Phase change applications can help to ensure thermal efficiency within the build environment.
WALLS & ROOFS looks at phase change materials’ (PCM) ability to enhance thermal performance within various built environment scenarios.
According to Construction and Building Materials, PCM is a substance with a high heat of fusion. Therefore it can store and release large amounts of energy in the form of heat during its melting and solidifying processes at the specific transition temperature. Christopher Kendrick and Nicholas Walliman stresses that PCMs can store much larger amounts of thermal energy per unit mass than conventional building materials.
As such, PCMs can help to conserve energy and improve buildings’ efficiency by maintaining comfortable room temperatures throughout the day. While traditional insulation works as a barrier to slow heat transfer, PCM goes beyond insulation to absorb and release excess heat as needed to provide a suitable ambient temperature.
Incorporating PCM in building materials has attracted a lot of research interest worldwide. This is due to the looming global warming crisis and PCMs’ ability to reduce energy consumption in buildings, thanks to their thermal energy storage capacity. In tests conducted by a major public utility company, 25% to 30% savings were achieved, according to Phase Change Energy Solutions.
Thanks to major design breakthroughs, companies such as Phase Change Energy Solutions are using environmentally-friendly bio-based PCMs to reduce energy consumption and CO2 emissions, shift peak electricity demand and reduce building operating costs, while creating a more comfortable environment for occupants.
“Phase change materials are used to store the latent heat absorbed in the material during a phase transition and can be used to maintain constant, comfortable building temperatures.”
Micro-encapsulation and phase change materials
The article “Development of phase change materials based micro-encapsulated technology for buildings: A review”, published in Renewable and Sustainable Energy Reviews, states that thermal energy storage (TES) systems using PCMs have been recognised as one of the most advanced technologies to enhance buildings’ energy-efficiency.
Internationally, current research focuses on suitable methods to incorporate PCMs in building design. The paper states that there are several methods and applications for PCMs in TES. Micro-encapsulation technology for thermal energy storage is one of the more advanced technologies to enhance the use of PCMs within building materials for walls, roofs and floors.
Within the built environment, PCM-based micro-encapsulation can be used for latent heat thermal storage (LHTS), which presents effective solutions to thermal energy storage and retrieval devices. Using micro-encapsulation PCMs for concrete and wall/wallboards are of particular interest, according to Renewable and Sustainable Energy Reviews.
According to Kendrick and Walliman, PCMs can be used for cooling a building in three conventional ways:
• Passive cooling: Cooling through the direct heat exchange of indoor air. PCMs are incorporated into the existing building materials, such as plasterboards.
• Assisted passive cooling: Passive cooling with an active component (for example a fan) that accelerates heat exchange by increasing the air movement across the PCM’s surface.
• Active cooling: Using electricity or absorption cooling to reduce the temperature and/or change the PCM’s phase to active cooling, and to a lesser extent supporting passive cooling.
Harnessing the sun with passive solar architecture
“Taking advantage of the normal tracking of the sun is becoming more necessary as we look for ways to reduce our carbon footprint,” states Phase Change Solutions. A building’s energy consumption is a major source of greenhouse gas (GHG) emissions. Passive solar architecture can help counter this, as it relies on solar and heat control techniques, heat amortisation and heat dissipation.
According to Comprehensive Renewable Energy, solar heat protection or exploitation can involve improving thermal insulation and controlling internal gains. The journal adds that strides have been made in creating state-of-the-art building fabrics, cool materials and intelligent control techniques. PCMs can play an important role in improving efficiency.
A trombe wall is a system for indirect solar heat gain. It optimises heat gain and minimises heat loss during cold spells, and avoids excess heat gain when it is hot outside. Although this passive heating system is not extremely common, it is a good example of combining thermal mass, solar gain and glazing properties to enhance human comfort.
The system consists of a dark-coloured wall with a high thermal mass facing the sun, glazing spaced in front to leave a small air space. The glazing traps solar radiation like a small greenhouse. An attached sunspace is essentially a trombe wall where the air space is so big that it is habitable.
Placing phase change material in trombe walls that are exposed to the sun at critical times during the day allows one to take full advantage of the transition of the phase change to decrease the heating load of the structure.
Placing a bio-based mat into the ceiling cavity of a structure – whether new or retrofit – is one of the easiest, most cost-effective phase-change applications. Numerous studies of actual structures have revealed significant savings on heating, ventilation and air-conditioning (HVAC) expenditures, according to Phase Change Energy Solutions.
Decorative wall panels
Simply placing phase change materials in decorative wall panels or behind existing artwork is a quick and effective means of saving money and increasing interior comfort.
For the past 20 years, significant research has been done on the potential use of PCMs in concrete. The results showed that PCM-concrete has some useful characteristics, such as better latent heat storage and thermal performance. “On the other hand, PCMs have some negative impacts on the properties of concrete,” states Construction and Building Materials. However, the journal states that the negative impacts can be minimised if an appropriate PCM and a suitable means of incorporation are employed during the production of the PCM-concrete.
Case study: Metal reroofing over an existing roof
In 2009 three experimental attics utilising different roof retrofit technologies were constructed at the Oak Ridge National Laboratory’s Field Exposure Testing Facility in the United States. Field-test data from the three test attics were collected between 13 November 2009 and 2 July 2010.
The first test attic represented the traditional way of roof retrofitting, where the old roofing materials were removed completely, disposed in landfills and replaced with a new roof cover. The two other attics used roof-over-roof technologies. Both technologies used metal roofing panels that could be installed directly over the existing roofs without having to remove the old materials. These metal panels contained a cool-roof coating to minimise solar heat gains. In the third test attic, roof-integrated PV laminate and PCM heat sink were used as well.
The test data demonstrated that roof-over-roof reroofing can be an effective way to refurbish the old roofing surface, as well as to improve an existing roof’s energy performance. During the winter and spring, the PV-PCM attic showed a 30% reduction in the heating load compared to the conventional shingle attic. On average, the maximum daytime temperatures were lower by about 15% in the PV-PCM attic compared to the shingle attic. This difference was higher in the late-spring and summer months.
Full acknowledgement and thanks are given to “Development of phase change materials based micro-encapsulated technology for buildings: A review”, Renewable and Sustainable Energy Reviews (Volume 15, Issue 2), February 2011; Construction and Building Materials Volume 46, 2013; Comprehensive Renewable Energy, 2012; Removing Unwanted Heat in Lightweight Buildings Using Phase Change Materials in Building Components: Simulation Modelling for PCM Plasterboard by Christopher Kendrick and Nicholas Walliman; www.ustainabilityworkshop.autodesk.com and www.ornl.gov for providing the information to write this article.