The Holcim Innovation Center’s Dr Matthieu Horgnies outlines the group’s novel approach to future energy storage needs.
In urban areas, heating networks centralise energy production and mutualise the cost, facilities, and constraints of heating for houses and buildings. They are also a powerful way to implement state-of-the-art power generation systems, thereby optimising energy efficiency.
One of the biggest challenges these networks face is the fluctuation of energy consumption, especially in variable weather conditions. This often leads heating network administrators to produce more energy than is needed, in order to cope with consumption peaks. As a result, a significant part of energy can in turn be wasted.
Fourth and fifth generation heating networks - that work at lower temperatures and enable an effective use of local renewable heat sources - are becoming the path to decarbonising heat production. Nevertheless, replacing continuous and controlled (mainly fossil-fuel based) energy sources with fluctuating and uncontrolled renewable energy sources such as wind and solar, exposes heating networks to the above risk even more.
In this context, thermal energy storage is key to develop such systems and rationalise smaller heating networks. The stored energy would then be released to overcome the consumption peak without having to produce more energy than needed. The need for energy storage solutions is therefore more relevant than it has ever been.
Cement-based materials used in thermal energy storage
Different technologies to store heat, including sensible heat, latent heat, and sorption materials, have different drawbacks. The first two are not suitable for long term heat storage because of heat losses. Sorption heat storage systems, be they based on physisorption or chemisorption, are more appropriate for a system with high-energy density and high volumes of released heat power.
Composite materials based on ettringite were proposed in several studies as thermochemical energy storage materials. As manufactured from a specific mix of cements, the particle-size of the ettringite-based materials can be easily managed and its material cost would be lower than any other thermal energy storage material. The energy storage system would consist of a type of reactor filled with a mixture of cement-based granules, allowing the formation of a very high amount of ettringite when hydrated. This would be >50%, while regular hydrated cement pastes contain a maximum of 10 - 15% of ettringite. Two types of reactions will occur according to the environmental conditions:
1. When there is excess energy being produced in the heating system, dry conditions in the reactor will trigger dehydration of the cement-based material. This converts the ettringite into meta-ettringite, which will then ‘store’ the available energy;
2. On the other hand, when the heating network requires additional energy, the reactor will switch to wet conditions, triggering the hydration process of the energy storage material (with transformation of meta-ettringite into ettringite) and thus releasing heat energy.
Infinite and reversible
This process, which is infinitely reversible, makes it possible to store different types of energy in the form of chemical potential energy. It is characterised by an absence of energy dissipation. It can easily ‘capture’ heat when the cement-based material (ettringite) is becoming dry and can release heat when the cement-based material (meta-ettringite) is hydrated.
As it is manufactured using an innovative pelletizing process, the particle-size of the ettringite-based materials can be easily managed. The cost of the materials required would be lower than all other thermal energy storage materials. The expected high energy density of the system (promising a small volume for the energy storage system) and a reported low working temperature of 60°C make ettringite suitable for storing low-temperature heat resources, including fourth-and-fifth generation heating networks..
A partnership with ENGIE and INSA-Lyon
The Holcim Innovation Center, ENGIE lab CRIGEN and INSA-Lyon (through the laboratory Centre for Energy and Thermal Sciences of Lyon) have signed a memorandum of understanding to develop a sustainable and efficient high-energy-density ettringite-based heat storage for heat networks. This will be characterised by high-performance and low cost materials and sophisticated process control to create a cost-effective heat storage solution.
The guideline of the project will be the multi-cyclic use per year of the thermal energy storage system. The objectives are threefold:
1. To better integrate renewable energies in low-temperature district heating networks;
2. To reduce the overall energy consumption of such systems;
3. To ensure wide penetration of this initial market, while developing the system for adoption in other contexts, ranging from inter-seasonal heat storage to electricity peak-load shifting.
Next steps
The studies planned under this agreement follow a first proof of concept (~TRL 3) undertaken during a first thesis funded by Holcim at INSA-Lyon (2017 - 2020, www.theses.fr/2020LYSEI056). They aim to optimise reversible chemical reactions, allowing cement-based materials to store excess energy produced in mineral form, then to restore it on demand by simple rehydration of the compounds formed.
In addition, the integration of these compounds within a pilot of an energy storage system is scheduled to model the contributions of this new technology in the context of different uses of heat production in an urban environment.
To foster this ambition, validation of the integrated technology at a realistic scale (TRL 5-6) will be accomplished. The target of the material development is a density higher than six times the energy density of water, i.e. a minimum of 300kWh/m3 in realistic operating conditions.
References
H. Lund, P.A. Østergaard, T.B Nielsen, S. Werner, J.E. Thorsen, O. Gudmundsson, A. Arabkoohsar, B.V. Mathiesen, ‘Perspectives on fourth and fifth generation district heating,’ Energy, vol. 227 (2021) 120520.
K. Ndiaye, M. Cyr, S. Ginestet, ‘Durability and stability of an ettringite-based material for thermal energy storage at low temperature,’ Cem Concr Res, vol 99 (2017) 106 – 115.
J. Kaufmann, F. Winnefeld, ‘Seasonal heat storage in calcium sulfoaluminate based hardened cement pastes – experiences with different prototypes,’ Jo Energy Storage, vol. 25 (2019) 100850.
L.G. Baquerizo, T. Matschei, K.L. Scrivener, ‘Impact of water activity on the stability of ettringite,’ Cem Concr Res, vol. 79 (2016) 31 – 44.
B. Chen, F. Kuznik, M. Horgnies, K. Johannes, V. Morin, E. Gengembre, ‘Physicochemical properties of ettringite/meta-ettringite for thermal energy storage: Review, Solar Energy Materials & Solar Cell.,’ vol. 193 (2019) 320 – 334.
B. Chen, K. Johannes, M. Horgnies, V. Morin, F. Kuznik, ‘Characterization of an ettringite-based thermochemical energy storage material in an open-mode reactor,’ J Energy Storage., vol. 33 (2021) 102159.
