A Method for Measuring the Creep Behavior of Pressurized Polymer Tubing

Polymer components have been proposed for use in domestic solar hot water heating systems. A polymer heat exchanger is under development for such systems. For heat transfer considerations, the heat exchanger will be comprised of many thin walled tubes. The heat exchanger must survive 10 years of service at high pressure (1.55 MPa) and high temperature (82°C). A novel method has been developed for evaluating the long term performance (creep) of the polymer tubing. Traditional creep testing, performed with dog bone test specimens can not be applied because the thin walled tubing has anisotropic material properties. Consequently, performance must be evaluated directly on the extruded tubing. The method entails wrapping a Constantan wire around the tube specimen to continuously record the hoop strain. For pressure loading of tubing, this method offers significant improvements over strain gage instrumentation. In this paper, the test method is described, an analysis of the strain transfer between the tubing and wire wrap is presented, and strain data for polypropylene tubing measured with a strain gage and wire wrap are compared. The data show that the wire measurement method can be successfully used for the characterization of long term mechanical behavior of polymer tubes.

SES/Boeing Dish Stirling System Operation

The data presented in this paper document the operation and service requirements for the 25 kW Stirling Energy Systems (SES)/Boeing Dish Stirling system. Among the information presented are the hydrogen used by the system, oil and water service requirements, and reliability data in the form of a component outage tree. Also presented are discussions of the problems that have occurred during the thousands of hours of on-sun operation of two dish/Stirling systems, which comprise two solar concentrators and three power conversion units (PCUs). We have identified a number changes to the design of the 15-year-old dish/Stirling system. However, based on the performance of the systems and the data presented in this paper, there appear to be no serious obstacles to the commercialization of this technology.

Performance of the SES/Boeing Dish Stirling System

The operation of Stirling Energy Systems’ Stirling Dish system and components, originally built and tested during the mid 1980s by McDonnell Douglas [Lopez, 1993] and operated in the Department of Energy Dish Engine Critical Components (DECC) Program since 1998 is presented in this paper. The operating time, performance, and system availability are presented. The data show that the Kockums Stirling engine has accumulated over 8,200 hours of on-sun operating time, has generated over 115 MWh of electrical energy, and has accumulated more than 15,000 hours of test cell operating time since April of 1998 in the DECC Program. Power measurements indicate that the system performs the same as it did in the 1980s. The daily energy plots show net energy efficiency between 24% to 27% when the daily energy available exceeds 600 kWh. System availability data during the 1998/1999 testing period shows that the system was available over 94% of the time when the insolation exceeded 300 W/m2. The data presented herein focuses on three power conversion units (PCUs) and two solar concentrators, which are tested in various combinations and as individual system components. During later parts of the testing cycle, one of the engines, PCU 209, included newly manufactured Stirling engine components (regenerators and coolers) as part of a manufacturing cost reduction program.

Ground-Coupled Heat and Moisture Transfer From Buildings: Part 1 — Analysis and Modeling

Ground-heat transfer is tightly coupled with soil-moisture transfer. The coupling is threefold: heat is transferred by thermal conduction and by moisture transfer; the thermal properties of soil are strong functions of the moisture content; and moisture phase change includes latent heat effects and changes in thermal and hydraulic properties. A heat and moisture transfer model was developed to study the ground-coupled heat and moisture transfer from buildings. The model also includes detailed considerations of the atmospheric boundary conditions, including precipitation. Solutions for the soil temperature distribution are obtained using a finite element procedure. The model compared well with the seasonal variation of measured ground temperatures.

Photovoltaic Phenomena in Thin Film ZnTe-CdSe Heterojunctions

Thin layer ZnTe-CdSe heterojunctions were produced by vapor phase epitaxial growth of ZnTe and CdSe layers on mica and single-crystal ZnSe substrates. These heterojunctions photosensitivity covers the wavelength region of 0.56–0.85 μm. The shape of photosensitivity spectral dependence of ZnTe-CdSe heterojunction depends on the components thickness and their doping level. Thin layer ZnTe-CdSe epitaxial heterojunction parameters under illumination of 80 mW/cm2 (AM1.5) are: FF=0.53, Uoc=0.72V, Isc=14.8 mA/cm2, efficiency η=7.1%. Thin film polycrystalline ZnTe-CdSe heterojunctions having the efficiency η=4.3%, Uoc=0.54 V, Isc=10.6 mA/cm2 were fabricated by using As or Cu doped ZnTe layers and In doped CdSe layers produced by HWT.

Frequency Distributions for Hourly and Daily Clearness Indices

The clearness index for hourly and daily radiation is an important parameter in describing solar radiation. Liu and Jordan demonstrated that the monthly average daily clearness index could be used to predict the long-term distribution of daily solar radiation in a month. This paper reviews recent literature on the prediction of hourly and daily frequency distributions and cumulative frequency distributions of clearness indices. Ten years of measured weather data for six cities in the US are used to investigate the nature of the hourly and daily frequency distributions. A second set of ten years of data for six cities is used to verify the predictions. A bi-exponential probability density function is proposed that fits the observed bimodal nature of the data better than existing models. A case is made for the function being universal.

Recent Progress in Reducing the Uncertainty in and Improving Pyranometer Calibrations

The Measurements and Instrumentation Team within the Distributed Energy Resources Center at the National Renewable Energy Laboratory, NREL, calibrates pyranometers for outdoor testing solar energy conversion systems. The team also supports climate change research programs. These activities led NREL to improve pyranometer calibrations. Low thermal-offset radiometers measuring the sky diffuse component of the reference solar irradiance removes bias errors on the order of 20 Watts per square meter (W/m2) in the calibration reference irradiance. Zenith angle dependent corrections to responsivities of pyranometers removes 15 to 30 W/m2 bias errors from field measurements. Detailed uncertainty analysis of our outdoor calibration process shows a 20% reduction in the uncertainty in the responsivity of pyranometers. These improvements affect photovoltaic module and array performance characterization, assessment of solar resources for design, sizing, and deployment of solar renewable energy systems, and ground-based validation of satellite-derived solar radiation fluxes.

Novel Ionic Liquid Thermal Storage for Solar Thermal Electric Power Systems

Feasibility of ionic liquids as liquid thermal storage media and heat transfer fluids in a solar thermal power plant was investigated. Many ionic liquids such as [C4min][PF6], [C8mim][PF6], [C4min][bistrifluromethane sulflonimide], [C4min][BF4], [C8mim][BF4], and [C4min][bistrifluromethane sulflonimide] were synthesized and characterized using thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), nuclear magnetic resonance (NMR), viscometry, and some other methods. Properties such as decomposition temperature, melting point, viscosity, density, heat capacity, and thermal expansion coefficient were measured. The calculated storage density for [C8mim][PF6] is 378 MJ/m3 when the inlet and outlet field temperatures are 210°C and 390°C. For a single ionic liquid, [C4mim][BF4], the liquid temperature range is from −75°C to 459°C. It is found that ionic liquids have advantages of high density, wide liquid temperature range, low viscosity, high chemical stability, non-volatility, high heat capacity, and high storage density. Based on our experimental results, it is concluded that ionic liquids could be excellent liquid thermal storage media and heat transfer fluids in solar thermal power plant.

Preliminary Results on Luminaire Designs for Hybrid Solar Lighting Systems

We report on the design of two hybrid lighting liminaires that blend light from a fiber optic end-emitted solar source with electric T8 fluorescent lamps. Both designs involve the retrofit of a commercially-available recessed fluorescent luminaire with minimal reductions in the original luminaire’s optical efficiency. Two methods for high-angle dispersion of fiber optic end-emitted solar light are described and the resulting spatial intensity distributions, simulated using ZEMAX, are compared with standard cylindrical fluorescent tubes. Differences in spatial intensity distribution are qualitatively characterized and potential design improvements discussed.

Experiences With Using Solar Photovoltaics to Heat Domestic Water

The solar photovoltaic (PV) industry continues to make progress in increasing the efficiency while reducing the manufacturing costs of PV cells. Economies of scale are being realized as manufacturers expand their production capabilities. Products are commercially available that integrate photovoltaic cells within building façade, fenestration, and roofing components. Legislation and incentive programs by government and commercial entities are supporting both reduced first costs and greater rates of return. The combination of factors support improved cost-effectiveness. As this trend continues, more options for using PV become possible. One such application is a stand-alone, PV-direct, solar water heating application. Solar water heating can be effectively accomplished by directly using the DC power production from solar photovoltaic modules. A simple controller having multiple power relays connects the PV modules with different combinations of in-tank resistive elements. The controller actively changes the resistive combination so that the photovoltaic modules generate power at or near their maximum output. The technology, which has been patented and licensed, is applicable to configurations that use a single water heater and to two water heaters that are piped in series. Prototypes using both tank configurations have been in operation at four field sites. This paper emphasizes the single-tank application and the field results from installations in Maryland and Florida.