Active Solar Shading

Solar shading can be used to decrease the cooling power demand and cooling energy use but that also reduces the possible benefit for heating with the incoming solar radiation when there is a heating need. The apparent solution is to shade when there is a cooling need and allow solar gains when there is a heating need. This paper presents energy use simulations on such a system set up in a theoretical office cell and a theoretical apartment in southern Sweden showing remarkable decrease in energy use for heating and cooling, or too high over indoor temperatures. INTRODUCTION As a part of decreasing the energy use, and thus carbon dioxide emissions within the European Union, focus has increased on low energy buildings. Buildings are commonly fitted with windows to enable a view out and, during parts of the day, let light in. The windows of a building are generally known to increase the use of energy. Usually, it is not a cooling need, and the window acts as a piece of wall with insufficient insulation most of the year. When there is solar radiation, it acts as a heat gain, usually increasing both annual heating and cooling (Hellström, 2008). Analogously, in dwellings, where it is uncommon to cool the apartments, windows can lead to increased heating demand due to high thermal transmittance and a lot of hours and degree hours of too warm indoor climate (Bagge et al., 2006; Bagge, 2007). If it is assumed that a certain window area is the case, due to for example architectural reasons, it is not an option to change the window area to decrease the energy use. Fixed, constant, solar shading can be used to decrease the cooling power demand and cooling energy use but that also reduces the possible benefit for heating with the solar gain when there is a heating need. There are systems that control the solar shading based on measured solar radiation for example against a certain façade of a building. When there is too much solar radiation, the windows are shaded. The risk is still that heat that could be beneficial is kept out from the room. The apparent solution would be to use a controllable solar shading and shade when there is a cooling need and allow solar gains when there is a heating need. To do this in practice, there is need for an output telling if there is a heating need or not, and a controllable solar shading system. This paper presents energy use simulations on such a system set up for a theoretical office cell and for a theoretical apartment in southern Sweden. Objectives A typical office cell was simulated regarding heating and cooling use for the room, and an apartment was simulated regarding heating use and over temperatures with different strategies of solar shading on the windows, amongst them active solar shading, which means that the windows are shaded from incoming solar radiation when there is a cooling need in the room. A number of parameters were changed to allow for an overview of the potential of active solar shading. Limitations The amount of windows was assumed to be given. Only shortwave solar radiation coming into the room was modeled. Window airing was not considered in the simulation model. Windows were also assumed to be used in one direction only. The internal heat loads were assumed to be constant and not in relation to the incoming light. Measurements of the proposed system are not presented in this paper. METHOD Commercial commonly used energy simulation softwares have not been found to handle active solar shading. Therefore, code was developed to implement this strategy. The different tested strategies were to • ‘Nev’ – never use solar shading • ‘Alw’ – always use solar shading to a ratio of 0.1 of the window solar gain coefficient • ‘Lim’ – shade at an optimal level of outdoor shortwave solar radiation normal to the simulated window to a ratio of 0.1 of the window solar gain coefficient to give the lowest sum of heating and cooling annual energy use in the office case. Eleventh International IBPSA Conference Glasgow, Scotland July 27-30, 2009