Concentrating Solar Power

Solar troughs are the most widely deployed and energy-efficient solar thermal technology.

Dish engine systems eliminate the need to transfer heat to a boiler by placing a Stirling engine at the focal point.

The PS10 solar power tower near Seville concentrates sunlight from a field of heliostats on a central tower.

Concentrating solar power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as a heat source for a conventional power plant or is concentrated onto photovoltaic surfaces . Although a wide range of concentrating technologies exist, the most developed are the solar trough, parabolic dish and solar power tower. Each concentration method is capable of producing high temperatures and correspondingly high thermodynamic efficiencies, but they vary in the way they track the Sun and focus light.

Concentrating solar power systems are divided into concentrating solar thermal (CST) and concentrating photovoltaics (CPV).

History

Concentrated sunlight has been used to perform useful tasks from the time of ancient China. A legend, since proved to be a myth, claims Archimedes used polished shields to concentrate sunlight on the invading Roman fleet and repel them from Syracuse. In 1866, Auguste Mouchout used a parabolic trough to produce steam for the first solar steam engine.¹ Over the following 50 years, inventors such as John Ericsson and Frank Shuman developed concentrating solar-powered devices for irrigation, refrigeration and locomotion.²

Concentrating solar thermal

CST can be used to produce renewable heat or electricity (this last one, generally through steam). CST systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as heat or as a heat source for a conventional power plant (solar thermoelectricity).

Although a wide range of concentrating technologies exist, the most developed are the solar trough, parabolic dish and solar power tower. Each concentration method is capable of producing high temperatures and correspondingly high thermodynamic efficiencies, but they vary in the way they track the Sun and focus light.

A solar trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The reflector is made to follow the Sun during the daylight hours by tracking along a single axis. A working fluid is heated up to 150-350 °C as it flows through the receiver and is then used as a heat source for a power generation system.³ Trough systems are the most developed CSP technology. The Solar Energy Generating Systems (SEGS) plants in California, Acciona's Nevada Solar One near Boulder City, Nevada, and Plataforma Solar de Almería's SSPS-DCS plant in Spain are representatives of this technology.⁴

A parabolic dish or dish engine system consists of a stand-alone parabolic reflector that concentrates light onto a receiver positioned at the reflector's focal point, . The reflector tracks the Sun along two axes. The working fluid in the receiver is heated to 250-700 °C and then used by a Stirling engine for power generation.³ Parabolic dish systems display the highest solar-to-electric efficiency among CSP technologies and their modular nature offers scalability. The Stirling Energy Systems (SES) and Science Applications International Corporation (SAIC) dishes at UNLV and the Big Dish in Canberra, Australia, are representatives of this technology.

A solar power tower consists of an array of dual axis tracking reflectors (heliostats) that concentrate light on a central receiver atop a tower, where there is a water (this can be sea water) deposit. The working fluid in the receiver is heated up to 500-1000 °C and then used as a heat source for a power generation or energy storage system.³ Power tower development is less advanced than trough systems and lower construction cost plus better energy storage capability. The Solar Two in Daggett, California and the Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain are representatives of this technology.

Concentrating Solar Thermal Power (CSP) is the main technology proposed for a cooperation to produce electricity and desalinated water in the arid regions of North Africa and Southern Europe, the Trans-Mediterranean Renewable Energy Cooperation DESERTEC.

Concentrating photovoltaics

Concentrating photovoltaics (CPV) is a term used when sunlight is concentrated onto photovoltaic surfaces for the purpose of electrical power production. Solar concentrators of all varieties may be used for this, often mounted on a solar tracker in order to keep the focal point upon the cell as the sun moves across the sky. (Reference: MSU-CSET Participation Archive with notation in the Murray Ledger & Times)

Simplest manifestation of the concentrator photovoltaics consists of a glass magifier in front of a solar cell. One of the earliest expression of concentrating photovoltaics (CPV) was presented at the 1979 Murray State University 6th Annual Regional Science Fair by Kalani Kirk Hausman. The 12-year-old local student presented concentration of solar energy onto single-layer silicon photovoltaic cells using glass magnifying lenses and plastic fresnel lenses.⁵

Serious reserach and development work on concentrator PV systems was conducted since 1970s. A linear trough concentraor system was tested and installed at Sandia National Laboratories in late seventies. The first modern point focus photovoltaic concentrating system was developed in the Sandia in the late 1970’s. This System used a point focus acrylic Fresnel lens focusing on water cooled Si cells and two axis tracking. Similar concept was used in other prototypes. Ramón Areces system, developed in the late 1970’s, used hybrid silicone-glass Fresnel lenses while cooling of Si cells was achieved with a passive heat sink.

Compared to conventional flat panel solar cells, CPV is advantageous because the solar collector is less expensive than an equivalent area of solar cells. CPV system hardware (solar collector and tracker) is targeted to be priced well under 3 USD/Watt, whereas silicon flat panels that are commonly sold are 3 to 5 USD/Watt (not including any associated power systems or installation charges). Semiconductor properties allow solar cells to operate more efficiently in concentrated light, as long as the cell junction temperature is kept cool by a suitable heat sinks. CPV operates most effectively in sunny weather, since clouds and overcast conditions create diffuse light which essentially can not be concentrated.

Low concentration CPV

Low concentration CPV are systems with a solar concentration 2-10 suns. For economic reasons, conventional silicon solar cells are typically used, and at these concentrations, the heat flux is low enough that the cells do not need to be actively cooled. The laws of optics dictate that a solar collector with a low concentration ratio can have a high acceptance angle, and thus does not require active solar tracking.

Medium concentration CPV

From concentrations of 10 to 100, the CPV systems require solar tracking and cooling, making them more complex.

High concentration CPV

These systems have concentrating optics consisting of dish reflectors or fresnel lenses that concentrate sunlight to intensities of 200 suns or more. The solar cells require high-capacity heat sinks to avoid thermal destruction, and to manage temperature related performance losses. Multijunction solar cells are currently favored over silicon, as they have a higher efficiency. The efficiency of both cell types rises with increased concentration; the multijunction efficiency also rises faster. Multijunction solar cells, originally designed for non-concentrating space-based satellites, have been re-designed due to the high current density encountered in CPV (typically 8 A/cm² at 500 suns). Though the cost of multijunction solar cells is roughly 100x that of a comparable silicon cell, the cell cost remains a small fraction of the cost of the overall concentrating PV system, so the system economics may still favor the multijunction cells.

Much of the original research into multijunction photovoltaics was sponsored by governments and the astronautics industry. More recently, the technical research and product development of CPV systems has grown due to investment in terrestrial electric generating systems. Recent technological advances in triple-junction solar cells by Spectrolab have yielded 40.7% conversion efficiency.⁶

In May 2008, IBM demonstrated a prototype CPV using computer chip cooling techniques to achieve an energy density of 2300 suns.⁷

Concentrating Photovoltaics and Thermal (CPVT)

CPVT technology produces both electricity and thermal heat in the same module that can be used in private homes. It increases the total energy output to 40-50%, as compared to a normal PV panel with 10-20% efficiency and produces more thermal heat in wintertime, compared to normal thermal collectors. The thermal system never overheats⁸.

Australian, American, Chinese researchers are exploring the Combined Heat and Power Solar (CHAPS) possibility, while Europeans are producing it ⁹.

By territory

United States

As utilities scramble to meet California's 20 percent renewable-energy target by 2010, Southern California Edison has signed the latest solar-thermal agreement with eSolar. ¹⁰

Contents