Daylight modelling and optimization of solar facades

Abstract

Improved daylighting is an efficient way of utilizing solar energy in buildings. It is relatively cheap and more efficient than common electric incandescent or fluorescent light. Daylight can be utilized with traditional facades with wall windows, skylight windows, or with multifunctional photovoltaic (PV) solar facades which can produce daylight, heat, and electricity for the building. In this thesis, the optimal use of daylight in solar facades is studied. A new daylight simulation tool, DeLight, developed in the Helsinki University of Technology, is used for the daylight calculations and optimizations. It is a more accurate tool than the simple daylight factor models but faster than the more sophisticated models commercially available.

The simulation tool DeLight uses a single room two-zone model with one wall window. It takes as input data hourly horizontal beam and diffuse irradiance measurements which are readily available at many weather stations around the world. The irradiance measurements are converted to illuminance values by the Perez luminous efficacy model, and the illuminance values are used to generate a sky luminance distribution by the Perez all-weather sky luminance model, because these models were found to agree best with year-round illuminance and irradiance measurements. The interior illuminance distribution is calculated from the room geometry and the sky luminance distribution by an interior light transfer model. By simplifying the geometry and omitting higher-order interior reflections, a fast but still accurate simulation tool is achieved. The average simulation error of DeLight is only 2-3% at illuminance levels of 300-500 lx when compared with year-round illuminance and irradiance measurements.

The simulation tool DeLight has been used to assess the daylight availability in a case study room for four facade orientations at four locations. Maximum yearly average daylight availability (DA) is achieved with continuous dimming and automated blind system control. About 70-90% of the yearly lighting requirement during the office hours can be provided by daylight in Southern Europe, and 50-70% in Northern Europe. With on/off switching and manual blind control, most of the electricity savings due to daylight are lost. The DA can be increased considerably by using diffusive glazing elements near the top of the facade where an outside view is not necessary. However, it is important to also have a clear glazing window at the centre of the facade to permit the occupants visual contact with the environment.

In multifunctional solar facades, the optimization of the window area becomes the key design issue. Increasing the window area reduces the electric lighting load but it also decreases the PV electricity production and increases the heating and cooling load. Considering the yearly total auxiliary energy requirement for heating, cooling, electric lighting, and appliances, the optimum window area is around 15-20% of the total south-oriented facade area. An optimal PV facade layout consists of a central window, diffusive glazing above the window, and PV panels covering the rest of the facade area.