1,721,010 research outputs found
Stochastic optimization of combined heat and power units: Industrial facility in Russian Federation
No full textThis study presents a two-stage stochastic optimization model for the short-term operation scheduling of a Combined Heat and Power (CHP) system. Given the design of a cogeneration system, comprising thermal energy storage, the model aims at determining the units’ operating schedule that will provide the minimal operating and maintenance costs. The proposed model takes into account ambient conditions, time-varying loads and Russian Federation policies on gas and electricity tariffs, as well as uncertainty. The original problem is a Mixed Integer Non-Linear Programming (MINLP) one. Where the nonlinearities are due to the performance curves of the units. Such curves have been Piece-Wise Linearized (PWL) to convexify the model. Daily and weekly problems are optimized to better manage the storage as well as start-up and shut-down procedures. The so defined weekly MILP problem can easily reach tens of thousands of variables, with thousands of them integers, making it even more challenging to deal with uncertainty parameters such as temperature, electric and heat loads, and market prices of electricity based on its historical data. The expected value is calculated for each of these parameters based on historical data. Additionally, a certain degree of uncertainty has been introduced in order to make the solution robust against stochastic parameters deviation, assessing their impact via sensitivity analysis. Uncertainty has been assessed by means of descdist function utilizing R software. The model is written in the AMPL modeling language and has been applied to a real test case – an assembly plant in Saint-Petersburg, Russian Federation. The computational results of the stochastic optimization are qualitatively discussed and benchmarked against the results of the equivalent deterministic model, with the expected value calculated for each uncertain parameter based on its historical data, and perfect foresight approach
Control schemes for district heating substations considering user-defined building's indoor temperature
No full textBuildings’ space heating (SH) systems are connected to the district heating (DH) systems via heat exchangers (HEs) located at substations. In the HEs, proportional–integral (PI) controllers adjust the DH mass flow rate by monitoring the SH system's supply temperature according to the set-points given by an outdoor temperature compensation curve without any feedback about a building's indoor temperature. We introduce two alternative controller schemes with improved configurations concerning energy savings and power peak shaving. The first scheme is a PI controller monitoring the indoor temperature according to its set-points by changing the source-side mass flow rate in the HE. The second one is a cascaded PI controller consisting of the inner and outer loops of PI logic. The inner loop monitors the SH system's supply temperature according to the set-points altered by the outer loop monitoring the indoor temperature based on its set-points. The simulation studies in the TRNSYS software environment show that the new controllers’ configurations with the tuned PI constants enable the flexible maintenance of indoor temperature, boosting buildings’ integration into modern DH systems’ operation. By tuning the PI controllers’ parameters, one can manage the trade-offs between indoor temperature maintenance, energy use, and thermal load peaks. Moreover, the cascaded PI controller may reduce thermal load peaks occurring during set-point changes by 26%. On top of that, such a controller's scheme can be developed to retrofit conventional controller's one, simplifying its implementation and accelerating the transition from the traditional generation of DH systems into the new one
Extensive techno-economic assessment of combined inverted Brayton – Organic Rankine cycle for high-temperature waste heat recovery
No full textThis work is a techno-economic study of the combination of inverted Brayton cycle, and organic Rankine cycle (combined IBC-ORC) applied for high-temperature waste heat recovery (WHR) of the engine exhaust energy. In IBC, exhaust gases expand to subatmospheric pressure in the turbine, transmit heat residuals to ORC, and restore pressure to 1 atm in the compressor. The system is analysed in the Aspen Hysys software in design conditions at the case study of 1.4 MW gas-fueled internal combustion engine as a high-temperature waste heat source (470–570 °C). Firstly, the paper shows the performance of the system optimised for different ambient temperatures. The role of water condensation contained in flue gas is emphasised for these bounds. Then, the paper presents a multi-objective optimisation illustrated by Pareto fronts for the objective functions of system electric efficiency and levelized cost of energy (LCOE) in the mentioned range of exhaust temperatures. TOPSIS-based Pareto-front analysis results in recommendations of the best sets of cycle parameters in this trade-off. For exhaust temperatures 470 °C, 520 °C, and 570 °C, optimal configurations identified via TOPSIS methodology demonstrate 10.8%, 12.1%, 13.3% efficiencies with LCOE equal to 185.5 /MWh and 146.1 $/MWh correspondingly
Off-design performance analysis of a novel hybrid binary geothermal-biomass power plant in extreme environmental conditions
No full textA novel configuration of a hybrid binary geothermal biomass power plant is proposed that generates electricity through the Organic Rankine Cycle (ORC), which receives the thermal power provided by a biomass heat source through intermediate geothermal fluid. The plants are located in regions with extreme environmental conditions where water is not available, making the use of air-cooled condensers in the ORC necessary, and the seasonal variations of the ambient air temperature are remarkable. As a further novel aspect, the modification of the biomass mass flow rate is used to overcome the simultaneous harmful effects of a considerable reduction in the geothermal fluid temperature during the operative life of the plant and the more significant seasonal change of the ambient air temperature. Our original approach involves developing a simulation model of the proposed plant using the commercial software Aspen® to determine the energy performance in off-design conditions, i.e., in the presence of the simultaneous changes of the biomass flow rate, ambient air temperature and geothermal fluid temperature. The biomass flow rate is controlled to maximize the net electric power or net thermodynamic efficiency of the plant with varying ambient air and geothermal fluid temperatures. In comparison to the first operating mode, the second enables a saving of the biomass used annually in the range of 28.3%–42.6%, corresponding to the maximum and minimum geothermal fluid temperature, respectively, with the resulting detrimental effect on the yearly produced electric energy in the range of 9%–23.6%
Energy efficiency analysis for a kilo-watt class vanadium redox flow battery system
No full textA new methodology for estimation of the key characteristics of commercial scale Vanadium Redox Flow Battery (VRFB) at different operating conditions is proposed. The method is based on a set of simplified correlations that allow estimating VRFB rated power, capacity and operation time directly from the geometry of stack and tank without detailed numerical simulation of the battery. The study is focused on investigation of a kilo-watt class VRFB system (5 kW/15kWh) considering a wide range of current densities (40–100 mA cm−2). The proposed simplified approach is validated considering the most representative cases of battery operation strategies related to slow and fast modes. It demonstrated high accuracy for the estimation of rated power and operation time (average error below 3%) as well as stored energy (average error below 6%) compare to results of detailed numerical simulation. As a result, the proposed methodology can be used as a simple tool for development of proper battery usage protocol (a schedule for battery usage), which could allow avoiding over/underestimation of committed battery energy and power during battery operation. In addition, the obtained results can be also used in order to improve the accuracy of techno-economic studies determining the most economically attractive cases for application of VRFB systems
Techno-economic analysis of combined inverted Brayton – Organic Rankine cycle for high-temperature waste heat recovery
No full textMany practical cases with waste heat recovery potential such as exhaust gases of reciprocating engines, cement kilns or heat-treating furnaces, are nowadays often integrated with organic Rankine cycle to convert waste heat to the mechanical power. However, when dealing with high-temperature waste heat, organic Rankine cycle faces efficiency limit due to the physical properties of the working and thermal fluids. That gives room for further enhancement of the waste heat recovery technologies via the investigation of different non-conventional schemes as one of the possible ways. In the present work, a system introducing the combined inverted Brayton plus organic Rankine cycle is under investigation. Aspen Hysys models of both conventional organic Rankine cycle and combined cycle were designed, orienting on waste heat recovery from the heavy-load gas-fueled reciprocating engine exhaust. In this way, the performance of the combined scheme was benchmarked versus the conventional organic Rankine cycle. An assessment of the organic Rankine cycle working fluids was provided, and pentane has shown the best thermodynamic performance. The study on inverted Brayton cycle defined the remarkable effect of the water condensation in the gas duct on the inverted Brayton cycle performance. Finally, both thermodynamic and economic optimizations of the models were conducted, setting the stage for the comparison of solutions. Results have shown the 10% advantage of the combined scheme over organic Rankine cycle in generated power and system efficiency. The levelized-cost-of-energy-based optimization for variable capacity factors has highlighted above 6% advantage of the investigated solution. The analysis of the sensitivity from machines’ efficiencies and heat exchangers’ pinches has shown that with some sets of parameters, the studied scheme may concede to the organic Rankine cycle
Detailed Li-ion battery characterization model for economic operation
No full textThe rapid adoption of Li-ion batteries in electric vehicles, merchant storage applications, and as providers of ancillary services to the electric grid, requires the development of tractable and accurate models for their reliable operation. For such operations, it is common practice to use generic and ideal storage mathematical models, i.e., models in which the storage efficiency and power limits are considered constant; resulting in a possible miscalculation of the flexibility and profit gained from the use of storage units. To promote a more accurate characterization of energy storage operation, we propose a linear model representing the variable efficiencies and power limits; for both battery charging and discharging processes. Based on a battery equivalent circuit model, we analyze the non-convex behavior of the power limits and efficiencies. Through the use of a methodology for the piece-wise approximation of a concave function, we derive a linear model that characterizes both the battery power limits and the efficiencies as affine functions of the level of stored energy and the power charged/discharged. The proposed linear model is compared with a generic one under a centralized economic dispatch and merchant storage operation. A reliability analysis of the use of generic storage models is presented, highlighting the impact on the system's flexibility derived from the simplifications present in this model. The use of the proposed model guarantees the feasibility of the battery scheduling, preventing additional compensation costs for both the system operator and merchant investor
Towards future infrastructures for sustainable multi-energy systems: A review
No full textIntegration of different energy infrastructures (heat, electricity and gas vectors) offers great potential for better managing energy sources, reducing consumption and waste as well as enabling a higher share of renewables, lower environmental impact and lower costs. This paper aims at reviewing the state-of-the-art energy system infrastructures in order to provide a comprehensive overview of technologies, operational strategies, modelling aspects and the trends towards integration of heat, electricity and gas infrastructures. Various technological domains are taken into account, ranging from energy distribution networks (thermal, electric and gas), components for the energy vector conversion (e.g. combined heat and power, power to heat, power to gas, etc.) and energy storage. Furthermore, the aspects related to smart management in energy systems are investigated, such as integration of renewable energy sources and energy recovery systems
Non-Ideal Linear Operation Model for Li-Ion Batteries
No full textCurrently, the characterization of electric energy storage units used for power system operation and planning models relies on two major assumptions: charge and discharge efficiencies, and power limits are constant and independent of the electric energy storage state of charge. This approach can misestimate the available storage flexibility. This work proposes a detailed model for the characterization of steady-state operation of Li-ion batteries in optimization problems. The model characterizes the battery performance, including non-linear charge and discharge power limits and efficiencies, as a function of the state of charge and requested power. We then derive a linear reformulation of the model without introducing binary variables, which achieves high computational efficiency, while providing high approximation accuracy. The proposed model characterizes more accurately the performance and technical operational limits associated with Li-ion batteries than those present in classical ideal models. The developed battery model has been compared with three modelling approaches: the complete non-convex formulation; an ideal model typically used in the power system community; and a mixed integer linear reformulation approach. The models have been tested on a network-constrained economic dispatch for a 24-bus system. Based on the simulations, we observed approximately 12% of energy mismatches between schedules that use an ideal model and those that use the model proposed in this study
Zero dimensional dynamic model of vanadium redox flow battery cell incorporating all modes of vanadium ions crossover
No full textA 0-D dynamic mathematical model for a single Vanadium Redox Flow Battery (VRFB) cell is proposed. The model is based on the conservation principles of charge and mass transfer focusing on the precise simulation of crossover with diffusion, migration and convection. The influence of these phenomena on the capacity decay was systematically analyzed, revealing considerable impact of convection component, which dominates under diffusion and migration and mainly responsible for observed capacity loss. The model allows to simulate main characteristics of VRFB systems (such as battery voltage, state of charge, charge/discharge time and capacity decay due to crossover) with high accuracy. The model was validated with experimental data in the wide range of current densities (40–100 mA cm−2), and the results demonstrated good agreement with experiments having an average error within 5% range. In addition, the model requires a modest computational time and power, and, therefore, it can be suitable for application in advanced control-monitoring tools, which are necessary for a long-service life and sustainable operation of VRFB systems
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