Solar 101

Understanding the Basics

Solar Panel Physics: Harnessing the Power of the Sun

The sun’s energy can be converted into electricity using a silicon solar cell designed to create a flow of electrons.

Silicon Solar Cells

A photovoltaic solar panel is made up of silicon. Silicon is a semiconductor and the second most abundant element on earth. Silicon crystals can be grown in a lab to make solar cells. As they are grown, very small quantities of other elements are incorporated into the crystal. Boron and phosphorous are typical elements added to silicon. This process is called “doping” silicon. 

Solar Cell P/N Junction

Phosphorous (n-type) has an extra electron in its electron cloud. Boron (p-type) lacks an electron in its electron cloud. Some of the extra electrons from the silicon doped with phosphorous (n-type) move to the boron side (p-type). Because the n-type loses an electron, it becomes positive while the p-type becomes negative. As the crystal is grown, boron is replaced by phosphorous and this creates a p/n junction, or a negative end and a positive end within the silicon cell.

 P-type              N-type

            Boron              Phosphorous

 Lacks electrons            Extra electrons

 Negative (-)            Positive (+)

Photoelectric Effect

Photons from sunlight hit the silicon cell and excite electrons due to the photoelectric effect. These excited electrons leave the n-type region because there are already extra electrons present. They are collected by metal electrodes on the n-type surface of the solar cell and flow through a circuit on their way to the p-type region. This circuit is completed by wires and can include other components like a simple light bulb. The electrons flow through another electrode into the p-type region. They then flow across the crystal to replace the electrons that left the n-type region, completing the circuit. That’s how sunlight produces electricity!

Building Up Solar Power

When electrons are excited they produce a direct current (DC). Your inverter will change this into an alternating current (AC), then send it to the breaker panel of your building, powering the circuits and devices in your house.

A silicon cell is 0.5 Volts. See MINI PHYSICS LESSON: I=V/R

Multiple cells make up a module. Multiple modules wired in series make a string. Multiples strings wired in parallel make an array.

Electrons are the only “moving” part of the solar system so panels can be used for decades.

MINI PHYSICS LESSON

I = V / R

Current (I) is equal to the amount of voltage (V) pushing against an amount of resistance (R). Another name for current is Amperes, and another name for resistance is Ohms.

Understanding Panel Specifications Data Sheet

A solar panel has multiple characteristics describing its electrical function. These can mostly be found on the back of a solar module or on its datasheet. Here are some common specs.

Open Circuit Voltage (Voc)
The voltage of a circuit that is not connected to any load.
Short Circuit Current (Isc)
The current output of a circuit that is not connected to any load.
Maximum Power Voltage (Vmp)
The actual voltage of the module when connected to any load.
Maximum Power Current (Imp)
The actual current output of the module when connected to any load.
Maximum Power (Pmp)
The amount of power (in watts) that the module produces.
Standard Test Conditions (STC)
The conditions that most modules are tested under for efficiency. Commonly 25℃ and 1000 W/m2
Solar Irradiance (W/m2)
The power per unit area of the sun’s electromagnetic wavelength.
Temperature (Celsius)
The degree or intensity of heat present in a substance or object; Celsius is the scale of temperature in which water freezes at 0° and boils at 100° under standard conditions.
Fire Rating
The duration for which a passive fire protection system can withstand a standard fire resistance test.
Class
Differences in fire performance between different materials can be evaluated by comparing flame spread ratings (Class A is the greatest resistance, followed by B and C) and heat release rate.
Different Panels Have Different Characteristics

Monocrystalline Panels

  • Rounded Edges

  • Darker Blue Color

  • More expensive

  • More efficient (2% more power in the same place)

  • Better performance in heat

  • Better performance in shade

Polycrystalline Panels

  • Square shaped molded

  • Crystal-like blue appearance

  • Less expensive (due to easier manufacturing process)

  • Less efficient

  • Performance affected by heat

  • Performance affected by shade

Electrical Characteristics

To understand the power of a solar panel system, it is best understand the Voltage-Current characteristics. The maximum power of any electrical system can be found by reading an I-V Curve. On the x-axis is voltage (V) and on the y-axis is current (I).

The graph above can be used to find the maximum power (Pmp) output of a solar panel. Power is current times voltage (P = I x V). Therefore, the point in the curve with the highest amount of current and highest amount of voltage will provide the most power. The highest power can be found at the start of the downward slope of each curve, the “knee” of the curve.

Each curve represents a different amount of solar irradiance. An increase in irradiance means there is a higher density of photons falling on the array. Interestingly, voltage changes very little with irradiance but irrandiance does increase current. Therefore, more solar irradiance provides more power (1000 W/m2 solar irradiance provides the highest amount of power in this graph).

 

Series Vs Parallel

In series, the negative end of a panel is connected to the positive end of the next panel, and so on. Panels connected in series increase the voltage. The current stays the same, and so does power (watts). It’s important to raise the voltage to closely match 120 Volts inside your home.

In parallel, the negative end of a panel is connected to the negative end of the next panel, and so on. Panels connected in parallel increase the current. The voltage stays the same, and so does power (watts). A steady, sufficient amount of current is important to power all the electrical appliances in your home.

Net Metering

A. DC Switch

B. Inverter

C. AC Switch

D. Utility Panel

Excess energy is monitored and sent back to the utility company in the form of credits on your bill. This is called Net Metering. During the day and summer, excess power is generated due to the copious amounts of sunlight available. The excess energy you produce is credited by your utility company and offsets the energy you would have paid for during cloudy days, nights, and winter months.