ABSTRACT

This study applies an advanced exergy analysis to a novel unified biomass-based tri-generation energy system for power generation, heating and cooling with an even simpler cycle configuration. The thermodynamic models which demonstrates the moderate energy and exergy efficiency of the proposed system was conducted using three refrigerants - R245fa, R1234yf, and R1234ze. Variation in the behaviour of the system was established based on several performance index of the system. Additionally, the system’s susceptibility to improvement in exergy efficiency via advanced exergy analysis presented a theoretical framework for the choice of optimum operating variables for this purpose. Models were also developed to provide for the exergoeconomic performance of the system. The results demonstrate that the exergy efficiency of the system is greatly enhanced by virtue of the new Organic Rankine Cycle (ORC) arrangement to include cooling and heating as products. In fact, the incorporation of the cooling arrangement led to an increase in exergy efficiencies in the order of 28.34 %, 22.32 %, and 29.61 % with refrigerants R245fa, R1234yf, and R1234ze in that order. Thus, the system is suitable for direct coupling with a Brayton topping cycle at controlled mass flow rates of flue gas. With any of the three refrigerants, the exogenous exergy destruction is greater than the value of endogenous exergy destruction. Accordingly, the greatest contribution to the exergy destruction rate is from the internal irreversibilities of the system due to the small average exergy efficiency of the plant. With refrigerant R245fa, the total system output diminishes at temperatures in excess of 120 oC, thus establishing a broad optimum temperature of the system at this point at a pressure range of 2.4 to 2.7 MPa. At this condition, the total system output varied from 49.388 kW to 49.70 kW. Consequently, it is optimally feasible to run the system at 2.5 MPa and 120 oC. Furthermore, at the basic operating parameters of the system, the electrical cost from the system due to exergoeconomic analysis is 0.1106 $/kWh, 0.06925 $/kWh, and 0.09813 $/kWh respectively, for refrigerants R245fa, R1234yf, and R1234ze. Accordingly, the least thermoeconomic cost for the system is obtained with R1234yf at 0.06925 $/kWh which corresponds to 24.93 N/kWh, about 10.07 N/kWh less than the national energy tariff, thus demonstrating the feasibility of the proposed energy system.