• Energy Research

Abstract In this paper, the specific consumption of the heat energy supplied by three most utilized heating systems (district heating, gas and electricity system) in the residential sector of the city of Kragujevac (Serbia) is analysed. Required data are provided by combining data from local utility companies with national census results. Comparison of present tariff systems shows that consumers in the residential buildings are the most motivated for energy savings if they use electricity or gas as energy source. The specific energy consumption in district heating system shows relatively high quantities, primarily as a consequence of dominant unmotivating tariff payment system. The average heat consumption from gas and electricity sources relatively fits to EU average, but the electricity consumption of the electric heaters is not acceptable in long term. Low electricity price in Serbia and high price of unit of energy from district heating system of the city of Kragujevac (and Serbia) will lead to an increase of electric energy consumption which will cause additional problems related to covering of that consumption from domestic energy sources.

An evaporative three finger thermosyphon (ETT), made entirely of glass, is comprised of three fingers, where one was an evaporator and two were condensers. As evaporating liquid and saturated vapor, water was used inside the ETT, and air was present inside the ETT's condenser fingers, at their tops. This air enables a variation of the effective heat transfer area inside the condensers and, thus, a control of the heat transfer from the evaporator to the two condensers. The cold water used to cool condenser 1 had a cross flow area smaller than that used to cool condenser 2. Experimental investigations revealed that the ETT behavior was controlled by the evaporator heater and by the cooling water in condenser 2. These control features change with elapsed operation time. The ETT behavior was not controlled by the cooling water in condenser 1.

In this paper influence of house walls on heating energy consumption and indoor thermal comfort was investigated. The simulations were carried out using an extended dynamical model of building-software ZID under transient conditions, like the first heating day after a long non-heating period. Four simulation cases were considered: two-layered thermal-insulation concrete house walls were used, where the thermal insulation was inserted in the inner, outer or middle positions according to the inside of the house, and one-layered concrete house walls. The influence of the house walls on internal thermal comfort and heating process is presented by two introduced factors: thermal comfort degree-hour and building envelope response factor (BER factor). This new BER factor could be a significant variable in considering house walls as a passive heating energy source. The conclusion is that under certain conditions the BER factor is at a maximum, at which point the influence of house walls on inside thermal comfort and heating energy consumption is also at a maximum.

Educational buildings are buildings with special social importance, and their energy consumption requires particular attention. Since the majority of educational buildings in Serbia were built between 1950 and 1990, with concrete frame, brick walls and poor thermal insulation, it can be concluded that these buildings have relatively high potential for thermal energy savings. In this paper, specific heat consumption of educational buildings (42 different institutions classified throughout educational levels) in the city of Kragujevac (Serbia) was analysed in order to determine their specific heat consumption (per heated floor area and per user) and to list the priorities for refurbishment. According to educational level, they were classified in preschool buildings, elementary and high schools and faculties (university buildings). Data about heat energy consumption were collected during several heating seasons in order to minimise the influence of seasonal climate variations. Values of specific heat energy consumption of the buildings were benchmarked with data on specific heat consumption of schools from different European countries. In order to accomplish the tasks, authors introduced the concept of weather normalised specific heat consumption. Results of the performed analysis pointed out preschool and elementary school buildings as refurbishment priorities since their average specific heat consumption is the biggest (186 and 176 kWh/m2/a, respectively). Besides, normalised specific heat consumptions of the educational buildings in Kragujevac are higher than in comparing countries, and the potential for their energy efficiency improvement is significant.

In an industrial building, several refuse hot-air flows (RHAF) are often available. We use a recovery heat exchanger (RHE) to heat incoming air for space heating by using a mixture of these RHAFs. A proper scenario for use of their heat contents is essential for efficient control of the energy system to achieve the greatest energy-saving efficiency. Software has been used in an industrial building with three RHAFs to control their mixtures. Depending on the temperatures of the colder and hotter zones of the industrial building and the outside temperature, we can find regions for use of RHAF mixtures of particular compositions and avoid energy losses while maintaining good thermal comfort in the building.

This paper presents a new solar collector design that combines flat absorber plates and glass tubes with a compacted automatic time-controlled (E–W) single-axis tracking mechanism. The mentioned design incorporates the advantages of the single-axis tracking system with a relatively small increase in the solar collector dimensions. According to it, a mathematical model was formed, and the test sample was fabricated. Using a bespoke test rig, the experimental investigation was carried out by simultaneously measuring the thermal performances of the solar collector in question and the flat plate solar collector with the same inclined position and absorber material. The experimental results (from July 15 to October 15, 2021) showed that compared to a reference collector, the new solar collector’s average daily specific useful heat was higher in the range of 14–23%, and the overall thermal efficiency was higher in the range of 9.7–17.9%. Using the obtained experimental and simulation data, the mathematical model was verified within an average deviation of 5.67%. Despite the partial reduction of the useful heat due to the mutual shading of absorbers, the mentioned solar collector shows promising results along with a compact design. We propose that this device will present promising opportunities in future applications.

We use a recovery heat exchanger (RHE) in an industrial building to heat incoming air for space heating by using the heat contents of some of the refuse hot-air flows (RAF). Several RAFs are often available with different flow rates and temperatures. A proper scenario for use of their heat contents is essential for efficient management of the energy system. Using a steady-state, bottom-up approach, we have obtained a set of equations that we have linearized. Linear programming (LP) is then used and LINE software has been developed. This software has been used for an industrial building with three RAFs to obtain a use scenario corresponding to the highest energy-saving efficiency and minimum life-cycle costs of the RHE. Depending on the RHE size, temperatures and flow rates of the RAFs, and the outside temperature, we can recover the heat contents of different RAFs in different ways.

Abstract This paper presents the results of a theoretical and experimental investigation of a double exposure, flat-plate solar collector with a flat-plate reflective surface. The main role of the reflector, which is placed below and parallel to the collector, is the reflection of solar radiation on the lower absorber surface. To enable absorption from the lower absorber surface, it is necessary for the insulation mounted on the lower part of the collector box to be removed and the lower box surface replaced by a glass cover. In order to determine the feasibility of the proposed concept, theoretical and experimental investigations of a double exposure and conventional flat-plate solar collectors were carried out. The experimental tests verified the developed mathematical models of the thermal behaviour of the mentioned solar collectors. The main advantages of the proposed collector–reflector system in relation to the previously investigated are: parallelism between the reflector and the collector, mirror reflective surface and mobility of the reflector in all three possible orthogonal directions. The proposed system is simpler because it consists of only one reflector. The experimental and theoretical results show that performance of a double exposure, flat-plate solar collector can be significantly higher than a conventional solar collector. The experimentally obtained relative difference of thermal power of these collectors is in the range of 41.79–66.44%, the highest achieved value of this difference in the reviewed literature is 48%.

Abstract Several refuse hot air flows with different flow rates and temperature potential are often generated by an industrial building. A recovery heat exchanger in an industrial building is used to heat incoming air for space heating by using the enthalpies of some mixture of these refuse hot-air flows. A proper scenario for use of their enthalpies is essential for efficient management of the energy system. Moreover, there are also in these buildings different heat gains so as from lightance and machines used in production process. Using a steady-state, bottom-up approach, a set of equations has been obtained. These equations have been linearized and linear programming used to develop LINEHG software. This software has been used for an industrial building with three refuse air flows, hotter and colder building zones and heat gains in these zones to obtain a use scenario corresponding to the highest energy-saving efficiency and minimum life-cycle costs of the recovery-heat exchanger. Depending on the heat gains in the colder and hotter zone of the industrial building, the enthalpies of different refuse-air flows can be recovered in different ways.