Monday, March 2, 2009

Does Solar Tracking make sense?

I want to know, so I'm going to try to work out a rough scenario.


Looking at Wattsun Tracker Datasheets, I've decided to use 12 175W Suntech Panels. See http://www.wattsun.com/prices/Wattsun_Tracker_Prices.pdf

Cost of Tracker Equipment: $6250.

Additional Installation Costs (Rough Guess): $3000-$4000 (lower costs if you can put together an out-of-work electrician, welder, and some laborers).

Panel Total cost at $4.50/W = $9450; Total Peak Watts: 2100W

Inverter Cost: $2500 (small inverter, for just this application).


Cost of Tracking System:

Using these rough estimates, the total cost of the Tracking System with Panels would range from $21,200 - $22,200. Just to assume the worst, I'll stick with $22,200, or $10.57/Watt.

The cost of JUST the Tracker and Installation ($4000), runs $10,250, or $4.88/Watt.


Cost of Stationary System:

Calculating a rough cost of an Installed Stationary System, I'll go with the above Panel Cost of $4.50/Watt, and using the Solarbuzz estimation, which suggests that the total installed cost of the system will be twice the cost of the panels (I believe that this would include the Inverter). So, for comparison purposes, I'll set the Installed Stationary system at a total of $18,900, or $9/Watt.


Insolation Comparison:

In a normal stationary scenario, the Installation would produce energy according to the usual local Insolation values. However, the fact that it's a tracker, leads to an INCREASE in the effective Insolation value. Using a US Government Insolation Reference, it looks safe to say that for at least a very large portion of the US, there's a 2 kWh/M2 difference in Annual Insolation between a "Flat Plate Tilted South at Latitude," and a "Two Axis Tracking Flat Plate." I know from previous calculations that 2 kWh/m2 is equivalent to an insolation ratio of 8.33%.

Let's put this percentage in terms of our original 2.1 kW System. Assume that the Stationary Installation is on a roof angled at latitude, in a region that recieves an average of 20% Insolation over the course of the year. In ideal conditions, this system will produce 2.1 kW*Year * 20% = 0.42 kW*Year = 3679kWh.

Now, let's put that same system on a tracker, thus increasing the effective Insolation Value by 8.33%. This system will produce 2.1 kW*Year * 28.33% = 0.59 kW*Year = 5212kWh.

We can see that an 8.33% increase of in the effective Insolation Ratio has increased the total Annual Energy Output by 29.5%!


Does the Tracker pay off?

To start out with, let's find out how much Energy each system will produce in 25 years. To be a bit more accurate to the real World, I'll take off 25% from each value to reflect Inverter losses, efficiency degredation over the 25 year lifespan, and variation from the Manufacturers Test Conditions that went into the initial rating of the Panels.

Stationary: 3679kWh/Year * 25 Years * .75 = 68,961kWh.

Tracking: 5212kWh/Year * 25 Years * .75 = 97,725kWh.

So, over the course of 25 Years, the Tracking System produces 28764kWh more than the Stationary System.

Since the Tracking System cost $3300 more than the Stationary System, this is our target to beat.

Taking the difference between the two outputs, and multiplying by a reasonable energy selling price ($.12/kWh) gives 28,764kWh * $.12/kWh = $3451, which, compared to the additional cost of the Tracking System ($3300) is a win over 25 Years, just barely.


Conclusion:

Yes, the tracker pays off slightly over 25 years, using rough estimations. Much would depend on the specific local conditions, and the Electricity Costs.


Final Comparison:

The Stationary Roof Installation had a Total Cost of $18900, or $9/Wp, and produced 68,961kWh over 25 Years.

$18900 / 68,961kWh = $.27 / kWh.

The Tracking Installation had a Total Cost of $22,200, or $10.57/Wp, and produced 97,725kWh over 25 Years.

$22,200 / 97,725kWh = $.23 / kWh.

From this, we can see quite clearly how, though the price per Peak Watt for a Tracking System is higher than for a Stationary System, the actual cost per unit of Energy of a Tracking System is lower.


Note:
Of course, there are many variables unaccounted for in these basic Calculations, including Government Subsidies, Interest on Loans, and Insurance Considerations. More detailed Calculations would have to be done on a specific case-by-case basis. I think this is good for a start.



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4 comments:

Chris Gueymard, PhD said...

I'm not sure how you obtained this "insolation ratio of 8.33%", and what you actually mean with this.
I have analyzed all the available U.S. data about this, and the result is that a 2-axis tracking flat-plate receives 25 to 40% more radiation than a fixed latitude-tilt collector. This ratio is directly affected by climate: the lower value is for the cloudiest areas, the higher value is for sunniest areas, i.e. in the southwest.

D Pickard said...

Hi Chris,

The Insolation Ratio that I mention the value of 8.33% for is roughly based on the difference between the measured values for tracking vs stationary insolation values found at http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/

Then See http://americansolareconomy.blogspot.com/2009/01/note-on-units-of-energy-and-insolation.html

So, what it ends up being, is that the insolation ratio that I calculated for the various colors on the Insolation Map is increased by 8.33%, which works out to roughly 30% increase in Total Average Energy over the year.

Of course, the numbers are rough, and will definately vary based on the local details, but I just wanted a rough estimate to use as a base for other calculations.

Fred Wetlaufer said...

The potential purchaser of a PV system will need much more information to make a decision. What is the expected life of a tracker ? What are the repair and replacement costs of trackers over the same 25 yr period? What would the picture look like if the purchaser applied the cost savings of a fixed system to a larger fixed system? What are the rebates and tax credits providing to the picture? What is the local electric provider offering in the way of incentives for grid-tie systems?

Anonymous said...

1st comment: the price difference between your stationary and two-axis systems are less than the cost of the tracking system parts themselves. Better then to assume that the two-axis system cost is = stationary cost + tracker parts.

2nd comment: The added effect from the tracker should be compared to the added effect of spending the same money on more panels.

3rd comment: you assume a cost of $0.12/kWh, which skews the equation in favour of bigger systems (if the cell is profitable at all). Calculating the average cost of production would be more comparable (or see 2nd comment).

4th comment: Some advantages of solar systems are no moving parts, low maintentance and building integration. The major disadvantage is high up-front cost. Tracking systems remove advantages and adds to the weakness.