• Energy Research

Liquid Droplet Radiators (LDR) and Liquid Sheet Radiators (LSR) are among the most promising technologies to realise lightweight heat exchangers for space applications. With respect to traditional solid surface radiators, favourable rejected heat flux over mass ratio (up to 1000W/kg) can be achieved provided that accurate optimisation of the global system is performed. In this work a calculation procedure to evaluate the performance of rectangular LDR is employed together with an optimisation algorithm to get the main characteristics of the LDR components as a function of the droplet sheet parameters and the working temperatures in the range 300–400 K. The influence of the intermediate heat exchanger coupling the radiator to the thermal bus system is considered in order to show the possibility to employ the LDR, in a wide range of space applications, by direct substitution of conventional solid surface radiators.

Regenerative magnetic refrigeration at room temperature has the potential to overcome various problems affecting vapor compression devices while providing competitive performance, but the effects of several loss mechanisms must be evaluated and accounted for. In actual devices, inactive sections in the regenerator originate dead volumes leading to possible non-optimal exploitation of the magnetocaloric material and the associated loss mechanism. While magnetic refrigeration gained attention, few studies have systematically investigated the effects of dead volume on system performance. In this work, a one-dimensional model valid for a generic magnetic refrigerator device (either linearly reciprocating or with continuous, or discontinuous, rotary motion) is used to study the effect of the dead volume. The device performance was assessed by comparing the characteristic curves (cooling power, and COP) of an ideal device (zero dead volume) to the corresponding characteristic curves for different dead volume ratios. The performance is negatively affected if the device is operated under the same working conditions and control parameter settings used in the ideal device. This effect is higher for higher temperature spans. Nevertheless, the device could approximate the ideal performance by adjusting its control parameters. The main measures to mitigate the negative effects of dead volumes can be summarized in the necessity to operate at lower frequencies, higher fluid mass flow rates, and higher torque. The results of this study prove that the proper control of such operative parameters is able to maximize the device performance and mitigate the performance losses due to the dead volume effects.

The plant in the Sport Palace “Carmine Romanzi” (Palacus, University of Genoa) consists of a Solar Assisted Heat Pump (SAHP) which integrates the two existing gas burners in the production of Domestic Hot Water (DHW) and Space Heating (SH). To manage the large number of control and measurement points along the installation, a data acquisition and control system has been introduced. Therefore, long inactivity periods have been recorded, due to the manual technicians’ intervention: they turned the SAHP off since their knowledge of the plant was not sufficient to manage its problems. This issue shows a lack of “acceptance” both from the end user, who doesn’t rely on the innovative plant, and from the technical staff, who doesn’t encourage the employment of new technologies. A case study concerning the solar thermal field in integration of the DHW production is provided to evaluate qualitative indicators of acceptance and therefore illustrate a measurement of acceptance. The data collection system requires a further integration to record real time data to objectively quantify the level of acceptance according to coefficients depending on the end users’ usage of the plant.A well performing data collection system and real time regulation criteria are the starting points to increase plant efficiency and reduce the need of active end users’ intervention on it. An efficient and self-managed plant presents implicitly a very high level of acceptance as well.

PurposeThe purpose of this paper is to carry out a numerical investigation to study laminar convection flow of Al2O3-water nanofluids within a three-dimensional rectangular section channel asymmetrically heated.Design/methodology/approachA three-dimensional model of the channel is designed and simulated by using Comsol Multiphysics. The finite elements method is used to perform the numerical simulation. A variety of cases are taken into account by considering Reynolds numbers ranging from 250 up to 1,000, concentration between 0 and 6 per cent, particle dimension of 20, 40 and 60 nm and inlet temperature equal to 293.15 and 320 K. A constant heat flux of 1,000 W/m2is imposed on the top surface of the channel.FindingsThe results demonstrate that nanofluids guarantee improved thermal performances with respect to the base fluid, as shown by the augmented Nusselt number. On the other hand, pressure drop shows a noticeable increase; therefore, an entropy generation analysis is developed to establish optimal conditions for the system under investigation.Originality/valueThe originality of this work consists in the analysis of a three-dimensional asymmetric heated channel with nanofluids in laminar convection. The present work would be beneficial to improve the design of devices with particular focus on solar thermal panel.

The present paper analyzes the Italian energy system focusing on the possible energy, environmental, and economic effects that the utilization of individual heat pumps for winter heating can produce. To this aim, a model of the Italian energy system is developed by employing the tool EnergyPLAN in order to develop an hourly simulation of the system at country level. Different scenarios in terms of heat pumps penetration, ranging between 10% and 50%, are simulated and sensitivity analyses in terms of average coefficient of performance of heat pumps are performed. The increase of heat pumps generation shows a steadily decrease of fossil fuel consumption for buildings heating, as well as a reduction of carbon emissions. The utilization of heat pumps for buildings heating leads to an increase of 10 p.p. in the load factor of the combined cycle gas turbine thermal power plants. Furthermore, an optimal heat pumps penetration is determined, namely 20%, which minimizes the energy system costs, calculated as the sum of fuel cost and externalities savings.

Abstract Magnetic refrigeration is becoming increasingly relevant as a means to achieve sustainability in refrigeration, thanks to its durability, efficiency, and low environmental impact. This paper regards the design of a new rotary magnetic refrigerator feasible for domestic use at room temperature. An interdisciplinary approach was necessary to perform the design, construction, and optimization of the device through mechanical design, thermo-hydraulic design, and integration into a proper test rig. Optimization of critical parts, such as the regenerator, the magnet structure, and the external heat exchangers, was performed using numerical models. The optimal values of control parameters were determined using a parametric sweep study. The results of all these steps of the study resulted in significant progress in designing and constructing an efficient rotary magnetic refrigerator. This work presents the insights obtained from the project implementation, including the design and construction aspects, presented in a practical way that can guide future projects in the field of magnetic refrigeration. Graphical abstract

Wastewaters treatment with biological processes is widely applied as a reliable solution to water pollution in urban areas. This process produces sludges and process waters. Sludges can be furtherly threated in dedicated plants to produce biogas, that can then be used to produce heat in specific plants, but it is sometimes simply burned using flares. A possible alternative use of biogas is the coupling with a CHP generator which can be used to produce electrical energy and heat. In the present paper, it is investigated the possible exploitation of the sludges produced by a wastewater treatment plant for feeding a sludge treatment plant, both in Genoa (Italy), in order to produce biogas. This work is based on the analysis of available data (2009-2018) provided by the sludge treatment plant technical staff, which showed strong variations in sludge flow rates as a result of the variation of incoming wastewater load. Since the available data is not always correlated with wastewater incoming flows and biogas production, starting from data provided for similar plants operating in Italy, it was possible to estimate a theoretical biogas production which was considered as the reference trend.

Abstract In this study, a very simplified dynamic lumped model for the simulation of small-scale single-temperature vapor compression refrigerators working between two thermal sources with finite thermal capacity is presented. The model is compact enough to be employed in actual regulation systems, but adequate to describe the basic underlying physical phenomena relevant to the transient response of the refrigerated cell. The dynamic behavior of the system is simulated taking into account the main heat capacities involved in the heat transfer processes between the system, the refrigerating fluid and the outside. The numerical model has been validated by comparing the calculated results with transient experimental data coming from an instrumented chest-freezer. After a steady state tuning phase, the model was able to predict the transient temperature response of the cell with good accuracy.