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In this work, the effects of proposed measures for improving environmental footprint of woody and non- woody biomass combustion in residential appliances, comprising partial insulation of combustion chamber and inclusion of combustion intensifier are numerically and experimentally evaluated. In the numerical part, a simplified modeling approach for wood pellets, based on Finite-Rate and Eddy- Dissipation model, is presented and the obtained combustion reaction maps are extensively discussed. The accompanying experiments with wood and agropellets are performed in a multi-fuel boiler equipped with rotary pellet burner. It is found that significant reductions in pollutant emissions are obtained (CO – 31 to 78%, PM – 28 to 56 mg/Nm3 at standardized oxygen content). The results also show that by combining both measures, CO emissions below 1000 mg/Nm3 at the standardized oxygen content (limitation for Class 4 according to EN 303–5:2021), without a significant drop in combustion efficiency (approx. 2%), can be expected for agropellets. The optimal combustion conditions can be achieved with lower excess air ratio, whereby a slight reduction in NOx emissions can be accomplished.

This paper presents differences in calculations of annual heating and cooling energy need performed by numerical dynamic simulations software IDA ICE and those carried out by the modification of simple hourly method from EN ISO 13790 EN ISO 13790, widely used for determining building energy need. A simple model of a nearly-zero energy building was created and all heat gains and set-points that could lead to a mismatch in initial or boundary conditions were analysed. The impact of those on the annual heating and cooling energy need was examined by adding and/or removing every single one of them. Boundary conditions in numerical dynamic simulations were set up to match those in simple hourly method. Such an approach enables evaluation of differences in results and definition of their origin. The comparison of results has shown that in most cases, annual energy need for heating and cooling calculated using numerical dynamic simulations software differs from that calculated using EN ISO 13790. Among the others, more detailed heat accumulation model of heat gains in building’s envelope in IDA ICE software was marked as the main reason. Fact that solar heat gains seem to be underestimated by EN ISO 13790 and differences in heat transfer towards ground contribute to the differences in results as well.

This paper presents numerical analysis on influence of different configurations on thermal performances of a polymer solar collector. In the first step two configurations including single cover plate and honeycomb cover plate are investigated. Results showed that collector with single cover plate is more efficient than collector with honeycomb cover plate at lower reduced temperature values. Honeycomb cover plate behaves like insulator, increasing overall collector top resistance to heat loss which results in higher thermal efficiency at higher reduced temperature values. This also leads to higher temperatures at stagnation regimes. Also, in this paper two passive overheating protection based on natural convection are examined. Channels inside honeycomb cover plate can be connected to channels inside absorber plate allowing for air to flow inside collector. Three different channel widths are considered 4.25, 10, and 20 mm. With such passive method, it is possible to reduce average absorber temperature up to ~20°C. Second overheating protection uses additional channel below the absorber plate allowing the flow of ambient air. With this method it is possible to reduce temperatures up to ~50°C but it also causes high temperature gradient, 50°C/m of collector length which can potentially result in deformation of polymer solar collector. Present work is a part of the research focused at development of a low-cost polymer solar collector that operates in a drain back system.

The paper presents results of analysing the most commonly used measures for improving combustion of wood and agro pellets in domestic combustion appliances. This includes numerical and experimental analysis of the combustion chamber partial insulation and inclusion of combustion intensifier in the conventional combustion system on the obtained combustion efficiency and reduction of pollutant emission. The results show that significant reductions in unburned pollutant emissions can be obtained by both measures, while the improvements in combustion efficiency and particulate matter emissions only with the inclusion of combustion intensifier.

This study aims at better understanding of the complex flow phenomena in plate heat exchangers. This is identified in the literature to be crucial for improvement of numerical models and generalized heat transfer and pressure drop correlations with respect to their applicability in heat exchanger optimization. 3D numerical simulations of the flow in corrugated channels of plate heat exchangers with different corrugation angles (β28° and 65°) were performed in order to investigate in detail complex flow patterns and their influence on the heat transfer and pressure drop at laminar, transient and turbulent flow conditions. The study is supported by the visualization and thermal–hydraulic tests (Re = 0.1–6037) carried out on the channels with corresponding chevron angles, using fluids of different viscosity to explore effects of different velocities at fixed Re numbers. Consequent alternation of flow patterns is believed to cause recorded differences in heat transfer and pressure drop data. Results from the simulations are analysed in different sections of the channels in light of the flow substreams, velocity distribution, wall shear stress and wall heat flux distribution. They confirm assumptions from the present visualization tests on the longitudinal and furrow substreams interactions and characteristics, as well as on the thermally and hydrodynamically developing flow conditions within a basic cell. A suitability of comparing simulations and experimental data in terms of Fanning friction factor and Nusselt number is discussed relative to the definitions of characteristic length, hydraulic diameter and mean velocity at different flow patterns. The computed heat transfer and pressure drop is in a relatively good agreement with the performed tests and literature data. The presented numerical-experimental analysis outlines importance of the substreams velocity on their interactions and flow components size, which in turn determines PHE channel heat transfer and pressure drop characteristics. The present findings may serve for improvement of CFD modelling, generalized correlations and definition of Re, Nu and f to allow for comparison of heat transfer and pressure drop data at different geometries and flow conditions.

This paper presents the research on improving combustion of wood and agro pellets in domestic hot water boilers. The novel biomass combustion system concept consisting of rotary burner and two types of combustion intensifiers is proposed, with the aim to allow simple application in the most of existing boilers, as well as to improve the efficiency and emissions to extent the related requirements of the relevant EU standards are met. In this regard, inclusion of the combustion intensifier based on honeycomb structure into conventional combustion system is numerically and experimentally investigated and the most appropriate placement in a furnace found. The results show that, with such implementation of the combustion intensifier, significant reductions in unburned pollutant emissions (CO >40% in case of wood pellets and >30% in case of agro pellets), particulate matter emissions (28–56 mg/mn3 at 10% of O2 in dry flue gases) and improvements in combustion efficiency (>2.6–3.7%) can be obtained. A detailed numerical analysis of the subsequent combustion process within the combustion intensifier lead to the novel combustion intensifier design, aimed at lowering manufacturing cost. Preliminary numerical and experimental results indicate similar or slightly less reduction in unburned pollutants of novel design.

This paper presents the procedure for conducting cost-optimal energy performance calculations for a new nearly zero energy hotel building in Croatia in the context of Energy Performance of Buildings Directive 2010/31/EU. For this purpose, dynamic simulation models of building and its technical systems were developed in IDA ICE simulating tool and coupled with GenOpt optimization software. Annual energy performance of the building is assessed by running hourly simulations. Energy, equipment and installation works costs were obtained by the open market analysis. After that, the global cost function was derived, and optimization performed to minimize the costs. The results show that the preferred heat source at the cost-optimal level of the building energy performance is air-to-water heat pump coupled with gas condensing boiler. Furthermore, an analysis of thermal comfort is provided to demonstrate the use of the proposed approach in preventing system oversizing, efficiency decrease and thermal comfort deterioration. At the end, energy performance calculations obtained by IDA ICE were compared to those obtained by the widely used EN 13790 and EN 15316 series of standards used for determining energy performance of buildings for legal purposes. Based on this, the recorded discrepancies in results are analysed and applicability of mentioned standards calculations on low-energy buildings is discussed.

This paper presents a novel 2D numerical and experimental approach which can be used to assess the thermal performances of a polymer solar collector as a part of the design process. Numerical simulations are performed using software package FLUENT. To simulate radiative heat transfer, different radiation models that are available within FLUENT are applied including the discrete ordinates (DO) and surface-to-surface (S2S) model. Solar incident radiation is taken into account as an energy source computed analytically or via ray tracing software or as an external beam which is a part of boundary condition in semi-transparent DO model. Different combinations of longwave and shortwave radiation modeling are tested, first on a segment of a typical flat plate collector. In the second step, these models are applied to the test box at different boundary conditions and materials. Numerical models are then applied to the geometry of polymer solar collector prototype to assess its thermal efficiency. Combination of longwave radiation computed with DO and S2S sub-model and solar incident radiation modeled analytically or via ray tracing software yields sufficiently accurate results on the all tested computational domains, thus any of them can be used to compute thermal efficiency of a polymer solar collector. On the other hand, when applying semi-transparent DO model with the solar incident radiation modeled by the external beam, the obtained results are unphysical. Present approach is a part of the research aimed at the development of a thermally efficient low-cost polymer solar collector running in a drain back system. The prototype will be then experimentally tested.

Biomass is one of the oldest energy source known to mankind. Progress made so far in understanding of a rather complex direct combustion process allowed for a constant technology development. Increasingly stringent environmental requirements of the EU, especially in terms of the pollutant emissions, require development of new design solutions of the combustion system for residential hot water boilers fuelled by some form of solid biomass. The emissions are becoming even greater problem if, various residues from agricultural production are used instead of a wooden biomass, as they are becoming more desirable fuel due to their lower cost. In this paper, various issues related to the combustion of agricultural biomass are discussed. Attention is given to the problems associated with high volatile matter contents, presence of nitrogen, sulphur, chlorine and low ash melting temperatures. At the end, novel design modifications of a conventional residential biomass combustion system are proposed, as the well-known methodologies for combustion and emissions control applied in industrial-scale boilers are in most cases not suitable for small-scale units.

Most of the physical parameters that are used to assess the satisfaction with the ambient thermal condition in a mathematical way are contained within the definition of operative temperature. This temperature, which can be used as a representative of indoor thermal comfort, is a function of the air temperature, the mean radiant temperature and the relative air velocity. In this paper, the room air, mean radian temperature and indoor air velocity were determined experimentally for wall-mounted convector and conventional radiator at controlled room conditions. The room air temperature and indoor air velocity were continuously measured at several positions and heights (0.75 m and 1.5 m) using calibrated T- type thermocouples and hot wire probes, while mean radiant temperature was calculated using the thermograms captured by the IR thermal camera and numerically computed radiation view factors. Each wall was divided into several sections with approximately similar temperatures (differences < 0.5 °C) for which view factors were determined. Thermal heat output of the tested heat emitters was derived according to EN 442-2:2014. Obtained results were analysed and conclusions about the achieved thermal comfort and related energy saving were made accordingly.

This paper presents a new methodology, based on the energy performance of buildings Directive related European norms. It is developed to overcome ambiguities and incompleteness of these standards in determining the delivered and primary energy. The available procedures from the present “Algorithm for determining the energy demands and efficiency of technical systems in buildings”, normally used for energy performance certification of buildings, also allow detailed analyzes of the influence of particular system components on the overall system energy efficiency. The calculation example is given for a Croatian reference dwelling, equipped with a solar hot water system, backed up with a biomass boiler for space heating and domestic hot water purposes as a part of the dwelling energy performance certification. Calculations were performed for two cases corresponding to different levels of the dwelling thermal insulation with an appropriate heating system capacity, in order to investigate the influence of the building heat losses on the system design and energy consumption. The results are compared against those obtained for the conventional system with a gas boiler in terms of the primary energy consumption as well as of investment and operating costs. These results indicate great reduction in both delivered and primary energy consumption when a solar system with biomass boiler is used instead of the conventional one. Higher savings are obtained in the case of the dwelling with higher energy need for space heating. Such dwellings also have a shorter payback period than the ones with better thermal insulation.

In this paper the new dynamic method for calculation of energy for heating, cooling and mechanical ventilation is presented. The method is based on a combination of the simple hourly method, from EN ISO 13790, and a modification of the existing calculation procedure for determination of the energy need for mechanical ventilation, described in Algorithm for calculation of the energy need for HVAC systems. Developed method, incorporated in Croa-tian national Algorithms used for energy certification of buildings, enables the determination of the hourly energy need for heating and cooling which includes all energy flows that occur during the air conditioning process (moistening, dehumidifying, etc.). Simulations are performed for a Croatian reference dwelling, equipped with two different HVAC systems. The results are compared against those obtained from the quasi-steady state monthly method, also described in Algorithms. The comparison proved that the application of the quasi-steady state method would preclude the correct calculations in buildings with air conditioning.

This paper presents new approach for determining buildings technical system energy performance. This new methodology describes a mathematical model for accurately predicting indoor temperature and heat losses of the space heating and domestic hot water system components. The entire model is described by system of ordinary differential equations which can be solved using standard numerical techniques. The innovative aspect of the method is the integral approach in mathematical modeling of buildings energy needs and technical systems heat loss, taking into account heat accumulation in all considered parts (building envelope + technical system). Such approach allows a detailed insight of the system behavior for chosen working conditions. This model can serve for energy performance calculations in a wide variety of buildings types and their technical systems. The calculation example is given for family house, equipped with conventional space heating and domestic hot water heating system, with the time step of 1 min and for characteristic day of each month within a year. The results are compared against those obtain from EN ISO 13790 and standard series EN 15316. The comparison shows significant differences in determination of the annual delivered energy to the heating system (33%), as a consequence of difference in estimation of the energy need for heating (15%) and calculation of the technical systems recoverable heat losses utilization factor, which seems to be underestimated. The delivered energy to the space heating and domestic hot water heating system differs 25%, while the energy delivered to the generation system differs 4%.

Characteristics of the flow in chevron plate heat exchangers are investigated through visualization tests of channels with β = 28° and β = 61°. Mathematical model is then developed with the aim of deriving correlations for prediction of f and Nu for flow in channels of arbitrary geometry (β and b/l). Thermal and hydraulic characteristics are evaluated using analytical solutions for the entrance and fully developed regions of a sinusoidal duct adapted to the basic single cell. The derived correlations are finally adjusted so as to agree with experimental results from tests on channels with β = 28° and β = 65°. f and Nu calculated by the presented correlations are shown to be consistent with experimental data from the literature at Re = 2–10, 000, β = (15– 67)° and b/l = 0.26–0.4.

The paper presents the results of experimentally supported numerical analysis performed in order to investigate the possibilities to improve the thermal efficiency of plate solar collectors. Different numerical models were developed in order to asses the influence of design and operating parameters such as bond conductance between absorber plate and tube, tube diameter, glass cover to absorber plate distance, optical properties of absorber and flow rate on thermal efficiency of collectors. Following the results, two designs of collector without tubes, with parallel flat and corrugated absorber plates of chevron type, is further considered and shown to be an effective way to increase the thermal efficiency of solar energy conversion beyond that of commercial glazed and unglazed solar water heaters. Based on the results, the guidelines for design of a new collector prototype consisting of chevron type corrugated plates normally used in plate heat exchangers are provided.

This paper presents the optimized solution of a solar system for providing domestic hot water through the results of a system work simulation done on an hourly basis for a motel with capacity of 35 persons located in climate areas of Zagreb and Split. The analyzed system consists of two tanks for domestic hot water, capacity 1600 liters each, and appropriate collector area (43,7 m² for Zagreb and 30,4 m² for Split). The basic criterion for optimization was the shortest investment payback period with a primary goal to cover the energy demands for domestic hot water in July and August. This paper presents monthly and yearly results of performed simulation for both climate areas and description of the calculation procedure for estimating solar radiation and energy gain., U ovom radu je prikazano optimalno rješenje solarnog sustava za zagrijavanje potrošne tople vode kroz rezultate satne simulacije za motel smještajnog kapaciteta 35 osoba smještenog u klimatskom području Splita i Zagreba. Simulirani sustav se sastoji od dva spremnika volumena 1600 litara i odgovarajuće površine solarnih kolektora (30,4 m² za Split i 43,7 m² za Zagreb). Kriterij optimizacije sustava je bio što kraći period povrata investicije uz osnovni zahtjev da sustav pokrije energetske potrebe objekta za PTV-om u ljetnim mjesecima. Dani su mjesečni i ukupni godišnji rezultati simulacije za oba klimatska područja, kao i opis proračuna Sunčevog zračenja i prikupljene energije.

SAŽETAK: U radu su dani opis i rezultati ispitivanja mlaznica za stvaranje vodene magle koje se upotrebljavaju u sustavima za gašenje požara u strojarnicama i pumpnim stanicama na brodovima. Ispitivanja su provedena u skladu s propisima International Maritime Organisation te uključuju ispitivanja konstante protoka, raspodjele mlaza te otpornosti na zaprljanje. Ispitivanja su provedena kao dio postupka ishođenja tipskog odobrenja za mlaznicu tip WM5-90 od ovlaštene osiguravateljske kuće., SUMMARY: The paper presents the tests on the water mist nozzles commonly employed in fire extinguishing systems in machinery spaces and pump rooms. The tests were performed according to the procedures defined by the International Maritime Organisation and comprise determination of the flow constant, flow distribution characteristics and the clogging tests. The tests were conducted as a part of the type approval procedure for the nozzle type WM5-90, as requested by the accredited insurance company.

The paper presents the investigation on the influence of different geometric parameters such as the chevron angle β, pressing depth b and corrugation wave length l on thermal-hydraulic performances of corrugated channels, commonly employed in plate heat exchangers. In order to fully understand a rather complex flow phenomenon occurring in corrugated channels, a number of dye based visualization tests have been conducted on two channels with chevron angles β=28° and β=61° at low Re-number flow conditions. The existence of two flow components has been recorded. The influence of flow components on the heat transfer depends mainly upon the angle β and Re-number as well as on the aspect ratio b/l. The exception are plates with the angle β, U članku su predstavljeni rezultati istraživanja utjecaja različitih geometrijskih parametara, poput kuta V-žljebova β, dubine žlijeba b i širine žlijeba l, na toplinsko-hidrauličke karakteristike valovitih kanala kakvi se uobičajeno koriste u pločastim izmjenjivačima topline. Kako bi se u potpunosti objasnio složeni mehanizam strujanja u valovitim kanalima, proveden je određen broj vizualizacija strujanja u dva kanala s kutovima V-žljebova od β =28° i β =61° pri malim vrijednostima Re-brojeva. Zabilježeno je postojanje dviju komponenti strujanja. Njihov utjecaj na izmjenu topline najviše ovisi o kutu β i Re-broju, kao i o omjeru b/l. Iznimka su ploče s kutom β

This text is intended to the basic course ‘’Renewable energy technology’’ and the advanced course ‘’Applied solar technology’’ held within the International Master of Science Programme ‘’Sustainable energy engineering’’ at the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb. The text comprises basics on solar energy and its utilization for conversion into useful thermal energy, as well as some more advanced topics needed for designing and sizing of the solar systems and equipment. The information provided here are based on the relevant literature from the field and on the author’s working experience in designing, researching and testing of the solar systems and collectors. Since many engineers in technical sciences have not had any training in solar energy, this text may be of use to them as well as to everyone beyond universities interested in the solar energy applications.

The work represents an attempt to fully understand and evaluate characteristics of single phase flow in the corrugated chevron channel as a constitutive geometry in the plate heat exchangers. For this purpose the visualization tests have been carried out at the laminar and transient flow conditions for the plates with different chevron angles as the most influencing geometrical parameter. The measurements of the heat transfer and pressure drop characteristics accompanied the visualization tests at the Re numbers varied within the range Re=2&#61624 ; ; 1400. The first part of this work gives a general overview of the plate heat exchangers related features as design, applications and calculation procedure of performances. The subsequent text concerns the physical and mathematical model of the flow in the channel being supported by the detailed information related to the fundamental analytical solutions provided for the laminar flow in the simplified models and ducts of similar but simpler geometry. A comprehensive literature survey is included in order to enable comparison with obtained results from the visualization and thermal-hydraulic measurements as well as from the mathematical model. A separate part of the work describes the measurement procedure and results, comprising mentioned comparison as well as an analysis of the influence of the sine duct geometry and working conditions on the thermal-hydraulic characteristics of the flow through the sinusoidal corrugated chevron channel. A comprehensive literature survey proved there is a lack of systematized reliable data concerning thermal-hydraulic performances of plate heat exchangers (PHEs) as well as it indicated the uncertainties related to the complex flow mechanism involved in the mass, heat and momentum transfer in the corrugated chevron channels. The results from the visualization tests indicate simultaneous occurrence of the furrow and longitudinal component in 3D flow. Their relative size and influence on performances is dependent upon the angle, aspect ratio b/l and velocities, especially for the channels with higher angles (&#61538 ; ; >45o). The thermal-hydraulic tests in the channels with the angles 28o and 65o performed using fluids with significantly different viscosity, outlined the influence of the velocities on the flow pattern. Recorded different values at same Re numbers call for redefinition of the dimensionless groups involved in presentation of thermal-hydraulic characteristics. Significant decrease of the heat transfer at Re

This paper presents the optimized solution of a solar system for providing domestic hot water through the results of a system work simulation done on an hourly basis for a motel with capacity of 35 persons located in climate areas of Zagreb and Split. The analysed system is consisting of two tanks for domestic hot water, capacity 1600 litres each, and appropriate collector area (43, 7 m&sup2 ; ; for Zagreb and 30, 4 m&sup2 ; ; for Split). The basic criteria for optimization was the shortest investment payback period with a primary goal to cover the energy demands for domestic hot water in July and August. This paper presentes both monthly and yearly results of performed simulation for both climate areas and description of the calculation procedure for estimating solar radiation and energy gain.

In this paper, current advances in cooling techniques, and temperature control of photovoltaic (PV) panels in general, are analyzed and discussed. Namely, it is well known that a decrease in the panel temperature will lead to an increase in the electrical efficiency, so in recent years different cooling techniques have been proposed and tested experimentally. The efficiency drops with the rise in temperature, with a magnitude of approximately 0, 5%/°C. Several cooling techniques have been tried, mostly based on active water and air cooling, as these are the simplest techniques. Other cooling techniques include conductive cooling, phase-change material cooling, etc. Increase in electrical efficiency depends on cooling techniques, type and size of the module, geographical position and the season of the year, and usually corresponds with a rise of 3-5% in overall efficiency. Finally, a perspective on the other cooling techniques for PV panels will be also elaborated on and discussed in this paper.