Q: What are the different types of concentrating solar energy technologies? Why are they limited to the southwestern United States? -- Bertha Z., Berea, KY
There are two main types of concentrating solar energy technologies: concentrating photovoltaics (CPV) and concentrating solar thermal (CST). Together they are commonly referred to as concentrating solar power (CSP), although sometimes CSP is used interchangeably with CST.
1. Concentrating photovoltaics (CPV) uses lenses or mirrors to focus or increase the sun's light on a photovoltaic solar cell or panel. This technology includes both a low-concentration approach, which increases the sun's magnification by less than 5 "suns," and high concentration approach, which can increase the magnification by hundreds of suns. High-concentration CPV uses focusing lenses to concentrate the sun's rays on a single, high efficiency solar cell that is very small, on the order of 1-centimeter square.
When you hear about a new world record for PV efficiency that exceeds 40%, it is generally this type of technology they are utilizing. CPV's "better mousetrap" uses less photovoltaic material (tiny, high efficiency cells), concentrates the sun and increases performance, hopefully enough to offset any additional costs.
2. Concentrating solar thermal (CST) technology uses mirrors to focus the sun's light on a heat capturing point, the heat from which can then be either used directly or converted to electricity. The three basic designs of CST are troughs, towers and dish-engine systems.
Troughs are set-up in large horizontal fields that contain long loops of piping (many kilometers for large installations). The pipes collect the 600+ degree (F) heat from light reflected off mirrors that concentrate the sunlight in a line on the pipes. Troughs have the longest proven operating history and the least number of unknowns for CSP technology project development.
Towers use a mirror field that is set-up around the tower. The mirrors focus sunlight on a heat receiver at the top that collects the heat and transfers it to piping inside the tower where is it circulated and used to make electricity. The design minimizes the field of piping to the vertical tower height to a few hundred meters and can reach temperatures in excess of 1000 degrees (F). While currently there are very few commercially operating tower installations, based on announcements, this technology may grow rapidly.
Dish-engine systems look like satellite dishes and focus light on a Sterling engine mounted on an arm in front of the mirrors. Each dish-engine is an autonomous generator—unlike the other CSP technologies that use a central power plant design—and utilizes a temperature and pressure difference to produce kinetic movement inside the engine, which is then converted to electricity.
An interesting development for troughs (and possibly towers in the future) is the interest on the part of utilities in "hybrid-solar power plants," which include the pairing or retrofitting of natural gas or coal power plants with the thermal input or boost from CSP.
The one thing that is common among the different kinds of concentrating solar power technologies is that unlike traditional photovoltaic panels, they need "direct normal" solar radiation, i.e. sunlight that can cast a shadow. A certain percentage of solar radiation is made up of diffuse or scattered light, caused by clouds, humidity or particulates. Solar resource measurements are reported as either "direct" normal radiation (no diffuse light) or total radiation (diffuse + direct).
The southwest has the highest percentage of "direct normal" radiation of nearly anywhere in the world, making this one of the best regions for development of CSP. However, there is one CSP trough project in Florida—a hybrid CSP plant that will augment a natural gas plant—and a number of trough and tower projects in Spain. CSP will work in both areas, but performance will be commensurately reduced based on the direct normal radiation profiles.
The CSP industry is growing fast in Spain and the United States, and SEPA is tracking over 5,000 MW of new project announcements that are slated for development over the next five years. Not all of them will be built—permitting, financing, technology and other factors need to fall into place first—but the industry is poised for rapid growth regardless of any individual project's outcome.
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