The present paper reports the experimental results and the potential performance of the investigation on flat solar thermal collectors using nanofluids as innovative heat transfer fluids for solar energy applications. The straight use of heat-transfer nanofluids in traditional solar flat panel revealed some technical issues, due to the nanoparticles sedimentation. Therefore, sedimentation has been investigated both in standard solar flat panels and modified ones made from transparent tubes. The results of the first tests showed that the main sedimentation parameter is the flow velocity and to better control it a standard flat panel was modified changing the cross-section of the lower and top header of the panel, that have been tapered to keep constant the fluid axial velocity. The modification of the panel shape (patent pending) enabled a negligible particles deposit. After different nanofluids were tested on the panel prototype, water–Al2O3 was chosen as heat transfer fluid. All tested nanofluids were prepared in batch and their thermal conductivity and convective heat transfer coefficient were measured prior of their use as heat transfer fluid in the solar panel. A thermal conductivity enhancement up to 6.7% at a concentration of 3 vol% was observed, while the convective heat transfer coefficient increased up to 25%.

In the present work a systematic investigation on several mechanisms affecting the thermal conductivity of Alumina based nanofluid, such as layering, Brownian motion, clustering, ballistic phonon motion, thermal boundary resistance and mass difference scattering, is presented. The effect of mass difference scattering is for the first time suggested and studied in the present work. Both theoretical and experi- mental approaches have been carried out in order to analyze the competition of these phenomena and to identify the most relevant. This was obtained by comparing micrometric and nanometric particles suspended in liquid water (293 K), frozen water (253 K) and diathermic oil (293 K). Each of the above- mentioned conditions was selected to make dominant only one of the mechanisms that affect nanofluid thermal conductivity. The main results of this investigation concern the mass difference scattering, which has been found to be the most intensive mechanism reducing the nanofluid thermal conductivity with respect to the microfluid one.

In this paper a critical investigation of layering phenomenon has been carried out, by means of experimental and numerical analyses, to explain the differences in thermal conductivity between nanofluids based on metal (Cu) and metal oxide (CuO) nanoparticles. Particularly, molecular dynamics simulations have been developed to investigate the adsorption of water molecules surrounding Cu and CuO nanoparticles of various sizes. Furthermore, different volume concentrations of nanoparticles in water have been analyzed. The numerical results revealed two shell-like formations of water molecules (layers) close to the Cu nanoparticle surface, differently from CuO nanoparticle, where no significant layering phenomenon has been observed. This result can explain the higher thermal conductivity of Cu-based nanofluid with respect to CuO-based one, which has been experimentally measured. The numerical and experimental results lead to the conclusion that layers of ordered water molecules surrounding metal nanoparticles play an important role in explaining experimental data of nanofluid thermal conductivity.

This work deals with the efficiency and the energy behavior of Ground Source Heat Pumps (GSHPs) used for heating and cooling of buildings. In particular horizontal type heat exchangers have been investigated for different configurations, in order to evaluate the characteristics of these systems in the most common layouts and in different working conditions. The main results pointed out the heat fluxes transferred to and from the ground and the efficiency of the system. The calculations were made with the CFD code Fluent and the simulations covered one year of system operation, both in summer and winter for typical climate conditions of the South of Italy. The most important parameter for the heat transfer performance of the system resulted the thermal conductivity of the ground around the heat exchanger and the optimal ground type was that with the highest thermal conductivity (3 W/m K in the cases analyzed in this work). The choice of the velocity of the heat transfer fluid inside the tubes was another key factor. The depth of installation of the horizontal ground heat exchangers did not play an important role on the system performance. The helical heat exchanger arrangement resulted as the best performing one.

Cooling of electronic devices is one of the main challenge of new generation technology. The extreme miniaturization has high benefits, but the heat to be dissipated per unit of surface increases in uncontrolled way. In this paper the application of a new generation of heat transfer fluids, nanofluids, to electronic devices is analyzed. Even if the use of nanofluids is not still common, there are many papers that deal with this topic, reporting both experimental and theoretical results. The development of this technology could be one of the key elements that could give an important impulse to further miniaturization of electronic devices and at the same time increase their energy efficiency.

This paper describes a programmable electronic system for controlling the environmental parameters and managing the electrical functions of a civil/industrial thermo-solar plant. The device acquires data from temperature and light sensors, processes these information and commands external equipments (pumps, electric valves and power supplies) with dedicated relay outputs for the optimization of plant performances in order to maximize efficiency and energy saving. Recently several researches, in the field of solar thermal energy production, have demonstrated that nanofluid-based solar collectors present higher conversion efficiency. In this context, the designed control unit can be used to detect physical parameters in order to compare to monitor, at the same time, the two different types of solar collector in similar environmental conditions and to show on touch screen display the detected performances.

The paper analyzes data about recharge of electric cars in Rome during 2013 as a part of a national research project (P.R.I.M.E.). The electric vehicles were recharged through the public Enel Distribuzione recharging infrastructure. For each recharge, the initial and final time were registered together with the electricity absorbed from the grid. The total number of recharges was about 7700. The first step of the investigation is the statistical analysis of the distribution of recharges in daily time slots in order to analyze the recharge behavior of Italian drivers. For each day and for each time slot, literature data from the Italian national grid operator (Terna) were used to retrieve the energy mix used to produce electricity in that day and in that time slot. In the third step, electricity generation mixes were used to obtain emission factors for greenhouse (CO2) and pollutant emissions (CO, NOx, HC and particulate). Using information about the electric consumption of vehicles registered in Rome, the emission factors in g/km were obtained and compared with the limits set by European legislation for conventional (gasoline and diesel).

An experimental study on new high temperature parabolic trough collector (PTC), with transparent receiver tube, based on gas-phase nanofluid, has been carried out for the first time in this work. Two-axes solar tracking PTC, with 4 m2 reflecting surface has been realized. Besides, two coaxial quartz tubes, with vacuum in the inner space were used as receiver pipe, with air-dispersed CuO nano-powders as working fluid. The aim of this work was to investigate the technological issues related to the use of gas-based nanofluid coupled with transparent quartz receiver and to evaluate the performance of the first prototype, comparing numerical and experimental results. The experimental campaign highlighted a critical issue related to nanopowder deposition within the receiver pipe, due to humidity. Moreover, in a day of measurement, the fluid temperature higher than 145 °C has been maintained for about 10 h, reaching a maximum value of 180 °C, with a mean efficiency of about 65%.

Global biodiesel production grew by 23% per year between 2005 and 2015, leading to a very strong expansion of the sector in a decade and, at the same time, the interest in the use of liquid biofuels/biodiesel in compression ignition engines has grown quickly. Taking into account that the use of biodiesel in IC engines directly affects their coolant temperature, with impact on performance, in this study an experimental campaign has been carried out on a 4-strokes single cylinder engine, aimed to assess whether the use of nanofluids, instead of water, could be a valuable solution to reduce peak engine temperature. Such nanofluids were characterized by higher thermal conductivity compared to conventional fluids, due to CuO nanoparticles added at different concentrations within the base fluid. Measurements of temperature were recorded at steady and unsteady conditions, by proper thermocouples located around the exhaust valve seat in the cylinder head and in the exhaust valve spindle. Particularly, temperatures of the exhaust valve spindle and exhaust valve seat in the cylinder head were measured at part and full engine loads, using water as coolant and then CuO based nanofluids. Experimental results showed that, at 100% engine load in unsteady conditions, it was possible to achieve a temperature reduction up to 13.6% on the exhaust valve seat and up to 4.1% on the exhaust valve spindle, when nanofluid at 2.5% volume concentration was used.

Nanofluids belong to a new generation of heat transfer fluids. Their thermal properties make them suitable to be employed in high-performance energy systems. In this paper a new setup for investigating the interactions between microwaves and nanofluids is presented. This is a new issue in this field and only one other experimental campaign has been carried out in the scientific world so far. The design of this experimental setup together with the preliminary results on two different water-based nanofluids (Al2O3 and CuO nanofluids) opens a new frontier in the field of heat transfer in nanofluids.

In this study, a modified flat panel solar thermal collector was built and thermal efficiency was measured with two heat transfer fluids: distillated water and Al2O3–distillated water based nanofluid at high concentration (3.0%) volume fraction of solid phase. In this work for the first time nanofluid with high nanoparticle concentration has been used thanks to a modified solar thermal collector, based on patent WO2011138752 A1, which consists in bottom and top headers properly shaped in order to reduce sedimentation of clusters of nanoparticles. Thermal efficiency has been measured through an experimental setup, according to EN 12975-2 standard. Experimental results showed that an increase of thermal efficiency up to 11.7% compared to that measured with water has been obtained by using nanofluid. Besides effect of nanofluid on thermal efficiency is greater at high temperatures.

The work reported in this paper shows experimental results from a study on a new type of heat transfer fluid, nanofluids, using diathermic oil as base fluid. These kinds of heat transfer fluids find application in those areas of heat transfer where high efficiency, compact volumes and high energy fluxes are required. In literature there are not many experimental data on diathermic oil based nanofluids because many experimental campaigns are focused on water nanofluids. On the other hand diathermic oil nanofluids are very important in those applications where high temperatures are reached or where the use of water is not suitable. Samples of nanofluids, with nanoparticles of CuO, Al2O3, ZnO and Cu, having different shapes and concentrations varying from 0.0% up to 3.0%, have been produced and their thermal conductivity has been measured by means of hot-wire technique, according to the standard ASTM D 2717-95. Measurements were carried out to investigate the effects of volume fraction, particle size of nanoparticles and thermal conductivity of base fluid. The effect of temperature has been also investigated in the range 20°C - 60°C. A dependence was observed on the measured parameters and the results showed that the heat transfer performance of diathermic oil enhances more than water with the same nanoparticles.

Basi della termodinamica - Cicli termodinamici - Aria umida - Trasmissione del calore

Greenhouse farming, where energy con- sumptions are mainly related to the greenhouses heating, is one of the sectors consuming the most energy in the agricultural industry. High costs and the uncertain availability of fossil fuels constrain the use of heating applications. Among possible solutions, the utilization of renewable heating systems such as geothermal energy through ground-source heat pump systems (GSHPs) at competitive prices has to be taken in consideration. The competitiveness of these systems depends mainly on the characteristics of the end-users, i.e., the annual heating loads. Few studies focusing on the potential of using these systems start with an analysis of the thermal re- quirements and end with a cost evaluation in tune with local assets, geo-climatic conditions, and landscape pro- tection. This paper analyzes the greenhouse crop indus- try in the Apulia region in southern Italy, as a potential end-user of GSHP systems. Data collected from an area mainly devoted to greenhouse crop production have been used to (a) describe greenhouse farms, (b) define the heating requirements of a greenhouse model repre- sentative of the most used typology in the investigated area, and (c) examine the economic viability of green- house heating with GSHP systems. Both vertical and horizontal ground heat exchanger (GHE) configurations are compared with conventional fossil-fuel heating sys- tems. In all scenarios considered, the observed payback periods appear reasonable and worthy of consideration. The results suggest that these technologies can fully satisfy the winter heating requirements in a cost- effective way and they can support the planning of measures aimed to improve the sector competitiveness.

The efficiency of cooling system is critical for wind turbines, particularly during the hot season, when high temperatures could damage the electric generator and mechanical parts of the turbine. The cooling system proposed in this paper is able to increase the efficiency of heat transfer with the use of nanofluids and the wind turbine tower as a heat exchanger to dissipate waste heat in the environment. In this study the use of Al2O3-water nanofluids has been considered. The results of this investigation appear encouraging because they have shown that the proposed new solution is able to assure highly efficient heat transfer and to limit thermal stresses on the electrical and mechanical components of wind turbines.

Facendo uso dell’ambiente di programmazione TRNSYS, si è condotta un’analisi sulle prestazioni di impianti integrati solare-geotermico utilizzati per la produzione di acqua sanitaria e per la climatizzazione invernale. Il caso di studio di partenza è quello di un’abitazione mono-familiare sita a Brindisi. Creati e testati, dapprima in maniera separata, i modelli di impianti solare e geotermico sono, poi, stati integrati nell’ottica di utilizzare l’acqua calda proveniente dal collettore solare termico nel circuito idronico di scambio a terreno della pompa di calore per mantenerne alta la temperatura di evaporazione. Per questa applicazione, i miglioramenti in termini di consumi e di efficienza dell’impianto prodotti con l’approccio integrato si sono rivelati più ridotti delle attese. Gli studi hanno, in seguito, riguardato le prestazioni dell’impianto integrato in diverse configurazioni caratterizzate da differenti proporzioni tra la superficie dei collettori solari e l’accumulo termico. Lo stesso modello è stato poi applicato a utenze del tipo ufficio con risultati più incoraggianti.

In dual-fuel engines, a combustible mixture of air and generally a gaseous fuel is ignited, thanks to the injection and autoignition 6 of a small amount of liquid fuel. It is well-known that dual-fuel engines suffer from poor combustion when operated at low loads. This 7 behavior, due mainly to the presence of an overlean mixture into the combustion chamber, leads to unacceptably high levels of carbon 8 monoxide and unburned hydrocarbons emitted at the exhaust. In order to solve this problem a possible solution could be to split the pilot 9 injection of liquid fuel into two split injections, the second having the function of boosting the combustion of gaseous fuel also during the late 10 combustion phase. In this paper this solution has been implemented on a diesel common rail single cylinder research engine converted to 11 operate in dual-fuel mode. The composition of the gaseous fuel, indirectly injected, simulated a typical producer gas. The liquid fuel used 12 during the experiments was biodiesel, injected by means of a common rail injection system. The first section of results describes the tests run 13 for comparison purposes, performing only one pilot biodiesel injection and varying its timing on a wide range. The second section of results 14 then presents the tests run for different timings, varied on a wide range, of the first split injection, and different dwells between the first and the 15 second injections. The engine behavior has been discussed in terms of heat release rate, fuel conversion efficiency, and nitric oxides, total 16 hydrocarbons, and carbon monoxide emission levels at the exhaust. The results demonstrate that splitting the pilot injection leads to an 17 increase of fuel conversion efficiency and a reduction of both total hydrocarbons and carbon monoxide. This final result allows to state 18 that splitting the pilot injection is an effective way for sustaining the gaseous fuel combustion in dual-fuel engine late during the combustion 19 phase.

An overview of innovations introduced in the field of flat solar thermal collectors is presented using information from different literature sources. Despite the large number of publications available about this specific issue, this review is focused on the last ten years period, to underline the actual trend of the scientific and technologic world so far. Bearing in mind this choice, in this study a selection of the most valuable papers has been done, considering different points of view and aspects. Even if this work cannot be considered exhaustive of the complete literature about this field, it can be taken into account as a quick reference to have an overview about new materials, geometries, heat transfer fluids etc., available and tested in the last decade.

Aim of this work is the design of a programmable electronic system for monitoring the environmental parameters and managing the electrical functions of a thermo-solar plant. The designed control unit detects data from temperature and light sensors, processes acquired information and commands external equipments (pumps, electric valves and power supplies) in order to optimize plant performances and maximize efficiency and energy savings. Recently several researches, in the field of solar thermal energy production, have demonstrated that nanofluid-based solar collectors present higher conversion efficiency. In this context, the designed control unit can be used to detect their operation parameters in order to compare the performances of nanofluid- based solar collector with those of traditional one. The electronic experimental setup is capable to monitor, at the same time, the two different types of solar collector in similar environmental conditions and to show on touch screen display the detected performances. In order to have reference data, experimental measurements have been carried out by using traditional water and Al2O3–based nanofluid thermo solar collectors. The obtained experimental data showed the benefit in terms of efficiency in the use of nanofluid as heat transfer fluid in such a system.

A new model of solar reactor based on a double-loop fluidized bed involving CeO2 nanoparticles and two gas streams, N2 and CO2, for efficient thermochemical fuel production, is presented. The fluidized bed reactors are commonly used to carry out a variety of chemical reactions, due to solid granular materials, which play the fundamental role of catalyst. In the system under investigation, the overall reaction CO2→CO+1⁄2O2 is achieved, by means of a thermochemical two-step cycle, based on CeO2 nanoparticles. The first step (CeO2 thermal reduction) has been implemented with a solar-driven endothermic dissociation of the metal oxide to lower- valence metal-oxide. The second step (CO2 splitting) has been carried out with an exothermic oxidation of the reduced metal-oxide, which is produced in the first step, to form CO. The use of nanoparticles as catalyst allows maximizing the surface area of reaction, and at the same time, the reactor based on double-loop fluidized bed allows continuous operation, without alternating flows of inert sweep gas and CO2. The thermodynamic analysis of the system under investigation showed a calculated maximum ideal efficiency of about 63%.

È oggetto di questo studio l’elaborazione di un modello matematico sviluppato in ambiente Mathcad in grado di simulare il funzionamento di scrubber verticali ad acqua in controcorrente (torri di lavaggio) per la pulizia di syngas da gassificazione di biomasse lignocellulosiche, ai fini del suo utilizzo in un motore a combustione interna. Il modello calcola il numero di stadi necessario a soddisfare le specifiche prestazionali dello scrubber in termini di efficienza di abbattimento del particolato e di raffreddamento della corrente gassosa e fornisce indicazioni progettuali sulla scelta dei parametri geometrici e funzionali. La variazione imposta dei parametri funzionali consente, poi, di condurre un’analisi dell’output progettuale, in particolare al variare del rapporto tra portata d’acqua immessa e polverizzata e portata di syngas, della dimensione media dei contaminanti da rimuovere e della temperatura di ingresso nello scrubber.

In this paper, the design of a double-loop fluidized bed solar reactor, involving CeO2 nanoparticles and two gas streams of N2 and CO2, for efficient thermochemical fuel production, has been optimized in a six-dimensional parameter space by means of a multi-parameter optimization algorithm. The system under investigation is capable to develop a thermochemical two-step cycle, producing CO by means of the overall reaction CO2→CO+1/2O2. The use of nanoparticles as catalyst allows maximizing the performance of the reactor; actually, nanoparticles increase surface area of reaction, with respect to common catalysts and, at the same time, allow realizing the reactor as double-loop fluidized bed, which can operate without alternating flows of CO2 and inert sweep gas. A genetic algorithm coupled with a quasi-random Sobol design population has been used, to find the optimal configuration of the double-loop fluidized bed solar reactor. The results highlighted the very important role of several factors, as radius of fluidized beds, mean residence time of reactor, mass of nanoparticles within reactor, solar concentration ratio, etc., on the performance of the system under investigation and allowed to find the best configuration of the system, reaching the mean global efficiency over a period of time of 1 year equal to 29.96%, with a maximum of 59.46%.

Air conditioning is one of the areas that has a high electrical energy consumption, mainly during summer, in hot and humid climates. The major part of the air conditioning systems are based on the vapor compression cycle, but in the last decades solar cooling technology focused the interest of the scientific community and industrial world. Solar cooling deals with a heat driven cycle for cold production. This technology is well represented by absorption refrigerators and desiccant cooling systems. However, in hot and humid climates the latest cycles are not well developed, therefore the systems based on these cycles cannot face the full cooling load needed by the utilizer. Efforts have to be done in order to change their configuration and improve their efficiency. The aim of this paper is to propose new configurations for solar cooling systems and their adaptation to hot and humid climates.

A numerical method, named WEST (Wind Energy Study of Territory), has been developed and applied to a specific geographical area in south of Italy. This method, through actual historical meteorological and geophysical data of a territory, allows characterizing anemometric fields and, therefore, potential available wind power. WEST has been developed in such a way to be effective in both studies of large area and siting. Particularly, this method is composed by different calculation algorithms, which altogether constitutes the numerical model, which allow obtaining useful information on the technical feasibility of installing wind turbine in an area. In this work, by means of WEST, three-dimensional wind fields of Apulia Region (Italy) have been reconstructed, obtaining the wind power density maps at several heights: 35 m, 60 m, 80 m and 100 m above ground level.

Investigations on the potential thermal efficiency of an innovative nanofluid solar thermal collector have been performed using a commercial software (RadTherm ThermoAnalytics rel. 10.5). The Al2O3-nanofluid has been simulated as working fluid of the solar thermal collector, varying the nanoparticles concentration from 0%vol of Al2O3 nanoparticles (pure water) up to 3%vol of Al2O3 of nanoparticles. The numerical model has been validated with experimental data, obtained with a real prototype of the simulated solar thermal collector. Real thermal properties of the nanofluids at different concentrations have been used in the simulations. The boundary conditions used for the simulations have been those of real weather conditions. An increase in thermal efficiency (up to 7.54%) has been calculated using nanofluid with a volume fraction of 3% and the influence of nanoparticles concentration on the thermal performance of the solar collector has been pointed out.

Lubrication of large two stroke marine diesel engines typically is performed by specially blended lubricants with high CaCO3 concentration in order to prevent sulphuric acid corrosion. The feed rate of lubricant, which is injected into the engine, is strictly related to neutralization reaction of sulphuric acid. At the state of the art, its amount is established following a function of engine load and sulphur content of fuel oil, but regardless the stoichiometric quantity needed to neutralize acid corrosion effects. As result of this lubrication strategy, feed rate of lubricant often results higher than the minimum stoichiometric quantity, yielding unnecessary costs, but sometimes feed rate of lubricant and its content of CaCO3 cannot be enough to completely neutralize sulphuric acid, producing corrosion. Taking into account that concentration of CaCO3 within lube oil can be estimated by measuring refractive index, this work aimed to study SPR sensors, capable to measure in real time small variation of lubricant optical properties, in order to adjust lubricant feed rate, according to the real needs of neutralization. Therefore, a numerical optimization of SPR sensors for lube oil characterization has been carried out, analysing several cases, different for laser source, optical prism and thickness of 3 metal film layers. Mathematical results allowed to find the best sensor in terms of sensitivity. This work is the first step towards the development of a semi-closed loop lubrication control system.

A promising new generation of solar thermal collector able to enhance the thermal efficiency is the DASC (Direct Absorber Solar Collector). In this paper we report optical absorption measurements performed on several water-based nanofluids (Al2O3, CuO, TiO2, ZnO, CeO2, and Fe2O3) as a function of nanoparticles concentration. These measurements are of fundamental importance to assess the possibility to use the above mentioned metal-oxide nanoparticles in liquid-based nanofluids for direct absorption low temperature flat panel solar collector. The obtained results show different optical behaviors of the nanofluids depending on nanoparticles material and concentration. In all measurements the transmittance rises passing from visible to infrared region and in some cases, when the nanoparticles concentration is too low, the extinction distance grows up to values larger than the typical diameter of a solar receiver.

The use of nanofluids as working fluids in direct absorption solar collector is growing up and the study of optical properties of nanoparticles is an important step for the success of this new technology. In this paper we report optical absorption measurements performed on several metal oxide nanoparticles (ZnO, CeO2, Fe2O3) as a function of temperature in the range 25–500 °C, in order to study their optical properties, and to investigate how several heating cycles could affect nanoparticle structural stability and absorption characteristics. These are quite important issues to be investigated in order to assess the possibility to use such metal-oxide nanoparticles as gas-based high temperature nanofluid in Concentrated Solar Power (CSP).

Artificial drying, using industrial devices (dryers), helps to reduce the residual humidity content in biomass in a relatively short time. Convection is one of the most common mode of drying (referred to as direct drying). Heat is supplied by hot air/gas flowing over the surface of the solid. The heat for evaporation is supplied by convection to the exposed surface of the material; the evaporated humidity is carried away by the drying fluid. Indirect dryers (working by conduction) are more appropriate for particulate and granular materials or for very wet solids; while radiative dryers use various sources of electromagnetic radiation with wavelengths ranging from the infrared to microwaves. In this work, two mathematical models of cross flow and rotary dryers (both convective dryers) have been proposed. Both dryers treat wood chips. The two models allow to calculate the thermal efficiency and residence time of wet solid wood chips, as a function of the residual moisture content, as well as the analysis of the behaviour of the outlet wet solid and drying gas, in consideration of the dryer length and of the feeding material conditions in the dryer. The models have been developed in the Mathcad software environment.

A thermal analysis of a new photovoltaic–thermal (PV–T) solar panel design, called thermal electric solar panel integration (TESPI), has been performed using radtherm thermoanalitics software. Combinations of different water flow rates and different panel configurations have been analyzed to determine which one produces best performance in terms of optimal PV efficiency and available thermal energy. Higher total panel efficiencies (thermal and electrical) were achieved in configurations utilizing the highest water flow rates, independently from the chosen configuration. However, high water flow rates translated into minimal net temperature differences between the PV/T panel inlet and outlet.

The work reported in this paper shows the experimental results from a study on diathermic oil based nanofluids. Diathermic oil finds application in renewable energy, cogeneration and cooling systems. For example, it is used in solar thermodynamic or biomass plants, where high efficiency, compact volumes and high energy fluxes are required. Besides diathermic oil is very important in those applications where high temperatures are reached or where the use of water or vapor is not suitable. Therefore an improvement of diathermic oil thermo-physical properties, by using of nanoparticles, can increase the performance of the systems. In literature there are not many experimental data on diathermic oil based nanofluids because many experimental campaigns are focused on water nanofluids. Samples of nanofluids, with nanoparticles of CuO, Al2O3, ZnO and Cu, having different shapes and concentrations varying from 0.0% up to 3.0%, have been produced and their thermal conductivity has been measured by means of hot-wire technique, according to the standard ASTM D 2717-95. Measurements were carried out to investigate the effects of volume fraction, particle size of nanoparticles on the thermal conductivity of the nanofluid. The effect of temperature has been also investigated in the range 20°C - 60°C. A dependence was observed on the measured parameters and the results showed that the heat transfer performance of diathermic oil enhances more than water with the same nanoparticles.

An analytical overview of experimental results about the heat transfer capabilities of nanofluids is presented, using widely scattered available information from diverse literature sources. It is shown that, despite the large number of publications available about this issue, only few studies provide quantitative estimates on a complete set of experimental conditions so far and many studies are not coherent. Bearing in mind this problem, in this study a selection of the most valuable papers has been done, taking into account different points of view and hypotheses. Even if this work cannot be considered exhaustive of the complete literature in the field of nanofluids, it can be taken into account as a quick reference guide to have an overview of the different heat transfer phenomena in nanofluids and how the most important parameters (size, shape, concentration, materials etc.) influence the expected thermal performance of nanofluids.

The interest in the absorption refrigeration systems continuously grows in many applications as they can exploit either waste heat or both traditional fuels and renewables (natural gas, sun, geothermal, biomass) as primary energy. Among several configurations of absorption cycles the GAX (Generator/Absorber/heat eXchanger) one plays an important role and allows to achieve good energy efficiencies of energy utilization with no relevant increase in system complexity. Here the analytical models of both GAX and GAX Hybrid (GAX-H) absorption cycles using ammonia–water mixture as working fluid are presented. Developed in Matlab R2010a, they simulate mass and energy conservation laws in each system component. The thermodynamic properties of ammonia-water solution are calculated based on the equations by Ziegler and Trepp, on the equations of state by Schultz and on the explicit relations for the bubble point and the dew point temperatures by El-Sayed and Tribus. The developed models were run to investigate the influence on the overall cycles’ performance of the generator, the condenser and the evaporator temperatures, of the absorber pressure and of the heat exchanger effectiveness. Results show the general trends of the efficiency of the cycles and allow the comparison between GAX-H and GAX cycles. When a pressure ratio of 1.5 between the absorber and evaporator pressure is maintained, the GAX-H COP (Coefficient Of Performance) is 50% and 32% higher than the conventional GAX cycle one operating with a degassing range of 0.2, 0.3 respectively.

Nanofluids have excellent potentiality in the field of heat transfer fluids and particularly for solar energy systems such as concentrated solar power plants. However they present many issues to be fixed in order to have a large diffusion. One of these is sedimentation. In this paper, stability, viscosity, FT-IR spectra, cluster size and thermal conductivity of Al2O3 – Therminol nanofluids have been investigated as heat transfer fluid in high temperature solar energy systems. Al2O3 – Therminol nanofluids have been prepared to investigate and to improve stability of the suspensions, varying temperature during mixing with magnetic stirrer, amount of surfactant and sonication time with ultrasonic vibrator. Stability of the nanofluid samples was investigated through backscattering technique and for cluster size analysis Dynamic Light Scattering (DLS) was used. Thermal conductivity of the sample was measured in order to evaluate not only the effect of both volume fraction and temperature, but also the influence of the surfactant (oleic acid). Stability of nanofluids depends on temperature during sample preparation and sedimentation phenomenon is inversely proportional to temperature during mixing with magnetic stirrer. Influence of concentration of surfactants was studied through preparation of samples having a solid phase particles concentration of 0.3 %vol, 0.7 %vol and 1.0 %vol, respectively. The presence of surfactants creates some bonds with nanoparticles, which mainly helps nanofluids long-term stability. On the other hand, the presence of surfactants inside the nanofluids does not influence their thermal conductivity. From DLS measurements, a dependence of cluster size on volume fraction was observed for all nanofluid samples. Experimental data show: viscosity increases by increasing volume concentration; nanofluids with and without surfactants show a non-Newtonian behavior and viscosity of nanofluids increases by increasing cluster size.

Obiettivo del lavoro è l’analisi del fenomeno della sedimentazione dei nanofluidi all’interno di pannelli solari piani, finalizzata ad adottare gli accorgimenti necessari alla sua attenuazione o alla sua completa eliminazione. Lo studio è condotto su pannelli a tubi trasparenti attraverso analisi ottica. In una fase iniziale si è realizzato un pannello solare piano a tubi trasparenti, con le dimensioni di un classico pannello disponibile in commercio, all’interno del quale si è fatto fluire un nanofluido a base di acqua e ossido di alluminio (Al2O3). Le zone interessate dalla sedimentazione della fase solida erano il tubo collettore di ingresso, quello collettore di uscita e i tubi traversi La quantità di fase solida depositata è risultata essere dipendente dalla velocità del nanofluido nei tubi di ingresso e di uscita. Nella fase successiva si è realizzato un pannello solare piano, delle stesse dimensioni del precedente, con una modifica sui tubi di ingresso e di uscita, volta a eliminare il fenomeno della sedimentazione per effetto di una opportuna variazione di velocità del nanofluido. La forma era tale da garantire una velocità costante lungo tutta la loro lunghezza. Essa è stata ottenuta agendo sulla sezione di passaggio all’interno dei tubi attraverso l’introduzione di un solido opportunamente sagomato. Il nanofluido utilizzato era a base di acqua e Al2O3 ed era alimentato nelle stesse condizioni adottate per il pannello della prima fase. Per effetto delle modifiche apportate il fenomeno di sedimentazione si è ridotto a livelli trascurabili e, in certi punti, è stato completamente eliminato. Il pannello solare modificato, dotato di tubi a sezione variabile, è stato oggetto di domanda di brevetto per invenzione industriale depositata all’Ufficio Italiano Brevetti e Marchi (N° LE2010A000006) e successivamente è stata depositata anche la domanda di brevetto internazionale (Application number: PCT/IB2011/051988).

Le pompe di calore geotermiche hanno avuto in Italia nell’ultimo decennio sviluppo e diffusione grazie soprattutto al fatto che consentono consistenti risparmi energetici quando utilizzate per soddisfare il fabbisogno di climatizzazione estiva e invernale degli edifici. In questo lavoro vengono analizzati i diversi modelli matematici presenti in letteratura per il calcolo delle prestazioni delle pompe di calore dotate di scambiatori orizzontali e verticali e per le relative metodologie di dimensionamento semplificate. A seguito dei risultati di questa analisi è stato concepito e implementato un modello matematico in ambiente Matlab-Simulink che integra le metodologie per la progettazione dei sistemi geotermici verticali e orizzontali e valuta le potenze termiche scambiate con il terreno correggendo i valori tabellari di riferimento e largamente in uso nella progettazione semplificata degli impianti di piccola taglia, in funzione delle particolari condizioni operative. Il modello è tarato con il riferimento a risultati sperimentali e ai valori in uscita dalle procedure di calcolo del software RETScreen.

Solar concentration system with thermo-vector fluid made up of gas-based nanofluids and with suitably shaped receiver element of the solar radiation.

Solar concentration system with thermo-vector fluid made up of nanofluids and with suitably shaped receiver element of the solar radiation.

Solar collector for applications with nanofluids or biphasic heat transfer fluids, including both a top pipe ( 1CT) and a bottom pipe ( 1CB ) wherein at least one of these pipes has a variable cross section in order to avoid sedimentation of nanofluids and other biphasic heat transfer fluids that flow inside the tubes of the solar collector itself.