• Energy Research

Abstract This study focuses on the economic analysis of the heating network modernization by using preinsulated pipes and twin-pipes. The cost analysis is based on different type, diameter, and technological diagrams of different heating network pipe variants (from DN25to DN100) and various types of insulation. Net present value (NPV) analysis comparing a five variants of the heating network pipes (Case 0: Pipes with insulation PUR standard; Case 1: Pipes with insulation PUR plus; Case 2: Pipes with insulation PEX standard; Case 3: Twin-Pipes with insulation PUR; Case 4: Twin-Pipes with insulation PEX). The value of NPV, which is the highest (NPV = 7000 €, DN65, case 2 and NPV = 5200 €, DN50, case 4) among presented variants is the best choose to consider during the investment. Despite the higher prices of Twin Pipe system, they are more profitable due to shallow heat transfer losses. Heat losses are calculated for various heating networks, including preinsulated and twin-pipe heating networks. The remaining heat loss calculation is based merely on temperature levels and thermal resistance factors (in the ground), determined using the pipe dimensions and materials. It is found that the most cost-effective from an economic point of view is the use of TwinPipe system. These pipes are more expensive, but the heat losses that occurs during their use are much smaller compared to preinsulated pipes. Short return time of such an investment (6–7 years) with 100 m of pipe installed and positive NPV causes that this type of pipes is the best investment in the case of district heating networks. The results shown that Case 3 (PEX insulation) is more economicaly beneficial than Case 4 (PUR insulation).

The objectives of this work are: (a) to present a new system for building heating which is based on underground energy storage, (b) to develop a mathematical model of the system, and (c) to optimise the energy performance of the system. The system includes Photovoltaic Thermal Hybrid Solar Panels (PVT) panels with cooling, an evacuated solar collector and a water-to-water heat pump. Additionally, storage tanks, placed underground, are used to store the waste heat from PVT panels cooling. The thermal energy produced by the solar collectors is used for both domestic hot water preparation and thermal energy storage. Both PVT panels and solar collectors are assembled with a sun-tracking system to achieve the highest possible solar energy gain. Optimisation of the proposed system is considered to achieve the highest Renewable Energy Sources (RES) share during the heating period. Because the resulting optimisation problem is nonlinear, the classical gradient-based optimisation algorithm gives solutions that are not satisfying. As alternatives, three heuristic global optimisation methods are considered: the Genetic Algorithm (GA), the Particle Swarm Optimisation (PSO) algorithm, and the Jaya algorithm. It is shown that the Jaya algorithm outperforms the GA and PSO methods. The most significant result is that 93% of thermal energy is covered by using the underground energy storage unit consisting of two tanks.

Abstract In this paper, a recently proposed optimization algorithm named as ‘Jaya algorithm’ is used for the dimensional optimization of a micro-channel heat sink. Two case studies are considered and in both the case studies two objective functions related to the heat transfer and pressure drop, i.e. thermal resistance and pumping power, are formulated to examine the performance of the micro-channel heat sink. In the first case study, two non-dimensional design variables related to the micro-channel depth, width and fin width are chosen and their ranges are decided through preliminary calculations of three-dimensional Navier–Stokes and energy equations. In the second case study, three design variables related to the width of the micro-channel at the top and bottom, depth of the micro-channel and width of fin are considered. The objective functions in both the case studies are expressed in terms of the respective design variables and the two objectives are optimized simultaneously using the proposed Jaya algorithm. The results are compared with the results obtained by the TLBO algorithm and a hybrid multi-objective evolutionary algorithm (MOEA) and numerical analysis. The results obtained by the application of Jaya algorithm are found much better than those reported by the other approaches.

Abstract The PV panels cooling allows one to increase the photovoltaic conversion efficiency, as well as to obtain the waste heat, which can be utilized for various purposes. This paper presents the model of the radiator used for photovoltaic (PV) panels cooling. The cooling liquid is a water-glycol mixture, the PV panel electrical power output is 280 W. The radiator consists of a various number of segments (U-tube), that are located at the PV panel width. The three-dimensional control volume method formulation is used to calculate the temperature distribution within the PV panel surface (glass, silica layer, protective foil), and aluminum radiator. The one-dimensional energy equation is used to calculate the cooling liquid outlet temperature. The effect of an increasing number of radiator segments and the mass flow rate is studied to optimize the heat transfer performance and decrease the manufacturing costs. The simulations are conducted for various conditions, including the cloudy and sunny days. The simulation results are compared with experimental data collected at a stand installed at the laboratory of the Department of Energy at the Cracow University of Technology. The measurement results and the numerical simulation shows quite good agreement. For the studied case, it is shown that lowering the temperature of the panels increases the gross efficiency of the entire system. Moreover, the waste heat obtained from the PV cooling may be utilized as low-temperature heat for domestic hot water preheating or heat pump system applications.

To understand usage of solar air collector there are factors that have influence on the performance. The goal of this paper is to investigate a solar air collector prototype in order to perform a performance optimization using the possible min imum of equations. The thermal efficiency, fluid outlet temperature, heat increase and heat losses of the collector are calculated depending on the collector geometry, fluid properties, fluid inlet temperature, air flow rate, solar insulation and ambient temperature. The calculations were performed using actual data of temperature, wind speed and the global-horizontal radiation Hh over the year 2011 at the international airport of Annaba city. Comparison of results reveals that optimal efficacy is obtained for south facing panels with an inclination angle ß comprised in the interval [15°, 35°]. These conditions provide a useful energy exceeding 4300 W and an efficiency of about 51 %.

The preparation process of the nanocomposites and nanofluids strongly affects their general physical properties and stability in general. They are very sensitive with respect to the quality of the preparation process which should be obtained carefully to produce quality and reliable homogeneous samples. This paper provides a useful analysis in different preparation procedures with the focus on the nano-enhanced phase change materials (NEPCM) and fluids (nanofluids) due to their wide application areas. The preparation processes were identified and categorized together with the key parameters that have an important impact on the preparation process in general. The main problems and issues during the preparation process were discussed and elaborated in detail, gained by a detailed analysis of commonly applied preparation procedures. Since the preparation process of both nanofluids and nanocomposites should be carefully planned regarding the working safety, an overview of the safety equipment was provided as well as an assessment of the toxic potential of nanomaterials. Quality and well obtained preparation process can contribute to the rational utilization of the nanomaterials, and by that, it could reduce negative environmental footprints released from production as well as application of the nanomaterials. The main research findings of this paper provide useful guidelines for researchers dealing with the preparation of nano-enhanced fluids and phase change materials that should be helpful to get insights directed to the complexity of the preparation process and ensure an efficient preparation process.

Abstract In this paper, a novel gas-liquid compressed air energy storage prototype, installed in the laboratory of DIAEE Department of Sapienza University of Rome, is studied. Similar to the Compressed-Air Energy Storage (CAES), the Gas-Liquid Energy Storage (GLES) technology is based on gas compression/expansion, where the liquid-piston compression and expansion are utilized. This paper reports on the experimental performance of the first GLES prototype and presents the results from a validated mathematical model. The results show that the proposed system has a high energy efficiency (indicated) over 90%, and then to achieve high values of round trip efficiency (RTE), it is important to improve and optimize the efficiency of the auxiliaries (Motor/Pump and Turbine/Generator). Two different Test, with two different speed of charging phase were done. From the results of the experimental measurements done with the prototype built in the laboratory of the DIAEE Department of Sapienza University of Rome, it can be seen that for slow compression the RTE of the system is around 72%, instead for the fast compression phase, the RTE is around 70%. A mathematical model was implemented and tested with the experimental measurements. From results it can be seen a good agreement between the experimental and numerical analysis, with a maximum error in the Test B (slow compression) equal to 2.5% and 1% respectively for charging and discharging phase. From the parametric analysis it can be seen that only the volume of the tank and the pressure ratio are needed to predict the round trip efficiency of the system.

Abstract In this paper, a building energy simulation tool is exploited to study the impact of multiple radiative inter-reflections exchanges in an urban environment. The aim is to evaluate their influence on the thermal energy demand of buildings. A street canyon model validated in a previous work is used in TRNSYS to investigate the effects of the related urban radiative trapping. Due to multiple shortwave and longwave reflections, the actual radiation exchanged by the buildings facades is different if compared to a street canyon building, where only shadowing phenomena due to canyon geometry are considered. Buildings energy simulation commercial codes do not take in account inter-reflections inside urban canyons. The objective of this study is to evaluate how multiple shortwave and longwave reflections affect thermal energy demand (cooling and heating) of a street canyon building depending on its orientation, its transparent/opaque surfaces ratio and on the solar absorption factor of the envelope surfaces. Increases in cooling demand up to 50% and decreases in heating demand up to 20% are found.