Terrestrial Solar Energy Applications

Unfortunately, the story of solar energy (S) applications is one of high cost and low efficiency Inexhaustibility, and cleanliness also though Some use the majority of available S, but for others, system efficiencies of nearly 10 % are sometimes considered good. The most common methods of terrestrial S use are solar thermal, photovoltaic, and simple air heating. Solar thermal is a term for use of S for heating fluids. Parabolic mirrors and reflectors are often used, with a pipe filled with fluid at its focal point. Distribution of S components (discussed in previous features) is a consideration because only direct S energy is efficiently collected using this method. Thus, a reason for considering cloud occultation, shadowing fraction, etc. for forecasting this. System efficiencies can be quite large, dark pipes absorbing most incident S. Photovoltaics is a term for semi-conductors which convert S to electricity. When exposed to S, electrons flow across the surface of solar cells. System efficiencies are very small though, only seldom greater than 20 %. Efficiencies of typical silicon-based solar cells are 10-18 % (most at the low end), and galluim arsenide solar cells 18-24 %. Thus, when researchers speak of breaking solar cell efficiency records, such is analogous with running a quarter mile during about 42 sec while a drag racer can do that in about 6 sec Peak absorption for solar cells is for near infrared electromagnetic radiation (about .8-.85 µm), very little absorption of blue (thus diffuse S) occurring. Solar air heating is achieved by exposing dark walls to solar energy, a method which can be used for much more than heating air. This is largely efficient because such materials absorb S well.

When solar power is mentioned, residential applications are often thought of. Other S applications include solar ponds, in which algae develops during warm and sunny conditions (which can be used as fuels), lights, radios, appliances, and personal computers. The most logical application (though requiring plenty energy) is solar air cooling. Unlike for solar air heating, S is most available when cooling is desired. Being completely free from external energy sources is possible (though presently rather expensive). Well-designed systems can provide as much power (main site) as would otherwise be required. I don't discuss these applications much, but abundant WWW resources exist.

I discuss topics I've been more involved with, e.g., geometry and estimation of S collection. Photovoltaic arrays consist of solar cells, typically grouped in a module. Each module is typically about a half square meter, though sizes greatly vary. Typical terrestrial solar arrays are only slightly more that 10 % efficient, such that a .5 m² array converts about 60 W with an incident S flux of 1000 W/m². Such a S flux with a standard spectral distribution, optical air mass 1.5, and 25 C temperature is called standard test conditions, which can be simulated and used as a reference for solar cell and array characterization. It is so called, because it approximates typical Summer midday conditions at midlatitudes. E.g., if a solar array module is rated at 60 W, such is approximately the energy obtained under typical sunny conditions facing a direction generally toward our sun. Collection is often worse, though tracking arrays can collect close to their rated amount for several hours during a day. If a solar array must be stationary, it is best if tilted equatorward at a slope angle approximately equal to the noontime solar zenith angle. If it must be stationary during an entire year, a slope angle equal to the local latitude is approximately best. Reflection of surroundings, mirrors, etc. should also be considered, and such affects can be modeled for various sky conditions, and averages obtained using climate data for specific time periods (e.g., a week, month, or year). Such data is available from the National Renewable Energy Lab, though a user should be aware that data for most locations is estimated using correlations with weather data from other sites. Such estimation is not always accurate, and the site includes discussion of such issues.

Perhaps the most exciting solar energy applications involve transportation. Solar boats have been constructed, some of which circumnavigate, and solar cars have been built and raced. I believe the first solar car race was the Tour de Sol in Switzerland during 1985. During 1982, Hans Tholstrup constructed and drove a solar car, Quiet Achiever, across Australia, from Perth to Sydney. During that time, he began envisioning a race for such vehicles across Australia, which became the World Solar Challenge (WSC). It involves professional, university, and high school teams, and is raced on a 3010 km route in the beautiful Australian desert from Darwin to Adelaide during the Southern Hemisphere Spring (late October or early November). The race is during a time of year before heat becomes awful, but temperatures can be more than 100 °F at northern locations not near shore. Rules, dates, and related info can be obtained from race organizers. You may notice that the race route is chosen such that cars go poleward, mostly with the sun at their backs (the noon solar beam is approximately directly over Katherine that time of year). Such increases solar energy collection for car's arrays, most which which slope backward for aerodynamic smoothness. Northern Territory University's entire solar car tilted sideways for increased solar energy collection during the 1993 race ! The first WSC was raced during 1987, won by GM's Sunraycer. A part of the Stuart Highway (the main race route) in the Northern Territory was not yet paved then - a tire change being required entering & exiting it ! Paul MacCready greatly contributed to its design and construction, and George Ettenheim (then from AeroVironment) to the Sunraycer Team's efforts with weather, logistic, and strategic info. The race has recently been dominated by Japanese teams, Honda's Dream winning the previous 2 races, requiring only 4 9-hour days with a few media stops during 1996. The main reason for absence of North American teams among the leaders is differing race regulations between the WSC and Sunrayce, which only allows terrestrial grade solar cells and lead acid batteries. Most collegiate teams can't afford a million dollar solar array for the WSC !, and corporate teams have not recently entered.

Another race planned to commence about a month from now is Sunrayce. Established with large contributions from the U.S. Department of Energy and General Motors, it involves collegiate teams from North America. The present race is a 10-day event from Indianapolis, IN to Colorado Springs, CO. University of Michigan won the first 2 races, during which I was a fortunate participant (and our WSC teams which finished 3rd & 11th). Though the 1990 Sunrunner was very well designed, our 1993 Maize & Blue was particularly aerodynamically smooth and shaped well for solar energy collection, and well-engineered, though a few problems existed. Perhaps I'll include more info regarding this (particularly solar energy estimation) as a much larger separate feature

The Winston Solar Challenge is a race for high school students, and was developed with a more educational than competitive philosophy, though it is also becoming a road race as those previously mentioned. Maybe someday they'll win the WSC

Text is copyright of Joseph Bartlo, though may be used with proper crediting.