Phase change materials

This Issue This is a part of the Technology feature

By - , Build 152

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.

Figure 1: Heat stored in thin wood structure impregnated with phase change material.
Figure 2: Research facilities at Tamaki Campus, University of Auckland.
Figure 3: Measured indoor room temperatures during summer.

THE USE OF phase change materials (PCMs) in timber homes and offices to increase the comfort of these lightweight constructions 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. Paraffin, 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 effective 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 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 solidification, 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 73with PCM. The shaded area is the melting range of the PCM in which energy is stored as a latent heat of melting.

Figure 1: Heat stored in thin wood structure impregnated with phase change material.

Improving by encapsulating

Microencapsulation of PCM is a technique that allows PCMs to be durable. Because of the nature of PCMs, bulk usage of PCMs in buildings can be problematic. The advantages of microencapsulation are:

  • increasing surface area, which gives better heat exchange properties
  • protecting the PCM from leaking to the outside environment and vice versa
  • allowing the core material to undergo phase change and volume change without affecting the bulk structure or integrity of the building.

Microcapsules have two parts – a core material and a shell material. The core material consists of a PCM, and the shell material is usually a cross-linked polymer such as polymethyl methacrylate.

A common PCM that is encapsulated is a paraffin wax, with a suitable melting point. The two major steps in microcapsule formation are emulsification and the polymerisation process. Both have an effect on the microcapsules produced.

It is desirable to produce spherical microcapsules containing a high proportion of PCM and low polymer concentrations to enable the product to retain most of its thermal characteristics. Details of the encapsulation technology developed at the University of Auckland are available in published reports.

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 the use of PCM.

Figure 2: Research facilities at Tamaki Campus, University of Auckland.

Tests show temperature damping

Typical measurements generated from the test facilities show significant damping effects to the interior temperature in the office with PCM. This utilises the free cooling available at night, which is used to solidify the PCM (see Figure 3).

The daytime temperature of the PCM office is about 5°C cooler than the other office, except when the ambient temperature during the night before was not low enough to solidify the PCM.

Figure 3: Measured indoor room temperatures during summer.

Peak load shifting

Applying PCM in buildings can also create electricity peak load shifting.

When PCM is used in a home’s walls and ceilings, the heating or air conditioning can be switched off for an extended period without changing the indoor temperature significantly. This means electricity use could be limited to low-peak periods, helping to level the electricity peak load. If variable price-based electricity is provided, this will also translate to electricity cost saving.

Savings on cooling

In another study, cooling by ventilating at night combined with PCM-impregnated gypsum boards was experimentally investigated using the same facilities.

The two test offices equipped with smart control systems were used to test the 74concept. Initially an air conditioning (AC) unit, without night ventilation, was used in both huts to charge the PCM during low peak periods, showing very little savings in electricity.

However, when night ventilation was used to charge the PCM instead, weekly electricity savings of 73% were achieved (see Table 1).

Table 1
Energy and electricity cost saving achieved using night ventilation system

  DAY 2DAY 3DAY 4DAY 5DAY 6DAY 7DAY 8TOTAL
Energy saving 57% 64% 55% 48% 83% 92% 76% 73%
Electricity cost saving 67% 34% 58% 52% 85% 93% 73% 67%

Need to use the right scenario

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 energy savings and comfort were assessed.

By varying the heating set point and the phase change temperature of the PCM, significant benefit could be observed.

Some poor scenarios show that the integration of PCM can increase both the discomfort by up to 6% and the energy requirements of up to 25%. On the other hand, appropriate scenarios bring significant energy savings of up to 33% and comfort enhancement of up to 31%.

Good design is necessary

This highlights the need for clever design when integrating PCM into buildings. The goal is to find a trade-off between energy savings and comfort enhancement. The PCM with a phase change temperature range of 21–26°C shows the best results.

Our analysis also showed that the pay-off period could be as long as 15 years if PCM is not used wisely. However, this could be reduced to less than 5 years if the PCM quantity and the location where it is used are optimised.

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Articles are correct at the time of publication but may have since become outdated.

Figure 1: Heat stored in thin wood structure impregnated with phase change material.
Figure 2: Research facilities at Tamaki Campus, University of Auckland.
Figure 3: Measured indoor room temperatures during summer.

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