bluuebunny/crossref_metadata_2025 huggingface dataset:
High Density Thermal Energy Storage With Supercritical Fluids
Authors: Ganapathi Gani B. , Wirz Richard
Published: July 2013

A novel approach to storing thermal energy with supercritical fluids is being investigated, which if successful, promises to transform the way thermal energy is captured and utilized. The use of supercritical fluids allows cost-affordable high-density storage with a combination of latent heat and sensible heat in the two-phase as well as the supercritical state. This technology will enhance penetration of several thermal power generation applications and high temperature water for commercial use if the overall cost of the technology can be demonstrated to be lower than the current state-of-the-art molten salt using sodium nitrate and potassium nitrate eutectic mixtures. An additional attraction is that the volumetric storage density of a supercritical fluid can be higher than a two-tank molten salt system due to the high compressibilities in the supercritical state.

This paper looks at different elements for determining the feasibility of this storage concept — thermodynamics of supercritical state with a specific example, naphthalene, fluid and system cost and a representative storage design. A modular storage vessel design based on a shell and heat exchanger concept allows the cost to be minimized as there is no need for a separate pump for transferring fluid from one tank to another as in the molten salt system. Since the heat exchangers are internal to the tank, other advantages such as lower parasitic heat loss, easy fabrication can be achieved.

Results from the study indicate that the fluid cost can be reduced by a factor of ten or even twenty depending on the fluid and thermodynamic optimization of loading factor. Results for naphthalene operating between 290 °C and 475 °C, indicate that the fluid cost is approximately $3/kWh compared with $25-$50/kWh for molten salt. When the storage container costs are factored in, the overall system cost is still very attractive. Studies for a 12-hr storage indicate that for operating at temperatures between 290–450 °C, the cost for a molten salt system can vary between $66/kWh to $184/kWh depending on molten salt cost of $2/kg or a more recent quote of $8/kg. In contrast, the cost for a 12-hr supercritical storage system can be as low as $40/kWh. By using less expensive materials than SS 316L, it is possible to reduce the costs even further.
Simulation Study for the Performance of a Large Solar Hot Water System Constructed by Natural Circulation Solar DHW System Modules
Authors: Yang Ru , Yu Kuan-Hsiang
Published: July 2013

This research is aimed to study the system performance for a large solar hot water system constructed by connecting a series of small domestic natural-circulation systems. This type of large solar hot water system has been recently installed in Taiwan without either practical or theoretical support. There are few studies on this type of large solar hot water system available. This paper presents the numerical simulation study for the control and the system operating parameters effects on the system performance to provide important information both for users and system designers.
Screening Tool for the Evaluation of Combined Cooling, Heating, and Power Systems in Existing Office Buildings
Authors: Fumo N. , Mago P. J. , Spencer J. D.
Published: July 2013

Combined Cooling, Heating, and Power (CCHP) systems have been widely recognized as a key alternative for heat and electricity generation because of their ability to consume fuel more efficiently, which translates into a reduction in carbon dioxide emission, considered the main factor contributing to global warming. However, economic analyses do not always favor the implementation of this technology. Even though CCHP systems offer other benefits such as power reliability, power quality, and fuel source flexibility, they are often negated as a feasible alternative because of these poor economic indicators. Therefore, a more comprehensive evaluation of the system should be considered. This is particularly true in an environment where economic, environmental, political, and logistical problems associated with increasing centralized electrical power production are becoming more difficult to overcome. In addition, as consumers continue to be more involved and to develop a better understanding of energy choices, the demand for technology that better meets their energy needs is increasing.

To promote the development of CCHP projects, it is important to facilitate, without any cost, a first order analysis of this technology to determine if a more cost intensive, in-depth analysis should be performed. This analysis can be done by using screening tools such as the CCHP Screening Tool for Existing Office Buildings (CCHP-ST-EOB) proposed in this study. Screening tools should be as accurate as possible while maintaining the simplicity of their data input in order to make it easy to use by a broad audience that may include building owners and managers without engineering background. In this sense, the CCHP-ST-EOB uses a methodology that translates energy consumption from utility bills as input into hourly energy consumption for a more accurate analysis of the matching between the demand and supply sides. This tool takes into consideration partial load efficiencies for the power generation unit and absorption chiller for a more realistic simulation of the system performance. Results are presented in terms of cost, primary energy consumption, and CO2 emission. The tool is available to be downloaded free of charge at http://microchp.msstate.edu/thankyou.html.
A Screening Methodology for Climatic Evaluation of the Cooling Potential of Night Ventilation in Buildings
Authors: Endurthy Akhilesh Reddy , Reddy T. Agami
Published: July 2013

Night ventilation is a well known strategy for passive cooling of residences and small commercial buildings. The building’s thermal mass can be cooled at night by ventilating the inside of the space with the relatively lower outdoor air temperatures, thereby lowering indoor temperatures during the warmer daytime period. Numerous experimental and theoretical studies have shown the effectiveness of the method to significantly reduce air conditioning loads or improve comfort levels in those climates where the night time ambient air temperature drops below that of the indoor air. One could develop/adapt computer programs with detailed mathematical component models to simulate and evaluate the performance of night ventilation strategies in a specific location for a particular building. A more basic problem is to develop a methodology whereby potential designers can screen various climatic locations and regions in order to perform a preliminary evaluation of which months of the year are good candidates for implementing such a scheme. Only after completion of such a phase is a detailed evaluation warranted for specific buildings. In this paper, effectiveness of night ventilation is quantified by a parameter called the Discomfort Reduction Factor (DRF) which is the index of reduction of occupant discomfort levels during the day time from night ventilation. Two different thermal network models which provide such insights are evaluated. Daily and monthly DRFs are calculated for two climate zones and three building heat capacities for the whole year. It is verified that night ventilation is effective in seasons and regions when day temperatures are between 30 °C and 36 °C and night temperatures are below 20 °C. The accuracy of these preliminary screening models may be lower than using a detailed simulation program but the loss in accuracy in using such tools is more than compensated by the insights provided, along with better transparency in the analysis approach and results obtained.
Methodology for Optimization of Stand-Alone Solar Photovoltaic Systems
Authors: Fumo N. , Bortone V. , Zambrano J. C.
Published: July 2013

The concept of Net-Zero Energy in building refers to a building which has an annual balance of energy flow at the utility meter of zero. The concept implies that the building may consume energy from an external provider at times in order to satisfy the building demands, but at other times it must produce enough on-site energy to compensate for this energy. The use of renewable energy technologies is implicit as the source of energy to compensate for any energy used from an external provider. Solar photovoltaic is a proved technology for achieving Net-Zero Energy building but economic factors has limited its broad use. The design stage of a solar photovoltaic project is critical to make a project feasible. In the design stage, the equipment sizing must be optimized in order to reduce the initial capital cost and, therefore, improve the economics of the project. For houses, which is the focus of this paper, a stand-alone solar photovoltaic system must supply the house energy demand at all times since it is not connected to the electric grid. As a means to size the system, data of solar energy availability must be used to ensure that the system will provide enough energy to satisfy the energy demand as well as provide energy to charge the batteries that will provide the energy required when the solar energy is not available. In this paper, a methodology to optimize the size of the photovoltaic array and the battery bank is proposed. The methodology accounts for Typical Meteorological Year data (TMY3) to ensure that the system, based on accepted statistical data, will be able to satisfy the energy demand at all times. An example that uses energy demand data obtained from the simulation of a house using the software EnergyGauge is used to illustrate the implementation of the methodology.
Performance and Operation of the Southern Nevada Water Authority High Concentration Photovoltaic Plant for the Last 800 Days
Authors: Ayubi Omer , Sahm Aaron , Boehm Robert
Published: July 2013

The Southern Nevada Water Authority (SNWA) obtained six of the Amonix, Inc. multi-junction Concentrating Photovoltaic (CPV) systems in 2009. A description of the CPV systems, plant layout, and operating description is given. Data showing the power and that the systems have generated over 1,200 MWh of grid energy during the last 33 months are presented in the paper. An estimate is given of the net annual energy generated based upon the last 33 months of daily energy performance. Data is also presented showing the daily peak power divided by peak DNI and the energy performance during this period. The effect of shading is discussed and data are presented showing its effects on the annual field performance. Data are also given on the operating reliability and the long-term performance of the plant. A discussion and pictures are given of how the terrain is returning to its natural state and how the habitat has accepted the CPV systems.
Solar STIG Cycle Annual Analysis
Authors: Livshits Maya , Kribus Abraham
Published: July 2013

The Steam Injection Gas turbine (STIG) cycle offers a way to use solar steam at a low temperature and pressure, generated by low-cost concentrators, in order to augment the power output of the turbine. In conventional STIG, the steam is generated from the gas turbine exhaust and injected into the combustion chamber. In previous work we proposed the solar augmentation of a STIG cycle, using solar concentrators to generate steam in much larger amounts compared to the natural limit of heat recovery in the conventional STIG. In the current work, an annual analysis of the Solar STIG cycle is presented for two sites with moderate and high annual DNI, under two scenarios: constant power with a varying Solar Fraction (SF), and variable power with a nearly constant SF. Results show typical annual SF in the range of 20–30%, and solar to electricity efficiency of around 15%, similar to the annual efficiency of current parabolic trough plants that operate at much higher pressure and temperature. The variable power scenario improves the SF with only a minor decrease in efficiency.
Optimal Sizing of Second Life Battery to Reduce CO2 Emissions
Authors: Keeli Anupama , Sharma Ratnesh K.
Published: July 2013

Batteries used in electric vehicles cannot be used in the vehicle once the battery capacity falls to 70% to 80%. The remaining capacity of these batteries can be used in various applications so that they can be kept out of landfills. One of the applications is to use the second life batteries to reduce carbon dioxide (CO2) emissions from the grid. The CO2 emissions from the grid are high during the non-peak hours when only the base plants are operating. We charge the second life battery when the power from renewable resources is available and discharge the battery when there is peak load. With the increase in renewable generation such as photo voltaic, wind energy the carbon foot print changes with the hour of the day. The demonstrated technique optimizes the use of second life battery to reduce the emissions considering uncertainties in the load and availability of renewable power.
Numerical Simulation of Natural Convection in Solar Cavity Receivers
Authors: Yuan James K. , Ho Clifford K. , Christian Joshua M.
Published: July 2013

Cavity receivers used in solar power towers and dish concentrators may lose considerable energy by natural convection, which reduces the overall system efficiency. A validated numerical receiver model is desired to better understand convection processes and aid in heat loss minimization efforts. The purpose of this investigation was to evaluate heat loss predictions using the commercial computational fluid dynamics software packages FLUENT 13.0 and SolidWorks Flow Simulation 2011 against experimentally measured heat losses for a heated cubical cavity model [1] and a cylindrical dish receiver model [2]. Agreement within 10% was found between software packages across most simulations. However, simulated convective heat loss was under predicted by 45% for the cubical cavity when experimental wall temperatures were implemented on cavity walls, and 32% when implementing the experimental heat flux from the cavity walls. Convective heat loss from the cylindrical dish receiver model was accurately predicted within experimental uncertainties by both simulation codes using both isothermal and constant heat flux wall boundary conditions except at inclination angles below 15° and above 75°, where losses were under- and over-predicted by FLUENT and SolidWorks, respectively. Comparison with empirical correlations for convective heat loss from heated cavities showed that correlations by Siebers and Kraabel [1] and for an assembly of heated flat plates oriented to the cavity geometry [3] predicted heat losses from the cubical cavity within experimental uncertainties, while correlations by Clausing [4] and Paitoonsurikarn et al. [8] were able to do the same for the cylindrical dish receiver. No single correlation was valid for both receiver models. Different turbulence and air-property models within FLUENT were also investigated and compared in this study.
Computational Analysis of a Pipe Flow Distributor for a Thermocline Based Thermal Energy Storage System
Authors: Afrin Samia , Cordero Eduardo , De La Rosa Sebastian , Kumar Vinod , Bharathan Desikan , Glatzmaier Greg C. , Ma Zhiwen
Published: July 2013

The overall efficiency of a Concentrating Solar Power (CSP) plant depends on the effectiveness of Thermal Energy Storage (TES) system [1]. A single tank TES system consists of a thermocline region which produces the temperature gradient between hot and cold storage fluid by density difference [2]. Preservation of this thermocline region in the tank during charging and discharging cycles depends on the uniformity of the velocity profile at any horizontal plane. Our objective is to maximize the uniformity of the velocity distribution using a pipe-network distributor by varying the number of holes, distance between the holes, position of the holes and number of distributor pipes. For simplicity, we consider that the diameter of the inlet, main pipe, the distributor pipes and the height and the width of the tank are constant. We use Hitec® molten salt as the storage medium and the commercial software Gambit 2.4.6 and Fluent 6.3 for the computational analysis. We analyze the standard deviation in the velocity field and compare the deviations at different positions of the tank height for different configurations. Since, the distance of the holes from the inlet and their respective arrangements affects the flow distribution throughout the tank; we investigate the impacts of rearranging the holes position on flow distribution. Impact of the number of holes and distributor pipes are also analyzed. We analyze our findings to determine a configuration for the best case scenario.
A Hybrid Measure-Correlate-Predict Method for Wind Resource Assessment
Authors: Zhang Jie , Chowdhury Souma , Messac Achille , Castillo Luciano
Published: July 2013

This paper develops a hybrid Measure-Correlate-Predict (MCP) strategy to predict the long term wind resource variations at a farm site. The hybrid MCP method uses the recorded data of multiple reference stations to estimate the long term wind condition at the target farm site. The weight of each reference station in the hybrid strategy is determined based on: (i) the distance and (ii) the elevation difference between the target farm site and each reference station. The applicability of the proposed hybrid strategy is investigated using four different MCP methods: (i) linear regression; (ii) variance ratio; (iii) Weibull scale; and (iv) Artificial Neural Networks (ANNs). To implement this method, we use the hourly averaged wind data recorded at six stations in North Dakota between the year 2008 and 2010. The station Pillsbury is selected as the target farm site. The recorded data at the other five stations (Dazey, Galesbury, Hillsboro, Mayville and Prosper) is used as reference station data. Three sets of performance metrics are used to evaluate the hybrid MCP method. The first set of metrics analyze the statistical performance, including the mean wind speed, the wind speed variance, the root mean squared error, and the maximum absolute error. The second set of metrics evaluate the distribution of long term wind speed; to this end, the Weibull distribution and the Multivariate and Multimodal Wind Distribution (MMWD) models are adopted in this paper. The third set of metrics analyze the energy production capacity and the efficiency of the wind farm. The results illustrate that the many-to-one correlation in such a hybrid approach can provide more reliable prediction of the long term onsite wind variations, compared to one-to-one correlations.
Optical Analysis of a Novel Linked Heliostat Tracking System for Distributed-Scale Concentrated Solar Power
Authors: Dunham Marc , Kasetty Rajan , Mathur Anoop , Lipiński Wojciech
Published: July 2013

In this work, a novel concentrating solar power system consisting of a small heliostat field utilizing simplified two-axis tracking is proposed for distributed-scale solar thermal power generation. Monte Carlo ray tracing is used to characterize the optical performance of the system and to parametrically evaluate its design. Radiative flux distributions are obtained in the receiver plane for solar irradiation at an example location of Albuquerque, NM, and date of June 21. The system delivers an 8-hour daily average optical efficiency of 64.4%, flux concentration ratio of 122.8 suns, and daily average thermal power of 47.3 kWt for a receiver of 0.35 m radius. The peak optical efficiency at solar noon was found to be 97.9% with a concentration ratio of 201.3 and thermal power of 77.5 kWt for the base simulation parameters.
Analysis of Dairy Manure and Food Manufacturing Waste as Feedstocks for Sustainable Energy Production via Anaerobic Digestion
Authors: Rankin M. J. , Trabold T. A. , Williamson A. A. , Augustine M.
Published: July 2013

Anaerobic digestion is a waste-to-energy conversion process that offers potential economic and environmental benefits of organic waste diversion and renewable energy generation. However, these systems are often not feasible for small-to-medium size food processors, due to the significant capital investment involved. The key objective of this study is to identify the volume and composition of dairy manure and liquid-phase food manufacturing waste streams available in New York State (NYS) to make co-digestion of multiple feedstocks in centralized anaerobic digester facilities an economically attractive alternative. Organic waste volume and property data were obtained via Freedom of Information Law (FOIL) requests at the county and municipal levels for each of the 62 counties in NYS. Spatial analyses of dairy confined animal feeding operations (CAFO) locations relative to food manufacturing facility locations were analyzed using Microsoft MapPoint imaging software, which identified concentrations of high strength liquid-phase waste in the upstate corridor extending between Buffalo and Albany. The results show that if anaerobically digested, dairy CAFO manure and food manufacturing waste can contribute significantly to the State’s renewable energy portfolio. A laboratory scale two-phase anaerobic digester (bioDrillTS-AD200©) can help establish the correlation between waste properties (e.g. total solids, etc.) and quantity and quality of biogas produced.
Characterization and Cost Analysis for the UC Merced Campus Load Including Effects of Solar Farm Variability
Authors: Marquez Ricardo , Davis Bron , Kaur Amanpreet , Coimbra Carlos F. M.
Published: July 2013

Variability and uncertainty in renewable energy generation has a direct impact on the cost of electricity prices because of the potential impacts on the operation of existing power generating units. The purpose of this paper is to present a case study detailing the variability added to the UC Merced campus load after including a 1-MW photovolataic (PV) plant to supply a significant amount (25–30% of maximum demand) of the energy load. Cost estimates of the required mechanisms to compensate for the variability are calculated based on utility prices and the PV power output fluctuations. The resulting cost estimates are then compared with solar power variability metrics in order to show the economic impacts of solar variability. Additionally, a simulation is performed for a integrated battery storage system to mitigate the PV fluctuations by solving a revenues optimization formulation. The simulated costs are also correlated with three variability metrics including the standard deviation of the fluctuations and the mean absolute values of the fluctuations. In order to extrapolate results from the case study to a more general scenario, we show that the obtained correlations of normalized variables can be useful for providing estimates on the financial impacts of variable generation resources based on more widely available solar irradiance data.
Numerical Analysis of Airfoils Used for Vertical Axis Wind Turbine
Authors: Uzarraga-Rodriguez N. C. , Gallegos-Muñoz A. , Parra-Santos Maria T. , Belman-Flores Juan M.
Published: July 2013

A numerical analysis of a three-bladed straight vertical axis wind turbine with NACA0015 airfoils-shaped is presented. The effect generated on the moment coefficient and power coefficient of the wind turbine rotor by the twist angle variation at the chord ends was analyzed. The configurations included the variation of blade twist angle of 15° and 30° located at 70%, 80% and 90% of chord length from leading end of the straight blade. The numerical study was developed in a commercial Computational Fluid Dynamics (CFD) using FLUENT®. This code allows to solve the Reynolds averaged Navier-Stokes equations and the transport equations of the turbulence quantities. The results show the aerodynamic performance for each configuration of the blade twist angle in the wind turbine, and are compared with data obtained from straight blade without twist angle. The wind turbine performance decrease about 67% as the blade twist angle increases, due to an increment in the drag force causing a negative moment against the rotation of vertical axis wind turbine. Also, the surface pressure distribution in a VAWT’s is presented.
General Performance Metrics and Applications to Evaluate Various Thermal Energy Storage Technologies
Authors: Ma Zhiwen , Glatzmaier Greg C. , Wagner Michael , Neises Ty
Published: July 2013

The solution proposed in this paper presents a new modeling approach that integrates a generalized thermal storage performance model into a concentrating solar power (CSP) plant. The overall performance, including round trip efficiency, for a thermal energy storage system is highly dependent on the operating parameters and operation strategy of the complete power plant. Previous methods used for analysis of thermal storage have followed one of two approaches: The first requires time-intensive customized detailed performance models of the thermal storage system and the power cycle to account for the effects of charging and discharging storage on conversion efficiency and heat transfer fluid (HTF) return temperature to the solar field. The second method uses a simple energy balance with “derate” factors that do not accurately predict the effects of storage on other plant components. In this paper, we develop a generalized method based on efficiency metrics and discuss the application in TES sizing and performance evaluation for an early concept study. The method is an integral approach and complements the detailed models that simulate yearly operation of a CSP plant.
Heliostat Characterization by Optical Techniques and Image Processing
Authors: Peña-Cruz Manuel I. , Arancibia-Bulnes Camilo A. , Iriarte-Cornejo Cuitlahuac , Cabanillas Rafael E. , Estrada Claudio A.
Published: July 2013

In the framework of the National Laboratory of Solar Concentrating and Solar Chemistry Systems (LACYQS, for its Spanish acronym), a Heliostat Test Field (HTF) was built in México. This research facility is located 10 km away from the city of Hermosillo, in the state of Sonora. The main purpose of the HTF, at the present stage, is to serve as platform for the development and testing of heliostat technology.

In order to evaluate the performance of heliostats, various optical tests have been implemented. In the sun tracking test, the heliostat is operated as a solar tracker. A camera is attached to the heliostat, which is pointed directly to the sun. Images are captured throughout the day to quantify the wandering of the solar disc in the image. In the reflected spot test, the image produced on the Lambertian target by the concentrating heliostat, due to the reflection of the sun, is recorded by a CCD camera throughout the day. Image processing algorithms calculate the centroid of energy of the image and evaluate the position and wandering across the white screen at all times. After this information is gathered, and the influence of wind and external factors eliminated, data are interpreted to characterize the behavior of solar projection algorithms and mechanical components.

In the fringe projection analysis, also known as deflectometry, fringe patters are projected at night on a Lambertian target. The image of the pattern reflected by the heliostat is recorded with a camera. Distortions in the fringes, due to mirror stress and canting, allows the characterization of the surface error of the facets.
Meeting the Challenge: A Stochastic Assessment of the U.S. Light-Duty Vehicle Fuel Economy Standards
Authors: Bastani Parisa , Heywood John B. , Hope Chris
Published: July 2013

The U.S. Department of Transport and EPA have recently proposed further regulation of the light-duty vehicle corporate average fuel economy and GHG emissions for model years 2017 to 2025. Policy makers are setting more stringent targets out to 2025 in a context of significant uncertainty. These uncertainties need to be quantified and taken into account systematically when evaluating policies. In this paper, a stochastic technology and market vehicle fleet analysis is carried out, using the STEP (Stochastic Transport Emissions Policy model), to assess the probability of meeting the proposed CAFE targets in 2016 and 2025, and identify factors that play key roles in the near and midterm. Our results indicate that meeting the proposed targets requires (a) aggressive technological progress rate and deployment, (b)aggressive market penetration of advanced engines and powertrains, (c) aggressive vehicle downsizing and weight reduction, and (d) a high emphasis on reducing fuel consumption. Three scenarios are examined to assess the likelihood of meeting the proposed targets. The targets examined here, 32.5 and 34.1 mpg in 2016 and 44 and 54.5 mpg in 2025, are reduced from the nominal CAFE values after allowing for the various credits in the proposed rulemaking. The results show that there is about a 42.5% likelihood of the passenger cars average fuel economy falling below 32.5 mpg and a 5.3% likelihood of it exceeding 34.1 mpg in 2016, and about a 4% chance of it exceeding 44 mpg in 2025, under the plausible-ambitious scenario. Under the EPA/DOT preferred alternative scenario, the likelihood of passenger cars average fuel economy meeting or exceeding 34.1 mpg in 2016 and 44 mpg in 2025 increases to about 74% and 34.5% respectively. The probability of meeting these combined CAFE targets drops to less than 1% in both near and mid terms, once light trucks are included in the mix. This analysis quantifies the probability of meeting the targets therefore to enable risk-based contingency planning, and identifies key drivers of uncertainty where further strategic research is needed.
The Role of Small Distributed Natural Gas Fuel Cell Technologies in the Smart Energy Grid
Authors: Rhodes Joshua D. , Nagasawa Kazunori , Upshaw Charles , Webber Michael E.
Published: July 2013

As the utility grid evolves to transmit information along with energy and water to the end-user, the traditional grid model is changing. The Pecan Street Smart Grid Demonstration Project in Austin, TX is at the leading edge of the evolution of the smart grid. Currently, over 100 homes, soon to be 1,000, have electricity demand information being measured on a 15 second interval. Using the highly granular energy use and solar generation data from Pecan Street, we attempt to estimate the potential for small natural gas fuel cells as distributed firming power for intermittent renewables in the built environment. Micro-grids have traditionally relied on the macro-grid for stabilization in the event of local interruptions in generation. In this paper we analyze the utility, economic, and system efficiency impacts of small distributed natural gas fuel cells as an alternative to the macro-grid for stabilization. Using our unique dataset, we have determined that the average home could utilize a 5.5 kW fuel cell either for total generation or backup, and the average home could operate as its own micro-grid while not sacrificing core functionality. We also explore the utility of matching the thermal output of a possibly smaller fuel cell, used in combined heat and power mode (CHP), to an absorption refrigeration system in place of traditional space cooling. With these types of energy assets, homes could possibly participate with local electricity markets, or the grid at large, in a highly dynamic way. A home energy network could, given homeowner set-points, adjust home uses of energy and sell high priced electricity back to the grid, possibly from both solar PV and fuel cell production, possibly eliminating energy bills. Lastly, we estimate that the system efficiency could possibly double by transporting natural gas to the end user to be converted into electricity and hot water as compared with traditional methods of using natural gas for power generation followed by electricity delivery.
The Net Impact of Wind Energy Generation on Emissions of Carbon Dioxide in Texas
Authors: Meehan Colin , Webber Michael , Nagasawa Kazunori
Published: July 2013

This analysis will examine the relationship between increased levels of wind energy generation and emissions per unit of electricity produced using historical data for electricity output and CO2, SO2 and NOx emissions in the Electric Reliability Council of Texas (ERCOT). Renewable Portfolio Standards (RPS) are generally seen in part as a policy tool for reducing overall system CO2 emissions, although renewable energy goals do not directly regulate such emissions. Limiting this analysis to ERCOT provides two important advantages: transmission of wind energy output is constrained by the physical boundaries of the ERCOT grid, simplifying the analysis and avoiding associated ‘leakage issues’; and ERCOT has the highest level of wind generation as a percentage of total system demand of any grid in the continental U.S.

The intermittent nature of wind generation has resulted in the need to ramp conventional thermal generation up and down to compensate for variability in wind output. Such ramping leads to inefficiencies in many fossil-fueled power plants that increase emissions of CO2, SO2, and NOx relative to a respective unit’s peak efficiency emissions rate.

Using EPA’s Clean Air Markets hourly emissions data, we calculate the total combustion emissions of CO2, SO2 and NOx per MWh of electricity output for the ERCOT system from 2003–2011. The EPA database includes CO2, SO2 and NOx emissions reported by facility owner and operators on an hourly basis in a manner that incorporates facility inefficiencies during ramping periods, allowing us to fully evaluate the CO2 emissions reductions achieved in ERCOT as a result of increased wind generation. The study is ongoing as we wait for emissions statistics from the final quarter of 2011 to be released by the EPA in early 2012.
Using High Resolution Solar Measurement in PV Variability Studies
Authors: Sengupta Manajit
Published: July 2013

Clouds, aerosols, water vapor and other atmospheric constituents influence solar energy reaching the earth’s surface. Each of these atmospheric constituents has it’s own inherent scale of temporal and spatial variability and they in turn influence the variability in the amount of solar radiation reaching the earth’s surface. This combined influence of the atmospheric constituents and their separate variability characteristics makes solar variability modeling a complicated task.

Output from photovoltaic (PV) power plants is dependent on the amount of solar energy reaching the surface. Therefore variability in solar radiation results in variability in PV plant output. The issue of variability in PV plant output has become important in the last couple of years as utility scale PV plants go online and increase in size. Understanding variability in PV plant output requires an understanding of (a) the spatial and temporal variability of solar radiation; (b) the influence of this solar variability on PV plant output.

The goal of this paper is to understand what temporal and spatial scales of variability in Global Horizontal Radiation (GHI) are important to a PV plants and what measurements are needed to be able to characterize them. As solar radiation measuring instruments are point receivers it is important to understand how those measurements translate to energy received over a larger spatial extent. Also of importance is the temporal nature of variability characterized not at a single point on the ground but over large spatial areas.

In this research we use high temporal and spatial resolution measurements from multiple time synchronized solar radiation sensors to create solar radiation fields at various spatial and temporal scales using a wide range of interpolation techniques. These solar fields are then used to create plant power output for various size PV plants. As various interpolation schemes can produce different distributions we investigate the impact of interpolation schemes on GHI and power output distribution. While power output from PV plants is an important quantity the temporal variability of power is a matter of concern to utilities. In this paper we show how PV plant output varies across different time scales.
Heat and Mass Transfer Characteristics of a Zeolite 13X/CaCl2 Composite Adsorbent in Adsorption Cooling Systems
Authors: Chan K. C. , Chao Christopher Y. H. , Bahrami M.
Published: July 2013

The performance of the adsorption cooling system using the zeolite 13X/CaCl2 composite adsorbent was studied using a numerical simulation. The novel zeolite 13X/CaCl2 composite adsorbent with superior adsorption properties was developed in previous studies [11]. It has high equilibrium water uptake of 0.404 g/g between 25°C and 100°C under 870Pa. The system specific cooling power (SCP) and coefficient of performance (COP) were successfully predicted for different operation parameters. The simulated COP with the composite adsorbent is 0.76, which is 81% higher than a system using pure zeolite 13X under desorption temperature of 75°C. The SCP is also increased by 34% to 18.4 W/kg. The actual COP can be up to 0.56 compared to 0.2 for zeolite 13X-water systems, an increase of 180%. It is predicted that an adsorption cooling system using the composite adsorbent could be powered by a low grade thermal energy source, like solar energy or waste heat, using the temperature range of 75°C to 100°C.

The performance of the adsorber with different design parameters was also studied in the present numerical simulation. Adsorbents with smaller porosity can have higher thermal conductivity and may result in better system performance. The zeolite bed thickness should be limited to 10mm to reduce the thermal response time of the adsorber. Addition of high thermal conductivity materials, for example carbon nanotube, can also improve the performance of the adsorber. Multi-adsorber tube connected in parallel can be employed to provide large heat transfer surface and maintain a large SCP and COP. The desorption temperature also showed a large effect on the system performance.
Classification of Commercial Building Electrical Demand Profiles for Energy Storage Applications
Authors: Florita Anthony R. , Brackney Larry J. , Otanicar Todd P. , Robertson Jeffrey
Published: July 2013

Commercial buildings have a significant impact on energy and the environment, utilizing more than 18% of the total primary energy consumption in the United States. Analyzing commercial building electrical demand profiles is crucial to understanding the relationships between buildings and the electrical grid for assessment of supply-demand interaction issues and potential; of particular importance are supply- or demand-side energy storage assets and the value they bring to various stake-holders in the Smart Grid context. This research develops and applies a systematic analysis framework to a Department of Energy (DOE) commercial building database containing electrical demand profiles representing the United States commercial building stock as specified by the 2003 Commercial Buildings Consumption Survey (CBECS) and as modeled in the Energy-Plus building energy simulation tool. The analysis procedure relies on three primary steps: 1) discrete wavelet transformation of the electrical demand profiles, 2) energy and entropy feature extraction from the wavelet scales, and 3) Bayesian probabilistic hierarchical clustering of the features to classify the buildings in terms of similar patterns of electrical demand. The process yields a categorized and more manageable set of representative electrical demand profiles, inference of the characteristics influencing supply-demand interactions, and a test bed for quantifying the impact of applying energy storage technologies.
High-Temperature Solar Selective Absorbers Based on Interface Effects in Refractory Metals Coated With Transparent Conductive Oxides
Authors: Shimizu Makoto , Iguchi Fumitada , Yugami Hiroo
Published: July 2013

At present, the utilization of thermal energy from sunlight has been widely adopted as the working principle of concentrated solar power (CSP) generation systems. In this research, we suggest a CSP technology based on the properties of transparent conductive oxide (TCO) thin films on metal substrates which is compatible with mass production of solar selective absorbers that can be utilized at high temperatures. Since the plasma wavelength of TCO materials is in the infrared region, electromagnetic waves with wavelengths longer than the plasma wavelength are reflected at the surface, whereas electromagnetic waves with shorter wavelengths pass through the surface layer and reach the substrate. In other words, a TCO thin film behaves as an antireflection film only in the transparency range of TCO coating. This phenomenon is demonstrated through numerical simulations based on rigorous coupled-wave analysis (RCWA). The prepared samples also show favorable spectral selectivity and satisfactory performance as solar selective absorbers, with a solar absorptance of 0.76, a thermal emittance of 0.12 at 800°C and a spectral selectivity of 6.5 at 800°C.
An Artificial Neural Network in Short-Term Electrical Load Forecasting of a University Campus: A Case Study
Authors: Palchak David , Suryanarayanan Siddharth , Zimmerle Daniel
Published: July 2013

This paper presents an artificial neural network (ANN) for forecasting the short-term electrical load of a university campus using real historical data from Colorado State University. A spatio-temporal ANN model with multiple weather variables as well as time identifiers, such as day of week and time of day, are used as inputs to the network presented. The choice of the number of hidden neurons in the network is made using statistical information and taking into account the point of diminishing returns. The performance of this ANN is quantified using three error metrics: the mean average percent error (MAPE); the error in the ability to predict the occurrence of the daily peak hour; and the difference in electrical energy consumption between the predicted and the actual values in a 24-hour period. These error measures provide a good indication of the constraints and applicability of these predictions. In the presence of some enabling technologies such as energy storage, rescheduling of non-critical loads, and availability of time of use (ToU) pricing, the possible DSM options that could stem from an accurate prediction of energy consumption of a campus include the identification of anomalous events as well the management of usage.
Post-Industrial Ceramics Compatibility With Heat Transfer Fluids for Low-Cost Thermal Energy Storage Applications in CSP
Authors: Calvet Nicolas , Meffre Antoine , Gomez Judith C. , Faik Abdessamad , Olivès Régis , Py Xavier , Glatzmaier Greg C. , Doppiu Stefania
Published: July 2013

This paper investigates the possibility of using a post-industrial ceramic commercially called Cofalit as a promising, sustainable, and inexpensive ($10/ton) thermal energy storage material. This ceramic presents relevant properties to store thermal energy by means of sensible heat in the temperature range of concentrated solar power (CSP) plants from ambient temperature up to 1100 °C. In the present study, the compatibility of this ceramic was studied with two conventional heat transfer fluids: nitrate molten salts for medium-temperature applications (200 to 500 °C) and air for high-temperature applications (500 to 900 °C). The use of this ceramic in direct contact with the heat transfer fluid should significantly reduce the cost of thermal energy storage systems in CSP applications and help to achieve the U.S. Department of Energy’s SunShot Initiative cost targets.
Effects of Ground Loop Design Parameters on Short- and Long-Term Operational and Economics of Ground Source Heat Pumps in Hot, Semi-Arid Areas
Authors: Gaspredes Jonathan L. , Masada Glenn Y. , Moon Tess J.
Published: July 2013

Deployment of ground source heat pumps in Texas and the Southwest has been limited by high initial cost and potential ground heating. To address these limitations for a residential application a sensitivity study of the ground loop design parameters was completed. The study uses an integrated building load-ground source heat pump model that is designed to be a test bed for assessing the short- and long-term performance of GSHPs. This study examines a 195 m2 residential house located in Austin, Texas with a 14.6 kW heat pump and 4 vertical boreholes each with a length of 68.5 m. The performance effects and the long-term economics of the total ground loop length, spacing of the boreholes, placement of the boreholes, grout thermal conductivity, and supplemental heat rejector sizes are compared. The study shows the importance of proper sizing, design, and placement of the borehole in locations with severely unbalanced heating and cooling loads.
Computational Evaluation of Latent Heat Energy Storage Using a High Temperature Phase Change Material
Authors: Reid Michael R. , Webb Rebecca N. , Lilly Taylor C. , Scharfe David B.
Published: July 2013

Latent heat energy storage systems have higher energy density than their sensible heat counterparts and have the added benefit of constant temperature operation. This work computationally evaluates a thermal energy storage system using molten silicon as a phase change material. A cylindrical receiver, absorber, converter system was evaluated using the heat transfer in solids with surface-to surface radiation physics module of the commercially available COMSOL Multiphysics simulation software. The progression of the solidification and melting fronts through the phase change material was modeled for two different methods of concentrated solar irradiation delivery. Heating the core of the PCM rather than the top of the PCM decreased the required solar input by 17%, decreasing the solar collector area required as well as lowering overall system weight.