We like this characterization of thermal solar concentrators, “thermalvoltaic,” because it calls to mind the fact that thermal energy can be concentrated and turned into electricity just like light can - “photovoltaic.”  And as we review solar thermal here, it is important to note that the sun is only one source of thermal voltaic power - geothermal energy is another prime example. 

A parabolic trough array glows in the sunlight Photo: Schott Solar

Unlike the emerging photovoltaic concentrators, thermalvoltaic concentrators, more commonly referred to as solar concentrators or solar thermal arrays, have been around a long time. 

There are three primary designs of solar concentrators, all of which use mirrors to concentrate sunlight onto a heat transfer fluid which collects enough energy to drive a turbine which turns an electric generator: 

The “power tower” design consists of a field of mirrors which track the sun all day, each of them moving in a pattern that precisely bounces the sunlight onto a centrally located boiler that sits on a tower in the middle of the field.  The combined heat from hundreds of these mirrors causes the fluid running through the tower to super-heat, driving a turbine. 

A variation on this design consists of a field of parabolic mirrors, similar in shape to satellite dishes, with individual boilers heating fluids on each individual mirror instead of pointing to a central tower.  Each unit independently tracks the sun across the sky all day, pointing precisely at the sun so the entire surface of the parabolic mirror reflects sunlight onto the heating fluids.

The third, and apparently most cost-effective version of solar thermal concentrators is referred to as the “parabolic trough” design.  This design consists of a field of parallel mirrored troughs, each one of which can be hundreds of feet in length.  In the center of each trough, at the focal point of the mirrored surface, runs a tube that absorbs the concentrated solar rays and heats a transfer fluid. 

Because the parabolic trough design only rotates on an east-west axis, it is not quite as efficient as the other two designs which rotate on an east-west and a north-south axis in order to point directly at the sun all day.  But because the rotation is only on one axis, combined with the fact that parabolic trough units can be hundreds of feet long each, appears to give this design a cost advantage over other designs.

Late last year, when we caught up with Alex Marker, a Research Fellow with Schott Solar, our first question was “why aren’t there more of these thermal voltaic installations in the sunny spots around the world?”  Marker noted that the biggest - and until recent years one of the only - commercial scale complex of thermal voltaic arrays are in California’s Mohave Desert, built between 1984 and 1992.  (There are nine solar thermal power stations in California’s Mohave Desert, operated by Florida Power and Light, with a combined output of 354 megawatts.)  Marker claims, probably correctly, that until now “there wasn’t a compelling need for utilities to change their thinking on how to produce electricity.”

This is clearly true, since - aside from a hybrid solar thermal plant using parabolic trough design constructed about five years ago in Rajasthan, India, producing 140 megawatts - only now are solar thermal, utility scale generating plants being constructed again.  Currently they are mostly being built in Spain and the southwest of the USA.   Schott Solar has been involved, along with partner Solargenix, in the construction of a 64 megawatt parabolic trough array in Boulder City, Nevada, which broke ground early in 2006 and went online on March 2nd of this year.

According to Marker, the costs for solar thermal electricity could come down to around $.07 per kilowatt-hour, which is definitely a competitive price.  To get there, said Marker, the installed base in the world would need to more than quadruple, to around 4 gigawatts, so the expertise would be in place to basically start “cookie cutter” production of the stations.

One of the most interesting things about solar thermal power is that the necessary additions to the balance of plant in order to store some of the accumulated heat is not significant.  This means that the thermal energy generated during the day can be stored and used to continue generating power through the night.  This is a significant advantage.

Marker stated the power output per acre of solar thermal arrays was about five acres per megawatt.  Photovoltaic power, based on 10 watts per square foot, requires about half that much space, as 2.5 acres per megawatt.  And as we have demonstrated in “Power the World with Photovoltaics,” there is plenty of land available to pursue the solar electricity option, whether it is with photovoltaic, or thermalvoltaic technologies.