IMPACT OF PHOTOVOLTAIC CELL MATERIAL IN PV CHARACTERISTICS

The impact of photovoltaic cell material in PV characteristic is discussed in this section by considering a PV system with resistive load as shown in Figure 5.7.

The set-up consists of a PV array followed by a DC-DC boost converter feeding a resistive load. The boost converter is employed for MPPT operation as it presents a variable resistance to the PV array. In addition, the voltage generated by the PV string is not sufficient to feed the load directly. Here, boost converter plays the major role to increase the voltage to the required level.

The operation of boost converter for MPPT is as discussed below. The input-output relations of the voltage, current, and resistance can be written

as

FIGURE 5.7 Schematic diagram of the PY system feeding a resistive load through a PY array and boost converter.

Here, R_?y and R_o are the input and output resistance of the PV panel and D is the duty ratio of the boost converter.

When a constant resistance is connected at the output of the converter, the input resistance (on an average) is dependent on the duty ratio D of the converter as given by eq 5.5. By varying the duty ratio, the PV module can be made to operate at different voltages, and the operating point is given by the point of intersection of the load line and the I-V characteristic as depicted in Figure 5.8.

Thus, as the load line varies from R_{ to i?₄by varying the duty ratio from D_l to D_r various operating points on the I-V curve can be accessed. Out of these points, one of the points will be the MPP, corresponding to a load of R . Thus, during MPPT, duty ratio is varied such that the reflected load resistance on the PV array is R

^J moo

FIGURE 5.8 Variation in PY operating point with reflected load resistance.

Moreover, from Figure 5.6 it is clear that the series and shunt resistances, R_s and i?_sh, respectively, and the module’s temperature coefficient for current will vary based on the irradiation as well as the material of the PV cell; and hence, the PV power, open circuit voltage, short circuit current, MPP voltage, and MPP current. Similar is the case with i? , the reflected load resistance on the PV array at MPP.

CLASSIFICATION OF PVMODULES BASED ON MATERIAL USED

Different types of PV modules available in the market and its classification based on the material used is discussed in this section. Solar PV modules currently used in PV power plants are of crystalline silicon and thin film modules (Fig. 5.9). Based on the process of growth of silicon crystal, crystalline modules are further classified as monocrystalline and polycrystalline modules. Whereas, thin films modules are classified based on the compounds used in fabricating the modules.

(A) Crystalline Silicon (c-Si) Modules

c-Si modules are made of solar cells encapsulated between a transparent front glass and a backing material made of plastic or glass. As mentioned above, crystalline modules are further classified as monocrystalline and polycrystalline modules. The monocrystalline silicon wafers are made through Chocklarsky process by slicing a large single ciystal ingot. Whereas, polycrystalline silicon wafers are made by slicing the cast molten multi-silicon ingots and are larger than mono-crystalline wafers. Polycrystalline cells are cheaper than monocrystalline cells, but are less efficient than monocrystalline ones.

FIGURE 5.9 (a) Thin film, (b) monocrystalline, and (c) polycrystalline PY modules.

(B) Thin film Modules

Thin film modules are broadly classified as amorphous silicon, cadmium telluride, copper indium (gallium) di-selenide, and heterojunction with intrinsic thin film layer (HIT) as discussed below.

(a) Amorphous Silicon (a-Si)

hi a-Si module, atoms form a continuous random network unlike as in c-Si. a-Si modules are thinner and are available at low cost than c-Si owing to the higher efficiency of a-Si to absorb light than c-Si’s, and also due to the possibility of depositing on both rigid and flexible low-cost substrates. Thus, facilitating a-Si as an ideal choice for different applications where the concern is of low cost rather than efficiency.

(b) Cadmium Telluride (CdTe)

CdTe modules are derived from cadmium and tellurium. This module is fabricated by depositing a semiconductor film stack on a transparent conducting oxide-coated glass. These modules are capable of producing high energy output across a wide range of climatic conditions.

(c) Copper Indium (Gallium) Di-Selenide (CIGS/CIS)

CIGS/CIS is a semiconductor module derived from copper, indium, gallium, and selenium with a capability of capturing more light than c-Si. However, these modules need thicker films than a-Si PV modules. Also, these modules are capable of offering the highest conversion efficiency among all the thin film PV module technologies.

(d) Heterojunction with Intrinsic Thin Film Layer (HIT)

Heterojunction with intrinsic thin film layer solar cell is composed of ultrathin amorphous silicon layers as the surrounding material for a mono-thin-crystalline silicon wafer. Even though HIT modules are more expensive than crystalline modules, their reliability and efficiency are far more than crystalline modules.

A comparison between different types of PV modules is as furnished in Table 5.2 shown below. Crystalline silicon dominates the solar market globally because it has the highest energy efficiency and longer lifespan compared to other PV modules.

TABLE 5.2 Comparison of PY Modules.

IMPACT OF AGING OF PV MODULES (DEGRADATION) IN RENEWABLEPOWER GENERATION

In addition to the high cost of PV panels, yet another challenge in the solar power sector is the aging of the PV modules leading to the degradation of the photovoltaic (PV) module, which in turn reduces the output power, owing to the extreme environmental operating conditions. Due to degradation, there is a considerable change in the series and shunt resistance of the PV module leading to reduced output power. PV modules experience fast degradation than expected since, the PV modules are operated for a long time in the outdoor conditions without proper maintenance and due to its corrosive nature.¹ However, the degradation rate depends upon the operating conditions.

Considering the diverse climatic conditions, the PV modules exposed to hot and dry climatic zones experience higher degradation rate that is 1.55% per year.² The degradation of PV panel can be categorized as optical degradation, cell degradation, degradation of cell/module interconnects, light induced degradation, temperature-induced degradation, and potential induced degradation.³ Owing to the degradation in the PV module, there will be considerable changes in the shunt resistance (i? _h), series resistance (R_ie), voltage at maximum power point (V ), current at maximum power point (/_mpp), and fill factor leading to severe power loss in the PV system. Degradation can lead to either increase in R,_e or decrease in R,_h.

EFFECT OF RSH AND RSE DEGRADATION ON VMpp

Due to degradation, as R,_h decreases from its nominal value with a constant irradiation for example, at 1000 W/m² at standard test conditions (STC), V of the panel increases and vice versa. Similarly, as the series resistance R increases from itsnominal value at STC, the V of the panel

se mpp ^A

also decreases and vice versa.

EFFECT OF DC LINK CAPACITOR MATERIAL IN POWER CONVERTERS/SOLAR INVERTER

Inverters are inevitable for interfacing PV modules to the utility grid, which plays key role in the efficiency, cost, lifetime, and stability of PV systems. The performance of PV inverters mainly depends on the power electronic devices used in the power converters. Si devices are widely available in market in all forms of semiconductor devices with a maximum junction temperature limit of 150°C. Hence, while designing any semiconductor device using Si, the temperature of the device should be below the specified limit. Moreover, fabricating higher voltage rating Si MOSFETs is not feasible due to the high cost as the device requires a large silicon die area as the breakdown voltage increases. In addition, the switching frequency of Si devices is also limited due to the heat generated by the devices. Due to the above-mentioned limitations of Si devices, now people are looking for power electronic devices with reduced size, weight, cost, power density, and improved efficiency close to cent percentage. Hence, there is an increased attention toward other wide bandgap semiconductors such as silicon carbide (SiC), Gallium arsenide (GaAs), Gallium nitride (GaN), Diamond, etc. A few properties of these wide bandgap semiconductor materials are given in Table 5.3.⁴

TABLE 5.3 Properties of Wide Bandgap Semiconductor Materials.

It is clear from the table that, diamond is the best choice among these materials. However, cost is the barrier in designing semiconductors with diamond. Finally, considering the practical difficulties, now manufacturers have chosen silicon carbide (SiC) in place of Si devices. However, GaN is also a possible solution.

SiC-based devices were commercialized by CREE Inc. in 1987 with SiC-based Schottky diodes with a voltage blocking capability of 600 V. Whereas, the voltage blocking capability of Si devices is limited to 300 V. Nowadays, SiC semiconductor devices are being designed for high- temperature, high-power, and high-radiation conditions as well unlike Si-based semiconductors. This invention is making revolutionary changes in the semiconductor design field since SiC devices are suitable for high voltage and temperature ranges with superior switching characteristics than silicon-based devices.