Solar thermal energy: Solar plants that work at night

To the north of the rugged foothills of the Spanish Sierra Nevada at an altitude of around 1,000 metres lies the high plain of Hoya de Guadix, one of the more remote areas of Andalusia. And yet, this secluded spot has been integral to the Spanish energy transition ever since 2007. This was when Andasol 1, the first sizable solar thermal power plant in Europe, went online. Since then, Andasol 2 and Andasol 3 have also been connected to the grid. Each of the three units feeds up to 50 megawatts into the Spanish power grid every day, even at night.

This is possible because the power plants do not convert the sun’s rays directly into electricity in the same way as photovoltaic (PV) systems do. Rather, they concentrate the sun’s rays to generate heat. This heat is then stored in molten salt tanks and saved for night-time use, for example. The stations use this heat to produce steam, which then – just like in a conventional power plant – powers turbines, which in turn drive a generator.

Parabolic trough plants and solar power towers

Two types of solar thermal power plants have proven effective thus far. Solar power towers use an innumerous number of mirrors to focus the sun’s rays onto a heat receiver installed on a tower or hill. The mirrors move automatically to face the sun, so that the rays are optimally directed onto the receiver every hour of the day. Generally, a thermal oil or salt is heated first, which then drives the water steam cycle.

Andasol 1 to 3 are parabolic trough power plants. They take advantage of the physical phenomenon that every ray of sunlight that hits the inside of a parabolic reflector is reflected through the same focal point. One power plant consists of several rows of one-dimensionally rounded mirrors, so that the focal point becomes a focal line with blackened receiver piping filled with working fluid running through it. The reflective rows run in a north-southerly direction. In the morning, their reflective side faces the east and over the course of the day, they turn westward to follow the sun.

Thermal energy storage for more full load hours

The concentrated sunlight in the Andasol power plants hits the heat receivers with about 80 times the energy and heats the thermal oil contained within to as much as 400 degrees Celsius. This is enough to heat thermal energy storage units as well as drive the water steam cycle. The 28,500 metric tons of molten salt can store enough heat to last more than seven full load hours.

This not only allows for phases with insufficient solar radiation to be bridged, but also means the operating time after sunset can be extended to 24 hours – particularly in the summertime. Using thermal energy storage technologies is a relatively inexpensive way to increase the annual operating time of solar thermal power plants and thus improve their profitability.

Competitive production costs

In principle, solar thermal power plants are only worthwhile in places with very strong solar radiation. For although PV systems produce electricity right from the first ray of sunshine, the generated steam needed to drive the turbines uses a lot of energy at once and therefore has a considerable lead time before the necessary operating conditions are met (steam parameters). However, according to calculations by investment bank Lazard, once they are up and running, the average production costs per megawatt hour of electricity could fall below 100 US dollars – provided a thermal energy storage unit extends the capacity utilisation of the generators.

Although this would still mean the resulting electricity would be more expensive than that from large photovoltaic power stations, solar thermal power plants with storage facilities are better able to produce electricity reliably, even if electricity prices rise – when the PV systems run out of sunshine, for example.

A pillar of the energy transition?

Load following is also conceivable in relation to solar thermal power plants. This means that electricity production can be adapted to the current demand on the grid which would have a stabilising effect. At present, however, this business model is not yet considered to be profitable, as capacity utilisation and efficiency suffer. In a carbon-neutral energy system, however, they could play this part.

Even greater flexibility could become possible by integrating other energy sources. At the Solnova solar complex near Seville, for example, natural gas or biomethane can be used to keep the water steam cycle going if there is not enough sunshine. Solar thermal power plants are also considered to be an important alternative to PV power plants, because they need significantly less space. Spanish energy company Iberdrola, for example, states that Núñez de Balboa in the southern Spanish province of Badajoz, which is currently the largest solar farm in Europe, has an annual generation capacity of 832 gigawatt hours (MWh) – spread across an area of 1,000 hectares (ha), i.e. 0.832 GWh/ha.

By way of comparison: The solar field of Andasol 3 – 75 percent of which, incidentally, is owned by German companies, including RWE Renewables – occupies only 50 hectares and supplies 142 GWh per year, or 2.8 GWh/ha. As it happens, the world’s largest solar thermal power plant is located in the Mojave Desert in California. The three Ivanpah solar power tower units have a rated output of 392 MW and, according to data from the US energy agency EIA, supply around 730 GWh of electricity per year. Four percent of this is generated with natural gas, with the rest using solar energy.