A Swedish research team has developed a way of storing solar energy in liquid form, but it is still a way off being commercially available. Meanwhile a competing technology is already shortlisted for a major renewable energy prize. In both cases the energy stored is delivered as heat.

Liquid solar energy

The solution of researchers at Chalmers University of Technology in Sweden is a chemical liquid that can tranport solar energy and then release it as heat whenever it is needed. The research, described in March’s edition of Energy & Environmental Science, describes how the team came up with a way of copying the means by which plants store solar energy – in molecules.

Transforming it into bonds between atoms in a liquid chemical makes it possible to transport it as well as store it.

“The technique means that that we can store the solar energy in chemical bonds and release the energy as heat whenever we need it,” says Professor Kasper Moth-Poulsen, who is leading the research team.

“Combining the chemical energy storage with water heating solar panels enables a conversion of more than 80 per cent of the incoming sunlight.”

The research project has come a long way since it began six years ago when the solar energy conversion efficiency was 0.01 per cent and the expensive element ruthenium played a major role in the compound.

Four years later, the system stores 1.1 per cent of the incoming sunlight as latent chemical energy – an improvement of a factor of 100, and ruthenium has been replaced by much cheaper carbon-based elements.

“We saw an opportunity to develop molecules that make the process much more efficient,” Moth-Poulsen says.

“At the same time, we are demonstrating a robust system that can sustain more than 140 energy storage and release cycles with negligible degradation.”

The process is based on the organic compound norbornadiene, which upon exposure to light converts into quadricyclane.

Hybrid panels

The rooftops of buildings can take advantage of the benefits of installing both solar water heating and photovoltaic modules.

Typical efficiencies for photovoltaic modules are now at least 20 per cent. Solar water heating systems have an efficiency of between 20-80 per cent, depending on the application, location and the required temperature.

Solar water heating systems make use of the full solar spectrum, whereas photovoltaics can only harvest a much more limited proportion.

Some companies have used this difference to design hybrid panels which contain both solar water heating and photovoltaic cells, particularly since the water can be used to stop the photovoltaic panels overheating, making them more efficient. The downside is the expense.

The Swedish researchers think that one of the potential applications for their technology, when it has become more efficient, will be a new generation of hybrid panels that utilise the heat, which can be released from the liquid storage medium.

Concept diagram for the hybrid solar panels

They say that combining solar water heating with their system allows for efficient usage of low energy photons for solar water heating combined with storage of the high-energy photons in the form of chemical energy.

Their simulations have persuaded them that these hybrid panels could be up to 80 per cent efficient. In terms of energy density they are comparable to a lithium ion battery.

The team will continue work on the technology to evaluate the potential cost and bringing it down by finding a way to mass produce the constituent chemicals, and to find a non-toxic solvent.

Sunamp’s technology

A totally different technology is from Sunamp, a British company that has developed its technology by collaborating with the University of Edinburgh School of Chemistry. It guarantees low-cost materials, exceptional long life, recyclability, safety and high energy density.

The technology has been shortlisted for the 2017 Ashden UK Awards alongside the work of the Passivhaus Trust and the Carbon Co-op, a community benefit society that helps its members to retrofit their homes.

Sunamp’s form of storage uses a salt as a phase change material. This absorbs and releases thermal energy during the process of melting and solidifying respectively.

Similar technology is used on a large scale with concentrating solar thermal power stations, typically located in hot, arid deserts.

In this case it is used for storing energy from photovoltaic panels, waste process heat, or heat from heat pumps and micro CHP (combined heat and power) systems, in order to increase efficiency.

How does it work? In the case of storing solar electrical energy, an electrical element connected to the solar panels heats up the salt, thereby melting it.

The salt is kept liquid in a vacuum-insulated container. When heat is required, cold water is passed through the liquid in a heat exchanger, absorbing the heat and causing the salt to re-solidify. The heated water passes to the tap and the salt is ready to be charged again.

Sunamp’s batteries come in various sizes and can be used in series, meaning they can be used in anything from small homes to large hotels, for example. They take up much less space than a hot water tank, can store heat for longer and are more efficient.

The battery can store heat at half the weight of hot water in a tank storing the same amount of energy. Whether they are cost-effective depends upon the location and pattern of usage.

David Thorpe is the author of a number of books on energy efficiency, sustainable building and renewable energy. See his website here.

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