A Performance Comparison of Air-cooled and Evaporative Refrigeration Condensers in a Typical U.S. Supermarket. ERIC CLIFFT (Rose-Hulman Institute of Technology Terre Haute, IN 47803) MICHAEL DERU (National Renewable Energy Laboratory, Golden, CO, 89401)

Supermarket refrigeration systems in the United States use enormous amounts of energy. However, energy efficient devices such as the evaporative condenser exist today that could cost effectively improve their future performance. Using eQUEST Refrigeration version 3.55, a supermarket near Denver, Colorado is modeled to experiment with the energy and cost savings incurred by replacing refrigeration system air-cooled condensers with their evaporative counterpart. As anticipated, the simulations convey both environmental and economic benefit associated with the use of evaporative condensers in a dry climate. In fact, the model predicts annual electricity savings of 208,800 kWh and reductions in utility costs by $12,322 each year. Clearly, evaporative condensers offer a practical way to mitigate U.S. energy use and foreign dependence, making them deserving of a spot in the supermarket refrigeration systems of the future.

Comprehensive Process Description of a Wet Scrubber Used for Cleaning Biomass-Derived Syngas in the Thermo Chemical Process Development Unit at NREL. KRISTIN WILLIAMS (University of Colorado Boulder, CO 80309) STEVEN PHILLIPS (National Renewable Energy Laboratory, Golden, CO, 89401)

The main product of biomass gasification is syngas, a gas composed primarily of hydrogen, carbon monoxide, carbon dioxide, and methane. Higher molecular weight hydrocarbons, commonly referred to as tars, also form during this process as undesirable byproducts and are removed in a scrubbing system. Mass balance calculations around the Thermo Chemical Process Development Unit (TCPDU) currently yield a carbon mass closure of approximately 85%. This project aimed to amass a data set that would improve the mass closure through enhanced analysis of the product gas stream and measurements from additional exit streams. The carbon mass balance was improved to 89.6% using the more comprehensive data, and the remaining carbon is attributed to accumulation within the system. A second goal was to develop an ASPEN simulation of the scrubber system. The model output was compared to experimental results with generally good agreement except for minor species. Reducing the output temperature of a heat exchanger was examined using the model, with the prediction that the concentrations of HCN and NH₃ in the exit gas will decrease. Preliminary tests investigated adding biodiesel to the TCPDU as a scrubber liquid. Biodiesel was mixed with a sample of scrubber water to examine the partitioning of organics between the diesel and water phases. Using Gas Chromatograph Mass Spectrometry and Liquid Chromatography, it was determined that essentially all of the tars remained in the water phase, except for approximately 15% of the phenol. Based on these results, biodiesel will not be added to the current scrubber configuration.

Development of a Calibrated Hydrogen Engine Model for the Evaluation of Multi-Cylinder Hydrogen Internal Combustion Engines (H2ICE). CIPRIANO DURAN (New Mexico State University Las Cruces, NM 88001) MATTHEW THORNTON (National Renewable Energy Laboratory, Golden, CO, 89401)

This research includes working with hydrogen internal combustion engines and the development of a calibrated engine model for use in stand-alone applications and in conjunction with vehicle systems-level analyses of H2ICEs. The goal is to implement preexisting test data from a hydrogen-fueled single-cylinder test apparatus (SCTA) in various multi-cylinder hybrid vehicle configurations. They include H2ICE/battery and H2ICE/fuel cell configurations, utilizing vehicle systems analysis (VSA) software to evaluate the performance and efficiency of hybrid vehicle powertrains versus conventional vehicle powertrains (e.g., gasoline ICE). Particular attention is given to calibrating the modeled combustion to a near-exact match of the empirical data to ensure that the model will be functional for sensitivity analyses and other engineering applications. This offers engineers the opportunity to create and evaluate various engineering options in a timely, efficient, and cost-effective manner. Anstalt für Verbrennungskraftmaschinen's (AVL) (or Institute for Internal Combustion Engines) one-dimensional engine cycle simulation code BOOST version 4.0.4 is the engine-modeling tool employed. This research demonstrates the feasibility associated with modeling empirical data for rapid result finding and R&D cost reductions while concurrently demonstrating the viability of H2ICE/battery and H2ICE/fuel cell configurations.

Enhancement of Air Cooled Condensers for Geothermal Power Production. BRIAN FRONK (Pennsylvania State University University Park, PA 16801) CHUCK KUTSCHER (National Renewable Energy Laboratory, Golden, CO, 89401)

Electricity generated from geothermal sources offers a more sustainable alternative compared to current methods of generation. Identified resources could provide up to 22,000 MW of clean sustainable power to the US grid. A large capital and operational cost of geothermal electric production are the air-cooled condensers found in the cycle. In order for geothermal to become more economically competitive, improvements in efficiency and reduction in costs of these units is necessary. Using a small-scale air cooled heat exchanger, different fin types and exchanger tube configurations were tested for optimal heat transfer and input fan power required. A plain fin was used as a control case and two experimental perforated fin types were tested. Tubes were configured in a conventionally staggered array, in-line and a 90 degree offset configuration. When heat transfer per bare tube area and hydraulic power required per bare tube area was compared for each fin / configuration combination, the conventional plain fin and standard staggered array performed the best. While the plain fin in a staggered array outperformed the perforated fins, this does not necessarily mean other types of enhanced fins won't provide improved performance. This research is part of ongoing work involving novel fin types and condenser optimization. It is the goal of NREL to reduce the cost of geothermal electricity by $0.01 kWh by the 2010.

Evaluation of the Energy Performance of the Site Entrance Building at the National Wind Technology Center. ANDREW TUPPER (Colorado School of Mines Golden, CO 80401) PAUL TORCELLINI (National Renewable Energy Laboratory, Golden, CO, 89401)

By incorporating energy efficient design and renewable power generation, the Site Entrance Building (SEB) at the National Wind Technology Center (NWTC) was built to achieve net-zero site energy use. The building neared the net-zero target in 2004, but has never achieved net-zero use for a full year. By analyzing long term SEB data, collecting information on plug loads, and observing building use, the following recommendations will be made to allow the building to achieve net-zero status. During the summer months, foil backed sheathing should cover the Trombe wall. The clerestory window should be open when cooling is required. The other windows shall remain closed unless the heat pump is turned off. Installing power strips, which are to be turned off when not in use, should minimize plug loads. The existing absorbed glass mat (AGM) batteries should be replaced with a larger bank of flooded lead cell batteries. An uninterruptible power supply (UPS) for the computer is redundant and should be removed pending battery replacement. The inverter, wind turbine, and PV voltage settings should be adjusted to increase system efficiency. By following these recommendations, it has been calculated that the building will be able to achieve net-zero status within the next year.

Modeling Bi-directional Thermosyphon Solar Water Heaters. KYLE BENNE (University of Missouri-Rolla Rolla, MO 63304) JAY BURCH (National Renewable Energy Laboratory, Golden, CO, 89401)

Solar water heating has the potential to significantly reduce the demand on fossil fuel energy sources. A typical solar water heating system includes a solar collector, a tank, and a pump to circulate flow through the system. A thermosyphon design eliminates the pump by utilizing the buoyancy of the hot fluid to naturally drive the flow. By eliminating the pump, a passive design has the potential to further reduce the demand on fossil energy, increase system reliability, and reduce the cost of a solar water heating system. The challenge is accurately modeling the flow rate generated by the siphon, particularly during the nighttime when reverse siphoning is possible. Reverse flow has traditionally been avoided though it can be utilized by consuming a portion of the tank's stored energy to prevent freezing. A program has previously been written for the TRNSYS software to model the flow rate in a thermosyphon, however it failed to simulate low and reverse flow cases. The present work corrects this limitation with several improvements. Most notably, the fluid head contributed by each internal node of the system's components was calculated in place of computing the fluid head based on the average temperature of each component. Secondly, the Crondt-Freidricks-Louie (CFL) condition was implemented, which states that the distance traveled by the fluid during a time step must not exceed the length of the element. As expected, the results provided by the model indicate a strong correlation between the height of the tank above the collector and the magnitude of the reverse siphoning. It has been observed that while reverse siphoning has the ability to prevent nighttime freezing, significant energy is lost from the storage tank. Using the tool developed in this work, it is possible to quantify energy lost in reverse thermosyphoning and determine the usefulness of reverse siphoning for freeze protection.

Modeling the Optical Performance of Parabolic Trough Solar Concentrators. ANITA BUDHRAJA (Northwestern University Little Rock, AR 72212) TIM WENDELIN (National Renewable Energy Laboratory, Golden, CO, 89401)

Parabolic trough solar concentration is currently the lowest cost solar power technology available on a large scale. Encouraged by the Western Governors’ Southwest 1000 MW Concentrating Solar Power Initiative as well as by state and federal incentives for solar energy, the National Renewable Energy Lab (NREL) Concentrating Solar Power research team is currently working to improve parabolic trough technology and increase its cost effectiveness in the U.S. and abroad. Specifically, the team devotes much of its parabolic trough development efforts to evaluating and improving concentrator optics. This paper examines how the optical performance of parabolic trough concentrators is affected by concentrator geometry, manufacturing imperfections and misalignment problems that occur in the field. More specifically, the optical effects of tracking offset, tracking twist, receiver misalignment, aperture width, receiver size, parabolic slope error and some combinations of these factors are investigated. Using the Solargenix trough design as a basis, optical analysis involves determining the exact shape of the trough using the VSHOT test procedure, building detailed but adaptable optical models of the parabolic trough system in Microsoft Excel, evaluating the optical performance of these models with SolTrace raytrace software, and analyzing SolTrace output data using Microsoft Visual Basic. This study has found that the geometry and alignment precision of parabolic trough concentrators can strongly affect their optical performance. For the Solargenix design in particular, tracking offset, tracking twist and receiver misalignment in the transversal direction are the most significant issues. Further, increasing the receiver size accelerates heat loss without significantly boosting optical performance, increasing the aperture width allows the trough to intercept more solar energy without significantly reducing optical performance, and reducing the parabolic slope error is worthwhile, especially if the receiver is made smaller. With the tools built in this project and the information presented in this paper, parabolic trough solar concentrating technology is better armed to shape a sustainable world.

Simulation Model of a Proton Exchange Membrane Electrolyzer Using PSCAD. BRIAN BUTTERFIELD (California Polytechnic State University San Luis Obispo, CA 93407) BENJAMIN KROPOSKI (National Renewable Energy Laboratory, Golden, CO, 89401)

Currently, staff at the National Renewable Energy Laboratory (NREL) Distributed Energy Test Facility are conducting research on integrating renewable power systems with electrolyzers to produce hydrogen from clean energy sources. One electrolyzer that is being tested uses a proton exchange membrane (PEM) stack that spits water with direct-current (DC) electricity into hydrogen and oxygen. To interconnect a renewable energy system such as a wind turbine, to the electrolyzer, a power electronic converter must be used to convert variable frequency voltage and current to constant voltage direct current electricity. To help in the understanding of how the power system for the electrolyzer works, a model of the electrolyzer based on empirical data is being developed in PSCAD (an electrical simulation tool). Based on this measured data, a model has been developed using curve-fitting and interpolation techniques. These techniques are translated into the PSCAD simulation software via control and circuit components. The PEM Stack model is voltage and temperature dependent, as is the actual component, and can reproduce data similar to that of the PEM Stack's actual measured data. This paper will describe the implementation of the techniques and processes used to model the PEM Stack, as well as verify the model's ability to reproduce data similar to actual measured data.

Simulation Model of a Proton Exchange Membrane Electrolyzer Using PSCAD Simulation Software. BRIAN BUTTERFIELD (California Polytechnic State University San Luis Obispo, CA 93407) BENJAMIN KROPOSKI (National Renewable Energy Laboratory, Golden, CO, 89401)

Currently, staff at the National Renewable Energy Laboratory (NREL) Distributed Energy Test Facility are conducting research on integrating renewable power systems with electrolyzers to produce hydrogen from clean energy sources. One electrolyzer that is being tested uses a proton exchange membrane (PEM) stack that spits water with direct-current (DC) electricity into hydrogen and oxygen. To interconnect a renewable energy system such as a wind turbine, to the electrolyzer, a power electronic converter must be used to convert variable frequency voltage and current to constant voltage direct current electricity. To help in the understanding of how the power system for the electrolyzer works, a model of the electrolyzer based on empirical data is being developed in PSCAD (an electrical simulation tool). Based on this measured data, a model has been developed using curve-fitting and interpolation techniques. These techniques are translated into the PSCAD simulation software via control and circuit components. The PEM Stack model is voltage and temperature dependent, as is the actual component, and can reproduce data similar to that of the PEM Stack's actual measured data. This paper will describe the implementation of the techniques and processes used to model the PEM Stack, as well as verify the model's ability to reproduce data similar to actual measured data.

Validation of an Indoor Calibration System for Pyranometers. AMY BOWEN (Baylor University Waco, TX 76798) STEVE WILCOX (National Renewable Energy Laboratory, Golden, CO, 89401)

Calibrated radiometers are essential to any climate change research or renewable energy applications that require accurate solar energy measurements. Instrument calibration produces a regulated instrument, by adjustment or assignment of a calibration factor, through comparison with a standardized reference. Because radiometers are used outdoors, their calibration is typically performed outdoors. However, outdoor calibration is often restricted by varying or poor weather conditions. Kipp and Zonen's indoor calibration system will provide flexibility in calibrations if it is compatible with different manufacturer's radiometers. Using a data set of 17 pyranometers, comparison of the outdoor responsivities with responsivities using the indoor calibration system provides insight into the functionality of the system. The pyranometers' responsivities from the two methods were typically within 1% of each other. However, two instruments deviated from their outdoor responsivities by about 6%, indicating that an instrument may occasionally respond differently to the indoor and outdoor light spectrums. Excluding the results from the outlier instruments, the mean bias error between indoor and outdoor responsivities was 0.06%. An infrared-correction method was also examined, and differences were slightly higher, averaging 0.33%. The indoor calibration system results have a 4.41% uncertainty and are recommended for use in applications where this is acceptable. Further investigation and experimentation concerning the unusual outlier case will be necessary to identify other instruments that may behave similarly.