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Energy storage

Making solar electricity last

Jürgen Heup and Sascha Rentzing, 8 Jul 13
Efforts to expand photovoltaics will only work if storage solutions are available that can offset fluctuations in solar energy. No cost-effective technologies exist as yet, but researchers are exploring some extremely promising avenues.

If you invest in a photovoltaic system today, you will also have to weigh up whether or not to purchase a storage solution. If your system is small, then batteries are a good idea because they can help you consume more of the lucrative electricity that you produce. In Germany, one kilowatt-hour (kWh) of solar energy costs EUR 0.15 to produce, while mains electricity from the plug socket costs an average of EUR 0.25. So clearly it makes sense for homeowners to supply themselves with as much energy from their photovoltaic systems as possible.

As for operators of large solar farms, acquiring storage technology makes sense because energy suppliers are likely to reduce the output of farms more often in the future. Solar electricity fluctuates heavily. Grids are flooded with it at midday, but there is none available at night. As more photovoltaic systems get connected to the grid, the balance between generation and consumption will become increasingly skewed. Storage technologies can keep grids stable by holding onto excess energy until it is needed. However, researchers still need to work out which kind of storage is best for which application. The technologies available range from small and large batteries to hydrogen, and thermal energy storage.

Lithium versus lead

About 50 companies in Germany now sell systems that combine solar panels and batteries. Most use modern lithium batteries, which are preferable to standard lead batteries since they take up less space, can store more solar electricity and, because their electrodes are more electrochemically stable, have a longer service life. “Lithium-ion batteries can last for up to 10,000 charge cycles, but lead batteries only manage about 3,000,” says Matthias Vetter, head of the Electric Storage Systems department at the Fraunhofer Institute for Solar Energy Systems in Freiburg, Germany. But lithium technology is also much more expensive. Vetter says that it currently costs about EUR 0.35 per kWh to store solar electricity in a lithium-ion battery. Of that, EUR 0.20 is down to storage costs alone. These kinds of figures are likely to scare off many potential investors.

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But Vetter thinks that bigger production runs and advances in technology could cut storage costs in half over the next three to four years. “If that is coupled with household electricity prices continuing to rise by five percent each year, using lithium-ion batteries will start to make financial sense as early as 2015.” Some manufacturers, among them Leclanché in Switzerland and Varta in Hanover, are already increasing production. Companies are also developing electrode materials that are more robust and more powerful. In most standard batteries, the anode is made of graphite and the cathode of lithium metal. The metal and the graphite are partners in the chemical reaction that occurs in the battery. Leclanché is now planning to use anodes made of lithium titanate, as they are quicker to charge than graphite and can withstand more charge cycles.

Solar electricity and heat pumps for the home

Batteries aren’t the only way to go if you have your own photovoltaic system and want to use more of the electricity it produces. You could choose a much simpler route and store the excess energy in water. In theory, all you need to do is put an immersion heater in your hot water cylinder. At midday, when your panels are producing electricity at full capacity and little is being used in the house, the excess energy will be stored up, which means you can use it later and the grid will get a little respite. The advantage of this solution over batteries is that most homes already have a water tank, so there is no need to purchase a special container. Not everyone thinks this is a good idea, though. Critics say that using solar electricity as heating is a waste, and that solar thermal collectors, which convert sunlight into heat directly, are a much better option. The trouble is, the cost of collectors has not fallen as much as it has for solar electricity systems in the past. Feed-in tariffs and the security they offer investors mean that photovoltaics have enjoyed much more growth than solar thermal technology has.

This is an abridged version of the article – the complete text is available in issue 4/2013 of new energy.

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