If one were to build a solar power plant to compete with coal or nuclear plants, a number of problems have to be overcome: cost, transmission, storage, and energy payback time. Solar shares with wind the problems of transmission and storage, but wind is cheaper. Solar thermal has an easy way to store energy for short periods, but this is not available for solar photovoltaic. Let us first consider the problem of cost. Solar cells made of silicon are expensive, but nonetheless 90% of installed cells are made of silicon because those were invented first. The fastest growing market nowadays is in thin-film solar cells, which are much cheaper. Led by First Solar of the USA, rapid buildup of solar power in Germany, China, and the USA is being done with thin-film cells.

To compete with standard energy sources in cost, the magic number of $1/W of peak installed power is sometimes quoted. Silicon cells have been working their way down in cost but are still far from this goal. Thin film, however, may have already reached “grid parity.” Where does this magic number come from? A rough calculation is given in Box 3.2 to show that it is quite reasonable.3435 The diluteness of sunshine means that central solar power would require lots of acreage. Box 3.3 shows that a solar plant generating the same power as a coal plant would occupy at least 100 km2 (10,000 hectares or 24,700 acres). Figure 3.28a shows what a solar farm looks like. It is a 100-hectare, 14-MW plant opened in 2008 in southern Spain. The 120,000 panels can handle 23 MW of peak power. Figure 3.28b shows an aerial view of the area, which was cut out of sunny wine-growing country. This amount of land is necessary to supply electricity for a small town of 20,000 homes.

Box 3.2 Price of Solar Cells for “Grid Parity” [4] [5]

Box 3.2 (continued)

which is three times higher. However, $1/W is the cost of the solar cells only. The cells have to be mounted, transported, and installed, and substa­tions have to be built to convert the low-voltage DC from the cells into high — voltage AC for the grid. Some mechanism must be built to store the energy for nighttime use, and long transmission lines have to be built to carry the electricity from the desert to population centers. The price for thin-film cells is reported to be as low as $1.18/W in 2009.35 However, First Solar executives estimate that the price of $1/W may have to be halved before grid parity is achieved.

Box 3.3 Covering the Desert with Glass

A typical large coal or nuclear plant produces 1 GW of electricity. How much area would a comparable central solar photovoltaic plant take up? Using the figure of 200 W/m2 given above for average solar radiation, we multiply by a solar cell efficiency of 8% to get a net power of 16 W/m2 from thin-film solar panels. More power is lost in the electronics and the inability to tilt the panels economically to follow the sun. A more realistic estimate for net power may be 10 W/m2 for a power plant. At this rate, a 1-GW power plant would require 100,000,000 m2 of space, the area of a square 10 km (6.25 miles) on a side. How much does it cost to cover such an area with solar cells? At $1 per peak watt or $5 per average watt, 1 GW would cost $5 billion for the cells alone. Compare this with covering the desert with other materials. Cheap plywood costs about $20 for a 4 x 8 foot sheet, 3/4-in. thick. This works out to $6.73/m2, or $670 million for 100 million m2, only about seven times less. Cheap win­dow glass costs about $58/m2 or $5.8 billion for 100 million m2. This is more than the $5 billion for solar cells! To produce photovoltaic cells at $1/W would be a remarkable achievement. Solar cells, which are glass coated with multiple delicate layers of semiconductor material, with electrodes, have to be manufactured at less cost than the retail price of ordinary glass!

With prices near grid parity, industrial investment in solar panels is expanding so fast that the numbers of dollars and megawatts given now will change rapidly. China is the largest manufacturer of solar panels, 99% of which are exported. China had only 140 MW of photovoltaic cells installed in 2009 but has plans to expand to 20 GW (gigawatts or thousands of MW) by 2020.36 The USA plans to have 5-10 GW installed by 2015. Spain added 2.3 GW in 2008, catching up with Germany’s 5.8 GW already in place.37 First Solar has ramped production to 192 MW/year, but at this rate many manufacturers will have to participate in the growth of central-station solar photovoltaic.