Because the earth loses her heat less efficient, the average global temperature rises. As a result, more extreme weather conditions occur. In urban environments, rain during these extreme weather conditions cannot be discharged immediately. A Polder Roof, a combination of a blue and green roof, provides a solution for this problem. This roof stores water under a green layer consisting of soil with vegetation. A Polder Roof is difficult to sell, because the investment of an user is earned back long term and the water storage does not have a direct influence on the user. This study aims to investigate the possibility to add a direct function to this water storage, by harvesting or transfer thermal energy out of or into this water, to heat or cool a building energy-efficiently.
To heat a building, a certain heat input is necessary to keep the building on the same temperature. This heat demand can be determined by standardized yearly values used in the sector. If these are distributed monthly over a year, a mean heat demand per second can be determined. This heat demand is provided by harvesting heat out of the water on the roof by using a heat pump. Therefore the temperature of the water changes. To state that the water may not turn into ice, a maximum daily heat drainage can be calculated. The total amount of thermal energy which can be harvested is determined by the water’s temperature and volume. The temperature of the water is influenced by the temperature of the air in contact with the water, incoming radiation and outgoing heat from the building underneath. The volume rises through precipitation on the roof and lowers by evaporation and the discharge from water of the roof. The influence of the air on the water temperature is modelled with the Cooling Law of Newton.
By modelling the described system, an equilibrium temperature difference between water and air can be found with a given heat demand. When cooling is needed, this difference will also occur because the heat pump transfers heat into the water and is cooled by the air. This method for heating a building can be used from March to November in the Netherlands. By using another isolation layer which can retain more radiation instead of the green layer, the applicability can be increased. Cooling with a heat pump can be applied if the cooling demand is low. A high water temperature is adversely for the efficiency of the heat pump and the vegetation. Because a Polder Roof isolates a building better than a conventional roof, the building has a lower cooling demand, so the temperature of the water will rise less. The use of a heat pump is more energy-efficient than a conventional installation, on the condition that the COP is higher than 1.33.