Concentrated solar

Concentrating solar power (CSP) comes in two main forms—defined by the technologies employed to concentrate sunlight. The most spectacular version is where a large field of reflecting mirrors (heliostats) focuses sunlight on a cylindrical receiver mounted on a tower in the centre of the area of mirrors.

Typival ‘power tower’ arrangementn of heliostats

In full sunshine, the receiver atop the tower is brilliantly illuminated (and extremely hot) and is visible from a long distance. Dubbed a ‘power tower’, the technology was first employed at scale at Barstow in California in the late 1970s.

The second variant is called a parabolic trough, or linear trough system.  It consists of lines of multiple linear parabolic mirrors that focus sunlight onto a pipe running along the focal axis of the mirrors. The pipe contains a heat transfer fluid which conveys heat to a secondary thermal circuit that generates steam which drives a turbine and generator.

Other forms of concentrating solar power systems are the linear Fresnel lens technology, and dish systems where a parabolic dish focuses sunlight onto a small heat engine that generates electricity.  However, the power tower and the linear trough systems are the most common arrangement and for the moment the most economically viable of the four technologies. 

Parabolic trough solar thermal system

An important difference between concentrating solar power and photovoltaic systems is that CSP technology only captures direct insolation.  Unlike PV systems which can generate power from diffuse solar radiation—even when it’s cloudy, CSP technology is only feasible in regions which have an abundance of direct insolation.

The graphics below show schematically the two main concentrating solar concepts. [1]

The two principal variants of concentrating solar technology

It should be noted that the power tower concept schematic shown  (which is from the US Department of Energy) does not show any thermal energy storage—which is considered essential for the economic viability of all concentrating solar power systems.

In 2017, concentrating solar power (CSP) registered an additional 100 MW of capacity coming online, bringing global installed capacity to around 4.9 GW.  The table shows that while there is considerable interest in this technology, its installation as a mainstream utility-scale power technology is advancing only slowly.  Nevertheless, its ability to provide dispatchable and baseload power—when thermal energy storage is part of the plant—is a huge advantage compared to more variable sources of renewable power like wind and photovoltaics.  

In the US, two large-scale power tower plants were operating in 2017. In California’s Mojave Desert, the  Ivanpah solar electric plant–the largest in north America has 173,500 heliostats focusing sunlight on three towers. At full power, the plant is capable of generating 392 MW of electricity. In Nevada, the Crescent Dunes solar energy plant generates 110 MW from a field of 10,347 heliostats[2].

Ivanpah power tower installation in California

In 2017, for the second year running, South Africa led the market in new addition. That country was the only one to bring new capacity online, commissioning the 100 MW Xina Solar One plant–which has 5.5 hours of thermal energy storage.

An estimated 13 GWh of thermal energy storage—based almost entirely on molten salts—was operational in CSP plants at the end of 2017.  The majority of CSP plants under construction will incorporate some form of thermal energy storage—which continues to be viewed as essential to the competitiveness of the technology.

Parabolic trough and power tower systems dominate the market with 0.9 GW of trough systems and 0.8 GW of tower systems under construction at the end of 2017.  China’s CSP market is also gathering speed—the government announcing 20 new projects of all types: parabolic trough, power tower, and Fresnel lens systems, with a combined capacity of 1 GW.  Five plants totalling 300 MW are slated to be operational before the end of 2018.  Other countries building CSP plants include India, Morocco, Israel, Saudi Arabia, Chile, and Australia.[3]  

The largest plant under construction is a 700 MW CSP plant in the Mohammed bin Rashid Al Mouktoum Solar Park in the United Arab Emirates—which has both a 200-metre power tower and a field of parabolic trough collectors.

Spain remains the global leader in existing CSP capacity with 2.3 GW operational at the end of 2017. The US is in second place with just over 1.7 GW of installed capacity.  These two countries account for over 80% of global CSP power production.[4]

While power from CSP plants is currently more expensive than wind or photovoltaics, there is widespread confidence that costs can be brought down, and that this advantage–when coupled with the technology’s ability to provide dispatchable and baseload electricity–will eventually make CSP a mainstream renewable energy technology (but only where there is abundant direct insolation).  One US projection sees the levelized cost of electricity for CSP plants falling to $0.06/kWh by the end of 2020—without subsidies—which would make CSP a very attractive solar power technology.[5] 

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Check out these sources for more information:

[1] See: The US Department of Energy website:  //www.energy.gov/eere/solar/articles/power-tower-system-concentrating-solar-power-basics

[2] See: Power tower system concentrating solar power basics, at //www.energy.gov/eere/solar/articles/power-tower-system-concentrating-solar-power-basics

[3] See: Renewables 2018 Global Status Report. Op. cit.

[4] Renewable 2018 Global Status Report. Op.cit.

[5] Renewables 2018 Global Status Report. Op.cit.