Thermochemical Materials for long-term loss free heat storage

HEAT STORAGE IN THERMOchemical MATERIALS (TCM)

Heat storage can be performed with different technologies and is currently mostly done in the form of water heating. The main drawback of this method is the high heat losses over longer storage periods (weeks or months). Therefor a different storage method should be developed to store loss-free, with a reasonable high energy storage density, but preferably not factors more expensive than water. In theory thermochemical materials fits these demands quite well. The reaction is drawn in figure 1. As the heat is stored by a reversible chemical reaction, the heat will not be released as long as the reaction is postponed. This is easily done by just storing the water and salt separately from each other. At the moment the water and salt are in contact with each other, an exothermal hydration reaction occurs which releases the stored heat.

TCM HEAT STORAGE

In Materials Solutions, we have our focus on understanding and development of TCM’s. One important goal is is to stabilize the TCM with a reasonable effort/cost, so the performance of the material and therefore of the heat battery is constant over all cycles. In the coming years the goals of our research would be to develop the heat battery from demonstration state to domestic application areas. The key requirements are therefore on material level:

• Operation temperatures: Generation of tapwater and realization of domestic heating
• Stable performance: > 1000 cycles
• Power output:   1-10 kW on 10 kg
• Energy density:   0.5-1.5 GJ/m³
   (under working conditions)

Within our group three focus areas for the TCM development are defined:

1. 1. READY-TO-USE TCM

TNO has identified and is currently developing various technologies to solve the challenges to have a ready-to-use TCM for domestic application. Salt hydrates have shown that they are unable to be used in bare form. The performance of the heat storage material drops as over the cycles the material falls apart, the bed permeability decreases and as a consequence the performance on system level is unpredictable. Techniques like spray-drying, matrix encapsulation and drum coating can be used to create a stable TCM. Focus is laid on a practical approach whereby especially the scalability from lab to industrial scale is important .

2. 2. FUNDAMENTAL RESEARCH ON REACTION MECHANISM

Without fundamental knowledge, no breakthrough technology will be possible. Therefore, Materials Solutions collaborates strongly with research groups of the Eindhoven University of Technology. Together with the university we study the reaction kinetics starting on the crystal scale size. This work includes modelling (i.e. COMSOL) of single crystals, grains and reactor beds and lab scale experiments (i.e. TGA/DSC and vacuum TGA).

1. 1. 3. DEVELOPING OF NEW REACTOR CONCEPTS

   As the foreseen heat storage in any application will be a combination of a well behaving material inside a designed storage device, the material and device development should go together. As in our team, reactor and material experts are present, the limitations of the reactor and material are on an early stage identified and can be tackled by smart design and development on both fields. This resulted already in a few demonstrators and those will be further developed within the coming period.

   More information, or are you interested in heat storage in your heating system? Please get in touch with Pim Donkers or Olaf Adan.