Question

Draw the circuit diagram of a stand-alone photovoltaic system assisted with batteries and explain the function of each stage of the system.

Answer 1

Stand-alone PV systems are used in areas that are not easily accessible or have no access to an electricgrid. A stand-alone system is independent of electricity grid, with the produced normally being stored in batteries. A typical stand-alone system would consist of a PV module or modules, batteries and a charge controller. An inverter may also be included in the system to convert the direct current generated by the PV modules to the alternating current from by normal appliances   

While a major component and cost of a stand alone PV system is the solar array, several other components are typically needed. These include:

• Photo voltaic generator equipped with arrays of photo voltaic cells
• Charge controller
• Battery banks
• Inverter

The Charge controller, Battery banks, Inverter together called as power conditioning unit. The power conditioning unit make up the balance of the system. Depending on the type of load whether DC or AC, the power can be delivered to load directly or through DC to AC converter (inverter). The battery bank is designed in such a way to meet the load demands irrespective of the solar irradiance.

Photo voltaic generator
A photo voltaic generator consists of large number of photo voltaic cells connected together. The modularity in PV generator starts with array, an array is subdivided into modules, and each module has a predefined number of photo voltaic cells connected in series or parallel. The cells are encapsulated with various materials to protect them from environment

When two PV cells are connected in series the voltages add up, if connected in parallel the currents add up. The operating points of solar cells are chosen such that the power delivered to load will be maximum. A real solar cell is characterized by following parameters

• Open circuit voltage
• Short circuit current
• Properties of material used in the construction of solar cell
• Maximum power that can be generated (Pmax)
• Efficiency of conversion
• Fill factor

The interconnection of solar cells in the module can be done based on the current and voltage rating of the module. For example if a solar cell generates optimum power at operating point of 2 volts and 1 amp. Our need is to have a module which generates 4 volts and 1 amps, two such similar solar cells are connected in series so that the voltages generated by each will add(2 V+2 V=4 V) and the current through them remains same at 1 ampere.

Battery bank
The battery bank is one of the most important components of standalone PV system. Battery banks consist of one or more batteries. The excess energy generated during the day will be stored in the battery to meet the load demands during nights when daylight is not available. Without the battery, the system would be unable to meet the load demands outside daylight. Hence it acts as lifeline of standalone photo voltaic system.

Solar batteries are really deep cycle batteries which are capable of surviving prolonged, repeated and deep discharges which are typical in solar energy systems that are stand alone(off grid).

Following are some of the important parameters of batteries:

• Nominal capacity
• State of charge SOC
• Charge (or discharge) regime
• Efficiency of battery
• Lifetime of battery

Some of the typically used batteries in standalone PV systems are

Flooded lead acid batteries
Sealed Gel batteries
Sealed AGM (Absorbed Glass Mat batteries)
The gel type and AGM type batteries together are called as VRLA batteries. VRLA stands for valve regulated Lead Acid batteries. Due to reduced maintenance cost and elimination of gassing VRLA batteries are preferred compared to flooded lead acid batteries. But whenever cost factor, ruggedness, durability is desired flooded lead acid batteries can be used.

Charge controller
Charge controller’s function is to fully charge a battery without permitting overcharge or reverse current flow (generally during night). In standalone PV system charge controller is a control element that manages the energy flow to PV system, batteries and loads by collecting information on the battery voltage and knowing the maximum and minimum values acceptable for the battery voltage.

There are mainly three varieties of Charge controllers

• Ordinary ON or OFF control 1 or 2-stage control
• 3-stage and /or PWM
• MPPT (Maximum power Point tracking)

Ordinary ON, OFF control is the simplest charge controller which has only two operating states open or close connection from PV generator to batteries. It will open when a certain preset high voltage or low voltage is reached and reopens only after these levels improve beyond certain thresholds.

PWM charge controller became industry standard because of its high performance and cost effectiveness. It slowly lowers the amount of power applied to the batteries as the batteries get closer and closer to fully charged. It has 3 stages of charging, as follows:

• Bulk charge
• Absorption charge
• Float Charge

Maximum Power Point Tracking (MPPT) controllers are more power efficient compared to other two charge controllers. They match the output of the solar panels to the battery voltage to ensure maximum charge (amps). But MPPT’s are more expensive and larger in size compared to PWM controller.

Inverter
The PV arrays produce power in DC form. Therefore when the stand-alone PV system contains an AC load a DC/AC conversion is required. An inverter is a DC/AC converter where the power flow is from the DC to the AC side, namely having a DC voltage, as input, it produces a desired AC voltage, as output

There are several types of inverters such as

• Square wave inverters
• Modified sine wave inverters
• Sine wave inverters using oscillators

The inverters are characterized by a power dependent efficiency η and hence can be chosen based on optimum comprise between efficiency and cost.

Working

The array of solar panels must be large enough to power all needs at the site and recharge the batteries at the same time as i supply energy to appliances the battery gets discharged as they get discgarged continuously. I have to charge them again, the reason I want to prolong the amount of time the length of time that the PV system will actually operate so as we discharge the battterie to operate appliances I have to charge them in order to be able to it the next day and the next night

PV array it goes to a combiner box because we have several modules we have to combine them in order to increase the voltage , the current and therefore the power, then it wiil go to a circuit braker (safety precaution), it will go to charge controller this charge controller has one simple mission to control the amount and the level of charging of the battery

When the battery will be fully charged this controller will stop the continuity of the charging ,will stop charging the battery if the battery is always a already full after the controller and after the charge controller there's another circuit breaker notice that between each module we two circuit breakers, after the circuit breakers we have the battery pack.

The battery pack depends on the number of days or number of hour or nights and a power consumptions of appliances this will determine the size of the battery pack it's basically to be storage of more power. More batteries will enable longer time for the PV system ti work ( no gird is connected)

the battery charge the batteries , the battery bank goes to again circuit breaker which will go to an inverter, Inverter receives DC in the input provides AC the ouput. and this output again goes to a circuit breaker to the customer console, electrical console that distributes these AC electerical energies into the various appliances in the home we have no gird we have DC coming from the panels converted by the inverter and distributed to the different appliances

we also have a backup generator if the battery is completely discharged and we still do not lie ourselves to the gird the generator will come and will behave as a backup to the batteries. the generator is basically the AC generator its connected directly to the inverter supply enegry to it