Search

• Four different heat pump systems (CVI-1S, CVI-SR, CVI-2S and CVI-3S) are analyzed. • The COP of each heat pump is maximized by optimizing the heat pump design. • The optimizations involve 167 fluids as possible refrigerants in the heat pumps. • The optimal fluids and the maximal COP are reported in charts. • The refrigerants dme, r161, r40, cf3i, r435a, r510a are among the best fluids. In this paper, numerical optimization analyses are performed for four distinct heat pump systems, namely, the CVI-1S, CVI-SR, CVI-2S and CVI-3S systems. These analyses are performed for various combinations of hot side and cold side temperature values (i.e., for hot side temperatures ranging from 25 °C to 60 °C, and for cold side temperatures ranging from -40 °C to 10 °C). A total of 167 fluids are compared in order to obtain the highest COP value, for each temperature combination investigated. A robustness analysis is also achieved so as to identify the fluids that provide a COP value larger than 95% of the maximal COP value. Results are presented as charts that allow identifying the best fluids and the most robust fluids for each heat pump system. The results show that heat pump systems involving more equipment or including separators are more efficient than the others. Moreover, the results show that the refrigerants dme, r161, r40, cf3i, r435a and r510a are among the most robust fluids for all heat pump systems investigated. [ABSTRACT FROM AUTHOR]

In this paper, the objective is to develop new graphical tools that allow straightforward identification of the optimal design of a Rankine cycle that uses LNG being regasified as a heat sink. To achieve that objective, a numerical model is developed in order to predict the value of the specific work output produced by the system. Then, numerical optimizations are performed and the results are reported in a graphical form. These new figures provide the values of the maximal specific work (objective function) for a wide range of operating conditions. These figures also provide the corresponding values of the four optimized design variables. The main result is that ethylene is the best working fluid for most of the operating conditions considered. Furthermore, a sub-optimal analysis is performed for each fluid investigated and the resulting figures allow to identify the fluids that can provide a specific work output close to that provided by the best working fluids. Finally, optimal designs identified in this paper are compared with optimal designs of the direct expansion (DE) and of the double direct expansion (D2E) systems. • A numerical model is used to optimize the Rankine-LNG design. • The specific work output is maximized, and the working fluid is optimized. • Among the 10 fluids studied, Ethylene provided the best results. • The optimized Rankine-LNG system is compared to optimized Direct Expansion systems. • Charts provided in this paper allow straightforward design of Rankine-LNG systems. [ABSTRACT FROM AUTHOR]

In this paper, a new theoretical method for upscaling the thermal conduction equation in periodic composite materials is presented. That method is named “the method of quasiperiodic fields”, and it is based on the assumption that the temperature field is quasiperiodic at the small scale. More specifically, that method allows to predict the form of the averaged governing equations, and the values of the effective transport coefficients. The new methodology involves four steps: (i) a first level averaging, (ii) the statement of a quasiperiodic problem, (iii) the development of factorized quasiperiodic problems, and (iv) the development of the closed form of the averaged thermal conduction equation. The values of the effective thermal conductivity obtained with that new method were compared with values reported in literature, and also with values obtained from direct numerical simulations. [ABSTRACT FROM AUTHOR]

In this paper, a new theoretical method for smoothing the diffusion equation in periodic porous media is presented. That strategy is named ‘the method of quasiperiodic fields’ because it is based on the postulate that the concentration is a quasiperiodic field throughout the porous medium. The method proceeds in four steps: (i) a first level averaging of the diffusion equation, (ii) the statement of a quasiperiodic problem, (iii) the statement of factorized quasiperiodic problems, and (iv) the development of a closed form for the averaged diffusion equation. A first application of these four steps on the initial diffusion equation in a periodic porous medium provides an averaged diffusion equation in which the effective diffusion coefficient varies periodically at the small scale. A second application of the four steps provides a smoothed diffusion equation in which the effective diffusion coefficient is constant. The final smoothed diffusion equation is stated in terms of the double average of the concentration field. [ABSTRACT FROM AUTHOR]

In this paper, the design of an Organic Rankine Cycle (ORC) is optimized by means of numerical simulations. The systems of interest are the subcritical and transcritical thermodynamic cycles. Optimizations are performed with the objective of determining the design that maximizes the specific power output. The design variables include the operating parameters (pressures, mass flow rates), and the best working fluid is determined by comparing the performance of 36 refrigerants. Optimization runs are performed for a wide range of geofluid temperatures (from 80 to 180 °C), and for a wide range of condenser temperature (from 0.1 to 50 °C). The results are summarized in charts that may be used as efficient tools for designing optimal geothermal power plants. Finally, an approximate analysis allowed to develop a new correlation for predicting the maximal specific power output of an ORC. [ABSTRACT FROM AUTHOR]

In this paper, three different modifications of the basic Rankine thermodynamic cycle are proposed. The objective is to increase the thermal efficiency of power systems based on Rankine cycles. The three new systems are named “Rankine-1SCR”, “Rankine-2SCR”, and “Rankine-3SCR” cycles, and they consist of linking a refrigeration cycle to the basic Rankine cycle. The idea is to use the refrigeration cycle to create a low temperature heat sink for the Rankine cycle. These three new power plant configurations are modeled and optimized with numerical tools, and then they are compared with the basic Rankine cycle. The objective function is the thermal efficiency of the systems (i.e., net power output (kW) divided by heat rate (kW) entering the system), and the design variables are the operating temperatures within the systems. Among the 84 × 84 (i.e., 7056) possible combinations of working and cooling fluids investigated in this paper, it is shown that: (i) the Rankine-1SCR system is advantageous for 1338 different fluid combinations, (ii) the Rankine-2SCR system is advantageous for 772 different fluid combinations, and (iii) the Rankine-3SCR system is advantageous for 768 different fluid combinations. [ABSTRACT FROM AUTHOR]

In this paper, six different modifications of the Double-Flash power plants are proposed. These modifications are named “interstage heating” and consist of additional heat exchangers properly located in the system. The six interstage heating designs are analysed, optimized and then compared to an optimized Double-Flash reference power plant. The objective function is the power plant specific output (kJ/kg), and the design variables are the separator temperatures (°C) and the split fraction. Optimizations are performed for a wide range of reservoir temperatures (i.e., from 140 °C to 240 °C). Results show that interstage heating processes may increase the specific output of the plant by about 5%, decrease the liquid content in the low pressure turbine by about 50%, and decrease the required cooling capacity of the plant by about 10%. On the other hand, the analysis showed that the new designs proposed have negligible influence on the high pressure turbine liquid content or on the silica saturation coefficient. [ABSTRACT FROM AUTHOR]

Abstract: The equations that govern the various transport phenomena occurring in a polymer electrolyte membrane fuel cell (PEMFC) were formulated and implemented in a commercial finite element software, in order to predict the fuel cell current density with respect to the operating conditions. The numerical model showed polarization curves in accordance with literature. The catalyst utilization was then improved by optimizing the platinum distribution (design variable) in the fuel cell, so as to maximize current density (objective function) for a fixed total amount of platinum (constraint). The first analysis showed that, for equal anode and cathode catalyst layer thicknesses, maximal current density was achieved by placing more catalyst in the cathode than in the anode. The second analysis showed that, for equal anode and cathode catalyst layer density, maximal current density was achieved by using a catalyst layer that is thicker on the cathode side than that on the anode side. Finally, a topological optimization of the platinum density within the cathode catalyst layer was performed with a gradient based algorithm, and the results showed that at a high stoichiometric ratio, the best design has most of its platinum placed where the reaction rate is the highest, i.e., close to the membrane layer. [Copyright &y& Elsevier]

Highlights: [•] The Self-Superheating (SSH) process is a modification of geothermal Flash cycles. [•] Self-Superheating provides 5% more specific work than Flash cycles. [•] Self-Superheating reduces the turbines moisture content by 20–40%. [•] Optimal separator temperature of SSH cycles is 5°C lower than those of Flash cycles. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the dimensionless reaction rate density of a two-dimensional channel with catalyst on its boundary was maximized by optimizing its aspect ratio H/L. Order of magnitude expressions in the limit of small H/L and in the limit of large H/L were developed and then intersected to predict the optimal aspect ratio and maximum reaction rate. Numerical simulations were performed and the results corroborated very well with the scale analysis. The results showed that the optimal design of microreactors depends on the Schmidt number (Sc), the Bejan number (Be) and the Catalyst number (Ct). Correlations based on those numbers are proposed. The analysis was performed for steady-state laminar flows, using the dilute species assumption and a first order kinetic expression. [Copyright &y& Elsevier]

Abstract: In this paper, we address the fundamental problem of maximizing the reaction rate of a channel reactor with catalyst on its boundaries by varying its aspect ratio L/H. The volume, pressure drop, amount of catalyst and catalyst surface density of the channels were constrained. The expression of the reaction rate in the limits of small, moderate and large L/H values were obtained by scale analysis. A distinct behaviour appeared for each of these three geometrical limits: a kinetic controlled regime, an external diffusion controlled regime, and a regime limited by fluid flow rates. From this scale analysis, we predicted analytically that the length can be varied so as to increase the rate of reaction of a catalytic channel, up to a threshold value from which further change of the geometry does not alter significantly the reaction rate. Intersections of the three reaction rate expressions provided estimates for the maximal reaction rate and for the threshold lengths, as a function of the Bejan number, the Schmidt number and a catalyst number. Finally, CFD simulations of the channel reactors were performed, and numerical parametric studies agreed well with the estimated equations for the threshold L/H values. Correlations based on the numerical data are proposed for the design of catalytic channel reactors. [Copyright &y& Elsevier]

Abstract: The thermal inertia of building internal components may be used to shift the irradiation (solar) heat load, which could result in substantial energy saving. In this paper, a numerical model was implemented to determine the thermal performance of internal surfaces including Phase Change Materials (PCM). The model was exploited to compare a typical concrete floor with a floor with PCM. Thermal performance was defined by three different objective functions, each pinpointing different thermal characteristics of the heat load. First, parametric studies were performed to understand the influence of the thickness of a typical concrete floor. Then, the optimization of the melting temperature, thickness and position of a PCM layer included in a floor was performed. These analyses used either simplified or real weather conditions (for Québec City). Results showed that the thickness of the concrete floor could be optimized based on the three criteria retained. Also, the floor performance may be enhanced by the inclusion of a PCM layer. It was shown that the gain of performance brought by the internal surfaces thermal mass strongly depends on the weather conditions considered. This paper provides a fundamental understanding of PCM influence on internal surfaces. [Copyright &y& Elsevier]

Abstract: A numerical model is presented to determine the thermal shielding performance of an exterior wall (e.g., building envelope) containing layers of PCMs. The model is exploited to perform a parametric study to assess the influence of the position and melting temperature of one PCM layer. Results showed that benefits are to be expected when the interior and exterior temperatures are close. Then, the wall composition has been optimized with a genetic algorithm based on a yearly analysis with the possibility of including several PCM layers. Idealized weather conditions and measured weather conditions (including solar radiation) have been considered. Results showed that for Québec City, optimal south-facing wall includes one PCM layer when real weather data are considered. Its effect is to shield the heat transfer in the summer. This paper provides a fundamental understanding of multilayer walls with PCMs. [Copyright &y& Elsevier]

A straightforward procedure is developed to determine dissipation fields (e.g., exergy destruction in conduction, pumping power, Joule heating) from the linear boundary conditions applied on their associated linear diffusion problem (e.g., heat conduction, Darcy flow, electrical flow). The mathematical tool proposed in this article takes advantage of the quadratic (nonlinear) behavior that comes with dissipative fields equations. The first objective of this article is to build a mathematical formulation expressing any dissipation field as a combination of fundamental dissipative fields resulting from decomposed boundary conditions, similarly to linear problems where fundamental solutions may be added with the superposition principle. The second objective is to demonstrate the numerical advantage of this formulation when applied to heat transfer and fluid flow optimization problems. Application of the mathematical tool proposed in this article to boundary control problems provided significant computational time reductions. [ABSTRACT FROM AUTHOR]

Abstract: The objectives of this paper are two: (i) propose an evolutionary algorithm for optimizing the topology of a heat-generating area cooled by a high thermal conductivity material in contact with a heat sink, and (ii) compare the evolutionary procedure and optimal structures found to the ones predicted by constructal theory. The evolutionary algorithm starts with an initial random topology, for which control volumes are repositioned based on temperature and heat flux information in order to minimize the hot spot temperature. Significant diminution of the hot spot temperatures have been achieved by the algorithm. Results revealed three families of geometrical configurations for the optimal conductive pathways generated by our algorithm, depending on the values of the conductivity ratio and amount of high thermal conductivity material: few radial blades growing on the heat sink, tree-shaped networks, and networks with loops. The optimal structures are compared with the heat tree architectures generated by constructal theory, and several similarities in terms of performances and geometries are revealed. [Copyright &y& Elsevier]

• Dual pressure transcritical ORC is usually the best in the range of conditions tested. • Specific work output between 5 and 70 kJ/kg were obtained from simulations. • Five fluids (R218, R115, R227ea, RC318, RE245cb2) stand out in the optimization. • Optimal values of cooling tower range are lower at colder brine and hotter cold sink. • Parasitic loads (pumps, cooling) range from 15 to 45% of the gross power produced. Four variants of the Organic Rankine Cycle (ORC) applied to geothermal power plants are optimized by means of numerical tools. These variants are: (i) the subcritical ORC with single-pressure heater (ORC/S/SC), (ii) the transcritical ORC with single-pressure heater (ORC/S/TC), (iii) the subcritical ORC with dual-pressure heater (ORC/D/SC), and (iv) the transcritical ORC with dual-pressure heater (ORC/D/TC). All the systems are recuperative and include a wet cooling tower. The objective function is the specific work output and design variables include operating pressures, mass flow ratios between the brine and the working fluid, superheaters effectiveness and cooling tower range. The systems are optimized for 20 different potential working fluids. The optimization is performed for inlet brine temperatures from 80 to 180 °C, and for ambient air wet bulb temperatures from 10 to 32 °C. The results show: (i) the superiority of ORC/D/TC for most of the cases, (ii) the relevance of using a dual-pressure heater at high sink temperature and low brine temperature, and (iii) the importance of choosing the right cooling tower range for an optimal power plant design. [ABSTRACT FROM AUTHOR]

Abstract: This review presents when and how Genetic Algorithms (GAs) have been used over the last 15 years in the field of heat transfer. GAs are an optimization tool based on Darwinian evolution. They have been developed in the 1970s, but their utilization in heat transfer problems is more recent. In particular, the last couple of years have seen a sharp increase of interest in GAs for heat transfer related optimization problems. Three main families of heat transfer problems using GAs have been identified: (i) thermal systems design problems, (ii) inverse heat transfer problems, and (iii) development of heat transfer correlations. We present here the main features of the problems addressed with GAs including the modeling, number of variables, and GA settings. This information is useful for future use of GAs in heat transfer. Future possibilities and accomplishments of GAs in heat transfer are also drawn. [Copyright &y& Elsevier]

The heat contained in internal combustion engine exhaust gases can be converted into mechanical energy by using an Inverted Brayton Cycle (IBC). In this paper, five different IBC versions are numerically modeled and optimized to maximize their specific work output: (i) basic IBC, (ii) IBC with liquid water drainage (IBC/D), (iii) IBC with liquid water drainage and a steam turbine (IBC/D/S), (iv) IBC with liquid water drainage and a refrigeration cycle (IBC/D/R), and (v) IBC with liquid water drainage, a steam turbine and a refrigeration cycle (IBC/D/S/R). The three latter cycles are presented for the first time in literature. The optimization is performed for a wide range of inlet gases temperatures (600–1200 K) and heat sink temperatures (280–340 K). Among the five IBCs, the IBC/D/S/R has the highest specific work output for the whole range of operating temperatures. A comparison with the subcritical Rankine cycle and Organic Rankine Cycles using isobutane and benzene shows that an IBC system might be a better choice for specific operating temperatures. Liquid water addition in the IBC/D/S/R leads to optimized designs using only the steam turbine at high inlet gas temperatures, indicating that a Rankine cycle is better suited for these conditions. • The inverted Brayton cycle is a good candidate for heat recovery from exhaust gases. • It is best combined with water drainage, vapor-compression and open Rankine cycles. • Work output of 270 kJ per kg of exhaust at 1200 K can be reached with 280 K cold sink. • It outperforms ORC and Rankine cycle for certain hot and cold source temperatures. [ABSTRACT FROM AUTHOR]