Andrea Lazzaretto

Evolutionary algorithms for multi-objective energetic and economic optimization in thermal system design

Thermoeconomic analyses in thermal system design are always focused on the economic objective. However, knowledge of only the economic minimum may not be sufficient in the decision making process, since solutions with a higher thermodynamic efficiency, in spite of small increases in total costs, may result in much more interesting designs due to changes in energy market prices or in energy policies. This paper suggests how to perform a multi-objective optimization in order to find solutions that simultaneously satisfy exergetic and economic objectives. This corresponds to a search for the set of Pareto optimal solutions with respect to the two competing objectives. The optimization process is carried out by an evolutionary algorithm, that features a new diversity preserving mechanism using as a test case the well-known CGAM problem.

Parameter Setting for a Tubular SOFC Simulation Model

Several empirical assumptions deriving from observations and measurements of the physical processes are involved in the modeling of Solid Oxide Fuel Cells (SOFCs). An insight of the main models proposed in the literature is given to present the characteristics and limits of these assumptions for the various existing configurations. The basic structure and equations of the models are discussed in details, focusing particularly on the parameters that are to be set to achieve reliability and accuracy. According to this discussion, a zero-dimensional model for a tubular Solid Oxide Fuel Cell (SOFC) is then presented. The model demonstrates good capability in predicting SOFC characteristic curves as they appear in the literature.

Cross-flow fan design guidelines for multi-objective performance optimization

Abstract More than one century has passed since Mortiers first cross-flow fan patent in 1891, but the design is still based on empirical considerations and on the re-adaptation of previously developed configurations. The complexity of the flow field inside the machine is the main reason for the lack of an established design procedure, because the characteristics of the eccentric vortex that forms within the impeller are deeply influenced by the geometry of the impeller and also of the casing. In this paper, the objectives that should be pursued in the design procedure are discussed and defined, and guidelines for cross-flow fan design are determined through the analysis of an experimental database of fan performance and efficiencies, which is obtained by the systematic variation of the most important geometric parameters of the casing in combination with different impellers.

A New Thermoeconomic Method for the Location of Causes of Malfunctions in Energy Systems

Diagnosis procedures primarily aim at locating the control volumes where anomalies occurred. This is not a simple task since the effects of anomalies generally propagate through the whole system and affect the behavior of several components. Some components may therefore present a reduced efficiency, although they are not sources of operation anomalies, due to nonflat efficiency curves. These induced effects are a big obstacle in the use of thermoeconomic techniques for the search of the origin of the anomalies. On the other hand, the real cause of the alteration of component behavior is the modification of its characteristic curve, due to degradation or failures. According to this concept, a new approach, based on an indicator measuring the alteration of the characteristic curve of the component affected by the operation anomaly, is proposed and applied to a test case power plant.

A thermodynamic approach to the definition of the HAT cycle plant structure

Abstract An optimization procedure of the Humid Air Turbine (HAT) cycle plant structure is presented here aimed at maximizing the total plant efficiency. The procedure is based on the design optimization of a “basic configuration of the plant” including “basic components” (compressor, turbine, combustion chamber, regenerator and saturator), always existing and connected in the same way in the plant structure, and the heat exchanger section which is viewed as a “black-box” where the heat transfer between hot and cold thermal flows occurs independently of the number and interconnections of the heat exchangers. The optimal boundary conditions between basic components and black-box are determined by means of a suboptimization procedure performed in each step of the main optimization procedure using rules of Pinch Technology and Second Law insights. Rules of pinch technology and second law insights are then used to determine the heat exchanger networks (HENs) within the black-box that fulfill the optimum design conditions at the black-box boundaries. Accordingly, “optimal” structures of the plant are determined by the combination of basic components and “optimal” HENs.

Dynamic Modeling of the Microalgae Cultivation Phase for Energy Production in Open Raceway Ponds and Flat Panel Photobioreactors

A dynamic model of microalgae cultivation phase is presented in this work. Two cultivation technologies are taken into account: the open raceway pond and the flat panel photobioreactor. For each technology, the model is able to evaluate the microalgae areal and volumetric productivity and the energy production and consumption. Differently from the most common existing models in literature, which deal with a specific part of the overall cultivation process, the model presented here includes all physical and chemical quantities that mostly affect microalgae growth: the equation of the specific growth rate for the microalgae is influenced by CO2 and nutrients concentration in the water, light intensity, temperature of the water in the reactor and by the microalgae species being considered. All these input parameters can be tuned to obtain reliable predictions. A comparison with experimental data taken from the literature shows that the predictions are consistent, slightly overestimating the productivity in case of closed photobioreactor. The results obtained by the simulation runs are consistent with those found in literature, being the areal productivity for the open raceway pond between 50 and 70 t/(ha*year) in Southern Spain (Sevilla) and Brazil (Petrolina) and between 250 and 350 t/(ha*year) for the flat panel photobioreactor in the same locations.

Forward sweep to improve the efficiency of rotor-only tube-axial fans with controlled vortex design blades

Common blade design techniques are based on the assumption of the airflow laying on cylindrical surfaces. This behaviour is proper only for free-vortex flow, whereas radial fluid migration along the span is always present in case of controlled vortex design blades. The paper presents a design procedure to increase aeraulic efficiency of fan rotors originally designed using a controlled vortex criterion, based on the assumption that a blade section positioning taking into account the actual airflow direction could be beneficial for rotor aeraulic performance. The proposed procedure employs a three-dimensional aerofoil positioning and blade forward sweep. The procedure is applied to a rotor-only tube-axial fan featuring a 0.44 hub-to-tip ratio, an almost constant swirl velocity distribution at the rotor outlet and a quite low blade Reynolds number. Rotor prototypes deriving from step-by-step blade modifications are experimentally tested on an ISO 5801 standard test rig. Results show the importance of considering radial fluid migration for highly loaded rotors.

Superimposition of Elementary Thermodynamic Cycles and Separation of the Heat Transfer Section in Energy Systems Analysis

In a wide variety of thermal energy systems, the high integration among components derives from the need to correctly exploit all the internal heat sources by a proper matching with the internal heat sinks. According to what has been suggested in previous works to address this problem in a general way, a “basic configuration” can be extracted from the system flowsheet including all components but the heat exchangers, in order to exploit the internal heat integration between hot and cold thermal streams through process integration techniques. It was also shown how the comprehension of the advanced thermodynamic cycles can be strongly facilitated by decomposing the system into elementary thermodynamic cycles which can be analyzed separately. The advantages of the combination of these approaches are summarized in this paper using the steam injected gas turbine (STIG) cycle and its evolution towards more complex system configurations as an example of application. The new concept of “baseline thermal efficiency” is introduced to combine the efficiencies of the elementary cycles making up the overall system, which demonstrates to be a useful reference to quantify the performance improvement deriving from heat integration between elementary cycles within the system.

Energy Integration in the cement industry

Cement production is an energy intensive industrial process that requires heat to be supplied at high temperature levels under the constraints of gas-solid heat exchange phenomena and the kinetics of chemical reactions. In this paper, the use of Pinch Analysis and Process Integration techniques to optimize the energy efficiency of the cement production will be explored. The aim is to use process modeling to characterize cooling and heating requirements of the process, focusing on the gas-solid heat exchanges while including waste fuel utilization. The heat cascade model is adapted to account for gas-solid and gas-gas heat recovery used to calculate the heat recovery in the process. A mixed integer linear programming problem is solved to calculate the integration of the available heat; this model optimizes the heat recovery and the energy conversion efficiency considering different fuels, heat recovery options and process operating conditions.

Thermodynamic and Pinch analyses for improving efficiency and structure of a CRCC plant with natural gas reforming and CO2 absorption

The need for improving performance and efficiency of thermal systems while reducing costs and environmental impact generally implies complex structures with several components and re-circulating material and energy flows to be considered. Some of the links among these components are often imposed by technological or ambient constraints, other links are decided by the designer to pursue one of more desired objectives (typically efficiency maximization, reduction of costs and environmental impact). In particular, the objective of efficiency maximization requires to correctly exploit all thermal flows available, that is to match hot and cold streams in order to minimize losses for the overall system. To do that, a method was suggested by the first author that consists in maximizing the efficiency of a so called “basic plant configuration” in which hot and cold thermal flows are considered independently of the structure of the heat exchanger network that will realize the heat transfer, whereas the other components have a fixed position in the structure lay-out. The method is applied here to a Chemically Recuperated Combined Cycle (CRCC) power plant with natural gas reforming and CO2 absorption. Optimum matching of hot and cold streams is obtained using thermodynamic and Pinch Technology evaluations at each step of an overall design improvement procedure of the total plant. Results show how the information obtained by combining Pinch Analysis and thermodynamic evaluations can be useful to obtain guidelines for improving the design configuration and parameters of complex energy systems.© 2004 ASME