Assessment of novel applications for nano-porous thermal insulation in district heating pipes and building walls

Doctoral thesis, 2016

In nano-porous thermal insulation there is a strong relation between the pressure and the thermal conductivity, even at pressures close to atmospheric pressure. This thesis presents research on applications of nano-porous insulation in hybrid insulation district heating pipes and in building walls. A concept of hybrid insulation district heating pipes has been investigated where the innermost layers of insulation consists of nano-porous insulation and the outer layers consists of polyurethane foam insulation. The concept has been investigated through a mix of laboratory measurements, field measurements and simulations. The presented research indicates that vacuum insulation panels (VIPs) can be used in district heating pipes to reduce the heat losses. For the evaluated configurations the heat losses were reduced by a magnitude of 30%. The heat losses from the supply pipe in a twin pipe were reduced by 50%. The two main considerations with using vacuum panels in district heating pipes are the thermal bridges and the long term performance. It is shown that the position of the thermal bridges in the panels has a large effect on the thermal performance of the twin pipes and the results indicate a preferred configuration to minimize heat loss. The thesis presents a model to evaluate the long term performance of the VIPs through temperature measurements. After three years of field measurements on pipes connected to a district heating network with temperatures up to 90C, there is no sign of any uncontrolled deterioration of the VIPs. Results from the investigation show that the use of aerogel can reduce the thickness of a load bearing stud wall with 40% compared to the use of conventional insulation. If done wrong, this can lead to some new consideration regarding mold growth risk. The fact that the thermal conductivity of nano-porous insulation is strongly influenced by the pressure in the pore gas can be used to create variable insulation, where the thermal properties of the insulation is changed to match the current circumstances. By putting the insulation in a diffusion tight bag, connected to a vacuum pump, the pressure in the material can be changed and thereby the thermal conductivity. The pressure was varied in a fumed silica and an aerogel blanket sample between 1 kPa and atmospheric pressure, which gave a variation in the thermal conductivity of 1.5 for the aerogel blanket and 3 for the fumed silica. Transient measurements during evacuation and refilling show that the thermal performance will be influenced by some transient effects, such as influence from the temperature of the inserted air, but the time scale is too small to have any large effect on the energy performance. When the variation is used in simulations of the energy use for an office building, an interesting result is that a variable construction gave a higher optimum U-value, corresponding to thinner walls.