Renewable Energy Global Innovations features: A novel heat exchanger concept for latent heat thermal energy storage in solar power towers: Modelling and performance comparison

Significance Statement

Solar radiation is a very important renewable resource and conversion technologies are broadly characterized as either line or point focus depending on how they convert it into solar power. Due to the cyclic nature of solar energy, there is a need for energy accumulation and efficient storage. Various research works have aimed at increasing the utilization of renewable energy options. A considerable hurdle to the successful implementation renewable technologies remains energy storage. Without storage renewable resources will never compete with other baseload technologies like coal and nuclear.

Doctor Heinrich Badenhorst from the University of Pretoria in South Africa created an innovative alternative energy storage concept for solar power towers. The research is now published in the peer-reviewed journal, Solar Energy. “The work may completely revolutionize the way thermal energy storage is done for industrial power generation” says Dr Badenhorst.

Recently latent heat storage technology is being pursued to acquire a high storage capacity. This technology is has not yet been implemented in plants, instead the salts are used in their molten form and energy is stored as sensible heat. The cost of these salts are high and they comprise a significant portion of overall plant cost. If the storage capacity of the salt can be increased by utilizing latent heat in addition to sensible, the costs can be significantly reduced for the same storage capacity.

However technical development of this approach is held back by the very low thermal conductivity of the salts, especially in solid form. This makes it very difficult to extract the stored energy using traditional heat exchanger designs. Instead a radical new concept is required which is free of the limits of conventional heat exchanger design when dealing with a solid-liquid phase transition. The study focused on the conceptual feasibility of recovering both latent and sensible heat from a liquid salt stream via granulation inside a solar power tower. Three important designs were considered: retrofitting the Gemasolar plant with the new technology, a Greenfield design and an idealized system. Tower height, steam conditions, salt composition and other operating parameters are considered in optimizing the design.

Phase change material and salt composition play significant role in the author’s suggested system. Salt choice can improve the enthalpy of fusion and sets the phase transition temperature. This in turn sets the maximum achievable steam generation temperature for turbine operation. To achieve a feasible design several operating parameters were adjusted. An energy balance approach was used to lower the amount of phase change material required.

This study combined the existing molten salt designs with the concept of a prilling tower to form solar power tower granulator. The design model explored the influence of many parameters like particle diameter and salt composition as these demonstrate operational feasibility and techno-economic viability. The introduced system in this study was found to be cost effective with 20% reduction in the overall cost of existing plants. Further optimization and implementation of Greenfields designs yield additional cost benefits, making this a very attractive option for large scale energy storage for baseload electricity generation using renewable and sustainable solar energy.

About The Author

Dr Heinrich Badenhorst: Research at the SARChI Chair for Carbon Materials is aimed at developing new functional materials for energy applications. In the Bulk Carbon research group, led by Dr Heinrich Badenhorst, the objective is to develop novel approaches to thermal energy storage.

The primary focus is on maturing graphite nano-platelet additives, derived from naturally mined graphite to improve thermal conductivity. The work includes the construction of tailor-made measuring equipment to characterise difficult substances such as phase change materials and their composites. In addition, the research aims to concurrently develop practical implementations of the new technology, for example the integration of prilling or granulation into the solar power tower design as a unique way of rapidly extracting the captured solar energy. Related efforts in the group have centred on the development of direct solar absorbers using carbon black nanospheres.

This work is currently being used to develop a solar thermal flash based sea and brackish water desalination technology. Dr Badenhorst completed his PhD at the University of Pretoria on nuclear graphite technology, where he currently teaches heat, mass and momentum transfer.

His work has been acclaimed through the international Green Talents competition, as well as the highly prestigious Meyring Naude Medal awarded by the Royal Society of South Africa.  

Journal Reference

Heinrich Badenhorst, A novel heat exchanger concept for latent heat thermal energy storage insolar power towers: Modelling and performance comparison, Solar Energy, Volume 137, 2016, Pages 90–100.

Department of Chemical Engineering, University of Pretoria, Lynnwood Road, Pretoria 0083, South Africa.

 

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