] [+]
In quantity, this unit sells for $700. That calculates out to $25 per watt. By way of comparison, the initial capitalization cost for a conventional power plant is on the order of $0.75 to $1.00 per watt. That makes the solar "alternative" 33 times more expensive than the conventional power plants of today, and we haven't even figured in the additional cost of the inverters and power storage systems that solar needs (or the land acquisition costs).
Solar proponents would be quick to point out that, while the capitalization costs may be higher for solar, they don't need to purchase the expensive fossil fuels that conventional plants use. While that is true, what they aren't telling you is that the cost of financing the much higher initial debt load for solar, is greater than the cost of the fuels that conventional plants use. (TANSTAAFL !)
Is there any use for solar power that makes sense?
Yes, solar power makes sense in those limited applications where the customer does not have convenient or economic access to the power grid, such as with remote country or mountain top homes. It is also useful for powering mobile or portable equipment such as utility, emergency, scientific devices, etc., where it is not otherwise feasible to hook to the power grid.
But other than those narrow exceptions, it makes no economic, engineering, ecological or practical sense to use solar power as a replacement for, or even as a compliment to, conventional power plants. Solar may have its' own specialty niche, but in no way does that rise to the level of an "alternative" to conventional power plants.
I agree with your general premise that the whole dynamic will change if PV cell cost decreases dramatically while efficiency increases. But here is the problem with the increasing PV cell efficiency. Notice in the table that the solar constant is 1,370 watts per square meter. While that is a very high intensity, it is spread over a very wide spectrum of wavelengths. PV cells have a sensitivity to a much narrower range of wavelengths. If the wavelengths of the sun's output matched the PV cell, efficiency would probably be in the 40-50% area. This lack of broadband detection is not a new problem and has affected virtually all applications of silicon/germanium detectors since their invention. I would speculate that billions have been spent on attempts to increase their sensitivity, but with very little progress to show for the effort. Does that mean that I believe it will "never" happen? I don't discount the possibility, simply because the word "never" (and variants like "always") are some of the most dangerous words in science and I don't believe in tempting fate!
harpseal says: "...since an average home needs in the range of 5KWhrs ..."
You might want to check that number. The average home uses closer to 24kW-hr of energy every day, or about 720 kW-hr per month.
--Boot Hill
(PS I didn't use watt-hours in my table, just watts per square meter averaged over all the variations found during a 24 hour period.)
--Boot Hill
It doesn't alter any of the data in the table whether you start your calculation with the solar constant or after atmospheric effects. The numbers all remain the same.
--Boot Hill
My PG&E bill runs in the $10-20 range...
Government should not be funding anything like this. Which, however, does not mean that science will never find a way to convert it to power which does make sense. The whole concept is in it's infancy. No way to know if it will ever pan out.
Choosing where to put our limited research dollars has always been a challenge. If you try to spread the money over all possibilities, you will get no where. But if I had to choose between, say, solar and fusion, I darn sure wouldn't pick solar.
--Boot Hill
You couldn't tell it from your vanity post. You aren't just comparing apples and oranges--you are comparing mustard seeds and watermelons. There "are" a few different sorts of solar cells, with widely differing efficiencies and costs. Hell, you don't even give the BASELINE PARAMETERS for your assumptions. Add to that the "strawman arguments" about hailstorm damage (the answer to that particular one is called "polycarbonate").
Your choice, but don't forget that, short of a hydrogen bomb, no-one has ever gotten more power out of fusion than was put in.
Solar is routinely producing power now.
Also, a diffuse power source (mutiple small generators) is much more robust, and less of a terrorist target than one big expensive central facility...
There is a very broad middle ground here, based on paradigm of localized use. It doesn't get rid of the need for large, non-solar power plants, but it does offer some relief.
What you've done is made a case against solar cells in their entirety. Your set of "narrow exceptions" misses a whole range of possibilities.
For example, it's possible to run a home air conditioning unit using power generated by roof-top cells. The highest demand and best opportunity to use the system occur at the same time: during periods of sunlight.
It's true that there are cost/energy return issues. But those are not unsolveable.
--Boot Hill
I can guarantee you that you are not getting 800 watts from a 2 m2 PV cell array.
--Boot Hill
--Boot Hill
I agree, which is why I started my previous post with the statement I made.
My guess is that this venture will be successful, but I wonder about maintence costs as well since these things do have moving parts. The design looks good (and you should be able to read it online in a couple weeks, Discover doesn't post the online issue until after the print issue has been out a while and this is the August 2003 issue). I bet that they will need maintenance every year or 6 months, at a miniumum to wipe of the glass enclosure that protects the unit from the weather but probably also to check the moving parts.
And how many watts at midnight, with a new moon, in mid-winter? In other words, what is the average output over a full year, which would tend to average out daily, seasonal, and weather variation?
You just couldn't do it and make any kind of economic sense (without government subsidies, that is). Let's look at an example.
1. We have a 3,000 watt window air conditioner unit running 8 hours per day for 180 days per year at a utility rate of 15¢ per kW-hr. Cost per year = $648.
2. When calculating PV output we will use no loss for night time, sun angle or dirt. The starting value of 790 W/m^2 is reduced by PV loss (85%), packaging loss (20%), heat loss (15%), inverter loss (20%), leaving 64.5 W/m^2. For 3,000 watts, that's 46.5 m^2 of PV panel array needed. The area of the SP-150 (for instance) is 1.32m^2, meaning that you need at least 35 of them at $700/unit or $24,500. Plus another $3,000 for the 3kW inverter and another $5,000 for someone to install and wire this system. Now we're up to $32,500. I'll give you the building permits for free.
3. But for that same $32,500, I could have run the air conditioner off the power grid for 50 years! And this does not even begin to account for the time-value of the $32,500 I had to plunk down on day one to start this venture!
So maybe you could explain to me again just why it would be such a good idea to buy the solar cells to run the air conditioner?
--Boot Hill
Bump.
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