How trackers can make or break a solar project
While the global growth of solar installations is expected to slow in 2017, the adoption of solar trackers for utility-scale projects is expected to continue to boom—and for good reason. By dramatically improving energy production, solar trackers meaningfully improve the overall economics of solar power plants. Since the tracker is literally the foundation of the power plant, the equipment selection can make or break a project over the long term. When it comes to solar trackers, which must perform every day for 30 years, reducing complexity means increasing reliability and durability. Understanding the following three important solar tracker selection elements will help EPCs and asset owners make smart equipment decisions that will achieve the desired long-term results.Number of motors: More or fewer?
Less is more. Solar trackers that employ linked row tracker architecture drastically reduce the number of motors needed for a solar project, which greatly increases the 30-year reliability and ease of maintenance. Scientific methods used to analyze the likelihood and severity of failure in system design provide evidence that fewer motors reduce project cost of ownership. A failure modes effect analysis (FMEA) is often a first step of a system reliability study. It involves reviewing as many components, assemblies and subsystems as possible to identify failure modes, their causes and effects. The FMEA, when used to analyze tracking systems, validates that fewer motors reduce failure rates and increase reliability.
For example, a 100-MW array built with individually motorized row architecture will have at least 28-times the motors than a linked row project of the same size. Assuming the motor type and quality is equal and therefore the mean time between failure (MTBF) is consistent, then the sheer quantity difference dictates that there will be 28-times the motor failures for an individually motorized row tracking system. Keep in mind that in this comparison we are only considering one component, the motor of the system. The integrated simplicity of a mechanically linked row tracker architecture reduces electrical components to only two for every megawatt. With individually motorized row trackers, there are roughly seven different electrical components for every row (30 kW). When compared on a megawatt basis, an individually motorized row system has about 231 electrical failure points versus only two for a linked system.
Powering trackers: Battery versus grid?
The way trackers are powered is another area in which reliability is increased by decreasing complexity. By using a very small amount of electricity from the grid, trackers can provide reliable performance without increased complexity and maintenance. Trackers that run on individual batteries, however, are another matter. If you’ve used a cellphone on a cold winter day, you may have had the experience of the phone shutting down due to the battery operating temperature being below freezing. Or you may have experienced the opposite extreme if you’ve ever left your cellphone in the car on a hot summer day, only to find the phone inoperable due to elevated temperatures. Battery-powered trackers typically use lithium-ion batteries similar to cellphones. This type of battery is dramatically affected by storage temperature, operating temperature, level of charge, depth of discharge, temperature cycling and aging.
What does this mean for tracking systems reliant on batteries for power? If the temperature is below freezing, the battery may be damaged by charging and should not be operated. Conversely, elevated storage and operating temperatures severely impact battery life. Unlike your cellphone, battery-powered trackers live outside, in the elements, exposed to the heat and the cold and are charged from a small PV module, which is an intermittent charging source with limited power. This restricts the environmental and operational controls that need to be employed for optimum battery life. Batteries are recognized as one of the highest maintenance components in an electrical system even when kept in a climate-controlled environment. What happens to the reliability when they are exposed to the elements on a daily basis? Some battery-powered trackers use 34 or more batteries per megawatt, while grid-powered trackers have zero batteries. For a 100-MW site, that is 3,400 batteries versus zero!
Load mitigation: To stow or not to stow?
Another important consideration is how the tracker will handle structural wind and snow loads. For solar asset owners looking for the lowest cost of ownership for 30 years, a key is to select a tracker that is built to withstand inclement weather without the operational risk associated with the need to stow in a storm. If a tracking system is carefully designed to withstand the wind in any position, the likelihood of system structural damage is minimized and the long-term costs associated with maintaining the stow function are reduced. Tracking systems built to withstand the loads in any position avoid this 30-year structural risk. A reliable and robust no-stow system allows asset owners to “set it and forget it and sleep well at night.”
Many emerging tracker designs, in an effort to offset costs associated with added complexity, reduce the metal structure of the tracker. The result is a lightweight structure that must actively stow in lower wind speeds for survival. They require battery back-up, anemometers and communications to survive. This results in even more complexity and risk. Furthermore, these active stow systems typically have no redundant back-up, which means that the failure of just one small component can result in a catastrophic structural system failure.
Less is more
Solar trackers are now widely recognized as necessary components that provide a smooth flow of power from dawn to dusk and deliver meaningful economic value to utility-scale solar projects. But the choice of tracker equipment is critical. The right tracker choice will deliver the lowest levelized cost of electricity over the 30-year plant life with the right combination of installed cost, performance, cost of ownership and mitigation of risk. System complexity equals less reliability and results in higher maintenance costs. When it comes to solar trackers, less is more!