Latent heat storage
A latent heat storage (also phase change or PCM storage) is a special type of heat storage which stores a large part of the thermal energy supplied to it in the form of latent heat (e.g. for a phase change from solid to liquid). The stored heat is hidden ( Latin latere, "to be hidden"), because as long as the phase transition is not fully completed, the temperature of a substance does not rise any further despite the supply of heat. Latent heat accumulators can therefore store very large amounts of heat in a small temperature range around the phase change and exceed heat accumulators that only use the sensible heat of a substance, such as hot water storage tanks .
Since many substances with different melting points can be used as phase change material (PCM), many storage applications from cold to high-temperature heat storage can be covered with this technology. The best-known commercial applications are currently still cold packs and heat packs.
Latent heat storage systems work by utilizing the enthalpy of thermodynamic changes in the state of a storage medium. The principle most frequently used is the use of the phase transition solid-liquid and vice versa ( solidification - melting ).
When charging the contents of commercial latent heat storage media, special salts or paraffins are usually melted as a storage medium, which absorb a lot of energy ( heat of fusion ), such as. B. dipotassium - hexahydrate . Discharging takes place during solidification, with the storage medium releasing the large amount of heat previously absorbed into the environment as solidification heat .
For technical applications as latent heat storage, undercooling of the melt is generally undesirable. Therefore, suitable nucleating agents must be added to the material , which cause crystallization just below the melting temperature .
In Wärmekissen is often sodium - trihydrate used. It is liquefied at a melting temperature of 58 ° C, which is usually achieved by placing the heat pad in boiling water. When heating, the pillow must be completely covered by water at all times ( water bath ), otherwise locally particularly hot salt will melt the bag. Even at temperatures far below the melting point - in some cases down to −20 ° C - the material remains liquid as a supercooled melt in a metastable state , since the salt dissolves in its crystal water ; the water molecules form a kind of crystal lattice of their own that dissolves first. If a metal plate (similar to the one in a cracking frog ) is pressed in the heat pad, this triggers crystallization. The cushion warms up again to the melting temperature, whereby the complete crystallization and thus the release of latent heat can extend over a longer period of time.
Possible triggers for the crystallization of the supersaturated solution are:
- the pressure wave that is triggered by pressing the metal plate,
- the resulting release of microscopic crystallization nuclei that get stuck in small cracks in the metal with each crystallization.
The advantage of this heat storage technology is based on the fact that a large amount of thermal energy can be stored in relatively little mass in a small temperature range determined by the melting temperature of the storage material used . If the medium is merely heated, however, a larger temperature range is required in order to store comparable amounts of heat.
In the case of the heat cushion, the metastable state of the supercooled solution is also used. In this way, the heat can be stored without thermal insulation and losses.
For example, when water solidifies or freezes - the phase transition from liquid water to solid ice at 0 ° C - roughly as much heat is released as is required to heat the same amount of water from 0 ° C to 80 ° C. The specific phase change enthalpy is therefore relatively high compared to the specific heat capacity (for water: melting enthalpy 334 kJ / kg, specific heat capacity approx. 4.19 kJ / (kg K )), which means that the energy density is considerably greater than that of hot water storage tanks . In combination with a heat pump , a water-latent heat storage system enables heat to be provided to evaporate the refrigerant , especially during the heating season .
The usable amount of heat depends on the maximum and minimum usable working temperature. It consists of two components:
- the specific heat multiplied by the temperature difference
- the amount of heat that is released during phase transitions in the usable temperature range.
Water with a melting temperature of 0 ° C is unsuitable because it is not in the working range.
Therefore, one is dependent on substances with melting temperatures between 40 ° C and 70 ° C and with high heat of fusion. Hard paraffin with a melting temperature of around 60 ° C and a melting enthalpy between around 200 and 240 kJ / kg (water: 333 kJ / kg) is therefore well suited. The heat generated during solidification is about a third less than that of water, but it is within the useful range.
The latent heat storage at a high temperature level can be realized in metals, for example. An example of a metallic phase change material (English m etallic P hase C hange M aterial, mPCM ) is an aluminum-silicon alloy with a melting temperature of 577 ° C. Since the range of usable working temperature is larger with high-temperature storage, more thermal energy can be stored in the sensitive area. However, the specific melting enthalpy is also greater for this material with a high melting temperature than for water or paraffins (aluminum-silicon: 560 kJ / kg).
Chemical heat storage
The use of the enthalpy of reversible chemical reactions , such as absorption and desorption processes based on chemisorption, follows a similar principle . This happens in so-called thermochemical heat storage systems , which enable an even higher energy density.
Modern latent heat storage materials based on salt or paraffin have physical properties developed for various applications and are available for almost all temperature ranges. They are used in hot plates for the catering industry or in the heating and building materials industry as heat-buffering building materials .
Latent heat storage systems based on salt or paraffin are also used in vehicle technology, for example to store excess engine heat and release it during a cold start. In addition, storage systems based on metallic phase change materials (mPCM) are currently being developed for use in electric vehicles. Storage systems with high energy densities and, at the same time, excellent thermal performance potential are to take over the heating capacity in electric vehicles in the future. As a result, the energy required for heating would not have to be taken from the traction battery, which could reduce necessary battery capacities or increase the range of electric vehicles in winter.
Phase change materials (PCM) are also used in functional textiles . This allows them to absorb, store and release body or ambient heat. In this way, they enable the temperature of a “feel-good area” to be buffered both downwards and upwards.
When using latent heat storage for solar heat storage of heating energy for the winter, the investments are higher, but the system saves a lot of space compared to the use of water tanks or gravel and can give off heat more evenly than these because of the use of latent heat.
A calculation example should clarify the order of magnitude. To heat a well-insulated house with an energy requirement of 100 kWh / (m² · a) and 89 m² living space, 890 liters of heating oil or 890 m³ of natural gas are required (see the " Calorific value " article ). This corresponds to an annual heat requirement of 32,000 MJ. In order to generate this amount of heat in summer using solar absorbers , assuming 100 days of sunshine and a yield of 4 kWh / (m² · d), around 23 m² of solar absorber surface are required. Around 200 m³ of paraffin are required in a tank to store the 32,000 MJ of heat generated by solar absorbers in summer for the winter in the form of latent heat. In 2008, individual small containers filled with paraffin in a water tank are common. The 200 m³ correspond to a round tank 8 meters high and a diameter of 5.6 meters. With the approx. 200,000 liters of heating oil that fits into such a tank, the same house could be heated for 225 years.
In the recycling plant Augsburg part of the waste heat is produced by combustion in containers since January 2013 as part of a pilot project sodium stored. These are then transported by truck to nearby Friedberg , where the heat is used to heat a school center.
The applications in construction are now very diverse, for example in room enclosures. They are thermally passive or fitted with plastic capillary tube mats through which water flows as thermally active storage plates. The performance over time of these storage disks can be viewed as individual elements (e.g. underfloor heating, wall heating, cooling ceilings) and can be determined in great detail with numerical simulation models. If the storage disks are to be examined together with the thermally coupled room, a complex simulation with the simulation model is appropriate.
A new type of facade element (" solar wall ") stores as much heat in a four-centimeter thickness as a 30-cm-thick brick wall. During the day, heat is stored and the element keeps the temperature constant at the melting temperature of the PCM, 27 ° C. Double glazing keeps most of the heat “under glass”. In summer, a prismatic screen prevents sunlight from being absorbed from an angle greater than 40 °.
Another idea that has not yet been implemented is its use in washing machines and dishwashers so that the thermal energy from previous cleaning processes is not discharged into the wastewater without being used. If that z. B. 60 ° C hot waste water from the wash cycle fed to a latent heat storage, part of the heat can be used to heat the next wash water to 40 ° C and thus electrical energy can be saved.
- Patent US2114396 : Heating pad. Published April 19, 1938 , Inventors: Roland Lyman McFarlan, Neck Marblehead, John Bowles.
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- Patent DE2917192A1 : Reusable heat pad. Published November 6, 1980 , inventor: Gustaf Arrhenius (A61 F7 / 03, C09K5 / 06).
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