Energy efficiency in buildings - thermal insulation as the key

Author: Dr. Hans-Peter Ebert, Division Manager Energy Efficiency, ZAE Bayern (As of April 2015) Heat energy has its price and it evaporates  all too easily in the form of heat losses. That's why thermal insulation is used in many energy-efficient systems where heat is needed, whether buildings or industrial equipment, machinery or transportation vehicles. Examples are to show in the following the importance of thermal insulation measures in different areas.

Nearly 40% of the final energy demand in Germany is accounted for by the building sector. This energy use is associated with one third of all CO2 emissions. An important starting point for increasing energy efficiency in this sector is the thermal insulation of the building envelope. This plays a key role in determining a building's energy requirements. Innovative material approaches are being used to create increasingly efficient thermal insulation systems in order to reduce the remaining energy required for heating and cooling buildings. These systems are said to be efficient in several respects: they have an outstandingly low thermal conductivity, and at the same time fewer valuable raw materials are needed to produce insulation systems with sufficient insulating properties. Examples are today's highly developed vacuum insulation panels (see Fig. 1),  or nano-structured materials such as aerogels, so-called nanofoams. One advantage of these high-performance insulation systems is that 'very low insulation thicknesses are needed to achieve the required thermal insulation. This makes these systems particularly interesting whenever building land is expensive, there is actually no space for subsequent thermal insulation, or an architecture is to be realized that is not impaired by thick layers of thermal insulation.

Another way to support energy-efficient air conditioning of rooms is to use so-called low-emitting surfaces, or low-e surfaces for short. During heating periods, walls or textiles covered with such low-e layers would reflect heat from internal heat sources. For the occupant, this results in a higher sensible wall temperature and thus significantly improved thermal comfort. At the same time, such surfaces also radiate significantly less heat than conventional building materials. Such low-e surfaces can be realized by using metallic layers. However, reflective metal surfaces generally meet with little acceptance for aesthetic reasons - especially in private homes. ZAE Bayern has therefore developed low-e coatings that can be adjusted to the desired color impression in the optical spectral range, but reflect most of the long-wave thermal radiation like a metal coating. This achieves a separation between function and optical appearance and thus higher user acceptance.

Figure 2 shows the realization and mode of action of a low-e textile, which here was stretched under a wooden ceiling structure of an ice rink. While the construction is inconspicuous in its visual appearance, the thermal image shows that the textile surface acts like a mirror and the cold ice surface as well as the warm lamps are "reflected" in it. Thus, the heat that collects underneath the ceiling structure as a result of the rising warm air is transferred to the cold ice surface  only to a small extent. In terms of building physics, the advantage is that the problem of condensation on the wooden structure is alleviated and, at the same time, less energy is required to prepare the ice surface.

Technical insulation, as thermal insulation is known in industry, also plays a prominent role in making industrial processes energy- and thus cost-efficient. One thinks here only of the chemical industry or the metal production and processing. Almost 80% of the process heat used in Germany is required at temperatures above 100°C, with 70% of this above 250°C alone. Almost 40% of the industrial energy demand is required for the operation of industrial furnaces. These must be insulated with corresponding efficiency.

Current research and development work at ZAE Bayern is aimed at using nanomaterials to provide even higher-performance insulating materials for various fields of application. Carbon aerogels are particularly promising in this respect. These materials consist of a three-dimensional network of interlinked carbon particles (see Fig. 3).

The pore dimensions, as well as the particle sizes, can be tailored to size dimensions ranging from nano- to micrometers; this is done by selecting suitable synthesis parameters during the production of these materials in a wet-chemical sol-gel process. In terms of thermal insulation properties at high temperatures, carbon aerogels are unbeatable (see Fig. 4).  Heat is effectively retarded  trans-ported in the highly porous material by conduction processes via many detours and narrow points in the finely branched carbon network - the proportion of the framework thermal conductivity to the total thermal conductivity of these materials is correspondingly low. The thermal conductivity of the gas within the pore structure is also severely limited, since at high temperatures the gas molecules are already severely restricted in their mobility at pore sizes below 1 µm. Finally, carbon is an efficient absorber of thermal radiation. Thus, the carbon matrix is almost impenetrable to thermal radiation and the radiation contribution to the total thermal conductivity is correspondingly very low. 

Unfortunately, we have not yet reached the age in which energy from renewable energy sources is available in abundance. Until then, innovative thermal insulation and surface coatings can reduce heat loss and help lower the energy demand required by our society, as well as the associated CO2 emissions.