1,721,064 research outputs found
Application of Membrane Reactor and PEMFC-based Micro-CHP System in Off-grid Applications
No full textThe performance of an innovative m-CHP, rated at 5 kWel, is assessed at full and partial load. The system is powered by a polymer electrolyte membrane fuel cell (PEMFC), and hydrogen is produced via ethanol reforming in a membrane reactor. The membrane reactor selectivity may decrease with time and the separated hydrogen may contain other gases with penalties on cell voltage and system efficiency. To account for this aspect, the system is simulated at steady-state conditions by 1D phenomenological models of the membrane reactor and the fuel cells stack, for different hydrogen purities and load fraction. The fuel cell efficiency is affected by the presence, at anode side, of CO and inert gases coming from the membrane reactor and from the cathode side by membrane electrode assembly (MEA) cross-over, as well as by voltage decay over time. Then the innovative micro-combined heat and power (m-CHP) system, coupled with a heat pump, and heat and electricity storage systems, is applied to two off-grid houses, located in different climatic regions: it allows about 20% of fuel saving per year compared to a conventional m-CHP based on an internal combustion engine. The fuel saving reduces to 15% in case of low-purity hydrogen, and 10% in worst conditions with low-purity hydrogen and degraded cells
Soiling of solar collectors – Modelling approaches for airborne dust and its interactions with surfaces
No full textThis literature review deals with the well-known problem of soiling in solar plants, which it severely affects the energy yield of solar power plants. A loss of reflectivity due to soiling reduces the entire productivity of the plant by limiting the energy harvested (i.e. the incoming direct normal irradiance is not properly reflected towards the right focus). On the other hand, the costs of maintenance and cleaning of the collectors represent a significant component of the plant operational costs. Therefore, in this paper, a multi-disciplinary literature review is conducted with the aim of collecting existing models for the key processes, organising them into a ‘dust life cycle’. This cycle is divided into four steps: Generation, Deposition, Adhesion, and Removal; with emphasis on the interaction between dust particles and solar collectors’ surfaces. Generation deals with the loading of atmosphere with dust particles, deposition concerns the processes that actually bring airborne dust onto the collectors’ surface, adhesion and removal represent the competing forces whose balance determine which particles remains adherent on the collectors and which are detached. The intent is to provide a complete framework for the development of a future physical model for the prediction and estimation of the actual soiling of the solar collectors, which engineers can implement in order to maximize the revenues of CSP plant, pushing towards more clean and sustainable energy production technologies
Improving the traditional levelized cost of electricity approach by including the integration costs in the techno-economic evaluation of future photovoltaic plants
Full text linkThe levelized cost of electricity (LCOE) is a techno-economic parameter used to evaluate the cost of a kilowatt-hour of energy produced from a selected power plant. The initial investment, annual operation and maintenance costs together with the annual energy production are some of the input data needed to determine the LCOE. The most typical approach to calculate the LCOE does not account for the interaction of the new power plant with the existing energy system, assuming indirectly the power plant as stand-alone. This can be misleading in scenarios with high variable renewable energy sources (VRES) penetration as costs related to overproduction, reinforcement of the grid and additional efforts of existing fossil fuels power plants to satisfy the electricity demand that is not instantly covered by VRES production are not accounted for. The aim of this work is to define a general methodology of easy application for the estimation of these additional costs, called integration costs, of the photovoltaic (PV) technology with the corresponding parameter called system LCOE. In order to demonstrate the importance of the new definition, the methodology is applied to the future Italian energy system and PV sector foreseen for the year 2030. The Italian PV LCOE in 2030 calculated with the usual methodology ranges from 12.55 to 15.93 €/MWh, while the system LCOE can be as high as 22 €/MWh with a relevant increase by on average 50%. In case of addition of storage to PV systems, the system LCOE after the addition of the integration costs ranges from 45 to 51 €/MWh. However, even when batteries and integration costs are included, PV remains competitive compared to the market price
Modelling of an existing neutral temperature district heating network: Detailed and approximate approaches
Full text linkThis paper deals with the modelling of an existing neutral-temperature district heating network, meaning that the distribution temperature is close to the ambient temperature, with decentralised heat pumps. The considered case is located in Ospitaletto, Italy. Heat sources are given by industrial waste heat at about 25◦C and aquifer wells at about 15◦C. Two models are used to analyse the network: a detailed model able to calculate local values of operating parameters and an approximate model focused on energy balances aggregating all users with a lumped demand. Both models include the behaviour of heat pumps, a feature not available in other district heating modelling tools. An entire year of operation is considered, with an hourly time resolution. Load profiles are provided as inputs, while the main outputs consist of energy balances and primary energy consumptions. The corresponding results are compared, showing a reasonable agreement, where the approximate model underestimates the overall electricity consumptions by about 15% with respect to the detailed model. On the other hand, the different information levels and execution times (the detailed model requires about 30 min to solve the considered network for a full year with hourly time steps, while the approximate model is almost immediate) make the two models suitable for different purposes, like the simulation of control solutions for the detailed one and scenario analysis for the other
MILP and MINLP models for the optimal scheduling of multi-energy systems accounting for delivery temperature of units, topology and non-isothermal mixing
No full textMulti-Energy Systems (MES) represent an increasingly important element in the undergoing energy transition, as they can effectively supply different energy vectors to their users through the coordinated local management of a set of different energy sources. This work presents an innovative Mixed Integer Linear Programming (MILP) formulation for the operational planning of MES featuring thermal generators arranged in series and/or parallel with different and/or variable heat delivery temperatures and filled-tank thermal energy storages. The proposed model can approximate the non-isothermal mixing processes occurring in water collectors and storage systems through the definition of a set of virtual headers at constant temperature levels. To assess the accuracy of the MILP model (called water-flow MILP), a detailed MINLP model of the MES is formulated (water-flow MINLP). The two models are compared with the energy-flow MILP model adopted in the literature (neglecting details of the internal MES arrangement and supply temperatures), considering a set of real-world case studies consisting in different possible MES designs designed to serve the District Heating Network of our university. Results indicate how literature energy-flow models tend to underestimate operating costs by as much as 16% and define an operational solution not able to reach the target heat delivery temperature. The proposed linearized water-flow MILP model allows to identify scheduling solutions which are not only feasible but also equal or better than the solutions found by solving the MINLP model. For MES with multiple dispatchable units (e.g., CHP engine, heat pumps and boilers), the MINLP model cannot be solved to global optimality within 1 h while the MILP water flow model can find feasible solutions with 6% lower cost
Preliminary investigation of the influence of equations of state on the performance of CO2 + C6F6 as innovative working fluid in transcritical cycles
Full text linksCO2 power cycle is the most investigated and most promising technology for replacing conventional steam cycle in CSP plants. Nevertheless, the efficiency of sCO2 power cycle is strongly penalized by high ambient temperatures which are typical of favourable CSP locations. This paper focuses on a new working fluid for power cycles which consists of CO2 blended with C6F6. The addition of C6F6 increases the fluid critical temperature allowing for a condensing cycle for ambient temperatures up to 45 °C. The calculated gross mechanical efficiency of the innovative cycle is around 42% when adopting a typical Peng Robinson equation of state with van der Waals mixing rules for a maximum operating temperature of 550 °C and a minimum cycle temperature of 51 °C. This performance varies just of ±0.1% if the prediction of the binary interaction parameter of the Peng Robinson is over- or under-estimated by 50%, but more significantly if other equations of states are adopted (up to 1% points). Moreover, a detailed analysis on the operating conditions of the cycle components highlighted that components design is affected by the adopted EoS. A sensitivity analysis is then performed to identify where the largest differences in predicting the efficiency of the cycle occur
Experimental and analytical study of an innovative integrated dual-source evaporator for solar-assisted heat pumps
Full text linkEnergy efficiency is an effective way to produce energy reducing the impact of fossil fuels in the energy sector. A promising solution for residential application consists of producing domestic hot water and space heating using Solar-Assisted Heat Pump. This work analyzes the performance of a Solar-Assisted Heat Pump with an innovative Integrated Dual-Source Evaporator connected to Photovoltaic/Thermal modules. The concept is firstly validated in laboratory where the Solar-Assisted Heat Pump is operated in real environmental conditions. Then, a numerical model is developed in Matlab® to identify the optimal design of the Integrated Dual-Source Evaporator. Numerical results show that the performance of the system are significantly affected by the solar irradiance and in a wide operating range the photovoltaic production overcomes the Heat Pump consumptions. Compared to a standard Air-Source Heat Pump, the proposed concept shows up to 14% higher Coefficient of Performanc
Optimization of PEM Fuel Cell Operation with High-purity Hydrogen Produced by a Membrane Reactor
No full textAn innovative micro‐cogeneration system (m‐CHP) based on membrane reformer and polymer electrolyte membrane fuel cell (PEMFC) is developed within the FluidCELL project. The purity of the hydrogen separated by the membrane reformer can decrease over time, due to membrane/sealing degradation, therefore a methanator is adopted to prevent CO poisoning of the fuel cell. This paper investigates the optimal control strategies of a polymer electrolyte membrane (PEM) fuel cell at different hydrogen purities by using a detailed 1D model, including the CO poisoning on the anode Pt‐Ru catalyst, and calibrated against experimental data. Simulation results show that the system is able to work also with low‐purity hydrogen thanks to the effectiveness of the methanator, the resistance to CO poisoning of the Pt‐Ru anode catalyst, the small voltage drop due to inert gases accumulation in the anode recirculation loop: at 0.3 A cm−2 as current density, the stack efficiency is always above 60% even when the membranes selectivity drops to 5 102
Renewable Energy Communities: Business Models of Multi-family Housing Buildings
No full textThe new European directive on renewable sources (RED II), which entered into force in December 2018, has opened new perspectives on the consumers and the decentralization of energy production. The purpose of this work is to analyze the inclusion of the energy communities in the Italian regulatory framework. The analysis focuses on the strategies that can be adopted by tenants to share rooftop photovoltaic module production and stored electricity and the consequent economic impact on their electricity bills. We have created a program that simulates various business models to be proposed to prosumers of multi-family housing buildings through which the economic return of every participant in the energy community is evaluated. The program receives as input the data of the renewable energy community, and it returns as output the electricity bill of each tenant, while considering the cost of the energy purchased from the grid and the economic revenues from self-consumption and from the sale of the excess production for each of the business model adopted. Therefore, the advantages and disadvantages of the models are highlighted. The net metering is profitable, but excluding it, energy sharing within communities would be the best scenario: In the considered case study, with 10 kW of PV installed power for a community, the reduction in costs is equal to 15%. Moreover, the convenience of a heterogeneous set of electricity demand profiles of the members is clearly evident (in the considered case study, this entails a 10% reduction in costs). Finally, the most proper business model must be selected, to assure the benefit for each energy community participant: The examined models result in a bill differential between −20 and 36% for each participant
A novel stochastic model for flexible unit commitment of off-grid microgrids
No full textThis work proposes a novel two stage stochastic optimization approach for the Energy Management System (EMS) of MicroGrids (MG). It combines the scenario-based uncertainty characterization with the use of piecewise affine correction rules for the recourse decision. These rules are used by the EMS to set the commitment status of the programmable generators and to correct the storage management as a function of the realization of the uncertainty, measured in real-time. The novel stochastic model is integrated in a hierarchical EMS, based on two control layers: the first one employs the proposed stochastic approach to determine the daily strategic scheduling of the MG, and the second layer optimizes the real-time dispatch. The proposed EMS is applied to a real-world case study of a rural MG. Results indicate that the proposed EMS outperforms state-of-the-art optimization approaches in terms ofservice reliability (99.8%) and fuel efficiency. Moreover, rolling horizon simulations of the proposed stochastic model showed 100% reliability, and 30% of fuel cost savings with respects to state-of-the-art methods. The novel EMS is then deployed on a laboratory-scale microgrid (Multi-Goods MicroGrid Laboratory, MG2lab), demonstrating secure and economic operations. A comparative analysis is made with respect to deterministic and standard stochastic two-stage approaches; the proposed solution outperforms the other models during real operations, with savings of fuel cost up to almost 10%
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