1. Basic Principles
1.1 Volts, Amps and Watts
Throughout this book there are references to Voltage, Current, Power and Resistance. It is important to understand what each of these means and how they relate to each other. The units for each are:
- Voltage: The potential difference between two points. Is measured in Volts (V) and has the symbol ‘V’.
- Current: The flow of electrons in a circuit. Is measured in Amps (A)and has the symbol ‘I’.
- Resistance: A material’s opposition to an electrical current. Is measured in Ohms (Ω) and has the symbol ‘R’.
- Power: The rate of doing work. Is measured in Watts and has thesymbol ‘P’.
- Energy: The capacity for work, the product of power and time. Has the symbol ‘E’. The basic unit of energy is the Joule, but electrical energy is normally expressed in Watt hours (Wh) or kilo Watt hours (kWh). One kWh is 1000 Wh.
The relationship between these units is:
P = VI
or V = P/I or I = P/V
V = IR
or I = V/R or V = I/R
P = I2R
Power equals voltage multiplied by current. This can also be expressed in the other two forms shown.
Voltage equals current multiplied by resistance. Again there are two other forms shown. This is known as ‘Ohm’s
law’.
Power equals current squared multiplied by resistance.
1.2 The Photovoltaic Effect
The photovoltaic effect is the means by which solar panels or ‘photovoltaic modules’ generate electricity from light. A solar cell is made from a semiconductor material such as silicon. Impurities are added to this to create two layers, one of n-type material, which has too many electrons and one of p-type material which has two few. The junction between the two is known as a p-n junction. This process is known as doping and is the same technique used to manufacture transistors and integrated circuits (silicon chips).
Light consists of packets of energy called photons. When these photons hit the cell, they are either reflected, absorbed or pass straight through, depending on their wavelength. The energy from those which are absorbed is given to the electrons in the material which causes some of them to cross the p-n junction. If an electrical circuit is made between the two sides of the cell a current will flow. This current is proportional to the number of photons hitting the cell and therefore the light intensity.
1.3 Solar Modules
A photovoltaic or PV module is commonly made from a number of cells connected together in series. This is because each cell only produces a voltage of about 0.5 Volts. It is usual for there to be 36 cells connected together to provide a voltage of about 18 – 20 Volts. This forms a module which can be used to charge a 12 Volt battery. Figure 2 shows a typical module. The separate cells can clearly be seen.
There are also ‘thin film’ solar modules where the separate cells are formed as part of the manufacturing process. Figure 3 shows such a module. This technique is employed for the small solar panels which are fitted to calculators and similar devices. They are much cheaper to manufacture but deliver lower efficiency. This means that less of the light which hits them is converted to electricity. Recent advances in technology, however, have made larger and more efficient thin-film solar modules available.
Often a number of solar modules will be connected together into a solar array in order to provide more power than a single module can provide.
1.4 Energy Storage
Photovoltaic solar modules generate electricity only when there is light falling on them, and the amount of power generated is proportional to the light intensity. This means that a way has to be found of storing the electricity generated and releasing it when it is needed. The normal method is to use the surplus power to charge a lead-acid battery. This is the same type of battery as used in cars, although the different requirements mean that a car battery is not suitable, instead a deep-cycle battery is needed.
A battery is made up of a number of cells, each consisting of two lead plates in a container of dilute sulphuric acid. Each cell has a nominal voltage of 2 Volts, so a number are connected in series, for example 6 cells forms a 12 Volt battery.
1.5 Control and Conversion
The electricity generated by the photovoltaic effect is low voltage direct current (DC) whereas mains electricity is much higher voltage alternating current (AC). This means that additional devices may be needed to control the battery charging process and convert the power to the correct voltage. The two most commonly used devices are the photovoltaic controller and the inverter. The controller makes sure that the battery is neither overcharged or over-discharged. The purpose of an inverter is to convert low voltage DC into higher voltage AC. It does this by first turning the DC power into AC and then using a transformer to step up to a higher voltage.
1.6 Operation
The principles of operation of a typical stand-alone solar power system are shown in figure 6. Electricity is generated in the form of low voltage DC by the photovoltaic solar modules whenever light falls on them.
This power is routed through a controller, which feeds whatever power is necessary to any DC appliances such as lights and uses any surplus to charge a battery. When there is less power being generated than the appliances are using, power flows from the battery to the appliances. The controller monitors the battery state of charge and disconnects the appliances if the battery becomes very discharged.
Any AC (mains) appliances are connected to the inverter. This is not connected to the controller but directly to the battery. It incorporates its own control mechanism to ensure that the battery is not over-discharged.