A tank full of sunshine

As an energy source the Sun seems to have it all. It is unlimited, clean, free to use, available the world over and abundant: the energy that reaches the Earth’s surface from the Sun every year could satisfy mankind’s needs 8,000 times over. Even the capital costs of solar energy, which have historically made it much more expensive than fossil fuels, have fallen markedly in recent years.

But a major hurdle remains. Like wind, sunshine is not available around the clock, and electricity generators must find ways of plugging the gap at night or when it is cloudy. If countries are serious about reducing their reliance on fossil fuels, says Ib Chorkendorff, a physicist at the Technical University of Denmark (DTU), new techniques for storing renewable energy are essential.

As he puts it: “Those who want to have fun after four must learn how to store”. One way to store energy is to use hydropower. This involves pumping water uphill when electricity demand is low and then allowing it to fall back down and turn turbines during peak periods. Suitable sites are hard to come by, however, and construction is expensive. Batteries, too, are costly and likely to find limited use for the foreseeable future.

Bottling up the Sun

This is where “concentrating solar power” (CSP) comes into its own. The basic idea is to use the energy from the Sun’s rays to heat up a suitable fluid that runs along pipes at the foci of parabolic mirrors laid out in rows. Part of that fluid passes through a heat exchanger to create steam that drives a turbine, which in turn produces electricity.

The remainder heats up a second substance – typically molten salt – via a separate heat exchanger. When the Sun stops shining, cool transfer fluid is heated by the hot, stored salt and then passed once more through the steam generator to continue generating electricity. Thus the system can run around the clock.

For CSP to be competitive with fossil fuels, its storage needs to be cheaper and more efficient. One solution is to combine within one material the roles of heat-transfer fluid and storage medium, thereby eliminating a heat exchanger. This arrangement could also potentially raise the storage temperature, which increases conversion efficiencies and reduces volumes. The 5-MW Archimede plant in Sicily, for example, uses molten salt to do both jobs and as a result can boost storage temperatures to 550˚C, compared to the roughly 400˚C of commercial CSP plants that use synthetic oils. But Archimede’s efficiency is limited by the need to heat the pipes, as its salts solidify below 240˚C, explains Fabrizio Fabrizi of ENEA, Italy’s national agency for energy research. Researchers have developed salts that have significantly lower melting points, but they are still too expensive.

A tower of light

Another option is to do away with parabolic mirrors and instead focus the Sun’s rays onto a single receiving module at the top of a tall tower via hundreds or thousands of small reflectors suitably arranged on the ground. These “power towers”, such as the 20-MW Gemasolar plant in southern Spain, operate at higher temperatures than conventional CSP plants and need far less molten salt, used both as heat-transfer fluid and storage medium. At night, the salt can be drained and kept in a liquid state using comparatively little energy.

A more radical alternative is to store the Sun’s energy by melting solid salt, rather than raising the temperature of already molten salt. A group at the Fraunhofer Institute for Solar Energy Systems in Freiburg, Germany, is looking to commercialise this techno­logy. It would allow more energy to be stored in a given volume and take place over a very limited temperature range, making it ideal for direct steam generation – a particularly cheap and efficient form of CSP that uses water as the heat-transfer fluid. According to Fraunhofer’s Verena Zipf, the institute has built a prototype device with a partner from industry and estimates that a full-scale device could appear on the market within three years.

Hydrogen from the Sun

This ability to divert some of the Sun’s energy for later use is a major selling point for CSP. It is not shared by photovoltaics – currently a far more popular form of solar power – since the incident rays in this case are converted directly into electricity, which is harder to store. However, research is underway to efficiently transform that electricity into the ideal storage mechanism: chemical bonds, notably hydrogen molecules.

Electricity is used to create hydrogen via electrolysis. When an electric current is passed through water (H2O), the positively charged anode strips hydrogen atoms of their electrons, causing the water molecules to break up and oxygen molecules to collect on the anode while hydrogen molecules accumulate on the negatively-charged cathode.

This process has been known for more than 200 years, but the trick is to maximise the fraction of the incident energy that ends up in the chemical bonds. With photosynthesis, plants typically do no better than about 1%. Scientists are aiming to develop marketable devices that have percentages in double figures.

In September 2014, Michael Grätzel and colleagues at the École Polytechnique Fédérale de Lausanne (Switzerland) announced that they had converted 12.3% of incoming solar energy into hydrogen. They employed their own “dye-sensitised” solar cells, but instead of using them to generate electricity they connected them to an electrolyser containing electrodes made from nickel and iron.

DTU’s Chorkendorff, meanwhile, hopes to combine energy collection and hydrogen production within a single device: a semiconductor that provides electrons and positive charges to attached oxygen- and hydrogen-generating catalysts. His goal is to employ two different semiconductors to extract as much energy as possible from both red photons and higher-energy blue photons. “Existing tandem devices are suitable for satellites but too expensive to roll out over square kilometres,” he says. “In the end the technology has to be dirt cheap.”

Transforming CO2 into natural gas

The hydrogen produced in this way could be used directly to power vehicles – by consuming it inside fuel cells – or be added to natural gas and burnt in a gas power station. Still, some researchers believe that the energy locked up inside hydrogen could be better used by making other fuels.

The German government set up the “iC4” project to improve the conversion of carbon dioxide (captured in fossil fuel power stations, for instance) into synthetic natural gas by combining it with hydrogen. Bernhard Rieger, spokesman for iC4, and colleagues at the Technische Universität München have increased the proportion of carbon dioxide converted into methane from around 65% to nearly 95% by employing a nickel-based catalyst and a new kind of heat-removing reactor. “We have demonstrated this not only in the lab but also now in a way that can be commercialised,” says Rieger.

But why go to the bother of producing methane through a process that itself requires energy? Rieger says it’s a question of infrastructure. There is currently no global supply system for hydrogen, though there is one for methane. Natural-gas pipelines cannot be used to pump gases consisting of more than about 4-5% hydrogen but are ideal for transporting methane, the main constituent of natural gas. Only by spending huge amounts of money on a new pipeline system, Rieger argues, can hydrogen become an attractive proposition. But, he adds, “I accept there are two different views on this. The future will show which one wins out.”

The heat batteries

One good way to use excess electricity is to transform it into heat, says David Smeulders of Eindhoven University of Technology. “Around 40% of our energy consumption goes to heating buildings. As heat pumps are very efficient, water represents a good way to store energy – provided one can keep the water warm long enough.”

Thermochemical materials (TCM) – porous materials made of special salts like sulfates – are much more efficient than water tanks. “One can charge a TCM with chemical energy by exposing it to hot, dry air,” he explains. “The latter takes away the water molecules found in the TCM and puts the salts in a state of higher energy. One can regain the energy later by putting the salts in contact with cold, wet air. They absorb the humidity and give out energy, effectively heating and drying the air.”

Excess electricity could be used in electric heaters to produce hot air and store energy in a TCM. Another idea is to incorporate such salts into building walls to absorb energy during the summer and release it in the winter.

Smeulders’ team at TU/e is looking for new, more efficient TCM compounds. “We’re in contact with major chemical manufacturers but also with government housing institutions, which will have to renovate their estates. I believe we could have something practical within five years.”



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