Part 1 - Performance Engineering for Solar

So, you’ve invested in a new solar plant, an electrical generator that uses solar are the means of creating electricity. Your dreams of reducing your electricity demand are at hand, maybe even the ability to cut-the-cord and never pay the utility again. You are green, clean, and have taken a big step towards energy independence. What could go wrong?

For many owners of solar, the dream of a clean source of renewable energy from the Sun starts with these very optimisms, but quickly gets clouded (yeh, pun intended) by the realities of system ownership and real operating conditions.

• Why didn’t I save as much as I expected?

• Did I generate less because of poor weather, or is there something wrong with my system?

• Will my system work better next month, or is this the best my system can do?

Having made a large initial investment, questions like these can create quite a bit of frustration and buyers-remorse for inexperienced system owners. Whether you have built solar on your new home, or invested in large utility-scale portfolio, this series is intended to help you answer these very questions and more.

What is Performance Engineering?

To understand whether your solar generation is working properly or whether you have a problem that should be addressed falls into the domain called ‘Performance Engineering’.  Performance engineering refers to both the activity and persons responsible for determining if a system is operating properly when exposed to specific conditions.  While many performance engineers are actually degreed engineers, many are also savvy business analysts and persons with a reasonable grasp of basic spreadsheet-level math skills. To make this post the most relevant for the widest audience possible, I will purposefully keep the mathematics clear and simple.

To understand and evaluate the basic performance of a solar system you first need to understand the conditions and factors that affect solar module operation. These conditions are the amount of sunlight (e.g. Irradiance) and temperature.

Irradiance, is the measure of the amount of solar radiation (e.g. sunlight) present in a specific area, and is expressed in the units watts-per-meter-squared (W/m2). Irradiance is the ‘fuel’ that solar modules require to produce electricity, where the photons from the Sun strike the electrons stored in the solar modules creating a buildup of electrical charge. The more irradiance, the higher the electrical charge, the more electricity.

How do you measure Sunlight (e.g. Irradiance)?

Irradiance is measured using a sensor called a pyranometer. Irradiance measurement values are typically observed from 0 to 1,000 W/m2, where 0 represents no sunlight and 1000 represents a very bright sunny day. Values greater than 1,000 are possible, but are often due to extremely arid conditions or the edge of clouds that cause a temporary gathering of sunlight into a more intense ‘flare’.

The level of irradiance, 0 to 1,000, is directly correlated with the power output expected from a solar system.

This leads us to our first performance engineering tool … comparison of irradiance to system power output. Or ‘Fuel vs Power’ as we often abbreviate.

To provide a simple example, and to start using a little math, let’s say we have a 100kW solar array installed on a house. If we measured an irradiance of 1,000 W/m2, we would expect the solar system to have a power output of approximately 100kW.

Similarly, if we measured an irradiance of 500 W/m2, we would expect the same solar system to have a power output of approximately 50kW. Half the fuel, half the power output.

First some math, then the explanation for why these estimates work the way they do.

With normal irradiance values having a range of 0 to 1000, we are really saying that our fuel level is at 0% when measured at 0 and 100% when measured at 1,000. 0 divided by 1,000 is 0 (0/1,000 = 0) and 1,000 divided by 1,000 is 1 (1,000/1,000 = 1). 1 represents 100%.

At 500 W/m2, the math is simply 500 divided by 1,000 is 0.5 (500/1,000 = 0.5 or 50%). When fuel is 50%, the output from the solar system should be approximately 50% too.

This simple comparison of the proportion of fuel to expected system power generation is true re4gardless of the size of the solar system. A 3,750kW solar system would be expected to generate 50% of its nameplate capacity (3,750kW * 50% = 1,825kW) in the same way a 2kW solar system would be expected to generate 50% of its nameplate capacity (2kW * 50% = 1kW) under the same 500W/m2 fuel conditions.

So, what about Temperature?

For a little clarification, when we talk about temperature, we’re referring to the temperature of the solar module cells … the actual part of the solar module where the electrical charge originates.  Solar module cells are actually a type of semiconductor (a material that either conducts or insulates electrical charges) whose electrical conductivity properties change based on their temperature. In the case of a solar module cell, the hotter they become, the less efficient of an electrical conductor they are.

This is important for performance engineers to understand, because seasonal temperatures can have a noticeable effect on the overall performance based on temperature. Put simply, when it’s cold outside (e.g. winter) solar modules are more efficient, and when it’s hot outside (e.g. Summer) solar modules are less efficient.

Before we talk about the specific math and magnitude of temperature on the performance of our system however, we need to talk about how solar modules are rated and what information is available to help us understand the effect of temperature.

Stay tuned for my next post on this topic, where I’ll discuss Standard Test Condition (STC), temperature correction, and performance ratio (PR).