Capture of Heat Energy from Diesel Engine Exhaust

Abstract

Diesel generators produce waste heat as well as electrical power. About one-third of the fuel energy is released from the exhaust manifolds of the diesel engines and normally is not captured for useful applications. This project studied different waste heat applications that may effectively use the heat released from exhaust of Alaskan village diesel generators, selected the most desirable application, designed and fabricated a prototype for performance measurements, and evaluated the feasibility and economic impact of the selected application. Exhaust flow rate, composition, and temperature may affect the heat recovery system design and the amount of heat that is recoverable. In comparison with the other two parameters, the effect of exhaust composition may be less important due to the large air/fuel ratio for diesel engines. This project also compared heat content and qualities (i.e., temperatures) of exhaust for three types of fuel: conventional diesel, a synthetic diesel, and conventional diesel with a small amount of hydrogen. Another task of this project was the development of a computer-aided design tool for the economic analysis of selected exhaust heat recovery applications to any Alaskan village diesel generator set. The exhaust heat recovery application selected from this study was for heating. An exhaust heat recovery system was fabricated, and 350 hours of testing was conducted. Based on testing data, the exhaust heat recovery heating system showed insignificant effects on engine performance and maintenance requirements. From measurements, it was determined that the amount of heat recovered from the system was about 50% of the heat energy contained in the exhaust (heat contained in exhaust was evaluated based on environment temperature). The estimated payback time for 100% use of recovered heat would be less than 3 years at a fuel price of $3.50 per gallon, an interest rate of 10%, and an engine operation of 8 hours per day. Based on experimental data, the synthetic fuel contained slightly less heat energy and fewer emissions. Test results obtained from adding different levels of a small amount of hydrogen into the intake manifold of a diesel-operated engine showed no effect on exhaust heat content. In other words, both synthetic fuel and conventional diesel with a small amount of hydrogen may not have a significant enough effect on the amount of recoverable heat and its feasibility. An economic analysis computer program was developed on Visual Basic for Application in Microsoft Excel. The program was developed to be user friendly, to accept different levels of input data, and to expand for other heat recovery applications (i.e., power, desalination, etc.) by adding into the program the simulation subroutines of the desired applications. The developed program has been validated using experimental data.