Basics of Photovoltaics

28.2.1 Basics of Photovoltaics

The density of power radiated from the sun (referred as “solar flow when a load is connected. The photocurrent (I ph ), which energy constant”) at the outer atmosphere is 1.373 kW/m 2 . is internally generated in the solar cell, is proportional to the Part of this energy is absorbed and scattered by the earth’s radiation intensity. atmosphere. The final incident sunlight on earth’s surface has

A simplified equivalent circuit of a solar cell consists of a

a peak density of 1 kW/m 2 at noon in the tropics. The tech- current source in parallel with a diode as shown in Fig. 28.2a. nology of photovoltaics (PV) is essentially concerned with the

A variable resistor is connected to the solar cell generator conversion of this energy into usable electrical form. Basic ele- as a load. When the terminals are short-circuited, the out- ment of a PV system is the solar cell. Solar cells can convert put voltage and also the voltage across the diode is zero. The the energy of sunlight directly into electricity. Consumer appli- entire photocurrent (I ph ) generated by the solar radiation then ances used to provide services such as lighting, water pumping, flows to the output. The solar cell current has its maximum refrigeration, telecommunication, television, etc. can be run (I sc ). If the load resistance is increased, which results in an from PV electricity. Solar cells rely on a quantum-mechanical increasing voltage across the p–n junction of the diode, a process known as the “photovoltaic effect” to produce electric- portion of the current flows through the diode and the out- ity. A typical solar cell consists of a p–n junction formed in a put current decreases by the same amount. When the load semiconductor material similar to a diode. Figure 28.1 shows resistor is open-circuited, the output current is zero and the

a schematic diagram of the cross section through a crystalline entire photocurrent flows through the diode. The relation- solar cell [1]. It consists of a 0.2–0.3 mm thick monocrystalline ship between current and voltage may be determined from the or polycrystalline silicon wafer having two layers with dif- diode characteristic equation ferent electrical properties formed by “doping” it with other impurities (e.g. boron and phosphorous). An electric field is

− 1) = I ph −I d (28.1) established at the junction between the negatively doped (using

I qV /kT =I

ph −I 0 (e

phosphorous atoms) and the positively doped (using boron where q is the electron charge, k is the Boltzmann constant, I ph atoms) silicon layers. If light is incident on the solar cell, the is photocurrent, I 0 is the reverse saturation current, I d is diode energy from the light (photons) creates free charge carriers, current, and T is the solar cell operating temperature ( ◦ K). The which are separated by the electrical field. An electrical volt- current vs voltage (I–V) of a solar cell is thus equivalent to an age is generated at the external contacts, so that current can “inverted” diode characteristic curve shown in Fig. 28.2b.

−I dark I c eg. diode

PV CELL

illuminated

I sc

eg. solar cell

(a)

(b)

FIGURE 28.2 Simplified equivalent circuit for a solar cell.

28 Power Electronics for Renewable Energy Sources 725

A number of semiconductor materials are suitable for the “square” the I–V curve is, given by manufacturing of solar cells. The most common types using silicon semiconductor material (Si) are:

Fill Factor = (V mp ×I mp )/(V oc ×I sc ) (28.2) • Monocrystalline Si cells.

For a silicon solar cell, FF is typically 0.6–0.8. Because sili- • Polycrystalline Si cells.

con solar cells typically produce only about 0.5 V, a number of • Amorphous Si cells.

cells are connected in series in a PV module. A panel is a col- lection of modules physically and electrically grouped together

A solar cell can be operated at any point along its characteris- on a support structure. An array is a collection of panels (see tic current–voltage curve, as shown in Fig. 28.3. Two important Fig. 28.4). points on this curve are the open-circuit voltage (V oc ) and

The effect of temperature on the performance of silicon short-circuit current (I sc ). The open-circuit voltage is the max- solar module is illustrated in Fig. 28.5. Note that I sc slightly

imum voltage at zero current, while short-circuit current is increases linearly with temperature, but, V oc and the maximum the maximum current at zero voltage. For a silicon solar cell power, P m decrease with temperature [1]. under standard test conditions, V oc is typically 0.6–0.7 V, and

Figure 28.6 shows the variation of PV current and voltages

I sc is typically 20–40 mA for every square centimeter of the cell at different insolation levels. From Figs. 28.5 and 28.6, it can be area. To a good approximation, I sc is proportional to the illu- seen that the I–V characteristics of solar cells at a given inso- mination level, whereas V oc is proportional to the logarithm lation and temperature consist of a constant voltage segment of the illumination level.

A plot of power (P) against voltage (V ) for this device (Fig. 28.3) shows that there is a unique point on the I–V curve at which the solar cell will generate maximum power. This is known as the maximum power point (V mp ,I ). To maximize

Radiation = 1000 W/m mp 2 the power output, steps are usually taken during fabrication, 6

the three basic cell parameters: open-circuit voltage, short- circuit current, and fill factor (FF) – a term describing how

5 Ta =273 [K]

Ta =280 [K] I/P

P mpp

I sc

Current [A]

I mpp

Ta =290[K] 1 Ta =310[K]

Ta =320[K]

V mpp

V oc

0 5 10 15 20 25 BP280 Voltage [V]

FIGURE 28.3 Current vs voltage (I–V) and current power (P–V) characteristics for a solar cell.

FIGURE 28.5 Effects of temperature on silicon solar cells.

PV Cell

PV Module

PV Panel

PV Array

FIGURE 28.4 PV generator terms.

726 C. V. Nayar et al. Ambient Temp. [300 k]

low load levels significantly increases maintenance costs and 5 G=1000 W/m 2 reduces their useful life. Renewable energy sources such as 4.5

PV can be added to remote area power systems using diesel G=800 W/m 4 2 and other fossil fuel powered generators to provide 24-hour

power economically and efficiently. Such systems are called 3.5

“hybrid energy systems.” Figure 28.8 shows a schematic of a

3 G=600 W/m 2 PV-diesel hybrid system. In grid-connected PV systems shown in Fig. 28.9, PV panels are connected to a grid through

2.5 inverters without battery storage. These systems can be classi- Current [A]

2 G=400 W/m 2 fied as small systems like the residential rooftop systems or 1.5

large grid-connected systems. The grid-interactive inverters

must be synchronized with the grid in terms of voltage and 1

G=200 W/m 2

frequency.

0 5 10 15 20 Generator Diesel

BP280 Voltage [V]

PV Panel

FIGURE 28.6 Typical current/voltage (I–V) characteristic curves for

Power Conditioning and

Load different insolation.

Control

and a constant current segment [3]. The current is limited,

Battery

as the cell is short-circuited. The maximum power condition occurs at the knee of the characteristic curve where the two

FIGURE 28.8 PV-diesel hybrid system. segments meet.