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Improving thermal-regulating properties for membrane structures

March 1st, 2008 / By: / Feature

New tensile fabrics with thermal-regulating properties: just add phase change material.

Membrane materials used for roof structures of buildings provide a relatively low thermal insulation capacity compared to the classic building materials of wood, steel, fiber mats, and tarpaper. Thus, a large amount of heat penetrates daily through such roof structures, especially during the summer months, leading to an overheating of the building’s interior. On the other hand, the nightly heat loss through these membrane constructions, specifically during the winter months, is significantly high. The low thermal insulation capacity of conventional architectural membranes can be greatly improved by using a membrane material with thermo-regulating properties. The thermo-regulation properties of the membrane material are provided by the application of phase change material (PCM) — a highly productive thermal storage medium.

PCM possesses the ability to change its physical state within a certain temperature range. When the melting temperature is reached in a heating process, the phase change from a solid to a liquid state occurs. During this melting process, the PCM absorbs and stores a large amount of latent heat. The temperature of the PCM and its surroundings remains nearly constant throughout the entire process. In the cooling process of the PCM, the stored latent heat is released into the environment in a certain temperature range, and a reverse phase change from the liquid to the solid state takes place. During this crystallization process, the temperature of the PCM and its surroundings remains nearly constant. The fact that a large amount of latent heat can be absorbed or released without any temperature change makes PCM highly desirable as a means of heat storage.

In order to compare the amount of latent heat absorbed by a PCM during the actual phase change with the amount of sensible heat absorbed in an ordinary heating process, let’s compare it with the familiar ice–water phase change process. When ice melts, it absorbs an amount of latent heat of about 335J/g. When the water is further heated, it absorbs a sensible heat of only 4J/g while its temperature rises by 1°C. Thus, water needs to be heated from about 1°C up to about 84°C in order to absorb the same amount of heat that is absorbed during the melting process of ice.

In addition to ice/water, more than 500 natural and synthetic PCMs — such as paraffi ns or salt hydrates — are known. These materials differ from one another in their phase change temperature ranges and their latent heat storage capacities.

The thermo-regulating effect

In a roof application, the PCM starts to absorb the heat provided by solar radiation during the day in the form of latent heat as soon as the membrane material’s temperature exceeds a given value. During the latent heat absorption by the PCM, its temperature and the temperature of the surrounding membrane material remain nearly constant. Therefore, the heat absorption by the PCM limits the heat flux into a building during the day. On hot summer days especially, the thermal comfort inside the building will be enhanced significantly as a result of the PCM’s latent heat absorption feature. The PCM releases the stored latent heat overnight in a reverse cooling process, which also limits the heat flux out of the building and, therefore, results in a significant reduction of the nightly heat loss through the membrane roof.

A PCM is not meant to absorb all of the heat provided by solar radiation during the day, since it would penetrate through the membrane roof into the building. Overheating of the interior is avoided by the latent heat absorption of the PCM starting at a given trigger temperature. And the interior temperature should be kept on a comfortable level without the use of additional air-conditioning capacity. Therefore, high peak demand energy requirements are prevented in hot climates.

Membrane material design

In an architectural membrane application, the PCM needs to be properly contained in order to prevent dissolution while in its liquid state. Although PCMs are often difficult to contain, I have found silicone rubber to be an appropriate carrier system. In order to fulfill the requirements in various geographical areas and in different applications, I have selected several PCMs for use in membrane fabrics. The melting points of the selected PCMs range from 30°C to 60°C. All of the chosen PCMs are non-combustible salt hydrates. They possess high latent heat storage capacities of up to 340J/g. Based on a PCM content of 40% in a 1.0mm-thick silicone rubber layer, latent heat storage capacities of up to 150 kJ/m2 are obtained. This is a substantial increase in the heat storage capabilities of architectural membrane structures. In order for an ordinary membrane material made of PVC-coated polyester with a similar weight to absorb the same amount of heat, its temperature would need to be raised by about 100°C.

The newly developed membrane materials with PCM treatment possess similar weights compared to common membranes made of PVC-coated polyester fabrics, PTFE-coated fiberglass fabrics, and silicone-coated fiberglass fabrics. However, the thickness of the newly developed membranes is slightly higher than the thickness of the membrane material used for comparison (Table 1).

Thermal performance

A PCM used in architectural membranes provides a substantial improvement of thermal performance by thermally controlling the heat flux through the materials. This thermo-regulating feature has a significant influence on the thermal management of the entire building. As a result, the thermal comfort of the enclosure will be enhanced, the overall heating and airconditioning demands of the facility will decrease, and the construction becomes more energy efficient.

In order to quantify the improvement of thermal comfort due to PCM use, studies were conducted of the temperature inside membrane structures with and without PCMs. For instance, a comparison test was done using two model buildings, one equipped with a roof structure made of the PVC-coated polyester fabric described in Table 1, and one equipped with a fiberglass fabric with a silicone rubber coating with PCM (Table 1). In both test configurations, only a single-layer membrane construction was used. Th e two membrane materials used in this test possess similar weights and show only a slight difference in their thickness. Temperature vmeasurements were carried out at the same distance underneath the two membrane structures. The temperature developments obtained for the two model buildings on the same day are shown in Fig. 1.

The test results indicate that there is a substantial delay in the temperature increase during the day due to the latent heat absorption by the PCM. The latent heat absorption by the PCM leads to temperature diff erences of up to 9°C between the two buildings. Furthermore, there is also a delay in the temperature decrease overnight due to the latent heat release of the PCM. The overall daily temperature fluctuations measured under the specific climatic conditions were reduced by about 10°C due to the thermo-regulating feature (latent heat absorption and latent heat release) of the PCM.

Energy savings

In order to quantify energy savings, a computer modeling procedure was used. For these calculations, a spherical membrane structure with a floor space of about 115m2 was used as a model building. The fabric structure consisted of approximately 300m2 of the fiberglass fabric, with a silicone rubber coating with PCM covering a volume of about 660m2. The latent heat storage capacity of the PCM applied to the roof totaled 45,000 kJ. This latent heat storage capacity leads to a significant reduction in air-conditioning demand on hot summer days, resulting in energy savings of up to 35%.

Material aging

The decrease of the daily temperature fluctuations leads to another benefit of the PCM application in membrane structures. High material temperatures and significant temperature fluctuations usually accelerate the material aging. Reducing the temperature increase in the afternoon and minimizing the daily temperature fluctuations, therefore, will substantially enhance the service lifetime of a membrane structure that is equipped with PCM.

Light transmission

The newly developed membrane material shows an interesting feature regarding light transmission. The translucency of the membrane material with PCM significantly exceeds the translucency of the common membrane materials summarized in Table 1. The test results are shown in Fig. 2.

Furthermore, the translucency of the membrane material equipped with PCM changes in the course of the day. The silicone rubber layer with the PCM becomes transparent as soon as the PCM is completely melted. On the other hand, when the PCM crystallizes, the silicone rubber layer with the PCM becomes opaque. The diff erence in the light transmission between the two states of the PCM incorporated into the silicone rubber that is coated onto the fiberglass fabric totals 15%.

Additional features

The newly developed membrane material can be used under ambient temperature between –50°C and 200°C. Tested in accordance with the standard method DIN 4102, it meets the requirements of the fire-protection classifi – cation B1. The membrane material possesses excellent mechanical properties, a high dimensional stability and an exceptional resistance to UV radiation and humidity.

In contrast to common silicone-coated fiberglass, the dirt repellency of the surface of the newly developed membrane material is very satisfactory. In a recent lab trial, dirt particles on the surface were easily removed with a cloth. In a building application, the dirt particles will be washed away by rain due to their low adhesion to the membrane’s surface.

Applications

The new membrane material is most suitable for applications in architectural roofing structures. However, the membrane material can be thermally beneficial when integrated in façade systems. Further possible applications include sunshades, blinds, and greenhouse coverings.

Summary

The newly developed membrane material with PCM treatment off ers a unique set of improved thermal performance capabilities previously unattainable in an architectural membrane fabric. These capabilities will allow for a substantial improvement of the thermal management of buildings with membrane enclosures. The enhanced thermal management reduces a building’s air-conditioning and heating demands, and thus makes the building more energy efficient. The reduced temperature fluctuations the membrane is supposed to buffer during the day may influence the material’s aging behavior in a positive manner that will lead to a longer service life. A unique feature of the new membrane material is the change in its light transmission as a result of temperature changes, which might be especially interesting for architectural applications. The thermal eff ects provided by the PCM application in membrane structures are durable. The thermal solution is maintenance free, cost effective, and does not require any external energy supply.

Barbara Pause, PhD, is president of Textile Testing & Innovation LLC, in Longmont, Colorado. Questions can be directed to her at: pause@textile-testing.com; Web site: www.textile-testing.com.

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