Heat stores are stores for thermal energy ( energy stores ). A distinction is made between stores for sensible heat , latent heat stores and thermochemical heat stores . Heat storage can be built in different sizes, ranging from decentralized small systems to large central storage. They are available as short-term as well as seasonal storage and, depending on the design, can absorb and release low-temperature heat for space heating or high-temperature heat for industrial applications. In addition to the storage of thermal energy, the most important goal of heat storage systems is to decouple the generation and use of heat over time.
Types of heat storage
- Sensitive heat storage
- They change their “perceptible” temperature during the charging or discharging process . B. Buffers . The heat capacity is one of the most important parameters for sensitive storage materials. Since this type does not undergo any phase changes , it can be used over a wide temperature range, especially in the high temperature range.
- Latent heat storage
- They do not change their “perceptible” temperature during the charging or discharging process, but the heat storage medium changes its physical state . Usually this is the transition from solid to liquid (or vice versa). The storage medium can be loaded or unloaded beyond its latent heat capacity , which only then leads to a temperature increase or decrease.
- Thermochemical heat storage or sorption storage
- They store the heat with the help of endothermic and exothermic reactions, e.g. B. with silica gel or zeolites .
In addition, a distinction can be made between open so-called aquifer reservoirs built into the ground and the usual container constructions.
- Utilization rate
- The degree of utilization of a storage facility is determined from the ratio of the stored usable energy and the energy supplied to the storage facility. With conventional water storage tanks, the degree of utilization decreases over time because heat is released into the environment. (Dependencies: surface of the storage tank, insulation material and thickness, temperature difference between storage medium and environment, see also: time constant ). This does not apply, or to a lesser extent, to thermochemical heat storage systems.
- Energy storage density
- The energy storage density describes the maximum amount of energy ( heat capacity ) that can be loaded into a storage unit in relation to its volume (or its mass) under given conditions .
- Loading and unloading time
- The time it takes to add or remove a certain amount of energy from the storage unit.
- Maximum loading temperature
- The maximum temperature of the tank.
- Feasible memory cycles
- The period between loading and unloading is called the storage period. The sum of loading, idle and unloading times represents the duration of a storage cycle. If irreversible processes take place during this process that impair the storage capacity, the number of executable storage cycles is limited. In the case of sorption storage (thermochemical heat storage), this requirement essentially relates to the stability of the adsorbents .
Areas of application
There are long-term and short-term storage.
Long-term heat storage can e.g. B. seasonal heat storage in the low-energy solar thermal . The most important types are: hot water heat accumulators (insulated containers with water), gravel / water heat accumulators (insulated containers with a gravel / water mixture), geothermal heat accumulators (soil up to 100 m deep is heated) and aquifer heat accumulators ( Groundwater and earth are heated - only works with standing groundwater). Thermochemical and most latent heat storage systems are also designed as long-term storage systems.
Short-term storage systems are those that only store heat for a few hours or days. Independently standing water storage tanks are mainly used for this, but thermochemical heat storage tanks can also be suitable.
Regenerators are short-term storage systems in which heat accumulates discontinuously, which is stored and released again. These heat accumulators are often used for air preheating in branches of industry where very large amounts of waste heat occur (e.g. iron or steel industry or blast furnace ( furnace gas ) in blast furnace systems). Regenerators in Stirling engines only have to temporarily store heat for a few milliseconds.
Short-term storage, also known as shift storage, are also used in the field of industrial solar thermal energy. They buffer solar energy for a few hours so that heat is available for hot water or heating purposes even during the night or electricity can be produced around the clock in solar thermal power plants.
Another use of short-term storage is storage heaters , in which electrical energy is stored during the night in the form of heat in fireclay bricks , which is then released again on the following day to heat the apartment. In the 1970s, asbestos-containing panels were common as a material, but their use has long been banned and can only be disposed of by appropriate specialist companies during renovation. The trade name “heat storage” is also common for the individual devices. The principle of storing heat in stones is also used in a large-scale experiment. With excess electricity, gigantic, well-insulated piles of stones are heated to 600 degrees using a fan heater, so that excess energy is stored in the form of heat. When electricity is required, fans suck the hot air out of the storage tank and feed it into a steam turbine, which in turn drives a generator. In 2019, an electrothermal energy storage facility in Hamburg with 1000 t of rock, 750 ° C and 130 MWh storage capacity was put into operation. Similar systems that can be charged with hot gas (including exhaust gases) are also built as mobile heat storage systems. Thus not only the temporal decoupling of heat generation and use is given, but also the spatial decoupling. In this way, excess heat from the steel industry, ceramic industry or glass industry can be used for further external use.
In addition to the differentiation according to storage duration, information about the temperature range can often be found. Until 2016, a distinction was made between low-temperature storage (<120 ° C) and high-temperature storage (> 120 ° C). This area has been expanded to include medium-temperature storage tanks since 2016. These are used at 120–500 ° C, whereas high-temperature storage systems have been assigned the range> 500 ° C.
Water for heat storage
Water is an excellent heat transfer medium because it has a very high specific heat capacity and is easy to use due to its low viscosity and toxicological harmlessness. The entry and withdrawal of thermal energy are uncomplicated and cheap.
The maximum storage tank temperature is usually limited to the boiling point (which is dependent on the system pressure) by regulating the system. An overheated water storage tank is also called a steam storage tank . Pressure relief valves or predetermined breaking points ensure a controlled pressure reduction before the danger of an explosion arises.
In a pilot project in Switzerland near Bern-Forsthaus, a geological water- molasses site at a depth of 200–500 m is to be drilled and used as a seasonal heat store. The groundwater, which is separated above it, is to be protected from warming by the vacuum-insulated passage of the pipes of the heat exchanger circuit.
Steam storage (Ruth storage)
A storage tank is mostly (e.g. 90%) filled with boiling water. The rest of the space above the water is filled with water vapor at the same temperature. If steam is withdrawn, re-evaporation begins. The heat required comes from the boiling water. Pressure and temperature drop. That is why one speaks of a gradient storage tank. It was invented by the Swedish engineer Johannes Ruths (1879–1935). The working range of the steam storage tank is defined by the initial and final parameters (pressure and temperature) as well as the initial degree of filling with boiling water. The decisive storage parameter is the ratio of the amount of steam that can be withdrawn per storage volume. It can be calculated with given boundary conditions. Once the minimum discharge pressure has been reached, heat must be fed back into the steam accumulator. As a rule, this is done by introducing water vapor , whereby the pressure must be above the withdrawal pressure at the beginning of the depletion. Typical applications are used to equalize the steam consumption, which in industrial processes can fluctuate greatly due to technological reasons.
High temperature storage (HTS)
High-temperature storage systems are short-term storage systems and usually consist of ceramic, or better still, metallic compounds. They are characterized by their high pressure fire resistance, thermal shock resistance and specific heat capacity. They are mostly simply set in heating boilers (constant temperature, low temperature, steam and hot water boilers) without damaging their substance. They are heated up by the burner flame. If the burner switches itself off, the HTS now continuously transfer the stored thermal energy to the heating system. This means that the repeated activation of the burner can be delayed. The Institute for Technical Thermodynamics of the German Aerospace Center (DLR) is now pushing research efforts in Germany to use this technology to store thermal energy that is generated in power plants.
Latent heat storage
Latent heat storage systems work by utilizing the enthalpy of reversible thermodynamic changes in the state of a storage medium, such as B. the phase transition solid-liquid (melting / solidification).
The most frequently used principle is to utilize the solid-liquid phase transition. When the contents of commercial latent heat storage systems are charged, special salts or paraffins are usually melted as a storage medium, which for this purpose absorb a great deal of thermal energy - the heat of fusion. Since this process is reversible, the storage medium gives off exactly this amount of heat when it solidifies. In addition, metallic storage media, so-called metallic phase change materials (English m etallic P hase C hange M aterials, mPCM ) may be used. These are characterized by a particularly high energy density due to a high maximum storage temperature and by a high thermal output due to good thermal conductivity.
The use of latent heat accumulators for long-term solar heat storage of heating energy for the winter is associated with higher purchase investments, but it is more space-saving and more even than the use of water tanks or gravel due to the utilization of latent heat. Hard paraffins melt at around 60 ° C, the heat of fusion is 200–240 kJ / kg, around a third lower than the heat of fusion of water, and the heat capacity is around 2.1 kJ / (kg · K), half as large that of water. In addition, there is the advantage that two thirds of the heat are permanently stored in the phase transition for months. When designing a paraffin storage tank, it must be taken into account that its volume is reduced by around 30% when it changes from liquid to solid.
Probably the best-known application of the latent heat storage principle is the regenerable pocket-sized hand-warming pillow based on an oversaturated sodium acetate trihydrate solution. One area of application that is currently opening up is the integration of latent heat storage systems based on metallic phase change materials ( mPCM ) in battery-electric vehicles. The added value in the electric car arises largely when the outside temperature is cold. Here, the necessary heating power for temperature control of the passenger cell can be provided by the thermal storage instead of the traction battery, which could reduce the necessary battery capacities or increase the range of electric vehicles in winter.
→ Main article: Thermochemical heat storage
Thermochemical heat accumulators use the heat conversion of reversible chemical reactions: The heat transfer medium used changes its chemical composition when heat is supplied, and when the conversion is initiated from the outside, most of the supplied heat is released again. In contrast to buffer and latent heat storage systems, thermochemical heat storage systems enable the almost loss-free storage of larger amounts of heat over longer periods of time. Therefore they are suitable for. B. as a seasonal storage for solar thermal applications in regions with high seasonal temperature differences.
For the successful demonstration of the use of a thermochemical sorption heat storage system in a solar-thermally heated passive house , the Fraunhofer Institute for Solar Energy Systems won the innovation award of the states of Berlin-Brandenburg in 1999.
Thermochemical storage systems were tested in technical applications as early as the 19th century. One of the first known applications of the technology was the soda locomotive , which went into operation in 1883 . Thermochemical heat accumulators are available today in many variants, including self-cooling beer kegs . The heat capacity is up to 300 kWh / m³, depending on the type of technology, and is thus around a factor of five higher than that of water.
- electric storage heater
- Hot water
- District heating storage
- Steam storage locomotive
- mobile heat storage
- For a water reservoir: see thermal resistance
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- Gütegemeinschaft Phase Change Material (PCM) e. V.
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