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C1: Waste heat recovery
The energy content of Internal Combustion Engine (ICE) exhaust gases is of the same order of magnitude of the mechanical energy provided to the driveshaft. On a second law of thermodynamics standpoint, it even displays a good recovery potential due to its relatively high temperature. However, this temperature will vary significantly with engine load, especially in the case of Direct Injection (both gasoline and diesel) engines.
On one hand, the power output of thermoelectric modules tends to grow with increasing temperature (unless the Seebeck coefficient is unsuitable for that temperature range). On the other hand, thermoelectric generators (TEG) are temperature limited. So, it is desirable that the temperature they will face will be as high as they are able to safely withstand. In the absence of temperature control, the protection against module overheating would have to involve the bypass of exhaust gases, that is, high power events would be wasted and not used for electricity generation. Another alternative would be to convert the temperature of the heat reaching the TEGs to the optimal level, irrespective of engine load.
Heat Pipe (HP) based heat exchangers can be used for very low resistance heat transfer between a hot and a cold source. Their operating temperature depends solely on the boiling point of their working fluid, so it is possible to control the heat transfer temperature if the pressure of the HP can be regulated, as in the case of the Variable Conductance Heat Pipes (VCHPs). This makes HPs ideal for the passive control of TEG temperature levels.
The Present work assesses both theoretically and experimentally the merit of the aforementioned approach. A thermal and electrical model of a Thermoelectric Generator with VCHP assist is proposed. Experimental results obtained with a Proof-of-Concept Prototype attached to a small monocylinder engine are presented and used to validate the model.
It was found that indeed the HP heat exchanger enables the TEG to operate at a constant, optimal temperature in a passive and safe way and with a minimal overall thermal resistance. Furthermore, the use of VCHPs was found to prevent thermal dilution under low loads by effectively reducing the active module area and not by deprecating the temperature level of the whole generator.
