Including thermal network operation in the optimization of a Multi Energy System

The combined production of different energy vectors with Multi Energy Systems is a very attractive opportunity to increase the generation efficiency, compensate the oscillations of renewable sources, and improve the flexi-bility in power generation. Optimizing their operation is a complex task, since the problem can easily reach high dimensions, representing a challenge for commercial solvers. The inclusion in the optimization of a thermal network whose simulation is based on temperatures and flowrates allows to significantly improve the applica-bility of the obtained results. In addition, the effect of the operating temperatures on the performances of thermal components should be included as well. With these purposes, the present study proposes a strategy for the operation optimization of a MES and its internal thermal network. The model relies on a decomposition approach, where the original problem is divided in two subproblems. In the first one, the MES operating costs are minimized without considering the effects of the thermal network, while in the second one, the thermal network operation is optimized in order to find the operating conditions that are more favourable to the ones found for the MES. These subproblems are iteratively solved until the process converges to a stable solution. Some efforts are taken to keep the mathematical formulation as simple as possible (the MES is a Mixed Integer Linear Pro-gramming, while the heating network is a Quadratically Constrained Programming). The developed model al-lows to find near-optimal solutions which satisfy the numerous physical and technical constraints addressed. The results provide an optimized schedule for the thermal storage in terms of mass flowrates and temperature. One of the strengths of the model is the relatively low computational time required to reach the convergence and, despite not being the global optimum, the high quality of the solution obtained

Integration of ThermoChemical Energy Storage in Concentrated Solar Power. Part 2: comprehensive optimization of supercritical CO2 power block

Abstract Among the various options of Thermo-Chemical Energy Storage, Calcium-Looping represents a promising alternative for Concentrated Solar Power plants, thanks to high operating temperatures, high energy density and absence of thermal losses. Finding the most suitable power cycle for this system is a task that has still to be solved and is not trivial because it consists in a complex process synthesis problem. From a preliminary analysis (Part 1), supercritical CO2 cycles results to be the most promising option. In the present work, the integration of this power block (pilot plant size, 2 MWe) is deeply investigated through a comprehensive analysis. Numerous thermal cycle layouts are considered and two options for the power block thermal feeding are assumed. The HEATSEP methodology (comprising genetic algorithm, pinch analysis and bisection) is adopted to optimize both components operating conditions and heat transfer processes in the discharging phase. The plant section devoted to the charging process is optimized and dimensioned taking into account the transient operation. Thanks to the complex problem structure developed, the algorithm is free to find the most suitable configuration between a huge set of feasible combinations. Both energy and economic optimizations are performed for the complete plant and, being in contrast between them, a multi-objective optimization is executed. The independent variables influence on the resulting configuration is assessed and intermediate layouts obtained from the Pareto curve are commented. Carbonator inlet temperature of reactants are observed to increase with plant efficiency. The maximum efficiency (21%) is obtained with the most complex power block (recompression, intercooling and reheating) exchanging heat directly on the carbonator wall. Less performing discharging processes are cheaper but determine higher costs of charging sections; the resulting effect is positive and the integration of simpler power blocks results economically convenient. A power cycle with single intercooling and thermal feeding performed on the carbonator outflows is the result of economic optimization (efficiency equal to 16.3%). The algorithm gives precedence to power block thermal feeding and then to reactants preheating. Novel plant layouts are designed for these configurations and data useful for further investigations are provided in the last part of this work

Integration of ThermoChemical Energy Storage in Concentrated Solar Power. Part 1: energy and economic analysis/optimization

Coupling of Concentrated Solar Power and Thermo-Chemical Energy Storage is a very interesting option because of the high efficiencies attainable with a renewable source and the large variation of solar radiation. Thermo-Chemical Energy Storage based on Calcium-Looping represents a promising opportunity thanks to high operating temperature, high energy density, null thermal losses and cheap calcium oxide precursor exploitable. The large variety of suitable power blocks and the importance of their integration in the discharging process makes it necessary to perform a coherent analysis of the selected alternatives, in order to compare them and establish the most convenient integration. Many aspects must be taken into account, such as system efficiency, investment costs and layout complexity. The purposes of the present work are: the development of a methodology to simulate the entire plant operations; the synthesis of heat recovery systems for both the charging and discharging processes; the execution of an economic analysis and the development of economic optimizations for the design/dimensioning of solar side and calciner side. Between the options investigated, power blocks based on supercritical CO2 are the most convenient both in terms of global efficiency (higher than 19%) and capital investment, keeping this advantage also for higher plant sizes. The methodology here developed and the results obtained are useful information for a deeper analysis of the most promising integration alternative, which is performed in the second part of this study