Tomáš Dvorák

Energy Demand Reduction in Nearly Zero-Energy Buildings by Highly Efficient Aluminium Foam Heat Exchangers

The capability periodically to store and release the latent heat of phase transition during melting and solidification of Phase Change Materials (PCMs) has been currently the main subject of interest with regard to cost reduction efforts for cooling, heating of interiors and Domestic Hot Water (DHW) necessary for the operation and maintenance of adequate thermal comfort in new modern as well as old renovated residential buildings. The main principle of PCMs facilities to reduce significantly the energy consumption in the building industry of the future is based on the ability of thermo-active heat exchangers to absorb and later to dissipate into the surroundings excessive heat which can be easily obtained from renewable sources (e.g. solar energy, geothermal heat, etc.) directly in a building or in its immediate vicinity. Smart interior tiling and furnishing systems can provide high energy efficiency by stabilizing the room temperature at a level ensuring sufficient thermal comfort basically governed by the thermal conductivity and heat exchange area between ceiling (respectively also wall and floor if necessary) heat exchangers (radiators) and the heat storage medium in the form of PCMs. Unfortunately, most conventional building materials, e.g. aerated concrete, bricks, gypsum, ceramic tiles, etc. are particularly characterized by very low thermal conductivity, which disadvantages them to be used for these purposes. However, highly porous metallic material such as aluminium foam prepared by powder metallurgy [10, 11] is on the contrary excellently heat conductive, which predisposes it to be used for light-weight design of supporting structure of very energy efficient indoor as well as outdoor thermo-active heat exchangers for building industry of the future. This contribution points to the possibility to apply aluminium foam for both the novel innovative roofing system to cover pitched roofs and the interior ceiling panels, with the minimum energy demands for maintaining the sufficient thermal comfort in future nearly Zero-Energy Buildings (nZEBs).

Applications of Nanocomposite-Enhanced Phase-Change Materials for Heat Storage

The development of efficient materials for heat storage has become recently a popular research topic as amount of energy gained from solar power depends significantly on day and night cycle. Thats why the right choice of material for heat storage directly affects the utilization efficiency of solar thermal energy. Research on heat storage materials nowadays focuses on phase change materials (PCMs) enabling repeatedly utilize the latent heat of the phase transition between the solid and liquid phase. Most currently used PCMs have low thermal conductivity, which prevents them from overcoming problem of rapid load changes in the charging and discharging processes. To overcome this obstacle and to obtain excellent heat storage possibility, various techniques have been proposed for enhancing the thermal conductivity of PCMs, such as adding metallic or nonmetallic particles, in-corporating of porous or expanded materials, fibrous materials, macro-, micro-, or nanocapsules, etc.The authors of this study report particularly the huge potential of oxide nanoadditives, such as titania (TiO2), alumina (Al2O3), silica (SiO2) and zinc oxide (ZnO), that are even in small quantities (up to 3 wt.%) able significantly to enhance the heat storage characteristics of conventional PCMs. Moreover, the microstructure of the granules produced by recycling of aluminum scrap refers to the possibility of their utilizing for the purpose of low cost solutions enabling to increase the thermal conductivity of PCMs. The above mentioned technical solutions are therefore the important keys to successful commercialization of materials for latent heat storage in future building industry.

Microstructure and Thermal Expansion of Hybrid - Copper Alloy Composites Reinforced with both Tungsten and Carbon Fibres

Tungsten as refractory material and high thermal conductive carbon fibres are promising candidates for production of copper matrix composites for high temperature applications. Three types of rod-like samples were prepared by gas pressure infiltration of different carbon/tungsten fibre preforms with copper and/or copper alloy (Cu-0.5Cr) respectively. The fibres are aligned parallel to rod axis and were combined with the tungsten wire cloth. The microstructure of prepared hybrid composites was examined. The samples were thermally cycled 3 times up to 550 °C at a relatively high heating/cooling rate (10 K/min) to touch real condition in applications where high heat is formed during short time. The thermal expansion behaviour in radial direction was also analysed. Results show that a combination of both types of reinforcements in rod-shapes samples insures good protection against composite disintegration during high temperature thermal loading.

Thermal Expansion of Advanced Materials for High Temperature Fusion Application

In this paper, two different technologies for preparation of the composite are presented: vacuum diffusion bonding and gas pressure infiltration. Samples contained different volume fractions of tungsten fibres (10%, 50% and 78%) that were arranged in unidirectional, cross-ply and circular architecture. The coefficient of thermal expansion (CTE) of Cu/W MMC samples was measured in the longitudinal and transverse direction to the fibre orientation and it strongly depended on arrangement of reinforcing phase and direction of measurement. For the unidirectional material the CTE was 4 - 6 ppm.K-1 and 11-12 ppm.K-1 in directions parallel and transverse to the fibre orientation, respectively. Samples in a shape of a ring contained reinforcement that was wounded around the sample centre (circumferential winding) or was cut-out from the Cu/W MMC plate. In the second case, the reinforced fibres had the woven arrangement. The CTE of ring measured in radial direction showed values from 5 ppm.K-1 to 20 ppm.K-1 for composites containing 72 and 10 vol. % of W-fibres, respectively.