Il laboratorio "Tecnologie solari a concentrazione e idrogeno da fonti energetiche rinnovabili" di Sardegna Ricerche

2009-03-26Sala anfiteatro, Via Roma 253, CagliariVerso l'idrogeno. Convegno divulgativo sulle iniziative per la diffusione delle tecnologie dell’idrogeno in Itali

Comparison of Medium-size Concentrating Solar Power Plants based on Parabolic Trough and Linear Fresnel Collectors

Abstract This paper compares the performance of medium-size Concentrating Solar Power (CSP) plants based on an Organic Rankine Cycle (ORC) power generation unit integrated with parabolic trough and linear Fresnel collectors. The CSP plants studied herein use thermal oil as heat transfer fluid and as storage medium in a two-tank direct thermal storage system. The performance of the CSP plants were evaluated on the basis of a 1 MW ORC unit with a conversion efficiency of about 24% and by considering different values of solar multiple and thermal storage capacity. The comparative performance analysis of the two CSP solutions was carried out with reference to the direct solar energy availability of Cagliari, Italy (1720 kWh/m2y) on a yearly basis by means of specifically developed simulation models. The results of the performance assessment demonstrate that CSP plants based on linear Fresnel collectors lead to higher values of electrical energy production per unit area of occupied land. The highest specific energy production of CSP plants based on linear Fresnel collectors is about 55-60 kWh/y per m2 of occupied land and it is achieved with solar multiples in the 1.74-2.5 range and storage capacities in the range of 4-12 hours. The highest specific production of the solutions based on parabolic trough collectors is about 45-50 kWh/y per m2 of occupied land and is achieved with lower solar multiples (around 1.5-2.3). Owing to their better optical efficiency, the use of parabolic troughs gives better values of energy production per unit area of solar collector (about 180-190 kWh/m2 vs. 130-140 kWh/m2)

Use of weather forecast for increasing the self-consumption rate of home solar systems: An Italian case study

With the aim of increasing the self-consumption rate of grid-connected Photovoltaic (PV) home systems, two main options can be implemented: the inclusion of an energy storage system, in particular a battery bank, and the adoption of a Demand Side Management (DSM) strategy. However, both the reshaping of the load consumption curve with the displacement of deferrable loads and the optimal management of the battery bank require estimation of the daily PV generation profile. The assessment of the on-site energy production can be carried out based on weather forecast data. However, the latter are characterized by uncertainty, which may affect the achievable self-consumption rate. This work investigates the influence of weather forecast errors on the performance of home PV systems equipped with a battery bank and characterized by a certain share of deferrable loads. Two different weather forecast services are considered, referring to the annual meteorological conditions occurring in Rome, and energy consumption data for 150 different households are analysed. The self-consumption rate is maximized by solving a suitable optimization problem, while different combinations of relative battery capacity, PV-to-load ratio and share of deferrable loads are considered. Two different approachesâ\u80\u94deterministic and stochasticâ\u80\u94are adopted and compared with an ideal approach where the PV generation profile is perfectly forecasted. The results show that the adoption of the deterministic approach leads to a reduction in the achievable self-consumption rate in the range of 0.5â\u80\u934.5% compared to the ideal approach. The adoption of a stochastic approach further reduces the deviations from the ideal case, especially in the case of consumption profiles with a high share of deferrable loads. Finally, a preliminary economic analysis proves that the use of a battery bank is not yet a cost-effective solution and a price reduction of the current battery prices is therefore required

Multi-objective thermo-economic optimization of biomass retrofit for an existing solar organic Rankine cycle power plant based on NSGA-II

Non-dominated sorting genetic algorithm (NSGA-II) was deployed in this paper for multi-objective thermo-economic optimization of biomass retrofit for an existing solar organic Rankine cycle (ORC) power plant. The existing plant consists of a field of linear Fresnel collectors (LFC), integrated directly with two-tank thermal energy storage (TES) system, which interfaces with ORC power block. The real solar-ORC plant currently runs at Ottana, Italy, albeit with some technical challenges basically due to inconsistent availability of solar irradiation. In order to upgrade the plant, a novel scheme had been proposed to install a biomass unit in parallel to the solar field, such that both LFC/TES and biomass furnace could directly and independently satisfy fractional thermal input requirement of the ORC. Being a retrofit system, existing design parameters of all the already operating units were imposed as equality constraints in this study, and the combustion excess air, as well as pinch point temperature difference of furnace heat exchangers that optimize the hybrid plant were investigated. Results showed that biomass mass flow rate of 0.133 kg/s and investment cost rate of 57 €/h are optimal for the studied biomass retrofit scheme. At this optimum point, excess air was obtained as 56%, furnace heater pinch point temperature difference as 28.8 °C and air pre-heater pinch point temperature difference as 38.5 °C. More generally, results showed that excess air value of less than 100%, furnace heater pinch point temperature difference of less than 80 °C, and air pre-heater pinch point temperature difference of less than 80 °C would optimize the studied biomass retrofit scheme. Keywords: Solar-Biomass power plant, Organic Rankine cycle, Hybrid renewable energy, Multi-objective optimization, Non-dominated sorting genetic algorithm (NSGA-II), Power plant retrofi

Impianti motori a vapore e a gas : con cenni sugli impianti combinati e sugli impianti idroelettrici

Il presente volume di “Impianti motori a vapore e a gas” è il frutto di una rielaborazione del testo di “Appunti alle lezioni di Sistemi energetici”, che per anni è stato un valido supporto didattico per gli allievi del corso di laurea in Ingegneria Meccanica dell’Università degli Studi di Cagliari. Il volume contiene tutti gli argomenti trattati nell’insegnamento di “Sistemi energetici”, sviluppati ed esposti con giusto approfondimento, e fornisce allo studente uno strumento completo ed esauriente, utile per lo studio della materia e per la preparazione all’esame. Oltre alla trattazione degli impianti motori termici a vapore e a gas, che costituiscono le tecnologie principalmente utilizzate per la generazione dell’energia elettrica, il volume riporta cenni sugli impianti combinati gas-vapore e sugli impianti idroelettrici, propedeutici ad approfondimenti sviluppati nell’ambito di altri insegnamenti specialistici.This volume of "Steam and gas power systems" is the result of a reworking of the text of "Notes to the lessons of Energy systems", which for years has been a valid teaching aid for students of the Mechanical Engineering degree course of the University of Cagliari. The volume contains all the topics covered in the teaching of "Energy systems", developed and exposed with the right depth, and provides the student with a complete and exhaustive tool, useful for the study of the subject and for preparing for the exam. In addition to the treatment of steam and gas thermal power plants, which constitute the technologies mainly used for the generation of electricity, the volume contains hints on combined gas-steam plants and hydroelectric plants, preparatory to insights developed in the context of other specialized teachings

Modeling and simulation of an isolated hybrid micro-grid with hydrogen production and storage

Abstract This work relates the study of system performance in operational conditions for an isolated micro-grid powered by a photovoltaic system and a wind turbine. The electricity produced and not used by the user will be accumulated in two different storage systems: a battery bank and a hydrogen storage system composed of two PEM electrolyzers, four pressurized tanks and a PEM fuel cell. One of the main problems to be solved in the development of isolated micro-grids is the management of the various devices and energy flows to optimize their functioning, in particular in relation to the load profile and power produced by renewable energy systems depending on weather conditions. For this reason, through the development and implementation of a specific simulation program, three different energy management systems were studied to evaluate the best strategy for effectively satisfying user requirements and optimizing overall system efficiency

A Study of a Packed-bed Thermal Energy Storage Device: Test Rig, Experimental and Numerical Results☆

Abstract This paper presents the experimental set-up built at the DIMCM of Cagliari University to study a thermal energy storage (TES) system based on alumina beads freely poured into a carbon steel tank using air as heat transfer fluid. The system is instrumented with several thermocouples to detect axial and radial temperature distribution as well as reservoir wall temperature. Experimental temperature distribution along the storage system was compared with the numerical ones obtained by a two-phase one dimensional Schumann model. Numerical results show good agreement with the experimental results if thermal properties are considered as temperature dependent and the experimental temperature profile at the top of the bed is used for simulations

Performance assessment of Adiabatic Compressed Air Energy Storage (A-CAES) power plants integrated with packed-bed thermocline storage systems

Among energy storage technologies, compressed air energy storage (CAES) systems have undergone a real development since the 70s, although only two large-size commercial plants are operating worldwide. CAES systems allow very large energy storage to be performed, accumulating compressed air to be used for electrical energy generation. In recent years, A-CAES (Adiabatic Compressed Air Energy Storage) plants have had an important role. This technology allows the storage of the thermal energy released during air compression to be used for heating the compressed air during electricity generation, avoiding the consumption of fossil fuels. The main objective of this paper is to propose an innovative system solution for large-size A-CAES plants. The proposed configuration is characterized by: (i) a compression train based on two axial compressors constantly operating at design conditions and a centrifugal compressor fully devoted to managing the pressure variation, (ii) a thermocline thermal energy storage (TES) system based on a packed bed of solid material located between the low-pressure and high-pressure compressors, (iii) an expansion train based on a high-pressure radial turbine and a low-pressure axial turbine. TES performance was evaluated with integration with the A-CAES plant through a dedicated numerical simulation model. Operating modes for managing the high-pressure and low-pressure turbines through air throttling and high-pressure turbine bypassing were also studied. Finally, an in-depth analysis of the off-design behaviour of the different A-CAES components was carried out. Globally the A-CAES round trip efficiency exceeds 0.7â0.75

Numerical Investigation of a Packed Bed Thermal Energy Storage System with Different Heat Transfer Fluids

Abstract This paper presents the results of a numerical investigation on the transient behaviour of a packed bed thermal storage unit using different fluids: oil, molten salt and air. The storage material consists of loosely spherical particles of alumina packed in a reservoir wherein the heat transport fluid flows from the top to the bottom in the charging phase, and in the opposite way in the discharging phase. The process of charge/discharge of the storage system gives rise to a typical temperature distribution along the flow direction defined "thermocline". The main objective of this work is to analyze the temperature distribution along the storage system and the formation of the thermocline for repetitive consecutive cycles, evaluating the progressive reduction of the stored energy in the solid material for every new cycle. The numerical investigation is based on a two-phase one-dimensional modified Schumann model, where thermodynamic properties of the fluid are temperature dependent