Energy alternatives : Solar

Not to take anything away from the miracle of photosynthesis, but even under the best conditions plants can only turn about 1% of the solar radiation that hits their surfaces into energy that anyone else can use. For comparison, a standard commercial solar photovoltaic panel can convert 12–18% of the energy of sunlight into useable electricity; high-end models come in above 20% efficiency. Increasing manufacturing capacity and decreasing costs have led to remarkable growth in the industry over the past five years: in 2002, 550 MW of cells were shipped worldwide; in 2007 the figure was six times that. Total installed solar-cell capacity is estimated at 9 GW or so. The actual amount of electricity generated, though, is considerably less, as night and clouds decrease the power available. Of all renewables, solar currently has the lowest capacity factor, at about 14%.

Solar cells are not the only technology by which sunlight can be turned into electricity. Concentrated solar thermal systems use mirrors to focus the Sun's heat, typically heating up a working fluid that in turn drives a turbine. The mirrors can be set in troughs, in parabolas that track the Sun, or in arrays that focus the heat on a central tower. As yet, the installed capacity is quite small, and the technology will always remain limited to places where there are a lot of cloud-free days — it needs direct sun, whereas photovoltaics can make do with more diffuse light.