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
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.
Power Law
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.
Electrical Circuits
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.
Series Connection
In a series-connected circuit, there is only one path for current to flow so the current flowing through each component is the same.
On the other hand, in a parallel-connected circuit, the voltage across each component is the same.
PV modules are usually connected in series to form a string. When this happens, every PV module on the string is forced to produce the same amount of current. We will discuss more about the implications of this in the design and performance in the next chapters.
If you have multiple inverters on your solar PV system, they will be connected in parallel to the point of interconnection with the house or building. These inverters will then have to produce the same output voltage.
Power and Energy
We have already discussed Power in terms of the power law but we don’t yet really have a concrete understanding of power in real life. Power is defined as how fast a load or a source consumes or generates energy. Energy, on the other hand, is defined simply as the ability to do work. This work can mean emitting light from a light bulb, producing sound from a radio, etc.
As we have already discussed, power is measured in watts (W) or kilowatts (kW). We commonly see this unit of measurement on our appliances’ power consumption. This means that, for example, a 600W microwave oven consumes energy faster than a 1.2kW (1,200W) one. The unit of energy is kilowatt-hour or kWh. A 600 W microwave oven would consume 600Wh or 0.6kWh of energy in one hour of continuous use.
PV modules are rated in watts. This rating is the maximum amount of power that the PV module can produce. The actual power that they produce, however, depends on other factors (this will be discussed more in the next chapters).
Engr. Jet Andal has 6 years of experience in the design and installation of residential, commercial and utility-scale solar PV systems. Together, and with the use of solar energy, let us help make the world a better place. You can click here to read all of our other blogs. For aspiring solar PV engineers, you can also check out his Solar PV Engineering Ebook on Amazon on this link.