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MySolarSystem Part II. The Problem with Solar: Mismatch

Introduction

At first glance, it might seem like everyone should go solar - the sun is an overwhelmingly abundant resource; however, there are several crucial issues that prevent it from being a viable option for everyone. The number one problem that faces traditional solar installations is known as mismatch. Mismatch causes all kinds of issues ranging from decreased power production to excessive maintenance as apparent in the image below.

What is Mismatch?

Mismatch describes the difference in performance between individual solar panels an array.

Panels 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 panels are connected in series and one panel is underperforming, due to shade from a tree for example, the entire "string" of panels produces less energy. A string of panels is simply a set of panels connected in series, and the underperforming panel is one that is "mismatched".

Think of a string of panels 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 rate of flow 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 that they could.

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. 

  1. Cloud shading and refraction: Clouds block and reflect irradiance (light energy moving to the panels from the sun) when passing over an array, and can lead to significant decreases in insolation, (the amount of energy hitting each panel) causing mismatch
  2. 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.
  3. Manufacturing mismatch: Since no two cells are identical, panel manufacturers “bin” their panels, selling them in ranges of power (typically +/-1.5% to +/- 5%), meaning 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.
  4. Thermal gradients: Panels towards the edge of an array receive greater air flow 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 panels.

II. Mismatch developed over time

Like all systems, solar installations degrade over time. This degradation causes mismatch in the following ways:

  1. Failed bypass diodes: A bypass diode is a safety component in solar systems, that keeps current from destroying panels by flowing backwards. Bypass diodes typically fail with the panels in operation, and this reduces 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 is difficult to detect by measurement equipment.
  2. Uneven soiling: Solar installations, like anything that sits on a rooftop 365 days a year, get dirty. Uneven soiling and dirt changes the insolation that each panel receives, leading to differing levels of power production in each panel.
  3. 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.
  4. Variable degradation: Solar panels degrade over time at different rates. NREL’s (the National Renewable Energy Laboratory) analysis of panel degradation showed that most panels 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.
  5. Accumulated wear and tear: System problems build up over time, such as mechanical or electrical faults, causing additional mismatch. Panel components can age and crack, humidity can short 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 individual 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, and Tigo has the most versatile solution on the market, designed with the intention of expanding the viability of solar energy.

 

 

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