Author: Dr. Klaus Hassmann, Energy Technology Cluster (as of August 2017)
Problem definition
In technical parlance, accumulators (rechargeable batteries) differ from batteries for the storage of electrical power; accumulators are rechargeable, batteries are not. Accumulators are used today primarily in the lower kilowatt (kW)/kilowatt-hour (kWh) range as a system component with photovoltaics (PV), for example in decentralized rooftop applications in single- or multi-family homes, but increasingly also in larger PV parks. Batteries are also being built in conjunction with wind turbines that feed at a higher voltage level; a trend toward outputs of several MW can be observed. Both battery types, small and large, are smart solutions to store renewable electricity and have it available not only, but especially, at times of high electricity prices during supply shortages - when there is no sun shining and/or no wind blowing - or also for grid support.
The battery stores direct current and also releases direct current. Systems that interact with the electrical grid require an inverter that must be matched to the voltage level of the feed-in and power output. System management wind/sun/accumulator during charging and discharging, optimization of consumption and grid feed-in is also an important issue that changes from project to project. In the systems, the space required varies greatly depending on the battery size and application.
Today, battery systems are of interest even without linking to renewables in the multi-digit MW range. The higher the grid voltage, the more powerful the battery systems can be. One can read of hundreds of kW in the low-voltage grid, of several megawatts (MW) in medium voltage and of hundreds of MW in the high-voltage and extra-high-voltage range; the hundreds of MW segment would be occupied by the pumped storage systems in the context of the energy turnaround if there were continuous additions; in this context, reference should be made to the article "Electricity generation from hydropower in Germany - robust, reliable and technically mature" in this portal.
The above power limits are fluid; they will adapt technologically in the future to advances in weather forecast accuracy as well as in grid expansion at all voltage levels with new lines/cables and operating equipment (power electronics).
Types
1. Lead-acid battery
Construction and mode of operation
The most widespread and therefore (still) the best-known representative is the lead-acid battery; in principle, it consists of an acid-proof housing and two lead plates or plate groups; they act as positive and negative electrodes, respectively. In the charged state, the positive electrodes consist of lead oxide, while the negatively polarized electrodes consist of finely divided, porous lead. Between them is the electrolyte, a 37 percent sulfuric acid. The electrode plates are separated by separators; these prevent the plates with different polarity from touching and causing a short circuit. During discharge, lead sulfate is formed at the negative electrode with the release of 2e-, and lead sulfate and water are formed at the positive electrode with the absorption of electrons after current is released to a consumer.
The following chemical processes occur during discharge:
Negative pole:
Positive Pole:
When charging, the processes occur in the opposite direction. The overall reaction when discharging:
The nominal voltage of a cell is 2 V, but the voltage varies between about 1.75 and 2.4 V depending on the state of charge and the charge or discharge current. The energy density is 0.11 MJ/kg (30 Wh/kg), while modern cells reach almost three times this value. A few more remarks on the lead battery: It has proven itself for decades not only as a starter battery for mobile applications but also stationary. Lead has a large mass relative to volume compared to other types, as well as a low energy density of 0.11 MJ/kg. All battery types operate fundamentally on the same principle as the lead-acid battery, even though they use different electrode materials and electrolytes. Large-scale test facility in Germany (selection)
- Berlin-Steglitz: in 1986, a lead-acid battery plant with a peak power of 17 MW was put into operation; at rated power, the battery could be emptied in 20 minutes. Before the fall of the Wall, the plant served to support the island grid in the western part of the city. It was shut down in 1994, as uneconomical.
2. Lithium-ion battery
Construction and classification
The lithium-ion battery is now considered to have the greatest market opportunities due to its proven excellent electrical characteristics. This is mainly due to the fact that both in the context of the sector coupling of electricity transition as storage for renewable electricity and the mobility transition in hybrid or pure electricity operation of motor vehicles, two major application areas of the energy transition are occupied by this type of battery.
In Germany, many renowned research institutions as well as commercial enterprises are involved in this technical development. Targets: To significantly reduce costs and risks
- Simplify the design of the batteries, e.g. through improvements in design and manufacturing; further reduce weight/volume per kW and kWh
- Improve the function, e.g. through the highest possible number of charge-discharge cycles, at a high energy density and nominal voltage; deep discharge without consequential damage
- achieve the longest possible service life and a high level of operational safety (exclusion of fire hazard through operating parameters or safety standards or standardization).
- Ensure disposal or recovery of valuable materials
Large-scale test facilities in Germany (selection)
- Accumulator storage power plant Schwerin: A lithium-ion battery system with a capacity of 5 MW and a capacity of 5 MWh went into operation in 2014 and serves primarily to compensate for short-term grid fluctuations.
- A lithium-ion system with an output of 1 MW and a capacity of 1.4 MWh incl. inverter, transformer and control system for the provision of primary control power was ordered by Stadtwerke Schwäbisch Hall at the end of 2016; commissioning is scheduled for fall 2017.
- The municipal utility Eins Energie in Chemnitz has built a large-scale lithium-ion battery with a capacity of 10 MW and a capacity of 15.9 MWh for the marketing of primary control power; the cells come, as is also common in other projects, from a foreign manufacturer. As investment volume an amount of 10 million € is indicated.
3. Redox flow accumulators
Construction and classification
In the word redox, "red" stands for reduction, means electron uptake and "ox" for oxidation equals electron release. In redox storage, the power (determined by the stack size) and the energy or capacity (determined by the tank volumes) can be set independently of each other depending on the application.
The system consists of the so-called stack for absorbing and releasing electrical power as the central component, and two peripheral, separate circuits, each with a tank filled with the respective electrolyte for charging and discharging. Two separate pumps deliver the electrolyte into the stack via the associated piping, depending on the mode of operation; ion exchange takes place there; this circuit prevents the two electrolytes from mixing, in which case the charges would cancel each other out. The pumps are operated only in the case of current delivery (discharge) or current collection (charge). Technical details can be found in this portal in the article "Redox flow batteries: Technical and Economic Investigations".
For stationary applications, as mentioned above, this type has the advantage of separating power and the capacity; however, for mobile applications in passenger motor vehicles (cars), this type is too bulky and too heavy.
As with the lithium-ion battery, various well-known research institutions in Germany are engaged in the further development of the redox-ion storage system; the topics are similar to those under 2; in addition, some commercial enterprises, some of them start-ups, are participating in the development and in the construction and operation of field tests close to the market.
Large-scale test facility in Germany (selection)
- For example, Stadtwerke Trier is operating a vanadium redox flow battery with an output of 10 kW and a capacity of 100 kWh under the project title "Parking Garage of the Future." The battery stores PV electricity, which is provided in a charging station for passenger cars. The container module, which weighs about 11 t, measures 4500 x 2200 x 2403 mm.
In this article, only the battery types currently prioritized by experts in Germany are discussed. Of course, there are other battery technologies such as nickel-cadmium or sodium-sulfur. To deal with them and others would clearly exceed the given scope.
Outlook
The great advantage of power storage batteries is their short start-up time at full load. At the moment, the lithium-ion battery is ahead in terms of characteristics related to aging, efficiency, flexibility in operation, to name just a few important features. In general, all technologies are still too expensive. From the results of the various field tests in competition of the different types as well as from further efforts in battery research, improvements, simplifications as well as reductions in costs can be expected. The battery application in the electric car will contribute above all with the unit numbers to be expected in the future to it, which technology will make finally the race for the market. In a report in the Handelsblatt was to be read with reference to a market research institute that in Germany in the year 2030 with a portion of the renewable ones of the power supply of 50 per cent the current storage achievement would have to increase on 21 gigawatt. In this forecast, the greatest potential is predicted for storage batteries, with investments of 30 billion euros.