- Considerations for System Sizing
- How to Get Solar Radiation Data
- Specific Aspects of PV System Siting and Sizing
Siting Solar Collectors
- Siting Domestic Hot Water Collectors
- Siting Active Space Heating Collectors
- Estimating Rooftop Collector Shading on the Future Home
- Seasonal Variations in Foliage and Ground Cover
Considerations for System Sizing
The basic questions that need to be answered to determine the size of a solar collection system are:
- How much heat or electric energy is needed?
- What type of collector will be used?
- How much energy will be lost between the collector and the point of use?
- 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.
- 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
- Do not count any half-hour periods shaded by evergreen trees, as they cast shadows year-round.
- 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.
- 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.