31 MAY 2012 • ECOLIBRIUM FEATURE Change agent Phase-change materials remain one of few green building tools yet to become mainstream. Recent advances in the way they can be incorporated into buildings and the experience of successful installations might soon pave the way for their wider uptake, Sean McGowan reports. The concept of phase-change materials (PCMs) is not a new one. Scientists have been studying the phase-change qualities of a range of materials and substances for decades, while ice and ice storage remains one of the oldest and most commonly used forms of this phenomenon. Interest in incorporating phase-change materials in buildings is also not new. However, the building and construction industry has continued to grapple with the risks and challenges associated with successfully incorporating it to even out the heat loads in modern buildings. In climates where buildings are required to move from a nominal heating mode to cooling mode in the course of the same day, PCMs are seen as having the greatest potential. This is particularly relevant in the temperate regions of Australia, where high diurnal temperature swings are more common. Like any other technology implemented in buildings, the use of these materials needs to be designed carefully to maximise their potential ’ According to Paul Bannister, M.AIRAH, managing director of Exergy Australia, incorporating PCMs into a building design effectively offers the potential to shift the heat load from one part of the day to another. “The phase-change material helps move the coolth from the cold parts of the day to the warm parts of the day, and the heat from the warm parts of the day to the cold parts of the day,” Bannister explains. “This then minimises the energy required to maintain space conditions.” And although PCMs can be used in conjunction with a building’s thermal mass, he says it is important to differentiate between the two, despite their similar characteristics. “When thermal mass stores heat, it warms up,” Bannister says. “When PCMs store heat, they do so at a fixed temperature while the phase change is occurring. This makes the heat storage both easier and more useful.” The Rotterdam Floating Pavilion features phase-change material in the walls of its conference room. These materials absorb warmth (liquid phase) above 21°C or heat up the room when the temperature is below 21˚C (fixed phase).
Transcript
31MAY 2012 • eColI b R I u M
F E A T U R E
Change agentPhase-change materials remain one of few green building tools yet to become mainstream. Recent advances in the way they can be incorporated into buildings and the experience of successful installations might soon pave the way for their wider uptake, Sean McGowan reports.
The concept of phase-change materials (PCMs) is not a new one. Scientists have been studying the phase-change qualities of a range of materials and substances for decades, while ice and ice storage remains one of the oldest and most commonly used forms of this phenomenon.
Interest in incorporating phase-change materials in buildings is also not new. However, the building and construction industry has continued to grapple with the risks and challenges associated with successfully incorporating it to even out the heat loads in modern buildings.
In climates where buildings are required to move from a nominal heating mode to cooling mode in the course of the same day, PCMs are seen as having the greatest potential.
This is particularly relevant in the temperate regions of Australia, where high diurnal temperature swings are more common.
like any other technology
implemented in buildings,
the use of these materials
needs to be designed
carefully to maximise their
potential’According to Paul Bannister, M.AIRAH, managing director of Exergy Australia, incorporating PCMs into a building design effectively offers the potential to shift the heat load from one part of the day to another.
“The phase-change material helps move the coolth from the cold parts of the day to the warm parts of the day, and the heat from the warm parts of the day to the cold parts of the day,” Bannister explains. “This then minimises the energy required to maintain space conditions.”
And although PCMs can be used in conjunction with a building’s thermal mass, he says it is important to differentiate between the two, despite their similar characteristics.
“When thermal mass stores heat, it warms up,” Bannister says. “When PCMs store heat, they do so at a fixed temperature while the phase change is occurring. This makes the heat storage both easier and more useful.”
The Rotterdam Floating Pavilion features phase-change material in the walls of its conference room. These materials absorb warmth (liquid phase) above 21°C or heat up the room when the temperature is below 21˚C (fixed phase).
eColI bR I u M • MAY 2012 32
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This ability to phase change while maintaining a near-consistent temperature is what makes the potential use of PCMs so exciting. So too is their typically lightweight nature, and the fact that in many cases only a fraction is needed to deliver a similar effect to that of a comparably large and heavy thermal mass.
Phase-change material
helps move the coolth from
the cold parts of the day to
the warm parts of the day,
and the heat from the warm
parts of the day to the cold
parts of the day’Materials offering such advantages now include a range of waxes, gels and salt mixes, typically encapsulated in small spheres. These are either stored en masse in thermal tanks or, as has more recently been developed, embedded in building
materials such as plasterboard or cement-based floor screed.
Although these materials are becoming more commonly available, like any other technology implemented in buildings, the use of PCMs needs to be designed carefully to maximise their potential.
effICIent PHASe CHAngeMelbourne’s CH2 building was the first commercial building in Australia to demonstrate the benefits of PCMs to store coolth generated by conventional efficient chillers at the coolest points of the day.
Five years on, it remains just one of a handful of Australian buildings to have incorporated the technology successfully.
That it does so using PCM thermal tanks makes it all the more unique.
The phase-change material used at CH2, comprising a mixture of non-toxic salts and organic compounds known as
eutectic salts, is encapsulated in 100mm diameter metallic spheres. The freezing and melting point of this material can be modified by adjusting the make-up of the compounds used, but in this instance was designed to be at 15°C.
Approximately 30,000 of these metallic spheres are stored in three tanks located in the basement, taking up approximately three car spaces.
According to David Jarratt, a director of WSP Built Ecology who oversaw the PCM design at CH2, the system was intended to minimise the energy consumption of the building’s cooling systems while also reducing the operation of the building’s chillers during the day.
“When overnight temperatures are appropriate (an external wet bulb temperature below 11°C) the PCM tanks are charged using cooling towers and shower towers,” Jarratt explains.
The cooling towers and shower towers use evaporative cooling to generate cool water
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that travels through the tank and charges (or freezes) the PCM between the hours of 1am and 6am.
five years on, CH2 remains
just one of a handful of
Australian buildings to
have incorporated the
technology successfully’When external conditions do not allow the use of cooling towers to charge the PCM system, efficient chillers are used.
“Once charged, the PCM system is used to provide chilled water for the chilled ceiling panels during the day,” Jarratt says. “In this way, the PCM offsets the daytime space cooling to the early hours of the morning.”
A separate sealed water system passes through the tank to deliver this water around the building.
In the event that the PCM system charge is exhausted (that is, melted) during the day, the chillers produce the high-temperature chilled water required for the chilled ceilings as would ordinarily be the case.
Jarratt says this type of PCM system was designed to complement the built form, which features extensive exposed thermal mass in the building interior in the form of pre-cast concrete.
“Heat is stored in the structure throughout the day and is purged through openable windows overnight to cool the structure,” he says. “The PCM system is designed to complement and extend the benefits of thermal mass that the Melbourne climate has to offer.”
AggReSSIVe HeAt tRAnSfeRUnlike thermal mass, the effective transfer of heat from PCMs requires a more aggressive strategy.
“PCMs are a relatively high-energy density store, which means that they need a relatively aggressive heat transfer mechanism,” Bannister says. “Night purge, and in particular passive night purge, really doesn’t have the capacity to do this.”
This makes the challenge of designing an effective PCM system a little less straightforward and their control anything but trivial, as Bannister found during the design of the AA3 building at Charles Sturt University’s Albury-Wodonga campus.
In a location characterised by cool winters, hot summers, high solar loads and high diurnal temperature variations, the building was constructed from rammed earth and was intended to provide a good level of thermal comfort via this mass with low energy use.
However, close analysis of the design revealed a number of shortfalls to these expectations, and the design
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eColI bR I u M • MAY 2012 34
F E A T U R E
was subsequently modified during construction. Thermal comfort was realised in summer by augmenting the high thermal mass of the building with a phase-change material, while heat was removed by a conventional hydronic in-slab system using water cooled by a cooling tower operating during the night.
The PCM selected was a hydrocarbon wax, encapsulated in tiny spheres with a diameter of 2-20 µm (micrometres). These were introduced into the building by being embedded into 12mm thick gypsum plasterboard and in 50mm thick cement-based floor screed.
The plasterboard was fixed to the underside of the cement slab using a cementious adhesive to ensure a good thermal bond was achieved. In this way, the hydronic system in the slabs was able to remove or add heat to and from the PCM as well as the indoor environment.
In this example, the PCM wax melts at 23°C, all the while absorbing the required latent heat from the plasterboard or cement slab.
According to Bannister, this has two effects.
“Firstly, the ability of the slabs to store coolth is enhanced, and secondly the surface temperature of the ceiling or floor is kept at a melting temperature while latent heat is absorbed,” Bannister says. “Hence, there is the unusual effect that as heat is transferred from the office space to the building fabric the temperature of the fabric stays constant and cool, thus holding down the operative temperature even if the ambient air temperature increases.”
He says the results have been impressive.
“The benefit as we have seen at AA3 is a stunning level of passive temperature control,” he says. “We managed to get the building to maintain below 30°C all summer in spite of the fact that someone had turned the cooling system off.”
Where PCMs are implemented, challenges around control are commonplace. The development of a control strategy that can detect when the PCM has melted or frozen is both difficult and potentially a waste of time, money and effort if later proved incorrect.
However, Bannister expects the extent of such challenges to ease as PCMs are more widely adopted and involved in experimentation.
“We are only recently beginning to understand the broader interactions between the impact of the PCM as a means of maintaining space temperature as opposed to a means of controlling the radiative temperature of the occupied space,” he explains.
“This is a complex field, and our work in this area is suggesting that there may be new approaches that allow simplification of control. But this demonstrates that there are challenges.”
tHe next PHASeSuch has been the documented success of these buildings, and a handful of others in Australia and around the world, that phase-change materials are now receiving stronger consideration in appropriate projects locally.
Although PCMs can theoretically be applied to any building type, Richard Jelbert, an associate with Norman Disney & Young, says the decision to use PCMs requires successful assessment of a building’s generation and demand profiles.
“Understanding when your system is required to charge and discharge, and determining the amount of energy required to be stored, which directly impacts on the storage capacity, are critical issues,” Jelbert says.
“That said, there are limitations to where it would realistically be applied in order to deliver a practical and effective solution.”
He says a major driver for its consideration should be the availability of and demand for heat or coolth at different times of the day. A major limitation to this is the availability of a PCM to operate within the required temperature range.
To this end, Jelbert believes that in the correct application, the PCM would be an enhancement of an existing system design rather than a component around which the rest of the system is designed.
With PCMs more widely available to the Australian industry now than in the past, it is likely that it will emerge as another weapon in the ESD consultant’s arsenal.
Jelbert points to a number of scenarios in which the application of PCMs may be advantageous. These include load-constrained sites that have electrical peak demand limitations. Here peak load
reduction or load-lopping strategies using PCMs to store energy and shift the time-load demand to an off-peak period could be used.
He says greenhouse gas emissions reduction due to trigeneration system may also be optimised by storage of thermal energy.
“In Europe, where greenhouse gas emission intensity of grid-supplied electricity is much lower than in Australia, successful co and trigeneration schemes need to use this approach.”
PCMs might also have a role to play in the retrofitting of thermal mass into lightweight building structures, particularly with new PCM plasterboards entering the market.
However, those hoping for a new wave of “melting buildings” like that found at the University of Washington’s campus in Seattle only have to look to the many failed ice storage systems that became popular in the 1980s to have this expectation tempered somewhat.
Although one would hope a slow and methodical adoption of PCMs in Australia would avoid this technology going the same way, the relatively small number of example installations and limited knowledge base here continues to present a major risk factor to consultants, developers and owners.
The cost premium associated with the low-volume production of PCM materials only adds to this risk.
“Owing to the diverse range of solutions available in the building services industry, whatever issues PCMs can solve have generally been solvable to date by another method at lower cost,” says Jelbert.
“However, as building services designs are increasingly tuned to achieve better and better performance, and to target more stringent performance or benchmarking outcomes such as NABERS and Green Star, the margins available to be trimmed in order to achieve lower building greenhouse gas emissions are becoming finer.
“The application of PCMs may be one of the technologies that prove to be viable in that position, where it will ultimately come to be competing with higher-cost alternatives such as renewable energy.” ❚
