• Energy Research

This paper considers the basic thermodynamic optimization problem of extracting the most power from a stream of hot exhaust when the contact heat transfer area is fixed. It shows that when the receiving (cold) stream boils in the counterflow heat exchanger, the thermodynamic optimization consists of locating the optimal capacity rate of the cold stream. At the optimum, the cold side of the heat transfer surface divides itself into three sections: liquid preheating, boiling and vapor superheating. Numerical results are developed for a range of design parameters of applications with either water or toluene on the cold side. It is shown that the optimal design is robust, because several of the design parameters have only a weak effect on the optimal design.

Abstract We present herein the development, experimental validation, and application of a volume element model for 3D dynamic building thermal simulation. The 3D spatial domain in the VEM is discretized with lumped hexahedral elements using ray crossings and ray/triangle intersection techniques that yield sufficiently accurate geometric representation of a building. Subsequently, energy balance is applied to each element in the mesh, and the resulting system of ordinary differential equations is integrated in time to obtain spatiotemporal indoor temperature and relative humidity fields. In this work, we adjusted the model by comparing the simulated indoor air temperatures to the experimental measurements as we calibrated model parameters with high uncertainty. The adjusted model was validated using different experimental data sets, and the numerical results were in a good agreement with the measurements. We employed the validated model to conduct a case study in which the sensible heat gain and loss as well as the time lag were evaluated as functions of different envelope thermal masses. Results showed the trivial effect of floor thermal mass on heat gain, and the changes in sensible heat gain and envelope thermal mass were not linearly proportional, alluding the existence of an optimal envelope design.

Abstract A Stirling engine configuration consisting of two cylinders, a regenerator and a sliding disc actuating mechanism (“swashplate”) is considered in this paper. A mathematical model, which combines fundamental and empirical correlations, and principles of classical thermodynamics, mass and heat transfer accounting for variable heat transfer coefficients, is developed. The proposed model is then utilized to simulate numerically the system transient and steady state response under different operating and design conditions. A system global optimization for maximum performance in the search for optimal parameters that lead to maximum cycle efficiency is performed with low computational time. Appropriate dimensionless groups are identified and the results presented in normalized charts for general application. The numerical results show that the two-way maximized system efficiency, η max , max , occurs when two system characteristic parameters, the ratio between the total swept volume during the expansion, and the total swept volume, φ , and the ratio between the heat transfer area of the hot side heat exchanger and the total heat exchange area, y, are optimally selected, i.e., ( φ , y ) opt ≅ ( 0 . 5 , 0 . 4 ) . The two-way maximized cycle efficiency found with respect to the optimized parameters is sharp, in the sense that a 225% variation of the calculated efficiency values was observed within the range of tested configurations in this study, and “robust” (i.e., relatively insensitive) to the variation of several parameters, thus stressing the importance to be considered in actual design. It is also found that the twice-maximized cycle efficiency and the total engine work output increase monotonically with the temperature of the hot source, Th. As a result, the model is expected to be a useful tool for simulation, design, and optimization of Stirling engines.

Abstract This work addresses the problem of configuration of gas and steam turbine combined cycles for ships by simultaneously considering increased efficiency and reduced weight as design objectives. The performed analysis provides basic information to produce systems with simultaneous advantage in both aspects. The combined cycle considered, with total constant power of 20 MW, is modeled as a gas turbine in standard configuration coupled to a simple Rankine cycle. Calculation of system's weight includes the machinery as well as the fuel required to guarantee a given time at sea. To estimate the machinery components weight, some scaling relations have been developed and used. The results presented include an analysis of the predicted weight and efficiency of the combined cycle respect to varying design parameters such as amount of heat recovered, time at sea, steam turbine exit quality, steam generator pinch point, and gas turbine performance. When compared against gas turbines in simple cycle mode, combined cycles produce a fuel requirement reduction that can overcome, in terms of weight, the size increase experienced by the plant. However, it is in general observed that minimum weight and maximum efficiency configurations do not necessarily coincide, as both objectives compete at intermediate values of heat recovery. Therefore, the particular choice of the final design depends on the relative importance assigned to each objective for the considered system. Notably, minimum weight and maximum efficiency solutions are very different for short trip periods but become basically the same for very long ones. Regarding gas turbine operation parameters, they have a strong influence on both total weight and efficiency. An interesting consequence is that a low efficiency gas turbine could produce better results than a high efficiency one, given a large enough temperature for the exhaust gases.

Abstract This paper improves previously published models by the authors for a single solid oxide fuel cell (SOFC), and introduces a procedure to optimize its external configuration and operating conditions, so that the net power is maximized. The previous models are hereby improved to include: i) a constant offset overpotential in total potential drop; ii) heat generation associated with all the potential losses; iii) temperature-dependent thermo-physical properties of fuel and air, and iv) pumping power to maximize fuel cell performance. The thermodynamic model is derived from physical laws (e.g., the first law of thermodynamics, Fick's law, Fourier's law) to obtain the temperature and pressure spatial distribution in the SOFC. The electrochemical model is validated by direct comparison with experimental data from the Pacific Northwest National Laboratory (PNNL), and allows for the computation of the SOFC voltage, current, and power output. Based on the simulation results, the structural design, the active three phase boundaries regions at the electrodes and the fuel utilization factor, and their impact on the SOFC performance are discussed. Subjected to fixed total volume, the optimal geometric and operating parameters are pursued so that the net power of the SOFC is maximized through a 4-way-optimization procedure. The method used is general and the numerically obtained maxima are sharp, taking into account that up to a 631% single SOFC performance variation was observed within the studied parameters' range. The fixed volume constraint was then relaxed, and the effect of total volume variation on performance was investigated, delivering the general optimal parameters for the 4-way maximized SOFC net power output within the studied total dimensionless fuel cell volume range. These findings show the potential to use the model as a tool for future SOFC design, simulation and optimization.

Abstract This paper presents a dynamic three-dimensional volume element model of a parabolic trough solar collector coupled to an existing semi-finite optical model for simulation and optimization. The spatial domain in the volume element model is discretized with lumped control volumes (i.e., volume elements) in cylindrical coordinates according to the predefined collector geometry. The spatial dependency of the model is therefore taken into account without the need to solve partial differential equations. The proposed model combines the laws of thermodynamics and heat transfer as well as empirical correlations to simplify the modeling and expedite the computations, and the resulting system of ordinary differential equations is integrated in time for temperature. The model was validated with the experimental data provided in the literature, and was employed to evaluate the sensitivity of the collector performance described by the first and second law efficiencies to receiver length, annulus gap spacing, concentration ratio, incidence angle, inlet fluid temperature and flow rate. This work also examined the effects of inlet fluid temperature and temperature differential on dynamic collector performance in the transient case study. Results showed that the first law efficiency was most sensitive to the inlet fluid temperature with the maximum variation of 30%, whereas the incidence angle and concentration ratio affected the second law efficiency the most with the maximum variations of 375% and 300%, respectively. The remaining parameters featured trivial effects in all cases. In the transient analysis, higher temperature differential and lower inlet fluid temperature yielded higher total heat gain while the total exergy gain was insensitive to both parameters. The first law efficiency should therefore be of greater importance than the second law efficiency in the control of dynamic collector performance based on these two parameters.

Abstract This paper presents the mathematical formulation and unique capability of a system-level ship thermal management tool, vemESRDC, developed to provide quick ship thermal responses in early design stages. The physical model combines principles of classical thermodynamics and heat transfer, along with appropriate empirical correlations to simplify the model and expedite the computations. As a result, the tool is capable of simulating dynamic thermal response of an entire ship, characterized by intricate thermal interactions within a complex ship structure, within an acceptable time frame. In this work, vemESRDC is demonstrated through three case studies in which transient thermal responses of an all-electric ship to different ship operation modes, weather conditions, and partial loss of cooling are investigated. The analysis examines particularly the following: (1) the required cooling capacities to maintain each ship component within its design limit; (2) equipment temperature variations with respect to partial cooling loss in battle mode; and (3) the assets of installing seawater heat exchangers to pre-cool deionized freshwater before chillers. For the notional all-electric ship conceived and assessed in this work, the results verify the capability of vemESRDC to capture dynamic thermal interactions between shipboard equipment and their respective surroundings and cooling systems, e.g., the tool provides practical insights into pulse load cooling strategy, and different solutions are obtained for distinct weather conditions. In addition to the case studies performed in this work, vemESRDC can be employed to conduct diverse studies based on which concrete ship thermal management strategies can be formulated in early design stages.

This paper presents a method to perform real-time system-level shipboard thermal simulations. The modeling approach is described in detail as the method is applied to the cooling facilities of the CAPS 5 MW MV laboratory. The derived simulation is studied using standard model validation tools such as factor screening and uncertainty propagation. Error bounds of system response variables are compared to results from experiments on the CAPS cooling facilities published previously. The goal is to provide a first step in the development of real-time system-level thermal simulations suitable for future HIL simulations employed in the engineering of future all-electric ships. It is envisioned that these simulations will be useful for the development and testing of future system-level controls.

Superior, scalable grid topology that may provide for higher quality of service, survivorship capacity, and reconfiguration capability is a feature of modern grid architecture. The reported research is aimed at delivering a possible, constructal solution that is based on a minimumredundant, scalable, reconfigurable topology that helps increasing the grid immunity to faults. We assume that a tree network serves nodes of consumption that are evenly distributed throughout the territory. All nodes are equally important and network survivability means delivering electrical energy to as many consumers as possible. The models and loads estimation is based on the load momentum method.