Concentrated solar power: how it works
Tags: Technology, Solar Energy, Solar Power, Sustainability, Renewable Energy

Concentrated solar power: how it works

A fast growing solar energy technology is based on a technique familiar to Girl and Boy Scouts, writes Catherine Bischofberger

The most widespread CSP technology used in plants around the world is parabolic trough

There are two kinds of solar power plants: photovoltaics (PV) and concentrated solar power (CSP). PV works by capturing sunlight on modules (panels) covered with one or two layers of a semi conducting material, usually silicon, and converting it into electricity.

Using a magnifying glass to capture sunlight, as all Girl and Boy Scouts know, is a very efficient way to heat things up. CSP is based on the same principle, using lenses to concentrate sunlight to heat steam engine generators.

CSP is currently on a strong growth trajectory, according to the Renewables Global Status Report 2018. Spain has the largest installed capacity, followed by the United States, South Africa, India and Morocco.

Storing energy

Most CSP plants incorporate some form of thermal energy storage (TES). This means that they can store heat during the day and convert it into electricity at night time. This is one of the important selling points for CSP together with its high degree of production reliability.

CSP plants are built in sun-drenched areas, from Andalusia in the south of Spain to the edge of the Sahara desert in Morocco, guaranteeing predictable production levels and ensuring grid stability.

Four different CSP technologies are used in the various CSP plants installed around the world:

1. Parabolic trough — a mirror-based parabolic solar collector which reflects light so that it heats a tube containing a fluid to a high temperature. The heated liquid is then used in a heat exchanger to produce Steam which then goes into a Steam turbine driving an electric generator to produce power.

2. Solar power tower — a series of flat movable mirrors called heliostats focus the sun’s rays upon a collector tower with a receiver situated at its top. This device transfers the sun’s heat to a fluid which produces Steam and generates power in the same way as above.

3. Linear Fresnel — flat mirror stripes concentrate the sunlight onto a receiver containing tubes located above them. These reflectors use the Fresnel lens effect (a concentrating mirror with a large aperture and short focal length). Fresnel is the name of the French physicist who invented that type of lens. Power is generated in the same way as above.

4. Dish — the devices look similar to TV satellite dishes, but they are made of curved mirrors and lenses. Solar thermal energy is concentrated onto a single receiver point situated above the reflector dish. The dish can be connected either to a Steam engine or to a Stirling engine. A Stirling engine operates by cyclic compression and expansion of air or gas at different temperatures leading to a conversion of heat energy to mechanical work which then drives a generator.

Molten salts

While various fluids (water, synthetic oil…) have been used to produce Steam, new installations are increasingly employing molten salts. The heated salts generate Steam and are easy to store, decoupling energy collection from energy generation, which is crucial for TES.

According to SolarPACES, an International Energy Agency (IEA) network of researchers and industry experts, the most widespread CSP technology used in plants around the world is parabolic trough, followed by solar tower. The number of linear Fresnel plants is increasing, but remains low in comparison. In the network’s global listing, there are only two dish/engine projects.

Zoom forward

A large amount of research and development work is focusing on technology which can lower the costs of CSP and make it more efficient. Its drawback is that it is more expensive to generate than electricity produced by other forms of renewable energy sources, such as solar photovoltaic (PV) systems.

Research projects include an US lab’s high-temperature falling particle receiver, in which sand-like ceramic particles are heated as they fall through a beam of highly concentrated sunlight focused by an array of mirrors. The falling-particle receiver can process heat at significantly higher temperatures than existing CSP technologies (up to 1000 °C and higher), which can increase power cycle efficiencies and reduce costs. Like molten salt, the particles can be stored conveniently.

Alternative salts are also being considered, for instance chloride. The chemical compound can reach higher temperatures than the sodium/potassium nitrate used today.