Solar Thermal Information

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6. PERFORMANCE ESTIMATION:
The performance of any solar energy system obviously
depends on weather. As such, it cannot be precisely
predicted from one day to the next. In the case of
a solar combisystem, performance also depends on
both the space heating and domestic hot water loads
of the building it serves. Both loads are highly variable
depending on personal preferences, energy saving efforts
and habits of the building occupants. Compounding this
situation are issues such as collector shading at specific
times or the possibility of snow on the collectors — even
when the sun is out.
Because of the highly variable nature of the source
as well as the demand for energy, the performance
of solar thermal systems is often estimated through
computer simulation. In some cases, these simulations
take place on an hour-by-hour basis, or possibly even
on a 15-minute basis. Because there are 8760 hours in a
year, these calculations are laborious and only capable of
being done using a computer.
EXPECTATIONS:
Many people with little more than a philosophical interest
in solar heating tend to overestimate the performance
of solar energy systems. Upon seeing a typical three-
to eight-collector array on a roof, a common question
to the building’s owner might be: “Can those heat your
entire house?” Implicit in this question is optimism that
solar collectors might eliminate the need for heat from
conventional fuels, such as oil, gas or electricity. Although
a lofty goal, this is almost never the case.
Constructing a very large solar thermal system that
could approach the ideal of 100% solar heating would
be very expensive, and unable to pay for itself in savings
over the useful life of the system. Such a system would
also generate much more heat than could be used
during warmer weather. In short, such a system would
be far from economically justifiable. Without economic
justification, such systems would have very limited
acceptance and contribute very little to widespread use
of renewable energy.
The most economical solar combisystems always use a
combination of solar and auxiliary energy.
Increasing the size of the solar portion of a combisystem
(e.g., more collector area and a larger storage tank)
demonstrates the law of diminishing returns. Each time
the collector area is increased by a fixed amount, say by
adding 100 square feet of collector area to the array, the
resulting savings will be less than that associated with
adding the previous 100 square feet. The total savings
goes up, but at a constantly diminishing rate.
From an economic standpoint, the goal is to find the
best combination of solar-derived and auxiliary heat that
produces the lowest life-cycle cost for a given project.
This requires knowledge of or accurate estimates for
many factors, such as the cost of various-size systems,
the thermal performance of the same, the cost of auxiliary
heating and its likely rate of inflation several years into
the future, the expected cost of system maintenance
and insurance, and how the system’s economics are
influenced by current incentives, such as tax credits and
subsidized loans. Having detailed and reliable information
on all these considerations during the design process
is virtually impossible. The best that can be done is to
make estimates, then use a software tool to gauge the
sensitivity of the design to variations in these estimates.
f-CHART ANALYSIS:
One established method of predicting the monthly
and annual performance of solar combisystems was
developed at the University of Wisconsin, Madison during
the mid-1970s. It is called f-chart. The letter f stands for
fraction—specifically, the fraction of the combined space
heating and domestic hot water load that is supplied by
solar energy.
f-chart was originally developed as a simplified method
for predicting the performance of solar thermal systems
for residential-scale applications at a time when the
computational performance needed for hour-by-hour
simulations was only available on mainframe computers,
and thus unavailable to most individuals. f-chart is based
on empirical correlations of the results of thousands of
hour-by-hour simulations done with a specialized solar
simulation program called TRNSYS. In its initial form,
the f-chart methodology could be completed using a
scientific calculator. Although this is still possible, the
methodology has been translated to software and is now
available from at least two sources.
Like other solar design tools, f-chart requires several
variables to describe the system being modeled. These
include:
• Location of installation (weather database)
• Collector area
• Collector efficiency intercept (FRta)
• Collector efficiency slope (F
R
U
L
)
• Collector slope
• Collector azimuth