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The share of solar thermal energy in the residential sector is increasingly stagnant compared to other forms of solar energy (e.g., photovoltaics). Further share increase might be achieved through improvement of thermal efficiency and/or a reduction in production costs. Nowadays, thermal performances of the mostly used state-of- the-art classic flat plate solar collectors have already reached high values and it is not expected that significant improvements of thermal performances can be achieved. For this reason, the development of solar technology is increasingly directed towards the use of polymeric materials in solar collectors to further decrease production costs. This paper presents evaluation of thermal characteristics of a novel prototype polymer solar collector design. The assessment of thermal characteristics on a segment of a polymer solar collector comprises experimental and numerical approach. Experimental part of research is performed using open test rig. Useful heat gain and efficiency factor is calculated for different inlet water temperatures. Functional dependence of efficiency factor on insolation, water and air temperature is computed. In order to validate numerical model developed in ANSYS FLUENT software package, simulations are firstly performed on a segment of a polymer solar collector and results are correlated against experimental ones. Afterwards, simulations are conducted for a whole collector that comprises 8 proposed segments and for such efficiency curve coefficients (η0, a1 and a2) are given. The estimated efficiency of proposed polymer collector design is 30-37% lower for typical summer operating regime compared to the state-of-the-art FPC. In addition, stagnation temperature measurements are performed according to EN ISO 9806:2017, when stagnation temperature of 125.1°C is determined after applying protective measures from overheating.

In the process of development of a new solar collector the creation of a prototype is a key step in verification of the new design and a basis for further improvements to the design. Due to the novel geometry and materials used in the extrusion process for the prototype collector’s parts and the inability to run the experimental extrusion in many iterations, the parts available for the prototype had significant deviations from the required tolerances and desired geometry. This presented a problem for the assembly of the prototype as the envisioned standardized fittings would not conform to the obtained geometry of the parts. It was therefore necessary to use 3D scanning to obtain the exact size and geometry of every single part with sufficient precision to enable 3D modelling of the necessary fittings required to assemble a prototype collector. Due to the application of this method, a prototype collector could be assembled using 3D printed fittings and tested. This paper provides an overview of the insights and practical considerations that were discovered in this process and suggests improvements in the process which could potentially save time and resources for future projects.

Phase change materials have been considered as a form of energy storage in buildings, but this research has mostly been limited to space heating applications. However, solar thermal applications operate at a higher temperature than is necessary for modern hydronic heating systems. This is both due to the need to achieve a set minimum temperature of domestic hot water in its storage tank to avoid health risks and by the high temperature of stagnation of solar collectors. In order to evaluate the applicability of various phase change materials as mediums for energy storage and as passive protection mechanisms for collectors in stagnation, an overview of available materials was made and individual materials were evaluated and ranked based on their performance in comparison to a classic purely hydronic system, as well as the economic feasibility of the proposed system. The performance of such a system was then simulated for a typical family house solar domestic hot water system.

Flat plate solar thermal collectors are a widespread solar thermal technology, but they can experience overheating during periods of high insolation and low demand for hot water. This imposes considerable limitations on materials that can be used in their production. While in some commercially available collectors the overheating problem is mitigated by employing design features which reduce the stagnation temperature of the collector, there is currently no method that has gained wide use or provided an effective solution for the problem. Therefore, there is still a need for a cost effective and preferably passive method of limiting or preventing overheating, which leads to the development of increasingly complex designs. For solutions which employ complex geometries an analysis of the optical properties and thermal characteristics of the collector is required. While the use of ray tracing computational tools in designing and analyzing concentrating collectors is now commonplace, no evidence of its use in flat plate collector design has been found in the literature. This is despite the fact that some new flat plate collectors are now already of such complex design which requires ray tracing analysis to enable reliable modeling of the thermal performances in a CFD software. To address this issue, this paper analyzes the viability of using ray tracing analysis for complex designs of different types of flat plate collector and develops a simulation model which is compared with the measurements on a model collector. The results of these simulations are then used as input values for CFD modeling of the collector thermal characteristics. The overall simulation results proved that ray tracing analysis yields reliable input values for computing thermal performances of flat plate solar collectors of both simple and complex designs.

Infracrvena termografija je beskontaktna metoda mjerenja i bilježenja temperature i njezine raspodjele na površinama objekata koje se bilježe infracrvenom kamerom. Nakon mjerenja ostaje trajan zapis o izmjerenim veličinama - termogram. Termografija i suvremeni termografski uređaji postigli su zavidnu brzinu razvoja i pružaju nove mogućnosti mjerenja temperature koja se mogu primjeniti u različitim područjima proizvodnje biljaka koja su povezana s promjenama temperature unutar ploda i krošnje. Upravo ta činjenica omogućuje primjenu infracrvene termografije u određivanju vodnog stresa u biljkama. U ovome radu provedeno je istraživanje na uzorcima presadnica industrijske rajčice (Lycopersicon ecvulentum Mill.) i paprike (Capiscum annuum L.). Mjerenja temperature su provedena termoparovima i infracrvenom termografskom kamerom. Analizirana je promjena temperature i indeksa vodnog stresa usjeva (CWSI) tijekom dana. Zaključeno je da je uz korištenje metode automatskog termografskog snimanja koja proizvodi termograme dovoljno visoke kvalitete, automatska obrada termograma za izračun CWSI pouzdano primjenjiva.

Praćenje ponašanja biljke i pouzdano određivanje potrebe za navodnjavanjem ekonomski je, agronomski i inženjerski izazov te je predmet istraživanja koja se temelje na mjerenju brojnih pokazatelja (temperatura lista, temperatura i vlažnost tla te temperatura i vlažnost zraka). U tom pogledu se infracrvenoj termografiji kao beskontaktnoj metodi mjerenja i bilježenja temperature i njezine raspodjele na površinama objekata, pridaje se sve veća pažnja. S obzirom da svako tijelo emitira infracrveno (IC) zračenje, ovisno o njegovoj temperaturi, postoje razlike vidljive IC kamerom što omogućuje primjenu infracrvene termografije u detekciji vodnog stresa biljaka. U ovome radu provedeno je istraživanje na presadnicama industrijske rajčice (Lycopersicon esculentum Mill.). Mjerenja temperature su provedena termoparovima i IC termografskom kamerom. Istovremeno su provedena i mjerenja vodnog potencijala lista (LWP). Analizirana je promjena temperature i indeksa vodnog stresa biljaka (CWSI) tijekom dana. Na osnovu vrijednosti CWSIa dobivenih pomoću više modela analize i mjerenih vrijednosti LWPa, izrađeno je više modela koji opisuju vezu tih dviju vrijednosti. Dobiveni modeli su pokazali statistički značajan stupanj korelacije između tih vrijednosti. Vrijednosti CWSIa dobivene ručnom obradom termograma su dale najveći stupanj korelacije, no i neke automatske metode obrade su pokazale rezultate usporedive s ručnima.

U ovome radu provedeno je istraživanje na uzorcima presadnica industrijske rajčice (Lycopersicon ecvulentum Mill.) i paprike (Capiscum annuum L.). Mjerenja temperature su provedena termoparovima i infracrvenom termografskom kamerom. Na uzorku rajčice su provedena i mjerenja bioloških svojstava biljke poput vodnog potencijala lista (LWP). Analizirana je promjena temperature i indeksa vodnog stresa usjeva (CWSI) tijekom dana. Na osnovu vrijednosti CWSIa dobivenih pomoću više modela analize i mjerenih vrijednosti LWPa napravljeno je više modela koji opisuju vezu tih dviju vrijednosti. Dobiveni modeli su pokazali statistički značajan stupanj korelacije između tih vrijednosti. Vrijednosti CWSIa dobivene ručnom obradom termograma su se dale najveći stupanj korelacije, no i neke automatske metode obrade su pokazale usporedive rezultate s ručnima. Zaključeno je da je uz korištenje metode automatskog termografskog snimanja koja proizvodi termograme dovoljno visoke kvalitete automatska obrada termograma za izračun CWSI pouzdano primjenjiva. Research was conducted on samples of tomato (Lycopersicon ecvulentum Mill.) and bell pepper (Capiscum annuum L.) plants. Temperature measurements were carried out using thermocouples and an infrared thermal imaging camera. Biological properties of the plant, such as the leaf water potential (LWP), were measured only on the sample of tomatoes. For both samples, an analysis of the change of temperature and crop water stress index (CWSI) during the day was conducted. For the tomato sample the obtained CWSI values and the measured LWP values were used to create multiple models describing their relationship. The obtained relationships showed a statistically significant degree of correlation. Values of CWSI obtained by manual analysis of thermal images produced the highest degree of correlation, but some of the automatic analysis methods produced results comparable to those obtained using the manual methods of analysis. It was concluded that if an automatic thermal imaging method which produced thermal images of sufficiently high quality is used, then the automatic method for the calculation of CWSI developed here is confidently applicable.

Ovaj rad opisuje proces tehničke konstrukcije i proračuna solarne dizalice topline za pripremu potrošne tople vode (PTV) u akumulacijskom spremniku. U sklopu rada su opisani princip rada i trenutna područja primjene u praksi solarnih dizalica topline. Zatim je napravljen termodinamički proračun lijevokretnog procesa za solarnu dizalicu topline koja može pripremati potrošnu toplu vodu u spremniku od 800 litara. Uz to je napravljen i termodinamički proračun izmjene topline u kondenzatoru, a koji je cijevna spirala unutar akumulacijskog spremnika potrošne tople vode. Usporedno s proračunom procesa solarne dizalice topline napravljen je i odabir potrebnih komponenti. Za navedenu dizalicu topline napravljen je simulacijski model pomoću kojega se može simulirati njen rad u različitim pogonskim uvjetima. Pri tome su za provedene simulacije korišteni podatci za referentnu godinu. This paper presents the design and analysis of a solar-assisted heat pump with direct expansion for heating of domestic hot water (DHW) in an accumulation tank. Within this paper the basic principles and the current applications of this technology are described. A solar-assited heat pump was then designed, complete with a calculation of its basic thermodynamic process (taking into account the required heating capacity and operating conditions) and a calculation of the heat exchange process in the spiral pipe (within the accumulation tank) which serves as the heat pump's condenser. Alongside the calculations the selection of appropriate parts was made. A simulation model was then developed in order to analyze the operation of such a system in various conditons. The simulation was conducted for a number of chosen dates within a reference year.