Adding thermal mass during refurbishment
“How effective are phase-change materials at increasing thermal mass and delaying the need for cooling during the day? Jeremy Sumeray of Armstrong Ceilings has the answers.”
Greening’ buildings is not a modern phenomenon. Using elements of them to conserve energy and their impact on the environment is a practice that goes back to prehistoric times when cavemen intuitively recognised the value of thermal mass in creating a comfortable indoor environment.
Hundreds of years before the adoption of the Kyoto Protocol in 1997 brought climate change to the attention of the contemporary masses, the Vikings took that most obvious of eco-friendly signs, the green roof, with them when they settled throughout the Northern Hemisphere in the eighth to 11th centuries. And it was back in Germany in the 1960s that the modern trend for green roofs really started to gain momentum.
At around that time, scientists started experimenting with PCMs (phase-change materials) which absorb heat when temperatures are high, typically during the day, and release it when temperatures drop, typically at night.
This technology is now incorporated in a plethora of building products, ranging from wall boards through plaster and screed to ventilation systems. But 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 20°C at night.
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| Simulating thermal mass using phase-change materials incorporated into the ceiling has the capability of maintaining lower temperatures by day and avoiding temperature fluctuations compared with a standard ceiling. |
These passive units store and give up latent heat and can be exchanged for standard ceiling tiles with minimum disruption. They delay the requirement of air conditioning by up to eight hours by reducing high-usage peaks and help reduce energy consumption by 50 to 70%.
These are just two of the results of a 2-year research-and-development programme during which more than 50 different PCM materials, configurations and conditions were tested. Fire resistance and acoustic performance were tested alongside the thermal element.
A trial of PCM at a mid-rise masonry office building in central London saw a number of PCM metal cassettes replace 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 600 mm-square 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 phase-change temperature of 23ºC, providing a total heat storage capacity of 136 Wh/m².
They covered 60% of the 47.5 m2 ceiling in the basement room that had an air circulation rate of 13 l/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 at night, the room used 20 to 70% less energy compared to a similar untreated room.
In addition, three individual tests (conducted as part of BSRIA job number 55749 and reported in BSRIA report 55749/1) by BSRIA Ltd (Building Services Research & Information Association) using three types of air-conditioning system (displacement ventilation, overhead air and overhead air ducted return) in a 16 m² insulated thermal test chamber were carried out at the organisation’s independent Laboratories in Bracknell, with the following results.
1 PCM tiles provided useful energy storage with 30 to 50% ceiling coverage, more coverage allowing for greater thermal storage.
In this first test, 0%, 30% and 57% PCM coverage gave 0, 662 and 1260 Wh of thermal storage, respectively. Adding 662 Wh of thermal storage delayed the operation of the air conditioner by 1 h 20 min with a 30 W/m² load in the displacement ventilation configuration while 1260 Wh delayed this by 4 h 27 min. The PCM tiles typically reduced temperature variations on the ceiling to ±1 K.
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| Armstrong Ceilings’ Coolzone ceiling panels incorporating a phase-change material are used in this building in Madrid to delay the need for air conditioning during the day. |
2 It was possible to discharge the accumulated heat using airflow rates typical in HVAC systems.
This test, with 0% and 30% coverage, showed PCM can be successfully purged with airflow rates typically used for overhead air and unducted return system. Purge times are affected by the amount of energy stored in the ceiling (662 Wh), purge-air temperature and purge-air velocity. The key result was that lowering the purge temperature from 18 to 14ºC decreased the purge time by 3 h 7 min.
3 Selection and design of HVAC played a key role in ensuring optimal efficiency of PCM ceilings.
Again with 0% and 30% coverage, the test showed thermal mass is used most effectively in displacement ventilation types of systems which typically feature low airflow rates and greater amounts of stratification. Ceiling-based thermal mass also worked well in ‘well mixed’ overhead air systems, although the storage does not last as long as in displacement-ventilation systems.
Tests are now also being carried out on two naturally-ventilated classrooms at schools in Bath and Nottingham where PCM coverage is 47% for a 62 m² room and 53% (52 m²), respectively.
With increasing energy costs and greater awareness of carbon emissions, so the old lessons of thermal mass, natural ventilation and low-energy cooling are becoming part of modern energy strategies.
And with most of the software packages available for the energy modelling required during a building’s design stage now including the possibility of adding PCM to the analysis, the development of more effective cooling and ventilation strategies is available from the word go.
Jeremy Sumeray is with Armstrong Ceilings. He is senior segment manager for sustainability for Europe, Africa and the Middle East.
