Page 74 - Build 152

Technology
Phase change materials
Phase change materials that store then release energy could one day be incorporated into homes, cutting energy costs and reducing system load. This sounds futuristic, but the technology is well advanced.
BY MOHAMMED FARID, DEPARTMENT OF CHEMICAL AND MATERIALS ENGINEERING, THE UNIVERSITY OF AUCKLAND
THE USE OF phase change materials (PCMs) in timber homes and offices to increase the comfort of these lightweight construc- tions is being examined at the University of Auckland.
Increases thermal mass
PCMs are materials with a high heat of fusion that melt and solidify within a narrow temperature range releasing and storing a large amount of heat while doing so. Para n, inorganic compounds, fatty acids and fatty acid esters are common PCM that are already commercially available.
PCMs are particularly useful for latent heat storage. The latent heat of melting PCMs should be higher than 150 kJ/kg, and they should be safe and inexpensive. For use in buildings, they should melt and solidify between 18–25°C.
When incorporated into building materials, PCMs substantially increase the thermal mass of the buildings without increasing their physical mass. They can reduce the current dependence on heating in winter
and air conditioning in summer and provide an e ective way of shifting electricity peak load and lowering electricity costs.
Better thermal comfort
With walls and ceilings that contain PCMs, the temperature fluctuation inside the building can be significantly damped. This increases the thermal comfort of the building while reducing the amount of energy consumed for heating and cooling.
The amount of solar energy received by a typical residential building on a daily basis is
PCM
MELTING
POINT
many times more than its total daily energy requirement. However, for most modern buildings, this energy is largely wasted due to the lack of heat storage capability.
When solar energy is stored as latent heat using PCM during the daytime in winter, it can be released at night-time upon solidi ca- tion, providing passive heating.
In summer, the PCM can absorb heat when it melts during the day, providing free cooling in the process.
Figure 1 shows the increased thermal mass of a thin wood structure when impregnated
PCM in wood Wood
Temperature (°C)
Figure 1: Heat stored in thin wood structure impregnated with phase change material.
72 — February/March 2016 — Build 152
FEATURE SECTION
Heat stored