PCM-STOVE – Innovative PCM-based heat storage integrated in wood stoves

With typical thermal efficiency of 70-80 % at nominal load, modern wood stoves often produce more heat than actually required, especially in well-insulated, low-energy buildings and passive houses. At the end of the combustion, the heat release rapidly decreases, while the need for heating remains. The underlying idea of PCM-STOVE is therefore to extend the heat release from the stove by developing a compact heat storage using phase change materials (PCMs) to store excess heat and release this heat gradually to the room. PCMs are materials that, during melting and solidification, respectively store and release large amounts of heat at a nearly constant temperature. PCMs with a melting temperature of 100-150 °C, will store excess heat from the wood combustion and release it to the room over an extended time-period after combustion ends. Compared to current sensible heat storage solutions using e.g. soapstone, PCMs offer higher heat storage capacity and more stable heat release. Such systems may be retrofitted to existing wood stoves or included as an add-on to new products. The challenge of PCM-STOVE is, therefore, to develop a lightweight and compact heat storage solution for integration in the lightweight cast-iron stoves produced by JØTUL AS. This includes finding and characterising a suitable PCM, designing a prototype with optimal heat exchange properties and experimentally testing the prototype. Increased use of heat storage solutions for wood stoves is expected to result in a reduction in the overall wood consumption required to sustain a given level of thermal comfort and reduce the need for part load operation of wood stoves. These, in turn, will also lead to a reduction in particle emissions and other pollutant gases, benefiting society at large through increased air quality and health benefits thereof, and reduced environmental impact. Since the project IPN PCM-STOVE started in 2021, two NTNU Master of Science students have been involved in the research activities supervised by NTNU and SINTEF. The first student focused on heat transfer from the wood stove to the PCM, which is key to secure that a sufficient amount of energy is stored in the heat battery while firing the wood stove. Both CFD modelling and experimental work were carried out to test several solutions. The second student focuses on heat transfer from the PCM to the room, which is key to utilize the stored heat to significantly improve the users' thermal comfort. CFD modelling and experimental testing are also being used here. In parallel, several materials were tested to identify the best PCM candidate for the application. The PCM material shall have a high latent heat, to store as much energy as possible, and shall stand high temperatures without degrading. Several technical workshops have taken place in Autumn together with industry partner Jøtul. The discussions focused on further detailing the research questions, propose potential technical solutions, discuss possibilities for industrial production and establish the plan for the realisation of the innovation by Jøtul. To succeed, it is vital to ensure a tight collaboration with Jøtul's product development team.

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Thermal insulation is so effective in modern buildings that wood stoves often overheat the house within the first 2-3 hours: With typical thermal efficiency of 70-80 % at nominal load, modern wood stoves often produce more heat than actually required, especially in well-insulated, low-energy buildings and passive houses. At the end of combustion, heat release rapidly decreases, while the need for heating remains. The underlying idea of PCM-STOVE is therefore to extend the heat release from combustion by developing a compact heat storage using phase change materials (PCMs) to absorb excess heat and release this heat gradually to the room. PCMs are materials that, during phase change (melt or freeze), absorb or release large amounts of heat at a nearly constant temperature. A PCM with a melting temperature of 100-150 °C, will absorb excess heat from wood combustion and release it to the room over an extended time-period after combustion ends. Compared to current sensible heat storage solutions using e.g. soapstone, PCMs offer higher heat storage capacity and more stable heat release. Such systems may be retrofitted to existing wood stoves or included as an add-on to new products. JØTUL AS envisions a significant international market potential for such a solution being sold as an add-on product integrated in their wood stoves series. The challenge of PCM-STOVE is, therefore, to develop a light and compact heat storage solution for integration in light cast-iron stoves. This includes finding and characterising a suitable PCM, designing a prototype with optimal heat exchange properties and experimentally testing the prototype. Increased use of heat storage solutions for wood stoves is expected to result in a reduction in overall consumption of wood required to sustain a given level of thermal comfort. This, in turn, will also lead to a reduction in particle emissions and other pollutant gases, benefiting society at large through health benefits and increased air quality.