Jacob Karni, a professor in the Department of Earth and Planetary Sciences at the Weizmann Institute of Science, delivered a talk at the MIT Energy Initiative in September 2016 outlining a novel solar energy conversion system, based on almost 30 years of his and his colleagues’ work. The system is designed to maximize the efficiency of a solar-thermal plant, balancing the fundamental requirements of such a system and incorporating novel technologies in large-scale solar optics, solar receiver, thermal storage, heat engine, and fuel production from CO2 and water. Here, Karni talks with MITEI about the process of designing the system and how he plans to advance it.
Q: You’ve discussed how intricately different aspects of a solar power system interact to have an effect on efficiency. What are some of the biggest discoveries you’ve made over the years in understanding how to build an efficient solar power system?
A: 25 years ago, when we started to look into system configurations we realized that solar-thermal systems, in general, had an intrinsic problem: the efficiency of the primary optical component (the field of solar collectors/reflectors) of small systems—dish concentrators or small solar towers of up to about 1 megawatt electric (MWe)—was significantly higher than that of larger systems, and that had a strong effect on overall costs. On the other hand, cost reduction and improved efficiency of the heat-to-power conversion unit (i.e. the engine-generator) could only be achieved at much larger sizes—around 50 MWe or higher. This, I think, was the most important “discovery” we have made, because it clearly defined our (very challenging) problem.
Q: What kind of roadblocks have prevented this proposed system from being deployed, and how are you addressing them?
A: In the late 1980s, when we started working on solar-thermal systems at Weizmann, the focus was on key enabling components. By the mid 1990s, we developed a volumetric solar receiver capable of operating at high-temperature (>1000°C), high-pressure (>20 bar) and high radiation concentration (>5,000x). We also developed several secondary optical concentration methods capable of supplying such high concentration. When the euphoria settled down, we realized due to our “discovery” above that our “great” receiver and secondary optics were not enough for a breakthrough reduction in the (too-high) cost of solar-thermal systems. An overall system approach that considered all the system components and their integration was required. But our analyses indicated that developing cost-effective solutions was not simple. There were two major problems to confront: (i) finding a balance between the size of the primary optics and power conversion unit, and (ii) storage and transportation of solar energy.
Q: What has the process of choosing and evaluating potential solar power system components looked like over the life of this project?
A: In the last twenty some years we developed tools for analyzing the component & overall efficiencies and estimating the costs of solar systems. We evaluated many “conventional” and “novel” optical configurations. Based on concepts initiated at Weizmann, HelioFocus Ltd. developed a solar-thermal system based on a relatively low-cost, 500m2 dish-concentrator with excellent optical efficiency, a highly efficient solar receiver and a heat transmission system.
Another research focus at Weizmann, since the mid 1980s, was storage and transportation of solar energy, through the conversion of solar energy to chemical potential (i.e. solar fuel production). Our work includes various methods of solar fuel production (or enrichment). We developed novel reactors for reforming methane, worked on metal-oxides reduction using solar energy, thermal storage, and various other chemical energy conversion approaches. In the early 2000s we started to explore options of using solar energy to dissociate water and CO2 and making syngas—a mixture of CO and H2, which is a precursor for producing various fuels and other chemicals. Through cooperation with NewCO2Fuels Ltd. this project evolved into an enhanced electrolysis process, with very high-efficiency and relatively low cost.
However, we didn’t develop heat-to-power conversion units. We tried to integrate engines made by various companies with our solar systems, but didn’t get very far. Recent, on-going development of supercritical CO2 turbines, by several independent companies and research institutes, and other component developments at MIT and several industrial companies have allowed us to fit in all the pieces of the puzzle and assemble this novel system configuration.
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