Solar Cell Equivalent Circuit:

Solar cell is a current source, the electronic characteristic of which can be represented by an equivalent circuit as shown in figure below.

Equivalent Circuit

An ideal solar cell equivalent circuit consists of a current source (Iph ) and a diode. But in practice there are two extra resistances, one in series (Rs) and the other in parallel (Rsh). The series resistance is because of the fact that a solar cell is not a perfect conductor and represents the ohmic loss. The parallel resistance is caused by the recombination of electron-hole pair or leakage current from one terminal to the other due to poor insulation at edges.

From the equivalent circuit diagram,

Iph – Id – Ish = I

Where,

I_d = I_o*[exp(q*V_d / nkT) – 1]

V_d = V + I*R_s

I_sh = V_d / R_sh

Hence, I_ph – I_o*[exp(q*( V + I*R_s ) / nkT) – 1] – (V + I*R_s ) / R_sh = I

Iph = Current produced by the solar cell       Id = Diode current

I_sh = Shunt or leakage current                      I = Load current

I_o = Reverse saturation current of diode      q = Charge of an electron

V = Voltage across load                               n = Diode quality factor

k = Boltzmann’s constant                            T = Absolute temperature of the cell

Generally, series resistance is of very low value and shunt resistance is of very high value. Ideally, Rs = 0 & Rsh =  ∞. A shunt resistance of a few hundred ohms does not reduce the output power of the solar cell appreciably.  In reality, Rsh is much larger than a few hundred ohms and can in most cases be neglected.  The series resistance, however, can drastically reduce output power.

Short circuit current (I_sc) and Open circuit voltage (V_oc): Two quantities of interest for solar cell i.e. I_sc & V_oc can be calculated from the above equivalent circuit equation by neglecting R_sh .

Putting I = 0, V_oc = (nkT/q)*ln(I_ph/I_o +1)

Putting V = 0, I_sc = I_ph – I_o*[exp(qI_scR_s / nkT) – 1]

If we further neglect R_s i.e. R_s = 0, then

Isc = Iph

Types of wind energy converters

Types of wind energy converters:

Before we go into further details about the wind energy systems, let’s see the types of wind energy converters, or turbines. They can be broadly categorized into two types;

(1)   (1)Horizontal-axis wind turbines rotate about an axis that is horizontal. They include:

(a) ‘Dutch-type’ windmills-used mainly for grain grinding

(b) Multi-blade windmills-used for pumping

(c) High-speed propeller type windmills

'Dutch-type' Windmill

Multi-blade Windmill

High-speed Propeller-type Wind turbine

(1) (2)Vertical-axis wind turbines have their rotation axis vertical. They are of two types:

(a) The Savonius rotor

(b) The Darrieus rotor

The Darrieus Rotor

There exists a variant of the Darrieus rotor turbine, called the Giromill. In this the blades are straight rather than ‘troposkein’.

The Giromill

We saw in the previous post that Cp has a maximum value of 16/27. However, it also depends upon the type of turbine used.

Power Coefficient for varous wind converters

Solar Cell – Basic Principle of Operation:

 

 

 

Solar Cell Structure and Principle of Operation

Solar cells basically consist of p-type and n-type semiconductors with two metal contacts, one in p-type and the other in n-type as shown in figure. These two types of semiconductors form a p-n junction with n-type facing the sun.

Absorption of Solar Radiation

Sunlight is composed of photons containing energy which correspond to the different wavelengths of the solar spectrum. When photons strike a solar cell, depending upon the type of semiconductor used, some low energy photons (infrared) are not absorbed by the electrons in the semiconductor material while some high energy photons are absorbed for excitation of electrons into the conduction band and the excess of energy is released as heat.These excited electrons are attracted toward the n-type semiconductor. This causes more negative charges in the n-type and more positive charges in the p-type semiconductors. If an electric load is connected between the two types of semiconductors through the metal contacts,electrons start flowing from n-type to p-type semiconductors via the load. Thus photon energy is directly converted to electrical energy without an intermediate mechanical or thermal process.

The cell is a current source of DC type. This DC current is used directly in applications. AC current can also be produced by using inverters.