Unless energy can be squirrelled away at times of plenty to be unearthed at times of scarcity, clean technologies like solar and wind power will always need significant amounts of backup from conventional sources.
That’s a big issue, not least because the EU’s Renewable Energy Directive requires Member States to fulfil at least 20 % of their renewable energy needs by 2020, and it's one of the main challenges in making renewable energy viable for the mass market.
“‘Efficient energy storage is of great importance.’
Norway revealed a plan earlier this year to become ‘Europe’s battery’, suggesting that surplus renewable energy could be used to pump water uphill, driving its vast hydroelectric network when needed.
And in May, Elon Musk, the entrepreneur and CEO of electric car company Tesla, unveiled plans for a home battery that is charged by solar panels and cheap electricity from the grid.
European researchers are also on the case, and are already in the process of refining several technologies to tackle the problem.
For example, Swedish company Ferroamp is developing a device called the EnergyHub, a wall-mounted box that is able to make use of different sources of energy to provide power and charge batteries within the home.
The EnergyHub is not a storage solution in itself, but a clever way of routing electricity to balance the provisions of renewable energy sources with the realities of today’s power grids and battery technologies, thereby decreasing reliance on the grid.
‘Solar energy is by definition intermittent and provides energy only when the sun shines,’ said Mats Karlström of Ferroamp. ‘You (need to) charge batteries from solar panels during the daytime, and use this energy during the night at an even rate.’
Switch
One main problem in this area is the fact that most power grids and electrical appliances use alternating current (AC), while solar panels and batteries work with direct current (DC). To solve this problem, the EnergyHub contains a directional inverter – a device which can efficiently switch between the two electricity types.
When the sun is shining brightly, or the wind is blowing strongly, the EnergyHub directs incoming clean energy directly into any batteries it is hooked up to. Conversely, if the winter nights are long, or if there isn’t even the faintest rustle of leaves on the trees, the EnergyHub turns to the grid to recharge the batteries, by converting some of its AC into DC.
At any time, of course, the hub can supply normal AC power to a customer – drawing either on the grid or on batteries – to power lights, computers, kettles, and so on.
Hubs by other manufacturers also contain inverters, says Karlström, but Ferroamp’s is unique in being scalable and future-proof. That means it is designed to work with as few or as many renewable energy sources as a customer likes, and will work with any type of battery.
In a field where technologies are advancing rapidly, this is a key selling point.
‘One important design factor for EnergyHub was that the customer should be able to modify and add more battery resources any time during the (hub’s) lifetime, as technology and battery costs open up new doors,’ said Karlström, whose company has received EUR 50 000 in funding from the EU. ‘We use the same approach for (solar) panel technologies.’
Reducing our reliance on the grid even further means going beyond the management of energy storage into energy storage itself, and here researchers are coming up with some novel solutions.
The SOLENCO project, for instance, is developing a way to use hydrogen gas to store energy. The storage happens when electricity splits water into its constituents, hydrogen and oxygen, via a process known as electrolysis. To get the electricity back, the hydrogen can be used to drive an internal combustion engine or a fuel cell.
Another project, RESTRUCTURE, is looking at a more direct method of stockpiling solar energy, this time from concentrated solar power plants. In such plants, a field of mirrors reflects sunlight onto a central tower, which heats up a gas to drive turbines.
Midday sun
In the midday sun, concentrated solar power plants can reach temperatures of 600 degrees Celsius or more. Come evening, however, the facilities cool down and can no longer generate useful amounts of electricity.
The RESTRUCTURE researchers aim to get around this by coating metal oxides onto sponge-like honeycomb materials in the tower. When the metal oxides get hot, they lose their oxygen constituent. Later on, the reverse chemical reaction, oxidation, releases the heat from the material for renewed energy generation.
Dr George Karagiannakis, an associate researcher at the Aerosol and Particle Technology Laboratory who is coordinating RESTRUCTURE, said he and his colleagues have already proved the concept in the lab and are now trying to scale it up.
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