New method for Thermochemical Heatstorage
Thermochemical heatstorage is one of the newer methods for storing and transporting heat. Although most of these storage systems are still in experimental phases, there are yet some smaller applications available in the commercial circuit, such as the widely used sodium acetate heat pads for household and medical use.
They contain a supersaturated solution of sodium acetate, where crystallisation is triggered by flexing a small flat disc embedded in the liquid. The crystallisation process releases the stored heat while the liquid transforms into a crystal powder. By heating the powder, it transforms into luiquid again, and it can be reused this way. Heat can be reused at once or being stored for days, months, or even many years, which shows the unequalled power of this method. None of the best isolated tanks and thermosses are able to store heat for such a long time without any losses of energy.
At this moment there are also a few large scaled projects using thermochemical heat storage for offices, houses, greenhouses, garden sheds, etc. A real break trough hasn't been realised yet because there are still technical issues remaining concerning adding heat to the system and releasing the allready stored heat.
Recently I have been developping a few systems which could make thermochemical heat storage easier and more efficient, thereby hopefully leading to the break trough we're still waiting for. These systems contain a 'mobile' and a 'static' version where details about it are explained below.
1. Static version
This appliance consists of a number of compartiments containing the same liquid as used in the sodium acetate heat pads. The metal disc is mounted on a steel plate which is built equal with an electromagnete. There's also a sonde built in the tank with liquid, which measures the electro conductivity of the liquid. The measured results (expressed in Siemens or microSiemens are sent to a chip or microcomputer. When sodium acetate is liquid, the measured results will be lower than when it's crystallized, because a salty solution conducts electricity very well.
The computerchip is programmed, so it will consider all measured values below x microSiemes as being a luiquid (charged) solution, and higher values are interpreted as an uncharged or crystallized solution.
When heat is demaned, and the solution is 'charged', a small electric pulse will be sent to the electro magnete, causing the steel plate bending to the magnete. This way the metal disc will be bent too and starting the crystallisation process while heat will be released.
Via a heat exchanger, which is built in the tank with liquid, the released heat will be transported to a water circuit which can be used for the central heating system or for sanitary purposes. A second heat exchanger, also built in the tank, is connected to the heat source such as a wood burner or solar collector, so the tank can be recharged after (or during) use.
Folowing picture shows a schematic presentation of such a compartiment in the storage tank:
The compartiments are built on eachother as being a part of a vertical mounted tank. Each compartiment consists of a sonde, electro magnete with steel plate and metal disc, and the two heat exchangers. When heat is demanded, one of the compartiments will be activated while the separation between the compartiments prevents the whole tank from crystallisation during one short 'heating session'. The chip/microcomputer is programmed in a way that the highest charged compartiment will be uncharged first, working its way down as the discharging process continues. This because heat tends to rise and the crystals in the discharged compartiments will act as an isolator, preventing the remaining heat to escape. Charging goes opposite, and will be more efficient that way because the eventual remaining heat will be allready absorbed by the crystals in the upperlying compartiment, and fasten the charging process.
The tank might look as shown in following picture:
Note that a completely charged tank will act as a normal boiler in a solar thermal collector. This because the liquid sodium acetate can be heated too as if it was ordinary water. The storage tank can be given a 'second charge' that way, increasing the storage capacity dramatically. One may choose to use this physical heat during summertime and store the chemical heat for use in wintertime or during colder periods. This could be implemented by programming the micro computer i a way that you can switch the system in a 'winter' and 'summer' mode. When switched in 'summer mode', the electro magnete will be disabled. A complete warming circuit using this system, might look as in following sheme:
2. Mobile version
Because this system can store heat virtually forever, a mobile version using batteries must be considered. The tank containing the sodium acetate solution is portable and could be shaped as a real battery. It contains the same elements as the static version. Now the solar collector or other heat source is connected to an adaptor which is built on the circuit of a regular heatin system. The adaptor consists of two heat eschangers, one for the charging process and one for discharging. The heat exchangers are built in such a way that the heat exchangers of the battery can be slided over the heat exchangers in the adaptor. Batteries can be designed as real batteries, with the same looks as a penlight battery and with +/- markings to put it in the adaptor correctly. This because people are used to work with batteries in their daily life. In an alternative version the battery could be equiped with hinges, so you can click it open, slide it over the heat exchanger in the adaptor, and close it again. Batteries could be produced in 5, 10, 15 or 20 kg versions for example. The adaptors can hold one or more batteries of a particular size in serial or parallel depending on the heating demands of your system.
Following (simplified) setups are making use of the 'hinge system' for battery operated heatstorage:
1. Unopened battery:
2. Opening:
3. Inside of opened battery:
4. Complete system with adaptor and batteries:
This system is easily accesible, even for those with a low budget, because one may choose to install a very basic system with solar collector, adaptor and a couple of batteries, and buy additional batteries at later times as the budget allows it. With each extra battery the yearly heating costs will decrease until a point is reached where you have enough batteries to store the needed warmth for winter heating and heating costs will be zero. The batteries have an infinite amount of charge/discharge cycles and will never expire like electric batteries do.
