Introduction
The sun is an abundant resource; however, several crucial issues prevent it from being a viable option for everyone. The number one problem that traditional solar installations face is mismatch. Mismatch causes various issues, from decreased power production to preventative maintenance, as shown in the image below.
What is Mismatch?
Mismatch describes the difference in performance between individual solar modules in an array.
Modules are connected in two ways - in parallel and in series. Imagine that you have a set of tubes of the same length. If you connect these tubes end to end, they are connected in series. If you lie them down side by side, they are in parallel.
When modules are connected in series, and one module is underperforming due to shade from a tree, the amount of power degradation varies with the type of shading, the type of module, the location of the shading, and the solar inverter's maximum power point tracking capabilities.
The underperforming modules are "mismatched." This is an appalling example of an array mismatch, but hopefully, it is rare.
Think of a string of modules like a road. When part of a road is under construction and lanes are reduced to one, all the cars on the road have to slow down. Fewer cars get where they need to go on time because there is a reduced rate of flow on the road.
In electrical systems, this flow rate is called current, and it represents the number of electrons (the number of cars) that can cross a given section of a circuit at a given time. Below are listed the most common causes of mismatch that keep solar installations from producing the power they could.
Common Mismatch Myths
The most commonly spread myth about shading is the Christmas tree effect. In short, shading one module will not "drag down the entire string." Read this post to learn more.
No Virginia, there is no Christmas tree Effect
Sources of Mismatch
I. Out-of-the-box mismatch
These sources of mismatch are always present in solar arrays from the moment an installation is commissioned.
- Cloud shading and refraction: This is the number one cause of mismatch worldwide! Clouds block and reflect irradiance (light energy moving to the modules from the sun) when passing over an array and can lead to significant decreases in insolation (the amount of energy hitting each module), causing mismatch.
- Rooftop Shading and Orientation: Research has shown that the USA residential roof space is limited in its ability to accommodate traditional solar arrays due to tree shading, roof obstructions, and problematic roof construction. Out of 100% of the pitched rooftops, (which are 92% of the entire residential roof space), less than 25% can support solar systems with no mismatch. See the figure below for an illustration, source: GTM Research.
- Manufacturing mismatch: Since no two cells are identical, module manufacturers “bin” their panels, selling them in ranges of power (typically +/-1.5% to +/—5%). This means that panels are mismatched as soon as they are produced. Some installers may re-bin their panels before installation to sort them into tighter groups, but this is considered uncommon.
- Thermal gradients: Modules towards the edge of an array receive greater airflow and run cooler than panels in the center of the array. Since most crystalline silicon panels change their power production by about 0.44% per degree Celsius, a 20°C change in temperature leads to a 9% difference in power output between hot and cold modules.
II. Mismatch developed over time
Like all systems, solar installations degrade over time. This degradation causes mismatch in the following ways:
- Failed bypass diodes: A bypass diode is a safety component in solar systems that prevents current from destroying modules by flowing backward. Bypass diodes typically fail while the panels are in operation, reducing a panel’s voltage by one-third (in a 3-diode panel). Since this is only about 2-3% of a string’s total voltage, it isn't easy to detect by measurement equipment.
- Uneven soiling: Solar installations get dirty like anything that sits on a rooftop 365 days a year. Uneven soiling and dirt change each module's insolation value, leading to differing power production levels in each panel.
- Voltage drop: With long cables between strings and inverters, losses due to system wiring add up and contribute to mismatch, especially in large-scale systems.
- Variable degradation: Solar modules degrade over time at different rates. NREL’s (the National Renewable Energy Laboratory) analysis of module degradation showed that most degrade at a rate of up to 1% per year, but some were between 1-4%, so if one degrades at 1%, while another degrades at 4%, a large mismatch is caused.
- Accumulated wear and tear: System problems, such as mechanical or electrical faults, build up over time, causing additional mismatch. Module components can age and crack, humidity can short out exposed connections, and thermal changes can separate mechanical connections.
Conclusion
Mismatch occurs in systems of all sizes, from residential to commercial to utility-scale, and typically results in a 2-5% energy loss in a new, unshaded array, with further losses growing over time. However, these losses can be recovered by using panel-level power electronics. By keeping each panel working at its peak power point, panel-level power optimizers can increase the energy output of any solar array, as demonstrated in the next article. This greatly increases the viability of solar energy for the entire residential market. Tigo has the most versatile solution on the market, designed to expand the viability of solar energy.