CPD 1 2013: Phase-change materials

A phase-change material is a substance that requires a relatively large amount of heat to change its state from a solid to a liquid, and is therefore capable of storing and releasing large amounts of energy. Phase-change materials have been used for a range of applications over recent decades, but only relatively recently, with advances in their durability, in construction.
For use in buildings, they are typically contained in a cassette that is embedded in a ceiling tile or wall, and play a similar role to that of the thermal mass in stone or concrete. As temperatures rise during the day, heat is absorbed into the material, helping to maintain a comfortable temperature within the space. When this heat is purged, either through night-time cooling or mechanical ventilation, the PCM is “reset” for the following day so that it can again begin to absorb heat as the room warms.

The benefit of PCMs over thermally massive materials is that they are far lighter, storing relatively large amounts of heat per unit of volume, and they can be relatively easily installed as part of a refurbishment programme to increase the thermal mass of lightweight structures and therefore improve their thermal performance and the efficiency of ventilation systems, and lower cooling or heating energy consumption.

How do they work?

When heat is applied to a substance, the energy transfers in one of two ways. The first is that the substance gains heat. For example, if heat is applied to water, it will rise in temperature to a maximum of 100°C — its boiling point. Likewise, if heat is removed, the temperature of the water will fall, to a minimum of 0°C, or its freezing point. This type of heat transfer, or storage, is called sensible heat.

However, adding heat does not always cause a substance’s temperature to rise. If heat is added to water that is already boiling, it remains at 100°C, and the absorbed heat instead causes the water to turn from a liquid into a vapour.

This is a phenomenon common to all pure substances. As they absorb heat, they eventually reach a melting point (in solid form) or evaporation point (in liquid form), at which point they change state — from solid to liquid, or from liquid to gas. During this process, they absorb heat but do not get hotter. This type of heat storage is known as latent heat.

It is this latent heat that enables PCMs to control room temperature. The PCMs used in construction typically change from solid to liquid at 23-26°C. (Computer simulations show that 26°C is the optimal phase-change temperature for passive summer heat reduction in buildings, while 23°C is needed for situations where PCMs are part of a mechanical air-conditioning system.) As they melt, they begin to absorb heat from the room, rather than simply gaining heat themselves. In this way, the room temperature can be kept constant until the change of state — or phase change — is complete. The PCM can be returned to its solid state by night-time ventilation (as long as the night air is cooler than the phase-change temperature), or by mechanical means in hotter climates. The phase-change cycle is then ready to begin again the next day.

Types of PCM

There are many types of PCM but not all are suitable for use in buildings. Water, for example, has transition temperatures of 0°C and 100°C, neither of which are conducive to a comfortable living or working environment. The selection criteria when choosing a PCM include:

• A melting temperature in the desired operating range — in construction this would be 23°C or 26°C.
• A high latent heat of fusion per unit volume — in other words, they can store a large amount of heat per unit of volume, minimising the area of PCM tiles that are needed.
• High thermal conductivity. The quicker the PCM reacts to changes in temperature, the more effective the phase changes will be.
• Minimal changes in volume — substances expand or contract when they change state. Because PCMs in construction need to be contained within a cassette, large changes in volume could create problems.
• Congruent melting. This means that the composition of the liquid is the same as that of the solid, which is important to prevent separation and supercooling.
• A completely reversible freezing/melting cycle.
• Durability over a large number of cycles.
• Non-corrosiveness to construction materials.
• Non-flammability.

The two main types of PCM used in construction are inorganic salt hydrates and organic paraffin or fatty acids, and both materials have a set of advantages and disadvantages that must be taken into consideration.

Inorganics: salt hydrates

Advantages: Salt hydrates are a low-cost, readily available PCM. They have a high latent heat storage capacity and high thermal conductivity. They are also non-flammable.

Disadvantages: The volume change between the solid and liquid states is very high. Another problem with the solid-liquid transition is the danger of supercooling. This is when the temperature of a liquid is reduced to below its freezing point without it becoming a solid.

Additives called “nucleating agents” can help with this process, but they become less effective over time. Salt hydrates are also very hygroscopic, which means they trap humidity. By doing this, the water content varies and the melting point varies as well. This is a danger for long-term stability.

Organics: paraffins and fatty acids

Advantages: Paraffins and fatty acids do not expand as they melt, and freeze without much supercooling, so they do not need nucleating agents. They are chemically stable, compatible with conventional construction materials and recyclable. Paraffins are hydrophobic, which means they are water-repellant. As a result, their phase-change points are reliable. Pure paraffins are also highly durable, and do not degrade in contact with oxygen. Nor can pure materials, consisting of a single substance, separate from themselves — unlike salt hydrates, which could break away from their water content when cycled frequently.

Disadvantages: Organic PCMs are flammable and have low thermal conductivity and low latent heat storage capacity. Impurities reduce heat capacity further, so it is very important that the paraffins used are in a pure state. This, however, raises the cost, as they have to be completely refined of oil.

Construction materials

Microencapsulation
Construction applications use phase-change materials as they change between their solid and liquid states, rather than between a liquid and a gas state, as the volume change is far less. This does present the practical problem of containing the material in its liquid state. An effective solution here is microencapsulation.

The idea is that the PCM, in the form of a wax, is contained in an extremely hard plastic shell. Each capsule is tiny — for example, the BASF Micronal DS 5000 X microcapsules used in Armstrong’s CoolZone products have a diameter of about 2-20 microns — or 0.002-0.02mm. Because the capsules have a very large surface-volume ratio, they allow a high level of heat transfer, while also protecting the paraffin to keep it in its pure form.

Pure paraffin is a suitable material for the wax because it undergoes less expansion than other PCMs, maintains its form in a liquid state and is highly durable — after 10,000 test cycles of the BASF Micronal DS 5000 X microcapsules (which use pure paraffin) there were no damaged capsules. The formulation of the paraffin wax can be adjusted to give a melting point of either 23°C or 26°C.

PCMs in ceiling tiles

Because heat rises, an effective use of PCM microcapsules is to place them in a cassette and add them to a suspended ceiling tile. As paraffin is flammable, the PCM insert must be sandwiched between tiles in a material with a good fire reaction performance, such as metal. A metal tile also offers good thermal conductivity, pulling the heat through into the PCM. A typical loading of 50% of the ceiling in PCM tiles will maintain the temperature in an typical mechanically ventilated office at 24°C for up to four to five hours. After that, the room will continue to heat up as before, until the heating load reduces. The other 50% of tiles can be service tiles or standard acoustic ceiling tiles. PCM tiles should not be cut and so are not suitable for perimeter cuts or service penetrations.

With cooler night-time temperatures, the PCM will return to solid form, transferring the heat energy back into the room. This means that the room is not too cool first thing in the morning but at a comfortable working temperature, and the PCM tiles are reset for another working day.

Using metal PCM ceiling tiles in this way can lead to significant reductions in energy use. For example, 10sq m of Armstrong’s CoolZone tile can store up to 2kWh of energy. Over a 30-year lifecycle, this saves 6MWh of thermal energy, which would create approximately 1,140kg of CO2, if supplied by mechanical cooling.

A metal PCM ceiling tile such as Armstrong CoolZone can be dropped into a standard suspended ceiling grid system, making installation simple. Each PCM cassette weighs approximately 9kg, so grid strengthening may be required.