Solar Energies 2016
The fifth Winton Symposium was held at the Cavendish Laboratory on November 3, 2016 on 'Solar Energies'. This year's theme revolved around how we convert sunlight into useful energy and how our understanding of the underlying science can direct future advances that are needed to meet the growing demand for energy.
Professor Sir David King, the UK’s Foreign Secretary's Special Representative for Climate Change provided an overview of the Paris agreement, and the importance for future human survival of controlling climate change. Agreement is in place to limit average global temperature rise to below 2 degrees, however the risk is still high that this will not be met. To drive these technologies forward requires large-scale collaborative projects for which Mission Innovate Programme has been established. This has support from twenty-two major governments, each with a commitment to double their governmental clean energy RD&D over the next five years, with a total budget of $30 billion a year. David concluded that bringing together leading scientists and technologist can turn the biggest risk that humanity faces into the biggest opportunity of our age.
Professor Greg Engel from the University of Chicago explored what we can learn from the design principles adopted in nature for photosynthesis. Deploying femtosecond spectroscopy his group is able to study the coupling of states in these complex systems that have led to the appearance of surprising long coherence, which could help design new model molecules. The goal of the work is not to make plants more efficient, but to learn how they work and then transfer these ideas to molecules and polymers to make more efficient solar cells.
Professor Albert Polman from FOM Institute AMOLF in Amsterdam, explained that there is a fundamental efficiency limit of solar cells due to the low energy photons not been absorbed and high-energy photons losing energy as they thermalize to the band edge; this is referred to as the Schockley-Queisser limit which for a single junction cell is 34%. In practice solar cells are not able to reach this limit and there is considerable headroom for improvement, through carrier and light management. The later is the focus of the research of his group, and he provided examples of how nanotechnology can be used to make solar cells more efficient.
Link to presentation Light Management in Photovoltaics using Nanotechnology
Professor Tonio Buonassisi from MIT talked about technology enablers for large scale photovoltaics with a focus on Si solar cells. To meet the Intergovernmental Panel on Climate Change (IPCC) targets, the annual growth of solar cell has to increase more rapidly an current levels, which be a difficult in an industry that has high capex and low and variable margins. His activities are focusing on improving efficiency which will in turn will reduce price which is a key driver for the growth required.
Dr Frank Dimroth from the Fraunhofer Institute for Solar Energy Systems Systems in Freiburg described how multi-junction solar cells can be optimised to produce up to 43.3% efficiency in daylight with a concentrator. This technology has been deployed in South Africa with a 44MW system. To meet our climate change targets $13 trillion investment in renewable energies is needed by 2030 so there will be a bright future for solar scientists.
Professor Henry Snaith, Department of Physics, Oxford University provided an overview of solar cells made with organometallic lead halide perovskite semiconductors. Work from his group has taken these to efficiency levels now approaching those of silicon in the space of 4 years. The big surprise is that these materials can operate in solar cells with much higher levels of impurities than can tolerated in silicon, and this makes it possible to process very cheaply. One route to commercialisation they are exploring is to produce a tandem cell with a top layer of perovskite material that absorbs the higher energy, visible spectrum photons on a lower layer of standard silicon cell that absorbs the infra-red.
Professor Jenny Nelson, is Professor of Physics at Imperial College and talked about solar energy with the focus on solar cells made with organic, molecular semiconductors. The organic materials used in these solar cells can be processed from solution, such as by direct printing, and so offer the opportunity to move to low cost and low energy production. These systems had efficiencies of 2.5% in 2001 and have now reached 11.5% through optimisation of the band gap and band alignments and tuning the microstructure. There is still considerable headroom for improvement which requires further R&D to understand and reduce the sources of energy loss.