|Solar Home Systems: Product Information Sheet (GTZ, 1990)|
What a photovoltaic system looks like
A solar module works just like an electric generator. Hence, it is simplest to connect an electrical device such as a lamp directly to the module. Like most simple solutions, this, too, has certain drawbacks - the most obvious one being that in this case the lamp could only be used during the daytime, when the sun is shining.
To compensate for these limitations, a battery is added to the system. The battery is charged by the solar generator, stores the energy and makes it available at the times and in the amounts needed.
However, the integration of a rechargeable battery into the system poses a new problem. Batteries are devices which are sensitive to overcharging and deep-discharging, both of which may damage the electrodes and require the battery to be replaced after a very short time. For this reason, it is necessary to have a charge regulator, which keeps the state of charge within the acceptable limits.
A solar-powered system of this type can run quite a lot of consumer devices, but only, of course, if the energy demand does not exceed the generator output. A solar module generates a direct current (DC), generally at a voltage of 12 V. Many appliances, such as lights, TV's, refrigerators. fans. tools etc., are now available for 12 V DC operation. Nevertheless' the majority of common electrical household appliances are designed to operate on 110 V or 220 v alternating current (AC). They cannot be operated by an SHS!
The solar module
Solar modules are e an array of solar cells which are interconnected and encapsulated behind a glass cover. The stronger the light on the cells and the larger the cell surface, the more electricity is generated and the higher the current.
Three types of basic materials are currently used in solar modules. Monocrystalline silicon has the best efficiency - about 14% of the sunlight can be utilized - but it is more expensive than multicrystalline silicon, which tropically has 11% efficiency. Amorphous silicon is widely used in small appliances such as watches and calculators, but its efficiency and long-term stability are significantly lower; consequently, it is rarely used in power applications.
Modules are rated in peak watts (Wp). A watt is the unit used to express the power of a generator or the demand of a consumer. One peak watt is a specification which indicates the amount of power generated under rated conditions, i.e. when solar irradiance of 1 kW/m² is incident on the cell at a temperature of 25°C. This level of intensity is achieved when weather conditions are good and the sun is at its zenith. No more than a cell of 10 x 10 cm is necessary to generate a peak watt. Larger modules, 1 m x 40 cm in size, have an output of about 40-50 peak watts.
Most of the time, however, the irradiation is below 1 kW/m². Furthermore, in sunlight the module will warm up beyond the rated temperature. Both effects will reduce the module's performance.
You can typically expect an average output of about 6 Wh per day and 2000 Wh per year per peak watt. To give you acme idea of how much that is, 5 Wh is the energy consumed by a 50 W lamp in 6 minutes (50W x 0,1h = 5Wh) or by a small radio in one hour (5W x 1h = 5Wh).
Although some differences still exist in product quality, most international companies produce fairly reliable units which can be expected to work for 20 years. Meanwhile, suppliers guarantee the specified power output for a period of up to 10 years.
The most decisive criterion for the comparison of different modules is the price per peak watt. In other words, you will get more power for your money with a 50 Wp module which costs $ 400 (8 $/Wp) than with a "cheap" 40 Wp module that costs $360 (9 $/Wp). The rated efficiency of a system is a less important consideration.
The generator of an SHS includes up to four modules. In 12 V systems, all of the modules are switched in parallel, i.e. all positive and all negative terminals are connected to each other, in 24 V systems two modules are connected in series.
The generator should preferably be installed on the roof, where it is protected against damages and theft. In many cases, a piece of sturdy wire and some boards are sufficient for installation. In any case, all suppliers offer support structures for this purpose.
A battery stores the energy delivered by the solar generator and provides power for various appliances. As a component of an SHS a battery has to fulfill three tasks:
a) It covers peak loads which the generator cannot meet on its own
b) It provides energy during the night (short-term storage).
c) It compensates for periods of bad weather or of unusually high energy demand (medium-term storage).
Automotive batteries, which are available all over the world at reasonable prices, are the most commonly employed type of battery. However, they are designed to deliver high currents over short periods. They cannot withstand the continous cycles of charging and discharging that are typical of solar systems.
The industry has developed batteries, sometimes called solar batteries, which meet these conditions. Their main feature is low sensitivity to cyclic operation. Unfortunately, there are only a few developing countries in which such batteries are produced, and imported batteries may be exceptionally high in price owing to transport costs and customs duties. In such cases, a heavy-duty truck battery may be an appropriate, easily accessible alternative, even if it has to be replaced more often.
In the case of large SHS's, the capacity of one battery may not be sufficient. If so, more than one battery, but not more than 3, can be switched in parallel, i.e. all poles marked + and all marked - are connected to each other. Thick copper wires, preferably less than 30 cm long, should be used for the connection.
During charging, batteries produce gases which are potentially explosive. Thus, you should avoid using an open fire nearby. However, gassing is relatively low, especially if a charge regulator is used; the risk is thus no greater than that normally involved in the use of automotive batteries in cars. Nevertheless, the batteries need to be well ventilated. Therefore you should not cover them up or put them in boxes.
The capacity of a battery is indicated in ampere-hours (Ah). A 100 Ah, 12 V battery, for instance, can store 1,200 Wh (12 V x 100 Ah). However, the capacity will vary, depending on the duration of the charging or discharging process. In other words, a battery will deliver more energy during a 100 h discharging period than during a 10 h period. The charging period is indicated by an index to the capacity (C), e.g C100 for 100 hours. Note that suppliers may use different reference periods.
When storing energy in batteries, a certain amount of energy is lost in the process. Automotive batteries have efficiencies of about 75%, while solar batteries may perform slightly better.
Some of the battery capacity is lost in each charging-discharging cycle and eventually drops to a level at which the battery has to be replaced. Solar batteries have a longer lifetime than heavy-duty automotive batteries, which last about 2 or 3 years.
The charge regulator
A battery can only be expected to last several years if a good charge regulator is employed. It protects the battery against overcharging and deep-discharging, both of which are harmful to the battery.
If a battery is fully charged, the regulator reduces the current delivered by the solar generator to a level which equalizes the natural losses. On the other hand, the regulator interrupts the amount of energy supplied to the load appliances when the battery has discharged to a critical level. Thus, in most cases a sudden interruption in supply is not a system failure, but rather an effect of this safeguard mechanism.
Charge regulators are electronic components and, as such, may be affected by malfunctions and improper handling of the systems. Improved designs are equipped with safeguards to prevent damages to the regulator and other components.
These include safeguards against short circuit and battery reverse polarity (mixing up of the batteries' +/- poles) as well as a blocking diode to prevent overnight battery discharge.
Many models indicate certain states of operation and malfunctions by means of LEDs (light emitting diodes = small lamps). A few even indicate the state of charge; but do not rely on this too much! The state of charge is difficult to determine and can only be roughly estimated.
Due to their excellent efficiency and long lifetime, flourescent lamps should always be used. Fluorescent tubes or the new compact fluorescent lamps are suitable. In many cases, 18 W lamps have proved to be a good compromise between the need for sufficient lighting intensity, e.g. for reading and working, and the necessity of saving energy.
Such lamps require electronic ballasts to be operated with a DC system. The quality of such ballasts varies considerably and sometimes proves to be very poor. Low-quality ballasts will result in high costs for continuous replacement of worn-out tubes. It is important for ballasts to have a good efficiency, a high number starting cycles, reliable ignition at low temperatures and low voltages (10.5 V), and protection against short-circuit, open circuit, reverse polarity and radio interference.
Since dry-cell batteries are an extremely expensive source of energy, it is always advisable to substitute them with solar power. There are two technical solutions to this problem.
In order to connect a radio directly to a solar system, it is necessary to have a voltage ban. former, which adjusts the 12 V delivered by the battery. Such transformers can be switched to match the voltage levels of various radios. Many radios, however, are not equipped with an external DC input socket, and consequently the connecting wires have to be soldered to the contacts in the battery compartment. In such cases, the radio can no longer be moved.
Many of the voltage transformers available are of poor quality. The output voltage deviates from the rated values, they cause radio interference, and they cannot withstand higher load currents. Many failures occur because the maximum load current, which is often not indicated, is exceeded and the regulator is ruined as a result.
If the owner likes to carry the radio with him, the use of rechargeable nickel Cadmium pocket cells is a more appropriate - but also more expensive - solution. The cells can be recharged with a special charge regulator which is connected to the solar system.
Charge regulators for nickel cadmium cells have to fit the specific battery type called for by the radio. Most, but not all, devices can handle all of the commonly used battery types.
A simple means of avoiding unnecessary losses is to use appropriate cables and to attach &hem properly to the terminals. Cables should always be as short as possible. The ones connecting the different appliances should have a cross-sectional area of at least 1.6 mm². To ensure that the voltage loss does not exceed 3%, the cable between the generator and the battery should have a cross-section of 0.35 mm² (12 V- system) or 0.17 mm² (24 V-system) per metre and module. Thus, a 10 m cable for 2 modules would require at least 10 x 2 x0.35 mm² = 7 mm². Since cables with a cross-section exceeding 10 mm² are difficult to handle and even difficult to get, higher losses have to be accepted in some cases.
If a part of this cable is exposed to the elements, it should be of adesign that will withstand such weather conditions. Tolerance to ultraviolet rays may be an important feature.