• Energy Research

The use of downhole heat exchangers (DHE) in the exploitation of geothermal resources is characterized by an absence of mass withdrawal from the aquifer. Although this peculiarity reduces installation costs it also imposes limits on the heat flow withdrawable (generally less than 100 kW), and, therefore, on the use of DHEs in small applications such as greenhouses, small buildings or thermal baths. For this reason DHEs are mainly used in superficial geothermal aquifers (up to 30 m depth), usually with liquid-phase water at temperatures greater than 60°C. A study has been made of the influence of the position of the casing slotted section within an aquifer on the heat withdrawal rates using DHEs. This study numerically simulates an aquifer using the finite-element method to determine the heat flow that can be withdrawn by the DHE when the slotted section position is varied within a geothermal aquifer. The simulations carried out also enable us to determine the influence of the main characteristics of the aquifer and the extraction plant on the design of the tube casing slotted section. On the basis of the numerical results obtained, a particular configuration of slotted section is proposed where this is subdivided into different sections, one placed in the lower part of the aquifer and the other in the upper part. The results obtained have shown that this configuration optimizes the heat flow drawn by the DHE from the geothermal aquifer.

The valorization of residual biomass plays today a decisive role in the concept of “circular economy”, according to which each waste material must be reused to its maximum extent. The collection and energy valorization at the local level of biomass from forest management practices and wildfire prevention cutting can be settled in protected areas to contribute to local decarbonization, by removing power generation from fossil fuels. Despite the evident advantages of bioenergy systems, several problems still hinder their diffusion, such as the need to assure their reliability by extending the operating range with materials of different origin. The Italian project “INNOVARE—Innovative plants for distributed poly-generation by residual biomass”, funded by the Italian Ministry of Economic Development (MISE), has the main scope of improving micro-cogeneration technologies fueled by biomass. A micro-combined heat and power (mCHP) unit was chosen as a case study to discuss pros and cons of biomass-powered cogeneration within a national park, especially due to its flexibility of use. The availability of local biomasses (woodchips, olive milling residuals) was established by studying the agro-industrial production and by identifying forest areas to be properly managed through an approach using a satellite location system based on the microwave technology. A detailed synergic numerical and experimental characterization of the selected cogeneration system was performed in order to identify its main inefficiencies. Improvements of its operation were optimized by acting on the engine control strategy and by also adding a post-treatment system on the engine exhaust gas line. Overall, the electrical output was increased by up to 6% using the correct spark timing, and pollutant emissions were reduced well below the limits allowed by legislation by working with a lean mixture and by adopting an oxidizing catalyst. Finally, the global efficiency of the system increased from 45.8% to 63.2%. The right blending of different biomasses led to an important improvement of the reliability of the entire plant despite using an agrifood residual, such as olive pomace. It was demonstrated that the use of this biomass is feasible if its maximum mass percentage in a wood matrix mixture does not exceed 25%. The project was concluded with a real operation demonstration within a national park in Southern Italy by replacing a diesel genset with the analyzed and improved biomass-powered plant and by proving a decisive improvement of air quality in the real environment during exercise.

The analysis of the available literature highlights that the models proposed for gasification are calibrated and validated (at a single operating condition) for only one sample of sewage sludge (SS). In the present work, a numerical model of gasification, based on a restricted chemical equilibrium approach, is developed through the software Aspen Plus. The model is calibrated for one SS that is generated through the mechanical-biological -chemical treatment of wastewater (namely SS-A) and validated for another one that is produced by mechanical-biological processes in combination with phosphorous precipitation techniques (namely SS-B). The novelty consists in considering different sludge samples, one for calibration of the model and one for its vali-dation. Calibration and validation (for five different operating conditions) are based on experimental data on syngas generation in a fixed-bed gasifier under laboratory conditions. The developed gasification model is used to identify optimum temperature (900 degrees C) and equivalence ratio (0.2) through sensitivity analyses. Then the model is used to assess the combined heat and power generation potentiality of SS by integrating a gasifier with an internal combustion engine. This potentiality is predicted to be 2.19 and 2.53 kWh/kg SS as dry solid for SS-A and SS-B respectively. Energy recovery from SS through the proposed solution may supply around 50% of electrical energy demand to run wastewater treatment plants and from 60 to 75% of thermal energy needed for thermal drying of mechanically dewatered SS for gasification.

Abstract The need to address the global challenge of using clean energy, mitigating climatic changes and favoring sustainable development, has promoted the diffusion of new technologies for the use of renewable energy resources. Geothermal technologies can generate electricity and/or heating and cooling while producing very low levels Green House Gas (GHG) emissions and therefore play an important role in realizing these targets. In particular, low temperature applications have known a great development over the last years, thanks to the larger availability, compared to high temperature traditional ones, and to the increasing cooling demand that is also related to the global warming. A sustainable and profitable use of low enthalpy geothermal resources is strictly related to a correct analysis of ground thermal response to energy extraction/injection: in fact, sizing methodologies and optimization strategies are based on a balance of plant׳s energy demand and predictions of ground thermal variations due to these energy requirements. Therefore, an accurate mathematical modeling of thermo-dynamic behavior of the ground is fundamental for optimal design of geothermal plants for two reasons: it is the basis for the estimation of ground thermo-physical properties from the analysis of the Thermal Response Test, and it is essential in order to predict hour by hour (or short term) responses of the ground to continuously changing energy loads and therefore to estimate system energy consumption. Besides, there is a great interest in modeling thermo-fluid dynamic phenomena which occur in geothermal wells and geothermal heat exchangers as these can significantly affect the performance of the whole system. In this work, the numerical models that are currently available for simulation of the thermo-fluid dynamic phenomena occurring in low enthalpy geothermal energy systems are analyzed, underlying the main differences and recent advances in modeling approaches.

Abstract This paper presents a numerical procedure for the simulation of heat and fluid flow in a heat exchange system for exploitation of low enthalpy geothermal reservoirs. The authors employ for the first time the generalised model for the mathematical description of heat and fluid flow through saturated porous media in order to study down-hole heat exchanger, well and aquifer, using a single domain approach. Steady state operation of the system is considered and the results obtained are validated against experimental data collected for a geothermal convector prototype installed in an existing geothermal well on the island of Ischia in southern Italy. The comparison shows that the proposed procedure can be successfully used for the simulation of this type of problems, and represents an excellent tool for down-hole heat exchangers optimization. The results of the present model are employed here to analyse the approximate boundary conditions that were previously developed for the simulation of a simplified aquifer model.

Abstract This paper presents the performance of an organic Rankine cycle (ORC) powered by medium-temperature heat sources. A simulation model, developed by the authors, has been improved to this scope. The model is based on zero-dimensional energy and mass balances for all the components of the system. It is also strictly related to the geometrical and design parameters of its components, especially in case of heat exchangers. The model evaluates the energetic and economic performance of the system, for different operating conditions and design criteria. In particular, the model allows one to set the geometrical parameters of heat exchanger and evaluate the off-design performance of the system. Hence, it could be an useful tool in the preliminary design of the plant. The n -butane has been used as working fluid according to results of the previous authors’ work. Two types of simulations have been performed. The first simulation aims at selecting a design optimization criterion of some geometrical parameters of the shell and tube heat exchangers. The total cost of ORC plant has been selected as objective function. The parametrical analysis has been performed in steady-state regime. The second simulation evaluates the off-design performance of the ORC power plant. The thermal input of the cycle, i.e. diathermic oil coming from the heat source, has been varied in terms of mass flow rate and temperature to analyze the plant response to variations of boundary conditions starting from the design point. With respect to the total cost minimization, as objective function, the simulation results show that for all heat exchangers the higher the heat transfer area, the higher the net power generated and income. Instead, the evaporator shows different trends, hence it represents a key element in ORC design. The geometric optimization of heat exchangers allows the ORC to increase the economic benefit, the net power generated and the global efficiency of about 21.06%, 20.01% and 33.60% respectively. The results of the off-design analysis show that the heat source mass flow rate is a key parameter in net power generation. Fixed the heat source temperature on the upper bound of its variation range (185 °C), the net power generation shows both the maximum and minimum value, 335.4 kW and 269.3 kW, in correspondence of the lowest and the highest value of heat source mass flow rate respectively. Moreover, the results show that the plant efficiency decreases as both heat source mass flow rate and temperature increase. Its maximum value, 14.7%, is achieved for heat source temperature and mass flow rate equal to 155 °C and 18 kg/s, while its minimum value, 9.54%, is reached for heat source temperature and mass flow rate equal to 185 °C and 24 kg/s.

Abstract This paper experiences the potential of the use of Building Information Modeling (BIM) technique as a strategy to facilitate the energy performance analysis of existing buildings with historical relevance. The relationship between BIM and sustainability is an emerging concept which is becoming more and more interesting in the construction industry. The different methods for energy modeling of buildings provided in the literature usually imply the use of dynamic simulation software, such as EnergyPlus and TRNSYS, whose graphical interfaces are essential and not particularly user-friendly, if compared to the more popular CAD. Modelling in BIM environment, on one hand, helps to speed up certification procedures and, on the other hand, to define a new work philosophy during the design of energy efficiency interventions, thanks to the software’s interoperability. This study focuses on the analysis of the energy performance of the Maritime Station of Napoli, in southern Italy, located in the Angevin wharf and hosting the port terminal. The procedure employed for the study is a BIM working procedure, therefore involving different software tools. The authors have analyzed and compared the results obtained with different tools, to verify the efficiency of their interoperability, together with the parameters that most influence the analysis. Finally, the authors evaluate the dependency of the results from climatic conditions, conducting the analysis for other two Italian cities, located in different Italian climatic zones.