Canned sunshine

Recently the German energy service provider, Eon announced major investments in its pumped storage hydropower stations at the Edersee in Hesse, Germany, in anticipation of increased deployment of wind and solar power in its network.
“Pumped storage power stations can efficiently store surplus energy and are thus an excellent partner for integrating intermittent renewable energy supply. Such plants are an important building block of future energy supply, ” said Ingo Luge, the chairman of the board of management of Eon Energie AG. Eon’s rivals Vattenfall and Enel also recently told shareholders that investment in storage is part of their respective renewable energy strategies.
Investment by Europe’s leading energy players is good news for solar PV because there are synergies between storage and the growth in wind and solar power. “Solar and wind’s peak power production times do not always coincide with peak demand. In solar the peak is noon but peak demand is between 5 and 8 p.m. In wind, peak power production can sometimes be in the middle of a cold winter night,” said Alessio Beverina, a partner at venture capital fund manager Sofinnova Partners. He says that various forms of storage technology are emerging to enable managing the peaks. Battery storage is one of the technologies widely seen as particularly applicable to solar PV. One of the applications is cloud mitigation in large-scale PV plants. “Storage becomes critical when feed-in tariffs (FIT) are removed,” said Matthias Vetter, who heads up the PV Off-Grid Solutions and Battery System Technology department within Fraunhofer ISE’s Electrical Energy Systems division (EES).

Cloud mitigation

The phasing out of FITs in several major markets in Europe, and the lack of them in other markets abroad, forces solar plant owners to compete on an even playing field with other power producers. That is, large-scale solar parks – greater than one megawatt (MW) – have to negotiate and define production plans and contracts. These can be between the PV plant owners and the utility, a community, or even a remote mining operation, for example.
The problem is in the details of the planning. While the accumulated solar power production is quite precise over a one day period, greater granularity for the next 12 to 24 hour period is required by the power purchasing contracts. “Accumulated energy production of PV parks is predictable one day in advance using modern and reliable prediction tools. But power contracts between utilities and producers typically plan the demand and production in time intervals of 15 minutes,” said Vetter.
The producer might know that there will be clouds the next day, but not exactly when they will pass overhead. The timing can be off by minutes to half an hour. To address this problem, Vetter’s team worked on a battery storage solution that is able to provide short-term power backup for just this scenario.
Vetter’s team found that an operator of a one MW park does not need 100 percent backup. In fact, to be able to meet contractual requirements, a capacity of 250 kilowatt hours (kWh) of usable storage is sufficient to compensate for short-term weather disturbances. The optimal battery chemistry was also analyzed as part of the research. “You need storage with fast discharge characteristics and a certain pre-determined energy storage capacity to compensate for short-term fluctuations in power production,” said Vetter. His team researched the most suitable battery solution and found that lithium ion (Li-ion) technologies are good, especially Li-ion variations with very high cycle life times, but reduced energy densities.

Just-in-time power

It is the kind of solution that improves the business case for solar plant owners. “If PV power plants don’t deliver the power on time and in the volume contracted for, they can be punished in the future,” said Vetter.
Unfortunately the traits that make lower energy density Li-ion good for PV storage also make them “uninteresting” for electric vehicle applications (which means they won’t necessarily benefit from the high volume and lower prices expected due to the advent of electric cars).
There are other applications for battery storage. Vetter says that redox-flow, as well as high temperature batteries have a good potential where higher storage times and higher solar autonomy times might be used in isolated mini-grids, or when higher backup times are needed, for example. (See the table “Parameters of different battery technologies” for more details and comparison points for each battery type.) Key factors affecting battery selection are cyclic stability, calendar life times, efficiencies, self-discharge rates, reliability and safety. Some of these parameters are more important than others, such as battery lifetimes.

“Utilities are cautious and extremely conservative. They think in a twenty year time frame with investment scopes up to forty years. They don’t want a battery that needs to be changed every three years,” said Beverina of Sofinnova Partners.
All parameters in view of a specific application need to be considered for determining the costs for the stored energy. “For residential grid connected PV battery applications and especially for PV park applications where primarily short time storage is needed, Li-ion is one of the most attractive chemistries,” said Vetter.
But the cost of Li-ion batteries still doesn’t suit the markets and the niches they fit into. Vetter is optimistic that battery prices will fall. “The costs are heading in the right direction. They are getting cheaper,” said Vetter. The costs of capital and operating expenditures (CAPEX and OPEX) are still too high. “When the capital and operating costs of storage are included, we are far from grid parity,” said Beverina.

Utilities test innovative solutions

Besides batteries, other technologies are emerging to address PV power integration in the grid. Eon, for example, announced in November that it would be testing a new kind of hydrogen catalysis storage solution. It is investing five million euros to set up an electrolysis plant that will manufacture hydrogen for distribution in its existing gas network. The plant is aimed at finding a storage solution for Eon’s wind farms, but it could just as easily be used for large PV power plants.
One of the pioneers in safer hydrogen storage is France’s McPhy Energy, a startup company backed by Sofinnova Partners that has developed a solution for on-site production of hydrogen from solar power or other forms of energy. It is a low-pressure gas, making it safer than existing systems.
Hydrogen is competitive with batteries and could be competitive with compressed air and pumped hydro in locations that are not favorable for the latter two technologies, according to National Renewable Energy Laboratory (NREL) research (see graph “Ranges of leveled cost”). Utilities or other customers could use the hydrogen they produce with solar power as a gas product, or to power fuel cells converting it back to power in 100 kilowatt to one MW solid oxide fuel cells. “At the moment, FITs offer a better return on electricity from solar PV plants than the return that a plant owner could develop selling hydrogen. What we will see are several niches defined by market need and applications driven by the end of FITs,” said McPhy’s CEO Pascal Mauberger.
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Furthermore, a new market for hydrogen could emerge if the auto industry makes the shift to cleaner engines. The value of hydrogen in this application is attractive as one kWh is worth about 200 euros, according to Mauberger.
Projects testing McPhy’s hydrogen solution include GDF Suez, a northern France project with a gas company testing a 50 to 100 kWh solution. McPhy has four systems in industrial prototype in Japan with Iawatani Corp., Italy, the U.K. and Germany.
Mauberger says that several newer storage methods, like battery technologies, hydrogen and flywheels, as well as established ones like pumped hydro will all be part of the mix. He is not the only one saying that. The U.S. Department of Energy is promoting a heterogeneous future for storage by funding a range of storage technologies from flywheels to hydrogen and Li-ion chemistries.

Conclusion

Researchers at Germany’s ZSW (Centre for Solar Energy and Hydrogen Research Baden-Württemberg) say that by 2020 more than one in three kilowatt-hours will be generated by renewable energy sources. Today it is one in six kilowatt-hours. The German National Renewable Energy Action Plan calls for close to 52 gigawatts of photovoltaic power capacity connected to the grid in the same time frame.
Storage solutions in combination with an intelligent energy management system are an enabler of direct and large-scale grid integration of decentralized PV power production. A variety of lower cost storage technologies used in various applications from load leveling to cloud mitigation are no doubt going to be a part of that upcoming growth story.

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