Storing solar power

Researchers in a Franco-German project have come up with a power storage system that considerably increases direct consumption from roof arrays. Such applications are becoming increasingly important in countries like Germany, which has implemented a kind of real-time net-metering called "own consumption."

At the ZSW's solar test field in Widderstall, researchers conducted long-term tests on the Sol-Ion storage system.

Photo: ZSW

As a part of the Franco-German Sol-Ion research project, scientists at Baden-Württemberg's Center for Solar Energy and Hydrogen Research (ZSW) in Stuttgart have come up with a new storage system for solar power. In a test building, they were able to increase "own consumption" – solar power used directly within the household without being exported to the grid – by 26%. The ZSW says the amount of energy stored in the new system covers the average power demand in a single-family home of four people in the evening. In other words, the system turns fluctuating, intermittent solar power into a source of energy that can be consumed as need be.

The Sol-Ion storage system is about the size of a normal household freezer. It contains the power inverters needed for the solar array as well as a battery charge rectifier, both of which have a nominal output of five kilowatts. The system also contains electronics for device controls, which the researchers optimized. They then collected data during various long-term tests over six months. Lithium-ion batteries with a useful storage capacity of 6 kilowatt-hours are the centerpiece of the system.

In the long-term tests, the researchers demonstrated that the battery's capacity was utilized to a great extent. Power came from a 5.1 kilowatt array on a carport. "Even from February to mid April 2012, the battery was charged with four kilowatt-hours of solar energy on the average, and it was often even full," says Michael Powalla, head of photovoltaics at ZSW. "We look forward to seeing the results in the summer. When the sun shines for a long time, the energy stored can suffice from the late evening until sunrise," Powalla says.

Such technology has moved into focus particularly since the current governing coalition announced that it will only be offering feed-in tariffs for 80% of the solar power produced. The remainder is to be consumed directly. Rough calculations show that investments in PV rooftop arrays pay for themselves for homeowners under this policy regardless of the returns from feed-in tariffs. After all, the feed-in tariff of 19.5 cents for a kilowatt-hour of solar power is much lower than the retail rate in Germany, which ranges between 23 and 25 cents. And the lower feed-in tariffs drop, the more "own consumption" pays for itself. At the same time, the cost to society would be lower if only feed-in tariffs were offered; essentially, the "own consumption" policy pays homeowners a bonus to invest in storage systems and tailor their consumption to their own power production.

But up to now, power storage systems have been the main obstacle because they remain expensive. The bonus paid for "own consumption" does not currently suffice to pay for battery systems. If this problem is not solved in the next few years, the policy of "own consumption" might not allow Germans to install more photovoltaics then without the policy. "At present, the average household can only consume around 30% of its solar power directly without some kind of storage system," estimates Powalla – and that level is considerably greater than the 20% currently required by law. But the law could change and require more direct consumption.

The only way that consumers can respond without storage is to have some appliances – such as washing machines and refrigerators – run mainly when the sun is shining, but there are no such appliances on the market at the moment. For the time being, the only way to increase direct consumption is to store as much solar power as possible in the afternoon for consumption in the evening and at night. Unfortunately, the policy of "own consumption" is still limited to solar power; no such bonuses or incentives are offered for wind power (not even for small generators in backyards) or to people without solar roofs who nonetheless wish to adjust their consumption so that more photovoltaics can be installed. (Sven Ullrich / Craig Morris)