Solar energy utilization is vital for our society which faces the global warming problem and the increase in the prize of fossil fuels. Concentrating solar collectors are a promising technology for producing useful energy from the sun. In this study, an innovative concentrating collector with a paraboloid dish reflector is presented and simulated. The main idea of this collector is the use of a dish reflector for continuous heat production with a high concentration ratio. For this reason, a spherical receiver absorber with two linear insulated ducts is used. The cost of this system is relative low because there is not a glass cover but a tracking system is required. The final results show better performance than the typical collectors such as the flat plate collectors and the evacuated tube collectors, fact that makes this collector a promising solution. The design and the simulation are carried out with the commercial software Solidworks, in the flow simulation studio.

Concentrating solar collectors are proven to be efficient solutions for heat production in the order of 150-400 degrees C. These collectors utilize tracking reflective surfaces and employ line focus concentration. This paper deals with the optical and thermal investigation of a linear Fresnel collector with trapezoidal cavity receiver. The optical analysis was performed through the development of a ray-tracing model and showed the distribution of the solar radiation onto the absorber perimeter. The effect of the slope error is investigated parametrically. The peak instantaneous efficiency was calculated at 0.693, the concentration ratio was found 10.9 and the ground coverage 76.2%. The thermal analysis was performed through a CFD model. The simulation results showed that the absorber heat losses ranged from 181.2 W/m to 986 W/m, for inlet fluid temperatures from 150 degrees C to 375 degrees C, respectively. The heat losses from the receiver, absorber and glass cover were also calculated. Temperature distributions in the receiver components were extensively discussed.

•A solar driven underfloor heating system is investigated in this work.•The system is examined with and without phase change materials on the floor.•Different kinds of solar collectors are coupled to the heat pump.•The best configuration includes phase change materials and thermal PV collectors.•The electricity consumption can be reduced from 42% up to 67%.

The performance characteristics of an aircraft piston engine are affected mainly by the air–fuel mixture quality (i.e. condition of the fuel injection system) and by the spark timing and spark duration (i.e. condition of ignition system). Thus, the present work focuses on investigating the effect of both fuel injection and spark ignition systems on performance characteristics of two aircraft piston engines which are of the same type but have overhauled by two different workshops. The investigation is conducted by applying an existing diagnostic technique, which is based on the simultaneous recording and processing of two electric signals: one corresponding to cylinder pressure and the second corresponding to the ignition system. The basic characteristics of the proposed methodology are simplicity and field applicability on engines of this type. A detailed experimental investigation has been conducted on the aforementioned two aircraft piston engines on a dedicated test bench. From the results, it is revealed that the proposed diagnostic methodology provides reliable information for the effect of both the ignition and fuel injection systems on engine performance characteristics. The results derived from the specific work enable the comparative evaluation of the engines and their ignition and fuel injection systems. Finally, based on this first investigation, the proposed methodology seems to be promising, because it can be easily applied on any type of spark-ignited engine and especially on aircraft piston engine, where due to its geometry and multicylinder nature, the application of lab techniques on the field is, if not impossible, extremely difficult.

Over the last years, the thermal comfort level during the summer period has been significantly increased due to the use of conventional cooling and air-conditioning systems, leading to higher electricity consumption. Solar cooling systems may provide the solution and become the leading technology in the future. The aim of this paper is to present a small-scale solar thermal system for cooling an office building in Athens, Greece. The study documents the system design, the monitoring procedure and equipment, and presents the experimental results from the first complete summer period. The daily electrical coefficient of performance (COP) of the absorption chiller of 48.6 and the electrical COP of the solar system 10.9 indicate the potential of solar cooling in small-scale systems.

Parabolic trough collectors are the most mature technology for utilizing the solar energy in high temperature applications. The objective of this study is the thermal efficiency enhancement of the commercial parabolic collector IST-PTC by increasing the convective heat transfer coefficient between the working fluid and the absorber. There are two main factors which influence on this parameter, the working fluid type and the absorber geometry. For this reason three working fluids are investigated, thermal oil, thermal oil with nanoparticles and pressurized water. Moreover, a dimpled absorber tube with sine geometry is tested because this shape increases the heat transfer surface and increases the turbulence in the flow. The final results show that these two techniques improve the heat transfer coefficient and the thermal efficiency of the collector. More specifically, the use of nanofluids increases the collector efficiency by 4.25% while the geometry improvement increases the efficiency by 4.55%. Furthermore, collector parameters such as the heat loss coefficient, the exergetic efficiency, the pressure losses and the absorber temperature are presented for all the examined cases. The model is designed with Solidworks and is simulated by its flow simulation studio.

The energy performance of buildings can be estimated using various software which use different models for this simulation. It is important to know the value of heating and cooling loads of buildings in order to design the optimum system in every case for minimize the cost and the fuel or electricity consumption. In this study, the loads of a typical building are calculated by two different software, TRNSYS and eQUEST. These programs have different strategy in the loads calculation fact that makes this comparison important in order to validate the results. A one-zone building of 100 m2 area with four external insulated walls directed in the four orientations is analyzed. All the external walls include double glass windows except from the north wall. The building is located in Athens, where there are average heating loads and great cooling loads. Internal loads from lighting, people and equipment are taken into consideration in order to create a typical building. Simultaneously, this building is parametrically examined by changing the windows area, the infiltration rate, the building orientation and the insulation thickness of the external walls. The calculation of the loads in eQUEST are indirect and are become with the use of the coefficient of performance. Also the strategy for the lighting is different because of TRNSYS controls the outside irradiation and the eQUEST use inside controllers. The final results show that TRNSYS gives greater loads about 5 % in the study case and in the most cases of the parametrical analysis TRNSYS gives greater loads. Moreover, a monthly comparison is presented which proves that the results are close to each other.

In this study, the optical and the thermal performance of a compound parabolic collector (CPC) with evacuated tube are presented. In the first part, the optimization of the reflector geometry is given and in the next part the thermal analysis of the solar collector is presented. The design of the reflector has a great impact on the solar energy exploitation and for this reason is analyzed in detail. In the thermal analysis of the collector, the two most usual thermal fluids, the pressurized water and typical thermal oil, are compared. Pressurized water performs better and it is the most suitable working fluid for transferring the heat because of its properties; something that is analyzed in this study. Moreover, the optical efficiency of the collector for various solar angles (longitude and transverse) is investigated and the heat flux distribution over the absorber is given. In the last part, the temperature distribution over the absorber and inside the fluid are presented and a simple validation of the thermal model is also presented. The model is designed in commercial software Solidworks and simulated in its flow simulation studio. (C) 2016 Elsevier Ltd. All rights reserved.

Energy consumption in buildings for heating purposes consists of a great percentage of the total building energy consumption. The increasing rate of electricity cost leads our society to use different and more efficient ways for covering the heating loads. The utilization of solar energy is an efficient way to cover the heating needs with a green and low cost way. In this study a solar assisted floor heating system is modelled and analyzed with the commercial software TRNSYS. The solar flat plate collectors operate during the day capturing the solar energy which is stored in the storage tank by increasing the water temperature. The hot water flows inside the tubes under the floor of the house when the inside temperature is under the desired level. An auxiliary heater gives extra energy in order to keep the desired thermal comfort conditions inside the building. Also a controlled mix between the water building flows is made in order to have the desired temperature in the inlet of the building. The examined building has an area of 100 m2 and volume 300 m3 with 12 m2 double glasses. Infiltration and other internal gains (lights, people, and machines) are taken into consideration in the simulation. A parametric analysis is made in this study in order to optimize our system by the criterion of the lowest auxiliary energy consumption. The main parameters that influence on the system performance are the collecting area, the storage tank volume, the floor segmentation and the control strategy of the heating system. After the optimization, it is proved that a collecting area of 30 m2 with a storage tank of 2.5 m3 is able to cover the 90 % of the yearly heating load in the examined building. The desired inside temperature was selected at 22 °C and the auxiliary heater operates when the inside temperature is lower than 21 °C in order to decrease the auxiliary energy dramatically. Simultaneously, the case of the stand-alone solar system is analyzed and finally it is shown that a collecting area of 50 m2 is able to keep thermal comfort conditions in high levels.

Solar energy utilization is a promising Renewable Energy source for covering a variety of energy needs of our society. This study presents the most well-known solar concentrating system, the parabolic trough collector, which is operating efficiently in high temperatures. The simulation tool of this analysis is the commercial software Solidworks which simulates complicated problems with an easy way using the finite elements method. A small parabolic trough collector model is designed and simulated for different operating conditions. The goal of this study is to predict the efficiency of this model and to analyze the heat transfer phenomena that take place. The efficiency curve is compared to a one dimensional numerical model in order to make a simple validation. Moreover, the temperature distribution in the absorber and inside the tube is presented while the heat flux distribution in the outer surface of the absorber is given. The heat convection coefficient inside the tube is calculated and compared with the theoretical one according to the literature. Also the angle efficiency modifier is calculated in order to predict the thermal and optical efficiency for different operating conditions. The final results show that the PTC model performs efficiently and all the calculations are validated.

Solar energy utilization in buildings is vital because of the increasing demand of energy for heating and cooling. Also, improvements in building structure are needed in order to decrease the heating and cooling demand. In this study, the impact of insulation layer thickness in the solar coverage of two solar heating systems is analyzed energetically and financially. The examined building is a typical Greek Building located in Athens with area 100m2. The first heating system combines flat plate collectors with a fan coil system, while the other is an underfloor heating system coupled with flat plate collectors. An auxiliary heater is located in both systems in order to supply extra energy when it is needed. The final results show that 6cm of insulation lead to minimum cost and fan coil system is the less expensive solution. TRNSYS 16 is the simulation tool of the study and gives many features to the users for solar applications in buildings.

A numerical procedure is developed for the management of solar energy in buildings using passive systems, which are based on Phase Change Materials (PCM). For the simulation of the phase change process, the concept of effective thermal capacity function is used, which is determined experimentally and then generalized using triangular functions. The developed procedure is applied along the typical year in the Athens area buildings for studying the effect of main PCM parameters, including the phase change temperature range, the phase change heat and the thickness, location and way of embodiment of PCM within buildings elements. It is found that energy savings up to 33% may be obtained along the Athens typical year by placing at the proper building location PCM layers with the right thermal properties. The conclusions of the study may be used, apart from the Athens area, to regions of similar climate and moreover, because of their generality, most of them may be useful to any climatic conditions.

A parametric analysis is conducted for space cooling systems based on cold water flowing, during the night, within regularly arranged pipes embedded in a layer of phase change material (PCM), located among the structural layers of the ceiling. The introduced PCM layer in conjunction with night cooling add to the usual ceiling cooling systems offers the advantages of low energy consumption, high cool storage capacity, operation under reduced night electricity price, smoothing of electricity consumption by eliminating daily peak loads, improved thermal comfort and elimination of ceiling dripping. Our parametric analysis is based on a transient three‐dimensional finite‐difference solution of the related heat‐transfer problem for various values of all the main system parameters. PCM phase change process is simulated by using the effective thermal capacity function, which is determined experimentally for PCM suitable for air‐conditioning applications. Our tests showed that the main parameters of the system are pipe spacing, PCM layer thickness, pipe depth within the ceiling, cooling water inlet temperature, night cooling duration and PCM properties (thermal conductivity, phase change heat and ends of phase change temperature range). The effect of all the above parameters is analysed and suggestions are made for selecting the proper combinations of their values in order to obtain the lowest energy consumption in conjunction with the highest level of thermal comfort. Copyright © 2010 John Wiley & Sons, Ltd.

The recently presented thermal delay method is an improved version of the well-known T-history method, which is widely used for thermal properties measurement of phase change materials (PCM). The most important difference between the thermal delay and the T-history methods is that the former is based on the use of thermal delay (i.e. temperature difference) between PCM and a reference fluid at any specified time while the latter makes use of their time delay at any specified temperature. Although the thermal delay method has been documented in our previous publication, measurements are performed of the known and indisputable values of ethyl alcohol thermal capacity and the latent heat of the double distilled water (WFI), which confirm the accuracy of the method. Additional comparisons with values provided by PCM producing companies show disagreements lower than 1.7%. The main volume of measurements of the present study includes the following thermal properties of various practically interesting PCM: (a) the temperatures at the ends of the two-phase region; (b) the liquid and solid PCM thermal capacities; (c) the phase change heat; (d) the heat storage capacity during any specified temperature range; and (e) the effective thermal capacity function, which is a very important and useful property for practical applications. The above function is provided for each one of the PCM examined in the form of diagrams, as well as in the form of analytical expressions derived by curve fitting to the processed experimental values. All measurements were repeated 20 times and the results were averaged in order to minimize errors from accidental incidents.

A model and a corresponding numerical procedure, based on the finite-difference method, have been developed for the prediction of buildings thermal behavior under the influence of all possible thermal loads and the “guidance” of cooling control system in conjunction with thermal comfort requirements. Using the developed procedure analyses have been conducted concerning the effects of thermostat operation mode and cooling power in terms of the time, on the total cooling energy consumption for the ideal space cooling, as well as for various usually encountered real cases, thus trying to find ways to reduce cooling energy consumption. The results lead to suggestions for energy savings up to 10%. Extensive comparisons between the ideal and various real cooling modes showed small differences in the 24-h cooling energy consumption. Because of the above finding, our detailed ideal cooling mode predictions gain considerable value and can be considered as a basis for comparison with real cases. They may also provide a good estimate of energy savings obtained if we decide to increase thermostat set point temperature. Therefore, as the extent of cooling energy saving is a priori known, one can decide if (and how much) it is worthy to increase thermostat set point temperature at the expense of thermal comfort. All results of the study, which refer to the Typical Athens Buildings during the typical Athens summer day, under the usual ranges of thermal loads, may be applicable to other regions with similar conditions.

Using a finite-difference procedure, the dynamic energy response of indoor spaces under the influence of indoor energy pulses is analyzed. The method of analysis is simple and explicit and is based on the indoor surface thermal capacitance and heat-loss coefficient Cs and Ls respectively. It is demonstrated that these parameters characterize fully any specified indoor space, as far as its energy behaviour is concerned. Their values are calculated for an extended variety of indoor spaces, i.e. for various floor areas, floor dimensions ratios, indoor surface materials of envelope, partitions and furnishings, fenestration and indoor partitions areas. The range of validity of the present method of analysis is also defined and the corresponding deviations are quantified with reference to rigorous finite-difference solutions. The provided values of indoor space characteristics Cs and Ls may be used in a wide range of technological building applications, including comparisons and classifications of indoor spaces, design and selection of construction materials and furnishing as well as the investigation of effects from electric equipment, windows or doors opening, short-time ventilations, brief stay of visitors, etc.

Improvements are proposed to the well-known T-history method, which is widely used for thermal properties measurement of phase change materials (PCM). Our improvements refer to the experimental arrangement, to the way of measurement processing, as well as to the kind and presentation format of the final results. The proposed arrangement has a controlled indoor environment and is fully automatic, without need for staff attendance, even for repeated sets of measurements of the same or different PCM simultaneously. The proposed way of measurement processing is based on the use of thermal delay (i.e. temperature difference) between PCM and a reference fluid at any specified time and not in the use of their time delay at any specified temperature. This fundamental change leads to increased accuracy and considerable reduction of duration and labour of the measurement processing, as proved by the performed measurements of various PCM. The effective thermal capacity function as a final result is proved to be more useful than the results of the original method. The new procedure is a first step towards defining specifications for the measurement of PCM thermal properties.

A multi-zone model for calculation of the closed cycle of a direct injection (DI) diesel engine is applied for the interesting case of its operation with ethanol–diesel fuel blends, the ethanol (bio-fuel) being considered recently as a promising extender to petroleum distillates. Although there are many experimental studies, there is an apparent scarcity of theoretical models scrutinizing the formation mechanisms of combustion generated emissions when using bio-fuels. This is a two dimensional, multi-zone model with the issuing fuel jets divided into several discrete volumes, called ‘zones’, formed along and across the direction of the fuel injection. The model follows each zone, with its own time history, as the spray penetrates into the swirling air environment of the combustion chamber. Droplet evaporation and jet mixing models are used to determine the amount of fuel and entrained air in each zone available for combustion. The mass, energy and state equations are applied in each zone to provide local temperatures and cylinder pressure histories. The concentrations of the various constituents are calculated by adopting a chemical equilibrium scheme for the C–H–O–N system of eleven species considered, together with chemical rate equations for calculation of nitric oxide (NO) and a model for net soot formation. The results from the computer program, implementing the analysis, for the in cylinder pressure, exhaust NO concentration and soot density compare well with the corresponding measurements from an experimental investigation conducted on a fully automated test bed, standard ‘Hydra’, DI diesel engine located at the authors’ laboratory, which is operated with ethanol–diesel fuel blends containing 5%, 10% and 15% (by vol.) ethanol. Iso-contour plots of equivalence ratio, temperature, NO and soot inside the cylinder at various instants of time, when using these ethanol–diesel fuel blends against the diesel fuel (baseline fuel), shed light on the mechanisms underlying the combustion and pollutants formation. They reveal how the widely differing properties of ethanol in these blends, against the normal diesel fuel, affect greatly the combustion mechanism and the related emitted pollutants.

Given the importance of buildings on the energy balance in Greece, an attempt has been made to study their energy behaviour and thermal comfort. Our primary purpose is to provide an estimation of the building's energy consumption and examine how this affects the comfort conditions. This includes the definition of thermal conditions acceptable for various activities at different times of day during each month of the year. We cannot underestimate the value of real measurements and observations of the building's energy systems, but such data are not always available. The best opportunities for improving energy performance occur early in the design process. Our simulation results can give an indication on which end uses are the most energy consuming, the "weaknesses" of a building and thus urge the owner or engineer to take effective conservation energy measures.

A multi-zone model for calculation of the closed cycle of a direct injection (DI) Diesel engine is presented and applied for the interesting case of its operation with vegetable oil (cottonseed) or its derived bio-diesel (methyl ester) as fuels, which recently are considered as promising alternatives (bio-fuels) to petroleum distillates. Although there are many experimental studies, there is an apparent scarcity of theoretical models scrutinizing the formation mechanisms of combustion generated emissions when using these fuels. The model is two dimensional, multi-zone with the issuing jets (from the nozzle) divided into several discrete volumes, called ‘zones’, formed along the direction of the fuel injection and across it. The model follows each zone, with its own time history, as the spray penetrates into the swirling air environment (forming the non-burning zone) of the combustion chamber, before and after wall impingement. Droplet evaporation and jet mixing models are used to determine the amount of fuel and entrained air in each zone available for combustion. The mass, energy and state equations are applied in each zone to yield local temperatures and cylinder pressure histories. The concentrations of the various constituents are calculated by adopting a chemical equilibrium scheme for the C–H–O–N system of 11 species considered, together with the chemical rate equations for the calculation of nitric oxide (NO). A model for evaluation of soot formation and oxidation rates is included. The results from the relevant computer program for the in cylinder pressure, exhaust nitric oxide concentration (NO) and soot density are compared favorably with the corresponding measurements from an experimental investigation conducted on a fully automated test bed, standard ‘Hydra’, DI Diesel engine installed at the authors’ laboratory. Iso-contour plots of equivalence ratio, temperature, NO and soot inside the combustion chamber at various instants of time when using these fuels against Diesel fuel (baseline fuel) shed light on the mechanisms underlying the combustion and pollutants formation and reveal how the widely differing properties of these fuels, against the normal Diesel fuel, affect greatly the spray formation, combustion mechanism and the related emissions.