Harvesting Energy from Sun

The Mission:

Students will be able to understand how application of photo-electric effect can be helpful to meet the electric energy supply in urban/rural/remote areas. The students are expected to create a game in which they must make an electric grid of solar panels that can produce adequate electric output that can meet the demand of small town with population of 5000 people.

2. Concepts to Understand

2.1 Power generation by Photo-electric effect in Solar cell

Solar cells work on principle of photo-electric effect. They capture sunlight and convert the sunlight to electric energy.

The work function of the metal is W_o. This metal captures the sunlight and gives off an electron.

The kinetic energy of the photon is given by

KE = eV_o

Here V_o­ is the electric potential at which photoelectron is ejected.

The energy of the photon is given by,

E = hv

According to the Einstein’s photo electric equation we have:

KE = E – W_o­

This equation gives the voltage output of solar cell.

The output of 1 cell is  0.5 V and current of 35 mA.

This current is direct current also known as DC.

2.2 Series Connection of Solar panels

Considering a solar cell as an electric cell, if we connect 4 cells in series then their voltage gets added but the current remains the same because the current produced by 1 cell must pass through all the other cells.

Cells connected in series

Thus, total voltage output V = V₁ + V₂ + V₃ + V₄ = 0.5 +0.5 +0.5 +0.5 = 2 V

But total current output I = I₁ = I₂ = I₃ = I₄ = 35 mA.

Thus, total electric power produced by this configuration is  W.

Disadvantage of series connection of cells is, if one cell damages or stops working due to shade then the entire panel stops working. Hence a diode is connected to each cell in parallel to provide alternate route to current to flow in the circuit without breaking the circuit. It also protects the panel from voltage overloading or current overloading.

When an appliance needs to work on high voltage, then series connection is preferred.

Diode protection in solar panel

2.3 Parallel connection of cells

When cells are connected in parallel to each other, their output voltage remains the same, but their current adds up.

Consider 4 solar cells connected in parallel configuration as shown

The output voltage of this configuration is

V = V₁ = V₂ = V₃ = V₄ = 0.5 V

And the output current of this configuration is

I = I­₁ + I­₂ + I­₃ + I­₄ = 0.035 + 0.035 + 0.035 + 0.035 = 0.14 A = 140 mA.

And electric power produced in this configuration is

Thus, in the parallel connection the current generated by each solar cell gets added while potential remains the same.

When an appliance which needs to draw more current, parallel configuration is preferred.

In case of damage to either one of the cells, the rest cells in this configuration still work and generate the same current. Only their current output gets reduced.

2.4 Array of solar cells

To maximise current output at certain voltage, an array is formed.

It consists of a matrix of solar cells with cells in each row connected in series and each row of cells connected in parallel.

Consider the following example, here 8 cells are arranged in an array. 2 rows of 4 cells connected in series. These rows are connected in parallel.

The total voltage output of each row is 2 V. And as these rows are connected in parallel hence the output voltage of the array configuration is 2 V.

The total current output of each row is 35 mA. And these rows are connected in parallel hence the current output of the array configuration is 70 mA.

Thus, total power output obtained in this array configuration is

Thus, power and current both are maximised by array formation.

 2.5 Commercial Solar power plant

A Solar panel consist of 36 such cells which are connected in series with each other.

Total voltage produced by 1 solar panel is in range of 12 V – 18 V.

And maximum total current output of the solar panel is 35 mA in 1 second.

The maximum power output of 1 solar panel at 18 V is

For current driven output, these panels are connected in parallel, and for large voltage driven application these panels are connected in series.

 2.6 Commercial Solar power plant

There are generally three configurations observed in commercial solar power plants

a) Off grid: This is generally practiced in the remote areas or small villages where electric distribution by electric power plant is difficult. In order to meet the demands of electricity an off grid solar power plant is setup in an open non-agricultural area.

Depending on the demand of electricity, the array of solar panel is created. The electricity produced by the power plant is stored in battery bank which can supply the output voltage of 240 V.  These batteries are connected to a device that changes the direct current to alternating current at 240 V and 50 Hz cycle and then the power is distributed to the grid.

Disadvantage of off grid power plants:

1. It cannot serve to mass demand and can only support to small population.
2. Cost of installation of such powerplant increases due to the cost of batteries.
3. Cost of maintenance is very high
4. No government subsidiary received for installing such power plants in cities. For rural population very little subsidiary received from government.
5. When installed for individual house, no incentives are received.

Advantages of such power plants are

1. Due to battery bank, continuous electric supply is ensured during night also.
2. When installed for small population, electricity is distributed at very affordable rates.

b) On grid: Such power plant is generally erected near an electric power plant. It can also be practiced in small town where electric power is distributed from an electric power plant. Consider a small building where a small solar power plant is installed to meet their energy demand. The energy created by this power plant is measured by a smart meter and tracks the output for 8 hours. The electricity produced by such power plants are not stored in batteries instead it is fetched to the grid of a conventional power plant. The consumer of the building instead uses the power from the conventional electric power plant.

There are incentives received to the grid owners from the conventional power plants and their electricity bills are very minimum.

Advantages of such solar power plant is

1. It reduces the dependency on coal/fossil fuel and makes the electricity generation cheaper
2. It generates revenue to the solar power grid owner
3. It does not require battery bank and hence reduces the cost of installation
4. Government subsidies are available to install such smart on grid power plant
5. Even a small house can install an on grid electric power plant and earn incentives or save on their electricity bills

Disadvantages of such solar power plants are:

1. As there are no battery bank hence no storage of electricity. Hence, we need to depend on the conventional power plants during night-time for our need of electricity.

c) Hybrid: To overcome the challenges of both off grid and on grid power plants, hybrid power plants

are created where electricity generated by array of solar panels is stored in battery bank and it is also sold to the conventional power plants.

The incentive structure remains the same as that observed in on grid power plant.

Advantages of such power plant are

1. The owner of hybrid power plant does not need to be dependent on the electric supply from conventional power plant for their needs during night-time.
2. They earn high incentives compared to on grid power plant owners
3. It is easily installable for factories, amusement parks, small towns, etc.
4. Owner can be willingly be on or off the grid.
5. Cost of maintenance is lower compared to off grid power plant.

Disadvantage of hybrid system:

1. Cost of installation is very high due to battery storage and battery banks need to be created at various places adding cost to infrastructure.
2. Cost of maintenance is high compared to on grid power plant.

Did you know:

In India, a floating solar power plant is planned to construct on Indira Sagar Dam in Madhya Pradesh. It is proposed to have a capacity of 1 Gigawatt electricity production. It will be on grid type power plant and continue to produce electricity along with the hydroelectric power plant built with Indira Sagar Dam. It will be able to serve nearly 1/4^th population of the state of Madhya Pradesh