Electricity Basics for Solar PV
Updated: Jun 24, 2020
You won’t need an advanced degree in electrical engineering to be a solar PV engineer but you will have to have a solid grasp on its basic concepts. In this chapter, we will study these basic concepts and understand how we will use them in designing solar PV systems.
Ohm’s Law is the law that relates the three most basic electrical parameters, namely: voltage, current, and resistance. Before we go into what Ohm’s Law says, we first need to understand what these three parameters mean:
Current (symbol: I) – or electric current, is the flow of charged particles (usually electrons) electrons from a high energy state to a lower energy state. It is responsible for the flow and transfer of energy from the source to the load. It is measured in Amperes (A). In PV modules, electrons are put in a higher energy state by absorption of photons (light particles). These electrons in a higher energy state become free to move around in the circuit to release the energy absorbed from light.
Voltage (symbol: V) – in simple terms, voltage is the force that pushes the charged particles which constitute the electric current. It is also called potential difference because it pushes charged particles from a point of high potential or energy state to a lower potential. It is measured in Voltage (V). In PV modules, voltage is produced from the positive terminal to the negative terminal.
Resistance (symbol: R) – is a material’s opposition to the flow of current. Conductors like copper and aluminum have very low resistance. This means that electric current can easily flow through them. This is why wires are usually made up of copper and aluminum. Insulators like plastic and rubber, on the other hand, has a very high resistance. PV modules are usually made from Silicon which is a semiconductor. Semiconductors have a resistance that is in between those of conductors and insulators. It is measured in Ohms (Ω).
Ohm’s Law states that the voltage across an object is proportional to the current through it. This simply means that the higher the voltage, the higher the resulting current is. Its formula is:
We can also interpret it as:
The resulting current is directly proportional to the voltage source. A higher voltage source will produce a higher current when applied to the same load. This is why an appliance rated at 110V should not be plugged to a 220V outlet. The resulting current will be twice as much, destroying the appliance.
The resulting current is inversely proportional to the resistance of the load. Insulators like plastic and rubber used in protective equipment have a very high resistance, and because of this, only a harmless amount of current flows.
The Power Law states that power produced or absorbed by a device is equal to the product of its voltage and current. Power is measured in Watts (W) or kilowatts (kW). Its formula is:
This formula can be applied to every part of the solar PV system, so it is very important to know and understand. The power produced by the PV modules is equal to its output voltage multiplied by the output current. The power dissipated by the wires in the solar PV system is equal to the product of the voltage on it and the current passing through it. The power produced by the inverter is equal to the product of its output voltage and current.
What kills? Voltage or Current?
This is a common question asked by everyone that is studying basic electricity. Electricity is dangerous only because when current flows into anything, it produces heat. When too much current flows, like in a short-circuit, too much heat is also produced. This can cause serious burns or fire in the household. So, it is really the current that kills. To prove this, and to get a really good grasp of what I just explained, let us look at a device called the Van de Graaf generator.
You may have already seen pictures or videos of people touching these in science museums. If you touch it, it makes some of your hair stand up. The reason behind this needs a whole other discussion that involves static electricity. What we will look at is the amount of voltage that this device is producing. Did you know that it can produce up to 200,000V? Yes, the metal ball that the people are touching is actually at 200,000V! But why don’t they get electrocuted? This is because it is producing only a little amount of power. And remember the power law? The power produced by a device is equal to the product of its output voltage and current. With only a little amount of power and a very large amount of voltage, only a small amount of current can be generated. And since I have said earlier that it is the current that is dangerous, this device is then harmless. Any amount of current over 10 milliamps (0.01 amp) is capable of producing painful to severe shock. Currents between 100 and 200 mA, on the other hand, (0.1 to 0.2 amp) are lethal.
An electrical circuit is a path in which electric current flows. The simplest electric circuit is composed of a source, a switch, and a load.
Current will only flow on the circuit when the switch is closed. This completes the loop and forms a closed circuit. If the switch is opened, the loop is broken and therefore current cannot flow. This is called an open circuit.
Series and Parallel Connections
When electrical components are connected in a daisy chain, they are said to be connected in series. When they are connected in separate “branches”, it is then called parallel connection. The pictures below illustrate how each is implemented in a circuit.
In a series-connected circuit, there is only one path for current to flow so the current flowing through each compon