A Brief Overview of the Components in a Solar System
To the uninitiated, a solar power system may seem complicated and bewildering, with many seemingly incomprehensible technical components that play unknown roles in the system.
This article attempts to give a brief overview of what the main components are, as well as define what roles they play in the system.
The diagram below shows the main components of a typical solar power system and how they are normally connected together:
Starting from the top of the diagram, lets start with the solar PV panels.
Solar PV Panels
Solar photovoltaic (PV) panels are semi-conducting devices that cause an electric current to flow when sun light of sufficient strength falls on the panel.
In the diagram, they are wired together in series, in two “strings” of three panels (6 panels in total). By wiring the panels in series in “strings” the voltage is increased which has the benefit of keeping the current flowing in the cable to a minimum and thus reducing losses in the cable, as well as allowing thinner cables to be used (especially since the solar cables joining the solar panel array to the MPPT are usually quite long in many installations).
The losses in the cable are proportional to I2R (the current squared multiplied by the resistance). So by halving the current, you reduce the losses by a factor of four. Hence its very desirable to keep the current flowing in those long cables to a minimum.
To work out how many panels you might need, try using our calculator.
In the diagram, three 30V panels are connected in series to achieve a string voltage of 90V. The only limitation to how many panels you can connect together in a string is the voltage rating of your components, particularly the MPPT. You should be careful to not exceed the maximum input voltage of the MPPT.
Which brings us to the next device in the diagram – the MPPT (Maximum Power Point Tracker).
Maximum Power Point Tracker (MPPT)
All solar panels generate direct current at constantly varying voltage and current strength depending on the strength of the sunlight and other ambient conditions. At any particular set of circumstances, the relationship between voltage and current strength produced by the solar panel has a characteristic curve such as the one shown below:
The power generated by the solar panel for any combination of voltage and current can be represented by the rectangle you can fit under the curve. In the graph above you can see three different rectangles – yellow, pink and blue. Each represents a different combination of voltage and current values that are defined by different points on the curve (points A, B and MPP in the graph). The area of each rectangle represent the power generated by the panel. You can see that the rectangle with the largest area is the pink one. So point MPP in the graph above represents the combination of voltage and current at which this panel will produce the maximum amount of power for the given conditions represented by that curve.
A Maximum Power Point Tracker (MPPT) is thus a clever electronic component that intelligently manages the voltage and current accepted from the solar panel(s) so that the maximum power is extracted from the panels under any circumstances. MPPT’s typically extract 30% more energy from a solar panel array than an unintelligent charge controller.
The other roles of a MPPT are to convert the voltage accepted from the panels, to the lower voltage of the battery bank, as well as to intelligently control the charging of the batteries, using the power received from the solar panels.
The Battery Bank
Solar panels generate energy at specific times when the sun is shining. But in most homes, that energy is required at other times, particularly at night when the sun is not shining. So in those cases we store the energy generated during the day in a bank of batteries for later use at night.
Most batteries available are rated at 12V (although you also get 2V and 6V batteries).
Again it is desirable to keep the currents flowing in the cables to a minimum to reduce losses. So we connect the batteries together in series to increase the battery bank voltage. In the diagram, two sets of four batteries are wired together in series (8 batteries in total). By wiring four batteries in series we achieve a battery bank voltage of 48V.
To work out how many batteries you might need, try using our calculator.
You need to select this battery bank voltage based on what your MPPT and Inverter can handle. 48V or 36V are common good choices.
This brings us to the last component in the system shown in the diagram – the Inverter.
The role of the Inverter is to convert the direct current (DC) received from the batteries into alternating current (AC) which your household appliances require.
Alternating current flows backwards and forwards in a wave that has a specific shape (called a sine wave).
The other consideration to consider with an inverter is that it should be able to deliver sufficient power to drive your respective appliances. especially when more than one is being used at a time. You should try to measure, or estimate, the maximum demand power that you will need during the day and make sure your inverter is rated above that.
Another thing to consider is the short duration spike in current flow caused by certain types of appliances (appliances with motors or other inductive coils) when they are switched on. This is referred to as “in-rush” currents. These short duration currents can be very high and may overwhelm your inverter causing it to shut down, or even damage it. Please read here for more information.
The output of the inverter will be “normal” 220V AC which can be connected to selected breakers in your home’s DB board by a qualified electrician, or used directly to drive selected household appliances. Please ensure that you conform to the relevant standards and regulations in this regard. A brief overview of the safety concepts is available here. Please note that all neutral wires of all appliances that will be powered by the inverter must be kept separate from the other neutral wires in the house – a common mistake.
Some inverters include built-in MPPT’s, all in one unit. So you don’t need to buy those components separately, which can be useful, as well as perhaps more cost effective. Although that may limit your flexibility for growth if you intend building the system up over time.