FEATURE
SECTION
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)
and air conditioning in summer and provide
many times more than its total daily energy
in timber homes and offices to increase
an effective way of shifting electricity peak
requirement. However, for most modern
the comfort of these lightweight construc-
load and lowering electricity costs.
buildings, this energy is largely wasted due
tions is being examined at the University
of Auckland.
to the lack of heat storage capability.
Better thermal comfort
When solar energy is stored as latent heat
With walls and ceilings that contain PCMs,
using PCM during the daytime in winter, it
Increases thermal mass
the temperature fluctuation inside the
can be released at night-time upon solidifica-
PCMs are materials with a high heat of
building can be significantly damped.
tion, providing passive heating.
fusion that melt and solidify within a narrow
This increases the thermal comfort of the
In summer, the PCM can absorb heat
temperature range releasing and storing a
building while reducing the amount of
when it melts during the day, providing free
large amount of heat while doing so. Paraffin,
energy consumed for heating and cooling.
cooling in the process.
inorganic compounds, fatty acids and fatty
The amount of solar energy received by a
Figure 1 shows the increased thermal mass
acid esters are common PCM that are already
typical residential building on a daily basis is
of a thin wood structure when impregnated
commercially available.
PCMs are particularly useful for latent
heat storage. The latent heat of melting
they should be safe and inexpensive. For use
in buildings, they should melt and solidify
between 18–25°C.
Heat stored
PCMs should be higher than 150 kJ/kg, and
PCM
MELTING
POINT
PCM in wood
Wood
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
Temperature (°C)
Figure 1: Heat stored in thin wood structure impregnated with phase change material.
72 — February/March 2016 — Build 152
Technology
FEATURE
SECTION
Testing facilities at the university
Office-size construction facilities at the
University of Auckland were constructed
to test the potential benefit of using PCM in
buildings (see Figure 2). Two of the offices
were identical, except the interior gypsum
board in one was impregnated with PCM
that had a melting point of 18–22°C.
Solar radiation, humidity and ambient
indoor and wall temperatures have been
continuously monitored for the two offices
during the last 7 years. Each office also has a
heat pump for heating and cooling, providing
information on annual energy savings with
Figure 2: Research facilities at Tamaki Campus, University of Auckland.
the use of PCM.
Tests show temperature damping
with PCM. The shaded area is the melting
Microcapsules have two parts – a core mate-
Typical measurements generated from the
range of the PCM in which energy is stored
rial and a shell material. The core material
test facilities show significant damping
as a latent heat of melting.
consists of a PCM, and the shell material
effects to the interior temperature in the
is usually a cross-linked polymer such as
office with PCM. This utilises the free
polymethyl methacrylate.
cooling available at night, which is used to
Improving by encapsulating
Microencapsulation of PCM is a technique
A common PCM that is encapsulated
that allows PCMs to be durable. Because of
is a paraffin wax, with a suitable melting
The daytime temperature of the PCM office
the nature of PCMs, bulk usage of PCMs in
point. The two major steps in microcapsule
is about 5°C cooler than the other office,
buildings can be problematic. The advan-
formation are emulsification and the poly-
except when the ambient temperature
tages of microencapsulation are:
merisation process. Both have an effect on
during the night before was not low enough
the microcapsules produced.
to solidify the PCM.
●●
increasing surface area, which gives better
It is desirable to produce spherical micro-
heat exchange properties
●●
●●
solidify the PCM (see Figure 3).
protecting the PCM from leaking to the
capsules containing a high proportion of PCM
Peak load shifting
outside environment and vice versa
and low polymer concentrations to enable
Applying PCM in buildings can also create
allowing the core material to undergo
the product to retain most of its thermal
electricity peak load shifting.
phase change and volume change without
characteristics. Details of the encapsulation
When PCM is used in a home’s walls and
affecting the bulk structure or integrity of
technology developed at the University of
ceilings, the heating or air conditioning
the building.
Auckland are available in published reports.
can be switched off for an extended period
without changing the indoor temperature
significantly. This means electricity use could
35
be limited to low-peak periods, helping to
Temperature (°C)
30
level the electricity peak load. If variable
price-based electricity is provided, this will
25
also translate to electricity cost saving.
20
15
Savings on cooling
In another study, cooling by ventilating at
10
night combined with PCM-impreg­­nated
Hut 1 inside temperature
Hut 2 (with PCM) inside temperature
5
gypsum boards was experimentally investigated using the same facilities.
0
0
24
48
72
96
120
144
168
Time (hr)
192
216
240
264
288
312
The two test offices equipped with smart
control systems were used to test the
Figure 3: Measured indoor room temperatures during summer.
Build 152 — February/March 2016 — 73
FEATURE
SECTION
Technology
concept. Initially an air conditioning (AC)
discomfort by up to 6% and the energy
with a phase change temperature range of
unit, without night ventilation, was used in
requirements of up to 25%. On the other
21–26°C shows the best results.
both huts to charge the PCM during low peak
hand, appropriate scenarios bring significant
Our analysis also showed that the pay-off
periods, showing very little savings in
energy savings of up to 33% and comfort
period could be as long as 15 years if PCM is
electricity.
enhancement of up to 31%.
not used wisely. However, this could be
However, when night ventilation was
reduced to less than 5 years if the PCM
used to charge the PCM instead, weekly
Good design is necessary
quantity and the location where it is used
electricity savings of 73% were achieved
This highlights the need for clever design
are optimised.
(see Table 1).
when integrating PCM into buildings. The
Need to use the right scenario
For more
Visit www.engineering.auckland.ac.
goal is to find a trade-off between energy
nz/en/about/our-research/energy-research/
savings and comfort enhancement. The PCM
phase-change.html.
Computer-based simulations of a typical
New Zealand home construction were
performed using the simulation software
Design Builder and Energy Plus to show the
benefits of using PCM in real homes. Both
Table 1
Energy and electricity cost saving achieved
using night ventilation system
energy savings and comfort were assessed.
By varying the heating set point and the
DAY 2
DAY 3
DAY 4
DAY 5
DAY 6
DAY 7
DAY 8
TOTAL
Energy saving
57%
64%
55%
48%
83%
92%
76%
73%
Electricity cost saving
67%
34%
58%
52%
85%
93%
73%
67%
phase change temperature of the PCM,
significant benefit could be observed.
Some poor scenarios show that the
integration of PCM can increase both the
74 — February/March 2016 — Build 152