INTRODUCTION

Characterization and testing during and after manufacture play important roles for ensuring quality and performance [1, 2] of Solar Photovoltaic (SPV) modules. The in-situ characterization during various process steps ensures that good quality modules with acceptable power output are produced. Testing of SPV modules has additional aspects of authenticity and accuracy of the test results. As the output power of the finished modules decides the DC output at system level, some standards and protocols are to be followed during testing to ensure that correct values are measured. Standards are applicable to the tester, which is also known as a ‘Sun simulator’. The protocol demands that the tester is always calibrated against some reference module during testing of finished modules. The reference modules are to be certified by authorized centres such as NREL, Fraunhofer, CIMET, etc. The modules are tested and the electrical parameters, including power are rated under Standard Test Condition (STC), which is 1000 Wm-2 irradiance incident normal to the plane of module face maintained at 25°C. These protocols and standards ensure that the testing of any module produces the same electrical characteristics, within the accuracy band of the tester used, wherever the module is tested. The accuracy required for the Sun simulator is also specified through a standard. These are very important as the accuracy of the rated electrical behaviour, particularly the power output of the modules, ultimately decide how much energy can be extracted from the SPV system using these modules. Inaccurate measurement of power can have serious financial implications for the seller if the measured power is less than the actual value and for the buyer if the measured power is larger than the actual value. It is also important to measure other electrical parameters, such as VOC, ISC, Vm and Im as the string design for SPV system (Chapter 9) requires accurate values of such parameters. It is possible to have the value of the power output very close to the rated value, but other electrical parameters may have large deviations/inaccuracies. The test protocol and standards addresses such issues as well.

Reliability determines the long-term behaviour of any device or product such as a SPV module. This is particularly important as the modules are supposed to produce power for 25 years or more.

INTRODUCTION

Solar Photovoltaic (SPV) modules occupy an important position in the value chain [1–5] (see Figure 9.1). Crystalline silicon (c-Si) is currently the preferred technology with a market share of about 85%. c-Si modules are made using crystalline silicon (Si) solar cells as the starting material. Several such cells are connected to make modules. The manufacturing process for c-Si modules is less complex than that for thin film modules. However, the value chain is quite long (see Figure 9.1) and more process steps in cell manufacture are required prior to module manufacturing. There are also processes, such as single crystal growth in the value chain, which require a substantial amount of electrical energy.

Thin film modules are made with an entirely different approach. These modules are made using a full size substrate (actually superstrate), typically glass with transparent conductive coating and use deposition techniques such as Plasma Enhanced Chemical Vapour Deposition (PECVD). For a-Si cells, layers of p, i and n are deposited sequentially to form the junction for PV conversion. Expensive and energy-intensive crystal growth required in c-Si technology is thus avoided. Historically, CdS/Cu2S were the first thin film cells invented in 1954. But, these were not commercially successful due to low efficiencies and degradation with time. Nowadays semiconductors such as amorphous Si (a-Si), CdTe or CIGS are used in thin film cells. Amorphous-silicon uses PECVD deposited a-Si as the active material. Single, as well as tandem junction a-Si films can be used to form a SPV module. A composite technology using a combination of a-Si and c-Si, called Heterojunction with Intrinsic Thin Layer (HIT) has also been developed. Cadmium Telluride (CdTe) and Copper Indium Gallium Selenide (CIGS) are the other two thin film materials that are being used for commercial SPV technology. Thin film technology has a much shorter value chain with lower electricity consumption than c-Si technology, PECVD being the only complex process. The cost per WP and payback period of thin film technologies is therefore lower than for c-Si technology. Another difference is that the temperature co-efficient of power output is less for thin film cells. This is an advantage in a tropical country such as India. Nevertheless, c-Si is still preferred due to higher efficiency and reliability.

INTRODUCTION

An overview of the Solar Photovoltaic (SPV) systems has been presented in Chapter 9. The entire system [1–9] consists of: (a) solar field; (b) structure; (c) Balance of System (BOS) comprising DC-DC converter, Maximum Power Point Tracking (MPPT), inverter/ Power Conditioning Unit (PCU) and other accessories such as LT panel, cables, combiner box, connectors, etc.; (d) storage, if required, comprising battery and charge controller and (e) transformer for on-grid systems. A solar field provides a varying DC output decided primarily by insolation and ambient conditions. The design and implementation of the solar fields consisting of solar modules connected in series and parallel, known as ‘string and array design’, has been discussed in some detail in Chapter 9. Structures are required to mount the modules. The structure for a fixed system is relatively simple. Tracking structures are more complex as these require provision for movement of the modules according to the Sun's position. Motors with movable structures are, therefore, required for such installations. The movement of the motor can be programmed to ensure that the Sun's rays fall perpendicular to the module face whenever possible. Dual axis tracking is even more complex than single axis tracking. It may be noted that the power required for driving the motors are provided from the power generated from the SPV plant.

The DC-DC converter along with MPPT takes the output of the solar field as an input and converts this to a maximum possible stable DC output. In case storage is provided, a battery is required to store some amount of energy for later use. A charge controller is also required to manage the charging and discharging of the battery. The inverter provides the AC output to the corresponding loads. A solar inverter consists of DC-DC with MPPT control and DC-AC converters. A Power Conditioning Unit (PCU) is essentially a solar inverter along with a charge controller. Accessories, such as cables, connectors, combiner box, LT panel, etc., are required to implement the string and array design (DC side) and manage the interface between inverter/PCU output and load (AC side). An on-grid system, which is also known as ‘SPV power plant’, sends the generated power to the electricity grid local substation.

INTRODUCTION

Our conventional energy reserves are limited and have severe environmental impact. There has been all round focus on the development of renewable energy primarily due to these facts, as well as from the perspective of energy security, climate change and energy access. Solar energy has been the Earth's most available energy source, capable of providing many times the total energy demand. Solar Photovoltaic (SPV) deals with conversion of sunlight into electricity. Governments across the world have realized the importance of solar power and over 60 countries have introduced feed-in tariffs, capital subsidies and incentives for productions to promote wider adoption and advancement of SPV. The average growth rate of global SPV capacity has been close to 40%, with the total installed capacity now approaching 350GWP. Renewable energy initiatives, particularly SPV, have picked up pace in India also. The Jawaharlal Nehru National Solar Mission (JNNSM) has been a major initiative of the Government of India to give an impetus to the domestic solar power industry. It sets an ambitious target of 20 GW of solar power capacity by 2022. Several State Governments have announced independent policies in SPV.

Solar PV systems [1–7] occupy a very important place in the SPV value chain (Figure 9.1). As it comes at the end point of the value chain, it decides the amount of power finally supplied. The power generated by a SPV system depends on the previous operations of the value chain as well. In the crystalline silicon (c-Si) technology, the type of wafers (mono or multi) and the efficiency of the solar cells, decided by the cell manufacturing technology, play a very important role. Assembly of the solar cells to make modules has been somewhat straightforward for c-Si technology. Thin film technology (a-Si, CdTe, CIGS, etc.) takes a different route in the value chain diagram (Figure 9.1). The modules are made directly by the Chemical Vapour Deposition (CVD) technique to deposit thin layers of appropriate materials on a conductive glass substrate to form p-n junction solar cell. In this case also the efficiency is decided by several factors such as material (Si, CdTe, CIGS, etc.), structure (amorphous, microcrystalline, etc.) and configuration (single junction, tandem junction, etc.).