There are many design and financial evaluations that go into selecting the correct type of renewable resources in a region. One of these evaluations is choosing between fixed-tilt and axis tracking systems in PV solar fields. KED’s solar market partner, Stellavise, is sharing their experience in the thought processes in selecting a tracking system.
Solar photovoltaic (PV) power plants utilize three main types of racking systems: (i) fixed-tilt, (ii) single-axis tracker, and (iii) dual-axis tracker. A fixed-tilt system positions the modules at a “fixed” tilt and orientation, while solar tracker systems automatically adjust the positions of the PV array so that the PV modules consistently “track” the sun throughout the day. Compared to a fixed-tilt PV array, single- or dual-axis tracking systems will increase the energy production by 15-30% for the same size array.
Figure 1. Fixed vs. Single-Axis Tracking (MWH/MW).
Source: Solar PV Balance of System (BOS) Markets: Technologies, Costs and Leading Companies, 2013-2016
Single-axis trackers are the most common tracking systems installed today. Although dual-axis trackers can increase total energy production by 5-10% above a single-axis tracker, single-axis trackers are more cost-effective and reliable. The additional energy produced by a dual-axis tracker rarely, if ever, outweighs the additional land, installation and operations and maintenance (O&M) costs required. Most importantly, the rapid decline in PV module pricing has led investors to prefer increasing a system’s power capacity (number of modules) to achieve energy targets rather than using more complex tracking systems that improve efficiency.
So why not build every project with single axis trackers? The answer is that single-axis trackers are not always technically or economically viable.
The technical viability issues are mainly wind loads, site topography and soil conditions.
Industry leading manufacturers have designed trackers capable of withstanding wind speeds up to 135MPH at 3-second gusts. If potential wind speeds are higher a tracking system is likely not an option. It should also be noted that although the tracker can withstand those wind speeds, the modules are stowed in a flat position when gusts exceed 60MPH. If the project is located in an area with high winds during daylight hours, there will be production losses due to the modules being stowed.
Site topography also limits tracker viability because trackers need to be installed in relatively flat locations. Each tracker segment is typically 250-275ft (72-80 PV modules) in length and cannot be installed at grades exceeding 5-6%. Additionally, ground undulations within the tracker segment need to be leveled to a certain tolerance. With these restrictions, sites with sloping or unleveled terrain will require extensive grading and additional costs which typically make trackers uneconomical.
Figure 2. Typical Tracker Grading Requirements.
Source: NEXTracker. http://nextracker.com/product-services/design-engineering/site-grading/
Soil conditions are another main issue to explore for technical viability. Trackers put additional torque on their piers compared to fixed-tilt racking systems. The additional torque load requires larger and deeper piers to be installed. Many sites have underlying bedrock which limits the depth of installation requiring drilling and additional cost. Furthermore, sites with poor soil or contamination may not allow for underground disturbance (e.g. landfill) and would be better suited for a ballasted fixed-tilt racking systems which avoid underground conditions entirely. Ballasted trackers are not currently available, resulting in poor soil being the selection criteria for fixed-tilt ballasted systems.
After ensuring the technical viability of trackers at a site, economic considerations must be taken into account. The gain in efficiency must be measured against the additional cost of the tracker to see if the tracker system will improve the overall economic return of a solar project. The four main considerations are cost of equipment, cost of installation, cost of additional land and cost of additional O&M.
Trackers are more expensive to procure and install than fixed-tilt racking systems. This is the most basic additional cost to include in a financial model. Tracker systems generally add a premium of 15-20% to the overall system costs when compared to a fixed-tilt system of similar size.
Tracker systems also have larger footprints per megawatt when compared to fixed-tilt systems. The cost of the additional land must be factored into the financial model. Typically fixed-tilt systems require 4-5 acres per megawatt while tracking systems need 4-7 acres per megawatt. An additional 20% of acreage for tracking systems is a good rule of thumb for checking the economics.
Lastly, tracking systems are more expensive to maintain over the 25-year life of a system. They include motors, sensors, and moving parts that fixed-tilt systems do not have. These require additional maintenance and add cost. It is not uncommon to see as much as 10-15% added to O&M plans that utilize tracking systems. A 10% increase in O&M costs is a decent rule of thumb for the conservative developer.
Single-axis trackers will provide additional energy production resulting in higher project revenue. Careful evaluation of the technical and financial characteristics of a project will reveal whether this additional revenue will support the higher costs of a single-axis tracker system.
For more information, contact Dan McGee at email@example.com.