“Phase change materials” integrated in vertically perforated clay units  – innovative passive air-conditioning systems for energy efficient buildings – Abstract

1 Introduction and objectives

The fundamental objective of this research project was to ­experimentally determine the temperature-dependent, specific heat capacity of three designated phase change materials (PCMs) in order to subsequently investigate the effects of their thermal properties on building components.

Various experiments were conducted for the purpose of appraising the constructional-physicostructural benefits of PCM incorporation for purposes of temperature stabilization and for reducing the heating and cooling requirements of buildings. Another aim of this research project was to clarify the extent to which, if at all, increasing the heat capacity of heavy clay materials would affect the energetic performance of buildings exposed to different sets of outdoor-climatic conditions.

With the aid of the “buildopt“ simulation program, an eight-zone model of a single-family home was studied for different design variants of vertically perforated clay units in various attitudes, thicknesses and PCM fillings with different phase change temperatures. Then, the effects of those ­parameters on the building‘s heating and cooling requirements and on the resultant operative room temperature were investigated.

In addition, it was demonstrated for an existing single-family home with a partial basement (approx. 130 m²) which rates of energy consumption can be achieved under different sets of climatic conditions for different modes of construction and, accordingly, different heat storage capacities, and which optimization options are available.

This study provides a foundation for the development of a new, PCM-filled clay unit for exterior and interior walls. With the help of PCMs, new-generation bricks will be designed to give rooms high heat storage capacity, hence improving hot-weather thermal insulation while minimizing or even obviating the consumption of energy for air-conditioning purposes. A minor reduction in heating demand, depending on the share of glazing, appears as a positive side effect.

2 Findings and conclusions

Phase change materials, also referred to as latent heat stores, are innovative products with a capacity to considerably reduce CO₂ emissions in the construction sector by increasing the energy efficiency of buildings. The use of PCMs in building construction therefore appears to offer outstanding opportunities for achieving long-term passive hot-weather thermal insulation.

Phase change materials actually can increase the heat capacity of vertically perforated clay unit masonry by a factor of 10. Due to the temperature dependence of the phase change, the material has to be chosen to harmonize with the intended application and location. Latent heat-storing materials significantly augment the heat storage capacity of buildings, hence reducing the occurrence of temperature spikes and making the room climate accordingly more comfortable. Lower summer-month room temperatures can be considered one of the main advantages of PCM integration.

As documented by the preliminary simulations, this also has the effect of reducing the annual overall ultimate energy demand.

For central European climatic conditions, cooling can be dispensed with almost entirely for all three window options, because temperatures of 26 °C and higher are only registered on approx. 13 days a year for structures with more than 50% glazed area in the exterior walls.

However, the appropriate phase change temperature and required quantity of PCM should be precisely determined in advance for each object in order to ensure that the overall employed volume of PCM will suffice to optimize the achievable decrease in summertime operative room temperatures.

Subsequent investigations should address the application of PCM layers in vertically perforated clay units, i.e., the configuration and optimization of the brick-cavity structure and how best to fill it. Naturally, before a mass-producible product can be brought to market, various other aspects will also have to be clarified, e.g., its mechanical and thermal stability and hygrothermal and acoustic properties.

Further-ranging investigations should deal with latent-heat-accumulating salt hydrates. In comparison with paraffins, as the sole type of PCM considered in the present project, salt hydrates have higher heat storage capacities and cost less. They are, however, very aggressive.

The relevant investigations and findings will be presented in detail in Zi Annual 2010.