Conceptual design and evaluation of a hybrid energy system based on a tri-level waste heat recovery: an approach to achieve a low-carbon cogeneration system

The exhaust waste heat of the fossil-fired power plants and some power generation technologies (such as fuel cells) is a rich source of recoverable thermal energy. Meanwhile, the effective use of the fuel cell's exhaust waste heat in the downstream processes and units can improve the thermodynamic superiority, reduce the environmental impacts caused by the release of waste heat in the environment and enhance the attractiveness and sustainability of the energy process. Here, the multi-variable analysis (thermodynamic, economic and environmental assessments)/optimization of a low-carbon cogeneration process (power and cooling capacity production system) are proposed and investigated. The proposed cogeneration process works under waste heat from the exhaust of a fuel cell, an ejector refrigeration cycle and an organic Rankine cycle (ORC)-driven tri-stage waste heat recovery system. A bi-objective optimization is based on identifying the maximum value of exergy efficiency and the minimum value of the total cost of the system. To highlight the advantages, the behavior of the considered cogeneration process is compared with a system that works under a compressor refrigeration cycle and a single-stage waste heat recovery system (as a reference process). According to the results, the considered process can provide approximately 10% higher exergy efficiency compared to the reference process. Moreover, the overall cost of the considered process can be about 20% lower compared to the reference process. The amount of emitted CO2 from the proposed process can be mitigated by nearly 11 thousand tons compared to the reference process.