Wednesday, 08 January, 2003

More on Solar Energy

Talk once again in the media about our dependence on oil in general, and foreign oil in particular.  The latest is The Detroit Project, an outgrowth of Americans for Fuel Efficient Cars.  Their claim:  driving an SUV is like sending a check to Osama bin Laden.  The idea certainly isn't new—see my November 3, 2002 entry.  But I think they're going a bit overboard.

With all the talk about our dependence on foreign oil, people are starting to bring up alternative energy sources again.  Today I saw a post from somebody claiming that solar is the "one true way."  There is an astonishing number of supposedly bright people who think that government and private industry have conspired to deprive us of this oh-so-wonderful power source.  I never was much of a believer in conspiracy theories, so I thought I'd do some research and run some numbers.

A reasonably good summary of what I learned is in this paper on Photovoltaic Power Generation.  According to my research, a good average number for the amount of sunlight that hits the earth's surface is about 1 kilowatt per square meter (1 kW/m2).  During daylight hours.  On a clear day.  The linked article has a map that shows the average daily sunlight, in kilowatt hours per square meter, for different parts of the country.  For Central Texas, that number is between 5.0 and 5.6.  Unfortunately, photovoltaic cells are not very efficient.  The high end for new, clean, commercial cells is about 17 percent, making that 5.6 kWh/m2--about 0.95 kWh/m2per day.  So how big an array would I need to power my house?

I pulled out our electric bills for the last year.  We average about 3,000 kWh per month, or about 100 kWh per day.  Dividing by 0.95 comes out to 105 square meters, or about 1,130 square feet.  So if I covered half of my roof in solar cells, I could generate enough power to run my house.  What would such a thing cost?

A good rule of thumb for the cost of photovoltaics is about $5.00 per watt.  A system to power my house needs to generate 100 kWh per day, but do it in 5.7 hours.  That means that I need a 17.5 kW system which will cost $87,500 for the photovoltaics alone.  I still need to add a system to store the generated power so I can use it at night or when it's cloudy.  The cost of photovoltaic cells typically makes up between 25% and 50% of total system cost.  So I'll have to spend at least another $87,500 on a storage system and associated hardware, bringing the total cost of my power system to more than I paid for my house.  Not friggin likely.  True, if the cells were 100% efficient, I could save about $70,000, but that does nothing for the storage system.  Even if I could do it for $15,000 it doesn't make sense.  At 5 cents per kilowatt hour (what I currently pay), it'd take me 100 months (8.5 years) to make up the cost, and by that time I'd have to replace the cells.

I'm all for cleaner energy, but not at those prices.