Global Radiation. Passive and Active Solar Systems

The effective solar radiation on a building (on the surfaces which are aligned with the direction of radiation at the time) is referred to as the global radiation E_eg. This is the sum of the 'direct' and 'diffuse' solar radiation (conditioned by the Earth's atmosphere and due to the scattered radiation caused by the varying conditions of the sky), given in W/m² or in Wh/m² per month or per day or per year. In the case of diffuse and direct radiation, the component of the radiation which is reflected from neighbouring buildings, roads and bordering surfaces, for example, must be taken into account (particularly when such reflections are strong).

Global radiation can be employed as a source of heat, directly for 'passive use' through structural measures (e.g. glass surfaces to utilise the greenhouse effect or internal heat storage walls) (18), or indirectly by 'active use' (e.g. using collectors, solar cells) – (18) for the energy requirements of a building. Also, the proportion of global radiation received directly determines the effective heating influence of the sun on the cooling load, which has to be calculated in the layout of heating and ventilation systems for each type of building.

The necessary global radiation on buildings and collector surfaces for the utilisation of solar energy must be determined. This is related to the location of the building, and can be obtained as an energy parameter.

- (19) shows the horizontal irradiance in W/m² due to the sun E_eS and the sky E_eH as a function of the elevation of the sun for clear skies. The horizontal global irradiance E_eg is the sum of the components generated by the sun E_eS and the sky E_eH.

Application: In order to be able to determine the actual amount of solar energy to be used, the contributions must be presented as functions of the inclination and, if necessary, the orientation of the surfaces of the building, corresponding to – (11). The horizontal irradiance can be obtained from – (19).

- (20) shows the reduction of the incident level of solar radiation as a consequence of the different inclinations (0-90°) and orientations.

In the case of a vertical surface, only about 50% of the annual horizontal global irradiance can be utilised.

The quantity of radiation incident on a vertical, but differently orientated, surface under a cloudless sky can be read off the graphs in – (21), at least for the highest and lowest positions of the sun.

Passive and active solar systems. The energy requirement for a building in northern Europe during the 8-month period of heating in winter is relatively high in comparison to that required during the months from May to August. During the months of September and April, although the global radiation component is not very intensive (see – (22)), part of the energy requirement of a building (heating, domestic water, ventilation etc.) can be covered by the use of the thermal energy of the surroundings, which again places emphasis on the problem of long-term storage.

In the application of solar energy, a distinction is made between two main systems according to their principle of operation: active or passive.

Active systems are those in which the heat gain and heat output processes are driven by equipment installed in the building. They are also referred to as indirect systems, since the heat output occurs after the conversion processes. The operating principle of an active system is represented in – (23) as a heat cascade. The heat gain can be achieved by means of solar collectors or something similar.

In passive systems, the solar energy is used 'directly'. This means that where the form of the building, the material, the type of construction and the individual components are suitable, the incident solar radiation is converted into heat energy, stored and then given out directly to the building.

Four physical processes which are important to the heat gain, conversion and output are described below.

(1) Thermal conduction – (24), (1). When a material absorbs solar radiation, this energy is converted into heat. Heat flow is caused by a temperature difference, and is also dependent on the specific thermal capacity of the material concerned. For example, if the temperature of the surroundings is lower than that of a heated wall, then the 'stored' heat energy is transferred to the surroundings.

(2) Convection – (24), (2). A wall or other material heated by solar radiation gives back the available energy to the surroundings, according to the temperature difference. The greater the temperature difference between wall and surroundings, the greater the amount of heat given up. Air that is heated in this process will rise.

(3) Thermal radiation – (24), (3). Short-wave solar radiation is converted into long-wave (infrared) radiation on the surface of the material. The radiation is emitted in all directions, and is dependent on the surface temperature of the materials.

(4) Collectors – (24), (4). Sunlight penetrates glass surfaces which are orientated towards the south. Solar radiation converted inside the room (long-wave radiation) cannot pass back through the glass, and thus the inside of the room is heated (greenhouse effect) – (24), (4).

In any application of the systems described above, account must be taken of storage, controllability and distribution within the building.