A Guide to Understanding Solar Panels Power System Installations

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A solar cell can convert the energy contained in the solar radiation of the sun into electrical energy. Due to the limited size of the solar cell it only delivers a limited amount of power under fixed current-voltage conditions that are not practical for most applications. 

In order to use solar electricity for practical devices, which requires a particular voltage and/or current for their operation, a number of solar cells have to be connected together to form a solar panel, also called a PV module. For large-scale generation of solar electricity solar panels are connected together into a solar array. A typical solar panel installation for a residential apartment is shown below:
For a residential apartment, a hybrid/grid connected system is the most appropriate. The system will be supplied electricity during the day through the solar panels as well as store energy in the batteries while the batteries will supply electricity during bad weather or night hours or grid outage. The system can also be supplied electricity through the electricity grid or a portable generator. With the portable generator or the grid, electricity is supplied to the loads and energy is also stored in the batteries. It is a stand-alone system that is self-sufficient. Note that the portable generator is only required in the worst-case scenario where the weather is bad/during night hours when the stored energy in the batteries is used up and there is no electricity from the grid.

In the process of generating the electricity from Solar panel installations, several important components are connected to produce electricity. Which components are required depends on whether the system is connected to the electricity grid or whether it is designed as a stand-alone system.

The objective here is to describe the common components used for stand-alone systems commonly used in residential buildings.

Components of a Solar Panels Power System Installations

The following components typically make up a solar electricity power system for a residential system. These components can also be found installed in large commercial power applications:
1. Mounting Structure
2. Solar Panels
3. Energy storage system
4. Charge controllers
5. Inverters
6. Generators (optional but required for places with poor electricity grid supply)
7. Cables.

Mounting Structure

A mounting structure is used to fix the Solar panels and to direct them towards the sun. Proper positioning of the solar panels will ensure that the maximum solar radiation is captured in a given location and ensures the Solar power system performs as required. There are fixed mounting structures as well as those designed to track and follow the maximum radiation from the sun. Most residential Solar power systems are mounted on a fixed structure on the ground or most commonly on building roofs.

Solar Panels

The main building blocks of a Solar energy power system are solar panels. They are the units that trap the sun’s solar radiation and converts it to electricity. This electricity is then used to supply electrical loads as well as stored in batteries for stand-alone systems. Solar panels are typically mounted on structures or on building roofs for most residential solar PV installations.

Energy Storage Systems

Energy storage is a vital part of stand-alone systems because it assures that the system can deliver electricity during the night and in periods of bad weather. Usually, batteries are used as energy storage units. During the day when the sun’s radiation is very high, the solar PV system supplies electricity as well as charge the batteries during bad weather or night hours. Deep cycle batteries are usually applied for this purpose – batteries that can withstand repeated charge and discharge cycles.

Charge Controllers

Charge controllers are DC-DC converters that are used in stand-alone solar power systems to convert the solar panels variable electrical output voltage to a fixed voltage output that can be used to charge a battery or used as input for an inverter in a grid-connected system. Typically, the voltage output from a solar panel varies depending on the time of the day and the weather conditions which also makes the output current variable. Both a variable current and voltage are not good for good battery performance hence the need for the charge controller. The charge controller also helps to disconnect the Solar panels from the batteries when they are fully charged. This helps to prevent overcharging which affects battery performance and life.

All charge controllers use Maximum Power Point Tracking (MPPT). Owing to variations in the current and voltage output from a solar panel or array installation due to changes in solar radiation (technically called irradiance) and temperature, there exist a Maximum Power Point (MPP) on the I – V characteristic of the installation where the highest power is generated for a given irradiance and temperature. The voltage and current corresponding to MPP is Vmpp and Impp.

Given that MPP is dependent on ambient conditions, any changes in irradiance and temperature will shift the position of MPP on the I-V (current/voltage characteristics) of the solar panel/array installation. Therefore, changes in the I-V characteristics have to be tracked continuously with adjustment in the operating point to correspond to MPP after changes in ambient conditions. This process is called Maximum Power Point Tracking or MPPT. The devices that perform this process are called MPP trackers and they are built-in to modern charge controllers used in solar panel/array power installations.

Inverters

Inverters are DC-AC converters that are used to convert DC voltage produced by the Solar panels to AC voltage to supply alternating current loads. Appliances and loads used in residential apartment usually utilize alternating current hence the need for an inverter. Furthermore, for grid connected solar power systems, there is need to convert the DC voltage from the solar panels before transmission into the power grid. For low power application as are common in small residential PV systems, single-phase inverters are used. They are connected to one phase of the grid. For higher powers, three-phase inverters are used that are connected to all phases of the grid.

Electric Cables

The overall performance of Solar electricity supply systems also is strongly dependent on the correct choice of the cables. Selecting the wrong cable size can significantly affect the performance of the Solar PV power supply system.

The cables must be chosen such that resistive power losses are minimal. The power dissipated across a cable is proportional to the square of the current flowing through the cable. Hence, as the current doubles, four times as much heat will be dissipated at the cables. Therefore, modern solar panels have connected all cells in series.

Color Conventions for Cables

Solar Panels PV systems usually contain DC and AC parts. For correctly installing a solar panel system, it is important to know the color conventions for wiring. 

For DC cables:
Red is used for connecting the positive (+) contacts of the different system components with each other.
Black is used for interconnecting the negative (-) contacts

For AC wiring, different colour conventions are used around the world.

European Union,
Blue is used for Neutral,
Green-yellow is used for the protective earth 
Brown (or another color) is used for the phase.

United States and Canada,
Silver is used for neutral,
Green-yellow, Green or a bare conductor is used for the protective earth
Black (or another color) is used for the phase.

In India and Pakistan,
Black is used for Neutral,
Green is used for the protective earth 
Blue, Red, or Yellow is used for the phase.

Therefore it is very important to check the standards of the country in where the Solar  PV system is going to be installed.

Solar  Electricity Systems with Solar Panels
Types of solar PV Power Supply System