Solar Energy for Industrial Rooftops : An Economic and Environmental Optimization

Industrial applications present a largely undeveloped market for solar technologies, especially heat applications in the medium temperature range. At present, the vast majority of industrial process heat demand is met by natural gas, but several solar technologies are available which could potentially displace a meaningful fraction of the gas used in this application. The flat rooftops of factories can support solar thermal (ST) collectors, photovoltaic (PV) collectors, or a mix of both. However, determining the best solar mix is not straight-forward due to the number of parameters involved. To address this challenge, this paper conducts a multi-pronged optimization for several metrics in several characteristic locations around the world to maximize solar performance and minimize the system cost and environmental impacts. The environmental impacts considered here include the embodied energy and embodied greenhouse gas emission during the solar technology manufacturing process. Annual optimizations were conducted for each location using TRNSYS and Genopt. A particle swarm optimization (PSO) method was used in all optimizations to find the global optima of the stochastic trends. Thus, monocrystalline PV and linear Fresnel solar thermal collectors were compared via an hour-by-hour optimization of various objective functions was performed in— Sydney, Australia; Andir, China; and Prescott, USA. The objective functions were the embodied energy payback time, embodied greenhouse gas emission payback time, system performance, levelized cost of energy, and a single function that combines these indicators. Our results indicate that in locations where the direct normal irradiation is high, the optimum ST fraction increases. In Andir, China, Sydney, Australia, and Prescott, USA the resulting system performance indicated that ST system should occupy (50, 30, and 53) % of the available area, respectively. However, in the same locations using embodied energy, greenhouse gas emission and the economic indicators, the optimum ST area to available space ratio is (100, 70, and 100) %, (45, 0, and 90) %, and (19, 0, and 100) %, respectively. A sensitivity analysis of the solar technologies cost variation, interest rate, natural gas, and subsidies was also conducted. Although these variables are significant for the absolute LCOE and system feasibility, it was found that the optimum solar mix is impacted by the interest rate or subsidies and the natural gas price has only a marginal impact on the optimum solar mix. Overall, we believe this study provides guidance on how to compare different solar technologies based on critical indicators.