Solar power: new technologies

Concentrated Solar Power Technologies

Presentation of the main solar power technologies

You can design sun power plants with 5 main concentrated solar power technologies: Parabolic Troughs, Central Receiver Tower (or Power Tower), Fresnel, Stirling Dish and Solar Chimney technologies.

Parabolic Troughs

It is a proven technology. For an output of 50 MW it requires an investment of 250 Mio€. A 50 MW wide solar plant uses about 350.000 m² if there is no storage and 500.000 m² with a storage module.
The schemes are quite easy to understand. A collector is built to receive the sun heat and to redirect it with its mirrors to a receptor tube. The heat transfer fluid is then used in a classic turbine generator.

The collector has 3 key elements: the mirrors, the structure and the tube.
Specifications of one module of CCP:

• 12m long,
• 6m high,
• 28 mirrors,
• 3 tubes.

There exists different structures - Solargenix, Skal-ET and LS3 - but the mirrors have all the same dimensions: 1 500 cm x 1 700 cm and 1 641 cm x 1 700 cm.

In the tubes, the tempature reaches 400° Celcius. There are only 2 providers of tube worldwide!

Central Receiver Tower or Power Tower

There were some demo tests in the 80s but the first real commercial project works since 2007 (PS10). There are at the moment two other commercial project on the way (PS20 and Gemasolar).

The biggest existing project has a total output of 20 MW. The next image describes elements in a power tower field. It is always the same standard turbine process which convert solar power into energy it is just the way you receive and manage the solar power that really changes. The simplified goal is to make steam to power a turbine.

It is for smaller projects, from 2 MW to 20 MW. A 20 MW wide project needs about 175.000 m² of mirrors without storage. For 17 MW there needs about 300.000 m² of mirrors with storage.
The mirrors are always flat (but a small curvature of 5mm is possible by the customer on the structure), there is no standard dimensions it depends on the project. The pads and gluing are done by the customer.

In comparison with the CCP technology the molten salt used in central tower can reach higher tempature than the oil generally used in the CCP's tubes (520° Celcius vs. 400° Celcius). However the molten salts are very corrosive and expensive.
CCP structure moves on one axe and heliostats can move on two axes.
The technology is new so there are bigger technological risks in Central Receiver Tower since CCP have been working since around 1980. But cost reductions are said to be higher in Power Tower than in CCP and it is easier to adapt a Central Tower field to a particular topography.

Fresnel

The goal with Fresnel technology is to concentrate beam radiation to a stationary receiver at several meters height. This receiver contains a second stage reflector that directs all incoming rays to a tubular absorber.
The biggest existing project is Kimberlina (5MW) by Ausra. It reaches up to 280º Celcius.
This technology requires 3 mm flat mirrors with a small width (about 500 mm). The curvature is given by customer on the structure itself. It is finally less effective than Parabolic Trough but it also has a lower energy production cost.

Stirling Dish

The sunlight strikes the dish up onto to a single point above the dish, where a receiver (stirling engine) converts heat into electricity. It works on double axes and requires parabolic mirrors.
The main advantage is its higher concentration and it is suited for small projects (25 kW). But the drawbacks are a high maintenance and a high cost (because of a rigid frame and a strong engine). The biggest projects are in the US: Solar 1 and Solar 2, 1.7 MW with 70.000 dishes!

Solar Chimney

This technology is based on greenhouse and the lighter hot air effect. The first prototype was built in Spain in 1982 and its output was 50 kW. There are two main ongoing projects: the first in Spain too (50 MW) and a bigger in Australia of 200 MW.