Solar-Electrochemical Devices for Water Splitting and Chlorine Generation

Solar technologies hold the potential to ultimately counteract the effects of climate change. Nevertheless, the intermittency of sunlight poses technical challenges regarding the storage of the surplus production and the supply of the deficit. The generation of chemical fuels is of great interest to store energy for prolonged periods. Hydrogen has been a key candidate to address the issue. The possibility to generate hydrogen exploiting sunlight has been widely investigated in recent years. Fully decoupled devices seem the most promising pathway. The exploitation of earth-abundant elements is indispensable for the future of solar hydrogen, as they are cost-effective and they are inclined to meet the requirements of generation at a global scale. We have designed and tested a solar hydrogen generator solely employing earth-abundant materials; its efficiency is the highest reported to date for such class of devices. The solar-to-hydrogen efficiency we have recorded is > 14%, improving by more than 40% the prior state of the art. We have employed tailor-made silicon hetero-junction solar cells, coupled to an alkaline electrolyzer. The main hurdle that prevents the large deployment of solar hydrogen technologies is their cost. Those financial barriers can be overcome by coupling hydrogen evolution with a more cost-viable oxidation reaction. Generation of halides is a promising pathway to substitute oxygen evolution. The chlor-alkali process is a major electrochemical operation in the industry, and generates hydrogen, chlorine and sodium hydroxide. In my work, I have designed and tested a novel solar chlor-alkali reactor, which integrates a novel solar concentrator developed by a local startup and multi-junction solar cells. I have recorded solar-to-chemical conversion efficiencies > 25% under real outdoor illumination, doubling prior values reported in literature. Chlorine and chlorinated products can be deployed for disinfection of drinking water. Liquid solutions of sodium hypochlorite can be preferred over chlorine gas for such applications. I have demonstrated and compared the performances of solar-powered devices for hypochlorite generation, employing multi-junction technologies or silicon hetero-junction solar cells. I have characterized the efficiencies under real outdoor conditions, revealing solar-to-chemical conversion efficiencies of 10 â 14% for silicon-based cells, and confirming > 25% values for multi-junction cells. I have then developed a computational model that predicts efficiencies, throughputs and production cost in three locations of choice. The results suggest that solar-hypochlorite can outcompete traditional hypochlorite. Deployment of solar hypochlorite generators in developing countries can address the lack of access to drinking water those communities suffer. I have evaluated the performances of a simpler and more robust batch-type hypochlorite generator. The results highlight that the throughput can be enhanced by structuring the electrode plates. I have assessed the economics of hypochlorite production via solar-powered batch reactors; the cost is higher, but is still competitive with respect to the supply of bulk hypochlorite. My work holds the potential to inspire and spur further innovation in the field of solar-electrochemical operation, in which the platforms I have demonstrated and assessed can represent a starting point towards a new generation of devices.

Thermo-Economic Optimisation of Solar Tower Thermal Power Plants

James Spelling

Amidst a backdrop of rapidly increasing world-wide electricity demand, solar thermal power generation shows great potential for supplying the electricity needs of numerous countries in the Sun-belt regions of the world. In these regions, the absence of significant biomass, hydrological or geothermal reserves makes solar thermal power the most promising solution for meeting that most fundamental of demands: energy. Amidst the different options available for harnessing the Sun’s power, solar thermal technologies have been shown capable of producing electricity at the most economically viable rates [32]. However, all currently existing solar thermal power plants have been based around the use of Rankine, or steam turbine, cycles which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. With their high concentration ratios, solar thermal power tower systems are amongst the best options for making use of the Sun’s potential. As the energy delivery temperature of central receiver systems is higher, they harness the solar radiation at a higher exergy level [4]. This higher temperature also opens the road to the use of more advanced thermodynamic cycles. Hybrid fossil-fuel – solar systems are also imaginable [13], as the energy delivery temperature is compatible with standard gas turbine cycles. Hybrid systems can help mitigate the requirements of thermal energy storage, as well as reducing the perceived risk when investing in new solar technology, which stimulates research and development, as well as providing employment. Recent developments in the field of high temperature volumetric receivers [10] along with rock- based packed-bed storage systems have opened up an interesting possibility. High temperature receivers allow the use of higher-efficiency combined-cycle setups, whereas packed-bed units offer the possibility of cheap storage. Larger storage volumes allow pure-solar systems to extend their power production into the night, making hybrid options less attractive. This is compounded by the common practice of guaranteeing a preferred tariff for electricity sales from pure-solar sources With this in mind, a complete dynamic model of a power plant based on the pure-solar concept has been elaborated. The performance of the setup is evaluated over a range of days, using solar insolation profiles obtained from satellite data. In order to examine different design options and their impact on the performance of the power plant, a multi-objective, thermo-economic optimisation of both the power plant superstructure and operating conditions was performed using the new, dynamic models. By means of an evolutionary algorithm [17], a family of Pareto-optimal points were obtained, representing the trade-off between increased power plant efficiency, and lower levelised energy costs. Through use of optimisation, it was shown that exergetic efficiencies in the region of 21-27% can be achieved, for relatively low power outputs of between 3 and 11 MWe. These configurations correspond with levelised electricity costs in the region of 14-21 UScts/kWhe. In most regions, the use of solar power is encouraged by the guarantee of a preferential tariff for electricity sales from renewable sources. Sale of electricity at a price of around 24 UScts/kWhe [32] should therefore be imaginable, ensuring the economic viability of solar thermal power tower systems. It can be concluded that, when properly designed, solar thermal power plants based on combined cycles are both economically and thermodynamically promising. By raising the efficiency of the plant, the size of the solar collector field is diminished, increasing the otherwise low energy densities of solar power systems. By reducing the levelised electricity costs, solar thermal power plants become more economically viable, accelerating their construction and thus, hopefully, reducing our dependence on fossil-fuels.

2009

Assessment of Solar Thermal Power Technologies

James Spelling

In the present climate of growing awareness about the dangers of global climate change, the development of new renewable energy technologies is primordial in the efforts to reduce emissions of carbon dioxide and other greenhouse gases. Rapid increases in the price of oil and worries about the stability and security of the extraction of fossil fuels have lead to renewed interest in the development of local energy sources, thereby reducing dependence on foreign fuel sources. Amid the plethora of option available, one of the more promising solutions is the increased use of solar power. Two main contrasting technologies exist for the conversion of the Sun’s energy into useful work, namely photovoltaic cells and solar thermal systems. The development of photovoltaic power has shown massive growth in recent years despite high costs, due mainly to the modular nature of the cells which allow their installation by private citizens, coupled with a number of government incentives. Development of solar thermal power generation has however been reasonably limited until recently, with the only continually operated commercial plants being the Luz SEGS I-IX plants at Kramer Junction in the United Sates. Solar thermal power generation shows great potential for supplying the base electricity needs of numerous countries in the sun-belt regions, stretching from the tropics to the Mediterranean. This has lead to a recent renewal in the development of such systems, notably with the construction of the PS10 central receiver plant in Spain. The Spanish government plans to bring the total solar generation capacity to a total of 300 MW by 2013. In other countries around the world, interest is growing in solar thermal power, with increased government and private spending fuelling research and innovation. Most solar thermal conversion technologies are still at the experimental stage, and no single technology has yet to impose its dominance. Different countries and research groups hold different solutions dear, but if solar thermal power is to become a fully mature electricity generating technology, the best of these diverse solutions must be combined in order to achieve optimal performance of the power plants.

2008

Evaluation of a Combined Cycle Setup for Solar Tower Power Plants

James Spelling

As our awareness of the dangers of global climate change grows, the development of new renewable energy technologies is of primary importance in the effort to reduce emissions of carbon dioxide and other greenhouse gases. Rapid increases in the price of oil and worries about the stability and security of the extraction of fossil fuels have lead to renewed interest in the development of local energy sources, thereby reducing dependence on foreign sources. Amid the plethora of option available, one of the more promising solutions is the increased use of concentrating solar thermal power. Solar thermal power generation shows great potential for supplying the base electricity needs of numerous countries in the sun-belt regions, stretching from the tropics to the Mediterranean. This has lead to a recent renewal in the development of such systems, most notably with the construction of the PS10 and PS20 central receiver power plants in Spain. However, all currently existing solar thermal power plants have been based around the use of Rankine or steam turbine cycles, which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. Recent developments in the field of high temperature solar receivers [13] have made it possible to imagine the use of solar thermal energy to drive gas turbine units, potentially allowing the use of combined cycle setups in solar thermal power plants. In order to evaluate the potential improvement in efficiency that can be achieved by using combined cycles in solar thermal power plants, a detailed simulation of such a setup will be performed. By taking into account the requirements of thermal energy storage and the design of the necessary heat exchangers, an objective evaluation of the performance of the combined cycle can be obtained. As a key element in the energy conversion process, particular attention will be given to the modelling and simulation of the high temperature receiver. Due to the highly variable nature of the solar flux, the performance of the receiver will be evaluated over a range of operating points. In order to obtain a detailed breakdown of the sources of losses and irreversibilities within the system, a complete exergy balance will be performed on the power plant. Combined with the standard energy balance, this analysis will allow identification of the key areas for improvement in the design of solar thermal power plants. Finally, in order to determine whether such a setup is economically viable, the construction, operation and maintenance costs will be evaluated in order to predict the levelised energy cost associated with electricity production in a combined cycle plant.

2008

Thermo-Economic Optimisation of Solar Tower Thermal Power Plants

James Spelling

2009

Evaluation of a Combined Cycle Setup for Solar Tower Power Plants

James Spelling

2008

Assessment of Solar Thermal Power Technologies

James Spelling

2008