Tower power

solar thermal
At the top of the tallest solar tower (left), a receiver converts the absorbed solar radiation into heat. Image credit: DLR

Pamela Largue profiles Germany’s landmark solar thermal test centre.

The Deutsches Zentrum fàƒ¼r Luft- und Raumfahrt (DLR) is Germany’s research centre for aeronautics and space. The DLR operates the Institute for Solar Research, which researches and develops concentrating solar power (CSP) for solar thermal power plants that convert the sun’s rays into electricity, heat and fuel. One of the major experiments underway at the institute is a solar thermal power plant centred around a solar tower located in Jàƒ¼lich, some 60km (37mi) from Cologne. The project has been operating as a solar thermal test facility for commercial tower power plants since 2009 and is unique to Germany.

Thisà‚ articleà‚ wasà‚ originallyà‚ publishedà‚ inà‚ Power Engineering International Issue 1-2021. Read theà‚ mobile-friendlyà‚ digimagà‚ orà‚ subscribe to receive a print copy.

Solar thermal plants utilise energy from the sun to heat a fluid to a high temperature and the fluid then transfers its heat to water, which in turn becomes superheated steam. The steam is then used to turn turbines in a plant with the energy converted into electricity by a generator. There are different types of solar thermal plants ” linear concentrating systems, including parabolic troughs and Fresnel reflectors; solar towers; and solar dish systems ” although they all make use of mirrors to reflect and concentrate sunlight on a point. Parabolic troughs are composed of long parabola-shaped reflectors that concentrate sunlight onto a pipe that runs into the trough. The receiver pipe can reach temperatures upward of 400à‚°C as the trough focuses sun’s rays at up to 100-times its normal intensity. The fluid in the pipes is heated, returns to heat exchanges, where the heat is transferred to water generating superheated steam. The steam moves the turbine and produces electricity.

The site hosts more than 2,000 heliostats that direct the Sun’s rays to the top of the tower

Parabolic dishes are thought to concentrate sunlight more efficiently than troughs, with the internal fluid reaching temperatures upwards of 750à‚°C. In solar towers, the tower acts as a receiver for sunlight by standing amidst a large number of solar mirrors, or heliostats. Within the tower is a mounted heat exchanger where the heat exchange fluid is warmed. Electricity is produced when the hot fluid is used to create steam to run a turbine and generator.

We are striving to further increase efficiency and also reduce the cost of electricity production

DLR’s uber solar towers

The Jàƒ¼lich project consists of two solar towers that contain four test chambers, where solar irradiation experiments can be carried out. The Jàƒ¼lich site hosts more than 2,000 heliostats that concentrate the solar radiation and direct the sun’s rays to a central receiver at the top of the tower. The Institute of Solar Research explains the process that takes place at this point:

“The radiation heats a circulating heat storage medium to very high operating temperatures (around 560à‚°C for molten salt circuits, up to 900à‚°C for particle systems and up to 680à‚°C for air systems like in Jàƒ¼lich)”. The heat then generates steam that drives the turbines and generates emissions free power. For CSP generators, which store the sun’s energy as heat before converting it to electricity, molten salt allows power plants to continue generating electricity even at night, due to its efficient storage of heat.

Why use molten salt? Because it’s a medium to store large amounts of heat with relatively small volumes of fluid, ensuring a more stable power supply from intermittent sources like solar. The larger tower in Jàƒ¼lich stands at 60m high and can produce up to 1.5MW of electricity. The power can be fed into the local medium-voltage network, providing electricity for research purposes. In 2020, the research team expanded the test facility with a second tower with three test levels on which experiments can take place simultaneously; a process made possible by the control software of the mirror field which can align subgroups of mirrors to the different target areas of both towers. According to DLR, a particle receiver is being built on the upper level accommodating experiments with ceramic beads as a heat transfer, storage and transport medium. The middle level is equipped for process engineering applications, such as research on high-temperature processes for solar water splitting.

The lower level hosts the DLR’s current research on molten salt as a carrier medium for high-temperature heat. This is also where the pump, tank and heat exchanger for this system is installed and used. The aim of the DLR research at Jàƒ¼lich is to achieve higher temperatures and better efficiency in order to reduce electricity production costs, among other things. According to the site’s project information: “The focus is on mirror systems for directing and concentrating solar radiation, solar absorber and energy storage systems and their effective use, as well as theoretical and IT-supported analyses and developments in the field of fluid mechanics and heat transfer.

The new multi-focus tower (right) contains three test facilities. Image: DLR

“Depending on the development status and goal, individual components, functional groups or even a complete solar power plant system can be tested, evaluated and optimised.”

Miriam Ebert, project manager at the German Aerospace Centerà‚´s Institute for Solar Research, said: “We analyze how liquid salts behave at even higher temperatures. Our goal is to raise the salt temperature to 600à‚°C.

“In doing so, we are striving to further increase efficiency and also reduce the cost of electricity production. On a small scale, the molten-salt circuit in our pilot plant works almost like a larger, solar-thermal power plant. This means that our findings can be scaled up to an industrial level.”

These types of plants are thought to be more efficient in terms of converting steam to electricity, are more cost-effective due to storing heat rather than power, and are capable of producing dispatchable baseload power.
The low emissions and clean energy make the solar thermal power plant a popular choice for all countries beyond the borders of Germany. We look forward to seeing what the DLR Institute of Solar Research can deliver in terms of emissions-free solutions to drive the energy transition.

No posts to display