# Is there a clear explanation of Packing Factor (an option in the solar Array page)?

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Anonymous Jerk
Is there a clear explanation of Packing Factor (an option in the solar Array page)?

Hi all,

In SAM's PV simulation, the Array page has a section on Land Area, which is largely determined by a packing factor. The default on SAM is 2.5, but this explanation www.nrel.gov/docs/fy11osti/49417.pdf (p6) seems counter to the default.

How does SAM consider the packing factor value? And how should we calculate it for a simple, fixed-axis roof mount?

Many thanks!

Paul Gilman

For the PV component-based models, SAM uses the packing factor on the Array page to calculate the number of acres of land required for the project. When you specify a non-zero land cost in \$/acres on the System Costs page, SAM uses the number of acres to calculate the land-related costs, which contribute to the total indirect cost. For a rooftop system, an appropriate value for land area would be zero.

Best regards,
Paul.

Anonymous Jerk

Hi Paul,

Thank you for the reply. Maybe I should have explained my situation:

I'd like to see how much land area is required for arrays of differing PV modules in differing locations (rooftop, flat land, slopes). I know that Packing Factor affects this, so I'm hoping to find a way to accurately calculate packing factor. Let me try these questions:

What does a PF of 2.5 mean?
Under what conditions should PF be changed?
How should we calculate PF for different scenarios?

Many thanks!

Paul Gilman

SAM calculates the total land area by multiplying the packing factor by the total module area and converting it to acres. The calculation is explained in the Help topic for the Array page.

For example, the default Photovoltaics, Commercial case has a total module area of 1477.87 m2 and a packing factor of 2.5 (from the Array page). SAM shows the land area in acres as a calculated value on the Array page and the PV System Costs page: Total land area = 1477.87 m2 x 2.5 / 4,047 m2/acre = 0.91 acres.

The packing factor only affects the results of your simulation if you specify either land or land preparation cost in \$/acres on the PV System Costs page. In the default case, both of these variables are zero, so the packing factor does not affect the results.

The packing factor is designed to be used for large, ground-mounted systems where land area-related costs are a factor.

If land-related costs are important for your analysis, you may want to research the relationship between total module area and land area for different installed systems to get a sense of what value to use for your analysis.

Unfortunately, the term "packing factor" is also sometimes used to describe the density of solar cells in a PV module, which is a different use of the term than in SAM.

Best regards,
Paul.

Kloba

In answer to the original question:

Yes, there are different definitions of packing factor, and the one used in SAM is different from the one used in the document referenced above (http://www.nrel.gov/docs/fy11osti/49417.pdf) and both are different from how I've seen it stated elsewhere, such as this DOE glossary: http://www1.eere.energy.gov/solar/solar_glossary.html#P

In any case, the geometry depicted in the diagram you referenced is still useful in determining an appropriate packing factor.

Assuming the SAM definition of packing:

With flat (tilt = 0) modules, using a unity (1) packing factor gives only the area occupied by modules, so you will want to increase it to accommodate service access, inverter equipment, etc.

With tilted modules, the spacing chosen will determine when a module will become shaded by the adjoining module. This depends on latitude and tilt.

To get packing factor from row-to-row spacing:

p = (d / l) cos t, where
p = packing
d = row-to-row spacing
l = module width
t = tilt from horizontal.

Again, this only accounts for modules (and the spaces between rows), so increase somewhat for service access, inverter equipment, etc.

Fred Bloom

Had the same question and can't believe how the scientists can't seem to comprehend the practical application. We're looking at a 50 acre parcel or 500,000 sq. ft. rooftop and we need to know how many kW's or mW's to quote for the bid. Right? For example, Here is what I was taught to determine a fixed tilt:

300 watt Sun Power panel is 1.63 sq. M

300 Watts/1.63m2= 184 Watts per m2

184 = 1m2 therefore
1mW = 5434 m2 of panel surface per mW

If installed at 15% tilt and leave 33% space between rows to prevent shading:

Then add 48 % to surface area
also
Then add 7 % for aisles, etc.

then : 5434 X 155% = 8424 m2 per mW of area needed

1 acre=4046 m2

therefore: .48 mW DC per acre (fixed tilt at 15%)

What is the equation to determine maximum panels without shading for a single axis tracker?, please advise.

Paul Gilman

Dear Fred,

Are you asking how to determine the maximum number of panels that avoids self-shading for an array with one-axis tracking in SAM, or for a general equation that you can use outside of SAM?

In SAM, you can use the "Self-shaded" mode with one-axis tracking (on the PV Subarrays page), and adjust the ground coverage ratio for the area you have available for the array. Then you can adjust the number of modules (and inverter) on the Array page until you have a system with the cost and generation profile you want. The process in SAM is iterative: You can systematically try different layout options. In SAM, you can see the effect of array layout on self-shading and DC voltages and power at the inverter input to help you choose the best design for your application. See the help topic for the PV Subarrays page, particularly the section on Shading for one-axis trackers and ground coverage ratio for more details: SAM Help - PV Subarrays.

Note that in SAM the packing factor input is only used to calculate the number of acres for land cost calculations. The packing factor input is independent of the ground coverage ratio even though in an actual system the two values would be related.

Best regards,
Paul.

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