Economic Feasibility of Power/Heat Cogeneration by Biogas-Solid Oxide Fuel Cell (SOFC) Integrated Systems

Based upon the thermodynamic simulation of a biogas-SOFC integrated process and the costing of its elements, the present work examines the economic feasibility of biogas-SOFCs for combined heat and power (CHP) generation, by the comparison of their economic performance against the conventional biogas-CHP with internal combustion engines (ICEs), under the same assumptions. As well as the issues of process scale and an SOFC's cost, examined in the literature, the study brings up the determinative effects of: (i) the employed SOFC size, with respect to its operational point, as well as (ii) the feasibility criterion, on the feasibility assessment. Two plant capacities were examined (250 m(3)center dot h(-1) and 750 m(3)center dot h(-1) biogas production), and their feasibilities were assessed by the Internal Rate of Return (IRR), the Net Present Value (NPV) and the Pay Back Time (PBT) criteria. For SOFC costs at 1100 and 2000 EUR center dot kW(el)(-1), foreseen in 2035 and 2030, respectively, SOFCs were found to increase investment (by 2.5-4.5 times, depending upon a plant's capacity and the SOFC's size) and power generation (by 13-57%, depending upon the SOFC's size), the latter increasing revenues. SOFC-CHP exhibits considerably lower IRRs (5.3-13.4% for the small and 16.8-25.3% for the larger plant), compared to ICE-CHP (34.4%). Nonetheless, according to NPV that does not evaluate profitability as a return on investment, small scale biogas-SOFCs (NPVmax: EUR 3.07 M) can compete with biogas-ICE (NPV: EUR 3.42 M), for SOFCs sized to operate at 70% of the maximum power density (MPD) and with a SOFC cost of 1100 EUR center dot kW(el)(-1), whereas for larger plants, SOFC-CHP can lead to considerably higher NPVs (EUR 12.5-21.0 M) compared to biogas-ICE (EUR 9.3 M). Nonetheless, PBTs are higher for SOFC-CHP (7.7-11.1 yr and 4.2-5.7 yr for the small and the large plant, respectively, compared to 2.3 yr and 3.1 yr for biogas-ICE) because the criterion suppresses the effect of SOFC-CHP-increased revenues to a time period shorter than the plant's lifetime. Finally, the economics of SOFC-CHP are optimized for SOFCs sized to operate at 70-82.5% of their MPD, depending upon the SOFC cost and the feasibility criterion. Overall, the choice of the feasibility criterion and the size of the employed SOFC can drastically affect the economic evaluation of SOFC-CHP, whereas the feasibility criterion also determines the economically optimum size of the employed SOFC.