Comprehensive Notes on Diffuse Radiance, Irradiance, Voltage, Current, and Electrical Safety

Diffuse Radiance

• Diffuse radiance occurs after light bounces off an object. An example of diffuse radiance: cloud cover creates a day with no shadows.
• Absence of shadows indicates diffuse irradiance, where light is scattered rather than directly striking objects.
• Diffuse irradiance diminishes shadows.
• Solar sites can operate on diffuse irradiance, albeit at reduced capacity compared to direct normal irradiance.

Irradiance Measurement

• Irradiance is the energy from the sun that reaches the Earth.
• Measured in watts per meter squared \frac{W}{m^2}.
• Standard test condition (STC) for solar modules: 1000 watts per meter squared \frac{W}{m^2}.
• Module production output is measured by flashing with 1000 \frac{W}{m^2} of light.

Standard Test Conditions (STC) for Modules

• Irradiance: 1000 \frac{W}{m^2}.
• Temperature: 25 degrees Celsius.
• Atmospheric Density: 1.5 atmospheres.
  • Accounts for light traveling through the atmosphere at an angle.
  • Atmospheric density affects the amount of radiance reaching the surface due to "stuff" (particles, humidity) in the way.
  • Higher humidity (water) in the atmosphere diffuses sunlight, reducing direct irradiance.
• Module nameplate values are generated during STC.
• Deviation from STC results is allowed, typically within a plus or minus 5% range.
  • Module Wattage \pm 5\%
  • MPP Voltage \pm 5\%
  • MPP Current \pm 5\%

Voltage

• Voltage is the electromotive force (electrical pressure) between two points or conductors; it is the potential difference.
• Measured in volts.
• Static electricity example: shock from a car door.

Current

• Electrical current is the flow of electrons.
• Current flows from high to low potential.
• Current cannot flow if there is no potential difference.
• Analogy: Water flows from high pressure to low pressure; a garden hose cannot fill a fire hose due to pressure differences, only vice versa.

AC vs. DC

• Frequency: Alternating current (AC) has frequency, while direct current (DC) does not.
• Reactive Power: AC sources with frequency have reactive power which is a result of electrons having to move back and forth 60 times per second (60 Hz) to inspire movement of all the electrons along the circuit before any work (ie. light from a bulb) can be produced. DC circuits do not have this quality.
  Reactive Power Consumption: Energy required to start movement of electrons along the line before consumption of the electrical current by a load. Does not produce light, heat or mechanical movement. This is a necessary function for electrical current however it does NOT consume electricity. Measured on SCADA screens.
• Line Loss: DC conductors should be larger to mitigate friction without reactive power. AC uses reactive power to mitigate friction in the electrical lines.
• Bridging Gaps: Alternating current's reactive capability allows it to bridge across gaps because the electron begins to create an electromagnetic field as it moves back and forth. Direct current does not induce on anything. It travels around its conductor.
• Safety:
  • DC: Avoid contact; it cannot initiate current flow across a gap. It needs hard contact to complete a circuit.
  • AC: Follow minimal approach distances as shown on arc flash stickers.

Arc Flash

• Arc flash is related to current.
• Current: It represents moving electrons. No moving electrons (no current) means no heat from the arc.
• Voltage: It is the pressure behind the electron and how far away you can be before it hurts you.
• 250 amps of AC and 250 amps of DC have the same arc flash potential because amps are amps during an arc.

Hazard Stickers

• Hazard stickers often show minimum approach distances (shock hazard), but these may not be accurate in practice, especially in combiner boxes, where components are closer together than the sticker suggests.

Conductivity

• Pure H2O is an insulator. The minerals within water that allow for conductivity.
  Minerals Good conductors. Sodium, Magnesium, Calcium are all good conductors and electrolytes.
• Human Body:
  • Sweat: salty and conductive. This is an example of tracking.
  • Tracking: DC current finding a very small path. Change position if tracking is experienced, don't run.
• It's best to not over react because an over reaction is more dangerous than the potential fault. If a combiner box is about to start on fire, it will stay there, it is not trying to expand. AC instead always tries to find another way home and return to its source.

DC Current

While DC is unrelenting when it's flowing, it is also very happy. Compared towards an AC current which is always operating and flowing. What does Alternating Current do? Changes the STOP 60x a second. It is at either a high or low potential.
The reason DC it scares most people is because it is unrelenting compared to Alternating Current. However it is friendlier and by avoiding contact one is able to engage with a flowing bus and remain safe.
There still some similarites related to both though: How much heat is created by an AC current of 200amp and that of DC are relatively negligble and are enough to melt and burn everything.

Analogy: Gun

Electrical Currents Compared to a Gun:
Voltage: The force behind the bullet.
Current: The bullet itself.
While it important to not refute any electrical safety standards, it should always be asked why and what the reason it is to follow them.