bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermally Aware, Energy-Based Load Placement in Open-Aisle, Air-Cooled Data Centers
Authors: Demetriou Dustin W. , Ezzat Khalifa H.
Published: July 2013

This paper introduces a methodology for developing a reduced order model, using proper orthogonal decomposition (POD), to predict the IT rack's inlet temperature distribution within a raised floor air-cooled data center. The method used in this paper uses a limited set of computational fluid dynamics data at different useful IT levels and tile airflow fractions. The model was able to reconstruct these datasets to with 0.16 °C rms error and interpolate successfully for alternative configurations that were not included in the original dataset. The reduced order model can produce the temperature distribution in the data center in a fraction of a second on a standard personal computer. Several practical IT load placement options in open-aisle, air-cooled data centers, based on either geometrical traits of the data center, a prior physics-based knowledge of the airflow and temperature patterns or measurements that are easily obtainable during operation, are considered. The outcome of this work is the development of a robust set of guidelines that facilitate the energy efficient placement of the IT load amongst the operating servers in the data center. This work found that a robust approach was to use real-time temperature measurements at the inlet of the racks to remove the unnecessary IT load from the servers with the warmest inlet temperature. This strategy shows superior performance to the other strategies studied. The study considered the holistic optimization of the data center and cooling infrastructure for a range of data center IT utilization levels. The results showed that allowing for significant reductions in the supply air flow rate proved superior to providing a higher supply air temperature to meet the IT equipment's inlet air temperature constraint.
Exploring Advanced Metering Infrastructure Deployments for Commercial and Industrial Sites
Authors: Littman Allison , Lyon Geoff , Shah Amip , Vogler John
Published: July 2013

Smart meters have become increasingly common as an approach to benchmark and assess building energy use. In this paper, we explore what type of metering infrastructure may be required to derive value from the application of smart meters in the commercial and industrial sectors. As an example, we find that sole reliance upon a site-level smart meter—which has been the focus of most existing deployment models—provides sufficient data to extract summary statistics about how the energy use of a given site may compare to a typical ‘average’ site, but such installations fail to provide adequate detail about where the energy use is occurring or why any discrepancies might be occurring. To resolve these issues, we install a multi-tier advanced metering infrastructure (AMI) at a mixed use (industrial and commercial) campus. We use this AMI deployment to gain insight at different levels of the consumption hierarchy—from sites to buildings, panels, sub-panels, and end loads. The paper concludes by discussing the trade-offs associated with such augmented metering at each level within the hierarchy, with a view towards providing guidelines for AMI deployment at other industrial and commercial campuses.
Experimental Test of Multi-Stage Vertical-Axis Turbines for Cellular Communication Applications
Authors: Abraham John , Plourde Brian , Mowry Greg , Sparrow Ephraim
Published: July 2013

A multi-year research program has generated a working prototype for a vertical-axis wind turbine that is capable of powering cellular communication equipment. The turbine is designed to be affixed to already existing communication towers and thereby has a reduced cost of installation. The turbine is driven by air drag forces rather than by lift. It has a number of novel features including venting slots that are created to reduce the thrust loading on the communication tower. In addition, contoured caps are affixed to the upper and lower edges of the turbine blades to increase power production. As previously mentioned, the turbine design itself is a drag-based concept rather than the more typical lift-driven devices. The advantages of the drag-based design are: 1. lower startup wind speed, 2. slower rotation and a lessened vibrational load on the tower, 3. less sensitive to wind direction, and 4. it can be aligned with the tower.

The design of the device was carried out through a combination of numerical simulation and experimentation. The simulations have evolved from preliminary two-dimensional calculations to a fully three dimensional, unsteady, computational fluid dynamic analysis. Simultaneously, the experiments have included both in-field and wind-tunnel tests of various stages of the turbine design.

An outcome of the effort is a third-generation working vertical-axis wind turbine (VAWT) that is currently being evaluated with in-field tests. The results of the tests are positive and confirm the expectations that were developed during the product design phase. The turbine, which can be constructed with various rotor stages, has the capability of producing approximately 2–3 kW of power in wind-speed environments of 12–16 m/s. These power production levels are greatly in excess of that required to fully power the electronics equipment on a typical cellular communication tower. Unfortunately, subsequent tests showed that the turbine production dropped approximately sevenfold. The cause of the decrease in performance was friction in the mechanical components which coupled the rotating shaft to the support structure. This recognition reinforces the importance of low-resistance mechanical design for VAWTs.

Another aspect of the turbine design is the specialized electronics which allow the electronics to adapt to local wind speeds and consequently increase the efficiency of the power production.
CFD Simulation and Heat Loss Analysis of the Solar Two Power Tower Receiver
Authors: Christian Joshua M. , Ho Clifford K.
Published: July 2013

Solar Two was a demonstration of the viability of molten salt power towers. The power tower was designed to produce enough thermal power to run a 10-MWe conventional Rankine cycle turbine. A critical component of this process was the solar tower receiver. The receiver was designed for an applied average heat flux of 430 kW/m2 with an outlet temperature of 565°C (838.15 K). The mass flow rate could be varied in the system to control the outlet temperature of the heat transfer fluid, which was high temperature molten salt. The heat loss in the actual system was calculated by using the power-on method which compares how much power is absorbed by the molten salt when using half of the heliostat field and then the full heliostat field. However, the total heat loss in the system was lumped into a single value comprised of radiation, convection, and conduction heat transfer losses. In this study, ANSYS FLUENT was used to evaluate and characterize the radiative and convective heat losses from this receiver system assuming two boundary conditions: (1) a uniform heat flux on the receiver and (2) a distributed heat flux generated from the code DELSOL. The results show that the distributed-flux models resulted in radiative heat losses that were ∼14% higher than the uniform-flux models, and convective losses that were ∼5–10% higher due to the resulting non-uniform temperature distributions. Comparing the simulations to known convective heat loss correlations demonstrated that surface roughness should be accounted for in the simulations. This study provides a model which can be used for further receiver design and demonstrates whether current convective correlations are appropriate for analytical evaluation of external solar tower receivers.
Greenhouse Gas Reduction Potential With Combined Heat and Power With Distributed Generation Prime Movers
Authors: Curran Scott J. , Theiss Timothy J. , Bunce Michael J.
Published: July 2013

Pending or recently enacted greenhouse gas regulations and mandates are leading to the need for current and feasible GHG reduction solutions including combined heat and power (CHP). Distributed generation using advanced reciprocating engines, gas turbines, microturbines and fuel cells has been shown to reduce greenhouse gases (GHG) compared to the U.S. electrical generation mix due to the use of natural gas and high electrical generation efficiencies of these prime movers. Many of these prime movers are also well suited for use in CHP systems which recover heat generated during combustion or energy conversion. CHP increases the total efficiency of the prime mover by recovering waste heat for generating electricity, replacing process steam, hot water for buildings or even cooling via absorption chilling. The increased efficiency of CHP systems further reduces GHG emissions compared to systems which do not recover waste thermal energy. Current GHG mandates within the U.S Federal sector and looming GHG legislation for states puts an emphasis on understanding the GHG reduction potential of such systems. This study compares the GHG savings from various state-of-the-art prime movers. GHG reductions from commercially available prime movers in the 1–5 MW class including, various industrial fuel cells, large and small gas turbines, micro turbines and reciprocating gas engines with and without CHP are compared to centralized electricity generation including the U.S. mix and the best available technology with natural gas combined cycle power plants. The findings show significant GHG saving potential with the use of CHP. Also provided is an exploration of the accounting methodology for GHG reductions with CHP and the sensitivity of such analyses to electrical generation efficiency, emissions factors and most importantly recoverable heat and thermal recovery efficiency from the CHP system.
Evaluation of Combined Heat and Power (CHP) Systems Performance With Dual Power Generation Units
Authors: Knizley Alta , Mago Pedro J.
Published: July 2013

This paper evaluates the economic, energetic, and environmental feasibility of using two power generation units (PGUs) to operate a combined heat and power (CHP) system. A benchmark building developed by the Department of Energy for a full-service restaurant in Chicago, IL is used to analyze the proposed configuration. This location is selected since it usually provides favorable CHP system conditions in terms of cost and emissions reduction. In this investigation, one PGU is operated at base load to satisfy part of the electricity building requirements (PGU1), while the other is used to satisfy the remaining electricity requirement operating following the electric load (PGU2). The dual-PGU configuration (D-CHP) is modeled for several different scenarios in order to determine the optimum operating range for the selected benchmark building. The dual-PGU scenario is compared with the reference building using conventional technology to determine the economical, energetic, and environmental benefits of this proposed system. This condition is also compared to a CHP system operating following the electric load (FEL) and to a base-loaded CHP system, and it provides greater savings in operating cost, primary energy consumption, and carbon dioxide emissions than the optimized conditions for base loading and FEL.
Numerical Analysis of the Overlap Effect Between Blades at Four-Bladed Rooftop VAWT
Authors: Flores-Saldaña H. , Gallegos-Muñoz A. , Uzarraga-Rodriguez N. C. , Rangel-Hernandez V. H.
Published: July 2013

This work presents a numerical analysis of a four-bladed Rooftop vertical axis wind turbine (VAWT). The effects generated on the performance of turbine by the overlap variation between blades of wind rotor were analyzed. The numerical simulations were developed using commercial software based on Computational Fluid Dynamic (CFD). Each one of the models generated was built in a 3D computational model. A sliding mesh model (SMM) capability was used to present in dimensionless form the moment coefficient and power coefficient of the wind turbine based on the relationship between wind speed and rotor rotational speed. The results show that the aerodynamic performance is better with overlap between rotor blades, resulting in a significant increase in the moment coefficient and power coefficient. Having that in the cases of four-bladed Rooftop rotor with overlap both coefficients increase about 29% comparing with four-bladed Rooftop rotor without overlap between blades.
Considering Uncertainties in Research by Probabilistic Modeling
Authors: Eck Markus , Kretschmann David , Feldhoff Jan Fabian , Wittmann Michael
Published: July 2013

Technical and economical evaluation of solar thermal power plants constantly gains more importance for industry and research. The reliability of the results highly depends on the assumptions made for the applied parameters. Reducing a power plant system to one single, deterministic number for evaluation, like the levelized cost of electricity (LCOE), might end in misleading results. Probabilistic approaches can help to better evaluate systems [1] and scenarios [2]. While industry looks for safety in investment and profitability, research is predominantly interested in the evaluation of concepts and the identification of promising new approaches. Especially for research, dealing with higher and hardly quantifiable uncertainties, it is desirable to get a detailed view of the system and its main influences. However, to get there, also a good knowledge on the stochastic interrelations and its interpretation is required.

Therefore, this paper mainly assesses the influences of basic stochastic assumptions and suggests a methodology to consider suitable stochastic input, especially for parameters of systems still under research. As examples, the comparison between a parabolic trough plant with synthetic oil and direct steam generation is used.
Analysis of Operation Test Results of a High Temperature Phase Change Storage for Parabolic Trough Power Plants With Direct Steam Generation
Authors: Laing Doerte , Eck Markus , Hempel Matthias , Steinmann Wolf-Dieter , Meyer-Grünefeldt Mirko , Eickhoff Martin
Published: July 2013

For future parabolic trough plants direct steam generation in the receiver pipes is a promising option for reducing the costs of solar thermal energy [1]. These new solar thermal power plants require innovative storage concepts, where the two phase heat transfer fluid poses a major challenge [2]. For the regions where the heat transfer fluid is in a single phase (water or steam), sensible heat storage using molten salt [3] or concrete [4] as storage material can be applied. However, efficient energy storage in the two-phase evaporation/condensation region requires heat storage operation within a narrow temperature range.

For this two phase region, a high performance PCM storage technology was developed and demonstrated by DLR. A test module using 14 tons of PCM with 700 kWh capacity was built in 2009 and commissioned in 2010 in a direct steam test loop, set up at the power plant Litoral of Endesa in Carboneras, Spain [5]. The PCM-storage uses Sodium nitrate as phase change material with a melting temperature of 305 °C. Cycle testing has started end of 2010. Cycling tests have proven the expected discharge capacity of approx. 700 kWh for the PCM-storage module. System operation in constant pressure mode and sliding pressure mode has been conducted for the PCM-storage. While in the constant pressure mode a peak performance of the storage of more than 700 kW could be demonstrated, in the sliding pressure mode a constant power output over almost the whole charge and discharge period could be provided.

The paper discusses the test results and evaluation for different operation modes for the phase change storage for discharge operation. Charging of the phase change storage is always in a once-through mode. However, for discharge, the steam can be generated either in forced or natural circulation mode or in once-through mode, leading to very different effects for the two-phase flow and filling level inside the heat exchanger pipes in the storage. The effects for forced and natural circulation discharge will be analysed and described in the paper.
Characterizing the Influence of Land Area and Nameplate Capacity on the Optimal Wind Farm Performance
Authors: Chowdhury Souma , Zhang Jie , Messac Achille , Castillo Luciano
Published: July 2013

The development of utility-scale wind farms that can produce energy at a cost comparable to that of conventional energy resources presents significant challenges to today’s wind energy industry. The consideration of the combined impact of key design and environmental factors on the performance of a wind farm is a crucial part of the solution to this challenge. The state of the art in optimal wind project planning includes wind farm layout design and more recently turbine selection. The scope of farm layout optimization and the predicted wind project performance however depends on several other critical site-scale factors, which are often not explicitly accounted for in the wind farm planning literature. These factors include: (i) the land area per MW installed (LAMI), and (ii) the nameplate capacity (in MW) of the farm. In this paper, we develop a framework to quantify and analyze the roles of these crucial design factors in optimal wind farm planning. A set of sample values of LAMI and installed farm capacities is first defined. For each sample farm definition, simultaneous optimization of the farm layout and turbine selection is performed to maximize the farm capacity factor (CF). To this end, we apply the recently developed Unrestricted Wind Farm Layout Optimization (UWFLO) method. The CF of the optimized farm is then represented as a function of the nameplate capacity and the LAMI, using response surface methodologies. The variation of the optimized CF with these site-scale factors is investigated for a representative wind site in North Dakota. It was found that, a desirable CF value corresponds to a cutoff “LAMI vs nameplate capacity” curve — the identification of this cutoff curve is critical to the development of an economically viable wind energy project.
Heat Transfer Analysis of a Solid-Solid Heat Recuperation System for Solar-Driven Non-Stoichiometric Redox Cycles
Authors: Lapp Justin , Davidson Jane , Lipiński Wojciech
Published: July 2013

Heat transfer is analyzed numerically for a solid-solid heat recuperation system employed in a novel directly-irradiated solar thermochemical reactor realizing a metal oxide based non-stoichiometric redox cycle for production of synthesis gas from water and carbon dioxide. The system is designed for continuous operation with heat recuperation from a rotating hollow cylinder of a porous reactive material to a counter rotating inert solid cylinder via radiative transfer. A transient heat transfer model coupling conduction, convection, and radiation heat transfer modes is developed to predict temperatures of both components, rates of heat transfer, and the effectiveness of heat recuperation. Heat recovery effectiveness of over 50% is attained within a parametric study of geometric and material parameters corresponding to the design of a two-step solar thermochemical reactor.
Economic Chances and Technical Risks of the Internal Direct Absorption Receiver (IDAR)
Authors: Singer Cs. , Buck R. , Pitz-Paal R. , Müller-Steinhagen H.
Published: July 2013

Increased receiver temperatures of solar tower power plants are proposed to decrease the plants levelized electricity costs (LEC) due to the utilization of supercritical steam power plants and thus higher overall plant efficiency. Related to elevated receiver temperatures preliminary concept studies show a distinct LEC reduction potential of the internal direct absorption receiver (IDAR), if it is compared to liquid in tube (LIT) or beam down (BD) receiver types. The IDAR is characterized by a downwards oriented aperture of a cylindrical cavity, whose internal lateral area is illuminated from the concentrator field and cooled by a liquid molten salt film. The objective is the further efficiency enhancement, as well as the identification and assessment of the technical critical aspects. For this a detailed fluid mechanic and thermodynamic receiver model of the novel receiver concept is developed to be able to analyze the IDAR’s operating performance at full size receiver geometries. The model is used to analyze the open parameters concerning the feasibility, functionality and performance of the concept. Hence, different system management strategies are examined and assessed, which lead to the proposal of a cost optimized lead-concept. This concept involves a rotating receiver system with inclined absorber walls. The spatial arrangements of the absorber walls minimize thermal losses of the receiver and enhance film stability. The centrifugal forces acting on the liquid salt film are essential to realize the required system criteria, which are related to the maximal molten salt temperature, film stability and droplet ejection. Compared to the state of the art at a 200 MWel power level the IDAR concept can lead to a LEC reduction of up to 8%. The cost assumptions made for the assessment are quantified with sensitivity analysis.
Biobriquettes: As a Sustainable Composite Fuel for Rural Areas in Developing Countries
Authors: Ávila Adrian , Nova Víctor , Gomez Rafael , Verdeza Arnaldo , Mendoza Jorge M.
Published: July 2013

The province of Cordoba, Colombia, has a 250.000 tons production of corn, which generate about 45.000 tons of cobs per year, which do not represent any value for farmers. The disposal of this agricultural waste has become a source of contamination because is burned in open areas. On the other hand, this province has a considerable production of raw coal; nevertheless, it is characterized by its high sulfur content (1.65%) and low heat value (5111 cal/gr), as a consequence, it has a limited market, but is still used as a fuel. This study proposes the use of biobriquettes that are a composite fuel prepared from a mixture of biomass (corn cob) and coal in a low die press briquetting machine. They consist of different proportions of milled cob (up to 80%) and coal (up to 40%) mixed with a cassava starch based binder. For the mixtures proximal analysis, including sulfur content and heat values of the material was determined according to their composition. The experimental results showed that the biobriquettes compared with the raw coal have between a 92 to 32% lower sulfur content, while the heat value was reduced by 14 to 29%. Therefore, its use will reduce the amount of burned residue in open spaces and consequently the pollution.
Solar Collector Exergetic Optimization for a Multi Effect Humidification Desalination Prototype
Authors: González-Acuña R. , Pieretti P. , Malpica F.
Published: July 2013

Venezuela is a country with a great deal of water sources, as a consequence of its location in the Amazon Basin. In spite of this, about 1.6 million inhabitants are dispersed in remote regions throughout the nation, where water distribution is problematic due to the lack of this resource. Since 2007, The Institute of Energy of the Simon Bolivar University has been focused on the development of a 25 liters per day single stage solar Multi-effect Humidification (MEH) desalination plant using a flat plate solar collector. Once the construction of the prototype plant was completed, the characterization of its different components was performed. The one related to the solar collector unit was done on a testing rig designed and constructed according to the ANSI/ASHRAE 93-2003 standard requirements. In order to optimize the operation of this equipment is necessary to maximize the exergetic change of the working fluid across the solar collector, to achieve this objective a numerical simulation using a predictive algorithm and an available yearlong meteorological data was performed. The prediction algorithm was validated against experimental data obtained from the characterization tests. The numerical results obtained were in good agreement with previous related researches. It was found that the best way to operate the solar collector in remote areas and ensure the maximum exergetic gain of the working fluid was to maintain a fix mass flow rate of 0.006 kg/s (0.36 LPM). This corresponds with the climate conditions at Simon Bolivar University and an admission temperature of 54°C, which is the design value of the desalination plant.
Optimum Small-Scale Open and Direct Solar Thermal Brayton Cycle for Pretoria, South Africa
Authors: Le Roux W. G. , Bello-Ochende T. , Meyer J. P.
Published: July 2013

The open and direct solar thermal Brayton cycle is exposed to various weather conditions like changing solar irradiation, wind and surrounding temperature. The geometries of the receiver and recuperator and the turbine operating point as parameter can be optimised in such a way that they accommodate these weather changes and allow for high net power output throughout a typical year. In this paper, a method of obtaining these parameters based on total entropy generation minimisation is presented. A parabolic dish concentrator with a diameter of 4.8 m is used as well as an off-the-shelf turbo-machine. Results show that the net absorbed power at the receiver and maximum allowed receiver surface temperature play important roles in determining the optimum operating point and maximum net power output.
Thermodynamic Study of Advanced Supercritical Carbon Dioxide Power Cycles for High Performance Concentrating Solar Power Systems
Authors: Turchi Craig S. , Ma Zhiwen , Neises Ty , Wagner Michael
Published: July 2013

In 2011, the U.S. Department of Energy (DOE) initiated a “SunShot Concentrating Solar Power R&D” program to develop technologies that have the potential for much higher efficiency, lower cost, and/or more reliable performance than existing CSP systems. The DOE seeks to develop highly disruptive Concentrating Solar Power (CSP) technologies that will meet 6¢/kWh cost targets by the end of the decade, and a high-efficiency, low-cost thermal power cycle is one of the important components to achieve the goal. Supercritical CO2 (s-CO2) operated in a closed-loop Brayton cycle offers the potential of equivalent or higher cycle efficiency versus superheated or supercritical steam cycles at temperatures relevant for CSP applications. Brayton-cycle systems using s-CO2 have a smaller weight and volume, lower thermal mass, and less complex power blocks versus Rankine cycles due to the higher density of the fluid and simpler cycle design. The simpler machinery and compact size of the s-CO2 process may also reduce the installation, maintenance and operation cost of the system.
Optimisation of Tilt Angle for Different Photovoltaic Panels Under the Prevailing Environmental Conditions in Melbourne (Australia)
Authors: Hessami Mir Akbar , Lamande Sophie
Published: July 2013

The optimum tilt angle of a photovoltaic (PV) system was found by varying it between 9° and 49° at 5–10° increment over a period of three months and recording the output of the five different technologies installed. These technologies included mono-crystalline, poly-crystalline, HIT, thin film CdS/CdTe and thin film CIS modules. The yield and efficiency of these modules were determined experimentally and also by the SAM (Solar Advisor Model) computer software; these results correlated with each other to within ±15% with R2 = 0.72 (R = 0.85). This correlation improved when a longer period of time was considered in the simulations. An optimisation study found that the optimum fixed tilt angle was 31.8° for all technologies except thin film CIS modules for which the optimum angle was 27.9°. Furthermore, the yield of another PV system was recorded from April 2010 to July 2011 and compared with its prediction by the SAM simulations. It was found that there was an excellent agreement between the two sets of data. An optimisation study using SAM found that the increase in annual predicted energy generation as a result of changing the tilt of the panels from the current fixed value of 25° to the optimised tilt angle of 31.8° was only 1.1%. A similar optimisation study for a third PV system found that the increase in energy generation predicted after changing the tilt angle from 10°-12° to their optimum tilt angle was 1.9%. The optimum tilt angles found in this study were in good agreement with previous studies reported in the literature.
Optimization of an Integrated Energy Storage Scheme for a Dispatchable Wind Powered Energy System
Authors: Garrison Jared B. , Webber Michael E.
Published: July 2013

The intermittency of wind and solar power and the mismatch between when they are available and when demand is high have hindered the expansion of these two primary renewable resources. The goal of this research is to analyze an integrated energy system (named DSWiSS for dispatchable solar wind storage system) that includes a novel configuration of wind and solar together with compressed air energy storage (CAES) that is driven from excess nighttime wind energy and thermal storage energized by concentrated solar power in order to make these sources dispatchable during peak demand.

This paper builds off prior published work for the DSWiSS configuration with an analysis of actual historical meteorological data for West Texas solar insolation, generation output data for a wind farm in West Texas, recorded electricity demand data of the Electric Reliability Council of Texas (ERCOT) grid, and historical temperature data for West Texas to assess system performance. In this analysis, a comparison approach was taken by optimizing both the operation of a conventional CAES facility that does not incorporate wind and solar directly and the operation of a CAES facility directly coupled to a wind farm, which will be referred to as CAES-plus-Wind. Dynamic parameters for wind generation, electricity price, and ambient temperature were utilized in the optimization models.

Through the use of optimization models and the incorporation of a thermodynamic model of the CAES equipment, we found that in each season the electricity price is a key factor in determining whether the facility stores or generates energy. For the CAES equipment, the summer season yields the highest profits primarily because of the larger spread between highest and lowest daily price for electricity. Even though profits for the CAES equipment in the other seasons are small or negative, it appears that the value of the facility in the summer is greater than the costs in the other three seasons combined. Additionally, we found that the value of directly coupling the CAES facility to a wind farm versus operating the two entities separately yielded no significant increase in profits. Lastly, this analysis did not attempt to quantify the possible increase in wind farm generation output that could result from reduced curtailment with the use of an energy storage system such as is proposed in this paper. This additional source of revenue could be a major contributor to the economic justification for large scale energy storage.
Analysis of Drift Phenomena in Heliostat Images
Authors: Escobar-Toledo Martha , Arancibia-Bulnes Camilo A. , Peña-Cruz Manuel I. , Riveros-Rosas David , Peón-Anaya Rodolfo , Iriarte-Cornejo Cuitlahuac , Cabanillas Rafael E. , Estrada Claudio A.
Published: July 2013

Heliostat image drift is defined as the wandering of the irradiance spot produced by a heliostat on a receiver or observation screen. Two factors producing heliostat image drift have been analyzed theoretically in the present paper: errors in reference position, and time delay. Some regular behavior is found in the drift curves. Maximum deviations from target tend to be linearly dependent with either angular error, or time delay. In particular, the curves produced by this later effect are circular or elliptical. Heliostats at different distances from a tower have been analyzed. For heliostats far from the tower the drift curves due to errors in reference position also have a more or less elliptical shape. Results are presented as dimensionless quantities. Some practical implications of these results are discussed.
A Transverse Jet Reactor for Zinc Aerosol Hydrolysis
Authors: Nicodemus Julia Haltiwanger , Davidson Jane H.
Published: July 2013

Performance of a reactor designed for the hydrolysis of Zn in the two-step Zn/ZnO solar thermochemical cycle for hydrogen production is explored. Technically, complete hydrolysis of Zn in the hydrogen production step remains a major barrier to implementation, and much attention has been given to Zn nano-scale reacting aerosols as a solution. The success of this continuous process depends on achieving high particle yields and high conversions in the aerosol. A key challenge is to control the flow field in aerosol reactors to keep the particles entrained in the flow without deposition on the reactor wall. The ability of a new reactor concept based on transverse jet fluid dynamics to control the flow field and rapidly cool the Zn vapor is investigated. In the transverse jet reactor, evaporated Zn entrained in an Ar carrier gas issues vertically into the horizontal tubular reactor through which cooler H2O and Ar flow. Particles are formed in the presence of steam at ∼ 450K. The trajectory of the jet is controlled via the effective velocity ratio, R, which is the square root of the ratio of the kinetic energy of the jet to that of the cross-flow. A computational fluid dynamics (CFD) model indicates that the trajectory of the jet can be controlled so that the majority of the Zn mass is directed down the center of the reactor, not near the reactor walls for R = 4.25 to R = 4.5. Experimentally, maximum particle yields of 93% of the mass entering the reactor are obtained at R = 4.5.
Parametric Study Simulating the Daily Operation of a Thermocline Thermal Storage System
Authors: Valmiki M. M. , Li Peiwen , Chen Cholik
Published: July 2013

This work considers a packed-bed thermocline for use as an energy storage device in solar thermal power generation. A preliminary sizing procedure of a thermocline system for a 50 MWe plant is demonstrated along with predicted transient performance under various conditions. The inlet fluid temperatures are modeled as constant or time-dependent. The time-varying inlet conditions represent solar insolation variations throughout the day which roughly take a periodic form. Sinusoidal and periodic functions based on recorded data supply this inlet condition. A constant inlet temperature can be produced by circulating the heat transfer fluid in the collection field. Both possibilities are explored and the performance is compared for various charging time allotments. The study uses an efficient, accurate numerical scheme which has been developed to model the transient behavior of a porous thermocline tank. The model can be a useful tool for thermocline heat storage sizing. This can help take guess work out of the early stages of the design process as many simulations can be run efficiently and accurately.
Reducing Unwanted Gains During the Cooling Season From a Heat Pipe Augmented Passive Solar Heating System
Authors: Robinson Brian S. , Sharp M. Keith
Published: July 2013

The heat pipe augmented solar heating system significantly reduces heating loads relative to other conventional passive space heating systems [1–3]. Yet unwanted thermal gains during the cooling season from passive solar systems increase cooling loads and, in extreme cases, may even increase overall space conditioning loads relative to a nonsolar building. The objective of this study was to compare the effectiveness of several design modifications and control strategies for the heat pipe wall to reduce unwanted gains. MATLAB was used to simulate four different unwanted gains reduction mechanisms: 1. shading to block beam radiation from striking the collector, 2. an opaque cover to block all radiation from striking the collector, 3. a mechanical valve in the adiabatic section to eliminate convective heat transfer through the heat pipe into the room, and 4. switching the elevations of the evaporator and condenser sections of the heat pipe to provide heat transfer out of the room during the cooling season. For each mechanism, three different control strategies were evaluated: 1. Seasonal control, for which the prescribed mechanism is deployed at the beginning and removed at the end of the cooling season, 2. ambient temperature-based control, for which the mechanism is deployed if the forecast for the next hour (based on TMY3 weather data) is greater than 65°F, and 3. room temperature-based control, for which the mechanism is deployed if auxiliary cooling was required for the previous hour. For the seasonal strategy, the months for which the unwanted gains reduction mechanism should be deployed to minimize overall space conditioning loads were estimated with a season determination ratio (SD), defined as the monthly ratio of unwanted gains to heating load. Results suggested that SD may be a ‘universal’ parameter that can be applied across a range of climates for quick assessment of its optimal cooling season. With TMY3 data for Louisville, KY, the heat pipe system performed best with ambient temperature-based control. The mechanical valve was the best single mechanism. While in many cases the combination of the valve with a cover or shading produced slightly better performance than the mechanical valve alone, these additional reductions were small. Switching elevations of the evaporator and condenser sections produced little cooling, because of the low thermal emittance of the absorber and low thermal transmittance of the cover, and for the Louisville climate, small diurnal temperature swings during the summer.
A Reconfigureable Passive Solar Test Facility
Authors: Robinson Brian S. , Sharp M. Keith
Published: July 2013

A 12′ by 24′ passive solar test building has been constructed on the campus of the University of Louisville. The building envelope is comprised of structural insulated panels (SIPs), 12″ thick, (R-value of 45 ft2F/Btu) for the floor and walls and 16″ (R-63) for the roof. The building is divided into two symmetrical rooms with a 12″ SIPs wall separating the rooms. All joints between panels are caulked to reduce infiltration. Each room contains one window (R-9) on the north side wall, and two windows (also R-9) facing south for ventilation and daylighting, but which will also provide some direct gain heating. The south wall of each room features an opening that will accommodate a passive solar heating system so that performance of two systems can be compared side-by-side. The overhang above the south openings is purposely left short to accommodate an awning to provide adjustable shading. The calculated loss coefficient (UA) for each room of the building is 6.07 W/K. Each room is also equipped with a data acquisition system consisting on an SCXI 1600 16 bit digitizer and an SCXI 1102B isolation amplifier with an SCXI 1303 thermocouple module. Pyranometers are placed on the south wall and the clerestory wall to measure insolation on the solar apertures.

For initial tests, one room is equipped with an original heat pipe system previously tested in another building, while the other is equipped with a modified heat pipe system. Changes to the modified system include copper absorbers versus aluminum, an adiabatic section constructed of considerably less thermally-conductive DPM rubber than the copper used for the original design, and one of the five condenser sections of the heat pipes is exposed directly to the room air to provide early-morning heating. Experimental results will be compared to simulations with as-built building characteristics and actual weather data. Previous simulations with a load to collector ratio of 10 W/m2K, a defined room comfort temperature range between 65°F to 75°F, and TMY3 weather data for Louisville, KY, showed that the modified heat pipe wall design improves annual solar fraction by 16% relative to the original design.
Experimental and Simulation Study on a Solar Domestic Hot Water System With Flat-Plate Collectors for the Canadian Climatic Conditions
Authors: Yaïci Wahiba , Entchev Evgueniy , Lombardi Kathleen
Published: July 2013

This paper presents thermal performance results of an experimental and numerical simulation study of a solar domestic hot water system (SDHW) for Canadian weather conditions. The experimental test setup includes two solar panels, a solar preheat tank, and an auxiliary propane-fired storage water heater, and an air handler unit for space heating. Experiments were performed on the SDHW system during a different season of the year, over the period March through October 2011 to assess the system performance for different solar gain and water draw schedules. Sunny, partly cloudy and cloudy conditions were explored. The test results were analysed in terms of solar fraction, solar efficiency, and the effects of thermosyphoning and stratification in the solar storage tank.

Modelling and simulation of the solar thermal energy system using TRNSYS software was performed. The objective was to optimise key design parameters and to suggest an effective control strategy to maximise the heat extraction from solar collectors. The developed model was based on the experimental test setup. It was first adjusted and verified with the solar gain and water draw schedule experimental data. The results of the numerical simulations were then validated with experimental results obtained with other water draw schedule and weather conditions. Acceptable agreements between the predicted and measured values were obtained at this early stage of development. Further refinements in system and model validation are in progress in order to improve the accuracy of the predictions. Ultimately, as the final product of this investigation, this model will be used to predict the performance of solar domestic hot water and space heating systems in different Canadian locations, different operating conditions and water draw schedules.
CFD Simulation of Multiphase Flow in an Airlift Column Photobioreactor for the Cultivation of Microalgae
Authors: García Samira , Paternina Edgardo , Pupo Oscar R. , Bula Antonio , Acuña Francisco
Published: July 2013

Due to their better operational advantages, such as good scalability, operational flexibility, high mass and heat transfer characteristics, homogenous shear stress, good mixing and better control over fluid circulation, airlift column photobioreactors (PBR) have become a promising design alternative for microalgae culture, among bubble column and other types of photobioreactors. The good hydrodynamic environment for fragile microorganisms in airlift columns have led to numerous investigations in order to understand the hydrodynamic phenomena of multiphase flow, particularly gas-liquid, inside this type of PBR.

In the present paper, a CFD (Computer Fluid Dynamics) simulation in CFX, ANSYS Inc. 11.0, is developed following a multiphase flow model with an Eulerian-Eulerian approach. For evaluating turbulence, the modified k–ε model is chosen. Gas holdup, gas superficial velocity and liquid velocity profiles for different heights column are obtained by varying air volumetric flow rates at 2, 5 and 8 L/min. Also gas holdup, gas superficial velocity and liquid velocity contours are obtained for a sequence of five step times. An experimental video validation to compare the multiphase flow behavior has been made with a high-resolution video camera. The experiments are carried out by using an airlift column photobioreactor with air supply by a porous sparger.

The profiles of the hydrodynamic variables made for 5 different column heights show that the trends of gas holdup and gas superficial velocity are very similar and do not depend on the variation of inlet air flow. For the liquid velocity profiles, the trends don’t show the same behavior, the profiles at lower heights are off-centered and less symmetric and the maximum velocities are reached at h = 0.2 m.

The time sequences with the variable contours are made to enhance the visualization and understanding of the hydrodynamic behavior, especially when air supply begins and the bubble plume starts to form.
Geographical Limitations on Integral-Collector-Storage Collectors due to Collector Freeze
Authors: Schollenberger Frederick S. , Kreith Frank , Burch Jay
Published: July 2013

A major challenge for solar water heaters is to provide heat at a cost comparable to or lower than conventional fuels. Since the price of a passive integral-collector-storage (ICS) solar water heater has historically been less than that for active systems with freeze protection, they can potentially heat water at a lower cost. However, ICS panels are subject to freeze damage, as the collector generally has metal tubes carrying pressurized water that can freeze and burst. In order to delineate the geographical areas where ICS panels can be deployed safely, it is necessary to experimentally characterize the conditions causing freeze damage, to develop a model relating the freeze behavior to climatic conditions, to validate that model with experimental data, and to run the model against long-term weather data across the U.S. Two variations of an ICS panel and/or their bare tubes were tested in a walk in freezer and subjected to freezing conditions until freeze damage occurred. The units tested include both a single and double glazed tubular ICS panel. Key data includes the volume expansion of the tube(s) at burst and the collector loss coefficient near 0 degrees C. Under freezing conditions the insulated supply/return lines would freeze solid initiating a pressure-buildup and eventual burst in the collector tubes due to further internal freezing. An additional test on the single glazed unit was also conducted in which heat tape was installed on the inlet and outlet pipes to prevent them from freezing, which increases the freeze tolerance of the panel by forcing small internal interconnection pipes to freeze solid before damage occurs. Existing models for ICS thermal performance were modified to incorporate the freezing process, and have been validated with the experimental data. The validated models were used to predict regions of the country that are safe for installing the ICS panels. Simulations were run using 30 years of weather data available for all TMY2 sites, and maps were created to illustrate regions of safe installation throughout the US for both the with and without heat tape scenarios for the two ICS models. A correlation using record minimum temperature was developed to generalize the maps to any location for which the record minimum is known. The maps show quantitatively the expected conclusions: 1) that double glazing and higher insulation will extend the safe region; and 2) that the use of heat tape on the inlet and outlet pipes significantly increases the region in which ICS panels can be safely installed in the US.
Buoyancy Driven Mass Transfer in a Liquid Desiccant Storage Tank
Authors: Quinnell Josh A. , Davidson Jane H.
Published: July 2013

A new concept for long-term solar storage is based on the absorption properties of aqueous calcium chloride. Water, diluted and concentrated calcium chloride solutions are stored in a single tank. An immersed heat exchanger and stratification manifold are used to preserve long-term sorption storage, and to achieve thermal stratification. The feasibility of the concept is demonstrated via measurements of velocity, CaCl2 mass fraction, and temperature in a 1500 liter prototype tank during sensible charging. Experiments are conducted over a practical range of the relevant dimensionless parameters. For Rayleigh numbers from 3.4 × 108 to 5.6 × 1010 and buoyancy ratios from 0.8 to 46.2, measured Sherwood numbers are 11±2 to 62±9 and the tank is thermally stratified. Convective mixing is inhibited by the presence of a steep density gradient at the interface between regions of differing mass fraction. The predicted storage time scales for the reported Sherwood numbers are 160 to 902 days.