Reflective surfaces have been commonly used for thermoregulation in buildings across cultures since ages. Coating roofs with white paint to reflect the sun’s heat and keep the buildings cool is a widely prevalent practice. Recently, a team of researchers at Princeton University has devised a new strategy to achieve thermoregulation in buildings by managing heat flows. Based on important new insights into thermodynamics and reflective materials used in buildings, the research can prove invaluable in large-scale construction projects.
Researchers of the Engineering School at Princeton University have deeply studied every aspect of the existing methods of thermoregulation. Their findings indicate that thermoregulation in buildings can be drastically enhanced by coating vertical surfaces like walls with selective materials that can reflect specific wavelengths of heat to the sky.
• Based on new insights into thermodynamics and reflective materials used in buildings, a team of researchers from the Engineering School at Princeton University has devised a new strategy to achieve thermoregulation in buildings by managing heat flows.
• The research proposes the management of certain wavelengths of heat that are overlooked in existing technologies and extends the usage of vertical surfaces like walls for thermoregulation.
• According to the researchers, thermoregulation in buildings can be drastically enhanced by coating vertical surfaces like walls with easily available materials like polyvinyl fluoride that can reflect specific wavelengths of heat to the sky.
• Estimated savings using this method are energy savings worth $ 50 – 1,000 annually and a reduction in carbon dioxide emissions by 0.1 – 5 tons annually.
Buildings exchange heat with their surroundings mainly through radiation. Managing these heat flows that take place through electromagnetic waves can substantially alter a building’s thermoregulation capacities. Researchers of the Engineering School at Princeton University have recently discovered that radiant heat limited to specific wavelengths can be effectively managed using coatings made from easily available materials like polyvinyl fluoride, which lead to significant energy savings.
The novelty of the experiment emerges from two primary insights. First, apart from the different bands of the solar spectrum (UV, visible, and near-infrared wavelengths), buildings emit and absorb heat over the thermal infrared (TIR) wavelengths (2.5–40 μm). This wavelength is often overlooked in the past research regarding passive thermoregulation in buildings. Second, existing technologies have mostly focused on sky-facing parts of the building like roofs, while heat management using vertical facades (walls) is still underexplored.
The new radiative cooling model developed by researchers at Princeton University has the potential to address the challenges posed by traditional methods of thermoregulation in buildings such as ACs or central heating mechanisms. The new model uses carefully selected materials with high solar reflectance and long-wave infrared (LWIR) emissivity on vertical facades to achieve passive thermoregulation of buildings. The system is sustainable, cost-effective, and can radically enhance the energy efficiency of buildings.
The research leverages existing studies on the differential transmittance of heat in the atmosphere. Narrowband and long-wave infrared radiation is transmitted towards the sky, whereas broadband heat flows between terrestrial objects. According to the research, thermoregulation can be achieved by effectively managing the interrelationship between these different heat flows.
Existing thermoregulation technologies mostly rely on coating roofs with materials that reflect solar radiation. This takes place in the long-wave spectrum. The roofs freely reflect the sun’s heat to the sky. However this approach misses the role of vertical surfaces in heating or cooling the building. Vertical surfaces are surrounded by bare ground, roads and pavements, vegetation like trees, etc., and other residential and industrial buildings. The land and these objects heat up or lose heat depending on the time of the day, which in turn affects the thermoregulatory capacities of buildings drastically.
Ignoring the role of vertical surfaces in heat radiation leads to another problem. The presence of terrestrial objects also obstructs the free reflection of radiation back into the atmosphere. To counter this, researchers have proposed a way to direct the flow of heat between buildings back into the sky. On sky-facing surfaces, heat is radiated back into the sky in a narrow band of infrared spectrum. On the other hand, heat between buildings operates in a broad range. Existing technologies therefore use broadband-emissive coatings on vertical surfaces. However, the latest research proposes the use of selective LWIR-emissive coatings instead of broadband-emissive coatings like white paints and composites.
The intricacies of the material selection for this project are well thought out by the researchers. Broadband emitters are ruled out as they interact with a range of heat waves and terrestrial heat. Whereas, the LWIR-emissive materials proposed in this project operate only in the long-wave infrared range. The relatively lower interaction of LWIR-emissive materials with broadband heat waves leads to minimal heat gain in summer and minimal heat loss in winter.
The researchers at Princeton have conducted the experiments using easily available and cost-effective materials like metallized polypropene (PP), paint resins, ceramics, PMP sheets, etc. which have selective LWIR-emissive properties. The final experiments were carried out in the summer and winter seasons in Los Angeles and Princeton. The tests conclusively proved that a selective LWIR emitter achieves greater cooling compared to broadband emitters during hot days. A series of tests were conducted to test the effect of the materials under varying conditions like temperatures, sun’s rays, humidity, and wind speeds.
The team consisting of Jyotirmoy Mandal, Jyothis Anand, Sagar Mandal, John Brewer, Arvind Ramachandran, and Aaswath P. Raman have calculated the exact thermoregulatory potential of selective LWIR-emissive materials under diverse scenarios. Most importantly, the researchers have considered the impact of these materials on the infrastructural requirements of developing countries where low-cost effective thermoregulation is an urgent necessity.
Based on building-level energy simulations, researchers estimate that radiative thermoregulation using selective LWIR-emissive materials can save energy worth $ 50 – 1,000 annually. Moreover, this approach can reduce carbon dioxide emissions by 0.1 – 5 tons annually.Â
It is important to note that this mechanism is complementary to existing systems of thermoregulation in buildings. Thus, the energy saved by this method is added to the total energy savings leading to significant overall cost reductions. The simple idea of using LWIR-emissive coatings on vertical surfaces is easily replicable across varying geographical locations including developing countries where low-cost heat-management is a constant challenge. The researchers have ensured that the selective LWIR-emissive materials are easily available considering their potential wide-scale implementation in developing countries. They have tested PP and poly(ethene terephthalate) (PET) which can even be recycled from plastic waste such as discarded food containers.Â
The research undertaken at Princeton University marks an important shift in the study of thermoregulation in buildings. The research not only proposes a novel way of enhancing the energy efficiency of buildings but does so in simple and cost-effective ways ensuring their applicability across varying geographical conditions. Incorporating vertical surfaces in the thermoregulatory design of buildings can prove to be a game-changer in construction technology where large-scale energy efficiency is the key. The idea of using materials that are easily available and can be made by recycling as LWIR-emissive coatings is equally revolutionary.
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By Ravi Kumar
