Global radiation has its maximum in the direction of the respective position of the sun. Solar energy use in its simplest form is thus based on the optimal exposure of those surfaces foreseen or oriented for use in energy conversion, as well as their freedom from shadow.

The VDI-GUIDELINE 3789, Sheet 2 (1994) contains the foundations and significant working aids for the calculation of radiation.

The orientation of surfaces according to direction and angle of inclination leads to various amounts of absorbed radiation. Daily and yearly variations in the position of the sun must also be taken into account. (In fact, the noontime solar zenith varies by 47 degrees between the summer and winter solstices.) Maximum solar radiation thus shifts to more southerly oriented surfaces during winter because of the lower position of the sun.

A simple aid for the assessment of surface orientation in regards to usage of the annual solar radiation is depicted in Figure 3/6 from BIASIN and DIETRICH (1992).

This diagram assumes an average annual radiation of 982 kWh/m² on a horizontal surface. In the case of a southerly-oriented surface angled at 30°, a maximum annual radiation of 1,055 kWh/m² is possible.

In relation to this 100% theoretical maximum value, all other orientations in direction and angles of incline yield lower potential radiation values. When one considers the case of a building wall (angle of inclination 90°), one can ascertain a comparatively small change for orientations in the semi-circle from east through south to west in relation to the decreasing solar elevations in the east and west. That is, the annual solar radiation remains in the area of 60% to 68% of the maximum value. Admittedly, this denotes a much smaller amount of radiation than in the case of a horizontal surface, which is shown at 93% in the radiation diagram. Only the exposure area contained by the 93% curve signifies a radiation efficiency exceeding that of a horizontal surface.

The importance of vertical walls requires a differentiated consideration of the data for walls: Figures 3/7 and Figures 3/8 depict annual variations in global radiation (including for northerly wall orientations), distinguishing between average conditions (Figure 3/7) and almost cloudless days (Figure 3/8).

Figures 3/7 and Figures 3/8 lead to the following summarizing conclusions in relation to vertical exterior surfaces (walls):

Greater global radiation on nearly cloudless days implies lesser proportions of diffuse celestial radiation. The latter’s proportion on sunny days amounts to between 27% (in the case of a south wall) and 81% (north wall). On average (that is, under consideration of average cloud conditions), however, the comparable values for the proportion of diffuse radiation are 51% (south wall) and 94% (north wall).

The proportion of diffuse radiation under average conditions produces a certain equalization in global radiation at various orientations. The global radiation for east and west walls amounts on average (Figures 3/7) to 81% of that of the respective south wall, or only 73% on sunny days. The comparison values for the relationship of north wall to south wall amount to 48% on average, or 26% on sunny days.

During the months of June, July, and August, westerly and easterly oriented walls provide larger daily totals of global radiation than the south wall. This is especially true on sunny days with correspondingly large proportions of direct sunlight.

The energy-related advantage of a southerly orientation comes into play during the heating season. Above all during the months of November, December, and January, southward-facing surfaces show considerable deficits in radiation. In relation to the heating season, northern walls prove significantly less efficient in comparison to the year-round average.

A further point pertains to the southerly orientation of building walls as well as large window surfaces: The environment of the city with its close proximity of structures produces an artificial heightening of the horizon at low positions of the sun (mornings and evenings, especially in winter months), which results in later sunrises and earlier sunsets. Thus there exists a greater chance of freedom from shading for the southern sky, which provides additional importance to southward-facing surfaces. An orientation of building windows exclusively towards the south reduces energy use by about 10%.

The positive effect of a southerly orientation for individual rooms depends strongly on the proportion of window surface and the quality of the window materials. A more completely described model space in DUETZ and MAERTIN (1982) yielded the following conditions for the annual hours of full use of the heating system in the presence of double-glazed windows and a 50% proportion of window surfaces dependent on the orientation:

North: 100%, South: 83%, West: 94%, East: 93%,

Shading signifies a reduction in the astronomically possible sunlight through heightening of the horizon, e.g. from mountains or surrounding buildings. Primarily in the case of valleys and northerly-exposed surfaces, but also in areas of dense urban development, restrictions arise in the duration of sunshine.

Due to shading at low positions of the sun, northerly slopes with inclines up to 10° receive 10% to 30% less global radiation in winter than southerly-oriented faces. Development on northerly slopes should thus be avoided as much as possible, since these microclimatic disadvantages can only be insufficiently compensated through other built measures (DUETZ and MAERTIN, 1982).

Helpful for the planning are calculated solar maps which illustrate these relations. The figure 3/9a shows the direct sun radiation as a annual mean for the area of Stuttgart. In the figure 3/9b the global radiation is illustrated. The great differences  in radiation in dependence of the different slopes can be seen clearly.

Figure 3/9 reproduces the result of a computer model showing the distribution of solar energy for an area of development planned on a slope facing west to southwest (GORETZKI, 1990). Although at first glance one would expect good conditions for passive solar energy use because of the ideal exposure, the simulation shows a reduction up to 30% in radiation owing to the slope shading (as a consequence of the topography).

With the help of the polar coordinate diagram in Figure 3/10, the astronomically possible duration of sunshine for a location and its limitations due to horizon heightening and shading can be calculated from the various sun curves for particular times of the year. The diagram is strictly valid for the coordinates 49° 46" N, 9° 11" E, and thus in this form can be used with sufficient accuracy in the state of Baden-Württemberg. At this geographic latitude, the extreme values of the sun’s position at 12:00 local time (not Central European Time!) are:

64.5° (June 21, Summer Solstice)

17.6° (December 21, Winter Solstice)

The position of the sun for other dates and times can also be read from Figure 3/10.

The dashed lines give the time in Central European Time. The concentric circles are supplied with a scale in degrees for the elevation of the sun. The respective position of the sun is calculated from the intersection of a crosswise date curve with an upright time curve. At the intersection, one can read the angle of the solar elevation (concentric circles) as well as the solar azimuth (direction towards the sun in the sky), the latter of which comes from connecting the intersection with the middle point of the diagram and reading the resultant direction from the compass.

If one inserts into such a diagram a picture of the local topography or built environment from the perspective of a location to be examined, the portion of the solar track not obscured by heightening of the horizon shows the remaining potential for sunlight. Thereby the shading produced by existing or planned buildings in the southern part of the hemisphere can be assessed. This method is described in detail in TONNE (1954) (compare with Chapter 3.2.3).

The picture must be inserted into the diagram in a polar projection, similar to the reflected image on a mirrored half-sphere at the level of the horizon. The middle point of the diagram corresponds to the zenith, from which all vertical building lines emanate. The outer circle of the diagram corresponds to the horizon. Spatial lines running parallel to the horizon and horizontal building lines are transposed to concentric circles relative to their height.

Figure 3/11 shows an example taken from LOHMEYER et al. (1992) for such a study of shading from existing and planned development.

As a side note, the planning situation in this figure is the same situation depicted in the shading model of Figure 3/17. The wind tunnel study dealt with in Section 4.2.3 also concerns itself with the same case.

Recommendations for Planning

If these minimal requirements for the amount of sunlight cannot be fulfilled through correspondingly chosen distances between buildings, or where this is not desirable from an urban design perspective because of the gaps between buildings that would result, sun exposure for southern facades (realigning the longitudinal axes of buildings predominantly to an east-west direction) should be ensured by means of reduced building heights. Higher structural forms should thus be planned on the northern sides of buildings.

Studies of sunlight conditions frequently result from the need to plan and build effective solar protection to avoid overheating during summer. This takes place at best in the form of roof projections or balconies, which with the right dimensions provide shielding from solar radiation in summer without hindering the desired radiation during the winter heating period. Deciduous trees are equally suited to this task, as these shed their shade-producing leaves (in contrast to coniferous trees).