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Grid-connected Solar Electric Systems
Solar electricity - or photovoltaics (PV) - is the world's fastest growing energy technology. It can be used on a wide variety of scales, from single dwellings to utility-scale solar farms providing power for whole communities. It can be integrated into existing electricity grids with relative simplicity, meaning that in times of low solar energy users can continue to draw power from the grid, while power can be fed or sold back into the grid at a profit when their electricity generation exceeds the amount they are using. The falling price of the equipment combined with various incentive schemes around the world have made PV into a lucrative low carbon investment, and as such demand has never been higher for the technology, and for people with the expertise to design and install systems.
This Expert handbook provides a clear introduction to solar radiation, before proceeding to cover:
electrical basics and PV cells and modules
inverters
design of grid-connected PV systems
system installation and commissioning
maintenance and trouble shooting
health and safety
economics and marketing.
Highly illustrated in full colour throughout, this is the ideal guide for electricians, builders and architects, housing and property developers, home owners and DIY enthusiasts, and anyone who needs a clear introduction to grid-connected solar electric technology.
eBook
4 PV Cells, Modules and Arrays
In the previous chapter the many different types of PV cells and manufacturing techniques were discussed. This chapter explores how PV cells are used to create PV modules that can then be used to create a PV array, which is the principle component of a grid-connected PV system.
Box 4.1 Electrical basics
A basic knowledge of electrical terminology is essential to understanding the operation of a PV cell.
Current: represented by the symbol I, it is measured in amperes (A or amps). Current is produced by the flow of electrons; the higher the current the higher the rate of flow. There are two types of current:
AC: Alternating current is so named because the direction of the electron flow changes: electrons first flow one way and then the other - this switching continues at a constant frequency. Mains power uses AC current.
DC: Direct current does not switch but rather flows steadily in one direction. PV cells produce DC current.
Voltage: Always measured across two points, it is the change in potential energy per unit charge between those two points. It is represented by the symbol V and it is measured in volts (V).
Energy: Energy is measured in watt-hours (Wh) or joules (J) and is a measure of the ability to do work. A person eating a biscuit gains from that food energy that they can expend in doing work, e.g. walking up a flight of stairs. Kilowatt-hours (kWh) are commonly used to describe electrical energy produced by a PV system.
Power: Power is measured in watts (W) or joules/second (J/s) and is the rate at which energy is supplied. 1 watt is equivalent to 1 joule per second. Power is the product of current (I) and voltage (V):
P = I × V
Circuit: A circuit is the system of wires and electrical components (including PV modules) through which current flows. Current can only flow through a closed circuit.
Series connection: Two elements of a circuit are connected in series when they are connected one after the other, such that the current travels through them equally, while the voltage is divided between them (the largest voltage will occur across the largest resistance).
Figure 4.1
Resistors connected in series
Source: Global Sustainable Energy Solutions
Parallel connection: Two elements of a circuit are connected in parallel when they are connected across the same potential difference (i.e. the same voltage), and the current is divided between them.
Figure 4.2
Resistors connected in parallel
Source: Global Sustainable Energy Solutions
Book Chapter
9 Sizing a PV System
In Chapter 8 the choice of system components is discussed. While the client's requests and expectations of their system will give the designer a rough idea of the system size, it is important to carry out detailed sizing calculations to ensure that the array is matched to the inverter's input specifications and that all system components are appropriately sized to suit the site-specific conditions.
Book Chapter
5 Inverters and Other System Components
In addition to the PV modules, a PV system requires other components in order to interact effectively and safely with the power grid. This chapter discusses inverters, PV combiner boxes and metering (net and gross). PV arrays are covered in Chapter 4 and the PV array frame is discussed in Chapter 6.
Book Chapter
7 Site Assessment
Conducting a site assessment or site survey is an important step in the design and installation of a system. During the site assessment the installer should collect all the necessary information required to optimize system design and plan for a time-efficient and safe installation. A site assessment aims to determine the location of the PV array, the roof specifications, the amount of shading, the available area and other considerations.
Book Chapter
11 System Commissioning
The process of testing a PV system to confirm that it is producing electricity and interacting correctly with the electricity grid is known as system commissioning. Before an installer leaves the system to the customer it should be tested and inspected to ensure that the system is compliant with national and local standards and regulations, that all components have been safely installed and that all components are functioning as expected. Many utilities have rules or procedures that must be followed during the system commissioning process and in some cases the utility may wish to conduct a commissioning inspection. These requirements should already have been discussed with the utility when the interconnection agreement is made (see Chapter 10).
Book Chapter
8 Designing Grid-connected PV Systems
Following a successful site assessment at which all necessary information is collected, the system can be designed. The selection and design of system components are covered in this chapter. Most of the equipment described here provides safety as well as guaranteeing system longevity. It is important to note that in most countries all major points of system design are covered by national codes and standards, and designers need to familiarize themselves thoroughly with these.
Book Chapter