Sascha Rentzing, Contributor
May 08, 2013 | 25 Comments
It's always the same problem: You save energy but the electricity bills are still getting more expensive. Luckily, there is a solution: Solar power can be generated in Germany for 12 cents per kilowatt hour. In contrast, utilities currently charge an average of 25 cents for domestic electricity. What better reason to invest into your own photovoltaic system? Solar storage systems can increase on-site consumption by up to 70 percent. They absorb surplus solar power and pass on the energy as required — expensive grid power is hardly necessary. This makes the systems very attractive for consumers: According to an EuPD Research survey, almost 90 percent of solar operators are already thinking about buying an additional storage system.
Storage system providers are promising economical solutions that customers can’t refuse. Many companies are advertizing that their photovoltaic systems will remain economical despite rising electricity prices during the first twenty years of operation. Scientists, however, are skeptical as to whether these promises can be kept. Battery expert Uwe Sauer from the Institute for Power Electronics and Electrical Drives at RWTH Aachen has developed a model to calculate just how economical battery-based storage units are. It takes into consideration the efficiency of the system, the number of full cycles per year, capital costs and costs for storing power. This has rendered domestic storage systems as downright useless.
Sauer’s approach: First of all, the amount of energy the battery can absorb during its cycle life and pass on again needs to be calculated. For example, the company Deutsche Energieversorgung’s conventional lead-acid battery system may have 3,000 cycles for a capacity of 24 kilowatt hours, equaling 72,000 kilowatt hours in total. A 50 percent depth of discharge at which this cycle life can be achieved must be deducted. Another 80 percent must be deducted to cover the loss of efficiency to the whole system. During its cycle life, the battery has a capacity of around 30,000 kilowatt hours. At €6,300 for the system, storage costs come to 21 cents per kilowatt hour. If you add the 12 cents it costs to produce power on-site, the total cost comes to 33 cents. This sum is considerably higher than the current domestic electricity cost of 25 cents — meaning the system is not economical. Modern lithium-ion batteries offer even less value for money, according to Sauer’s calculations. Storage costs alone come to at least 35 cents for current systems.
According to battery expert Margarete Wohlfahrt-Mehrens from the Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) the situation is not quite as dramatic as that. She currently calculates the storage costs of classic lead batteries at around 10 cents and those of lithium ion batteries at 25 cents. These costs could soon drop, as she reckons that lithium technology could also become considerably cheaper. “We estimate that storage costs will be cut by half in the next few years,” says Wohlfahrt-Mehrens. Her reasons include the development of mass production and new innovations, which are helping the industry to develop more efficient production methods and higher-performance lithium ion technologies.
The cathodes and anodes of lithium ion batteries are produced by applying suspensions containing carbon and lithium as liquid electrolytes over a cylinder. It is now the manufacturer’s aim to use larger sheets to speed up the production process. In addition, companies are developing more robust and higher-performance electrode materials. Today’s batteries use graphite for the anode and lithium metal for the cathode. It serves as a chemical reactant for graphite. Manufacturers want to use new anodes made from lithium titanate in the future, which recharge faster and can withstand more load cycles than graphite.
Until economies of scale begin to take effect with more production and innovations, the German Federal Government intends to support the technology. For example, the government is considering granting low-interest loans from state bank KfW to solar installations with storage systems. Paying a subsidy of 30 percent of the costs for the battery is also under consideration. This would significantly reduce the payback period. Will solar batteries soon pay off or do they still have a long way to go?
This article was originally published on Solar Energy Storage and was republished with permission.