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Standalone PV system Introduction
Standalone photo voltaic systems functions independent of the electric grid hence the stand alone PV system. It is also called as off grid PV system as it involves no interaction with the utility grid. Many photo voltaic systems operate in standalone mode.
Description of standalone PV system
The block diagram of a standalone PV system is shown in the figure. Stand alone Photo voltaic system consists of following main components
- Photo voltaic generator equipped with arrays of photo voltaic cells
- Charge controller
- Battery banks
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.
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’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.
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.