New types of building products which absorb heat when temperatures are high, typically during the day, and release it when temperatures drop, typically at night, are coming into their own as a means of increasing the sustainability of buildings. These building products – usually relating to the building’s wall, floor, or ceiling construction – incorporate substances known as phase change materials (PCMs).
There are various different types of phase change material used in construction products based on water, paraffin or a salt hydrate. All are capable of absorbing and emitting heat with a comparatively small temperature change simply by a change in their physical state.
If a building reaches, say, 23°C the PCMs start to melt, soaking up heat from the room as they do so and helping to maintain a comfortable working temperature. When temperatures start to cool down towards 19°C, the PCM will return to a solid, giving out heat in the process and preventing the room from getting too cool overnight. Having a PCM incorporated into the ceiling, wall or floor is a bit like adding thermal mass to a building in that it helps to make the ambient temperature more comfortable.
Thus, PCMs can remove, or at least reduce, the need for heating and cooling when incorporated into some buildings. This technology is now used in a plethora of building products – ranging from wall boards through plaster and screed to ventilation systems – which are beginning to find their way into more new buildings. High-profile schemes using the technology include the iCon building in Daventry, Edinburgh Napier University’s new campus and the BRE’s stable block conversion in Watford.
One of the least intrusive solutions, especially for refurbishment purposes, is PCM ceiling tiles, which are ideally suited to climates like ours in the UK that drop below 20ºC at night. A number of these tiling products are now on the market, including Armstrong’s CoolZone. As heat rises the ceiling is the perfect place to install PCM and take advantage of its heat storage properties.
In ceiling cassettes the PCM is incorporated into the inserts which are sandwiched between the two metal panels. The tiles are designed to drop into a standard suspended ceiling grid system so installation is very quick and simple, although grid strengthening may be required as the PCM cassettes weigh approximately 9kg each. As the PCM is contained within the ceiling tile, the product is portable and can therefore be moved to problem areas, or taken to a new building entirely.
The PCM used in a ceiling tile system is microscopic polymer capsules containing a wax storage medium which is embedded in gypsum and then encased in the metal tile. As the PCM is microencapsulated it causes no health concerns. The material will not leak out of the ceiling tile and the plasterboard it is implanted in can be recycled in existing streams.
On heating during the day and cooling at night, the wax melts (at 23-26ºC) and solidifies. In this way the internal temperature is regulated, as explained above, ensuring a stable and comfortable environment in which to work. However, PCMs cannot replace air conditioning to manage internal humidity.
PCMs provide lightweight thermal mass. To create the same thermal capacity as 3cm plasterboard containing 30% PCM requires 14cm of concrete or 18cm of bricks. PCM allows high thermal mass properties to be added to lightweight construction which could not support bricks or concrete. By using lightweight construction it is possible to reduce building costs and the running costs of the project.
Ceiling tiles are one of the least intrusive solutions for PCM use
These passive LHS (latent heat storage) units can be exchanged for standard ceiling tiles with minimum disruption, and have been shown to delay the need for air conditioning by up to eight hours by reducing high-usage peaks and help reduce energy use by up to 50-70%. These are just two of the results of a two-year R&D programme which tested more than 50 different PCM materials, configurations and conditions for fire resistance and acoustic performance alongside the thermal element.
PCM on trial
For a trial of PCM at a mid-rise masonry office building in central London, a number of PCM metal cassettes replaced standard mineral tiles in the centre of the ceiling of a meeting room that was suffering from overheating and heavily reliant on air-conditioning.
The 600mm x 600mm PCM tiles, which are reversible and can be wholly recycled at the end of their life, comprise an infill of 25% PCM material with a melt point of 23ºC, providing a total heat storage capacity of 136.2Wh/m².
They covered 60% of the 47.5m² ceiling in the basement room that had an air circulation rate of 13l/s/m² managed by a split HVAC system incorporating a ventilation fan. Occupancy, temperature, airflow and air conditioner energy use were monitored for six months.
The pilot showed that when heat could be purged from the tiles at night – that is the temperature of the room cooled down because it was unoccupied – the room used 20% and 70% less energy for heating and cooling, compared to a similar untreated room.
PCM ceiling tiles are most appropriate for renovation projects – such as offices, schools, clinics or retail outlets – where the building spaces are overheating and the cooling system is working hard all day to maintain a comfortable temperature for the occupants.
They are probably not best suited to buildings that are occupied or in use 24 hours a day as the PCM would not have time to cool overnight. This would include buildings such as data centres where the servers that have to run 24/7 generate a huge amount of heat.
In addition, PCM should not be used as a replacement for insulation as it acts as a thermal storage unit, rather than blocking out or containing thermal energy. The system should also not be used on exterior walls as the solar gain via the walls greatly reduces the capacity of the PCM, leaving it inefficient. PCMs are about regulating ambient temperatures, not mitigating large temperature swings.
However, new lightweight buildings are appropriate for PCM as they typically introduce thermal mass as part of an energy-saving strategy. When designed in conjunction with a HVAC system to maximise efficiency, PCM can be used in passive and active systems, from naturally-ventilated spaces to integrated chilled ceilings.
Regardless of whether the building is new-build or being renovated, it is important to design in the night-time purge strategy as it is vital to remove the heat from the PCM during the night so it is ready to work the following day. This can be done using natural night-time ventilation – such as opening the windows with the BMS, or simply running ventilation fans for a few hours.
Integrating PCM tiles, which have a maintenance-free service life of 30 years, with building services such as lights, fire alarms, sprinklers and ventilation grilles should not pose a problem as in this respect they are no different to standard ceiling tiles.
They are easily relocatable within a room, or from room to room, or building to building, to handle areas of high heat load.
With rising energy costs and more awareness of carbon emissions, so the old lessons of natural ventilation, low-energy cooling and thermal mass are becoming part of modern energy strategies.
Most of the software packages available for the energy modelling required during a building’s design stage now include the possibility of adding PCM to the analysis, so the development of more effective cooling and ventilation strategies is available from the word go.
Jeremy Sumeray is senior segment manager for sustainability (Europe, Africa and Middle East) for Armstrong Ceilings. www.armstrong-ceilings.co.uk
PCMs can even out temperatures
Three individual tests conducted by BSRIA using three types of air conditioning systems – displacement ventilation, overhead air and overhead air ducted return – in a 16m² insulated thermal test chamber at the organisation’s independent laboratories in Bracknell, Berkshire, showed:
However, adding PCM does not guarantee energy savings. You must purge the accumulated heat during unoccupied hours. Further tests are now being carried out on two naturally ventilated classrooms at schools in Bath and Nottingham.
PCM ceiling tiles have also been installed at two BRE Innovation Parks — its latest in Scotland and the original in Watford.
In Scotland, they feature on the main seminar area of the 112.3m² visitors’ centre at the heart of the 1,125-acre Ravenscraig Regeneration Project. The carbon-neutral, closed-panel timber-framed building is designed to achieve a BREEAM “Outstanding” rating and represents the latest in off-site manufacture, technology integration and building management.
Some 40m² of the reversible PCM system, which is also wholly recyclable, has been used at Ravenscraig along with 20m² of PCM ceiling tiles installed as a pilot test by the BRE throughout the Victorian terrace at its flagship Innovation Park at Watford.
This stable block has been transformed into three energy-efficient homes to create a living refurbishment laboratory for testing energy products, materials, design solutions and installation techniques that make solid-wall homes more energy efficient and affordable to heat.
The main seminar area of BRE’s Ravenscraig project features PCMs