(420f) Next-Generation High-Efficiency Hybrid Solar to Hydrogen Conversion with Integrated Storage

By 2050 solar technologies are projected to deliver the majority of the worldâ??s electricity. Solar energy can be used to generate both heat and electrical power. Most solar panels are designed for only one of these purposes, so an electrical photovoltaic (PV) panel is typically less than 20% efficient.

It is well known that PV cells experience a deterioration in performance (efficiency) when they are operated at high temperatures, and that this leads to particularly high losses when the solar resource is at its highest. For example, a drop in PV cell efficiency in the range 10-20% can be expected, depending on the PV technology, when operating PV cells at temperatures of ~65 °C (or, 40 °C higher than room temperature), which is easily experienced in hot climates. Hybrid PV-thermal (PV/T) solar collector technology combines PV modules with the contacting flow of a cooling fluid (gas or liquid) in a number of configurations and geometries, and offers some advantages when space is at a premium and there is demand for both heat and power. Moreover, a major motivation in PV/T technology is to cool the PV cells and therefore to increase their electrical efficiency, while delivering a potentially useful thermal output (hot fluid stream). PV/T collectors are a highly efficient technology, capable of achieving system efficiencies (electrical plus thermal) in excess of 70%.

By far the most common use of the thermal-energy output from PV/T systems (in fact most solar-thermal collector technologies) is to provide hot water at 50-60 °C for households or commercial use, however, a wide range of opportunities arise at higher temperatures whence additional power-generation cycles (e.g., with organic Rankine cycles, thermoelectric generators, amongst other) or thermal-cooling technologies (e.g., with desiccant, ad/absorption refrigeration cycles, amongst other) can be combined with PV/T modules. These additional options become viable at temperatures typically above ~80 °C, and importantly, become increasingly efficient at progressively higher temperatures.

Operating PV/T panels efficiently at high temperatures for this purpose is a significant scientific and engineering challenge, since the two modes of heat-transfer loss that result directly in the drop in performance, namely convection and radiation, are both exacerbated at higher temperatures. The reduction of convective losses can be addressed by evacuation of the panel, while radiation losses can be decreased by using selective coatings. This talk will discuss recent advances in evacuated PV/T technology with optimized surfaces and selective coatings combined with a secondary power generation sub-system based on organic Rankine cycles (ORCs) for high-efficiency conversion of solar energy to electrical power and consequently hydrogen via electrolysis. Projections at this stage suggest an improvement of the order of 20-30% relative to PV-only systems, for the same area.

In this solar-to-electricity-to-hydrogen system, energy storage options include thermal storage, electrical storage and hydrogen storage. Of these, thermal storage is relatively low cost and can be performed over a range of scales with sensible heat storage (e.g., hot water tanks), phase-change materials (PCMs) covering a range of organic or inorganic substances (e.g., molten salt tanks), or via thermo-chemical energy storage which utilizes the energy alternately absorbed and released in reversible chemical reactions and has the highest theoretical storage capacity of these solutions. Electrical-energy storage on the other hand is highly dependent on scale. At small scales of application a variety of battery technologies exist or are currently under development, which are highly suitable solutions. At large-scales, on the other hand, the costs and potential environmental impact of the widespread use of batteries (arising from the use of hazardous or scarce substances) allow serious competition from mechanical or thermo-mechanical systems, which become the technology of choice. Relevant conventional electricity-storage solutions include pumped hydro (PHS) and compressed air energy storage (CAES).

A significant effort has been placed in recent years on the development of additional electricity storage technologies, driven by the increasing penetration into the grid of renewable energy technologies for electricity generation, many of which have an unpredictably intermittent nature. Of increasing interest are the so-called cryogenic â??liquid-airâ?? (or, liquid-nitrogen) energy storage (LAES) system and a novel storage technology known as Pumped Thermal Electricity Storage (PTES). LAES uses excess electricity to operate a refrigeration cycle with which to liquefy air, which is then stored in a tank at atmospheric pressure. When the electricity is needed, the liquid air is evaporated and used to drive a turbine. PTES, on the other hand, uses excess electricity to operate a heat pump cycle with which to convert the electrical energy into thermal energy. The thermal energy, both hot and cold relative to atmosphere, is then stored in two large reservoirs (hot and cold). The reservoirs contain a material with a high heat capacity, and are able to store the energy much more compactly than PHS. When required, the thermal energy can be converted back into electricity by running the heat pump in reverse, as a heat engine. The projected round-trip efficiencies of both LAES and PTES are lower than PHS, but these systems have a number of potential benefits, including low capital cost, no geographical constraints (i.e., elevations, caverns, etc.) in terms of deployment, and no associated environmental impact. This talk will present recent advances in the development of these technologies, compare the various options with each other and with alternative thermal and hydrogen storage in the context of solar-energy conversion and storage, and finally discuss the next-steps that would enable their successful deployment. In isolation, thermal-energy storage emerges as the technological option with the lowest cost and highest technical maturity, whereas at larger scales electricity storage can benefit as part of a wider storage, transmission and distribution network that includes additional intermittent generation from renewables.