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Appendix G – Application of a Movable Insulating Plate in DHW Tanks A 50 mm XPS plate will reduce the heat flux by 90% compared to a 5 mm plate. However, by doubling the plate thickness to 100 mm, the marginal reduction in the heat flux is less than 5 percentage points. Since the optimum thickness of the movable insulating plate depends on many technical and financial parameters, no optimisation has been carried out. However, by employing 50 mm XPS for the insulating plate, the heat flux between the hot and cold water volumes will be in the same order of magnitude as the heat flux between the water and the ambient air. As a consequence, 50 mm XPS was used when testing the hydrodynamic function and thermal performance of a movable insulating plate in a standard-sized DHW tank in the laboratory (ref. Section 4.2, Testing of a DHW Tank and a Movable Insulating Plate). G3 Important Factors Regarding Material Selec- tion and Plate Design The hydrostatic balance of the insulating plate is affected by the weight of the plate. If the plate absorbs water, the average density of the plate and the balancing weight may become larger than the density of the cold water in the tank, and the plate will sink. As an example, XPS has an average water absorption of 0.3 volume %. For an insulating plate having a thick- ness of 50 mm and a diameter of 495 mm, 0.3% water absorption will increase the weight by roughly 30 grams. Consequently, in order to avoid operational malfunction, the insulating material should have a very low water absorption, or the maximum water absorption should be taken into account when calculating the required mass of the balancing weight. The hydrostatic balance of the insulating plate is affected by the volume of the plate. If the plate expands during operation due to the elevated temperature at the upper part of the plate, the displaced water volume will increase and the hydrostatic balance will be altered. Consequently, if the average density of the insulating plate and the balancing weight becomes less than the density of the hot water in the tank, the plate will move upwards. For an insulating plate having a thickness of 50 mm and a diameter of 495 mm, 1% thermal expansion will increase the volume of the plate by roughly 0.1 litres, which corresponds to about 100 grams weight difference. Consequently, in order to avoid operational mal- function, the insulating material should have a very low thermal expan- sion coefficient, or the average thermal expansion should be taken into account when calculating the required mass of the balancing weight. G4PDF Image | Residential CO2 Heat Pump System for Combined
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