Development of high-temperature sensible thermal energy storage systems for advanced concentrating solar power generation
Creators: name: Mohan, Gowtham
Publisher: name: The Australian National University
Year: 2018
Subjects: Concentrating Solar Power
Thermal storage
Molten Salts
Eutectic
LCOE

description: Solar energy is increasingly considered as one of the most favourable alternative sources of energy to conventional fossil fuels to mitigate carbon emissions. Despite the advantages, the intermittent availability of the solar resource causes a gap between demand and supply, and solar power plants cannot meet energy demands at night without energy storage. Concentrating Solar Power (CSP) plants have the distinctive feature that thermal energy storage (TES) is relatively easy to integrate to support energy demand in the absence of solar radiation for several hours. Thus, development of energy efficient and cost-effective thermal storage systems is vital for the CSP industry. The selection of a particular thermal storage media requires understanding of thermo-physical properties, operating limitations, and chemical compatibility. This thesis focusses on the development of novel high-temperature TES media for next generation concentrating solar power plants. At present, the Levelised Cost of Electricity (LCOE) for CSP plants is higher than for conventional fossil fuel power plants. It is essential that efficiency improvements and cost reductions are made, to achieve cost-effective clean electricity through CSP. A key focus for next-generation CSP plants in to enable high-efficiency power cycles (such as the supercritical CO2 Brayton cycle, &gt;700ºC), requiring a higher maximum temperature compared to the state-of-the-art. An increase in temperature in the power block impacts design decisions for most components (including the solar field, solar receiver design, and containment materials) and choice of the heat transfer fluid and TES media. Nitrate salts used in state-of-the-art CSP plants cannot be used above 600°C, and possible alternative salts include carbonates, chlorides, fluorides, and hydroxides. Of these, based on extensive critical review of different alternatives, chloride salts are of particular interest due to their natural abundance, and hence low cost. A novel ternary eutectic salt mixture for high-temperature sensible thermal energy storage (HTSTES), composed of sodium chloride, potassium chloride and magnesium chloride (NaKMg–Cl) is developed and its thermo-physical properties are tested and compared with other potential binary and ternary mixtures. Based on the analysis, NaKMg–Cl has an acceptable melting point (387°C), reasonable heat capacity (1.18 J g-1 K-1), high thermal stability (&gt;700°C) and low storage media cost (4.95 USD/kWh). These combined traits of NaKMg–Cl make it a potential thermal storage solution for HTSTES. One of the desirable characteristics of HTSTES media is low melting point, to avoid freezing the pipes and other parasitic losses. In further attempts to identify a ‘sweet spot’ between the melting temperature of the mixture and storage media cost, multiple quaternary chloride mixtures (NaKMgZn–Cl) are developed by adding different proportions of ZnCl2 (10% to 90%) to the existing ternary NaKMg–Cl salt. The results show a significant reduction in melting temperature is possible, but with a steep increase in the storage media cost. The last part of the thesis explores the system level techno-economic analysis for three salt candidates (the ternary NaKMg–Cl and two of the quaternary NaKMgZn–Cl salts) identified for HTSTES application. LCOE calculations are made using the default System Advisor Model (SAM) power tower model, with cost and performance inputs based on 2020 SunShot CSP targets. The exception is the thermal energy storage system, for which costs are calculated separately and depend on the thermo-physical properties and cost of the salt media. For the salts identified, a trend of increasing LCOE with decreasing melting point is identified. The standout is the ternary NaKMg–Cl eutectic, which has the lowest LCOE of the salt candidates. However, there may be certain applications where even lower melting point is required, and in these cases, this work serves as a guide to the likely LCOE penalty for selection of a more expensive storage medium with more favourable melting point.