Friday, January 27, 2012

A step toward green

kw: solar energy, analysis

The Antelope Valley in southern California gets a lot of sunshine year-round. A company called eSolar and the city of Lancaster, plus the State, teamed up to take advantage of some of it. The resulting Sierra SunTower can produce 5MW of energy by heating two "sun towers" with sunlight focused by 24,000 large mirrors.

The array is impressive, as seen here in a Google Earth image. The entire site was originally 20 acres (8 hectares), of which the mirrors (heliostats) and their supports fill 14.4 acres (5.8 hectares). Two cleaned-off fields to the east indicate the company's intention to increase capacity by 50%.

The facility is presently used mainly to provide peaking power, and that mainly during air conditioning season, to about 4,000 homes. 5 million watts / 4,000 = 1,250W per home, about one-third of that required to run a central air conditioning system. When it is not needed for that, it is used for various experimental purposes. It is the only CSP (concentrating solar power) facility in North America.

The system is a hopeful introduction to what is needed to go for green energy in a big way. Peak solar input to the heliostat fields in June comes to about 32.5MW. Thus the areal efficiency is 15.4%, about the same as a ground-covering array of solar cells. In December, insolation drops to less than 19MW, and I presume that the system output also drops into the 3MW range.

How much ground would we have to cover to provide all the electricity needs for Lancaster city, provided that effective storage could take care of nighttime needs? City population is presently 157,000, so there are about 50,000 homes. I'll assume that half of these are single-family and half are apartments or condos, which require about 2/3 the energy of a detached home.

Assuming effective storage, the energy needed for a detached home is 2.7kW on a long-term basis during midsummer, and a little less during midwinter. Let's simplify matters by sizing the system for 3kW per single-family home and 2kW per condo or apartment. That comes to 250MW for all of Lancaster. A total of fifty SunTower facilities.

This shows a full section (36 square miles or 93 square km), which includes about 2/3 the city of Lancaster, and shows the SunTower facility just left of top center. It is not too intrusive just by itself, but what if we scattered fifty of these within the area of this image? Also, I couldn't find out what the construction cost of the facility was, but I suspect fifty of them would be a few billions of dollars.

Ongoing maintenance is a significant burden also. I read that the mirrors need cleaning after any windstorm that kicks up significant amounts of dust. Antelope Valley is short of water to begin with. How much water would be needed to clean 300-400 acres (120-160 hectares) of glass, once? That is just one element.

This is not to put down the effort. It is instead a clear-eyed look at the whole picture, so we don't go into such efforts with blinders on. We need to consider all aspects of a project like this, and put it in the perspective of the area it is meant to serve. 50x20 = 1,000 acres (400 Ha), or about 4.3% of the area shown in the image. That's what you need for a total solar solution, whether it is CSP or photovoltaic.

I am really a fan of solar energy. Solar cells also need cleaning, just as often as mirrors. I have invested in wind energy in the past, and at least windmills don't need frequent cleaning, but the "supply" is quite mercurial. CSP and solar cell technologies are at least beyond break-even, while wind is not, just yet. We need to put all these together with further innovation to survive the exhaustion of our fossil fuels, which is certain to happen during this century.

No comments: