The solar chimney

Today, thanks to the growing awareness of energy consumption related to built space and the resulting environmental damage, the need to redefine the values ​​that underlie building design has emerged. Until recently, the most widespread design model was dissipative and exploited non-renewable resources that were used according to a linear life cycle.

Over the decades, a very different model has emerged, a conservative one, thanks to which natural resources (climatic, geomorphological) are exploited. In this way, these energies are used, recovered and re-used, guaranteeing significant savings, in favor of environmental protection. This circular life model is a scheme that comes from the past but is essential today to ensure a better future for future generations. In fact, if the typical rural building of the past was characterized by a compact shape and east-west exposure, the buildings constructed after the Second World War were conceived as closed boxes managed by systems and completely indifferent to the context in which they arise.

With the progressive depletion of non-renewable resources and with the growing pollution, caused in part by energy-intensive buildings, the relationship between building and built environment has returned to being valued, considering the former a real living organism capable of influencing the urban scenario. From this perspective, sustainable construction presents itself as the only guarantee aimed at containing energy consumption, exploiting natural resources, saving money and ensuring the comfort of end users.
This situation can be summarized through a cyclical diagram that explains how each element is a contributory cause of the other.

As can be seen from the image, the increase in the Earth's temperature due to the greenhouse effect causes the growing need to cool domestic and work environments. To do this, we often resort to air conditioning systems, energy-intensive systems that exploit non-renewable sources. The latter are precisely the cause of the greenhouse effect, which in turn causes the increase in temperatures.

In order to remedy this “vicious circle” it is good to resort to alternative design methodologies that exploit the resources present in nature and that take into account the physical characteristics of the context in which one operates. Only by acting in this way, it will be possible to contribute to the improvement of air quality, with a consequent thermal comfort and a significant energy saving.

The solar chimney

Among the solutions adopted by eco-friendly design, we can include the solar fireplace, the use of which guarantees numerous benefits. It is a passive system that accumulates and distributes heat through non-mechanical means but by exploiting natural phenomena. Although it seems to be an innovative solution, its origins are very ancient: the first to adopt it were the Persian populations who needed to equip their homes with structures capable of providing optimal ventilation.

Later, the Romans also exploited the principle of rising hot air, produced by solar energy, in order to cool and heat domestic environments. It can therefore be deduced that the utility of the solar chimney has been perceived since ancient times and then underwent phases of changeable use depending on the historical period.

Today, this solution is increasingly interesting in design thanks to the indispensable function of passive heating of buildings. The solar chimney works with zero environmental impact and is used to improve the thermal and lighting conditions in internal environments, thanks to the exploitation of solar radiation.

A passive solar system is a set of technological components chosen for the purpose of controlling and regulating thermal exchanges between the building and the context. To this end, the elements on the facade and roof capture solar radiation and accumulate heat that is then transmitted to the internal component designed for this function. The thermal chimney is considered a passive solution with indirect gain: the accumulation surface is part of the roof and receives solar rays directly but only later releases them inside.

From a physical point of view, it is preferable to place the fireplace in the southern area of ​​the building because it is most exposed to solar rays. This solution consists of three main elements: the first is represented by an external surface, of variable dimensions and similar to a glass chimney. What matters is that it is located in the area of ​​greatest influx of solar rays and that it is made of highly filtering materials, so as to transmit all the light and heat to the internal environments. Furthermore, the dimensions of this portion are of fundamental importance: the larger the absorption surface, the greater the upward flow of air.

The second structure is represented by the ventilation shaft, a vertical duct made of reflective material and black walls, thanks to which during the day the solar energy accumulated on the roof is transmitted and as much heat is retained as possible. It is important to specify that the amount of light flow and heat transmitted is directly proportional to the diameter of the chimney and its length varies depending on the floors that affect the building.

The third essential component is the air passage ducts, through which the air passes before being released into the internal rooms of the house. Solar chimneys can be equipped with mirrored surfaces located in the upper part, capable of capturing and reflecting the light inside the solar chimney from top to bottom. This structure, thanks to the function performed by each component, guarantees the release, accumulation and transport of heat inside the domestic environments, thus ensuring the thermo-hygrometric well-being of the inhabitants.

operation

The thermal fireplace is considered an excellent ally for energy saving and consequently for containing costs in the bill. Thanks to the exploitation of the natural principle of rising hot air flows, this solution is able to ensure optimal thermal and visual comfort. To obtain maximum operation, it is preferable to install the fireplace on the south side of the building since this is the area most affected by the sun's rays.

Materials also play a fundamental role: light and reflective colors are banned, the entire structure must ensure good heat and light storage capabilities. Just like the ability to retain heat, the cooling function also plays an important role, as it pushes hot air currents outwards so as to maintain a comfortable temperature in the internal environments. During the day, the external collection surface exposed to the south attracts the sun's rays, accumulating the necessary light and heat. This flow is conveyed inside the well and once it enters the house, the warmer air current moves upwards. In this way, inside the living spaces the cold currents from below meet the heat coming from above and maintain an optimal internal temperature thanks to the escape of excess heat.

The peculiarity of the solar fireplace is that it can perform a double function, that of cooling the living spaces and at the same time heating them. In fact, through the insertion of the so-called Trombe wall, preferably oriented to the south, a large amount of heat is retained and then released to the internal environments.

Even in this case, the material plays a fundamental role in maximizing the performance of this wall, which must be sized and colored appropriately in order to retain as much heat as possible. This architectural element is in fact two-component: made up of an external surface in dark glass that acts as an accumulation mass and an internal wall in stone or concrete. When designing this type of system, it is necessary to use some precautions in view of the increase in thermal load during the summer: the solar radiation that affects the accumulation surface in this period of the year is greater and for this reason causes a considerable increase in thermal load with consequent unbalanced temperatures.

Through proper ventilation and the use of small attentions in the choice and sizing of the system components, this problem can be overcome, which in any case remains of a smaller magnitude than that related to artificial lighting (cause of a significant increase in the thermal load even in the absence of such passive systems). Furthermore, since ventilation is greater in the lowest section of the ventilation duct, in the design phase it is a good idea to calculate the height of the chimney, keeping in mind that it must be twice as high as the edge of the roof, in order to ensure effective ventilation. This measure must never compromise the appearance of the building but must be consistent with it, in order to guarantee functional and aesthetic integrity. The air inlet flows must be controlled through special closing systems to prevent harmful external gases and vapors from entering the home.

What are the benefits?

The thermal fireplace represents a sustainable solution that can bring numerous benefits from an energy, economic and personal comfort point of view. Powered by the renewable source of solar energy present in nature, it does not involve any cost for its operation.

Its purpose is essential in small homes or those located in high-density areas, where the quality of air and lighting is poorer. Thanks to the exploitation of solar radiation, free and with no negative impact on the environment, the use of non-renewable and polluting resources is excluded. Furthermore, the thermal chimney mechanism, thanks to which excess heat rises and is released towards the outside, guarantees a clear improvement in air ventilation in all types of environments, even those without openings.

This will result in spaces with higher air quality but also with adequate lighting for the use of each room, with an optimization of spatial usability. Furthermore, since in this way balanced temperatures will be obtained, the inclusion of air conditioning systems will no longer be necessary and the energy saving and consequently the economic one will be consistent. For this reason, these solutions take on great importance in the summer season, since through the exploitation of the upward motion, the fresh air entering and coming from the base of the building, heating up in the environment, evacuates outside through the chimney.

The underground ducts through which the external air enters the building guarantee the entry of a fresh flow into the environment, balancing its temperature. In this way, the convective motion promotes the natural exchange of air without requiring the use of air conditioners. The essential prerequisite for the correct functioning of the system is that the external temperature is lower than that desired for the domestic spaces. The optimization of the domestic microclimate leads to adequate thermal comfort and greater psycho-physical well-being for those who live in the environments.

Even the internal natural lighting will improve in quality and quantity and thus reduce the consumption of electricity during the day. For all these reasons, the presence of a solar chimney in the building increases the value of the property and guarantees a clear containment of management costs.

Reverse function: wind chimneys

While solar chimneys contribute to the accumulation of energy and heat, on the contrary, wind chimneys use the wind flow at a lower temperature to cool the interior spaces. The function of the wind chimney is therefore extremely important, especially during the summer, for the thermal cooling of the occupied spaces and some architectural examples are concrete evidence of this. A famous example is the South London neighborhood "BedZed" (Beddington Zero Energy Development) which boasts the merit of having zero impact on the environment.

At the basis of every design choice is respect for the environment through the use of recycled materials and energy obtained from renewable sources. From a technological point of view, the openings have been placed to the south to make the most of solar radiation and obtain a good level of lighting and there are no air conditioning systems. The maintenance of thermal comfort is instead the responsibility of the solar chimneys placed on the roofs of all the buildings which, in addition to fulfilling an important practical function, constitute an aesthetic element representing the residential complex.

Whether they are used to store heat to heat interiors (solar chimneys) or, on the contrary, to exploit cold winds to cool living spaces (wind chimneys), chimneys perform important functions for the well-being of the user and the protection of the planet. The growing attention to sustainability in architecture is a fundamental prerogative for respecting the planet and future generations. For this reason, passive systems will be progressively adopted in most of the built environment so that it can coexist in balance with the natural context in which it arises.

Tags