Which form of energy cannot be stored

Energy storage

A memory has always been required when the time lag between the procurement of an item and its consumption is too great. What can be removed later as required is then stored in the memory. On the right we see the granary (Kornhausboden) in the castle of Mildenstein Castle in Leisnig, Saxony. The floor made of fir wood in 1394/95 served as storage for the interest grain to be paid. The picture is taken from Wikimedia.

Energy stores are also filled with energy, and energy is withdrawn as required. The forms of energy for filling and extraction can be the same as the electrical energy in a storage capacitor. But they can also be different, such as the mechanical energy of the dammed water and electrical energy in a hydropower plant or electrical energy and heat in a night storage heater. For the accumulator we have the filling with electrical energy, the storage in chemical energy and the extraction of electrical energy. The accumulator also shows us the Achilles heel of some energy storage systems. It heats up when charging. So you don't get as much electrical energy out as you put in. The gas storage tanks in the gas supply system or the tanks for liquid fuels do not have this problem. They absorb chemical energy and release it again without loss.

A strong current interest in electrical energy storage arises from the increasing share of wind and solar energy in power supply systems. This increases the time discrepancy between possible generation and desired consumption and must be compensated for by so-called electricity storage. These are storage systems that take up electrical energy from the grid, store it temporarily in some form of energy and, if necessary, feed it back into the electricity grid as electricity for general supply.

How do these storage systems work, how quickly can they emit energy, what is the ratio of energy removed to energy input (overall efficiency) and what does the construction and operation of the storage system cost per unit of energy? There is also an overview article by Ausfelder et al. and further literature cited there or in Wikipedia. The following storage types are presented here: 1. Hydropower storage, 2. Compressed air storage, 3. Flywheel storage, 4. Heat storage, 5. Capacitors and coils, 6. Batteries / accumulators and 7. Electricity-to-gas storage.

Hydropower storage

Even an ordinary hydropower plant at a dam has a storage function, as the water is stored in the upper basin and the water turbine with an electric generator can be started up within a minute. Countries with a high proportion of hydropower in electricity generation (e.g. Norway or Canada) therefore have less need for additional energy storage.

A pumped storage plant can pump the water back from the lower basin to the upper basin. The first large pumped storage plant went into operation in Niederwartha near Dresden at the end of 1929, and in early 1930 the Koepchenwerk on Hengsteysee an der Ruhr reached the full installed capacity of 132 MW (now 153 MW). The water has a drop height of around 150 m in both of them, and the currently largest pumped storage plant, Bath County Pumped Storage Station in Virginia, USA, which has a capacity of 3 GW, comes at almost 400 m. In general, an efficiency of 75-80% is given for pumped storage plants. The transmission losses for transporting the electrical energy there and back must also be added.

The Sir Adam Beck electricity company at the Niagara Falls in Canada has a large pumped storage basin to generate an additional output of 174 MW at peak times. The picture on the right was taken from Wikipedia.

A total output of around 130 GW is currently installed in pumped storage plants around the world. 7.5 GW of this supply electricity between 4 and 8 hours a day in Germany. According to the AEE analysis, electricity storage in Germany took 7.8 from the grid in 2013 and returned 6.2 billion kWh back into the grid. With a gross electricity generation of around 600 billion kWh, this is around 1%. Wikipedia mentions 3 to 5 cents / kWh for the full cost of storing one kWh in a pumped storage power plant for one day. As investment costs per kW output of a system, Ausfelder give i.a. with 500-100 € per kW. When building new systems, tax requirements and expected environmental damage must be taken into account. For these reasons and because of the predominantly flat landscape, there are no plants under construction in Germany, and we are looking to neighboring countries for growth. The construction of pumped storage plants in former mines is still at the research stage, whereby a doubling of the investment costs can be assumed.

Compressed air storage

There are currently two demonstration systems for compressed air storage, see below. Air is compressed to a maximum of 75 bar with electrically operated pumps and stored in caverns about 1 km deep. When required, this compressed air operates gas turbines, which in turn generate electricity. Unfortunately, when it is compressed, heat is generated, and when it is relaxed, the air cools down, which requires additional cooling and heating. The former is a loss of energy; for the latter, energy even has to be added via a gas burner. This is why the demonstration systems only achieve an efficiency of 42% in Huntdorf (321 MW), Lower Saxony, and 54% in McIntosh (110 MW), Alabama, USA. The latter has a lower output, but a waste heat recovery, 14 minutes start time and, due to the larger storage volume, a stored amount of energy of 2860 MWh, almost five times as much as in Huntdorf.

No heat exchange with the environment may take place in an "adiabatically" operating compressed air reservoir. This requires that the heat generated during compression is stored and supplied again during relaxation. The efficiency could increase to 70%. The development project ADELE (adiabatic compressed air storage for the electricity supply) has examined the implementation possibilities for such a storage. However, the construction of the planned compressed air storage power plant in Staßfurt was discontinued in 2015 due to the lack of concrete market prospects.

Ausfelder specifies investment costs for compressed air storage as 1000 € per kW, among other things. They are therefore comparable to those of hydropower storage systems.

Flywheel storage

In every conventional power plant, kinetic energy is stored in the huge centrifugal masses of turbines and generators and thus a centrifugal mass storage is included. Even if the drive could be switched off suddenly, electrical energy would continue to be supplied. This also reveals two major problems with separate flywheel storage systems: The energy is only supplied for a few minutes and the investment costs are high. For a short-term emergency power supply in the range of minutes, flywheels made of carbon fiber composite rings or steel and an integrated electric motor can be used for charging or discharging. The greatest advantage of such a memory is the specified service life of 20 years. The biggest disadvantage is the self-discharge (halving of the rotating frequency of the flywheel after switching off the charging current without load current generation) within two hours. The use of this storage system makes economic sense if the energy can be charged and discharged in a time of up to 10 minutes. If you only consider the investment costs for the power, according to Ausfelder, a relatively low value of 100-360 € per kW results. However, if you relate the investment costs to the capacity, the result is 1000 € per kWh, a value that is 20-200 times higher than for the storage systems described so far. Therefore, despite the high efficiency of 89-90% and a deployment time of less than a second, only a few applications are found.

Heat storage

A heat storage from the past decades is the night storage heater, which, however, together with the other electrical heaters, is now outlawed as environmentally harmful. However, the hot water storage tanks, which are widely used in solar thermal systems, are environmentally friendly. They store the heat generated in the collectors via heat exchangers to the service water (drinking water) for hours or days until it is called up by the consumer. With the same household size, the typical solar hot water storage tank requires a significantly higher volume than a storage tank heated by gas and a significantly stronger thermal insulation in order to minimize heat losses even when stored for several days. However, applications for heat storage have now outgrown the domestic area. Since 2011, the Salzburg Nord thermal power station has had a 44 m high heat storage tank with a diameter of 29 m, which, when filled, offers a heat content of 1.1 GWh and an output of 60 MW for charging and discharging. The specific investment costs are 15 euros / kWh.

When 1 kg of water is heated from freezing point to boiling point, about 116 Wh are stored. With fireclay bricks you can already achieve a three-fold storage. Instead of this perceived or sensible heat storage in a temperature range without a phase transition, you can save the heat with the help of a phase transition. With water, for example, you need about 93 Wh to melt 1 kg of ice. Since the energy supply does not change the temperature during melting or evaporation, one speaks of latent heat or of latent heat storage. Such storage systems are particularly desirable for use in solar thermal power plants in order to store daytime heat for the evening. The largest test project to date with 700 kWh of storage capacity was in Carboneras, Spain, and successfully completed around 2950 operating hours and 95 cycles in the period 2010–2011. The storage tank contains 14 t of sodium nitrate. The phase change takes place at 305 ° C, which goes well with a steam turbine operating at 100 bar in a solar power plant.

Summarizing articles such as "The challenge of heat storage" by A. Thess et al. (2015) and "Thermal energy storage" by F. Scheffler (2019) describe the status of developments in this area in the specified years.

Capacitors and coils

The storage of energy in capacitors is already part of our everyday life if we use a bicycle that has a super capacitor built into the lighting. We have described energy storage in capacitors and coils on the energy side of electric and magnetic fields. Special features of the supercapacitors are a direct voltage of 2-3 V per single cell and a discharge time (half the voltage loss without load due to self-discharge) of about one month. The high investment costs, € 10,000 to € 20,000 per KWh, are the main disadvantage. The high loading and unloading capacity as well as the long service life of 105−106 Charge cycles are significant advantages over batteries. Coils as energy storage devices will remain in the research stage until superconductors are provided, which only require economically acceptable cooling. The use of superconductors is more likely to establish itself for energy transmission than for energy storage.

Batteries / accumulators

We also use the term battery for the rechargeable batteries, which in German usage are predominantly referred to as accumulators. We have explained the reason for this on the page accumulators. The most common types and their use in electrical storage systems and motor vehicles are also described there. Battery storage systems have also been used for emergency power supplies for a long time.

The largest European lithium-ion storage power plant in the Schwerin battery park with a capacity of 5 MWh is shown on a company picture on the left. The world's largest lithium-ion battery storage power plant at the Hornsdale wind farm north of Adelaide, Australia, has been in operation since 2017. The batteries have a total output of 100 MW and a capacity of 129 MWh. Decentralized battery storage systems are already being used on a large scale to maximize self-consumption of solar power. The currently largest virtual battery storage system is also decentralized: 12 sodium-sulfur battery systems with 4 MW each and 3 systems with 20 MW each, i.e. a total of 108 MW, are distributed over 10 locations in Abu Dhabi. They have been working with a storage duration of 6 hours since 2019, i.e. a total of 648 MWh, and can be controlled centrally.

The batteries are expected to have a service life of ten years and well over a thousand work cycles. So far, energy densities have barely exceeded 150 Wh / kg. The battery market is expected to exceed 300 Wh / kg in the current decade. Battery types with significantly higher energy densities are still at the research stage, see page accumulators.

Electricity-to-gas storage

Energy is stored when electricity is generated by electrolysis, which is then either liquefied or converted into methane, methanol or liquid fuels. The substances are used in chemical processes, for fueling cars with fuel cells or for generating electricity in power plants. Only when it comes to power generation do we have to deal with storage systems that take up electrical energy from the grid, store it temporarily in the form of a gas and, if necessary, feed it back into the electricity grid as electricity. Such an electricity-to-gas storage system has so far hardly been used for two reasons. Firstly, gas generation and electricity generation require expensive systems that only go into operation at peak times. Second, gas generation competes economically with natural gas, which can also be converted into electricity, which is currently much cheaper. However, all related problems are the subject of extensive developments, the status of which from the beginning of 2015 can be found in the overview article by Ausfelder et al. is shown.

When an electrical voltage is applied, water is separated into hydrogen and oxygen. The electrolysis processes mainly used for this are alkaline and PEM electrolysis (English: proton exchange membrane) as well as high-temperature electrolysis.

On the left is the colored tracing of an image that was already shown in the original on the fuel cells page. The gas battery from C. F. Schönbein and W. R. Grove contains dilute sulfuric acid in the four cells. The eight flasks surrounding the platinum electrodes were filled with oxygen or hydrogen before the gas battery was put into operation. A galvanic voltage is created. When a consumer is connected, a current flows and the gases in the flasks are converted via H.+ and O2− in water. In the figure, the consumer is again a galvanic cell (black vessel above) in which the reverse process takes place, i.e. the water in the flask is broken down into oxygen and hydrogen. This is the electrolysis of water in dilute sulfuric acid.

The water electrolysis technologies are described in a 2014 study by T. Smolinka. The alkaline electrolysis works in the temperature range 40−90 ° C with a liquid basic electrolyte, the charge carrier is OH. The anode and cathode areas are separated by a diaphragm that is permeable to the charge carrier. Cathode reaction or anode reaction are

The largest alkaline electrolysis power plant ever built has a system output of 156 MW and produces a hydrogen quantity of 33,000 Nm³ / h, see T. Smolinka. It stands at the Aswan Dam in Egypt.

The acidic PEM electrolysis works in the temperature range 20-100 ° C with a polymeric solid electrolyte, the charge carrier is H.+ and are cathode reaction and anode reaction, respectively

PEM electrolysers were able to prevail over alkaline electrolysers for niche applications with low capacities.

The high-temperature electrolysis works in the temperature range 700-1000 ° C with a solid oxide as the electrolyte, the charge carrier is O2− and are cathode reaction and anode reaction, respectively

Systems for high-temperature electrolysis are still under development. However, the above-mentioned other methods have extensive applications. However, they are used exclusively for the production of hydrogen, which is used in industrial processes, and could also produce hydrogen for fuel cell vehicles. Power generation is not profitable.

In the case of electricity-to-gas storage facilities, the focus is on the technologies for storing hydrogen. K. Müller first mentions hydrogen storage in the narrower sense. This includes gas storage in a physiosorbing carrier, an inclusion or a mixture of the gas or a reversible conversion with a liquid organic hydrogen carrier such as e.g. B. with toluene or with inorganic carriers such as metal hydrides. Energy storage with the help of hydrogen also takes place in the irreversible conversions described by K. Müller. The hydrogen is irreversibly converted into fuels such as methane and gasoline or into basic chemicals such as ammonia.

 


Last change: 04.03.2020