Passive use of solar energy

The passive use of solar energy is a special kind to gain renewable energy (see chapter Renewable energies), in this case solar energy. Solar gains happen largely by the building itself without major expenditure for apparatus or mechanical equipment. Auxiliary energy is not needed to operate the system or its application is very much restricted (= only a few percent of the obtainable energy gains).

Basically, the term "passive use of solar energy" is not the best choice to express what really has to be communicated. Because of the common meaning of the antithetical pair of words "active" ( = effective, successful) and "passive" ( = waiting, reluctant) this wording is endangered to be misleading. In fact, the characterizations "active" and "passive" do not refer to the performance of the respective systems but they describe the technical means which are applied in each case. Passive use of solar energy can be as effective as, or even better than active use of solar energy. Moreover, because of lacking mechanical devices and the absence of auxiliary energy, passive systems have the chance to be more economic.

Therefore, "energy-efficient solar building" would be a more precise characterization of this kind to use renewable energies. However, the term "passive solar" is internationally already introduced and reaches far back in the corresponding literature (since about 1975; USA, Iran, India). To avoid any confusion, this traditional terminology is respected.

The purpose of passive solar energy use is to gain solar radiation inside a building and to use it for heating. To this end, the sun must be able to enter the main rooms. This has many implications: site of ground, building orientation, building form, distances and heights of neighbouring buildings. All these problems refer to the master plan. Requirements have to make sure that the time of sunshine onto the relevant facades are sufficiently long, that the sun-facing facades are large enough, as well as that self-shading or shading by other objects are avoided as much as possible.

Passive use of solar energy is not a contradiction to the construction principles of low energy houses or to those of houses with optimized ventilation systems. It is much more an additional extension of these concepts. Excellent thermal insulation and optimized (with respect to hygienics and energy consumption) air exchange are even pre-requisites to assure, that passive solar heat gains do not flow away rapidly and without real use.

In order to give a quantitative impression how large possible gains from solar energy are for Germany, Table 1 lists the annual solar energies incident per m² solar aperture for a horizontal plane as well as for differently oriented vertical planes (including the ranges of local variations). Here, additionally, is distinguished between the heating days only, as well as the whole year. Days with heating are defined as those, where the average daily outdoor temperature is below 12°C. Table 1 shows, that mainly the south-facing facade, but also the others, receive significant amounts of solar radiation during the heating season, which are quite comparable with the corresponding energy losses.

Figure 1 sketches the essential physical principles and the constructive elements. Solar radiation falls onto the building envelope and through is transparent parts into the building and is partially converted to heat. This heat increases the temperatures of the building mass (storage of heat) as well as of the room air. Heat conduction, air currents due to free convection and IR radiation exchange between all surfaces are the driving mechanisms for heat flows. Solar heat gains are joined by internal heat gains due to persons and equipment as well as by controlled gains from the heating system. All gained heat is balanced by corresponding losses due to (instationary) conduction of heat through the building envelope and due to the external air exchange.

Figure 1: Principle of a passive-solar heating: The external solar radiation is collected through the transparent parts of the envelope and distributed through the building with the help of natural convection. The mass storage of the building or additional ones store the recovered heat.

The basic principle of passive solar heating is structured by six subsystems:

• collection of external solar radiation,
• distribution of incident radiation,
• control of solar radiation,
• storage of gained heat,
• distribution of heat flows,
• control of heat flows.

How far the principle of passive solar heating is realized, depends on the individual design and construction of a building as well as on its general energy concept. Within the limited frame of this chapter it would be too excessive to mention all possibilities in a detailed way. Instead, only three examples are given, which refer to collection of solar radiation by windows and walls as well as on storage and distribution of heat by hypocausts.

• Transparency of windows allows for solar gains. But, their U-values are by a factor 3 or even 4 worse than those of walls (windows: 0.6 W/(m²·K) up to 5.6 W/(m²·K); walls: 0.15 W/(m²·K) up to 1.4 W/(m²·K)). Low U-values of windows are achieved by multiple glazing, low-e coating and rare gas filling. Using multiple glazing and low-e coating reduce the total energy transmission (g-value), with the consequence that individual optimizations between U-value, g-value (energy efficiency) on the one side, and cost efficiency on the other side have to be performed. The (energetic) optimum percentage for windows on a facade depends on its orientation, on the local climate, on U- and g-value of the foreseen windows as well as on the U-value of the wall. South-facing windows with good U-values and temporary heat protection (shutters) usually gain energy during the heating season.
  
  
• Opaque external walls usually gain a very small fraction of the incident solar energy for its internal use (about 1% to 2%). If they are covered with transparent insulation materials (TIM), this fraction increases considerably: TIM weakens the incident solar radiation only slightly. The heat flow, generated at the interface between TIM and wall, branches into an outdoor and indoor route. Because of the rather low thermal conductivity of TIM, the outdoor branch is small, and the indoor branch can be bigger than the corresponding heat losses from there. This means, that the opaque parts of a building´s envelope, which normally are areas of heat loss can be converted to areas of heat gain. Therefore, the application of TIM is not restricted to new buildings but is also applicabe to the existing building stock. Caution: Heat gains may be as large, that additional measures for temperature control have to be added. This increases system costs for TIM (> 500 Euro/m²). Due to this fact, the corresponding applications are very restricted. For the moment, this technique is mainly used in demonstration projects.
  
  
• The storage of at least some fraction of gained solar energy is important for its useability. Oftenly the sun shines at moments in time when heat is not required for the building; at even more times heat may be needed and the sun is not available. In such cases a thermal storage helps, which is realized as a hollow floor (hypocaust) or wall (murocaust) and which serves as a heat reservoir for short times (some hours until very few days). Such floors with hypocausts consist of channels, built with bricks, which are passed by warm air (mostly from solar air collectors mounted on the roof) and which are covered at the top and bottom side by thermal insulation. The purpose of this insulation is to slow down the release of solar heat. The advantage of such a system is that materials and construction elements (e.g. floor, wall) are used as thermal storage which anyhow belong to a building. Therefore, additional costs may be low.

The simple fundamental ideas of passive solar heating, longevity of building measures, lack of expensive mechanical devices and equipment with the risk of malfunctions and last not least the evident success of passive solar concepts demonstrated for example buildings in the USA already since the end of 70´s and even more after the beginning 90´s in mid- and northern Europe (Germany, Switzerland, Austria, Scandinavia) confirm the certitude that this concept is generally applicable for moderate and cold climates. One has to consider, however, that passive solar concepts ever depend on local ambient conditions and climate. Therefore, it is not at all appropriate to believe, that a simple copy of a building at one specific site could be successful for another location, too.

There is a checklist of important issues and design aspects for passive solar houses, which have to be observed carefully. They are concluded subsequently in 10 points:

1. Site planning: Avoid self-shading or shading by trees and neighbouring buildings on windows or collectors during the heating season. Allow for the locally prevailing natural air flows during the warm season.
   
   
2. Excellent thermal insulation (see chapter Thermal insulation): Worth to note that through this measure overheating problems during the warm season are equally reduced.
   
   
3. Good air tightness and wind tightness (see chapter Ventilation): Besides the right selection of materials, components and constructions, especially a careful realization and control of all measures is required.
   
   
4. Intelligent thermal zoning of a building: Aggregation of rooms with less heat load in the northern sections of the building. Possibilities to distribute solar heat gains during the heating season and opportunities for increased ventilation in summer.
   
   
5. Orientation, positions and sizes of windows: Preference for south, with regard to summer case and winter case. Consideration of field of view to the outside, daylight use and glare. Bearing in mind their ventilation function as well as aspects of privacy and security.
   
   
6. Appropriate selection of glazings: Double or triple panes or double with low-e coating, with rare gas filling? Results may depend on climate. Moreover, they may be different for differently oriented facades within one building.
   
   
7. Kind of shading/temporary heat protection: Prohibition or reduction of disadvantageous heat loads in summer in order to avoid cooling and to maintain conditions of thermal comfort. Fixed or moveable shading elements? Decision on necessity or suitable selection of temporary thermal insulation.
   
   
8. Additional thermal mass for the storage of heat (relevant only for lightweight building constructions): Decision whether necessary, of appropiate additional thermal capacity and its spatial arrangement. Is intended to locally equalize heat loads or cooling loads, respectively, and to better use solar heat gains. This, simultaneously, improves the thermal comfort. Results may depend on the way how heating is controlled (permanent or intermitting heating system).
   
   
9. Richtige, saisonal veränderliche Lüftung: Möglichkeit zur verstärkten Lüftung während der warmen Jahreszeit zur Abfuhr von Überschußwärme bzw. zur Verbesserung des thermischen Komforts. Einbeziehung natürlicher Lüftungsstrategien (Querlüftung, Kamineffekt), evtl. Deckenventilator.
   
   
10. Optimized ventilation, (at least) seasonally variable: Option of increased ventilation during the warmer seasons (removal of cooling load, improvement of thermal comfort, e.g. by ceiling fans). Consideration of concepts for natural ventilation: Solar chimney, cross-ventilation. Ventilation with/without heat recovery during the heating season? Earth heat exchangers?

All these points make clear, that the very simple concepts of passive solar heating do not result in very simple rules for its building design. Instead, already the consideration of local ambient conditions and climate during the design process of a house, requires a more complex and more intelligent kind of thinking. This means to understand the whole network of design conditions and the need for integrated planning, right from the beginning of a project. For architects this is a very appealing as well as a demanding task. The widespread application of passive solar building depends on the question how many architects and clients can be animated by this concept, and how much of the required expertise can be acquired by planners and the corresponding guilds.

Helpful or even necessary is, that already in the stage of preliminary design the energy targets for a building are specified and that very early the corresponding planners are informed in order to develop a common concept about the building form, its room program, the useage of rooms, energy, ventilation, thermal comfort, daylight, technical equipment, costs etc. For answering these questions, tools or even simulation methods are applicable (see chapter Environmental design support tools), which help to optimize the design. Of course, it is important to apply such methods which are suitable to answer problems, which are really asked for. This means, that these methods are subjected to change, if the objects - preliminary planning/construction planning - develop during the working progress.