Exergetic Analysis of Solar-Powered Hybrid Energy Conversion and Storage Scenarios for Stationary Applications

The idea of this study is to investigate possibilities to use sunlight as the main energy source to generate and store electrical energy via different methods and technologies. Several systems consisting of photovoltaics, photoelectrolytic converters and solarthermal reformers in combination with fuel cells have been investigated in terms of efficiency and costs. A simple energetic approach would not account for these different kinds of energy and their differing availabilities (radiant, thermal, chemical, and electrical energy). To consider different forms of energy and compare them in a fair manner, exergy as the useful part of energy (the part that can theoretically be completely converted to work) provides a perfect instrument for dealing with complex energy conversion systems. In this study, four different scenarios have been investigated: Scenario A describes the direct conversion of sunlight to electricity by photovoltaics. The electric power is used in a Polymer Electrolyte Membrane (PEM) electrolyzer to split water to hydrogen which is stored in a pressure tank. A PEM fuel cell converts hydrogen to electricity on demand. Scenario B deals with a photoelectrolytic cell splitting water to hydrogen by solar irradiation combined with a storage tank and a fuel cell. In Scenario C, solar radiation is converted by photovoltaic cells to electricity which is stored in different types of batteries. Scenario D combines a methanol steam reformer heated by solar power with a PEM fuel cell to generate electricity. The reformate gas mixture can be stored at elevated pressure in a gas tank. In contrast to routes A–C, scenario D has two exergy inputs: Solar radiation and chemical exergy in form of methanol as fuel. All systems are analyzed for an average day in July and February in Central California, including a storage device sufficient to store the energy for one week. Scenario D reaches an overall exergetic efficiency of more than 25% in summer at the expense of an additional exergy input in the form of methanol. The exergetic efficiency of scenario C amounts to 10–17% in summer (4–6% in winter) depending on the battery type and scenarios A and B achieve less than 10% efficiency even in summer. The systems of scenarios A and C would cost around $20k–$45k per 1 kW average electricity generation during the day in July. Scenario D leads to significantly lower costs and scenario B is the most expensive design due to the current immaturity of photoelectrolytic devices.