Tri-Generation System Configuration Selection Based on Energy and Exergy Analyses

Abstract

A tri-generation system combining cooling, heating, and power generation can contribute to increased system efficiency and thereby reduce greenhouse gas emissions. This study proposed a novel concept using 100-kW polymer electrolyte membrane fuel cells (PEMFCs) as the basis for a tri-generation system with an integrated heat pump and adsorption chiller for greenhouse use. Three configurations of heat pump loop were designed to recover the waste heat from PEMFCs and used either for direct heating or cooling power generation in adsorption cooling. Analyses were carried out in terms of primary energy rate (PER) and exergy efficiencies. Of those investigated, the layout with a heat pump and internal heat exchanger demonstrated the best performance, with PERs of the cooling and heating modes at 0.94 and 0.78, respectively. Additionally, the exergy analysis revealed that the exergies are mostly destroyed at the expansion valve and evaporator due to differences in pressure and temperature. These differences are minimized when the system layout contains a cascade heat pump loop or an internal heat exchanger, thus resolving the problem of exergy destruction. As a result, the total exergy destruction in the system was decreased from 61.11% to 49.18% and 46.60%, respectively. Furthermore, the proposed configurations showed 36.1% and 31.4% lower values in terms of energy consumption compared with relevant works in the heating mode and cooling mode, respectively.

A unified deduction of the expressions of exergy and flow exergy

“…are the entropy and the number of moles of the ith constituent of R 0 . By substituting equation (24) in equation (23), one gets…”

“…If, in addition, system A is closed, the summation term in equation (24), that yields the summation term in equations (31), ( 38) and (39) is vanishing, because there are no changes in the mole numbers of R 0 . Thus, the exergy of a closed system is given by…”

“…Many scientific papers presenting results of exergy analysis appear every year in the literature. They concern, for instance, combined cycle power plants [1][2][3][4], steam Rankine cycles [5][6][7][8], organic Rankine cycles [9][10][11][12], refrigeration cycles [13][14][15][16], thermal and PV solar systems [17][18][19][20], fuel cells [21][22][23][24], heat exchangers [25][26][27][28]. Review papers on the exergy analysis of combined cycle power plants [29], steam power plants [30], solar energy applications [31], fuel cells [32], and heat exchangers [33] are available.…”