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Bridging the Carbon Gap: Fossil Fuel Use for the 21st Century

Coal gasification as a transition plan to build lead time to develop sustainable, climate-friendly energy technologies.
Fortnightly Magazine - November 15 2002

insolation to electric-grid-connected buildings, or installations with relatively large roof areas compared to the volume of occupied space. They could use photovoltaic "solar roofs" to generate electricity to offset the purchase of grid power by using the net metering concept. This option involves increases in use of grid power during periods of relatively low or zero insolation, plus a buy-back by the local utility of any excess power generated during periods of high insolation, all at the prevailing rates. That poses a rather complex technological challenge, since the relatively low-voltage DC power generated by photovoltaic modules composed of amorphous thin-film, single crystalline, and polycrystalline silicon must be converted efficiently to 120 volt, 60-cycle alternating current. Also, the conventional electric meter must be replaced by an installation that meters both purchased and self-generated power. Nevertheless, the Clinton administration launched a one million "Solar Roofs" demonstration project in the United States that utilizes the net metering technology of the cooperating utilities. It has shown promising results.

In a moderate climate zone with an average annual insolation of about 1,500 kWh/square meter, a 200 m2 roof installation on a single-family residence operating at 10 percent system efficiency could generate 30,000 kWh/year, but with wide fluctuations of daily and seasonal output. This total is probably in excess of typical annual residential power requirements, except when electric air-conditioning is used. Fortunately, peak air-conditioning requirements coincide with peak periods of insolation. The use of photovoltaic power for electric heating, even with efficient heat pumps, is more problematical, because peak heating requirements coincide with the lowest levels of insolation.

British Petroleum, which has acquired a large photovoltaic equipment manufacturer (BP Solar, formerly Solarex), is also installing "solar roofs" in 200 gasoline service stations in the United States to demonstrate the feasibility of industrial PV applications. The installed cost of amorphous thin-film silicon arrays for solar roofs in new residential housing is expected to fall to $2.70 to $3.00/peak watt by the mid-2000s. That would bring the levelized cost of electricity down to nine to 10 cents/kWh in southern California and 12 to 13 cents/kWh in southern New York-both competitive with retail electricity prices. 17 This assumes net metering, home mortgage financing, and credit for income tax deductions of home mortgage interest.

The Intermittency Problem. The basic problem of making distributed or centralized photovoltaic systems cost-competitive, however, is how to deal with the large daily and seasonal variations in output.

Of course, it would require an area of only about 150 x 150 km of photovoltaic arrays-about the size of New Hampshire-to replace all 3.5 billion kWh of U.S. power consumption in 2000. 2 That assumes PV arrays operating at 10 percent efficiency and at a Middle Atlantic insolation rate of 1550 kWh/m2/year. However, it seems impractical to construct large central installations to feed power to the existing grid and provide for energy storage of sufficient capacity to offset the large seasonal variations.

Electrolytic hydrogen is a promising photovoltaic energy storage medium, but only for decentralized systems, in spite of the combined efficiency losses of water electrolysis and hydrogen reconversion