Smart Grid: Combination of Vanadium Redox Flow and Lithium-Ion Battery


The example of Witstung shows the current problem of the energy transition. The small settlement near Bühl in southern Germany consists of ten buildings, every second of which is equipped with a photovoltaic system. On the balance sheet, the amount of energy generated by the PV systems is already as high as the energy consumed in Witstung over the year. Generation and consumption often widely differ, so that a large amount of electricity is fed into the local power grid. In the summer months, therefore, overloads of the power grid, so-called voltage band problems, often occur.

The joint project “Hybrid Optimal” now wants to show with a Smart Grid in Witstung how existing bottlenecks in the power grid, which hinder the further expansion of renewable energies, can be resolved cost efficiently. Under the direction of the Stadtwerke Bühl, the households of the settlement are equipped with Smart Meters, a commercial optimization software is purchased and a hybrid storage unit is installed. This storage unit consists of a vanadium redox flow battery and a lithium-ion battery and uses the optimal operating windows.

The vanadium redox flow battery (5 kW/45 kWh) is produced by the project partner SCHMID Energy Systems in Freudenstadt, and its operation is adapted to the requirements of the energy cell. The EverFlow® Compact Storage is intended to serve as a long-term storage facility – also with regard to other projects – and to supply the settlement overnight. Since with the vanadium redox flow technology the storage capacity can be scaled regardless of the performance by simply install larger tanks with more electrolyte, i.e. more storage medium. Thus, storage units with a capacity for consumption of several hours can be fabricated at low cost. Even storage capacities in the megawatt-hour range can be realized. In addition, there are virtually no limits for the service life and reuse of the electrolyte. On the one hand this is particularly eco-friendly and on the other hand economically interesting since it corresponds to a guaranteed value preservation of about one third of the system price.

Within the demonstration project “Hybrid Optimal” SCHMID will link the vanadium redox flow battery with the lithium-ion battery and will optimize it to become a powerful hybrid storage unit. Thereby the lithium-ion battery (40 kW/50 kWh) with its high partial load efficiency is to prevent grid overloads and is to serve as a short-term power source.

The energy cell takes part as a unit in the available energy markets within the limits of the current grid connection. For this purpose the Institute for Electrical Energy Systems and High Voltage Engineering of the KIT together with the Stadtwerke Bühl uses an optimization software. At the end of the project it shall be able to control consumption, purchase and sale in terms of profitability and profit maximization. In addition to the optimized marketing of excess PV electricity the energy cell can utilize the storage system in the winter months with little sunshine to flexibly stock up on the market with energy at lowest cost (e.g. excess wind power). As a result, all inhabitants of the settlement – both producers and consumers – benefit from the energy transition. Sociologists from the Georg-August-University in Göttingen will accompany the project and will carry out an acceptance research. In addition, the “Hybrid Optimal App” will provide the inhabitants and project partners with up-to-date information such as feed-in, charge status of the batteries or CO2 savings.

The state of Baden-Württemberg (Germany) is funding the “Hybrid Optimal” project, which is oriented toward scalability, with an amount of EUR 380,000. There is an abundance of examples for potential energy cells. While these could consist of housing blocks or streets in larger cities, isolated farmhouses or enclaves are an option in the rural area. As the example of Witstung shows, especially there a high potential for renewable energy generation (large roof surfaces and open spaces) encounters comparatively low-dimensioned grid connections.

Quelle: SCHMID Group

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