Research Of Multiple-Technologies Based Combined Systems Used For Large Temperature Difference Exhaust Waste Heat Recovery Of Diesel Engine

Energy conservation and emission reduction have always been topics of great concern. As the main source of motive power, internal combustion engines?ICEs? constitute a large proportion of global fuel consumption. Meanwhile, a large amount of fuel energy is released to the ambience in the form of exhaust?In general, it represents approximately one third of energy generated from fuel combustion? without effective utilization. If this part of waste heat could be exploited effectively, fuel utilization efficiency for ICEs would improve dramatically, thus alleviating the problem of energy shortage and environmental pollution.As a promising waste heat recovery technology, organic Rankine cycle?ORC? shows great potential for its desirable thermal efficiency, high safety and low maintenance requirements. However, the organic working fluid would resolve if the temperature exceeds its decomposition temperature, which is a clear limitation on its application. There exists a mismatch between the working fluid and high-temperature engine exhaust. Hence, this paper comes up with the idea of combining high-temperature waste heat recovery technologies with ORC to establish combined systems, i.e. dual-loop Rankine cycle?DORC?, CO₂ Brayton?BC?-ORC and thermoelectric generator?TEG?-ORC, so as to recover engine's high temperature exhaust heat completely.The engine we analyzed here is an inline 6-cylinder 4-stroke, and based on the experimental data, system parametric optimization is performed to obtain the optimal operating parameters regarding output power as objective function. Based on energy utilization parameters?output power, thermal efficiency, exergy efficiency, recovery efficiency? principally, together with related size/economic indexes?turbine size parameter SP, heat transfer capacity UA?, an elaborate comparison investigation among them is carried out under various engine conditions using the above-mentioned optimal operating parameters. The results indicate that in the case of BC-ORC system, CO₂ Brayton cycle gives better waste heat recovery performance than the conventional air Brayton cycle. DORC is superior to the other two strategies in terms of energy utilization capacity. In particular, it can generate 32.63 kW power, and the corresponding thermal efficiency is 26.55% under the rated condition, but it also gives relatively high values of SP and UA. TEG-ORC system is simple in structure and it's recommended to be applied when the engine operates under a relatively low load?exhaust temperature is relatively small?.The further optimization of DORC configuration proves that: applying TEG modules cannot promote DORC system performance as expected, as the conversion efficiency of TEG is still not satisfactory, and the integration of TEG causes sharply reduction in high temperature loop net output power.In this paper, ORC-based combined power systems which regard various technologies as topping cycles to recover engine's high temperature exhaust heat are proposed. Comprehensive evaluations are conducted to provide an adapted solution for engine exhaust waste heat recovery. Further optimization on the suggested combined system has also been made. It's found that ORC-based combined system is an effective way to recover engine exhaust waste heat completely, thus reducing fuel consumption and emissions.