The solar fireplace

Today, thanks to the growing awareness of energy consumption linked to the built space and the environmental damage that ensues, the need to redefine the values ​​that underpin the building design has made its way. 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, conservative model has emerged, thanks to which natural resources (climatic, geomorphological) are exploited. In this way, these energies are used, recovered and re-used ensuring significant savings, in favor of environmental protection. This circular life model is a pattern that comes from the past but is indispensable today to ensure a better future for subsequent generations. In fact, if the typical rural building of the past was characterized by compact shape and east-west exposure, the buildings built after the Second World War were conceived as closed boxes managed by plants and completely indifferent to the context in which they arise.

With the progressive depletion of non-renewable resources and with the increasing pollution, caused in part by energy-intensive buildings, the building-environment relationship has returned to value, considering the former a real living organism capable of affecting the urban scenario. With this in mind, sustainable construction is the only guarantee aimed at limiting energy consumption, exploiting natural resources, saving money and comfort of end users.
This situation can be summarized through a cyclical scheme that explains how each element is a contributing factor to the other.

As can be seen from the image, the rise in the earth’s temperature due to the greenhouse effect, causes the growing need to cool domestic and work interiors. To do this, air conditioning systems are often used, energy-intensive systems that use 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 keep in mind the physical characteristics of the context in which they operate. Only by doing so will it be possible to contribute to the improvement of air quality, with consequent thermal comfort and substantial energy savings.

The solar fireplace

Among the solutions adopted by the eco-friendly design, we can include the solar fireplace, the use of which guarantees numerous benefits. It is a passive type system that accumulates and distributes heat through non-mechanical means but 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.

Subsequently, the Romans also exploited the principle of rising hot air, produced from solar energy, in order to cool and heat domestic environments. It is therefore deduced that the usefulness of the solar chimney has been perceived since ancient times and then underwent phases of variable use depending on the historical period.

Today, this solution is increasingly affecting design thanks to the indispensable function of passive heating of buildings. The solar fireplace works with zero environmental impact and is used to improve the thermal and lighting conditions in indoor environments, thanks to the use of the sun’s radiation.

Passive solar system means a set of technological components chosen for the purpose of controlling and regulating heat exchanges between the building and the context. To this end, the elements on the facade and on the roof capture the solar radiation and accumulate the heat which is subsequently transmitted to the internal component designed for this function. The thermal chimney is considered a passive solution with indirect gain: the storage surface is part of the roof and receives the sun’s rays directly but only later gives it inside.

From a physical point of view, it is preferable to place the fireplace in the area south of the building as it is more exposed to sunlight. 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 the same is located in the area with the greatest influx of sunlight and that it is made up of highly filtering materials, in order to transmit the light and heat in total to the interior. In addition, the size of this portion is 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 in reflective material and black walls, thanks to which the solar energy stored on the roof is transmitted during the day and retained as much heat as possible. It is important to specify that the quantity of the luminous flux and the heat transmitted is directly proportional to the diameter of the chimney and its length varies according to the floors that affect the building.

The third indispensable component consists of the ducts for the passage of air, through which the air passes before being introduced into the internal rooms of the house. Solar fireplaces can be equipped with mirrored surfaces located in the upper part, capable of capturing and reflecting light inside the solar chimney from top to bottom. This structure, thanks to the function performed by each component, guarantees the introduction, accumulation and transport of heat within the home, thus ensuring the inhabitants’ thermo-hygrometric well-being.

Functioning

The thermal fireplace is considered an excellent ally for energy saving and consequently for the containment of costs in the bill. Thanks to the use of the natural principle of rising hot air flows, this solution is able to ensure optimal thermal and visual comfort. To obtain maximum functionality, it is preferable to install the fireplace in the south facing position of the building since it is the area most affected by sunlight.

Materials also play a fundamental role: clear and reflective colors are banned, the entire structure must ensure good ability to store heat and light. Just like the ability to retain heat, the cooling function also plays an important role, since it pushes the hot air currents outwards so as to maintain a comfortable temperature indoors. During the day, the external collection surface facing south, attracts the sun’s rays by 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 inhabited 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 lies in the fact that it can perform a double function, that of cooling the living spaces and heating them in the same way. In fact, through the insertion of the so-called wall of Trumpets, preferably oriented towards the south, a large amount of heat is retained, transferred later to the internal environments.

Also in this case, the material has a fundamental role in maximizing the performance of this wall which must be sized and colored properly in order to retain as much heat as possible. This architectural element is in fact two-component: consisting of an external surface in dark glass which acts as a storage mass and an internal stone or concrete wall. In the design of this type of system, some precautions must be used in view of the increase in the heat load during the summer: the solar radiation that affects the storage surface at this time of the year is greater and therefore causes a considerable increase in the thermal load with consequent unbalanced temperatures.

Through proper ventilation and the use of small attentions in the choice and sizing of the system components, it is possible to overcome this problem which remains in any case of lesser extent than that associated with artificial lighting (due to a high increase in the thermal load even in the absence of such passive systems). Furthermore, since ventilation is greater in the lower section of the ventilation duct, during the design phase it is good to calculate the height of the chimney, bearing in mind that it must be twice as high as the edge of the roof, in order to ensure a effective aeration motion. 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 interior of the home.

What are the benefits?

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

Its purpose is indispensable in small-sized homes or located in areas with high population density, where the quality of the air and lighting is poorer. Thanks to the use of solar radiation, free of charge and without negative impact on the environment, the use of non-renewable and polluting resources is excluded. In addition, the mechanism of the thermal chimney, thanks to which excess heat rises and is released to the outside, guarantees a marked improvement in air ventilation in all types of environments, even in those without openings.

This will result in spaces with greater air quality but also with adequate lighting for the use of each room, with an optimization of spatial usability. Moreover, since in this way balanced temperatures will be obtained, the insertion of air conditioning systems will no longer be essential and energy saving and consequently economic savings will be consistent. For this reason, these solutions take on great importance in the summer season, since through the use of the upward movement, the fresh air entering and coming from the base of the building warming up in the environment, evacuates outside through the fireplace.

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

Natural indoor lighting will also improve on a qualitative and quantitative level and thus reduce electricity consumption during daylight hours. For all these reasons, the presence of a solar chimney in the building increases the value of the property and guarantees a net containment of management costs.

Reverse function: the chimneys of the wind

If solar fireplaces contribute to the accumulation of energy and heat, on the other hand, those of the wind use wind energy at a lower temperature to cool the internal spaces. The function of the wind chimney is therefore extremely important, especially during the summer, for the thermal cooling of the occupied rooms and some architectural examples are concrete evidence of this. A famous example is represented by the South London district “BedZed” (Beddington Zero Energy Development) which boasts the advantage of having a zero impact on the environment. Respect for the environment is based on each design choice through the use of recycled materials and energy obtained from renewable sources. From a technological point of view, the openings have been located to the south to make the most of solar radiation and obtain a good level of lighting and air conditioning systems are absent. The maintenance of thermal comfort is the responsibility of the solar chimneys placed on the roofs of all buildings which, in addition to performing an important practical function, constitute a representative aesthetic element of the residential complex.

Whether used to store heat to heat indoor environments (solar fireplaces) or on the contrary to take advantage of cold winds useful for cooling living spaces (wind fireplaces), fireplaces perform important functions for the well-being of the user and safeguarding the planet. The growing attention to sustainability in architecture is a fundamental prerogative for respect for the planet and future generations. For this reason, passive systems will be gradually adopted in most of the built environment so that the same can coexist in balance with the natural context in which it arises.

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