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Johannes P. Kotzé Stellenbosch University – STERG

High temperature thermal energy storage and heat transfer using eutectic metals in concentrating solar power. Johannes P. Kotzé Stellenbosch University – STERG. Energy Postgraduate Conference 2013. Thermal energy storage – State of the art. Two tank molten salt 565 °C Subcritical steam

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Johannes P. Kotzé Stellenbosch University – STERG

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  1. High temperature thermal energy storage and heat transfer using eutectic metals in concentrating solar power Johannes P. Kotzé Stellenbosch University – STERG Energy Postgraduate Conference 2013

  2. Thermal energy storage – State of the art • Two tank molten salt • 565°C • Subcritical steam • Power block efficiency: • 36-40% • Excellent from a thermodynamic point of view but limited thermal efficiency

  3. Cost breakdown of a CSP plant – US-DOE • DOE – CSP calculation at 15 US$ cents/kWh • 41% of the costs are due to indirect costs and is site specific • Still, heliostats relates to 37.5% of the total hardware cost

  4. Most direct path for cost reduction • Consider a 10% increase in thermal efficiency from a usual subcritical steam power block to a ultra supercritical steam power block: • 26.3% savings in thermal input from the heliostats. • 26.3% less heliostats • And a 5.8% reduction in LCOE, and a 9.8% reduction in plant cost based on heliostats alone. • Requires higher temperature thermal storage temperatures

  5. Metallic PCM’s • Selected alloy for research is AlSi12: • 88% Aluminium, 12% Silicon (by mass) • Melting point [4]: ±577 °C • Heat of fusion : 490 to 546 J/g • Density@ 577 °C [5]: ± 2650 kg/m^3 • Thermal conductivity @ 577 °C [5]: ± 190 W/m.K • Low cost casting alloy known as AA4047 or LM6 • Higher storage temperatures possible • High thermal conductivity – Less complicated heat exchanger design • High density storage

  6. Metallic heat transfer fluids - NaK • Sodium – Potassium alloy (NaK) • Eutectic composition: • 77.8% potassium • Melts at -12.8°C (no trace heating) • Traditionally used in experimental nuclear reactors for safety reasons • Highly reactive with water • NaK46 is better suited for solar applications • 46% potassium • 20°C melting point • Higher specific heat capacity • Compact and economic receiver designfor even higher efficiency. NaK receiver capable of 3MW/m2 NaK eutectic system

  7. Concept evaluation • Combined storage and steam generator concept • Design tool: Flownex SE • 100MW electrical output • Subcritical steam cycle • Aim is to investigate design methodology and process control parameters

  8. Heat transfer analysis: Stefan problem • Two key components to analysis: • Heat transfer to the heat exchange pipes • Tracking and predicting the solidification front in the melt • This is a classical Stefan problem • Solved using a enthalpy tracking method • Heat of fusion is measured using DSC

  9. Experimental validation

  10. Conclusions • Higher storage temperatures relate to lower LCOE • Metallic phase change materials can offer storage temperatures that can be used with high efficiency power cycles • Metallic heat transfer fluids, like NaK can offer high temperature heat transfer solutions, will not solidify at night, and results in high performance receiver designs. • A concept was developed and evaluated. The design tools was evaluated using a experiment. • The experiment serves as a prototype, and is a proof of concept .

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