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A solar system with batteries can power many appliances, provided that their energy consumption does not exceed the power of the generator. Therefore, it is necessary to correctly determine the power of the system. The first step in this direction is the specification, i.e. technical description of the system.

Energy calculation

When designing a home photovoltaic system, you first need to make a list of all electrical appliances in the house, find out their power consumption and add them to the list.

The table below shows the average power consumption of some appliances for reference. However, it must be remembered that these are only rough estimates. To calculate the power consumption (E) of an inverter system (for AC appliances), a correction must be made (multiply the average consumption by a factor C to get the total power).

For the operation of other electrical appliances - refrigerator, iron, fan, electric stove, etc. You will need a larger and more expensive system. Since these systems are not subject to uniform standards, but depend on the specific needs of the consumer, the calculation must be performed by a specialist.

Secondly, it is necessary to estimate how much time during the day certain electrical appliances are used. For example, a light bulb in the living room burns 10 hours a day, and in the pantry - only 10 minutes. Record this data in the second column in the following table. Then make a third column in which you enter your daily energy requirement. To determine it, you need to multiply the power of the device by the time of its operation, for example: 27 W x 4 hours = 108 Wh. Write the resulting number in the third column - this is your total energy consumption per day.

Next, you need to determine the amount of solar energy that can be counted on in a given area. This data can usually be obtained from a local solar panel supplier or hydrometeorological station. It is important to take into account two factors: the average annual solar radiation, as well as its average monthly values ​​​​under the worst weather conditions (see general information in the chapter "Solar radiation").

With the help of the first value, the photovoltaic system can be adjusted according to the average annual solar radiation, that is, in some months there will be more energy than required, and in others less. If you go by the second number, you will always have at least enough energy to meet your needs, except for extremely long periods of bad weather.

Now you can calculate the nominal power of the photovoltaic module. Multiply the value of energy consumption (Wh per day) by a factor of 1,7 to correct for energy losses in the system, then divide by the amount of solar radiation (Wh per day), e.g. 280 (Wh/day) x 1,7 .5/ 96,2 (Wh/day) = 50 W. Unfortunately, the choice of nominal power of photovoltaic modules is limited. Using 50W modules, you can build a 100W, 150W, 95W generator, etc. If the power requirement is XNUMXW, a two-module system is best suited. If the total power of the modules is very different from your calculated value, you will have to use either an insufficiently powerful or too powerful generator. In the first case, the photocell will not be able to meet the total energy demand. It is up to you to decide whether partial provision of your needs suits you. In the second case, you will have an excess of electricity.

Battery sizing depends on the energy requirement and the number of PV modules. In the example shown, the minimum battery capacity is 60 Ah and the optimum is 100 Ah. Such a battery will be able to store 1200 Wh at 12 V. This is enough to supply electricity in the case described above, when the daily energy consumption is 280 Wh.

Constant pressure

In the past, almost all photovoltaic systems used a constant voltage of 12 V. 12 V devices powered directly from the battery were widely used. Now, with the advent of efficient and reliable inverters, batteries are increasingly using 24 V. Nowadays, the voltage of the electrical system is determined by the daily energy input during the day. Systems producing and using less than 2000 Wh per day are best combined with 12 V. Systems producing 2000-6000 Wh per day typically use 24 V. Systems producing more than 6000 Wh per day day, use 48 V.

Mains voltage is a very important factor that affects the parameters of the inverter, controls, charger and wiring. Once purchased, all these components are difficult to replace. Some components of the system, such as PV modules, can be switched from 12V to higher voltages, others - the inverter, wiring and controls - are designed for a specific voltage and can only work within this range.

Battery

The battery stores the energy generated by the solar module. The battery compensates for periods of bad weather or excessive power consumption (medium term storage).

The most commonly used are automotive batteries, which are affordable and available all over the world. However, they are designed to carry large currents for a short period of time. They do not withstand the long charge-discharge cycles typical of solar systems. The industry produces the so-called. solar cells that meet these requirements. Their main feature is low sensitivity to cyclic operation.

Unfortunately, only a few developing countries produce such batteries, and imported ones are too expensive due to shipping costs and customs fees. In such a situation, you can use powerful truck batteries - this is a more affordable option, although they will have to be changed more often.

For a large photovoltaic system, the capacity of one battery may not be enough. Then you can connect several batteries in parallel by connecting all positive and all negative poles to each other. For connection, you need to use a thick copper wire, preferably no longer than 30 cm. When charging, the battery emits potentially explosive gases. Therefore, you need to beware of open flames. However, gassing is negligible, especially if a charge regulator is used; so that the risk does not exceed the usual one associated with the use of a battery in a car. Still, batteries need good ventilation. Therefore, do not cover them and hide them in boxes.

Battery capacity is indicated in ampere-hours. For example, a 100Ah, 12V battery can store 1200Wh (12V x 100Ah). However, the capacity depends on the duration of the charging or discharging process. The recharge period is indicated as the capacity index C, for example "C100" for 100 hours. Note that manufacturers can produce batteries for different base periods.

When energy is stored in a battery, a certain amount of it is lost during storage. Car batteries are about 75% efficient, while solar cells are slightly better. Part of the capacity of the battery is lost with each charge-discharge cycle, until it is reduced enough that it must be replaced. Solar batteries last longer than powerful car batteries, which last 2-3 years.

Battery Sizing

It is important that the size of the battery allows you to store energy for at least 4 days. Imagine a system that consumes 2480 Wh per day. Dividing this figure by a voltage of 12 volts, we get a daily consumption of 206 Ah. So 4 days of storage equals: 4 days x 206 Ah per day, equals 824 Ah. If a lead battery is used, 20% must be added to this figure so that the battery is never fully discharged. This means that the capacity of our ideal lead battery is 989 Ah. If a cadmium-nickel or iron-nickel battery is used, the additional 20% capacity is not required, as alkaline batteries are not harmed by regular full discharge.

charge regulator

The battery will last several years only if it is used in conjunction with a quality charge regulator that protects the battery from overcharging and deep discharge. If the battery is fully charged, the regulator reduces the level of current generated by the solar module to a value that compensates for the natural loss of charge. Conversely, the regulator interrupts the supply of energy to consumer devices when the battery is discharged to a critical level. Thus, a sudden interruption of power supply may not be caused by a breakdown in the system, but the result of this protective mechanism.

Charge regulators are electronic devices that can also suffer as a result of malfunctions or improper handling of the system. More advanced models are equipped with fuses to prevent damage to the regulator and other system components. Among them are fuses against short circuits and polarity reversal (when the +/- poles are reversed), a blocking diode that prevents the battery from discharging at night. Many models are equipped with LEDs that indicate the status of operation and system breakdowns. In some models, even the battery level is noted, although it is very difficult to determine with accuracy.

Инвертор

The inverter converts low voltage direct current into standard alternating current (120 or 240 V, 50 or 60 Hz). Inverters range from 250 watts (about $300) to over 8000 watts (about $6). The electricity generated by today's sine wave inverters is of a better quality than that supplied to your home from the local power grid.

There are also "modified" sine wave inverters - they are not as expensive, but they are suitable for most household tasks. They can create slight interference, "noise" in electronic equipment and telephones. The inverter can act as a "buffer" between the home and the utility grid, allowing excess electricity to be sold to the public grid.

Cables

The best way to avoid unnecessary losses is to use the correct electrical cables and connect them correctly to the appliances. The cable must be as short as possible. Wires connecting different appliances must have a cross-sectional area of ​​at least 1,6 mm2.

To ensure that the voltage drop does not exceed 3%, the cable between the solar module and the battery must have a cross section of 0,35mm2 (12V system) or 0,17mm2 (24V) per meter per module. That is, a 1 m cable for two modules should not be thinner: 10 x 10 x 2 mm0,35 = 2 mm7. Because cable larger than 2mm10 is difficult to handle and even harder to find, higher losses sometimes have to be accepted. If part of the cable runs outdoors, it must be resistant to bad weather conditions. Its resistance to ultraviolet radiation is also very important.

Sun trackers

Photovoltaic modules work best when the photovoltaic cells are perpendicular to the sun's rays. Tracking the Sun can result in a 10% increase in annual energy production in winter and 40% in summer compared to a fixed PV module. "Tracking" is implemented by mounting a solar module on a mobile platform that rotates behind the Sun. First of all, you need to weigh the benefit of the extra energy gained from tracking the Sun against the cost of installing and maintaining a tracking system.

Tracking devices are not cheap. In many countries, it does not make economic sense to install solar tracking for less than eight solar panels (for example, in the USA). When using eight photovoltaic modules, we will get more power if we spend money on increasing the number of panels, and not on a tracking installation. Only with eight or more panels will the tracking device pay off. There are exceptions to this rule: for example, when photovoltaic panels directly feed a water pump, without a battery, then tracking the Sun is beneficial for two or more modules. This is due to technical specifications, such as the maximum voltage required to power the pump motor.

Lamps

Due to their high efficiency and long service life, energy-saving lamps are recommended for use in photovoltaic systems. Fluorescent lamps or the new compact fluorescent lamps (CFLs) are applicable in many applications. The 18 watt CFL replaces the traditional 100 watt incandescent bulb. If these lamps are powered by a DC system, they require an electronic ballast.

The quality of the ballast can be very different, up to unsatisfactory. Poor quality ballast will incur additional costs for the constant replacement of lamps. The ballast must be efficient, provide a high number of starts, reliable ignition at low temperatures and low voltage (10,5 V), as well as protection against short circuits, open circuits, polarity reversal and radio interference. Although most compact fluorescent lamps operate only with AC current, some companies offer such lamps that are powered by DC.

Service life and component pricing

A very important factor in economic analysis is the lifetime of the photovoltaic system. The lifetimes of the various components of the solar power supply are calculated based on the experience gained in recent years.

• The service life of photovoltaic panels is estimated at 20 years. Proper sealing and the use of tempered glass with a low iron content can lengthen this life.
• Galvanized iron frames and fixtures are used in most photovoltaic systems. Well-galvanized materials should last as long as panels, although they may need to be repaired.
• Battery. Depending on the nature of the charge/discharge cycle, the average life of so-called "solar cells" is 4 years.
• Battery chargers are designed for at least 10 years.
• Inverters typically last at least 10 years.

Sample data for pricing some components:

• Inverter - USD 0,50/W
• Frame (galvanized) - USD 0,30/W
• Controls - USD 0,50/W
• Cable - USD 0,70/m
• Local stationary batteries - USD 100/kWh
• Photovoltaic modules - USD 5 /W

See other articles Section Alternative energy sources.

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