Market entry of battery storage systems is imminent. Use of solar power set to become decoupled from production.

With a battery, homeowners can significantly increase lucrative selfconsumption

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 productiondependent
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 kilowatthours.
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 selfconsumption.

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.