3. System Components
In order to design a solar power system it is helpful to have a basic understanding of the various system components and their operation. The following paragraphs describe those components which will commonly be encountered.
3.1 Solar Panels
3.1.1 Types
As already discussed there are two basic types of solar module, crystalline and thin-film. The characteristics of these are similar but the method of manufacture is very different.
3.1.1.1 Crystalline
A crystalline module is made from a number of discrete cells, usually 36 for a 12 Volt module. These cells have to be assembled and soldered together by hand, which goes some way to explaining the relatively high price of crystalline modules. Each cell is made from a wafer, composed either of a single crystal (monocrystalline) or many crystals (polycrystalline) of a semiconductor material, usually silicon. The monocrystalline method produces cells of slightly higher efficiency, but for all practical purposes they can be regarded as the same. Polycrystalline solar modules can be distinguished by the obvious crystalline appearance of the cells.
3.1.1.2 Thin film
Thin film or “amorphous” solar modules are made by a different process. A thin film of semiconductor material is deposited on a substrate, usually glass. This substrate forms the body of the solar module. A laser is then used to score the material in order to produce individual cells, which produces the characteristic striped appearance. This method uses less of the semiconductor material and is easier to automate. The solar modules thus produced are therefore lower cost. Currently, however, commercially available thin film solar modules display significantly lower efficiencies than crystalline modules. This limits their use to applications where there is no size restriction on the solar array and adds to the cost of installation.
3.1.3 Operation
Figure 8 shows the relationship between voltage and current for an imaginary 12 Volt 36 cell module, the most common configuration. The two curves represent different insolation levels.
The current that a photovoltaic module will deliver into a short circuit is known as the short-circuit current or Isc. This is proportional to the insolation, so the more the sun shines the greater the current.
The open circuit voltage or Voc is determined by the number of cells in series, and is not significantly affected by the insolation. You can see that over the working voltage of a 12 Volt load such as a battery the current is nearly constant for a given value of insolation.
3.2 Batteries
3.2.1 Types
There are many different battery technologies available today. However it is one of the oldest, the Lead-Acid battery, which is most suited to stationary solar power applications. There are two main reasons for this; a large amount of energy storage costs very little compared to other technologies and it operates over a narrow voltage range which makes it ideal for powering common appliances. This type of battery does have its disadvantages, notably the fact that it is easily damaged by excessive discharge. Each cell of a lead-acid battery has a nominal voltage of 2 Volts, hence a 12 Volt battery is constructed of 6 cells in series. Lead acid batteries are usually available as 2 Volt cells or 6 Volt or 12 Volt monoblocs, i.e. a number of cells combined to make a battery. A standard car battery is an example of a 12 Volt monobloc.
3.2.1.1 Starting batteries
Starting batteries such as car batteries are easily available at very low cost. They are designed to deliver a very large current for a short time. Contrary to common belief this does not result in a heavy discharge, usually no more than 5% of the battery’s total capacity.
The demands of solar power systems require that the batteries are frequently discharged by 50% or more, and thus a starting battery is unsuitable. Attempts to use starting batteries in this way results in a very short life and is a false economy.
3.2.1.2 Deep-cycle batteries
The term ‘deep-cycle’ refers to batteries that are designed for regular discharging by 50% or more. The term is applied to many different forms of battery from small 6 or 12 Volt batteries to much larger batteries consisting of separate 2 Volt cells. Most traction batteries, that is those designed to propel electric vehicles such as fork-lift trucks, can also be considered to be deep-cycle. The majority of deep-cycle batteries have a liquid electrolyte (acid) which is vented to the atmosphere. Sealed types with the electrolyte in the form of a gel are also available, although their higher cost limits their use.
3.2.1.3 Leisure batteries
The term ‘leisure battery’ refers to a battery which is a compromise between the low cost of a car battery and the long life of a true deep-cycle battery. They have a much longer life than a car battery when regularly discharged and are much less expensive than a true deep-cycle battery. Their use is common in applications such as caravans, where the usage pattern is not as intensive.
3.2.2 Operation
3.2.2.1 Charging
The voltage at which a lead-acid battery is charged must be strictly regulated. If the charging voltage is too high, then excessive gassing will occur, leading to loss of electrolyte and possible plate damage. On the other hand, too low a voltage will lead to the plates becoming ‘sulphated’ which causes a loss of capacity. Figure 9 shows the relationship between voltage and current in a constant voltage charging regime.
3.2.2.2 Discharging
Batteries must be protected from damage by over-discharge. As the battery discharges the voltage at the terminals decreases. Figure 10 shows the terminal voltage of a lead-acid battery at differing rates of discharge. You can see from this how it is impossible to deduce the state of charge from the battery voltage alone, and therefore why some kind of over-discharge protection is necessary.
3.3 Solar Controllers
3.3.1 Function
The primary function of the controller is to regulate the charging of the battery. Many controllers also perform a number of secondary functions, the most common of which are the protection of the battery from overdischarge (low-voltage disconnect) and status monitoring by means of lamps or LCD displays.
3.3.2 Operation
A controller can function in two basics ways; shunt regulation, where the output of the solar array is shorted to control its output and series regulation, where the output of the solar array is disconnected by some form of switch. The switching method is usually electronic, although electromechanical systems may be encountered.
3.4 Solar Inverters
3.4.1 Function
The function of an inverter is to transform the low voltage DC of a leadacid battery into higher voltage AC which may be used to power standard ‘mains’ appliances. An inverter is necessary where appropriate low voltage appliances are unavailable or expensive or in larger systems where it is necessary to distribute the power over a wide area.
3.4.2 Operation
For our purposes there are two types of inverter; sine wave, which closely mimics the waveform of mains electricity and modified sine wave, which is more accurately described as a square-edged waveform with similar characteristics to a sine wave. Figure 11 shows the two waveforms. There are advantages and disadvantages to both types. The modified sine wave inverter is cheaper and tends to have both a higher capacity for overload and greater efficiency. However certain equipment may not operate correctly or may be noisy. These problems will not occur with a sine wave inverter, as the waveform is identical to that delivered by the mains.