Robert F. Boehm

FLOW AND HEAT TRANSFER AROUND A LINEAR ARRAY OF SPHERES

Abstract Laminar fluid flow and forced convection heat transfer over equally spaced linear arrays of spheres are analyzed using the finite element package FIDAP. For the arrays, sphere spacings of 1.5, 2, and 3 diameters are examined at Reynolds numbers of 40, 80, and 120 and Prandtl numbers ranging from 0.73 to 7.3. Average Nusselt numbers and drag coefficient data for a linear array of eight spheres (as an approximation to the developing region) and a single sphere with periodic boundary conditions (as an approximation to fully developed flow) are presented and correlated.

Performance of a Zero-Energy House

A comparative study is reported to measure the actual performance of a zero-energy house (ZEH) of typical tract design. Ideally, a ZEH produces as much energy as it consumes in a years time. Two identically sized tract houses (149.6 m 2 (1610 ft 2 ) ) were constructed side by side in southwest Las Vegas, NV One house is used as a base line (standard comparison) house and was built using conventional construction techniques. The other house, the ZEH, employs many energy saving features, solar power generation, and supplemental solar water heating. Both houses have been equipped with a network of sensors that measure virtually every aspect of energy usage in each home. Initially, both houses have been utilized as model homes in a tract housing development, so it was reasonable to believe that both experienced similar and consistent usage. Performance data are logged and are posted on the web. This paper describes the differences in construction details between the two houses. Results of monitoring are presented that contrast the performance of the two houses.

A Photoelectrochemical Model of Proton Exchange Water Electrolysis for Hydrogen Production

proton exchange membrane is proposed based on Butler-Volmer kinetics for electrodes and transport resistance in the polymer electrolyte. An equivalent electrical circuit analogy is proposed for the sequential kinetic and transport resistances. The model provides a relation between the applied terminal voltage of electrolysis cell and the current density in terms of Nernst potential, exchange current densities, and conductivity of polymer electrolyte. Effects of temperature on the voltage, power supply, and hydrogen production are examined with the developed model. Increasing temperature will reduce the required power supply and increase the hydrogen production. An increase of about 11% is achieved by varying the temperature from 30° C to 80° C. The required power supply decreases as the illumination intensity becomes greater. The power supply due to the cathode overpotential does not change too much with the illumination intensity. Effects of the illumination intensity can be observed as the current density is relatively small for the examined illumination intensities. DOI: 10.1115/1.2789722

Power “smoothing” of a commercial-size photovoltaic system by an energy storage system

Photovoltaic power fluctuations caused by cloud transients can cause power quality and voltage regulation-related problems in distribution systems with high PV penetration. This paper describes such fluctuations in a commercial size concentrating photovoltaic (CPV) system, and evaluates the use of an energy storage system (ESS) for power smoothing purposes. Recorded ramp rates at both 1-minute and 1-second sampling intervals are used in the simulations. A simple moving average is applied to determine the power to be generated or absorbed by the ESS at each step. Simulations show that the 1-second sampling is more suitable for this particular PV system size, and the level of smoothing depends on the ESS size to invest in.

Modeling a Passive Cooling System for Photovoltaic Cells Under Concentration

Cooling of photovoltaic cells under high intensity solar irradiance is a major concern when designing concentrating photovoltaic systems. The cell temperature will increase if the waste heat is not removed and the cell voltage/power will decrease with increasing cell temperature. This paper presents an analysis of the passive cooling system on the Amonix high concentration photovoltaic system (HCPV). The concentrator geometry is described. A model of the HCPV passive cooling system was made using Gambit. Assumptions are discussed that were made to create the numerical model based on the actual system, the methods for drawing the model is discussed, and images of the model are shown. Fluent was used to compute the numerical results. In addition to the theoretical results that were computed, measurements were made on a system in the field. These data are compared to the theoretical data and differences are calculated. Theoretical conditions that were studied included uniform cell temperatures and worst case weather scenarios, i.e., no wind, high ambient conditions, and high solar irradiance. The performance of the Amonix high concentrating system could be improved if more waste heat were removed from the cell. Now that a theoretical model has been developed and verified, it will be used to investigate different designs and material for increasing the cooling of the system.Copyright

Simulation and Optimization of a Concentrated Photovoltaic System

Reported here is the development, using results of analysis and experiments, and optimisation of a numerical model for a concentrated photovoltaic system. Models for the two major components of the system (cooling system and receiver) are developed separately from one another and then linked to simulate the performance for the entire system. The model is linked to yearly weather data and the optimization routines included in MATLAB are then used to select the input parameters (pump size, number of radiators, fan speed, etc.) which maximise the solar to electrical conversion efficiency of the system.

Laminar Buoyancy-Enhanced Convection Flows on Repeated Blocks with Asymmetric Heating

Laminar mixed convective flow across a block-mounted plate in a vertical channel is studied numerically. One plate with blocks is elevated to a constant temperature different from the inlet air and the other is adiabatic. The results show that the local Nusselt number is significantly affected by secondary flows induced by buoyancy and channel geometry. It is also noted that the heat transfer is mainly a function of the three geometric ratios and the Reynolds number. The average Nusselt numbers decrease with increasing Gr/Re2 values in the blocks and increase with Gr/Re2 at high Grashof numbers in the recirculation zones.

Effect of on/off charge controller on stand-alone PV system performance

Most battery charge controllers in stand-alone photovoltaic power systems shed portions of the PV array in discrete steps as the battery voltage approaches full charge. This paper analyzes the effect of such on/off algorithms on the system performance. A simple analytical model of a multi-step charge controller is presented and illustrated by an example. Computer simulations and field measurements of an actual system show that on/off chargers limit the use of PV energy hence resulting in lower performance than conventional models.

Comparison of time domain reflectometry performance factors for several dielectric geometries: Theory and experiments

We propose three nontraditional dielectric geometries and present an experimental and theoretical analysis and comparison of time domain reflectometry (TDR) performances for them. The traditional geometry (the probes inserted in material of essentially infinite extent) is compared to three nontraditional geometries where the probes are affixed outside of a core sample, inside of a bore, or flat on the surface of a semi-infinite solid. Our derivation relates the velocity of electromagnetic wave propagation to the complex permittivities and permeabilities of the media and the geometry for the three nontraditional configurations. Experimental results for air, styrofoam, dry sand, wet sand of varying water content, nylon, dry wood, and ferromagnetic steel are obtained for the three proposed configurations and are in fair agreement with the literature within the experimental uncertainties. Through experiments and theoretical analysis, the TDR performance is found to be the same within the experimental uncertainties for the three nontraditional geometries. The proposed geometries yield slightly lower sensitivities compared to the traditional geometry. Advantages and disadvantages of the geometries compared to the traditional geometry are also discussed.

Comparison of Two-Tank Indirect Thermal Storage Designs for Solar Parabolic Trough Power Plants

Two-Tank Indirect Thermal Storage Designs for Solar Parabolic Trough Power Plants by Joseph Kopp Dr. Robert F. Boehm, Examination Committee Chair Professor of Mechanical Engineering University of Nevada, Las Vegas The performance of a solar thermal parabolic trough plant with thermal storage is dependent upon the arrangement of the heat exchangers that ultimately transfer energy from the sun into steam. The steam is utilized in a traditional Rankine cycle power plant. The most commercially accepted thermal storage design is an indirect two-tank molten salt storage system where molten salt interacts with the solar field heat transfer fluid (HTF) through a heat exchanger. The molten salt remains in a closed loop with the HTF and the HTF is the heat source for steam generation. An alternate indirect two tank molten salt storage system was proposed where the molten salt was utilized as the heat source for steam generation. A quasi-steady state simulation code was written to analyze the key environmental inputs and operational parameters: solar radiation, solar field size, thermal storage system, heat exchangers, and power block. A base case with no thermal storage was modeled using design parameters from the SEGS VI plant and the effects of solar field size were analyzed. The two differing indirect two-tank molten salt storage designs were modeled and their solar field size and thermal storage capacity were treated as parameters. Results present three days of distinct weather conditions for Las