U. D.J. Gieseler, F. D. Heidta

University of Siegen, Division of Building Physics and Solar Energy Walter-Flex-Str. 3, D-57068 Siegen, Germany http://nesa1.uni-siegen. de, e-mail: heidt@physik. uni-siegen. de

Abstract

For low-energy buildings, passive solar gains can contribute significantly to the heat bal­ance. For the calculation and optimisation of the energy performance of low-energy build­ings, the precise quantification of solar gains is therefore quite important. The total solar gains can be calculated from the available solar radiation and the geometry of the building. However, the relevant part of the solar gains, which is really usable to substitute heating energy, is more difficult to obtain. This part depends on the actual temperature inside the buildings, which, in turn, is influenced by the amount of solar and internal gains, heat ca­pacity, as well as the building services. In this paper, a method is presented, to calculate the utilization of solar gains with a thermal building simulation software on a monthly basis. The results show, that for small to medium sized "passive houses" in standard German weather conditions, the utilized solar gains for the whole year is roughly (10 ± 2) kWh/(m2 a), and does not depend significantly on the details of the building construction.

Introduction

Solar radiation influences the heat flux through the transparent and opaque envelope of a building. Whereas solar radiation on the opaque envelope can reduce the transmission losses, the energy flux through the glazing of windows is denoted as "passive solar gains". In low-energy buildings, passive solar gains contribute significantly to the total heat bal­ance, consisting of heating energy, solar gains, internal gains, transmission losses and ventilation losses. Due to their time dependent and irregular availability, only a fraction of solar gains is really usable. The other part of solar gains obviously increases the indoor temperature above the desired level, i. e. it produces overheating. Purpose of this study is the accurate determination of the amount of usable solar gains, which may depend on the type of construction of the building, i. e. window type and their distribution on different ori­entations, effective heat capacity and heat loss coefficient or demand of heat energy, re­spectively. Thermal simulation methods allow to calculate both, the solar gains and the temperature distribution inside the building, by taking into account all relevant influences. The utilization of solar gains and the overheating hours are therefore calculated with TRNSYS (Klein et al., 1976). After the description of the calibrated building models, the calculation method for solar gain utilization is introduced, which is based on these models. Finally the results are presented, which are focussed on the utilized part of the solar gains for these buildings or buildings types, respectively.