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Thanks to their numerous economic and environmental benefits, Distributed Energy Systems (DES) are considered as a sustainable alternative to traditional energy supply. To maximize the DES operators' profits while also containing emissions, daily operation is crucial. In this paper, the operation problem is addressed for a real DES in Italy by considering economic and environmental aspects. The DES consists of a Combined Heat and Power system, condensing and conventional boilers, and absorption and electric chillers. The end-users are an office building, a theater hall, and a residential building cluster, which is connected to the DES through a district heating network (DHN). A multi-objective linear programming problem is formulated based on the real constrains for devices and DHN with the aim to find the optimal operation strategies of the DES, which maximize the operator's profit and minimize the net CO2 emissions, while satisfying the users demand. The Pareto frontier is found through the weighted-sum method, by using branch-and-cut. The method is implemented for a winter day of December, by using experimental data for electrical and thermal demand of end-users. The results show that the optimization method is efficient in finding good trade-off solutions between economy and CO2 emissions. Moreover, the economic/environmental performances of the DES with optimized operation are much better than those found for the current operation strategies. In addition, the effects of the Italian white certificates scheme on the DES performances are also investigated. İ 2018 IEEE.

The study of an innovative “active” cooling system of a wing leading edge of a hypersonic re-entry vehicle making use of water is addressed. In particular, a steady model is developed to study the critical discharge of the cooling water into a very low pressure ambient simulating the outlet conditions for both the re-entry and wind tunnel environments. Due to the strongly subcooled operating conditions, the model predicts no-flashing within the duct connecting the outlet (hot) manifold to the vacuum ambient. The mass flow rate needed to remove the aerodynamic heat load acting on the external surface is calculated by an iterative procedure. At each iteration, for a fixed value of the mass flow rate, the pressure within the outlet manifold is calculated and the exit section critical pressure is determined as well. Subsequently, a detailed thermo-fluid-dynamic analysis is conducted to evaluate the head losses within the pipes and the peak wet wall temperature. The iteration stops when the mass flow rate guarantees no-boiling conditions throughout the system. The findings arising from the steady model are confirmed by unsteady numerical simulations of the system start-up.

Efficiency of energy resource use is a key factor for a sustainable energy future. By matching the exergy levels of supply and demand, Energy Quality Management (EQM) of building energy supply systems may achieve more efficient use of energy resources. Beyond this, environmental impact of energy supply systems is another essential issue. This work addresses the influence of EQM on CO2 emissions in the operation optimization of a Distributed Energy System (DES). A multi-objective linear programming problem is formulated to reduce energy costs and increase the overall exergy efficiency. Total CO2 emissions are evaluated for the optimized operation strategies of the DES. The operators of the DES can choose the operation strategy from the Pareto front, based on their priorities and also aware of effects on CO2 emissions. Results demonstrate more efficient use of energy resources and reduction in CO2 emissions through EQM, as compared with conventional energy supply systems. © 2015 IEEE.

26th European Photovoltaic Solar Energy Conference and Exhibition; 778-781, Procedures applicable to concentrating solar collectors to determine how they modify spectrally and spatially the sun direct radiation are described in this paper. We use a radiometric apparatus provided with two cavities for light integration and attenuation (DCR) and coupled with an external spectrometer for spectral measurements. To measure spatial light distribution we use a CCD camera with its data acquisition system, a set of neutral filters, a receiver and the collector itself which has to be characterized. Finally, a heat flux sensor based on an inverse heat transfer method (IHTM) is being developed for the estimation of concentrated radiative heat flux by means of temperature measurements. The overall apparatus should be able to characterize a concentrated sunlight flux up to a concentration factor of 1000 X.

Thermal energy storage systems (TESs) are of fundamental importance for many energetic systems, essentially because they permit a certain degree of decoupling between the heat or cold production and the use of the heat or cold produced. In the last years, many works have analysed the addition of a PCM inside a hot water storage tank, as it can allow a reduction of the size of the storage tank due to the possibility of storing thermal energy as latent heat, and as a consequence its cost and encumbrance. The present work focuses on experimental tests realized by means of an indoor facility in order to analyse the dynamic behaviour of a hot water storage tank including PCM modules during a charging phase. A commercial bio-based PCM has been used for the purpose, with a melting temperature of 58°C. The experimental results relative to the hot water tank including the PCM modules are presented in terms of temporal evolution of the axial temperature profile, heat transfer and stored energy, and are compared with the ones obtained by using only water as energy storage material. Interesting insights, relative to the estimation of the percentage of melted PCM at the end of the experimental test, are presented and discussed. © Published under licence by IOP Publishing Ltd.

Thermal energy storage (TES) allows to extensively exploit solar thermal technologies by effectively handling the mismatch between energy production and demand thus possibly causing a downsizing of generation units. Among thermal energy storage technologies, those based on phase change materials (PCM) are particularly interesting because relatively large latent heat values may guarantee more compact systems (as compared to sensible TES). In this work, we report a numerical and experimental investigation on a hybrid latent-sensible heat storage characterized by a commercial hot water tank integrated with macro-encapsulated phase change materials. Those hybrid systems are interesting as they can possibly increase the overall thermal capacity of a sensible water tank. Despite all this, we demonstrate that increasing the effective storage capacity is a non trivial task with standard conditions and materials. To this end, three different numerical models have been developed and experimentally validated. The first model is based on the enthalpy porosity method and simulates the charge and discharge of a PCM storage unit in a climatic chamber. The second model is a one-dimensional description of the water storage tank without PCM. Finally, the third model is obtained by coupling the previous two and simulates the whole PCM-water thermal storage. This final model was validated through an experimental test, which consisted in inserting 94 modules of PCM in the water tank and observing the resulting thermal behaviour for three days, applying the load curve of a detached house of 200m2. The model proved to be very accurate, with determination coefficients between 92.10% and 99.80% for the considered physical quantities (temperatures and thermal powers). As a main contribution of this work, we proved that the hybrid thermal storage system did not exploit the full latent heat potential of the PCM, since only 40% of it actually changes its phase, due to is thermal transport properties that negatively affect heat transfer.

This work focuses on the experimental validation of a numerical tool realized to simulate a commercial hot water storage tank. The tool implements unsteady 1D models to simulate the temporal evolution of the temperature field inside the hot water storage tank, and the one relative to the heat transfer fluid flowing through the immersed coil heat exchanger. It has been implemented by means of the Simulink tool of Matlab. The first part of the paper is dedicated to the description of the indoor experimental facility used to realize the experimental test. Successively, the analytical models, and the numerical schemes and algorithms used to perform the numerical simulations are described. Finally, the results of the experimental validation of the tool, accomplished by comparing the experimental temperature profiles inside the tank, and the measured temperatures at the coil heat exchanger exit section over the entire experimental test duration, with the numerical results obtained from simulations performed using different correlations for the evaluation of the heat transfer rate between the tank water and the heat transfer fluid through the coil, are reported and discussed. © 2017 The Authors. Published by Elsevier Ltd.

In this work, the results coming from the schedule optimization of a residential natural gas-fired CHP system operating according to a novel heat-driven operation strategy, based on the possibility to dump part of the thermal energy produced by the CHP system, are compared with the ones relative to the case in which the CHP system operates following a heat-driven strategy allowing load partialization. The effects of the variation of the size of the thermal energy storage system on the optimization results are also investigated. The simulation of the two different operation strategies has been realized using a numerical code written in Matlab environment, and a heuristic algorithm has been used for the economic optimization of the operation relative to both the implemented strategies. From the economic point of view, results show that the difference between the best CHP system configuration in the case with heat dumping and the one relative to the case with load partialization is minimal, with the latter presenting a slightly higher economic performance. Moreover, conditions in which heat dumping could yield higher economic performances than load partialization are also presented in this work. As concerns pollutants emissions, results show that the impact of load partialization on a local scale is much higher, with CO emissions that can be up to three times the ones relative to conventional systems for the separate generation of electricity and heat. © 2016 Elsevier Ltd

Thermal energy storage systems have increasingly attracted researchers and engineers in the last decades, not only because they permit to overcome the mismatch between thermal energy supply and demand in solar thermal systems, but also because they have the potential to improve the power grid reliability and flexibility, as they allow, for example, to store the heat or the cooling energy produced by residential electric heat pumps for a later use. In this context, this work analyses the cooling energy charging and discharging of two different cold thermal energy storage units, based on the use of a phase change material (PCM), by numerical simulation. One unit consists of an aluminum cylindrical container partially filled with a biological PCM. The other one is similar, with the only difference that it also includes an aluminum metal foam immersed in the PCM. Two different 2D axisymmetric models have been developed, both based on the effective heat capacity method in order to account for the PCM latent heat, for the numerical simulation of the two different units, and the numerical results are compared with experimental ones, obtained by means of a climatic chamber, in order to assess, in each case, the best set of the model arbitrary parameters among the considered ones. Moreover, for each simulation model, a sensitivity analysis is performed to show the effects of two arbitrary model parameters on the numerical results. Simulation results relative to the unit with only the PCM indicate that the cooling energy charging is most affected by conductive heat transfer, and in particular by the thermally resistive layer of solid PCM that forms at the lateral wall of the aluminum container, and that heat transfer by free convection plays a crucial role in the cooling energy discharging process. As concerns the unit with the PCM and the aluminum foam, numerical results show that the charging process and the discharging one are about four times and two times faster than without the foam, respectively. Furthermore, numerical results show that, with the employed aluminum foam, free convection within the PCM is negligible in both the cooling energy charging and discharging process.

This work aims to present the results of an energetic and economic analysis of a biomass fueled CCHP system operating according to different design configurations. The investigated system consists of a biomass-fueled cogeneration unit, an absorption chiller, a thermal energy storage system and a cold one, providing electricity, heat and cooling to an Italian cluster of buildings. For each simulated configuration, the feasible investment cost of the CHP unit is evaluated considering the economic savings obtained with respect to separate generation of electricity, heat and cooling. The best configuration from the economic point of view is indicated, and the incidence of the variation of the absorption chiller and storage systems sizes on the feasible investment cost of the CHP unit is evaluated and discussed as well. Results indicate that the most influencing parameter is represented by the absorption chiller power. © 2017 The Authors.

In this work, an operation strategy for a biomass-fired combined cooling, heating and power system, composed of a cogeneration unit, an absorption chiller, and a thermal energy storage system, is formulated in order to satisfy time-varying energy demands of an Italian cluster of residential multi-apartment buildings. This operation strategy is adopted for performing the economical optimization of the design of two of the devices composing the combined cooling, heating and power system, namely the absorption chiller and the storage system. A sensitivity analysis is carried out in order to evaluate the impact of the incentive for the electricity generation on the optimized results, and also to evaluate, separately, the effects of the variation of the absorption chiller size, and the effects of the variation of the thermal energy storage system size on the system performance. In addition, the inclusion into the system of a cold thermal energy storage system is analyzed, as well, assuming different possible values for the cold storage system cost. The results of the sensitivity analysis indicate that the most influencing factors from the economical point of view are represented by the incentive for the electricity generation and the absorption chiller power. Results also show that the combined use of a thermal energy storage and of a cold thermal energy storage during the hot season could represent a viable solution from the economical point of view. © 2017 Elsevier Ltd

In recent years, Distributed Energy Systems (DESs) have been recognized as a good option for sustainable development of future energy systems. With growing environmental concerns, design optimization of DESs through economic assessments only is not sufficient. To achieve long-run sustainability of energy supply, the key idea of this paper is to investigate exergy assessments in DES design optimization to attain rational use of energy resources while considering energy qualities of supply and demand. By using low-temperature sources for low-quality thermal demand, the waste of high-quality energy can be reduced, and the overall exergy efficiency can be increased. Based on a pre-established superstructure, the aim is to determine numbers and sizes of energy devices in the DES and the corresponding operation strategies. A multi-objective linear problem is formulated to reduce the total annual cost and increase the overall exergy efficiency. The Pareto frontier is found to provide different design options for planners based on economic and sustainability priorities, through minimizing a weighted-sum of the total annual cost and primary exergy input, by using branch-and-cut. Numerical results demonstrate that different optimized DES configurations can be found according to the two objectives. Moreover, results also show that the total annual cost and primary exergy input are reduced by 20% - 30% as compared with conventional energy supply systems. © 2017 The Authors.

The heat storage system represents a key component for micro-cogeneration plants since it permits to store the unused thermal energy during electricity production for a later use. Nevertheless, it also represents a consistent additional cost that has to be taken into account in order to evaluate the profitability of the micro-CHP system with respect to the separate generation. In this paper the results of a technical and of a cost analysis of two different types of thermal energy storage systems for residential micro-CHP plants are presented. Indeed, in the present work hot water thermal energy storage systems and latent heat thermal energy storage systems have been dimensioned for different micro-CHP systems producing electrical and thermal energy for two different buildings situated in Italy. For each analysed micro-CHP system an adequate thermal energy storage capacity is estimated on the basis of the operational logic and of the electric and thermal loads, and the sizing of the cylindrical tank and of the coil heat exchanger relative to both types of thermal energy storage systems is performed. Comparisons in terms of components cost between hot water thermal energy storage systems and latent heat thermal energy storage systems are performed as well.

This work deals with the operation, modeling, simulation, and cost evaluation of two different cold storage systems for a single-family house in Italy, that differ from one another on the cold storage material. The two materials used to perform the numerical simulations of the cold storage systems are represented by cold water and a phase change material (PCM), and the numerical simulations have been realized by means of numerical codes written in Matlab environment. The main finding of the present work is represented by the fact that, for the considered user characteristics, and under the Italian electricity tariff policy, the use of a proper designed cold storage system characterized by an effective operation strategy could represent a viable solution from an economical point of view. © 2016 by the authors.

This paper focuses on the numerical characterization of a new sensor for the measurement of highly concentrated radiative heat fluxes, based on an inverse heat transfer method. The sensor will be coupled to the solar furnace that is being installed at the ENEA Portici Research Center. The highly concentrated radiative heat flux incident on the target surface of the sensor is estimated by implementing the inverse heat transfer method based on the Levenberg–Marquardt algorithm, which permits to compute the radiative boundary condition on the exposed surface of the target by measuring the temperature of its hidden bottom surface. Numerical simulations have been carried out to evaluate the sensor sensitivity to the following parameters: the emissivity of the target surface of the sensor; the synchronization error in the temperature recordings; the uncertainty in temperature measurements; the convection heat transfer coefficient; the misalignment between the axis of the target and the axis of the concentrated solar spot; the uncertainty about the values of the thermo-physical properties of the sensor components materials.

In this work the characterization of pollutants emission of two different commercial prime movers producing electricity and heat during the cold season for a multi-apartment housing situated in Italy is addressed. In particular, an internal combustion engine and a microturbine, both fed by natural gas, have been analyzed separately. For each prime mover, the emission of CO2, CO and NOx resulting from two different optimized operation modes are reported and discussed. The results show that the micro-CHP system with the internal combustion engine as prime mover involves a marked augmentation of dangerous CO and NOX emissions on a local scale. © 2015 IEEE.

Sustainable development requires not only the use of sustainable energy resources, but also the efficient use of all energy resources. The latter should be reached by considering the concept of energy as well as exergy - the true magnitude of thermodynamic losses. Exergy describes the quality of an energy flow as the percentage that can be completely transformed into any other form of energy. Reduction of exergy losses represents a more efficient use of energy resources, which is essential in the long run, but it is not captured by standard energy costs, which are crucial in the short run. In this paper, exergy analysis is used in the context of a multi-carrier energy district to match the supply and demand not only in quantity but also in quality. The innovative contribution of this paper is the offering of a trade-off between reducing exergy losses and energy costs, thereby attaining sustainability of the energy district. A mixed-integer programming problem considering several energy devices is formulated to minimize a weighted sum of exergy losses and energy costs while satisfying time-varying user demands. The problem is solved by branch-and-cut. Numerical results demonstrate that the optimized operation of the energy devices makes the energy district sustainable in terms of exergy efficiency and costs. © 2014 IEEE.

An innovative transient numerical model is presented for the simulation of a solar Domestic Hot Water (DHW) system. The solar collectors have been simulated by using a zerodimensional analytical model. The temperature distributions in the heat transfer fluid and in the water inside the tank have been evaluated by one-dimensional models. The reversion elimination algorithm has been used to include the effects of natural convection among the water layers at different heights in the tank on the thermal stratification. A finite difference implicit scheme has been implemented to solve the energy conservation equation in the coil heat exchanger, and the energy conservation equation in the tank has been solved by using the finite difference Euler implicit scheme. Energy conservation equations for the solar DHW components models have been coupled by means of a home-made implicit algorithm. Results of the simulation performed using as input data the experimental values of the ambient temperature and the solar irradiance in a summer day are presented and discussed.

In this work the scheduling optimization of micro-CHP systems producing electricity and heat for a single-family house situated in Italy is addressed. Three different commercial prime movers have been analyzed separately, and for each of them the operation scheduling that maximizes the revenues for the energy cogeneration with respect to the separate generation has been evaluated by means of an optimization algorithm. © (2014) Trans Tech Publications, Switzerland.

In this study the optimal operation of micro-CHP systems producing electricity and heat for a single-family house situated in Italy is addressed considering the effects of heat dissipation relative to thermal storage systems. Three different prime movers have been analyzed separately, and for each of them the operation scheduling that maximizes the revenues for the energy cogeneration with respect to the separate generation has been evaluated by means of an optimization algorithm. © 2014 IEEE.

This work has been realized in the framework of the Elioslab project, financed by the Italian Ministry of Education, University and Research (MIUR), which aims to create a research platform in order to develop components and systems for the production and utilization of medium and high temperature heat using concentrated solar energy. As regards high temperature heat production, a 30 kW solar furnace that consists of a heliostat with flat mirrors and a parabolic concentrator with off-axis alignment is being realized in order to achieve a solar radiation concentration peak of about 2000 suns. The energy flux relative to the concentrated radiation will be converted to high temperature heat by a cavity receiver cooled with CO2, and finally transferred to a device operating at high temperature consisting in a thermochemical reactor for hydrogen production. Due to the intermittency of solar radiation, a high temperature (>gt;800° C) packed bed sensible heat storage system, with alumina balls as heat storage material, has been developed in order to provide continuity to the user operation. This paper focuses on the parametric analysis that has been carried out by means of numerical simulations to evaluate the set of variable parameters that maximize the efficiency of the heat storage system of the solar furnace. The charging and discharging phases of the heat storage tank have been numerically simulated by means of an analytical model that takes into account the conductive, convective, and radiative heat transfer as well as turbulent diffusion due to the solid-fluid interaction. The results of the numerical parametric analysis are presented together with the experimental validation of the adopted analytical model accomplished by using a reduced-scale high temperature storage system. Copyright © 2013 by ASME.

An innovative “active” cooling system of a wing leading edge of a hypersonic re-entry vehicle making use of water is studied. In particular, a steady model is developed to analyze the critical discharge of the cooling water into a very low pressure ambient simulating the outlet conditions of both the re-entry and wind tunnel environments. Due to the strongly subcooled operating conditions of the fluid, the model predicts no-flashing within the duct connecting the outlet (hot) manifold to the vacuum ambient. For a fixed water mass flow rate, the pressure within the outlet manifold is determined, giving that flow rate in connection with the proper head losses. The exit section (critical) pressure is determined as well. These findings are confirmed by unsteady numerical simulations of the system start-up.

In the present work the unsteady numerical simulation of a solar domestic hot water (DHW) system composed of two flat plate collectors, a water tank for heat storage, and a coil heat exchanger is addressed. The simulations have been performed using two different solvers, namely a home-made code written in Matlab, and TRNSYS 17. In the first part of the paper, the analytical models used in the Matlab code, and the TRNSYS case are reported in detail. Successively, the results of the simulations realized by means of the two solvers are presented and compared. © 2014 The Authors. Published by Elsevier Ltd.