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Advanced construction layer Phase-change materials

In document A Handbook of Sustainable Building (Page 69-72)

The demand for the efficient use of energy encourages the construction of buildings with very low heat losses that can only be achieved with the selection of high-quality materials.

For this purpose, new materials are being developed in two ways:

1 materials based on vacuum insulation; and 2 materials that accumulate self-latent energy

(phase-change materials) (see www.corporate.basf.com).

Vacuum-insulating materials do not contain any air or gas pores and significantly reduce heat leaking. If, with the use of reflective materials, we also prevent radiation and additional nano-porosity, we could reach almost zero conductivity. Phase-change materials (PCMs) are materials that accumulate energy that is released in the process of changing aggregate states from solid to liquid and vice versa.

Microgranulation materials (or ‘special woof ’) have similar characteristics as ice. Ice changes phase when heated at 0°C and is converted to water. It absorbs a large amount of heat in the process and this results in cooling of the surroundings.

Phase-change materials have, for example, been employed in:

• the conditioning of buildings;

• heat pump systems;

• waste heat recovery;

• thermal energy storage;

• the heat depositor of passive houses;

• the construction elements of the building layer, etc.

In practice, we use PCM materials by building them in the form of micro- or nano-balls into concrete, bricks or plaster; when they change phase, they absorb a substantial amount of heat with a consequent significant rise in temperature.

When ambient temperature around a liquid material falls, the PCM solidifies, releasing its stored latent heat. Within the human comfort range of 20°C

to 30°C, some PCMs are very effective. They store 5 to 14 times more heat per unit volume than conventional storage materials such as water, masonry or rock.

A 15mm thick plaster plate with built-in micro grains of PCM is, for example, equivalent to a 12cm thick brick wall or a 9cm thick concrete wall, and it ensures that heat accumulates during the day, which is then released during the night when the temperature in the room falls (see Figure 2.5). As a result, the consumption of energy needed for heating and ventilation is minimized. During the summer season, temperature stability increases and the temperature inside the building is more pleasant. The temperature delay is much higher, as shown in Figure 2.6. With temperature stability, we mark the ability of construction to maintain stable temperatures in a room independently of outside changes. In order to achieve this, the ability of materials to accept and store heat and to release it when the temperature in the environment drops is very crucial.

Table 2.4 Overall heat transfer coefficients of the building envelope for recommended thickness of glass wool insulation

Building element (construction) Insulation thickness Heat transfer coefficient of (cm) the building envelope

U (W/m2K)

External walls 24–30 0.14–0.12

Ceiling below the non-heated attic 30–40 0.11–0.08

Roof above the heated attic 30–40 0.11–0.08

Floor above the ground 15–20 0.16–0.14

Figure 2.5 A wall with phase-change materials

Low-energy and passive buildings are becoming a reality in simple and massive construction. One of the possibilities for maximizing the heat stability of building construction is definitively through the use of phase-change materials. In the same way, we can also use latent depositors based on PCM in our heating/cooling systems.

Summary

Any building should be designed and constructed so as to increase its energy efficiency. The supply of energy to the building needs to employ innovative solutions that are technically feasible, are justified in terms of costs, are acceptable from environmental and social standpoints, and ensure a conventional level of living standard and comfort.

Engineering approaches for the economic use of energy, such as substantive insulation of buildings and the reduction of thermal bridges, has a limited influence on energy consumption. It is necessary, in addition, to apply certain active elements in housing – the exploitation of solar energy, ambient heat and wasted heat. Designing such systems requires a special approach to the problem; considering conventional energy sources as being economically and ecologically appropriate is no longer justified.

This chapter points out how important the integration of all relevant aspects within energy-saving houses is, with special emphasis on windows, which

contribute to a substantial amount of heat loss and, consequently, to environmental pollution. Therefore, it is necessary to encourage new ways of professional thinking promoted by the political directives of EU legislation, and to directly influence environmental standards, which are very important today. In the process of adopting such directives in EU member states, a crucial area is the promotion of insulated-window installation, with clear thinking about imposing a tax on CO2emissions per tonne or cubic metre of fossil fuel, cancelling purchase tax or value-added tax (VAT) for windows with insulated glass, and offering favourable loans and incentives for such purposes.

References

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Council Directive 93/76/EEC of 13 September 1993 to Limit Carbon Dioxide Emissions by Improving Energy Efficiency (SAVE), Official Journal L237, 222/09/1993

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In document A Handbook of Sustainable Building (Page 69-72)