The global trend of organic materials application for the construction low rise residential accommodation in the context of the sustainable development concept results in growing interest towards the usage of both known natural materials and their combinations as well as the search of new technological decisions. The trend of eco-buildings is starting to play an important role. The concept of eco-buildings means construction of low-energy buildings [1] that cause less harm to the environment due to minimised value of primary energy consumption and low rate of heat losses, cost of materials, etc. As Brojan, L. et al [2] states, one of the criteria, which can be applied for the assessment of environment damage, is an index of primary energy consumption, or primary energy input (PEI) index, MJ/m 2. In fact, this is the volume of necessary energy resources for the production and maintenance of a 1 m 2 surface of enclosing structures [2].

The best choice of energy efficient envelope from variety of available materials is still the challenge. Therefore, the attempt of thermal performance multi-criteria evaluation of some building materials of natural origin for energy-efficient envelopes is conducted in present paper. Such types of walls from natural energy-efficient materials are considered in comparison assessment: hempcrete, adobe, strawbale panel, earthbag, cordwood, SIP (plywood+ecofiber), hempcrete+straw and energy efficient block. The influence of thermal inertia time, internal areal heat capacity, as well dimensionless index of thermal inertia D, the total thermal resistance of the walls Rtot-value, mass of the wall assembly and its cost have been taken into consideration as important influence factors. The multi-criteria numerical assessment of envelope’s energy efficiency potential was performed by two popular methods – Analytic Hierarchy Process (AHP) as the subjective weighting method and Grey Relation Analysis (GRA) as the objective weighting method. Both of methods allow to arrange the alternatives and could be applied as decision support tools in decision making (DM) process of choosing the best alternative in terms of multi-criteria assessment. For more objective analysis, by taking into account the variety of physical and physical-mechanical parameters of the wall assembly material, the concept of generalized index of the envelope energy efficiency potential is proposed. Conducted research has shown that the best envelope type in terms of of generalized index of energy efficiency potential has the hempcrete wall and hemcrete+straw wall, almost three times smaller has the wall of the earthbags. The walls from adobe, cordwood and strawbale panels have practically the equal value of generalized index of energy efficiency potential. It could be observed that AHP method shown more inhomogeneous results, than GRA. The possible reason for that is the difference in evaluation attitude in techniques - AHP is considered as the subjective method with pairwise comparison matrixes, while GRA is objective method of comparison.

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The determinants of profitability and environmental friendliness of bioconversion are noted in the paper. As one of the major factors the reduction of thermal energy costs for the fermentation of organic wastes in their utilization is proposed to take into consideration. The expediency of using renewable energy sources for thermal stabilization of the fermentation process, especially in the thermophilic mode of organic fermentation, is emphasized. The energy efficiency biogas installation with structural and technological scheme is given as an example. It receives thermal energy to increase its efficiency from a solar collector and a heat pump. It is proposed to evaluate the energy efficiency of a biogas plant taking into account the cost of bioconversion products and the costs of providing this process. A mathematical model was proposed to substantiate the environmental-economic efficiency of a biogas installation with the minimum energy costs for the thermal stabilization of the biomass fermentation process. The model is based on fuzzy set theory which uses linguistic variables. Linguistic variables take into account the influence of quantitative and qualitative factors on the objective function.