The paper presents the results from a study of the thermo-physical properties of a new synthetic acrylic polymer used as material in road construction and as concrete on its basis. For the purposes of the study, the Modified Transient Plane Source Method (MTPS) was used. Data on the following thermal characteristics were obtained: thermal conductivity coefficient, thermal effusion, thermal diffusion and specific thermal capacity. A comparative analysis was made with the analogous indicators of conventional petroleum bitumen and asphalt concrete. The results of the study can be used for a more precise calculation and prognosis of temperature deformations and thermal stresses, as well as the crack propensity at low temperatures in asphalt concretes.

The paper presents results of a research for determination of a few general thermal-physical properties of environmentally friendly building materials made by clay, sand and straw. The aim of this study is to establish their heat insulating and energy-efficient capacity. All specific measurements were carried out by using the newest generation thermal conductivity analyser Mathis TCi. The results showed that the studied composite materials are good thermal insulators with thermal conductivity less than 0.5 W/m.K, which depends on the straw amount. Even less than 0.5 wt.% straw reflects on the insulating properties by decreasing the thermal conductivity coefficient with nearly 50 %.

The operation of an air-to-water heat pump system providing space heating production for an administrative building in Varna has been explored. For this purpose, discontinuous operational mode of the heat pump system during wintertime is considered. Study has been performed by use of real values for external ambient temperature and operating characteristics of the heat pump system. The seasonal efficiency of the heat pump operation has been calculated. Also, the minimal external ambient temperature until the heat pump may work in heating regime and operation duration at outdoor air temperatures higher than the design air temperature can be determined. The economic analysis of the heat pump system exploitation has been made finally.

The article is focused on the potential and application of biogas, as an alternative fuel from Renewable Energy Sources, for use mainly in gas-generator stations. Biogas fuel is basically a mixture of methane and carbon dioxide. Its composition depends on the type of raw material used for its production. Methane concentration in biogas is between 50÷80%. To be possible engine to work with maximum efficiency with different biogas fuels, it is necessary to modify specific adjustment parameters depending on the concentration of methane in the mixture. This requires the creation of a biogas simulation system for different concentrations of the main components. The aim is to investigate and determine the optimum and permissible biofuel blend concentrations and their impact on engine performance and fuel consumption. Biogas can be used as a fuel to produce electricity, heat or steam or as fuel for internal combustion engine, and its use will help to reduce harmful emissions into the atmosphere.

Annually, a huge amount of waste from plant biomass and animal manure is produced from agriculture and animal farming. Many studies provide information on the biomethane potential of agricultural and livestock wastes, but only a few studies have investigated the application of the substrates in combination. The objective of the study is to enhance the fermentation process in the digester for biogas production, obtained from animal and plant waste substrates. In four batch processes for three months, the temperatures and the residence time of the substrates in the fermenter were analyzed. Simultaneously, electricity and thermal energy were produced via cogeneration units, which were exported to the public grid and city heating network. The plant substrate is a silage mixture of corn and wheat waste. The animal substrate is a mixture of beef and pig manure. Animal and vegetable waste raw materials are collected and transported to the site, located in the region of southeastern Bulgaria. The total annual consumption of animal and plant waste is 17,971 t/year. The enhancement of the process leads to the production of 1,506,000 Nm³ CH₄/a of methane, the generation of which requires 299.63 MWh/a of electricity and 649.09 MWh/a thermal energy.

The focus of the present study is a small biogas power plant for anaerobic fermentation of several types of animal waste raw materials used for biogas production. The impact of some of the characteristics of substances such as composition, temperature, humidity, and pH of the mixture in the bioreactor has been considered. The above is vital for optimizing the fermentation process, and also to improve the biogas production process. The plant is located in Northeastern Bulgaria and the raw liquid manure is supplied by several neighboring small farms. The annual quantities of raw waste are as follows: cow manure - 1252 t / a; chicken manure - 427 t / a and pig manure - 639 t / a. The manure is collected in a preliminary tank and then pumped to the bioreactor. The fermenter itself is a hermetically sealed and thermally insulated tank where constant temperature is maintained. It is equipped with a stirring system, which helps the mixing and homogenization of the substrate. The tests were performed during three charges of the installation. The fermentation takes approximately 23 up to 25 days. The experiments were performed during the summer and autumn seasons when the ambient air temperature varies from 28 to 45 °C. The biogas can be used as an energy carried as the obtained organic fertilizer is suitable for agriculture purposes.