An exergy-based approach to the joint economic and environmental impact assessment of possible photovoltaic scenarios: a case study at a regional level in Italy

Nearly all energy conversion systems, and especially electricity generation plants do not operate at nominal conditions throughout their useful life: periodic or semi-periodic changes in the availability of the resource affect solar (thermal and PV), wind, hydraulic and geothermal plants, which in reality operate at off-design conditions for most of their life. To a smaller scale, fossil-fuelled plants may also be plagued by fuel availability problems. In spite of the ever growing net connectivity, since the demand curve in even large geographic regions displays a typical quasi-sinusoidal shape, fleet load modulation is unavoidable. Naturally, off-design operation and load cycling affect the cost of the generated kWh. This paper presents a general Thermoeconomic method to evaluate the economic and environmental effects of energy system integration, taking into account life cycle concerns (supply chains) and the effect of inefficiencies due to off-design operation of the systems. The method is here applied to a realistic case study of an Italian regional utility: an analysis of the implications of the variation of the productive mix between a photovoltaic power plant (PV) and a standard commercial, non-cogenerating Gas Turbine plant (GT) on the final cost of the electrical kWh. The demand curve is prescribed, and the effect of different mixes is assessed, both on the monetary and on the exergy cost of the electricity. The economic cost assessment is performed by standard Thermoeconomic techniques, whereas the exergy costs are evaluated using both the Extended Exergy Accounting (EEA) and the Thermo-ecological Cost (TEC) methods. The results show that a purely monetary cash flow accounting and thermo-economics lead to contrasting results, and also that the EEA and TEC cost indicators generate different rankings among the studied alternative GT/PV mixes.