Photovoltaics, space power

Since the first photovoltaic (PV) cells were fabricated for the U.S. space program in 1958, PV technology has evolved from once being a very high cost but essential and effective space power source to later becoming a small but diversified and enduring worldwide industry (1 6). Led by firms based in the United States, Japan, and Germany, this industry serves multiple markets (see Photovoltaic cells). 

We can combine one or more arrays with batteries to store the electricity generated by the photovoltaic cells. We use photovoltaic to power emergency phones along highways, satellites, the international space station, private houses, and even solar cars. 

Solar cells have been used extensively and successfully to power sateUites in space since the late 1950s, where their high power-to-weight ratio and demonstrated rehabiUty are especially desirable characteristics. On earth, where electrical systems typically provide large amounts of power at reasonable costs, three principal technical limitations have thus far impeded the widespread use of photovoltaic products solar cells are expensive, sunlight has a relatively low power density, and commercially available solar cells convert sunlight to electricity with limited efficiency. Clearly, terrestrial solar cells must be reasonably efficient, affordable, and durable. International efforts are dedicated to obtaining such devices, and a number of these activities have been reviewed (1). 

Photovoltaic devices typically consist of a series of thin semiconductor layers that are designed to convert sunlight to dkect-current electricity (see Semiconductors). As long as the device is exposed to sunlight, a photovoltaic (PV) cell produces an electric current proportional to the amount of light it receives. The photovoltaic effect, first observed in 1839, did not see commercial appHcation until the 1950s when photovoltaic modules were used to power early space sateHites. Many good descriptions of the photovoltaic phenomenon are available (7). 

A photovoltaic material generates a voltage when it is exposed to light and photovoltaic can be considered as a specialized area of optoelectronics. The principle has been known for many decades but it became a industrial reality only in 1958 when an array of photovoltaic cells, based on single-crystal silicon, provided power for a space vehicle. 

Photovoltaic (PV) solar cells, which convert incident solar radiation directly into electrical energy, today represent the most common power source for Earth-orbiting spacecraft, such as the International Space Station, where a photovoltaic engineering testbed (PET) is actually assembled on the express pallet. The solid-state photovoltaics, based on gallium arsenide, indium phosphide, or silicon, prove capable, even if to different extents and with... 

Cells made from GaAs are more costly than silicon cells, because the production process is not as well developed, and gallium and arsenic are not abundant materials. GaAs cells have been used when very high efficiency is needed regardless of cost such as required in space applications. They were also used in the Sunraycer, a photovoltaic-powered electric car, which won the Pentax World Solar Challenge race for solar-powered vehicles in 1987. It ran the 3000-km from Darwin to Adelaide, Australia at an average day time speed of 66-km per hour. The 1990 race was won by a... 

The use of fuel cells on the Gemini series (and all subsequent) space flights run by NASA is well known. However, there are no diurnal variations of solar light in space, so that photovoltaics can provide the power for most space stations (as in the Russian Mir ) and on longer space flights. 

While there is tremendous potential for solar energy to contribute substantially to the future carbon free power needs, none of the routes listed above are currently competitive with fossil fuels from cost, reliability, and performance perspectives. Photovoltaic solar cells have been around for decades and have been widely dep loyed in space vehicles. Terrestrially, their utilization thus far has been limited to niche applications or remote locales where less expensive electricity is not available. Costs for turnkey installations were 6 10 times more expensive in 1999 for solar... 

Stack of bandgaps perfectly matched to the solar spectrum, the ultimate conversion efficiency at 1 Sun intensity increases to about 66%. For practical purposes, the stacks have been limited to two or three p-n junctions actual efficiencies of about 32% have been reported in laboratory-scale photovoltaic cells with two cascaded p-n junctions, and a world-record 40.7% efficiency has recently been reported with three bandgaps, but with a solar concentration factor of 240 (press release, Spectrolab Inc., 6 Dec. 2006). Although the tandem cell efficiencies are very impressive, their cost is very high, and their main present use is for space applications. However, terrestrial applications using solar concentrators to reduce the net costs are being developed and may lead to competitive PV power costs (McConnell and Symko-Davies, 2006). 

Figure 12.13 illustrates a versatile experimental set-up for microwave conductivity measurements with the microwave source (8 0 GHz), a circulator and a detector, which monitors the microwave energy reflected from the electrochemical or photovoltaic cell. The cell and electrode geometries are designed in such a way that the microwave power can reach the energy-converting interface (losses in metal contacts or aqueous electrolyte should be minimised). Depending on the experimental conditions, time-resolved, space-resolved or potential-dependent measurements are possible as well as combinations (for further details, see Schlichthbrl and Tributsch, 1992 Wiinsch et al., 1996 Chaparro and Tributsch, 1997 Tributsch, 1999). 

NASA provides spacecraft with power by extensive use of photovoltaic cells. In this application, the lightweight of the cell, coupled with the almost continuous availability of sunlight in space, has made the photovoltaic cell a bargain almost independent of the cost. In space applications, collectors made from single crystals of silicon or gallium arsenide are used. These systems are very expensive. 

GaAs cells have often been used when very high efficiency is needed regardless of cost as required in space applications. This was also the case with the Sunraycer, a photovoltaic-powered electric car, which in 1987 won the Pentax World Solar Challenge race for solar-powered vehicles. It... 

Solar cells (photovoltaic cells) convert simlight to electricity. Photovoltaic cells are made of semiconductor materials such as silicon and gallium arsenide. When light strikes the cell, photons are absorbed within the semiconductor and create electron-hole pairs that move within the cell. This generates the energy that is used to power space vehicles. 

---