Project B2: Liquid metals for solar power systems

PI: Thomas Wetzel (KIT)
Partners: KIT, DLR, LUH, HZDR

1. Scientific case for the project

1.1 Background

The unique thermophysical properties of liquid metals make them an attractive option for cooling of surfaces with extremely high thermal load. Such surfaces occur, e.g., in the receivers of concentrating solar power (CSP) plants. While liquid metals are intensively studied as cooling liquid in special research applications like particle sources, transmutation systems and are also commercially used e.g. in electronic cooling, there is no recent utilization in CSP technology. It is known, however, that there are several potential benefits of such a liquid metal application: The receiver and solar field layout will benefit from the expected higher receiver flux capabilities when liquid metals are used as heat transfer medium, compared to state-of-the-art heat transfer fluids like molten salt. Thus, both the receiver and the solar field are expected to become smaller and therefore show higher efficiencies and lower cost. Thermal storage is another option considered important for CSP plants, leading to significantly higher capacity factors than other renewable energy sources. Together with hybridization (fuel co-firing) full dispatchability can be achieved, which becomes more important when high shares of renewables are foreseen. If liquid metals are used as a heat transfer medium in a CSP plant, their direct use or their suitable integration in a hybrid storage concept needs to be considered. In view of their superior heat transfer capabilities, there have been attempts to use liquid metal in CSP plants in the mid 1980’s. These were not successful because of lack in suitable monitoring, handling and technological standard at that time. However, substantial progress in these fields has been demonstrated within the last 30 years, but was not utilized yet in the solar thermal power field. This shall be changed with the project. The partners in this project will therefore bundle their expertise and experimental facilities in the fields of liquid metal cooling and liquid metal technology (KIT), technology and operation of CSP plants (DLR), liquid metal and magnetohydrodynamic process modeling and simulation (LUH) as well as advanced liquid metal measuring techniques (HZDR).

1.2 Most important goals and scientific program

The two overall goals of this project are
  • Planning, design, construction and operation of a small concentrating solar power demonstration system in the 10 kW range at KIT, with a liquid metal based receiver and intermediate storage for demonstrating continuous power supply capability.
  • Providing a complete design concept for a large pilot CSP plant based on liquid metal technology, up to evaluation of O&M cost and levelized cost of electricity (LCOE) to compare the liquid metal system to conventional systems, e.g., based on molten salt.
In a first step, the characteristics of various liquid metal options will be reviewed by KIT and DLR. An evaluation will be carried out to select a limited number of promising liquid metals. Criteria for this selection will be thermohydraulic properties (heat transfer, heat capacity, viscosity), temperature limitations (melting point, vapor pressure), handling, safety issues, and hardware considerations (pumps, piping, valves, HX), and operating experience with liquid metal systems.
The work on the small demonstrator system will then start at KIT with design and building of the liquid metal receiver, liquid metal piping, liquid metal based storage, flow control, liquid metal pumping, auxiliary heating systems, monitoring systems, etc. These components will first be tested using medium concentrating light sources in the 300 kW/m² range to demonstrate feasibility and gain operating experience. In a second step, tests at the large scale test facilities of DLR, namely the Xenon-High-Flux Solar Simulator and the high flux solar furnace in Cologne will be performed. Procedures for safe liquid metal handling, operation guidelines etc. will be developed and documented such as to be transferable for larger plants. Key scientific issues are the receiver thermohydraulic design – that can utilize existing experience with thermally highly loaded surfaces -, the design of a liquid metal based intermediate storage - due to the low specific thermal capacity of most liquid metals, hybrid solutions with solid or secondary liquid storage media need to be looked at here -, and pumping of the liquid metal – for which electromagnetic pumping is a solution with many technological advantages. It needs, however, substantial further development in view of its limited efficiency. All involved design processes for receiver, storage and pumping can substantially profit from the use of advanced simulation techniques. Here, the beneficial thermophysical properties, particularly the low Pr number of liquid metals, resulting in a scale separation between impuls and heat transfer, limit the applicability of conventional simulation models, particularly turbulence models. Therefore, special knowledge and careful validation are required and will be provided in this project. The development of validated simulation methods for the mentioned components does also allow the efficient transfer of the small demonstration scale to pilot or industrial scale application.
Parallel to the small demonstration system, preliminary designs for the receiver (incl. solar field layout), the power cycle, piping and BOP for a large pilot plant will be developed for the liquid metals selected in the first step, constantly using the knowledge gained in the small system. The following power cycles will be considered:
  • steam cycles: Tmax = 620°C
  • supercritical CO2 cycles: Tmax > 600°C
  • gas turbine cycles, various configurations: Tmax up to 1000°C
Several options for thermal energy storage, both direct and indirect, will be assessed on a conceptual level. Technological approaches, interface issues, cost aspects, and safety will be considered. Thermal and economic optimization of the selected configurations according to the specific characteristics of the chosen liquid metals will be part of the preliminary design. As next step the relevant configurations will be modeled in a system simulation tool (e.g. Ebsilon®). The system simulation will be carried out using hourly time series of a representative solar site. Actual operating conditions like ambient conditions, storage charge status, part load situations, etc., are accounted for. As a result, detailed annual performance characteristics are obtained, like annual electrical energy yield, parasitic power consumption and capacity factor. For the selected system components the cost will be estimated to obtain the total investment cost. With estimates for the O&M cost, the LCOE will be calculated from the annual net electrical energy output. The results of the analysis will be compared to reference systems, based on molten salt (state of the art and new developments) and/or particle receivers. For all these systems comparable assumptions will be made. The comparison shall consider LCOE, operational aspects and safety issues.

2. Existing competencies and infrastructure

The Karlsruhe Liquid Metal Laboratory (KALLA) at KIT has long term experience in experimental investigation of thermo-fluid dynamics, particularly in cooling of highly thermally loaded surfaces. This scientific work is complemented by vast operating experience of small and large liquid metal systems, development of all necessary auxiliary equipment like heat exchangers, control systems, etc. This knowledge has been utilized in many international projects [1,2]. KIT KALLA will be responsible for the design, construction and operation of the small demonstrating system described above. KIT will bring into the project most of the required hardware, the measurement and control systems, specially skilled workshop capacities, etc.
DLR has long term system and component expertise in the area of CSP and owns excellent solar research infrastructure. Recent studies on the use of liquid metals as heat transfer fluid in CSP plants have indicated a high potential for cost reduction [3,4]. DLR will bring in its capabilities with respect to receiver design for liquid metals, and for the layout and optimization of solar power plants. Special simulation tools are available for these tasks.
LUH (Prof. Nacke) will bring in its long term experience in mathematical modeling, numerical simulation and experimental investigation of liquid metal heating and cooling as well as practical experience with electromagnetic installations for melting, stirring and pumping of different liquid metal alloys [5,6]. Required simulation infrastructure is available at LUH.
HZDR will bring into the project its capability and experience with advanced liquid metal measurement techniques. The recently developed systems based on ultrasonic and electromagnetic tomographic principles represent a break-through for this purpose and allow an on-line monitoring of liquid metal flows. In addition and in case of necessity, possibilities at HZDR for the measurement of liquid metal material data might be used for the project.

3. Resource planning and budget justification

Within the project an experienced scientist will be needed at KIT, who will be responsible for the concept, design, built and operation of the experimental setup including measurement and control systems. This person will be accompanied by a PhD student, who will take a deeper look into the thermohydraulic design of the receiver, storage and pump components as well as in the operation behaviour of the liquid metal based demonstration system. A second PhD student will be located at DLR, performing the above mentioned detailed solar component layout, system analysis and assessment of the pilot CSP plant and providing 30 input to WP2. An experienced scientist will be required for the fluid dynamic, thermal and magnetohydrodynamic simulation and the electromagnetic pump design. This scientist will be located at LUH, to make best use of the available experience there.

Links: Close relations exist to the projects A1, B1 and B4.


[1] Th. Wetzel, et al., 2010, Experimental investigation of turbulent flow distribution in a hexagonal rod bundle for ADS prototype application. TCADS 2010, Karlsruhe, March 15-17, OECD NEA Proceedings, 159-171
[2] C. Fazio, et al., 2010, Development and assessment of structural materials and heavy liquid metal technologies for transmutation systems (DEMETRA): highlights on major results. TCADS 2010, Karlsruhe, March 15-17, OECD NEA Proceedings, 81-106.
[3] Singer, C., Buck, R., Pitz-Paal, R., Müller-Steinhagen, H., 2010, Assessment of Solar Power Tower Driven Ultra Supercritical Steam Cycles Applying Tubular Central Receivers with Varied Heat Transfer Media, Journal of Solar Energy Engineering, Vol.132, Issue 4, 041001.
[4] C. Singer, R. Buck, R. Pitz-Paal, H. Müller-Steinhagen, 2011, Economic Potential of Innovative Receiver Concepts with Different Solar Field Configurations for Supercritical Steam Cycles, ESFuelCell2011-54139, 5th Int. Conf. on Energy Sustainability, 7-10 August, Washington DC.
[5] Peşteanu, O., 1998, Contribution to the Calculation of Molten Metal Flow in Electromagnetic Channels. Studies in Applied Electromagnetics and Mechanics Vol. 13, Non-Linear Electromagnetic Systems, 783-786.
[6] Umbrashko, A., Baake, E., Nacke, B., Jakovics, A., 2005, Experimental investigations and numerical modelling of the melting process in the cold crucible. The International Journal for Computation and Mathematics in Electric and Electronic engineering, 24, 1, 314 – 323.