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Contact

Institute for Powertrains and Automotive Technology
Vienna University of Technology 
Getreidemarkt 9
1060 Vienna, Austria
Phone: +43 1 58801 31500
Mail: info[at]ifa.tuwien.ac.at

Office opening hours:
Mo-Fr: 8 am - 4 pm

Heat storage

As a sensible approach to reduce the fuel consumption of cars belongs the storage of waste heat of internal combustion engines and utilization at the next cold start to minimize the warm-up period. A new and innovative heat storage system should help to compensate various problems, such as low storage density, the need of thermal isolation of storage medium, losses at long parking duration and the difficult placement in the vehicle on the one hand as well as to improve different important criteria on the other.

The focus is on the utilization of the waste heat which is released from the engine to the coolant. The usable temperature level amounts to 95 °C. At this temperature level, salt-based sorption processes have very high energy densities. During the sorption process, an intercalation of sorbate molecules (water or alcohol) into the storage medium (salt, acts as absorber) takes place.

Basics of the heterogeneous evaporation
During the process, a gaseous supply of the sorbate is preferred. In terms of the phase transition gaseous / liquid, the overall volume can be kept very compact and the evaporation heat can be used energetically. In wide ranges the reactions runs isothermally, the temperature depends on the pressure. The pressure-temperature-diagram, Fig. 1, represents the vapour pressure curve of the reactants schematically (log p dependent on T).


pressure-temperature-diagram
In the heterogeneous evaporation, three vapour pressure curves exist: The left curve represents the vapour pressure over its pure liquid phase (sorbate B), the middle one represents the sorbate B over the absorber A and the right curve, the vapour pressure over the dried absorber material A. Besides the simple separation of the gaseous reaction products from the solid, advantages of the heterogeneous evaporation are the easy condensability of the sorbate B and the reduced space necessary for its storage (change of the phase state).

The process of the heat storage and the heat release can be described as follows: At the temperature level TICE, waste heat of the internal combustion engine is applied (engine waste heat) and so the sorbate B is desorbed out of the absorber. This gas condenses in the sorbate reservoir at an environment temperature level TEnv during isobaric cooling until pressure p1 is reached. Thereby, the condensation heat amount is released. After separating of the reactants of the reactor by disconnecting from the sorbate reservoir the liquid sorbate B can be stored at the temperature level of the environment for any length of time without any losses. Also the absorber A can cool down to the ambient temperature level, so the stored heat during this process amounts to engine waste heat - condensation heat.

For heat release (absorption heat) a valve between the sorbate reservoir and the reactor has to be opened and hence the sorbate B is evaporated by heat which is supplied from the environment. Thereby, the heat amount evaporation heat is supplied at the temperature level TEvap - this effect can also be used for the air-conditioning of the car. Subsequently the gas B flows through the valve and get absorbed in the absorber, heat is released at the temperature TABS. Although during the storage process the heat amount engine waste heat - condensation heat has been stored, now the heat released during condensation can be reclaimed again, when it is applied from the environment at the level > TEvap.

Heat storage prototype
After the basic suitability of salt hydrates with alcohol as sorbate was confirmed in a test bench, accordingly a modular heat storage prototype was developed. Basically, this storage system permits investigations adapted to automotive applications and delivers statements related to heat release during the absorption process. Fig. 2 shows the assembly of the heat storage prototype schematically. It consists of a reactor part (package with 19 cylindrical single reactors), which contains the absorber material and the sorbate reservoir, in which the condenser, a heating element and a window are placed.

prototype, schematic
In addition to its compact and modular design there are some constructive detailed solutions, which permit a quick change of the materials. The test bench is built up mobile for transport in a climate chamber to carry out low temperature investigations. Thus, exothermic reactions at temperatures up to -20 °C can be detected with different absorber materials.

Besides the determination of temperature and pressure profiles, it is focused on a reduction of desorption duration as well as optimization of the diffusion behaviour. The research shows, that reproducible measurements are possible and the chosen materials for the application are suitable in general. As well, the removal of evaporation heat can be verified, which results in a high temperature decrease.


Contact
Dr.techn. Michael Jakobi, MSc
Phone: +43 1 58801 31583
Mail: michael.jakobi[at]ifa.tuwien.ac.at