Passive solar design: What it is and how it works

Passive solar systems are architectural and technological solutions that utilize passive solar energy (solar radiation + building envelope components) to heat or cool environments without dedicated mechanical systems.

They operate through three actions: capturing the sun (integrated glass/collectors), storing heat (thermal mass), and distributing it within the building through natural processes such as radiation, conduction, and convection.

In construction, this bioclimatic strategy requires a balance between winter capture and summer shading, especially at mid and high latitudes. One of the most common systems is the bioclimatic greenhouse, which can help reduce energy needs when designed and sized correctly.

What is passive solar energy and how is it used in buildings?

Passive solar energy is the use of solar radiation to heat or cool a building without dedicated mechanical systems, exclusively utilizing orientation, building envelope, thermal mass, and shading.

From passive solar design, the so-called passive solar systems are created, which are architectural solutions integrated into the building capable of controlling the thermal exchange dynamics between the outside and inside, using solar radiation as a primary energy source.

How do passive solar systems work?

Passive solar systems utilize solar radiation incident on the surfaces of the building envelope and natural mechanisms (that is, without the aid of energy produced by thermal systems or imported from the grid) for the transfer of absorbed heat inside the building.

Passive solar systems significantly contribute to reducing energy needs, especially in buildings with efficient and well-insulated envelopes.

Since no mechanical systems are used, the energy consumption associated with their operation is zero. However, it must be noted that they need to be designed as an integral part of the architecture, anticipating proper sizing and optimal orientation.

Therefore, passive solar systems allow one to collect, store, and distribute thermal energy through natural processes, such as radiation, conduction, and convection, with the aim of regulating the thermo-hygrometric level of the environment.

Passive solar systems are based on three fundamental principles, which constitute the pillars of bioclimatic architecture:

• energy capture through greenhouse effect: this principle exploits the ability of glass to let in solar radiation while preventing the passage of long-wave radiation (about 11,000 nm) emitted by heated surfaces. The heat remains trapped and can be used in various forms, following the same principle that governs life on Earth;
• storage: energy is stored in heavy structures with high thermal inertia, such as walls and floors, with a time delay and attenuation of the thermal wave;
• distribution: the accumulated heat is then distributed and released into the surrounding environment, thereby reducing the energy needs of the building.

Elements that make up passive solar systems

The essential elements of passive solar energy capture systems are:

• collectors: made up of a transparent or translucent surface, integrated on well-exposed facades of the building or on the roof, and an absorber, a dark opaque surface, exposed to the solar radiation that penetrates the transparent surface;
• storage masses: composed of different types of materials designed to store heat and then release it even in the absence of solar radiation;
• control components: can be fixed or movable, manually or automatically operated and are shading devices to regulate the entry of solar radiation, reflectors to increase the radiation reaching the openings, various types of valves that act on the natural motion of heat transfer fluids between the elements, adjustable openings for controlling external air, etc.

Examples of passive solar design systems

Passive solar design systems can come in different types, each with its own specific characteristics.

Based on the relationship between the point where heat is captured and the actual place of its use, passive systems can be classified into:

• direct gain system: the predominant thermal exchange is direct radiative;
• indirect gain system: the predominant thermal exchange is convective, while the radiative exchange is indirect, through the storage wall;
• isolated gain system: the collection surface is separated from the thermal storage, and the exchange is indirect radiative with a mass that stores heat without directly introducing warm air into the environment.

The following comparative table classifies passive solar design systems based on the relationship between collection, storage, and the mode of heat transfer to the internal environment; the subsequent sections will explore the construction and performance specifics of each solution.

Direct gain system

Direct gain systems are the simplest application of passive solar design. They allow radiant energy to enter directly into the environment to be heated. This energy is usually stored by elements that have other functions, such as walls and floors, and then released inside the building for heating through conduction.

The system involves the installation of large glazed surfaces on south-facing facades, while opaque walls, especially those facing north, must be well insulated. Solar radiation passes through the glazed surfaces and is retained inside due to the greenhouse effect. Building structures, such as walls and floors, due to their mass, are particularly effective as heat accumulators. Therefore, it is advisable to position the insulation on the exterior of wall surfaces so that they can act as internal thermal buffers.

The glazed surfaces must be designed to minimize heat loss. Additionally, it is essential that ceilings, floors, and interior walls have enough mass to function as accumulators, which is not always the case due to the frequent use of lightweight materials like drywall, synthetic flooring, and wood in modern construction.

Direct gain

Indirect Gain System

In terms of indirect gain, heat is generated in a space adjacent to the inhabited area: sunlight is absorbed by a south-facing wall, which stores heat and, thanks to the airflow from the rear environment, transfers it to the interior space.

For better control of the internal temperature, a ventilation system is often included above and below the wall to help regulate the temperature.

Indirect Gain

The main indirect gain systems are represented by:

• solar wall;
• Trombe wall;
• roof pond (solar cover systems).

Solar wall

This system provides thermal gain through the capture of solar energy via specially designed glazing surfaces placed in front of a thermal mass wall, known as the solar wall, which absorbs, stores, and radiates thermal energy inward into the building. The system primarily consists of a capturing glazed surface located in front of a thermal mass wall (solar wall).
The glazed surface creates an air cavity in front of the solar wall, generating a greenhouse effect that increases the temperature of the thermal mass wall, which absorbs and stores the captured solar radiation. During the distribution phase, the solar wall radiates the accumulated energy inward through its inner face. The distribution of heat can be enhanced by convective motions activated by openings placed at the lower and upper areas of the wall, connecting the space between the glazed surface and the wall with the interior environment.

Trombe wall

In the Trombe wall, the storage element consists of a wall made with materials that have high thermal inertia, oriented to the south and painted a dark color to better absorb radiation. On the wall, upper and lower air inlets are appropriately positioned which can be opened or closed depending on the need, and in front of its outer side, a glazed surface is installed that acts as a capturing element.

Roof pond

Isolated system

The isolated solar gain system utilizes heat generated in a location distant from the environment that is to be heated, subsequently moving it via natural or forced convection. The advantage of using this latter type is its easy adaptability to existing buildings.

Isolated gain

Among the isolated systems are:

• air collectors with stone bed;
• solar chimneys Barra-Costantini;
• solar air collectors;
• solar greenhouses.

Air collectors with stone bed

This system is particularly suitable for single-family homes or one- or two-story buildings. The air collectors, positioned externally to the building, are sloped and lower, while a stone bed under the ground floor ceiling acts as a bulky heat accumulator.

It is generally not recommended for renovation, even for low buildings, as it would require significant modifications to the structure of the ground floor ceiling, if not its complete demolition. This could discourage the adoption of the system, unless there are severe deficiencies in the static or comfort requirements of the building’s base or large inter-floor heights that could be reduced with the introduction of the thermal mass and a new walking surface.

Solar chimneys

The described system involves the use of a insulated masonry inward with a glass collector and a black metal plate in the cavity that absorbs heat and increases the speed of the warm air. The warm air rises, passes through channels in the horizontal partition, enters the indoor environment, cools down by releasing heat, and returns to the cavity.

This system offers superior performance compared to a Trombe wall, but at a higher installation cost. Aesthetically, it requires attention due to the large glazed surfaces in front of dark elements. The airflow is controlled by dampers or valves. However, in recent residential construction, with minimum inter-floor heights, the adoption of this system is impractical without exemptions.

Solar air collectors

Solar air collectors are installed along the perimeter of the building and take advantage of natural convective movements to facilitate air circulation between the collector and the internal environments.

Solar greenhouses

Solar greenhouses (or bioclimatic) combine the characteristics of direct gain with those of a thermal mass wall. They operate as a direct gain system since they are directly heated by solar rays, and the heat is transferred to the adjacent environment through a thermal-conserving wall. Solar radiation is absorbed by the rear wall of the greenhouse, converted into heat, and a part of it is transmitted to the building.

The operation of a bioclimatic greenhouse is based on the concept of the greenhouse effect: it utilizes solar radiation to raise the internal temperature within the structure.

Specifically, the energy from visible light passes through the glass enclosing the greenhouse, is absorbed by the objects contained within the greenhouse, and then re-emitted at a longer wavelength (infrared light) which is not transmitted outside by the glass. The result is the heating of the interior of the greenhouse.

How a passive solar greenhouse supports sustainable heating

A solar greenhouse supports sustainable heating because it naturally captures and stores solar heat, reducing or even eliminating the need for external heating.

Made primarily of transparent material such as glass, it has the ability to maximize solar capture and, after passing the glazed surface, solar radiation transforms into heat, thanks to the well-known greenhouse effect.

By utilizing the greenhouse effect, it accumulates thermal energy during the day and slowly releases it inside the house; therefore, it is useful to include massive elements within that accumulate heat and then gradually disperse it into the environment.

They are a tool of high sustainability because they are designed to adapt to different seasons:

• in the winter season, they function as a true heat accumulator; solar radiation penetrates through the glazed surfaces, transforms into heat, and is retained inside. The thermal energy absorbed during the day is slowly released inside the house, making it useful to include massive elements that accumulate heat and then gradually disperse it into the environment. This helps to sustainably heat and light the home at almost no cost;
• in the summer season, on the other hand, shading systems, selective glazing, sunbreaks, or openings for natural ventilation are used to prevent internal overheating and ensure a good level of comfort, facilitating air exchange and utilizing natural currents.

Comparison between passive and active solar systems: main differences

An active system differs from a passive system in its use of mechanical or technological devices that consume energy; it employs systems and mechanical devices to collect and transport heat.

The mechanical systems of an active system are responsible for the capture, storage, and distribution of thermal energy, functioning independently of the form and technology of the actual architecture.

They differ from passive solar systems due to the following specifics:

• the collection system, whether it is water or air-based, is generally a modular component (solar collector), industrially manufactured, consisting of a transparent panel (generally glass), an air cavity, an absorbing black plate with an underlying insulating layer, and a metal casing that holds the aforementioned layers together;
• the distribution system is a real system circuit, consisting of pumps and pipes (for water) or fans and ducts (for air), that transports the fluid to the thermal exchange terminals, which can be traditional radiators or low-temperature coils (floor or ceiling) in the case of water fluid, or fan coil units or ventilation outlets in the case of air fluid.

Generally, buildings incorporating passive solutions into their architecture are not self-sufficient: this means they are unable to guarantee the designed comfort conditions throughout all times.

Thus, it is necessary to provide an auxiliary system to cover the needs and requirements that cannot be passively satisfied.

Among the main active systems that utilize solar energy, we can mention flat solar collectors or concentrators and storage systems.

The benefits of passive solar design for energy efficiency

Passive solar design is more than just a simple concept: it is a holistic approach that integrates architectural elements, materials, and the natural environment to optimize solar energy for home heating.

During the design process, it is essential to keep in mind the pillars of bioclimatic architecture and consider:

1. the placement of glazing openings: it is best that they are not obstructed or shaded by other buildings and that they are oriented towards the SOUTH (within 30°);
2. the insertion of the absorber: the absorber is the surface that covers the thermal storage element and absorbs the transmitted light and transfers it to the thermal storage; it can be, for example, the cladding of a wall or floor;
3. the insertion of an element with high thermal mass: this represents the material, beneath the solar light-absorbing cladding, that stores heat;
4. the distribution of heat: it is important, during the design phase, to consider how heat circulates and is distributed in spaces. Conduction, convection, and radiation are natural transmission methods, but it can also be facilitated with the help of fans or diffusers;
5. the insertion of solar control elements: blinds, shutters, and overhangs are elements used to shade glazing surfaces during the summer months.

Building section with key elements of passive design

A precise passive solar design, accompanied by the intelligent use of materials, allows to:

• significantly reduce energy consumption;
• reduce carbon emissions;
• achieve a higher quality construction;
• increase living comfort;
• improve quality of life.

FAQ on passive solar systems

What is passive solar energy?

Passive solar energy is the use of solar radiation to heat or cool a building without dedicated mechanical systems. It is based on the integration of orientation, glazed surfaces, thermal mass, and shading, allowing the architecture itself to capture, store, and distribute heat through natural processes.

What are passive solar systems?

Passive solar systems are architectural solutions integrated into the building that utilize passive solar energy to heat or cool spaces without the use of dedicated mechanical systems. They use glazed surfaces to capture solar radiation, high thermal mass materials to store heat, and natural processes such as radiation, conduction, and convection to distribute it within the building.

Their operation depends on correct orientation, appropriate sizing, and integration with solar control elements, such as shading and adjustable openings.

What are the main passive solar systems?

Passive solar systems are classified into three main categories based on the relationship between capture and use of heat:

• Direct gain, where radiation enters directly into the environment through glazed surfaces.
• Indirect gain, where heat is stored in a wall or intermediate mass before being released inside.
• Isolated gain, where capture occurs in a separate space and heat is subsequently transferred to the inhabited environment.

How does a direct gain system work?

In a direct gain system, solar energy passes through glazed surfaces preferably oriented to the south and is absorbed by walls and floors with high thermal mass. The accumulated heat is then gradually released inside the building, contributing to the natural heating of the spaces.

What is a Trombe wall?

A Trombe wall is an indirect gain system consisting of a massive wall with high thermal inertia, oriented to the south and protected by an external glazed surface. The wall absorbs solar radiation, stores heat, and releases it inward, also through convective motions regulated by upper and lower openings.

Are passive solar systems sufficient to ensure comfort year-round?

Generally no. Buildings that integrate passive solar systems are not completely self-sufficient and require an auxiliary system to cover energy needs during periods when solar input is not sufficient to ensure the design comfort conditions.

What is the difference between passive solar systems and active solar systems?

Passive solar systems integrate the capture, storage, and distribution of heat directly into the building’s architecture and operate through natural processes. Active solar systems, on the other hand, use system components such as modular collectors, pumps, fans, and distribution circuits to collect and transport thermal energy.