The gap between energy supply and demand keeps increasing. This has resulted in search for alternate fuels as well as using the conventional fuels in the best efficient way. For the latter cause, increasing the energy efficiency by incorporating various systems is considered. Such systems use the unused energy that goes waste. One such energy in automobile engines are the exhaust gas heat energy. A lot of heat is thrown away by the engine through the exhaust gas and all the energy in the gas is being wasted by just letting the air into the atmosphere. Various researches are being done on efficiently utilizing the energy. Waste heat recovery system is one such system that recovers the heat that goes waste from the engine and stores it elsewhere. The stored energy can then be used in subsystems where heat is required. For example, in places where heat is required to heat the water/air or for heaters that would need the use of hot air.
Latent heat thermal energy storage system (LHTESS) uses the principle of latent heat and stores energy. Latent heat is the heat required to change the phase of the material from either solid to liquid or from liquid to gas and vice-versa, by keeping the temperature constant during the process. Heat transfer is the basic principle that every such system follows. Heat gets transferred from a body of higher temperature to the body of lower temperature. We experience this in our day to day life. A hot coffee that is stored in a coffee mug as shown in Figure 1, cools down gradually over time, if we leave it open in contact with the air, by the same principle. Also, one can notice that the coffee would cool down at a much faster rate when it is stored in a much wider bowl than the mug. An increased contact surface gives rise to faster heat transfer. The heat from the coffee mug is transferred to the atmosphere which is colder than the coffee. So, the surrounding air gets heated. Now, this heat is not utilized, and it goes waste. But in a LHTESS, such heat is stored in a phase change material (PCM). A PCM changes phase from solid to liquid or a liquid to gas and vice versa on the application of a latent heat. The most practical PCM is always a solid-liquid transition material. The PCM used should also satisfy many thermal properties.
Figure 1: Illustration of heat transfer (Source : https://www.examfear.com/notes-dir/00/00/15/00001559.html) On integrating the LHTESS with an engine, the waste heat from the engine can be stored. The waste heat from the exhaust system of an engine is made to pass through a chamber. This chamber is filled with several pipes in order to increase the surface area for effective heat transfer. In between the pipes, PCM is used.
Figure 2: Working Principle of PCM (Source : http://andores.sell.everychina.com/p-107043515-cooling-thermal-energy-storage-using-phase-change-materials-paraffin-wax-pcm.html) During the charge stage, the hot exhaust gas passes through the chamber and crosses the pipe. The heat from the gas gets transferred to the surrounding pipes and consequently to the PCM. The temperature of the PCM gets increased and once the transition temperature is reached, it starts to melt. Using the latent heat, the PCM absorbs the energy and stores it at a constant temperature as depicted in Figure 2. This stored heat energy can then be released when the PCM is cooled down and the phase is changed back to solid. The released heat energy can be used in subsystems that require heat for its effective operation. This would help in reducing the power demand for the subsystems from the engine and the overall efficiency can be improved.