Energy-Efficiency Passive Strategies for Mediterranean Climate: An Overview

Abstract

Among all the activities in a society, construction has a key role in environmental, social, and economic pillars. Construction is also responsible for a considerable amount of waste production, energy consumption, pollutant gas emissions, and consumption of nonrenewable natural resources. Regarding energy consumption, a high demand for building operational energy has been observed in the last decades due to the more demanding requirements of the users with a continuous search for better thermal comfort in their homes, namely in developed countries. In Portugal, for instance, more than 20% of the electricity consumed is related to residential buildings, which is based on CO₂ emissions and other pollutants that negatively affect the environment. Much of this consumed energy is a result of the HVAC systems installed inside buildings to provide users with thermal comfort. One exciting opportunity to mitigate buildings’ operational energy consumption while contributing to thermal user comfort is the use of passive solutions. Even though several passive options are available and constantly under research, their use is still considered limited. This paper overviews and highlights the potential of energy-efficiency passive strategies, namely for Mediterranean-climate countries, where passive solar technologies can be set as a viable solution, as this climate is mainly known for its solar availability (solar hours and solar irradiance). A comprehensive overview of innovative and traditional housing passive solutions currently available is presented and discusses the main advantages, disadvantages, and concerns contributing to the optimal use of climatic conditions and natural resources in those regions.

1. Introduction

The European continent consumes about 1/5 of the energy at the worldwide level, and most of that energy has been imported due to the limited hydrocarbon resources on this continent. The European Union’s (EU) energy dependency did not substantially change over the last decade, with a minimum of energy imports of 53.9% in 2013 and a maximum of 60.5% in 2019 [1]. The previous data from 2020 indicate that 57.5% of European energy needs was imported. It is worth mentioning that more than half of the EU’s gross available energy was supplied by net imports, and the dependency rate has exceeded half of the needs. The dependency of the EU on energy imports, particularly oil and natural gas, forms the backdrop for policy concerns relating to the security of energy supplies [1].

The most energy-intensive sector in Europe is transport (32%), including road, air, river, sea, and rail transport. The buildings and construction sector follows with 27% while contributing with proportional CO₂ emissions [2]. In third and fourth places are the sectors of industry (25%) and services (13%). Therefore, the construction and buildings sector is the second most energy-intensive sector in Europe. These economic sectors are the ones where significant environmental mitigations can be achieved, as recently stressed in “Fit for 55”: delivering the EU’s 2030 Climate Target on the way to climate neutrality. Buildings are a major player that can help accelerate the “green transition” to a net-zero future by sustainable use of natural resources, eco-friendly materials, eco-designs, efficient use of the total energy, and valuation of ecosystem impacts [3].

Fuller et al. [4] showed that the location and size of the buildings are the dominant factors that explain the total energy used in the residential sector. Tzikopoulos et al. [5] showed that buildings’ energy efficiency decreased by 7% every 1500 degree hours in the Mediterranean climate, while building location also accounted for a variation of about 17%. Du et al. [6] used empirical data to show that the energy footprints are higher in low-rise suburban households than high-rise urban. It must be highlighted that, in 2020, about 14% of European inhabitants lived in a dwelling with a leaking roof, damp walls, floors, or foundation, or rot in the window frames or floor [7]. Besides, European citizens are increasingly spending more time indoors (80–90% on average) [8], and this was further stressed by the pandemic situation, in which remotely working became more common.

Building operational energy consumption has been under scrutiny with the European Commission’s agenda, as one of the front actions to promptly alleviate the global energy supply, climate change, and energy poverty [9,10]. The need to control the energy consumption of buildings and greenhouse gas emissions while guaranteeing users’ comfort has been constantly accompanied by standards and regulations [9,10]. Particularly, since the beginning of the 20th century, the efforts to promote clean solutions and improve building designs have resulted in the invention of many types of energy-production equipment and efficient heating, cooling, and ventilation.

From this perspective, passive solutions that improve buildings’ energy efficiency must be present and reflect upon the project’s early stages of development [11]. The main objective and function of these solutions are to contribute to the natural cooling or heating of the building, called passive solutions. Passive solutions are constructive technologies integrated within buildings. The exchange of thermal energy takes place by natural means that, when dimensioned correctly, can significantly reduce a household’s energy bills. Several passive strategies, namely natural ventilation, shading devices, overhangs, and daylighting, associated with the building envelope’s characteristics are important passive design parameters that should be given particular attention to highlighting the potential of buildings for energy efficiency.

Although there is already an increase in the concern with sustainable construction and solutions that can reduce energy consumption, passive solutions are still quite reduced in some countries, such as Portugal. The main passive solutions are direct gains, such as the orientation of the glazed openings to the south or the adoption of double/triple glazing and external protections [12].

The passive solar technologies are more important and efficient for buildings in Mediterranean climates. The Mediterranean climate is a particular variety of subtropical climates, with an average temperature above 10 °C in their warmest months (warm and dry summers) and an average in the coldest months between 18 °C and −3 °C. The European countries within the Mediterranean present 300 days of sunshine per year and an annual average air temperature of 16.3 °C, precipitation of about 726 mm/year, and relative humidity of approximately 63.2% [13], and they are excellent places to exploit bioclimatic concepts. They also have many natural and renewable resources, such as cork, whose insulating characteristics can be exploited. In the literature, it is possible to find a considerable number of research about the design optimisation of passive solar strategies, multiple passive technologies, or more focus on a target of a specific technology, such as glazing, sun shading, and Trombe walls. However, the application of solar passive strategies or technologies in Mediterranean countries, such as Portugal, is still lacking.

Since several member states still present an inefficient building stock, the European Union is committed to developing a decarbonised energy system, setting a goal to reduce greenhouse emissions by at least 40% by 2030, compared with 1990, and increasing the renewable energy consumed. This represents an opportunity that cannot be missed to ramp up the decarbonisation of the buildings and construction sector, contributing to achieving the Paris Agreement commitment and the United Nations’ Sustainable Developments Goals. Following that, the EU Green Tagging strategy includes recommendations for nearly zero-energy buildings (nZebs), a building with a very high-energy performance. The nearly zero-energy required should be provided by renewable sources, including on-site or nearby sources. The nZebs concept (Section 3) must rely as much as possible on passive solutions, which follows the Passivhaus concept (Section 2).

For many years, buildings have been artificially conditioned through mechanical and electrical appliances for heating, cooling, ventilation, and lighting. With growing environmental and energy concerns, it has become increasingly necessary to seek alternatives to solve this dependence, employing passive strategies of natural climatisation and alternative energy generation.

Passive solutions are some of the most sustainable and smarter options to provide maximum comfort for the occupants while reducing the energy needs for heating and/or cooling and lighting power [14,15,16,17,18,19]. In the heating season (winter), passive solutions rely mainly on maximising the capture of the sun through well-oriented and dimensioned glazed openings, to which massive elements can be associated, which will allow the storage of solar energy and its use in later hours. Regarding the cooling season (summer), the intent is to cool the buildings through cold sources, such as the outside air that at certain times of the day (morning and night) has lower temperatures relative to the interior temperature of buildings [20]. These solutions are permanent and constantly perform their functions without the user regulating their operation.

To take advantage of the local climatic conditions and natural resources, this study presents passive strategies for thermal comfort and energy efficiency of buildings, namely for the Mediterranean, considering the great solar availability in the region compared to other European locations. Section 2 and Section 3 introduce the Passivhaus concept and nZEB concept, respectively. Section 4 briefly discusses the importance of a correct building implementation and orientation. Section 5 and Section 6 describe several innovative and traditional passive solutions. Finally, the seventh section summarises and concludes the previous sections.

2. Passivhaus Concept

The Passivhaus concept was drawn back in 1988 and developed by Professor Bo Adamson and Doctor Wolfgang Feist due to the low-energy construction required in the 1980s for new buildings in Sweden and Denmark. The Passivhaus concept is based on three pillars: (i) the energy requirements are limited, corresponding to heating, cooling, hot water production, and electricity; (ii) the thermal quality requirement of the building; (iii) the construction of the building must be based on passive solutions to satisfy energy requirements at a profitable cost [21].

Afterwards, several projects were developed within the Passivhaus concept, such as Darmstadt (1990) and GroßUmstadt (1995), in Germany. In both cases, they were houses where the main objective was to achieve low energy consumption at reasonable costs for the German climate. Due to the success of the first building, the Passive House Institute was founded [22]. Since then, this institution has been dedicated to studying and developing energy-efficient buildings.

Over than 40,000 passive houses were built worldwide and 20,000 in Germany alone. The concept is gaining popularity due to the outstanding advantages it offers, but this is also due to its flexibility [23,24]. In fact, there are no restrictions on the type of construction, type of building, or specific climate. It can be adopted in solid wood construction, prefabricated or reinforced concrete construction, and residential, administrative, school, or hotel buildings, proving that the concept is suitable for any construction system, regardless of the function of the building. The Passivhausse standard is based only on physical principles [22]. The requirements to be applied in the design and construction phase of a building to verify the Passivhaus concept are:

• Heating and cooling needs.
• Primary energy consumption.
• Building water tightness.
• Thermal comfort for its users.

The limit for energy requirements for both heating and cooling is 15 kWh/(m²⋅year). The energy value is verified with the help of passive house planning package (PHPP), a tool used in the design of passive houses where the energy balance is calculated, as well as the planning of windows, the ventilation project, and the water heating project, determining the amount of energy for heating and cooling the building and estimating comfort in summer [22].

The primary energy consumption, such as water heating, electrical equipment, and indoor heating and cooling, should not exceed 120 kWh/(m²⋅year). The airtightness must be verified through the blower door pressurisation test at 50 Pa, whose limit of the hourly renewal rate of the indoor air must be equal to or less than 0.6 renewals per hour. The thermal comfort of the building requires that, during winter, the indoor temperature should not be lower than 17 °C in all rooms. On the other hand, the indoor temperature must not exceed 26 °C for 10% of the summer. Along with those requirements, it is important to consider the continuity of the thermal insulation of the building envelope. The value of the thermal transmission coefficient, U, must be limited to 0.15 W/(m²⋅K) in the entire environment, whether in cold or hot climates. Only 0.15 watts horizontally across the walls, slabs, and coverings of the envelope when they are subject to a temperature difference between the environments that each element separates in each square meter of surface. Besides, the thermal transmission coefficient of the windows, including the frames, must not exceed 0.8 W/(m²⋅K) for cold climates. Regarding the solar factor of the glazing, g, it should admit the highest possible value, around 0.5, on a scale from zero to one, which means that gains of approximately 50% would be obtained [23].

The Passivhaus concept also introduced heat recovery ventilation. With the help of a high-performance heat exchanger, this technology allows at least 75% of the heat that comes from heat recovery to be renewed, contributing to better indoor air quality and lower energy consumption. In addition to this factor, these systems must have a low acoustic level, below 25 dB. Another ventilation characteristic is that all building compartments must have an opening to provide natural airflow in summer.

It is estimated that the Passivhaus concept can save about 90% of energy compared to existing construction in Europe and 75% compared to new construction. This saving occurs in cold climates and hot climates during the cooling season [23].

3. nZEB Concept

The Energy Performance of Buildings Directive 2010/31/EU introduced the concept of nearly zero-energy buildings (nZEB) to promote buildings that are more efficient at fulfilling the current minimum energy performance requirements. According to this directive, a building with zero or almost zero energy balance represents a high-energy performance building. A large portion of its nearly zero energy needs is covered by locally produced energy. It was defined that all member states must take the necessary measures and create conditions so that, from 2020, all-new buildings constructed will be nZEBs. During the project phase, the building principles for an nZEB are:

• Reduction of energy consumption.
• Use of renewable energies.
• Reduction of greenhouse gas emissions.

In turn, it is essential to consider some other aspects such as solar orientation, local characteristics and constructive solutions, more specifically, the glazed spans of the materials to be used, among others. Traditionally, an nZEB uses electricity and natural gas sources only when its own production cannot meet its energy needs. The building uses renewable, non-polluting, and low-cost sources of energy. When the production of this energy exceeds consumption, excess production is exported to the public grid for later use.

Regarding energy production from renewable sources, technologies available during the building service life and with the lowest environmental impact must be preferred. These same technologies must have high availability and easy recurring resources, as with photovoltaic and solar water heating systems. When the aforementioned measures used in a Passivhaus (insulation, high-performance windows, ventilation with heat recovery, sustainable architecture) are combined with local renewable energy sources and with an energy consumption practically equal to the energy produced, it turns into a “near-net-zero energy”, “net-zero-energy”, or even “positive energy building”.

An nZEB building can be defined differently depending on the project objectives, the energy values desired by the design team, or the maximum costs intended by the building owner. In other words, there are nZEB buildings with different energy consumption, consumption that must be compensated with the corresponding production of renewable energy. These different perspectives are:

• Nearly zero-energy building (nZEB): the annual energy consumption should approximate the energy production by renewable energy sources.
• Net-zero-energy building (nZEB): annual energy consumption must be equal to or less than the energy produced by renewable energy sources.
• Net-zero site energy: annual energy consumption equals the building’s yearly own energy production from renewable sources, excluding nearby sources.
• Net-zero source energy: the annual energy consumption is at least equal to the production, considering the primary energy source used in the production and supply of energy to the building.
• Net-zero energy cost: the annual amount that the public grid pays for the supply of energy produced and exported by the building is, at least, equal to the annual amount that was paid to the grid for the annual energy consumption of the building. Building energy is zero or negative.
• Net-zero energy emissions: the amount of emission-free renewable energy produced annually is at least equal to the amount of annual energy consumed from renewable sources.

The choices made regarding solutions during the design phase are directly linked to the definition of the nZEB perspective indicated above. Although there is no standard approach to the design of nZEB, there is consensus that one should choose to start the project by introducing a passive sustainable design to reduce energy needs. In this way, passive solutions represent crucial elements in constructing an nZEB, since they are directly related to the heating, cooling, ventilation, and lighting needs.

4. Building Implementation

4.1. Buiding Integration and Orientation

The first energy-efficiency passive measure is an integration building study within the environment. Within the scope of bioclimatic architecture, the design and planning of a building consider all the environmental constraints in the vicinity [5]. This aspect, however, cannot be standardised and applied consistently, as numerous factors change depending on location, dwelling type, and specific climatic factors.

Housing orientation is one of the factors that can and should be controlled in the early planning stages, i.e., the positioning of the building considering the sun exposure, to allow efficient and effective use of the sun’s energy as a source of comfort (light and thermal). The difference in the sun’s angle of incidence in the different seasons allows a differentiated use of solar energy. The proper choice of orientation that the house will take can translate into solar gains that will reduce the nominal heating needs during the winter season; in addition, the proper and correct exposure of photovoltaic panels, eventually placed on the roof, could translate into a contribution to the reduction of annual energy needs, in line with the nZEB concept.

During the heating season, the height of the sun is lower, so a vertical surface facing south receives solar radiation longer than one facing in any other direction. During the cooling season, the sun has a higher position, causing the variation of the angle of incidence of radiation. Therefore, a vertical surface facing south will receive more energy, with the roofs of buildings registering the highest energy capture in this period.

Any surface facing north does not receive direct incidence of solar radiation, thus making it a less preferential orientation when it is necessary to decide the direction of exposure of spaces with greater permanence. The best solution is an orientation facing south, since it provides the space with the most significant time of exposure to solar radiation.

Therefore, solar radiation, especially in sunny countries such as Mediterranean countries, and the study of the sun’s trajectories are key factors during the design phase, being responsible for the natural lighting of the building and for heating the interior environment.

4.2. Building Shape

The volumetric impact on a building has a key role during its life cycle, reducing energy and natural resource consumption [14,17]. Compactness is a characteristic of the building volume; it is used to adjust the exposed envelope, depending on the useful area, as much as possible. This geometric relationship is represented by the shape factor (SFv), which is the ratio between the exposed building area and indoor volume. Buildings with a high shape factor are less compact, i.e., they have a large, exposed area for a given interior volume. This type of construction allows more significant heat losses. The exposed areas of the building must be reduced as much as possible since thermal losses through the exterior envelope are proportional to the product of the areas of the exposed surfaces and the thermal coefficient of the respective element. This means that two buildings with identical glazing characteristics and thermal coefficients, but different shape factors, also differ in the internal heat retention capacity. The impact of SFv varies considerably for buildings with varying properties in the thermal envelope and weather conditions [25]. A study of the impact of different thermal envelopes in buildings showed that more benefits are obtained by using materials with better thermal quality in the envelope when there is more exposed surface per m² [26]. However, many variables such as orientation, wind, and lighting must be considered for a correct architectural design.

4.3. Exposure to Prevailing Winds

Another feature to consider during the planning phase is the orientation of the building considering the prevailing winds of the place, since, with proper conditions, it may be used for energy production.

When the area over which its movement is made is restricted, the air changes velocity. The decrease in this area implies a considerable increase in its speed, which causes a certain degree of discomfort. However, it is possible to use this process to carry out ventilation operations, which makes the correct direction of the building or passive devices according to the prevailing wind direction, as depicted in Figure 1.

5. Passive Heating Solutions

After the planning phase of the building, taking into account the association of the building with its integration into the environment, it is necessary to consider the strategies to minimise the needing of equipment for maintaining the indoor temperature at acceptable comfort levels.

Passive heating solutions are integral to building structures or housing and can perform as collectors and accumulators of incidental solar energy. Passive heating distributes heat through natural transfer processes, contributing to interior comfort without using active air conditioning systems. They aim to maximise solar gains through well-sized and oriented glazed spans, to which thermal masses can be associated. Mediterranean-climate countries particularly fit those solutions [16,27,28], as this climate is mainly known for its solar availability (solar hours and solar irradiance). Therefore, it is logical that the spaces of permanence in the building must be oriented to the south to obtain greater thermal comfort due to the penetration of the sun. On the other hand, secondary spaces or divisions such as storage rooms, corridors, stairs, garages, etc., should be located in the north’s interior space.

It is also important to design the building in such a way that the south-facing side is longer than the east- and west-facing ones, so that this façade is warmer in winter (maximum radiation to the south) and cooler in summer (radiation to the south), with the proper shading [12]. Therefore, solar radiation and the study of the sun’s trajectories are key factors during the design phase, responsible for the building’s natural lighting and indoor heating.

Different types of passive heating solutions can be adopted, with the possibility of individual or integrated usage, benefiting from the combination of the advantages of each type and reducing or even eliminating the disadvantages that could exist, namely:

• Direct gains solutions.
• Indirect gains solutions.
• Isolated gains solutions.

5.1. Direct Gain Solutions

In the passive heating by the direct gains, the space to be heated has well-oriented glazed openings to allow the incidence of radiation in the space, as depicted in Figure 2, and in the surrounding thermal masses (walls and floors) [13].

Thermal masses play a stabilising role in an indoor environment, mitigating the thermal amplitude inside buildings. The higher the thermal mass, the smaller this variation will be. It will also be more difficult to heat the building, requiring a balance between thermal mass, insulation, and span area, depending mainly on the type of building and its location. During the daytime, the thermal mass absorbs the heat resulting from the direct incidence of solar radiation and, during the night-time, returns it to space. Attention should be paid to the insulation of the external environment elements to minimise the influence of external climatic demands [29]. The main advantages of direct gain solutions are:

• High performance, since the energy used per square meter, is maximum.
• Low cost.
• Reduction of energy use for heating.
• Use of natural lighting with direct light allows views to the outside.
• Simple and easy to build using current technologies and materials, even for solar shades.
• Easy architectural integration.

As for disadvantages of direct gain solutions, it can be pointed out:

• Lack of privacy and excessive lighting in the indoor space.
• No cooling characteristics.
• Short-term thermal storage with thermal fluctuations.
• High thermal losses through the glazing.
• Possible degradation of materials due to direct solar radiation.
• Local discomfort due to asymmetries in indoor temperature.
• Manual isolation devices, such as movable opening guards, require proper and careful attention to be effective.

Skylight

The skylight is a vitrified horizontal surface applied to roofs with reduced or completely flat slopes (see Figure 3). A skylight ensures a greater use of lighting, but it can become an asset from thermal gains when combined with an element with reflective capabilities. It is an element that requires a meticulous study of its application, since it may translate into excessive gains during the summer and insufficient gains in the winter [10].

It is essential to point out that, being a glazed surface, it usually has a thermal transmission coefficient relatively higher than the element surrounding it. Therefore, this passive solution presents thermal losses through the glazing that will be more penalising during the heating season.

5.2. Indirect Gain Solutions

In indirect gain solutions, the system’s thermal mass is interposed between the gain surface and the space to be heated. The thermal mass absorbs the incidental solar energy, later transferred to the room. This transfer can be immediate or delayed, depending on the air circulation strategy (or not) that is adopted. There is a divergence between the heatwave transmitted to space and the solar cycle radiation in these solutions. The areas where indirect gain solutions are located can take advantage of the delay and receive the energy absorbed during the day, late afternoon, and early evening. Still, it depends mainly on each building and its use. The building users manage the energy transfer using simple ventilation, consisting of two openings between the space and the hot zone. Indirect gain solutions can offer advantages such as:

• Heat gains during the day by convection of hot air and also during the night (most desired) by radiation of the energy stored in the thermal mass.
• Better control of heat transferred than indirect gains.
• It can contribute to cooling in the summer.
• Minor thermal fluctuations.
• Easy to install in a new construction with low additional costs.
• Possible aesthetic contribution to dwellings.

However, some disadvantages must be pointed out, namely:

• The thermal storage walls and stoves require a south orientation.
• Night heat losses can be high and require maintenance.
• Thermal storage masses can occupy habitable space and are additional loads to be considered.
• Need for night-time isolation, usually mobile, which requires manual action.
• Thermal storage walls reduce direct light and can create difficulties in natural ventilation.
• Stoves do not allow active cooling in the summer and have large thermal fluctuations inside, not being recommended for Mediterranean summers.
• High cost of high-performance glass stoves.
• Stove plants may create odours, excess moisture, and insects.

What follows are the main descriptions of indirect gain solutions.

5.2.1. Storage Walls

A storage wall consists of a glass wall, an airbox, and a solid wall. The heat transfer occurs through radiation by the glazing, which overheats the volume of air inside, i.e., greenhouse effect, and then the adjacent wall. This wall must be painted in dark colours and present high thermal inertia in order not only to capture the maximum heat energy but also to heat release during the night. After heating the wall, it radiates heat energy into the indoor area, as shown in Figure 4.

5.2.2. Dynamic Walls

Dynamic walls are characterised by an opening at the top of the wall that lets the air heated by the solar radiation pass through the two vertical surfaces, thus providing a relatively superior function compared to storage walls (see Figure 5). During low gains periods, the openings must be closed, and the wall will act as an accumulator wall.

5.2.3. Trombe Walls

The Trombe wall is comprised of exterior glazing, well oriented (south), an air wall, and a solid interior wall. The solid interior wall is usually dark in colour to increase the capture of solar radiation (see Figure 6). It can be made of stone, concrete, compact earth, ceramic material, or other material with good thermal storage capacity. Wall thickness varies according to material properties (density, specific heat, and thermal conductivity). The airbox typically has a thickness between 5 and 20 cm, depending on climatic conditions and the performance requirements established for the system. This wall can be ventilated.

The greenhouse effect is thus created between the glazing and the wall, where very high temperatures are reached (30 °C to 60 °C). The energy stored in that airbox during the day can be transferred to an indoor compartment during the night by conduction processes through the accumulator element. It is essential to use an occlusion system at night to reduce heat losses. This solution can be particularly interesting for cold or temperate climates and good insulation.

If, on the other hand, a ventilated wall is chosen, this energy transfer takes place through natural convection, thus heating the room. As needed, the walls have thermo-circulation holes at the bottom and top that contribute to heating and cooling. Thus, the heat transfer happens immediately, which means a smaller amount of energy is accumulated in the wall. For a proper function, the openings in the glass must be kept closed during the winter. In contrast, the holes in the wall must be kept open during the day and closed during the night or when the solar radiation is not significant to avoid heat losses due to inversion of air circulation. In addition to the holes, the Trombe wall must have shading devices, such as blinds or visors, to prevent overheating in the cooling season and avoid thermal losses during winter nights [20].

The thermal properties of the materials to be applied in the Trombe wall should be selected to reach effective thermal inertia. Besides, the Trombe wall must be designed to clean inside the glazing, since condensation may occur and degrade surrounding materials.

5.2.4. Water Walls

Water is responsible for storing and releasing the heat in water wall systems, as depicted in Figure 7. The storage capacity of water wall storage capacity is ten times superior to a masonry wall and five times superior to a reinforced concrete wall. As such, it constitutes an extremely effective solution for passive heating of residential spaces, but with relatively expensive drawbacks, since poor maintenance of the element can lead to serious pathologies associated with water infiltration into adjacent elements or even leaks into the interior of the dwelling.

Water walls have a similar functioning to the Trombe walls (see Figure 7). These are relatively dark in colour, still allowing the penetration of some solar radiation that will heat and illuminate the adjacent house. They can be complemented with floors of high thermal inertia, providing an increased absorption of this energy to reuse when needed. Instead of being a common material, such as brick, the thermal storage element is replaced by water. In the Trombe walls, there is a more significant time phasing between the energy absorption and its transfer to the interior [10]. The water containers must be at 90% of their storage capacity. A drawback of this system can be the water noise [10].

5.2.5. Water Roof

This solution is characterised by a mass of water on a roof slab of buildings, exposed to solar radiation and allowing the absorption and storage of heat. As a rule, a plastic cover is placed to prevent energy loss by convection to the outside. The material of the inner part of the roof must facilitate the conduction of heat to the interior of the building [13]. The water body is protected at the top by removable insulation, which:

• In winter, it is inactive during the day to maximise storage capacity, and at night it is activated to maximise the ability to retain this same energy.
• In summer, it is active during the day to prevent the room from overheating, even providing cooling of the compartment, while at night, it is deactivated to cause a cooling of the water body.

One of the main advantages of this approach is that it allows all compartments to have their radiant energy source with little concern about the structure’s orientation or the way it was built [10].

5.2.6. Floor Heating

Floor heating consists of a mass of water under the floor of a room. The solar radiation falls on a vitrified surface located outside a water heating receptacle, which will radiate heat energy through the floor, generating heating of the dwelling. It is also important to mention that the surface must have protection to collect energy efficiently in the winter and not be allowed to pass through in the summer, thus preventing the room from overheating.

5.3. Isolated Gain Solutions

In isolated gain solutions, the capture of solar gains and the energy storage are not located in the occupied areas of the buildings, so they operate independently of the building, such as stoves. Solar energy is transmitted to the space adjacent to the stove by conduction through the storage wall that separates them and by convection in case some holes allow air circulation.

5.3.1. Stoves

A stove is a closed space, protected by glazing, containing a thermal accumulator mass, generally the floor and the wall adjacent to the room to be heated. In this solution, the combination of thermal principles of direct and indirect passive solutions is verified. To guarantee the maximum reuse of this energy, elements with high weight are generally used, namely the floor slab and the separating wall. These two elements have high thermal resistance and will be responsible for storing and releasing the energy when needed.

For this solution to work effectively, some requirements must be met, such as stoves facing south, reducing the glazed areas facing east, west, and on the roof as much as possible, or making these areas completely opaque since they do not provide enough heat in winter and cause overheating in summer.

Another requirement is the need for insulation at night, both on the glazed surface of the greenhouse as well as on the walls and openings that separate the stove from the dwelling. An example of insulation would be the use of a blind, providing thermal insulation at night to minimise heat losses. On days with low insolation or at night, the stove acts as an intermediate thermal zone (buffer zone), concerning the adjoining compartment, contributing to reducing energy losses.

The greenhouse can function as a direct gain by capturing the heat and transmitting it to the interior of the building directly through air circulation or as an indirect gain through an accumulating mass, which retains the heat and transmitting it by radiation to the interior [12].

5.3.2. Thermosyphons

Thermosyphons use the relationship between the temperature of a fluid and its density (Figure 8). In other words, when air is heated, it tends to assume a higher position, thus creating a temperature renewal circuit if there is a constant heating source. In this case, this source is solar energy, which is adequately captured and stored to constantly heat the cold air that will later rise, pushing more cold air to be heated. For this purpose, a small greenhouse capable of absorbing energy from solar radiation is built. The heating is then absorbed by a gravel bed located in its vicinity. This bed will be responsible for heating the air consistently and consequently ensuring the predominance of the convection cycle (see Figure 8).

5.3.3. “Barra–Constantini” Solution

The “Barra–Constantini” consists of the total inclusion of the housing compartment in opaque elements responsible for absorbing the energy from solar radiation and consequently heating the cold air that travels between them (Figure 9). The existing convections originate between the differences in air density at different temperatures and generate heating of the house that is constantly reused, making a cyclic circuit.

6. Passive Cooling Solutions

The solutions that prevent and/or mitigate the heat gains, or strategies for heat dissipation, provide a reduction in cooling needs and an improvement in thermal comfort conditions in summer seasons. The application of passive or natural cooling depends on suitable environments that act as cold sources and create temperature differences.

Architectural options can consider preventing or protecting solar gains in all buildings. First, it is necessary to account for the type of glass used and the respective solar control. The best solution may be the use of external shading, as this prevents the entry of solar radiation into the interior of the building. If this is not possible, reflective glass solutions associated with internal shading solutions should be considered [20].

The use of insulation in the building envelope, especially if placed outside the envelope, leads to situations that reduce the thermal demands through the opaque envelope, thus reducing the building’s cooling load. Particular attention should be paid to the roofs since they receive solar radiation during the summer. Another aspect to consider is the colour of the building, as light colours translate into lower values for capturing solar radiation, favouring the thermal performance of buildings in summer. The attenuation of heat gains through the building envelope also depends on the building’s thermal mass. The passive cooling solutions can be classified as:

• Direct cooling solutions
• Indirect cooling solutions

What follows are brief descriptions of the main direct and indirect cooling solutions.

6.1. Direct Cooling Solutions

Direct cooling includes sun protection and all the procedures used to cool a space, placing that same space in direct contact with the cold source, which can be the coolest outdoor environment at night, underground pavement, water, or even through evaporative cooling and dehumidification cooling

6.1.1. Solar Protection

Use of deciduous trees

A straightforward strategy, taking into account the characteristics of the place and the availability, is the use of deciduous trees in the vicinity of glazing with orientations whose gains in the cooling season are harmful to the energy sustainability of the dwelling. Gains from glazing constitute a large part of the energy responsible for negative effects during the summer season and beneficial effects during the winter season. Thus, it is plausible to assume that the correct use of such a simple technique can easily enhance the use of these gains to obtain an ideal balance of these factors through the creation of shading that will prevent the passage of energy by unwanted radiation, facilitating its passage when its impact is beneficial, as illustrated in Figure 10.

Architectural design

It is also possible to obtain this solar protection through the architectural design of the building, the orientation of the openings, the position of the building regarding constructions, and through the volumetric impact and shape of the construction itself (see Figure 11).

Several constructive elements and accessories can provide the desired shading, such as visors (metallic, concrete, or stone) and manoeuvrable blinds (low weight and cost solution), that provide various degrees of transparency and regulation of the flow of solar radiation [13]. Despite being more expensive, external shading structures are about 70% more effective than interior solar protections, preventing solar radiation from reaching the glazing.

On the other hand, indoor devices are more cost-effective in terms of maintenance and installation with the benefit of being easier to adjust to any situation. In the case of devices installed inside, with double-glazed openings with ventilation openings to the outside, the advantages of the two mentioned cases are combined. In turn, the visors, when properly designed, are an exciting solution, as they allow shading in the summer and do not prevent the entry of solar radiation in the winter [13].

Another factor to consider is whether the devices will be mobile or fixed. The fixed devices are more common for the outside, to promote protection from solar radiation, and the mobile ones are more common inside for lighting. Therefore, the use of these shading devices and adjustable protection is essential, either to reduce heat losses by transmission in winter or to control heat input in summer.

Orientation also plays a key role, as it will influence the solar gains of the building. The shading of south-facing glazing is the easiest to achieve, using projected horizontal elements such as awnings, horizontal or reflective visors, recessed spans, etc.

When oriented to the north, these horizontal elements do not perform their shading function, being more valuable than vertical visors or recessed windows to block the low sun in the early morning and/or late afternoon of the surroundings oriented to the south. The windows facing east and west are, without a doubt, the most serious cases and the ones that show more concern in the summer. By shading in these orientations, the entry of sunlight is avoided when the sun is lower, since, during the summer, these are the hours of greatest gains, as the radiation falls perpendicularly on the glazing. Therefore, to avoid the choice of horizontal louvres in this case, it is always a good solution to choose external vertical mobile blinds made of slats or PVC. It is important to consider sun protection on opaque surfaces, mainly on the surfaces of the building envelope more exposed to solar radiation, such as the exterior cladding. If it is not possible to use effective shading devices, choosing exterior surfaces with a low absorption coefficient is important.

6.1.2. Natural Ventilation

Another type of cooling passive solution uses natural ventilation, capable of carrying out a correct renewal of the indoor air without resorting to mechanical devices and consequently without increased energy use.

Natural ventilation plays a significant role in removing heat from the interior of the building and establishing thermal comfort, either by lowering the interior temperature or renewing the saturated air due to human activities. However, it has some disadvantages, such as controlling the flow or airflow, since they are dependent on weather conditions and pressure differences resulting from temperature and wind variations. This lack of flow control can cause discomfort to the occupants due to the air currents that may arise and energy losses during the winter.

Normally, this type of ventilation occurs inside the dwellings and depends on the user opening the windows to renew the air. However, several constructive solutions allow natural ventilation, highlighted as follows.

Cross ventilation

The cross-ventilation solution favours the natural ventilation of the dwelling by creating openings commonly located on opposite facades; the windows entirely play their role. Some concerns should be stressed; namely, positioning these openings on sides whose characteristics of the incident winds are substantially different so that there is a pressure variation responsible for the induction of movement of the interior air, which is therefore one of the main reasons why there must be extra care when integrating the building.

Stratification ventilation

Stratification ventilation uses the concept of the difference in the behaviour of a fluid with temperature variation. Fresh air enters through an opening in the lower part of the opaque element, replacing the inside air. The opening must be located in an opaque element and placed at a higher level.

Solar camera

The solar camera solution is similar to stratification ventilation, but it assists in air movement by installing a solar camera in the upper part of the building. It has a similar function as a common chimney, to eliminate indoor hot air. An air chamber with a dark-coloured solar radiation collector protected by a glass envelope is applied to the roof of the space to be air-conditioned, as shown in Figure 12. Thus, the air present in the chamber is heated, becoming less dense and forcing its exit from the interior by stratification of the air. Therefore, the greater the incidental solar radiation, the greater the efficiency of the solar chimneys and the greater the efficiency. A good orientation is necessary for better intensity of solar radiation, which may be to the south, east, or west. However, good air extraction is not generated due to the chimney effect if the outside temperature is high. As with induced ventilation, there must be a difference in temperature from the outside and the warm air in the highest part of the space.

Static vacuums

The static vacuum cleaner system creates air movement inside space from air extraction in the cover, which is combined with the intake of outside air at the bottom of the circuit. These vacuum cleaners produce a depression in the indoor air due to the suction produced by a static device on the roof, as shown in Figure 13. Thus, the wind passing through the vacuum will create the Venturi effect, extracting the indoor warm air [24].

Wind Tower

The wind tower consists of the directional ventilation process’s inversion, as shown in Figure 14. The prevailing winds are taken advantage of by a tower located on the roof. They will develop pressures capable of forcing the indoor air to be renewed through an opening located on the opaque element in contact with the outside.

6.1.3. Underground Construction

Underground constructions use the temperature variation between the outside air and the ground during the summer season. For a variation of 1.2 m in height, changes in air temperature can achieve 10 °C. As such, it is plausible to use cooling tunnels with an air tower capable of capturing the outside air, directing and cooling it through the tunnels, and reusing it to provide the building’s natural ventilation (see Figure 15).

6.1.4. Evaporative Cooling

Evaporative cooling consists of a temperature decrease by changing the water state from liquid to gaseous, i.e., water evaporation is used to cool a space. In this process, water absorbs energy from the latent heat of evaporation without increasing its temperature. This increases comfort during the summer by making the space cooler and humid. For it to work accurately, there must be a large area of contact between air and water, such as water mirrors, swimming pools, or lakes, and it is still possible to optimise this solution if the water is in motion. It is a good solution for the Mediterranean climate, as better use is made when the solar incidence is more intense.

6.2. Indirect Cooling Solutions

Indirect passive cooling solutions use a radiant surface or a thermal storage element, which absorb heat from the building and cool it by transmitting energy to the outside or allowing fresh air to flow through it. There are two indirect cooling solutions: radiation and night ventilation of the heat storage elements.

6.2.1. Radiative Cooling

There is always heat transfer between two bodies by radiation from the body with the higher temperature to the colder body. This phenomenon can be applied to the cooling of buildings. Radiation losses are a continuous process that occurs both during the day and at night. However, their effects are felt more intensely during the night due to the absence of direct solar radiation [12]. Conditions for cooling a building can be created by using the roof as a thermal element. The horizontal roofs are elements with abundant external exposure, thus favouring radiative exchanges. However, thermal insulation is generally applied to these enclosures to minimise thermal losses in winter and gains in summer, reducing night-time radiative cooling. This problem is tackled by installing mobile insulation that is only activated during the day to avoid significant heat gains, thus optimising radiative cooling at night [31].

6.2.2. Night Cooling Ventilation of the Thermal Storage Elements

Cooling by night ventilation consists of circulating fresh air during the night and early morning hours, cooling both the interior rooms and the thermal storage elements. These elements will store heat during the day, cooling the interior spaces. The lower the temperature during the night, the more efficient the solution becomes. A variant of this solution uses a bed of stones located under the space to be heated. The fresh air from the outside cools the stones during the night, while the heat from the outside passes through the stones during the day, losing part of the energy and cooling the environment [12].

7. Summary and Conclusions

It is consensual that many passive solutions reduce the nominal heating and cooling needs of a building. However, it can be complex to indicate which is the best passive solution to be applied, since both the climate and the characteristics of the environment surrounding the building vary depending on the case.

This work presents a brief analysis of passive solutions currently available for energy-efficient buildings, namely in Mediterranean countries.

Table 1. Passive heating solutions summary.

Season	Bioclimatic Strategy	Passive Heating Solution		Building Type/Occupancy
Winter	Promote solar gain	Direct gains	Promote rapid space heating. The glazed spans should preferably be oriented to the south.	All buildings with night and/or day occupancy.
		Indirect gain	Store heat during the day to transmit it at night.	Building or compartment thereof to be used at night.
		Isolated gain	Stove: promote interior space heating during the day.	Building or area thereof with daytime occupation.
			Air collectors: allow the introduction of heated air in spaces with high air renewal needs during winter.	Building or area thereof with daytime occupation and with a high number of occupants (classrooms, auditoriums, etc.).

and

Table 2. Passive cooling solution summary.

Season	Bioclimatic Strategy	Passive Cooling Solution		Building Type/occupancy
Summer	Promote cooling	Direct gains	Cool a space by direct contact with the cold source, which can be the cooler outdoor environment, underground pavement, or water.	All types of buildings but most relevant in residential buildings.
		Indirect gain	Absorb heat from the building, radiating energy to the outside or cooling the interior environment with a fresh airflow.	All types of buildings.
		Isolated gain	Provide heat exchanges in an area separate from the environment to be cooled.	All types of buildings.

summarise the main passive solutions. In addition, some point must be emphasised:

• From the point of view of the geometric configuration of buildings, the lower the value of the shape factor, the better the energy performance.
• In buildings with a rectangular plan, the longest axis of the building should be oriented in an east-west direction, placing the longest facade facing south.
• From the point of view of distributing the areas of the glazed openings, they must be considered permanent and non-permanent areas of the building with building orientation.
• High inertia buildings will have more significant energy needs reductions than buildings with low or medium inertia.
• Light- or medium-coloured buildings have lower energy requirements than dark-coloured buildings.

Therefore, passive solar solutions are an exciting opportunity for new buildings, have great potential for energy savings, and can significantly contribute to achieving the objectives defined by the European Union to reach sustainable development.