Solar energy can be converted into practical energy in the form of solar power generation, solar fuel and solar heating. Basic research is needed to make breakthroughs in solar energy conversion technology. The DOE study identified 13 potential priority research areas.
World energy demand is expected to more than double by 2050 and more than triple by the end of this century. The incremental improvement of existing energy networks will not be sufficient to meet this demand in a sustainable manner. Finding enough clean energy for the future is one of the most daunting social challenges. So far, sunlight is the largest carbon free energy source. Earth gets more sunlight in an hour than it consumes in a year. At present, we use this energy through solar power generation. However, in 2001, solar power accounted for less than 0.1% of the total world power supply, while renewable biomass fuels accounted for less than 1.5% of the total energy supply. The huge gap between our current use of solar energy and its untapped potential poses a major challenge in energy research. Sunlight is a high-profile solution to meet our future demand for clean and sufficient energy. This paper has identified important scientific challenges and research directions. By the middle of the 21st century, scientific and technological progress will enable us to effectively and economically use solar energy resources to provide a large part of basic energy for the world. According to its basic energy products, solar energy conversion systems can be divided into three types: solar power generation, solar fuel and solar heating systems.
Solar power
The challenge of converting sunlight into electricity through photovoltaic solar cells is to significantly reduce the cost of transmitting solar power per watt, reduce the competition with fossil fuels and nuclear power generation by 5-10 times, and reduce the competition with primary fossil energy by 25-50 times. New materials that effectively absorb sunlight, new technologies that utilize the full spectrum of solar radiation wavelengths, and new methods based on nanostructures can all innovate solar power generation technologies. The technical development and successful commercialization of single crystal solar cells have proved the prospect and practicability of photovoltaic technology. New methods using thin films, organic semiconductors, dye sensitization and quantum dots provide attractive new opportunities for creating cheaper, more efficient and longer life systems. Many new methods need to be realized in the following aspects: First, significant progress in the fabrication of nanoscale structures has been achieved in the near future through top-down and bottom-up new technologies; Second, the progress made in nanometer level description by using electron, neutron and X-ray scattering and spectrometer; The third is advanced computer simulation of electronic and molecular behavior in nano semiconductor assembly using density functional theory. These advances in the basic science of photoelectric conversion, combined with the existing new semiconductor materials, can bring a revolution to the method of conceiving, designing and manufacturing solar cells.
Solar fuel
The inherent cycle of day and night and cloudy and sunny solar radiation requires an effective method to store the converted solar energy for future distribution and transmission. The most attractive and economical storage method is conversion to chemical fuel. The challenge of solar fuel technology is to directly use sunlight to produce chemical fuel in a perfect and cost-effective way.
For a thousand years, cheap solar fuel produced from biomass has become the primary energy on the earth. However, in the past two centuries, energy demand has exceeded biomass supply capacity. If existing plants are used, huge land area will be required to meet some primary energy needs. On the earth, almost all arable land needs to be planted with known fast-growing energy crops such as switchgrass to produce the energy currently obtained from fossil fuels every year. Therefore, the key research objectives are: first, to design plants and microorganisms by making use of breakthroughs in biology and biotechnology, so that they can become more effective “machines” for energy conversion; The second is to design an efficient, fully artificial, molecular level energy conversion machine based on the principle of natural photosynthesis. One element of these two methods is to use structural biology, genome sequencing and proteomics to give a continuous description of the structure and dynamics involved in the biological transformation of solar radiation into sugars and carbohydrates. Although the success of the laboratory has proved that solar fuel can be directly produced through artificial molecular machines, there is still a huge gap between the current technology level and available technologies. The existing laboratory system cannot achieve long-term stable performance, which is too expensive, inefficient and impractical for practical operation. Basic research is needed to develop various methods and systems to bridge the gap between scientific frontier and practical technology.
Solar heating system
The key challenge of solar heating technology is to find a cost-effective way to convert sunlight into heat that can be stored and distributed. The reactor is heated by the focused sunlight in the hot tower, and the temperature will reach above 3000 ℃. Therefore, raw materials can be used for chemical production of fuel without expensive catalysts. In order to promote this technology, it is necessary to use high temperature resistant new materials to manufacture solar thermal reactors. At a lower solar energy concentration temperature, solar heating can be used to drive turbines for mechanical power generation, which is more efficient than solar photovoltaic technology. Nanostructured thermoelectric materials provide hope for direct power generation using temperature difference, and the power generation efficiency is 20~30 at a temperature difference of several hundred degrees Celsius. The greater the temperature difference of the solar thermal reactor, the higher the efficiency will be. In order to maximize the cost-effectiveness of all centralized solar heating technologies, it is necessary to use low-cost, high-performance new reflective materials for focusing systems.
Priority research direction
The US Department of Energy has identified 13 priority research directions with great potential for scientific breakthroughs, which will promote the conversion of solar energy to the final use of electricity, fuel and heat energy. Many of these research directions involve more than one method or technology. These cross cutting problems include: patiently processing cheap materials to make them perform as well as expensive materials in electricity, optics, chemistry and physics; Develop new examples of solar cell design to break through the traditional efficiency restrictions; Find catalyst, so that solar energy can be converted into chemical fuel cheaply and effectively; Find out a new method of self assembly of molecular components as a functional integration system; Develop materials for solar energy conversion infrastructure, such as transparent conductors, reliable and cheap heat treatment materials.
By the middle of the 21st century, although there are still huge obstacles preventing existing technologies from using solar energy to produce the basic energy we need daily, energy experts believe that this goal can be achieved through basic research. This optimistic view is based to a large extent on the continuous and rapid progress of nano science in the world. New methods of nanoscale structure, description and simulation use tools that were not available five years ago, but now create new opportunities for understanding and operating the molecular and electronic pathways of solar energy conversion. The more optimistic view comes from the remarkable development of genome sequencing, protein production and structural biology, which will soon reveal the secrets of photosynthesis and natural biological catalysis. With a detailed understanding of these efficient natural processes, we can modify them and extend them to molecular reactions to directly produce solar fuel and fully access our existing energy network. The rapid progress made in frontier science such as nano science and molecular biology has laid a solid foundation for breakthroughs in solar energy conversion in the future.
