Diesel power plants remain essential for grid stability and off-grid electrification in developing countries, including Kenya. The conventional open cycle diesel generators, however, have a poor thermal efficiency of 35-45%, and most of the fuel energy is lost as heat by the exhaust gases and engine cooling systems (Saidur et al., 2012). This study focused on the technical feasibility of an open-cycle diesel power plant to be converted into hybrid cogeneration (CHP) and Organic Rankine Cycle (ORC) system. Wärtsilä 18V46 diesel engine was chosen as a case study for Kipevu III, Kenya. A steady state thermodynamic model was created in MATLAB and CoolProp (Bell et al., 2014). 44.2% efficiency was achieved during the baseline performance and 11,372 kW of exhaust heat and 3,908 kW of cooling heat were identified as recoverable. The exergy fraction values of the exhaust gas stream were 55.0% indicating its appropriateness to be used in power generation, and the exergy fraction value of the HT cooling circuit was 15.2% indicating its appropriateness to use in direct thermal application. The CHP subsystem recovered 1,834 kW of useful heat from the HT cooling circuit, which was 75% of the available low grade thermal energy. The ORC subsystem with R245fa recovered 9,666 kW of the exhaust heat (85% utilization) and produced an extra 1,256.6 kW of electrical energy, which is 7.2% more than the baseline. The overall efficiency of the integrated hybrid system was 52.0%, which was 7.8 percentage points higher than the baseline diesel plant. The sensitivity analysis showed that the most important parameter is the exhaust gas temperature where a 60°C incremental rise led to an increase in ORC power generation of 16.4%. The proposed system for the seven-engine Kipevu III plant will add 8.79 MW electrical capacity, 12.81 MW useful thermal energy, and will reduce the plant's annual CO₂ emissions by 44,424 tons which is equivalent to Kenya's Paris Agreement commitment (UNFCCC, 2015). The study concludes that it is technically possible to convert plants if they have a capacity greater than 50 MW and exhaust temperatures higher than 350°C, thereby providing a potential method for improving and upgrading the existing diesel infrastructure without complete replacement.

Keywords

Waste heat recovery, Organic Rankine Cycle, cogeneration, diesel power plant, exergy analysis.