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Research
Solar Energy Laboratory Research
Texas Solar Radiation Data Base
This project is being conducted by the University of Texas at Austin to
develop a solar radiation resource for Texas. Solar radiation
monitoring instruments located across the state record measurements
continuously. Data is downloaded at regular intervals and analyzed and
checked for possible errors. Once screened, it becomes part of the laboratory's database. Another comprehensive database is provided
where the user can access data for any particular station over any period of time since that
station became operational. The
project is supported by the State Energy Conservation Office (SECO).
The solar monitoring stations use two types of
instrumentation, Class I and Class II, to obtain total solar radiation
data:
- Class I Instrument Sites:
Five stations, Austin (The
University of Texas), Clear
Lake (NASA), Edinburg (UT Pan American), El Paso (UT El Paso), and Canyon
(West Texas A&M) have three separate instruments to measure the three
total solar components: global horizontal, diffuse horizontal, and direct
normal. These stations use two Eppley pyranometers, an Eppley
pyheliometer, and a SCI-TEC tracker with rotating shadow ball to obtain
the global, diffuse, and direct normal components. These five sets of
instruments are being provided on loan to the project from the National
Renewable Energy Laboratory (NREL).
- Class II Instrument Sites:
Ten stations, Overton (Texas A&M Agricultural
Research and Extension Center), Laredo (KGNS television station), Sanderson
(Texas Department of Transportation), Presidio (Texas Department of
Transportation adjacent to Presidio High School), Pecos (Texas A&M Agricultural
Research Station), Big Spring (Industrial Air Park), Menard (Menard High School),
Abilene (Abilene Christian University), Corpus Christi (Calallan High School) and Del Rio (Del Rio High School) use Ascension Rotating Shadowband Pyranometers
(RSPs) to obtain the three total solar components. The RSP is a single
photovoltaic sensor that measures the global horizontal component in
its stowed position. Every sixty seconds the shadowband passes over the
sensor and the instrument measures the diffuse horizontal component. From
these global and diffuse measurements (as well as from known time,
longitude and latitude) the instrument calculates the direct normal
component. These instruments have been purchased with SECO funds.
Future plans include installing more Class II
sites, and taking broad band spectral measurements at the existing five
Class I sites. At each of these five sites three pyranometers will be
installed, one with a clear dome and the two others with spectral
filters. These will permit obtaining data for three spectral bands.
In all, there will be at least fifteen sites located across Texas
with either the Eppley instruments or the Ascension RSPs installed, and each
site will provide the three components of total solar radiation: global
horizontal, direct normal, and diffuse horizontal.
Scale Prediction in Solar Hot Water Systems
Potable water can potentially result in scale formation on heated
surfaces. In solar hot water systems this can seriously degrade
system performance and, in extreme cases, clog fluid passages. In open
loop systems scale may form in the collector passages; in closed systems
scale may form on the potable water side of the heat exchanger. If the
heat exchanger has "passages" on the potable side clogging may occur
there. In wrap-around heat exchangers clogging would not occur, but
system performance might suffer over time.
Under an NREL sponsored project, software (SOLSCALE)
has been developed for use on a DOS platform computer for predicting scale formation
in solar hot water systems which include a heat exchanger. The user inputs information
on the system's location, the solar components (collector size, tank size, etc.),
the load, and the water chemistry. The software predicts the severity of scale
formation and indicates how the system component sizes may be altered to alleviate
the problem and how often to descale the system. This project is being continued
to validate the software with field data. In addition, more fundamental work
is being pursued to better quantify scaling rates.
Photovoltaic Powered Cooling Systems
Engineers at NASA are testing a variety of photovoltaic powered cooling
systems. They are interested in developing modeling tools to compare with their
experimental data in order to predict the performance of photovoltaic
powered cooling systems for terrestrial and space application.
Under a NASA grant, TRNSYS software is being used to model a variety of
solar photovoltaic powered cooling systems. The cooling concepts being
addressed are Thermoelectric Cooling, Stirling Cycle Cooling, and Vapor
Compression Cooling. For space application of the PV system, modeling of the
photovoltaic array performance must be modified to account for the vacuum
environment as it affects temperature and performance, and the heat
rejection for the cooling cycle must be accomplished with radiation panels.
Screening Software for Renewable Energy Applications
A software package, Texas Renewable Energy Evaluation Software (TREES),
has been developed with SECO funding to provide a
tool to
easily compare the economics of solar and wind energy with conventional energy
sources for a variety of thermal and electric applications. TREES allows
the user to relatively simply compare solar against fossil fuel energy
for various applications such as swimming pool and other water
heating. Similarly, the software allows comparison of solar
photovoltaic and wind generated electricity against conventional and
diesel generated electricity. The software assesses the attractiveness
of renewable energy applications in four categories: highly
attractive, attractive,
marginal, or unattractive. TREES is not a design tool, but rather, a
screening tool which helps the user determine whether a more detailed design
is needed.