Environmental and economic benefits of car exhaust heat recovery
Car exhaust heat recovery is being investigated in recent years as a way to increase the efficiency of Internal Combustion engines and simultaneously to reduce CO2 emissions by converting the thermal energy to electrical, employing either thermal fluid systems (mainly the Rankine Cycle) or Thermoelectric Generators. Research has shown that conversion of 10% of this waste heat into electricity may result to an increase of fuel efficiency of up to 20% Particularly, in the case of heavy duty vehicles there is evidence in the literature that assuming certain designs and manufacturing costs, a heat recovery system can increase the total powertrain efficiency by almost 30% This paper presents an analytic model for examining the environmental and economic benefits of car exhaust heat recovery using the aforementioned technologies. The main input parameters to the model are: the cost of ownership of the vehicle, which includes fixed expenses to purchase and own the vehicle and variable costs for its use and operation. Results presented in this paper show that, for heavy duty vehicles fitted with a heat recovery system the assumed basic cost increase could be paid back within approximately 1 to 9.5 years depending on the annual mileage, fuel price and fuel efficiency benefit varying between 20% and 10% Moreover, the CO2 emissions pay back time is estimated to be 1.4 years. It is therefore showcased that use of a car exhaust heat recovery system, in substitution of the conventional alternator, is a cost effective approach and a reduction of greenhouse gas emissions per heavy duty vehicle may be achieved. The relevant increase of system efficiency is dependent on an improvement of the efficiency of commercial Thermoelectric Generators, based on new materials and structures and a dedicated design of systematic structure in the case of Rankine Cycle systems, for the harvesting of exhaust heat.