Passive cooling: can we cool buildings with low to no energy consumption?
Passive cooling: can we cool buildings with low to no energy consumption?
Photo by Moshe Harosh from Pixabay
Passive cooling systems can also help mitigate energy poverty, despite potentially high ancillary costs such as installation and maintenance for some systems. However, for warmer regions in Europe, it is still premature to rely solely on passive cooling due to technological and cost-related challenges. The most sustainable approach is to combine passive strategies with low-emission active cooling methods, allowing the former to further reduce the energy consumption of the latter. Find out more in this overview article.
Introduction
Passive cooling is an approach that makes use of the building design and materials to keep a comfortable temperature inside buildings without mechanical or electrical devices. Instead of these it relies on natural heat transfer mechanisms, such as conduction, convection, and radiation to remove heat and maintain comfortable indoor temperatures. Passive cooling techniques often involve design strategies that maximise natural ventilation, shading, insulation, and thermal mass to minimise the need for conventional air-conditioning systems (vapour compression systems), which rely on electricity and external heat sources.
Space cooling, on the other hand, is defined as the amount of heat that must be removed from the indoor air to cool the space to ensure the thermal comfort of the occupants of enclosed areas. It follows that passive cooling is a different kind of space cooling technology than cooling methods that require the use of external energy (defined as active cooling).
The International Energy Agency (IEA) reported in 2018 that energy consumption for space cooling constitutes 20% of the total energy used in buildings worldwide, contributing to 10% of greenhouse gas emissions. Although the figures for Europe are lower, the continent faces challenges such as increasingly hot summers, rapid urbanisation, and an ageing population, making it more vulnerable to heat. Consequently, there is a growing need for space cooling systems in European homes, a requirement that was previously limited to southern regions and non-residential buildings.
This reliance on space cooling poses a sustainability problem: several studies confirm that the energy demand for cooling in Europe, both for residential and non-residential buildings, is bound to increase drastically unless the problem is addressed from the point of view of energy efficiency.
Social issues also arise, such as cooling (or summer) poverty, analogous to the better-known problem of heat poverty related to the winter months. The problem lies in the widespread inability to afford to keep one's home in appropriate temperature conditions during the warm months. Quoting the European Commission: ‘While cooling poverty is now well captured in the EU definition on energy poverty, European wide data is only available for the year 2012, with 19% of EU population reporting “not being able to keep their home adequately cool during summer”.’ This indicator was again collected in 2023, with the publication of the results expected in 2024.
Passive cooling, due to its passive and therefore zero-emission nature, is a useful tool to address these issues. However, Europe lacks comprehensive literature on the effectiveness of certain passive cooling strategies, which can vary significantly depending on geographical context. In northern regions, for example, nighttime ventilation may be effective. However, in Mediterranean regions, a combination of methods, such as ventilation, shading, glazing, and exploiting the thermal mass of building envelopes is necessary to enhance thermal comfort. The effectiveness of these methods may diminish in the future due to climate change.
Here we offer a non-exhaustive selection of the main passive cooling strategies organised according to a constructive/architectural principle:
- Natural ventilation: Enhancing airflow through the building using windows, vents, and architectural features.
Figure 1: An example of natural ventilation using a solar chimney and wind catcher. Source: final report of the project ABC21: D3.11-Final-Report-on-updated-technical-guidelines-and-tools_230531.pdf (abc21.eu)
Window shades and blinds and window retrofit: Blocking direct sunlight to reduce indoor heat gain and upgrading windows to improve thermal performance.
Figure 2: An example of window shading - Photo by Henry Han from Pixabay.
Building insulation: Reducing heat transfer through walls, roofs, and floors.
Figure 3 – Wall insulation -Photo by Alina Kuptsova from Pixabay
Phase change materials (PCMs): Materials integrated within the building envelopes that absorb and release thermal energy during phase transitions to regulate indoor temperatures.
Figure 4– Image of 3 layers of an organic PCM encapsulated in a poly/foil film - Photo by Bombarb.
Nature-based solutions: Implementing green roofs, green walls, urban green spaces, water features, etc.
Figure 5 – Green roofs - Photo by CHUTTERSNAP on Unsplash
Passive cooling is not limited to the individual building. The so called ‘urban heat islands’ are urbanised areas that encounter elevated temperatures compared to their surrounding rural areas. Man-made structures like buildings, roads, and infrastructure tend to absorb and release solar heat more effectively than natural environments such as forests and bodies of water. There are several effective solutions that utilise the principles of passive cooling to mitigate this phenomenon. Many of these are applied to the buildings themselves, both to reduce the building's ability to amplify heat to surrounding areas and to safeguard tenants who are affected by the heat produced in the heat island. Among these solutions is the planting of vegetation on roofs (green roofs) and the use of white roofs and light-coloured concrete that reflect much of the solar radiation.
Each passive cooling system, depending on its characteristics, adapts well to certain environments or climates. In general, passive cooling techniques alone cannot match the efficiency of vapour compression technologies in coping with extreme climates or particularly intense heat waves. However, a study [1] finds that, under specific climatic conditions, certain passive (or quasi-passive) methods can achieve temperature reductions comparable to those of active systems. Despite this potential, there are still impediments to the widespread application of passive cooling techniques.
The initial investment required to implement these technologies can, in fact, be significantly higher and prohibitively expensive for residential users compared to active methods. Although in the long run the initial investment in passive cooling can be amortised against the energy costs of active technologies, it is essential to consider not only the initial cost but also the space required, maintenance needs, and potential retrofit expenses, which can vary significantly between technologies.
A further aspect to be considered when comparing active and passive strategies is that of noise generation, a problem that characterises outdoor air conditioning units in particular. The problem is recognised and taken into account within the European eco-design requirements for air conditioners and comfort fans directive, which sets limits on the sound power level of these devices.
It is important, finally, to distinguish between integrating passive cooling strategies in a building through renovation and designing a new building with embedded passive cooling technologies. A large proportion of passive cooling techniques, including some of the most effective methods, such as phase change materials (PCMs), are often unsuitable for existing buildings due to issues of applicability, cost, or both.
Currently, the European market is dominated by conventional vapour compression technologies, which are the focus of much research aimed at developing more sustainable cooling systems. Many promising passive technologies, such as PCMs, still have a low Technology Readiness Level (TRL) and this prevents economies of scale.
Given the complexity of the subject in specific climatic contexts or during long heat waves or health-related issues, relying exclusively on passive cooling may not be sufficient. Therefore, the promotion of passive cooling should be part of a broader strategy involving a synergistic framework tailored to the relevant climate context, where passive technologies complement low-impact active ones, such as heat pump cooling systems powered by renewables, in addition to urban interventions to counter the heat island phenomenon.
This strategy, integrating passive cooling techniques with efficient active measures, can lead to a significant reduction in consumption, thus contributing to sustainability goals and simultaneously counteracting the problem of cooling poverty.
Figure 6 - The picture show how to cool buildings in a sustainable way by combining passive and active cooling - Source: https://www.eea.europa.eu/publications/cooling-buildings-sustainably-in-europe
EU policy framework in passive cooling
The Energy Performance of Building Directive (EPBD): The Directive, as indicated under recital 12 and 70, emphasises the problem of the increase in electric air conditioners, which create a peak load during the summer that can lead to increased energy costs as well as burdening the environment by stimulating the production of greenhouse gases. For this reason, the directive encourages the use of passive cooling techniques both at the individual building level and at the urban agglomeration level (urban heat island effect).
EU Energy Efficiency Directive (EED): EED focuses on energy efficiency with an eye on the problem of energy poverty of which it provides a clear definition. Cooling is central as an energy service, especially in this era when heat waves are becoming more intense and frequent. It is therefore necessary that access to space cooling is as wide as possible. While not directly referring to passive cooling, the implementation of passive design measures in buildings can contribute to achieving this target.
Renovation wave: Among the main areas of intervention to double the energy renovation rate of buildings in the European Union, this programme identifies decarbonising of heating and cooling. Passive cooling can be identified as a tool to help achieve the goal.
The 2016 EU Heating and Cooling Strategy: The non-legislative document provided an initial overview of the problem of emissions related to heating and cooling and strategies to tackle it. Although not explicitly referred to as passive cooling techniques, the importance of passive methodologies is highlighted.
Relevant organisations
Here we provide a non-exhaustive list of organisations that include passive cooling among their focus areas:
Representatives of European Heating and Ventilation Associations - REHVA
Air infiltration and Ventilation Centre - AIVC
American Society of Heating, Refrigerating and Air-Conditioning Engineers - ASHRAE
European Solar Shading Organisation - European Solar Shading Organisation
European Ventilation Industry Association - EVIA
European projects related to passive cooling
Life projects:
The CoolLIFE project began in 2022 and aims to address the need for sustainable solutions to the EU’s rising demand for space cooling in buildings, promoting innovative space cooling technologies including passive cooling techniques, and the use of available local renewable energy supply.
This project aims to lower air temperature during heatwaves and enhance public space conditions, improving overall quality of life. It involves four dimensions: COOL SKY (shading and cooling structures), COOL BUILDINGS (green roofs and cool coatings), COOL SOIL (green areas and cool pavements), and COOL LIVING (drinking fountains and a heat index app). Covering 5.8 ha in Almada, Portugal, the project expects to reduce temperatures by up to 3°C.
LIFE SUPERHERO aims to promote the adoption of ventilated and permeable roofs (VPRs) made of clay tiles, that offer sustainable and cost-effective cooling, reducing energy use by up to 50%, but are under-recognised in current EU regulations and standards, limiting awareness of their benefits.
The WATERCOOL project focuses on creating and testing new strategies to handle extreme temperatures, fluctuating water runoff, and droughts in cities affected by climate change. It leverages the urban water network to implement green solutions and cooling measures, aiming to enhance urban sustainability and improve the well-being of residents.
BIG4LIFE aims to co-design plans for the maintenance, monitoring, and evaluation of Building-Integrated Greenery (BIG) systems, such as green roofs and facades, in the Mediterranean climate. By using xeriscaping and networking with smart solutions, the project seeks to ensure the long-term viability and enhanced ecosystem services of BIG systems.
The LIFE iTS4ZEB project will develop advanced Thermal Energy Storage (TES) technologies using Phase Change Materials (PCM) and high-efficiency heat pumps for heating and cooling.
The COOLING DOWN project aims to envision a renewable cooling sector in Europe, providing policy recommendations to achieve it. Through research, expert consultations, and modelling, it will assess renewable cooling technologies' potential and their role in climate change adaptation, particularly mitigating the urban heat island effect.
Horizon Europe:
The HELIOS project seeks to develop innovative, energy-free passive cooling solutions by leveraging the features of urban surfaces, including buildings and outdoor areas. The project aims to create new materials that function like a ‘skin’, forming a surface layer that mitigates the impact of urban heat islands.
PRELUDE is an optimisation service aimed at enhancing building efficiency. It offers precise feedback and recommends cost-effective retrofitting actions using dynamic building renovation passports. Emphasising passive cooling methods like natural ventilation, it reduces reliance on energy-intensive HVAC systems.
FuturHist is a project aimed at futureproofing-built heritage, by developing standardised approaches for intervention and conservation, also via passive solutions.
H2020:
The ABC21 project seeks to explore and document traditional African and European architectural practices that utilise local materials and bioclimatic design principles.
Cooltorise aims to address summer energy poverty in Europe by enhancing indoor thermal comfort and cutting energy usage, lowering heat exposure and related health risks.
POWERSKIN + combines enhanced insulation and renewable energy technology based on photovoltaics in modular solutions for retrofitting existing curtain walls. Innovative smart technologies will improve thermal insulation and on-site energy generation and storage, delivering significant advantages in all types of environments, thereby addressing the building sector's substantial global energy consumption and CO2 emissions.
Conclusions
Passive cooling strategies are an important tool for achieving comfortable conditions in buildings. However, today, especially with respect to temperature, passive cooling alone may not be sufficient, particularly in areas affected by strong heat waves.
The optimal solution currently involves integrating passive cooling with efficient active measures. This approach significantly reduces energy consumption, drastically cutting both emissions and cooling-related costs, addressing both sustainability and socio-economic issues.
References
[1] Active and passive cooling methods for dwellings: A review, Oropeza-Perez and Alberg Østergaard, 2017, available here.
