Operating principle

Accumulator walls can function as a combined radiation and convection system with the application of openings in their lower and upper parts (trombe wall).

The trombe wall is a technology patented in 1881 by Edward Morse, patent US246626, and made famous in the 1970s by Felix Trombe (hence ‘trombe wall’). This system consists of a material with a high thermal storage capacity, which needs to be insulated from the outside by means of a transparent glass frame in order to take advantage of the greenhouse effect that occurs in the air gap between the glass and the storage material.

During the day, solar radiation falls on a thermal mass placed between the sun and the space to be heated. The radiation absorbed by the mass is transformed into thermal energy and transferred to the interior of the building after 5 to 8 hours, depending on the thermal properties of the material and its thickness. This means that the space heats up evenly towards the end of the afternoon, when it is most accurate. On the other hand, given that the space to be conditioned does not directly receive solar radiation, indirect gain systems offer the possibility of controlling interior temperature fluctuations, preventing overheating [2].

This type of system can provide the energy needed to meet night-time heating requirements. Operation should be carried out in the following sequence:

• Winter, daytime: ventilation holes should only be opened when the temperature in the air space exceeds the temperature in the room and heating is required. The air channels can also be opened to communicate with the outside for indoor air renewal;
• Winter, night: to reduce heat loss, any night-time occlusion device should be closed, as well as the ventilation holes;
• Summer, daytime: ventilation holes must be closed and the system must be shaded;
• Summer, night: to facilitate cooling of the wall, the external ventilation holes should be opened.

For the Trombe wall to perform its function correctly, it is essential that its constitution is properly studied and dimensioned:

• Adaptation to the climate zone in which it is located, the type of use and its architectural features;
• The material, thickness and area of the accumulator wall must be analysed, as well as the dimensional characteristics of the air box;
• The definition of a correct ventilation system is fundamental to the function that the introduction of this passive system is intended to fulfil;
• The type of glazing and shading devices must also be adjusted to the intended purpose.

Potential

The application of a ventilated Trombe wall to a residential building, whose area corresponds to only 3.20 per cent of the building’s floor area, reveals that energy heating needs can be reduced by 16.36 per cent. Thus, the inclusion of a Tombe wall system in the building envelope improves its energy efficiency [1].

The efficiency of the improved Trombe wall in heating the building can exceed 33.9 per cent, compared to the performance of the classic unventilated Trombe wall, whose efficiency is 21.7 per cent, which corresponds to an increase of 56 per cent.

[1] Ana Briga-Sá and others. Energy performance of Trombe walls: Adaptation of ISO 13790:2008(E) to the Portuguese reality. Energy and Buildings Volume 74, May 2014, Pages 111-119

[2] Mendonça, P.J.: Living in a second skin: Strategies for Reducing the Environmental Impact of Passive Solar Buildings in Temperate Climates. PhD dissertation. University of Minho. Guimarães, 2005.

Operating principle

An air/soil heat exchanger consists of a set of buried pipes located at the entrance to the ventilation system, underneath or next to the building. Depending on the geometry and size chosen, it is used to dampen the day or annual oscillation in order to avoid corresponding hot or cold temperature peaks.

The basic principle of using the earth to circulate underground air through heat exchangers is the soil’s seasonal thermal storage capacity, which results in a temperature lag relative to the outside temperature. This temperature difference makes it possible to use the ground for cooling in summer and heating in winter. Heat exchange should only be applied in climates with large differences between summer and winter temperatures, and between day and night.

 Heat exchange can be used to heat or cool the air inlet.

Potential

Lower energy costs, cooling, controlled air renewal (lower concentration of bacteria, spores and fungi in the intake air), possibility of reducing or avoiding a conventional cooling system.

This system, correctly dimensioned, is capable of meeting all the cooling needs of a building while at the same time renewing the indoor air. For the Portuguese climate, it is possible for peak loads of 30 to 40 W/m2 of floor space.

As it has a higher initial cost than a conventional HVAC system, it is important to carry out a proper study of the performance of this system in order to find the most favourable situation in terms of cost/benefit.

Project considerations

 Installing an underground heat exchanger requires a large amount of available floor space. In addition, a ventilation system, including fans and distribution pipework, etc. are required.

These systems require special attention to the selection of the diameter and material of the pipe; the position, depth and length of the pipe as well as the speed of the air inside, among other aspects that can improve the performance of these systems.

The system has to be placed at a certain inclination, the pipes have to be installed with a leak tightness guarantee. The filters have to be changed.

State of the Art

Although these systems have been applied for centuries in more or less traditional forms, a renaissance of the modern technique has been evident in Europe over the last decade, with the construction and critical analysis of test and demonstration facilities, as well as the production of simulation tools, making it possible to improve their performance and optimise their operation.

Innovation

By controlling the air intakes inside the house according to the actual weather conditions, its operation can be improved.

Operating principle

The effect of air temperature stratification in buildings can produce ventilation when there is no displacement of outside air. Thus, by placing an opening at the top of the space, warm air will tend to escape and be replaced by fresh outside air introduced into the building through openings located at a lower level.

The solar chimney can be used to remove unwanted contaminated air from the building, while at the same time drawing fresh air in from outside.

Potential

The strategy of air renewal through natural ventilation reduces the need for energy for ventilation and temperature control.

During the day on windless days, solar chimneys can also provide natural ventilation to a building.

Innovation

By controlling the openings in real weather conditions, the operation of this solar chimney can be improved.

Project considerations

If the outside temperature is high, there will be no good extraction due to the chimney effect. For it to work properly, there must be a temperature difference between the warm air in the upper part of the living space and the outside air.

For a solar chimney to function properly on a building, it must be exposed to the south and its height must exceed the level of the roof and have an air outlet that is not interfered with by prevailing winds. In this way, it can be seen as an aesthetic element given its presence in the building’s appearance.

A solar chimney can be combined with specially designed staircases or greenhouses or even be a solution that expands to underground pipes to supply fresh air.

These systems, when properly designed, can ensure hourly refurbishment rates of between 5 and 10.

State of the Art

There are a number of configurations that can be used to promote air renewal inside rooms, such as vertical or inclined absorption or transparent solar chimneys, cylindrical chimneys, etc.

There are solutions for solar chimneys that work like static hoovers. These hoovers produce a depression in the interior air due to the suction produced by a static device located on the roof. The wind passing through this device creates the Venturi effect, which causes the interior air to be sucked in. There is a wide variety of static vacuum devices, both in terms of size, which allows them to be adapted to different types of roof, and in terms of shape.

Operating principle

A greenhouse is an unheated transparent extension of the building, placed outside the building envelope, which acts as a thermal buffer. It increases solar capture while minimising heat loss to the outside.

Due to the large transparent areas, a greenhouse responds more quickly to changes in solar radiation, reaching higher and lower temperatures more quickly than a dwelling.

The greenhouse can be used to preheat the ventilation air in cold periods or to cool it down during hot summer nights.

Potential

Greenhouses are not designed for permanent residence. In cold and hot periods the climate inside will be too harsh. But when the climate is more moderate the greenhouse can be used as an additional occupational space.

A solar space can replace or complement conventional climate systems for space heating.

Innovation

A sunspace can also be integrated into the building envelope. The well-insulated space becomes more suitable as a living space in colder periods. Its function is also to capture solar radiation that can also be used to preheat ventilation air.

Project consideration

The first requirement for introducing a greenhouse into a home is to have a south-facing space.

Several aspects are important when designing greenhouses, including the following:

• Orientation: the greenhouse’s ability to capture solar energy depends to a large extent on this. South-facing is always the optimum orientation for an attached greenhouse, since the important thing here is to increase gains in the cold season;
• The type of structure: depending on the material used, the type of frame structure will influence the system’s obstruction factor, which is why it is important to optimise it, beyond the economic aspect;
• The transparent material used: the most important factor is the type of transparent material used. The amount of energy transmitted and retained depends on this, depending on the selectivity of its spectrum. Its characteristics determine its opacity at broad wavelengths and therefore determine the intensity of the greenhouse effect.

 Operating principles

Thermal mass is a term used to describe the ability of building materials to store heat (thermal storage capacity).

The basic characteristic of materials with thermal mass is their ability to absorb heat, store it, and release it later.

In summer, thermal mass absorbs the heat that enters the building. In hot weather, the thermal mass has a lower initial temperature than the surrounding air and acts as a heat sink. By absorbing heat from the atmosphere, the internal air temperature is lowered during the day, with the result that comfort is improved without the need for supplementary cooling.

During the night, the heat is slowly released by the passage of cool breezes (natural ventilation), or extracted by exhaust fans, or released back into the room itself.

In winter, the thermal mass in the floor or walls absorbs the sun’s radiant heat through the windows located to the south, east and west. During the night, the heat is gradually released into the room as the air temperature drops. This maintains a comfortable temperature for a few hours, reducing the need for supplementary heating during the early evening.

Potential

The use of heavy building materials with high thermal mass (cement slab on ground and brick insulated cavity walls) can reduce total heating and cooling energy requirements by up to 25 per cent compared to a house built with building materials with a low thermal mass (lightweight slab with wooden floor).

Thermal mass conserves energy within the accumulator mass by replacing or supporting heating systems.

Due to the effect of thermal inertia, thermal mass reduces internal temperature swings. It can therefore provide heating during cold periods and cooling during hot periods.

Project consideration

In order to dampen temperature fluctuations, heat must be stored in a thermal mass located inside the space that receives the radiation directly. Surfaces that receive solar radiation directly are slightly more effective than those that receive it by reflection.

When using thermal mass as a complement to the heating system or in certain rooms, whether north or south, careful thought must be given to the design.

In some cases, thermal mass can actually increase winter energy requirements. Where there is little possibility of solar gain to the north, or because the windows to the south are too small or obstructed (poor solar access), the benefits provided by the use of thermal mass will be minimal. Each time supplementary heating is used, the thermal mass needs to be heated before the air temperature rises, thereby increasing the heat energy required.

State of the art

The high thermal capacity of a room’s interior can be achieved with thermal mass walls, such as exposed concrete, or, when a lighter construction is preferred, with the use of phase change materials (PCM) embedded in the plaster.

Phase change materials (PCM), as they are called, are materials capable of accumulating latent heat by changing phase between the solid and liquid state. This phase change takes place at a constant temperature with a high accumulation capacity.

These materials have been used in accumulator walls because they allow for a smaller amount of accumulator mass, reducing their volume, and since they allow energy to be released at a relatively constant temperature, they reduce the possibility of overheating. They can also be applied to floors and roofs.

Innovation

With appropriate energy storage, the potential of the elements that absorb the sun’s direct radiation can be improved.

Operating principle

Translucent elements in buildings are the first and simplest type of passive solar system with direct gain, and as such the most widely used system. However, most of the time they are used empirically and unintentionally.

According to the principle of passive heating, windows behave like flat solar panels that capture radiation and let some of it pass through to the interior, where it is absorbed. In other words, energy is absorbed, stored and released directly in the room. The stored heat is redistributed by radiation (at infrared wavelengths) and natural convection, regulated mainly by the position of the thermal mass in relation to the living spaces [Mitjà, 1986].

Potential

In general terms, a window’s potential (ratio between gains and losses) in terms of thermal performance can reach a value of between 40 and 60 per cent, depending on the external conditions [Cusa Juan, 1999]. It is therefore understandable how important it is for windows to be properly sized and orientated to reduce heating needs.

There are numerous advantages to the direct gain system compared to other systems, including the following:

• The direct gain system is the most energy efficient, with the maximum radiation energy per square metre;
• It is one of the most economical construction systems, since the materials and systems used can be the most common, even without the need for additional thermal mass;
• The glazed surface provides natural light for the interior spaces and allows visibility to the outside (if glass or translucent material is used);
• The operating principle is simple.

The disadvantages of this system are as follows:

• It is necessary to provide a large amount of thermal storage mass in the building to avoid overheating that causes discomfort. Large catchment areas can increase the cost of the system, due to the glazing itself, the additional thermal mass and the thermal insulation devices needed to protect the glazing from losses at night;
• Large glass surfaces can lead to a lack of privacy and excessive natural light;
• A poor disposition of the occupant towards the glazed surface can cause glare;
• Direct radiation can cause asymmetries in radiant temperatures and can cause discomfort at peak sun hours.

Innovation

Innovations in this passive solar system have been carried out in order to reduce heat loss by transmission, increase solar protection or provide various controllable properties (heat and light intensity).

It has been possible to reduce the transmission heat loss of the translucent element by applying low-emissivity shields. These are useful in long-wave infrared ranges, as they improve selectivity, a property that allows the greenhouse effect to be exploited.

Thermal resistance can also be increased by using noble gases between the glass panes (argon, krypton), or by using night insulation.

Project considerations

The preconditions for efficient operation are:

• Correct window orientation, preferably to the south, so that gains outweigh losses in winter;
• A transparent window area in keeping with the thermal storage capacity of the room;
• Use of shading devices, as a means of avoiding excessive summer gains that could cause overheating;
• Efficient thermal insulation of the opaque elements to reduce heat loss and mobile night insulation.

The selection of glazing, which can be single, double or triple and can be installed in single or double frames, etc., must take into account its main characteristics, transmissivity,

1) The choice between single and multiple glazing is not always consensual. The choice of insulating double glazing, although initially a more expensive solution, increases thermal comfort in the areas close to the windows, offers greater flexibility in the selection of available products, reinforces acoustic insulation and reduces mechanical loads. Buildings that want to make substantial savings in energy costs should be fitted with insulating double glazing, particularly if the orientation of the openings is not the most favourable and if the effectiveness of any shading systems is not the most appropriate. In temperate climates, single glazing with adequate external shading can be effective even if there are significant levels of solar radiation.

2) Choose a moderate visible transmittance to control glare. A visible transmittance (Tv) of 50 to 70 per cent is a good starting point, depending, however, on the type of visual tasks, the dimensions of the glazed openings and the glare sensitivity of the occupants. The larger the glazing area, the more critical it is to control glare and, consequently, the lower the visible transmittance should be.

3) Take into account the climatic characteristics of the area.

Os sombreadores exteriores são, em geral mais caros de instalar e de manter e desempenham também um importante papel em termos estéticos, no edifício. As reduções das necessidades energéticas de arrefecimento conseguidas com a existência de dispositivos de sombreamento exteriores em vez de interiores são superiores a 70%, e em relação à inexistência de quaisquer dispositivos de sombreamento [Monteiro Silva, 2001].

In this endeavour to achieve good glazing performance, in addition to its correct sizing, the building’s interior envelope contributes a great deal. The parameters that can condition its sizing are its thermal inertia (the ability of materials to accumulate heat) and the colour and texture of the surfaces. The basic characteristics of these materials with thermal mass are their ability to absorb and accumulate heat and the lag they allow between the capture and return of this accumulated energy.

In some cases, thermal inertia can increase heating needs, especially when window areas are too small.

State of the Art

Other transparent or translucent materials, alternatives to glass such as acrylics, double and triple glazing with air gap insulation and polycarbonate sheets, are interesting solutions from the point of view of energy optimisation in buildings. By combining high transmissivity and consequent direct thermal gains with good thermal insulation capacity and thus maintaining heat inside buildings at night, even without the use of night-time occlusion systems, they are often referred to as transparent or translucent insulations (TIMs). These materials achieve a solar radiation transmission of more than 50 per cent and a thermal conductivity of less than 0.2W/m2.ºC.

More recently, aerogel has been applied to glass sheets. This material is a hybrid nanostructure resulting from the combination of glass and hybrid polymers (transparent materials) and aerogel (insulating material). They have a U below 0.7W/m2.ºC and a solar transmission of around 76%.