EEE: Renewable Energy

The main sources of renewable energy are:

1) Solar Energy

2) Wind Energy

3) Biomass Energy

4) Hydro Energy

5) Geo thermal Energy

6) River and Sea Current

7) Tidal Energy

The Sources of renewable energy are intermittent (irregular).Therefore storage device is very important for renewable energy to supply energy on demand. In the next section we will discuss on storage device in detail for solar system which is the most popular form of renewable energy.

Storage and Control Unit of Solar PV System

Basic block diagram of Solar PV System:

Storage Unit:

Battery is used as storage unit of a solar PV system. Solar cell is an intermittent source of electrical energy. Electricity generation from it depends on solar radiation. So it is not possible to get energy from it at night but most of the loads operate at night. Hence a storage unit (Battery) is needed to store energy at day from the solar call and use this energy at night or when solar radiation is not available. Another thing is that current of solar cell varies with solar radiation but constant current is essential for load therefore a battery is embedded with the solar PV system.

Control Unit:

A control unit (charge controller) is used to protect the battery from the overcharge and deep discharge problem. If the battery is full charged then charge controller disconnects battery from the solar panel. Again if battery is fully discharged then it disconnects load from the battery.

Two types of controller are used in control unit:

1) Series Controller

2) Parallel Controller

Battery:

An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery (or "voltaic pile") in 1800 by Alessandro Volta, batteries have become a common power source.

There are two kinds of battery:

1) Primary Cell

2) Secondary Cell

Primary Cell (Non- rechargeable battery):

A primary cell is any kind of battery in which the electrochemical reaction is not reversible. A common example of a primary cell is the disposable battery or dry cell.

Secondary Cell (Rechargeable battery):

A secondary cell or rechargeable battery or storage battery is a group of one or more electrochemical cells. They are known as secondary cells because their electrochemical reactions are electrically reversible. Lead-acid battery is an example of secondary cell.

Lead-Acid Battery:

Lead-Acid Battery is an electrochemical battery that uses lead and lead oxide for electrodes and sulfuric acid for the electrolyte. Lead-acid batteries are the most commonly used in photovoltaic (PV) and other alternative energy systems

Basic construction of a lead-acid battery:

Electrochemistry of Lead-Acid Battery:

The basic electrochemical reaction equation for both discharge and charge in a lead-acid battery can be written as follows:

Reaction in electrolyte

                                              H₂SO₄ <----> 2H⁺ + SO₄^-2

Reaction in Cathode

                                             PbO₂ + 4H⁺ + SO₄^-2 +2e^- <----> PbSO₄ + 2H₂O

Reaction in Anode

                                             Pb + SO₄^-2 <----> PbSO4 + 2e^-

Overall Reaction

                                             Pb + PbO₂ + 2H₂SO₄ <----> 2PbSO₄ + 2H₂O

Discharge:

During the discharge portion of the reaction, lead dioxide (positive plate) and lead (negative plate) react with sulfuric acid to create lead sulfate, water and energy.

Charge:

During the recharge phase of the reaction, the cycle is reversed: the lead sulfate and water are electro-chemically converted to lead, lead oxide and sulfuric acid by an external electrical charging source.

Effects that reduce performance of Lead-Acid Battery:

Deep Discharge and Sulfation Problem:

The biggest problem in lead-acid cells is sulfation due to constant undercharging. Here the sulfate ions have entered into deep bonds with the lead on the cell's plates. The sulfate ions can bond with the lead at three successively deeper energy levels. Level One is the bond we use when we normally charge and discharge the cell. After a month or so at Level One, some of the bonds form Level Two bonds which require more electric power to break. After several months of being at Level Two bond, the sulfate ions really cozy up to the lead and form Level Three bonds. Level Three bonds are not accessible electrically. No amount of recharging will break Level Three bonds. And lead-acid battery is permanently damaged.

Over Charge Problem:

During the recharge phase the lead sulfate and water are electro-chemically converted to lead, lead oxide and sulfuric acid by an external electrical charging source. If the battery is continuously charging over the full charge than water reacts with electrons given by external source and forms H⁺ and OH^- ions. It reduces the level of electrolyte of lead-acid battery.

Classification of lead-acid battery:

1) Automotive/Car/Thin plate/Shallow cycle lead-acid Battery:

Lead acid batteries designed for starting automotive engines are not designed for deep discharge. They have a large number of thin plates designed for maximum surface area, and therefore maximum current output, but which can easily be damaged by deep discharge. These batteries are not a good choice for a PV system.

12 V, 40 Ah Lead-acid car battery.

2) Industrial/ Thick plate/Deep cycle lead-acid battery:

Specially designed deep-cycle cells are much less subject to degradation due to cycling, and are required for applications where the batteries are regularly discharged, such as photovoltaic systems and uninterruptible power supplies. These batteries have thicker plates that can deliver less peak current, but can withstand frequent discharging.

Industrial/Thick plate battery is used in PV system

There is another type of lead-acid battery known as gel battery or maintenance free battery is used in UPS. Gel type electrolyte is used in maintenance free batteries.

Maintenance Free Battery

A 12V Lead-Acid Battery:

Lead-acid battery provides a nominal 12-volt potential difference by connecting six galvanic cells in series. Each cell provides 2.1 volts for a total of 12.6 volt at full charge. And at full discharge condition each cell provides 1.75 volts for a total 11.75 volt.

Measure Charging condition of Lead-Acid Battery:

Some terms related to charging condition of battery:

1) State of charge (SOC):

SOC represents a battery has how much charge. For a full charged battery, SOC is 100% and SOC is 0% for full discharged one.

2) Depth of Discharge(DOD):

DOD is just reverse of SOC. It represents a battery is how much empty. For a fully discharged battery the DOD is 100% but DOD is 0% for a fully charged battery.

From Above terms we get idea about charging condition of a Lead-Acid Battery.

For an automotive battery if DOD is 30~40% than the battery needs to charge again otherwise due to deep discharge the battery could be permanently damage.

For an Industrial battery 70~80% DOD is allowable after that battery needs to charge again.

Techniques of measuring Charge state of a battery:

1) By measuring relative density or specific gravity of electrolyte:

During the discharge the sulfuric acid reacts with lead and lead oxide and forms water. Density of water is less than density of sulfuric acid so due to discharge density of electrolyte reduces and by measure relative density of electrolyte we can identify SOC or DOD of a battery.

Specific Gravity:

Specific gravity is the ratio of the density (mass of a unit volume) of a substance to the density (mass of the same unit volume) of a reference substance. The reference substance is nearly always water. Temperature and pressure must be specified for both the sample and the reference.

Hydrometer (Measuring device of Specific Gravity):

A Hydrometer is used to determine specific gravity of liquid; that is the ratio of density of the density of water.