Energy efficiency of multi-apartment residential houses with individual heat supply

Владимир Новосельцев, Дина Новосельцева, Михаил Шеногин. Энергоэффективность многоквартирных жилых домов с индивидуальным теплоснабжениемThe aim of this work is to study the functioning of heating and hot water systems of the discussed houses, to find the reasons for increased energy consumption. Four energyefficient houses in the Brest region (Republic of Belarus) with apartment heat supply and a mechanical ventilation system with heat recovery were selected as the object of study. The research methodology of the heating and hot water supply systems of the houses consisted of the following stages: study of project documentation; thermal imaging inspection of building structures and instrumental studies of the operation of heating systems and hot water supply; questionnaire of residents to analyze the nature of their use of the studied systems; processing received data. The values of the operational specific consumption of thermal energy for heating were determined, which doubled the design values equal to 43 kW·h/m2. The main reasons for the excess of the operating specific consumption of thermal energy for heating and ventilation over the design value is the increased heating season compared to the design average for a month and the maintenance of elevated indoor air temperatures in apartments, on average, at ncreased heat losses. The functioning of the hot water supply system of energy-efficient houses is generally satisfactory for the used design solution, but sometimes due to the specific appartments planning leads to an excessive consumption of water and causes dissatisfaction of residents with a long waiting time for hot water with required temperature. Studies have shown that currently, the existing tariff policy in the Republic of Belarus due to low tariffs for gas and thermal energy for the population does not contribute to the efficient use of energy-efficient solutions by the masses of the population, which leads to an excessive consumption of thermal energy in modern residential buildings

Energy Storage In Cold Non-Elastic Deformation of Glassy Polymers

Experimental results on work W(epsilon), heat Q(epsilon) and stored energy U(epsilon) of deformation for glassy polymers such as linear PS, PC, PMMA, Polyimid, amorphous PET, thermotropic aromatic polyesters, Vectra T for example, crosslinked epoxy are presented. All the data was obtained by a deformation calorimetry technique. Loading and unloading of samples were performed at room temperature with strain rate epsilon = 10(-2) - 10(-4) sec(-1) under uniaxial compression up to engineering strains of epsilon(def) = 40-50%. During straining all polymers accumulate an excess of the latent energy U( e). Elastic fraction of the energy is released completely at sample unloading and only residual U-res(epsilon) energy is conserved in samples. The latent energy U-res(epsilon) grows up to epsilon(def) = 20-25% and levels off then. Shapes of the U-res(epsilon) curves are the same (S-shape) for all polymers. However, the saturation level is different for each polymer. The ratio U(epsilon)/ W(epsilon) was also measured. It was found that at strains epsilon(def) \u3c epsilon(y) (epsilon(y) - strain at the yield point) U(epsilon)/ W(epsilon) approximate to 100%. I. e. all W is stored by sample in a form of U. The ratio decreases up to 60-30% for different polymers at higher strains. Release of the residual energy U-res (DSC measurements) and strain epsilon(res) ( thermally stimulated strain recovery technique) was measured for deformed and unloaded samples at heating. It was found that about 85-90% of U-res stored by samples is released in glassy state of polymers (below T-g). The U-res is related to a small fraction of epsilon(res), only to 7-10%. The rest of U-res and epsilon(res) are recovered at the softening (devitrification) interval, around T-g. Computer modeling ( molecular dynamics) of an isothermal shear deformation was performed for 2-dimentional two component atomic glass containing 500 Lennard-Jones particles of two different diameters. It was found that localized deformation events are of anelastic nature. The epsilon(an) appears at early deformation stage in a form of localized shear events ( transformations). Such events are nucleated in a sample and merged and united at later deformation stages, when concentration of the events becomes high enough. Finally, merged transformations form kind of shear band crossing entire sample. On the basis of experimental data and computer modeling the deformation mechanism for glassy polymers is proposed. The first stage of the process is the nucleation of the carriers of non-elastic strain , anelastic shear transformations (ASTs). All these ASTs are energetically excited. The concentration of the ASTs is responsible for the amount of U-res(epsilon) stored by a sample. It is suggested that such nucleation is the rate-controlling step in non-elastic deformation of any non-covalent glass. Saturation of the stored energy is defined by the reaching the steady state regime in carrier\u27s concentration. In this regime the rates of nucleation and termination ( decrease of the stored local energy by AST) of carriers becomes equal. The termination proceeds spontaneously and easy ( fast). The decrease of local energy of ASTs follows by local uncoiling of chains and by an appearance of new, extended chain conformers. However, such uncoiling is not the rate-controlling step forentire deformation process. Suggested mechanism very well describes all existing experimental facts. Deformation mechanisms for glasses seriously differ from that operating in rubbers and crystals

Limited Thermal Conductance of Metal-Carbon Interfaces

The thermal conductance for a series of metal-graphite interfaces has been experimentally measured with time-domain thermoreflectance (TDTR). For metals with Debye temperatures up to ∼400 K, a linear relationship exists with the thermal conductance values. For metals with Debye temperatures in excess of ∼400 K, the measured metal-graphite thermal conductance values remain constant near 60 MW m−2 K−1. Titanium showed slightly higher conductance than aluminum, despite the closeness of atomic mass and Debye temperature for the two metals. Surface analysis was used to identify the presence of titaniumcarbide at the interface in contrast to the aluminum and gold-carbon interfaces (with no detectablecarbide phases). It was also observed that air-cleaved graphite surfaces in contact with metals yielded slightly higher thermal conductance than graphite surfaces cleaved in vacuo. Examination of samples with scanning electron microscopy revealed that the lack of absorbed molecules on the graphite surfaceresulted in differences in transducer film morphology, thereby altering the interface conductance.Classical molecular dynamic simulations of metal-carbon nanotube thermal conductance values were calculated and compared to the TDTR results. The upper limit of metal-graphite thermal conductance is attributed to the decreased coupling at higher frequencies of the lighter metals studied, and to the decreased heat capacity for higher vibrational frequency modes

Enhancing surface heat transfer by carbon nanofins: towards an alternative to nanofluids?

Background: Nanofluids are suspensions of nanoparticles and fibers which have recently attracted much attention because of their superior thermal properties. Nevertheless, it was proven that, due to modest dispersion of nanoparticles, such high expectations often remain unmet. In this article, by introducing the notion of nanofin, a possible solution is envisioned, where nanostructures with high aspect-ratio are sparsely attached to a solid surface (to avoid a significant disturbance on the fluid dynamic structures), and act as efficient thermal bridges within the boundary layer. As a result, particles are only needed in a small region of the fluid, while dispersion can be controlled in advance through design and manufacturing processes. Results: Toward the end of implementing the above idea, we focus on single carbon nanotubes to enhance heat transfer between a surface and a fluid in contact with it. First, we investigate the thermal conductivity of the latter nanostructures by means of classical non-equilibrium molecular dynamics simulations. Next, thermal conductance at the interface between a single wall carbon nanotube (nanofin) and water molecules is assessed by means of both steady-state and transient numerical experiments. Conclusions: Numerical evidences suggest a pretty favorable thermal boundary conductance (order of 107 W·m-2·K-1) which makes carbon nanotubes potential candidates for constructing nanofinned surface

Heat transfer from nanoparticles: a corresponding state analysis

In this contribution, we study situations in which nanoparticles in a fluid are strongly heated, generating high heat fluxes. This situation is relevant to experiments in which a fluid is locally heated using selective absorption of radiation by solid particles. We first study this situation for different types of molecular interactions, using models for gold particles suspended in octane and in water. As already reported in experiments, very high heat fluxes and temperature elevations (leading eventually to particle destruction) can be observed in such situations. We show that a very simple modeling based on Lennard-Jones interactions captures the essential features of such experiments, and that the results for various liquids can be mapped onto the Lennard-Jones case, provided a physically justified (corresponding state) choice of parameters is made. Physically, the possibility of sustaining very high heat fluxes is related to the strong curvature of the interface that inhibits the formation of an insulating vapor film

Radius and chirality dependent conformation of polymer molecule at nanotube interface

Temperature dependent conformations of linear polymer molecules adsorbed at carbon nanotube (CNT) interfaces are investigated through molecule dynamics simulations. Model polyethylene (PE) molecules are shown to have selective conformations on CNT surface, controlled by atomic structures of CNT lattice and geometric coiling energy. PE molecules form entropy driven assembly domains, and their preferred wrapping angles around large radius CNT (40, 40) reflect the molecule configurations with energy minimums on a graphite plane. While PE molecules prefer wrapping on small radius armchair CNT (5, 5) predominantly at low temperatures, their configurations are shifted to larger wrapping angle ones on a similar radius zigzag CNT (10, 0). A nematic transformation around 280 K is identified through Landau-deGennes theory, with molecule aligning along tube axis in extended conformationsComment: 19 pages, 7 figure2, submitted to journa

High thermal conductivity in electrostatically engineered amorphous polymers

High thermal conductivity is critical for many applications of polymers (for example, packaging of light-emitting diodes), in which heat must be dissipated efficiently to maintain the functionality and reliability of a system. Whereas uniaxially extended chain morphology has been shown to significantly enhance thermal conductivity in individual polymer chains and fibers, bulk polymers with coiled and entangled chains have low thermal conductivities (0.1 to 0.4 W m(-1) K-1). We demonstrate that systematic ionization of a weak anionic polyelectrolyte, polyacrylic acid (PAA), resulting in extended and stiffened polymer chains with superior packing, can significantly enhance its thermal conductivity. Cross-plane thermal conductivity in spin-cast amorphous films steadily grows with PAA degree of ionization, reaching up to similar to 1.2 W m(-1) K-1, which is on par with that of glass and about six times higher than that of most amorphous polymers, suggesting a new unexplored molecular engineering strategy to achieve high thermal conductivities in amorphous bulk polymers

Thermal Conductivity of Carbon Nanotubes and their Polymer Nanocomposites: A Review

Thermally conductive polymer composites offer new possibilities for replacing metal parts in several applications, including power electronics, electric motors and generators, heat exchangers, etc., thanks to the polymer advantages such as light weight, corrosion resistance and ease of processing. Current interest to improve the thermal conductivity of polymers is focused on the selective addition of nanofillers with high thermal conductivity. Unusually high thermal conductivity makes carbon nanotube (CNT) the best promising candidate material for thermally conductive composites. However, the thermal conductivities of polymer/CNT nanocomposites are relatively low compared with expectations from the intrinsic thermal conductivity of CNTs. The challenge primarily comes from the large interfacial thermal resistance between the CNT and the surrounding polymer matrix, which hinders the transfer of phonon dominating heat conduction in polymer and CNT. This article reviews the status of worldwide research in the thermal conductivity of CNTs and their polymer nanocomposites. The dependence of thermal conductivity of nanotubes on the atomic structure, the tube size, the morphology, the defect and the purification is reviewed. The roles of particle/polymer and particle/particle interfaces on the thermal conductivity of polymer/CNT nanocomposites are discussed in detail, as well as the relationship between the thermal conductivity and the micro- and nano-structure of the composite