In a bulk flow reactor (see Fig. 7), the TCM powder flows by means of gravity along a number of vertical plate heat exchangers. This concept has the lowest auxiliary energy use and allows for long reaction times, while heat and vapour transport may be optimised by keeping the layers of active material sufficiently thin (having a sufficiently large heat exchange area). However, special provisions will be required for the vapour transport and there is some risk of the TCM sticking to the heat exchanger plates. In addition, the system will need an active means to transport the TCM powder to the top of the reactor.

Fig. 7. TCM bulk flow reactor

For this system, no reliable value for the heat transfer between powder and plates was found in the literature. Therefore, a value of 300 W/m2K was assumed, being the lower limit for an extruder reactor, with the reasoning that also in the bulk flow concept some mixing of the material occurs. For the case of dehydration of the TCM, assuming that the heat transport would be the limiting factor for the power, for a 3kW reactor, this would lead to a reactor of about 20 cm in length and 10 cm x 10 cm base area, in which 50% of the reactor volume would consist of 11 parallel heat exchanger plates and the remaining volume would be filled up with TCM.

5. Conclusion

The practical realization of a separate thermochemical reactor would increase the feasibility of seasonal storage of solar heat in thermochemical materials. The present paper focuses on the boundary conditions for such a reactor and on reactor concepts that could fulfil these boundary conditions. Three compact powder reactor concepts for dehydration are shown. Further research is necessary to establish the practical feasibility of these reactor concepts for thermochemical storage.

References

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