Considerations for
System Sizing
The basic questions that need to be answered to determine the size of a
solar collection system are:
| 1. |
How much heat or electric energy is
needed? |
| 2. |
What type of collector will be used?
|
| 3. |
How much energy will be lost between
the collector and the point of use? |
| 4. |
How much of the sun's energy that
could potentially reach your collector surface actually will? A
percentage of the sun's radiation can be blocked by trees,
buildings, hills, clouds, dust, water vapor in the air, and other
things. |
| 5. |
How does the slope and orientation of
the collector affect the amount of solar energy received? |
The Solar Pathfinder instrument will help you answer the fourth
question. The Pathfinder is primarily designed to determine the percentage
of solar radiation blocked by permanent local features in the landscape
like trees, hills, and buildings. Climatic factors, such as the amount of
clouds, dust, and water vapor in the atmosphere are constantly changing.
We need to account for these climatic factors to determine the average
amount of solar radiation actually received at a certain location at a
certain time.
The best way to account for climatic effects is to take actual solar
radiation measurements with special radiation measurement instruments.
Clouds, dust, and water vapor scatter and absorb a part of the incoming
solar radiation. The amount of solar radiation that actually reaches the
earth's surface is measured with these instruments. The best solar
radiation data comes from locations that have been collecting this data
over a long time period. This data contains average, maximum, and minimum
values for the amount of solar radiation that strikes the collectors at
these locations. The problem is that there are only a very small number of
these locations around the world.
How To
Get Solar Radiation Data
Please Note: The Solar Pathfinder Assistant software has the NREL solar radiation data already in it.
In the United States, a major program was undertaken by NREL (National
Renewable Energy Lab) to correlate sparse solar radiation data with
available weather data at nearby sites. These correlations were used to
estimate solar radiation for 239 sites in the US with extensive weather
records. The data for the 239 sites is available in an excellent 250-page,
1994 publication called, Solar Radiation Data Manual for Flat-Plate and
Concentrating Collectors. This is available electronically in more of a
spreadsheet format
click here
to visit the website
NREL is an excellent resource, and though it tends toward the large
utility scale projects, it is funded by your tax dollars. If you have
additional technical questions, try 303-275-4626 or 4648. Like most NREL
numbers, you will likely get a machine.
The manual mentioned above has a page of tables for each location. The
data tables have columns for each month of the year and an annual total.
There are five tables: south facing fixed-tilt collectors, 1-axis
trackers, 2-axis trackers, direct-beam concentrating collectors, and
average climatic conditions. Units are metric (kilowatt-hour per square
meter per day). Most tables give average, maximum, and minimum values for
each month. The fixed and 1-axis tracker tables are broken down into tilts
of: horizontal, latitude minus 15 degrees, latitude, and latitude plus 15
degrees. We try to include a copy of your nearby locations with the
instrument for US orders.
Sandia Laboratories publishes a useful manual: "Stand-Alone Photovoltaic
Systems, A Handbook of Recommended Design Practices" (SAND87-7023) and
Photovoltaic Power Systems and the National Electric Code Suggested
Practices (SAND96-2797). "Stand-Alone PV" is a 400+ page reference that
has 70 pages of radiation data, much of it for locations outside the U.S.
Data is monthly for fixed array, 1-axis trackers, and 2-axis trackers.
Tilts include latitude minus 15 degrees, latitude, and latitude plus 15
degrees. Contoured world maps are included for each season of the year and
each of the three tilts above. This can be ordered from
Sandia Laboratories at
505-844-4383
For areas of the world with very little solar radiation data, NREL has a
crude global data set that uses data inferred from satellites. It gives
rough numbers, as it gauges what is happening at the bottom of the
atmosphere from above the top of the atmosphere.
There are a number of people and organizations working at different
computer based systems for middle and large scale projects that manipulate
radiation and solar Pathfinder data. This field is changing so fast that
individuals are encouraged to do their own research.
A simple installation of a few PV panels on a cabin project could use the
equivalent sun-hours per day chart that came with the PV panel literature.
This literature from some PV panel manufacturers will give rough numbers,
but will be very simple to use. PV panels are usually rated as producing
so many amps at near ideal or peak sun conditions. Then the amp rating for
the panel can be multiplied by the number of peak sun-hours, to get the
number of amp-hours per day that can be produced by the panel.
One peak sun-hour is equivalent to one kilowatt-hour per square meter per
day when the PV panels' tilt has been optimized for the entire year. The
kilowatt-hour per square meter per day is the unit for radiation data that
is provided in the Solar Radiation Data Manual available from NREL. The
suggested tilt angle that is optimum for the entire year is the one tilt
angle where a PV panel will produce the most electricity in a year's time.
The optimum tilt angle could be roughly estimated as being the latitude of
the location. Actually, the optimum tilt may be as much as 10 degrees or
more off a tilt equal to latitude. For instance, if the summers are much
sunnier than the winters, then you will want to tilt the collector more
toward the summer sun for maximum gain, which means your tilt will be more
horizontal (less than your latitude).
Of course, most of your energy use may be in the winter, so instead of
maximizing total energy for the year, you might want to maximize your
winter collection by increasing the angle of tilt.
There are many tradeoffs and considerations that will play into sizing and
orienting solar collectors and it will be wise to use an installer-dealer
with extensive background. (Many advertise in Home Power Magazine
referenced in Resources at the back of this manual.)
Specific Aspects of PV
System Siting and Sizing
Photovoltaic panels are affected by partial shade more than other types of
collectors. Shade over a portion of the panel can greatly limit power
output. Partial shade from towers, poles, deciduous trees and other
objects would be considered nearly the same as total shade for older
panels. PV manufacturers are gradually reducing this problem. Several
locations should be evaluated to find the one with the greatest collection
potential.
Output will also be affected by the use of trackers, or the number of
fixed panel adjustments made during the year. Much additional layman
information can be gathered from the Stand-Alone Photovoltaic Systems
Manual from Sandia Labs, referenced under How To Get Solar Radiation Data
above.
To find the expected power output from fixed solar panels:
Step 1: Use a South facing monthly sun path diagram for your site
tracing. Add the numbers in each half-hour period for which there is no
shading on the planned panel array. For large arrays, two readings should
be taken, at the east and west bottom edges of the planned array. Both can
be recorded on the same sun path diagram. Then add the numbers in the
half-hour periods when there was no shading on either tracing to find the
percent of solar radiation available for each month.
Step 2: Multiply this percentage by the radiation amount you
obtained from one of the data sources listed above. The radiation data
gives you the number of kilowatt-hours per day, or the number of peak-sun
hours, which number wise is essentially equivalent. (Peak sun-hours are
also referred to as "sun-hours" or "equivalent hours of peak output" or
"optimum sun-hours" by different sources. See an explanation of sun-hours
in How to Get Solar Radiation Data above.)
Step 3: Using the resultant number of sun hours from Step 2 above,
multiply by the optimum amps or ampere output under full sun (available
from the panel manufacturer) to find the expected average ampere-hours per
day for each month. Multiply by the panel voltage to get watts per day.
Example: Using the site tracing on page 10, add the numbers for the
half-hour segments in December when no shading will occur:
2 + 3 + 4 + 5 + 6 +7 +7 +8 +8 +8 = 58%. In the NREL publication under
Grand Junction, CO, in December, a south facing fixed collector tilted at
latitude (which is 39 degrees) receives on average 4.1 kWh/m2/day (4.1 X
.58 = 2.4 kWh/m2 day). This would be about 2.4 peak sun-hours per day for
December at this site. Multiplying 2.4 by the panel manufacturer's value
for optimum amps (let's use 3.02 amps) gives us 7.2 amp-hours per day.
Multiply by voltage (let's use 12v) gives us 86 watts per day.
Siting Domestic Hot Water
Collectors
The most ideal water heat collector orientation will typically favor the
winter months slightly, to make up for shorter days and the cooling effect
of colder temperatures. The panel tilt might equal the latitude plus five
to ten degrees. Although partial shading isn't as critical with thermal
collectors as with PV panels, we still need to compensate for a high
percentage of morning or afternoon shade. To do this, we need to aim the
panels more to the west or east, and possibly increase the tilt.
Step 1: Using the diagram shown on page 10, add the numbers in the
un-shaded part of the October sun path to find the site percent for
October. For example, the monthly sun path diagram on page 10 has an
October reading of 71%.
Step 2: Divide this number by two to find the half-day percentage.
71/2 = 35.5%; round up to 36.
Step 3: Start from the east edge of the October sun path, add the
un-shaded numbers until the total is nearly the same as the half-day
percentage. 2 + 3 + 4 + 5 + 6 + 6 + 7 = 33.
Step 4: Notice the place along the October sun path where the
half-day percentage is found, in relation to 12 noon (true south). In our
example, the half-day percentage is found between 11:00 and 11:30 AM.
Step 5: Overlay the clear angle measurement grid on top of the sun
path diagram. Using the angle numbers on the outside edge of the grid (the
azimuth angle), find the angle where the half-day point is located. Using
our example, for between 11 and 11:30 AM, the angle is 15 degrees.
Therefore, we would orient our collector 15 degrees east of south.
Step 6: To determine the collector's tilt, use the azimuth angle
found above (15 degrees in our example). Divide the azimuth angle by 5 and
add this figure to your latitude to find the collector's tilt. As an
example, use our 15 degrees azimuth angle and our latitude of 39 degrees
north. We would first divide 15 by 5 to get 3, and then add this to the 39
degrees of latitude to get a collector tilt of 42 degrees. This is due to
the greater percentage of energy coming in while the sun is lower in the
sky (i.e., morning or afternoon).
Siting Active Space
Heating Collectors
Since space heating is a concern primarily during the winter months, we
will use the information from the January sun path to orient the
collector. (A good rule of thumb for panel tilt is latitude plus 15
degrees.)
Step 1: Again using the diagram shown on page 10, add the numbers
in the un-shaded part of the January sun path to find the site percent for
January, and divide by two to find the half-day percentage. Our example
shows that the site percentage is 64%. 64/2 = 32.
Step 2: Starting from the east edge of the January sun path, add
the un-shaded numbers to find the half-day point. 2 + 3 + 4 + 5 + 6 + 7 =
27.
Step 3: Notice where this half-day point is in relation to 12 noon
(true south). In our example, the half-day point is between 10:30 and
11:00 AM.
Step 4: Overlay the angle estimator as above to find what your
angle is. Our azimuth angle is 20 degrees. We would orient our collector
20 degrees east of south.
Step 5: To determine the collector's tilt, divide the azimuth angle
by five and add this to your latitude plus 15 degrees. In our example,
20/5 = 4. If our latitude is 39 degrees, add 15 plus the additional 4
degrees to get a collector tilt of 58 degrees.
Estimating
Rooftop Collector Shading on the Future Home
Solar site analysis is difficult when trying to estimate how much sunlight
will strike the walls or roof of a building that hasn't been put up yet.
Usually the future rooftop will likely be less shaded than the building
site at the ground level.
Rooftop shading can sometimes be estimated by taking two readings, one at
ground level, and another one ten feet up a stepladder. Any shift in the
skyline between the two readings can be used to estimate the correct
rooftop solar reception by using proportions. For example, shading reduced
by half at the 10-foot level would be approximately halved again at 20
feet. A third reading could be taken from the roof of a nearby building
(if one exists) to help get a better perspective on shading patterns in
the area.
As long as there were not significant potential shade makers to the east
and west, we also could go north of the site, to a point at instrument
level that is on a line of sight with the future elevated collector.
Seasonal
Variations in Foliage and Ground Cover
Thumb rules:
1. Do not count any half-hour periods shaded by evergreen trees, as they
cast shadows year-round.
2. Do not count half-hour periods shaded by deciduous trees during the
leaf-bearing months; for thermal collectors, count these half-hour periods
at half their value during non-leaf-bearing months. For PV panels, these
half-hour periods should be assigned a value of zero, unless the
manufacturer can support a better figure.
3. Snow cover should cause an increase in the amount of solar radiation
that a sloped collector receives due to reflection. This increase depends
on the latitude, the collector tilt, and the kind of snow (new powder is
best, decreasing as snow becomes old and icy). The average increase of
solar radiation used for passive solar heating due to snow cover is only
around five percent, but PV panels on a powder-snow-covered, clear, cold
winter day often produce more watts than on the much longer sunny summer
days.
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