With a battery, homeowners can significantly increase lucrative self-consumption of solar power. However, there are hardly any storage systems on the market. Making battery storage systems fit for everyday use is the goal of the Franco-German research project Sol-ion. The experience gained after six months of continuous operation in the field test run at the Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (Centre for Hydrogen Energy and Solar Research Baden-Württemberg, ZSW) is available now: A storage system developed within the framework of the research project was able to increase the share of self-consumption in a test building in spring by 26 percentage points. The amount of energy stored covers the average electricity demand of a single-family home during the evening hours. With this type of storage, fluctuating and production-dependent solar power turns into a retrievable demand-oriented energy source.
Owners of new roof-mounted photovoltaic systems will save a few cents per kilowatt-hour in the future if they consume their own solar energy rather than feed it into the grid. This personal consumption is recently also required by German law. The amendment to the Renewable Energy Sources Act (EEG) from April 2012 stipulates 20 per cent. Consuming more might be difficult. "The lacking simultaneity between generation and consumption limits self-consumption without a battery to about 30 per cent," explains Professor Michael Powalla, Member of the ZSW Board and Head of the Photovoltaics Division. This holds true when annual generation equals the home's energy demand and no load-controlling measures are undertaken such as using the washing machine while the sun is shining. With battery storage systems, however, significantly greater self-consumption is possible. A part of the green electricity generated at noon is then saved for consumption purposes during the evening and night.
The Sol-ion storage system developed for the project is as large as a common household fridge and contains the usual inverters for solar power systems, batteries and control electronics. The first units were installed in 2011 at the ZSW solar testing facility Widderstall and in residential consumer homes. Together with the manufacturer voltwerk electronics GmbH, the researchers in Widderstall optimised the system control and implemented automated data collection. An extension to up to 20 private households and other research institutes is ongoing. The employed lithium-ion battery has a utilised capacity of 6 kilowatt-hours. Adding to that, there is a 5 kW-inverter and an identically rated battery charging rectifier.The tests carried out at the ZSW facility Widderstall have demonstrated high utilisation of battery capacity. A 5.1 kWp-system on a carport provides the power, the consumer is a testing facility building. "Even during the months of February to mid April 2012, the storage system could be charged at a daily average of 4 kWh of solar power and often to full capacity," says Michael Powalla. The amount of power stored covers the demand of a 4-person household in the evening hours even in spring. In the specific case measured here, the result was an increase in self-consumption of 26 per cent. "We are looking forward to the results in the summer. With longer hours of sunshine, the stored power may last from the late evening hours until the next sunrise," the researcher estimates.
With the lowering of the feed-in tariff in Germany down to 19.5 cents as of 1 April, self-consumption is more profitable than the subsidy. Every kilowatt-hour drawn from the energy suppliers currently costs 23 to 25 cents. Whoever consumes their own solar energy, instead of feeding it into the grid for 19.5 cents, is making a small profit. The difference will increase for new systems since EEG remuneration will continue to drop monthly. Currently, the additional costs for battery storage systems are not covered by this difference. This will change. Due to the falling prices for battery storage systems in the coming years, it will be more economical to use one's own generated energy than to purchase ever dearer energy from the utilities. Sol-ion wants to make a contribution.The Sol-ion project is funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) with € 4.3 million. On the German side, the inverter manufacturers voltwerk, the energy suppliers e-on, RWTH Aachen University and the Fraunhofer IWES are involved in addition to the ZSW research institute. French participants include the battery manufacturers Saft, the INES institute and the energy suppliers Tenesol. The test devices installed in France are optimised with the aim of ensuring operation in case of a power failure. The goal in Germany is to increase grid-connected self-consumption.
ZSW is one of the leading institutes for applied research in the area of photovoltaics, renewable fuels, battery technology, fuel cells and energy system analysis. In collaboration with Würth Solar, the institute successfully made CIGS thin-film photovoltaics ready for series production. There are currently over 200 scientists, engineers and technicians employed at ZSW’s three locations in Stuttgart, Ulm and Widderstall.