Phase-change material
A latent heat storage ( from the Latin latere be hidden ', so the term latent heat) is a device hidden the thermal energy loss, with many repeat cycles and over a long time can save.
One uses this so-called phase change materials (PCM, "Phase Change Materials " ), the latent heat of fusion, heat of solution or heat of absorption is much greater than the heat that they can store (without the phase transformation effect) due to their normal specific heat capacity.
Examples are heating pads, ice packs or filled with paraffin storage elements in the tanks of solar thermal systems.
- 3.1 Water
- 3.2 paraffin
Principles of Operation
Latent heat storage function by exploiting the enthalpy of thermodynamic changes of state of a storage medium. The most broadly used principle is the utilization of the phase transition solid-liquid and vice versa ( freezing - melting).
When charging the contents of commercial latent heat storage special salts or paraffins are usually melted as a storage medium, thereby absorb a lot of heat energy ( heat of fusion ), such as dipotassium hydrogen phosphate hexahydrate ). The unloading takes place as solidification, wherein the storage medium delivers the previously recorded large amount of heat as solidification heat back into the environment.
For technical applications as latent heat storage undercooling of the melt is generally not desirable. Therefore suitable nucleating agent must be added to the material of the short effect crystallization below the melting temperature.
Heat Packs
In Wärmekissen sodium acetate trihydrate is often used. It is liquefied at a melt temperature of 58 ° C, e.g. in the microwave. On heating, the cushion must constantly be completely covered by water (water bath), because otherwise locally particularly hot nascent salt to melt the bag. The material remains even at temperatures well below the melting point - possibly down to -20 ° C - as a supercooled melt in a metastable state liquid as the salt dissolves in its water of crystallization; the water molecules form a kind of their own crystal lattice, which dissolves first. If now a small metal plate (similar to a clicker ) pressed in the heat pad, solves the out crystallization. The pad heats up again to the melting temperature, wherein the total crystallization, and thus the release of latent heat can extend over a longer period.
The trigger for the crystallization of the supersaturated solution are:
- The pressure wave, which is triggered by the pressing of the metal plate,
- Thus caused the release of microscopic nuclei, which settle in small crevices of the metal at each crystallization.
One problem with the declaration by the pressure wave is that the crystallization experiment by sound waves, even by ultrasound, is not triggered.
Other salt hydrates may also be used, such as Glauber's salt with a melting point of 32.5 ° C or alum.
Features
The advantage of this technique relies on the heat storage to store in a small laid down by the melting temperature of the storage material used temperature range much heat energy in relatively little mass. The mere heating of the medium on the other hand, a larger temperature range is required to store comparable amounts of heat.
In the heat pad in addition to the metastable state of the supercooled solution is used. Thus, the heat can be stored without thermal insulation and losses.
Examples
Water
For example, during solidification or freezing of water - the phase transition from liquid water to solid ice at 0 ° C - about as much heat is released, as is required to heat the same amount of water from 0 ° C to 80 ° C. The specific phase transition enthalpy is thus compared to the specific heat capacity is relatively high ( for water: fusion 334 kJ / kg specific heat capacity of about 4.19 kJ / (kg · K ), whereby the energy density is significantly greater than that of hot water storage in conjunction with. a heat pump allows a water latent heat accumulator providing heat for evaporation of the refrigerant, and in particular during the heating period.
Paraffin
The usable amount of heat depends on the maximum and minimum effective working temperature. It consists of two components:
- Specific heat, multiplied by the temperature difference
- The amount of heat that is released during phase transitions in the usable temperature range.
Water is unsuitable with a melting temperature of 0 ° C, because this is not in the workspace.
Therefore one has to rely on materials with melting temperatures between 40 ° C and 70 ° C and with a high heat of fusion. (Water: 333 kJ / kg) Therefore, hard paraffin having a melting temperature of about 60 ° C and an enthalpy of fusion between 200 and 240 kJ / kg is suitable. The heat generation during solidification is about a third less than that of water, but they lies within the useful range.
Chemical heat storage
A similar principle is followed by the utilization of the enthalpy of reversible chemical reactions, for example by chemisorption based on absorption and desorption processes. This happens in so-called thermochemical heat storage systems that enable a greater energy density.
Applications
Modern latent heat storage materials of saline or paraffin base have been developed for various applications and physical properties are available for almost all temperature ranges. They are used for hot plates for catering, in latent heat storage for motor vehicles, in which excess engine heat is stored, to unlock them again during a cold start, or even in the heating and construction industries as wärmepuffernde building materials.
Phase change materials (PCM ) are increasingly being used in functional textiles. These can take the body or ambient heat, store and release. Thus, they allow for a buffering the temperature of a "comfort zone " down as above.
When using latent heat storage for solar heat storage of heating for the winter, even though investment is higher, the system saves compared to the use of water tanks or gravel but significantly space and, because of the utilization of the latent heat evenly release heat than this.
A calculation example is intended to illustrate the orders of magnitude. To heat a well-insulated house with an energy consumption of 100 kWh / ( m² · a ) and 89 m² living space 890 liters of oil or 890 m³ of natural gas are required (see the article " heating value "). This corresponds to an annual heat demand of 32,000 MJ. To produce this amount of heat in summer by solar absorber, about 23 m² of solar absorber surface are at an assumed 100 days of sunshine and a yield of 4 kWh / (m² · d) necessary. To save the generated by solar absorber in the summer heat quantity of 32,000 MJ for the winter in the form of latent heat, about 200 m³ paraffin are needed in a tank. In 2008, individual, filled with paraffin usual small container in a water tank. The 200 m³ corresponding to a cylindrical tank with height of 8 meters and a diameter of about 5.6 meters. With the matching in such a capacity of about 200,000 liters of fuel oil the same house, however, could be heated 225 years long.
In the waste incineration plant Augsburg a part of the waste heat generated during the incineration is stored in containers with sodium acetate since January 2013 as part of a model project. These are then transported by truck to nearby Friedberg, where the heat is used to heat a school center.
The applications in the construction industry are meanwhile very diverse, for example, in Raumumfassungen. They act thermally passive or water -carrying plastic capillary tube fitted as thermally active disks. The temporal power traces of these disks can be viewed as individual elements (eg floor heating, wall heating, cooling blankets ) are determined numerically very detailed with working simulation models. If the disks are examined together with the thermally coupled space, then a complex simulation with the simulation model is appropriate.
A new facade element ( " solar wall " ) stores in four centimeters thick as much heat as a 30 cm thick brick wall. During the day the heat is stored, and the element keeps the temperature constant at the melting temperature of the PCM, 27 ° C. A glazing keeps most of the heat " under glass ". In the summer prevents a prismatic lens that sunlight is absorbed from an angle above 40 °.
Another, not yet implemented idea is to use in washing machines and dishwashers, not unused to dissipate the heat energy from previous crossings cleaning the waste water. Is, for example, 60 ° C, the hot effluent from the wash cycle is supplied to a latent heat storage device, a portion of the heat for heating the washing water can be used next at 40 ° C and thus electrical energy can be saved.
Inventors and Patents
- Heating pad: U.S. Patent 2114396 - published: 1938-19-04, Inventor: Lyman McFarlan, Roland; Marblehead Neck; Bowles, John
- Thermophoric composition: U.S. 2118586A - published: 1938-24-05, C09K5/06, Inventor: Bowles, John; Lyman McFarlan, Roland
- Reusable Heat Packs: DE 2917192 A1 published: November 6, 1980, A61 F7/03, C09K5/06, Inventor: Arrhenius, Gustaf