Achieving Cooling in Buildings with Energy Saving

Like any other field, one of the primary targets of any research and/or development in the HVAC&R field is energy saving, because still now most of the cooling methods that we are using, are depending on the electrical energy. There lies the challenge as – even if we focus on the environment-friendly methods of cooling, we cannot completely eliminate the need of electric power, which is still mostly being generated through burning of fossil fuels.

Thus, energy saving is one of the topmost priorities in the HVAC&R field. With this backdrop, researchers at different educational institutes or universities are working to lower the power requirement for cooling. In the recent past, some such research workers have published their reports of progress. Today, I will focus on three such innovations in the field of building cooling.

New mechanism to cool buildings while saving energy

Researchers at UCLA and their colleagues have now found an affordable and scalable process to cool buildings in the summer and heat them in the winter.

Led by Aaswath Raman, an Associate Professor of Materials Science and Engineering at the UCLA Samueli School of Engineering, a research team has recently published the details of a new method to manipulate the movement of radiant heat through common building materials to optimize thermal management.

According to their finding, radiant heat, which is felt whenever a hot surface warms our bodies and homes and is carried by electromagnetic waves, travels across the entire broadband spectrum at ground level between buildings and their environments, such as streets and neighboring structures. On the other hand, heat moves between buildings and the sky in a much narrower portion of the infrared spectrum known as the atmospheric transmission window. The difference in how radiant heat travels between buildings and the sky versus the ground has long presented a challenge to cooling buildings with less skyward-facing surfaces. These buildings have been hard to cool in the summer as they retain heat from the ground and neighboring walls when the outside temperature is high. They are equally difficult to warm in wintertime as the outdoor temperature drops and the buildings lose heat.

Raman, who leads the Raman Lab at UCLA Samueli, pointed out, “If we look at historical cities like Santorini in Greece or Jodhpur in India, we find that cooling buildings by making roofs and walls reflect sunlight has been practised for centuries. In recent years there has been massive interest in cool roof coatings that reflect sunlight. But cooling walls and windows is a much more subtle and complex challenge.”

However, with the proven success of cooling buildings by using super white paint on the roofs to reflect sunlight and radiate heat into the sky, the researchers set out to create a similar passive radiative cooling effect by coating walls and windows with materials that can better manage heat movement between buildings and their surroundings at ground level. The researchers demonstrated that materials capable of preferentially absorbing and emitting radiant heat within the atmospheric window could stay cooler than conventional building materials in the summer and warmer than they could during the winter.

Detailing further Raman said, “We were particularly excited when we found that materials like polypropylene, which we sourced from household plastics, can selectively radiate or absorb heat in the atmospheric window very effectively. These materials border on the mundane, but the same scalability that makes them common also means that we could see them thermoregulating buildings in the near future.”

In addition to leveraging easily accessible cost-saving materials, the team’s approach also has the added benefit of saving energy by reducing the reliance on air conditioners and heaters that are not only costly to run but also contribute to carbon dioxide emissions.

Focusing more light on the mechanism, Jyotirmoy Mandal, the study’s first author and a former postdoctoral scholar in Raman’s lab, who is now a Civil and Environmental Engineering Assistant Professor at Princeton University, said, “The mechanism we proposed is completely passive, which makes it a sustainable way to cool and heat buildings with the seasons and yield untapped energy savings.”

According to the researchers, the new methodology can scale easily and will be especially impactful on low-income communities with limited or no access to cooling and heating systems that have seen increasing casualties resulting from extreme weather events across the globe.

High-tech windows for comfort and energy savings

According to the U.S. Department of Energy: for most homeowners, 30% of energy bill results from window inefficiencies. Now you imagine that the windows of your home don’t transmit heat, then how will that affect your energy bill? Obviously, your heating and cooling bills will go down; your energy consumption and carbon emissions will drop – and you will still be comfortable throughout the year.

AeroShield, a startup spun out of MIT, is poised to start manufacturing such windows. According to their observation, building operations make up 36% of global carbon dioxide emissions, and today’s windows are a major contributor to energy inefficiency in buildings. To improve building efficiency, AeroShield has developed a window technology that promises to reduce heat loss by up to 65%, significantly reducing energy use and carbon emissions in buildings, and the company has just announced the opening of a new facility to manufacture its breakthrough energy-efficient windows.

Commenting on their venture, Elise Strobach, Co-founder and CEO of AeroShield, said, “Our mission is to decarbonize the built environment. The availability of affordable, thermally insulating windows will help us achieve that goal while also reducing homeowner’s heating and cooling bills.”

Research on AeroShield’s window technology began a decade ago in the MIT lab of Evelyn Wang, Ford Professor of Engineering. The work focused on aerogels, remarkable materials that are ultra-porous, lighter than a marshmallow, strong enough to support a brick, and an unparalleled barrier to heat flow. Aerogels were invented in the 1930s and used by NASA and others as thermal insulation. The team at MIT saw the potential for incorporating aerogel sheets into windows to keep heat from escaping or entering buildings. But there was one problem – nobody had been able to make aerogels transparent.

An aerogel is made of transparent, loosely connected nanoscale silica particles and is 95% air. But an aerogel sheet isn’t transparent because light traveling through it gets scattered by the silica particles. After five years of theoretical and experimental work, the MIT team determined that the key to transparency was having the silica particles both small and uniform in size. This allows light to pass directly through, so the aerogel becomes transparent. Indeed, as long as the particle size is small and uniform, increasing the thickness of an aerogel sheet to achieve greater thermal insulation won’t make it less clear.

Teams in the MIT lab looked at various applications for their super-insulating, transparent aerogels. Some focused on improving solar thermal collectors by making the systems more efficient and less expensive. But to Strobach, increasing the thermal efficiency of windows looked especially promising and potentially significant as a means of reducing climate change.

The researchers determined that aerogel sheets could be inserted into the gap in double-pane windows, making them more than twice as insulating. The windows could then be manufactured on existing production lines with minor changes, and the resulting windows would be affordable and as wide-ranging in style as the window options available today. Best of all, once purchased and installed, the windows would reduce electricity bills, energy use, and carbon emissions.

Carbon dots-driven green radiative cooling coating for energy saving

The Hong Kong Polytechnic University (PolyU) researchers have developed an environmentally friendly Solar-driven Adaptive Radiative Cooling (SARC) coating for building roofs and walls. This coating can reduce a building’s surface temperature by up to 25°C and lower indoor temperatures by 2 to 3°C, all without consuming any energy. This non-toxic, metal-free and durable coating can be produced on a large scale, promoting an eco-friendly and energy-saving method to mitigate urban heat island effects and support the achievement of carbon neutrality.

Coating a building in a reflective material enables the self-regulation of its thermal environment to minimise indoor temperatures. However, traditional passive radiative cooling materials are unable to automatically adjust cooling capacity in response to environmental changes, which limits their applications. To address this challenge, a research team led by Prof. LU Lin Vivien, Professor of the Department of Building Environment and Energy Engineering at PolyU, along with key team member Dr Quan GONG, Postdoctoral Fellow of the same department, has invented a Carbon Dots (CDs)-driven SARC coating that can adjust cooling capacity based on solar irradiance. This new photoluminescent radiative cooling nanocoating can convert solar energy into light energy. As solar intensity increases, the coating’s solar reflectance is enhanced, preventing buildings from absorbing excessive heat.

However, traditional photoluminescent cooling materials typically rely on rare earth metals and perovskite materials, which pose environmental risks. To address these issues, Prof. Lu’s team has introduced groundbreaking, environmentally friendly polymer-based CDs as photoluminescent materials into radiative cooling coating. Nano-sized CDs were embedded into polymers to create a biologically harmless material. The polymer CDs were uniformly coated onto hollow glass particles to create Smart Cooling Beads, enabling the coating to effectively convert ultraviolet light into visible light photons and increase effective solar reflectance. This water-soluble SARC only requires the evaporation of water to form a coating on building surfaces without releasing any volatile organic compounds, thereby reducing air pollution.

Results have shown that, compared to conventional radiative cooling coating, the new SARC coating improved effective daytime solar reflectance from 92.5% to 95% and increased the cooling effect by 10% to 20%. For example, it can reduce the temperature by up to 25°C when applied to concrete rooftops.

By P. K. Chatterjee (PK)