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Intro
January 23, 2019 40 Comments

There are two general categories of PV systems:

  • Grid-connected PV systems, which are connected to the electric grid
  • Off-grid PV systems, which are not connected to the electric grid

Left: Grid-connected systems feed electricity into the national electricity grid. Right: Off-grid systems produce electricity that is consumed on site.

The two principal categories of grid-connected and off-grid systems also have further sub-categories. The overwhelming majority of systems will fall into the above categories. To understand how a solar system functions, it is essential to be clear about which type of system is being considered.

Intro
January 22, 2019 One Comment

There are two types of electricity:

  • Alternating current (AC) electricity, in which the direction of the electric current changes 50 or 60 times a second. AC electricity is delivered by the national electricity grid, diesel generators and inverters.
  • Direct current (DC) electricity, in which the direction of the electric current does not change. DC electricity is delivered by photovoltaic modules and batteries.

An inverter converts DC electricity to AC electricity. Most appliances run on AC electricity.

Most solar PV systems have inverters, either to convert DC electricity from the PV modules into AC electricity, or to convert DC electricity from batteries into AC electricity.

Sources of alternating current (AC) electricity and direct current (DC) electricity.

Symbols for alternating current (AC) electricity (left), direct current (DC) electricity (centre), and an inverter (right).

Intro
January 20, 2019 One Comment

The difference between power and electrical energy is very important when discussing PV systems and energy efficiency. Do not get them confused.

Power (P) is the rate at which energy is used by electrical equipment or produced by a generator:

  • The unit of power is the watt (W)
  • 1 kilowatt (kW) = 1,000 watts = 1 kW
  • 1 megawatt (MW) = 1,000,000 watts = 1 MW = 1,000 kW
  • 1 gigawatt (GW) = 1,000,000,000 watts = 1 GW = 1,000 MW = 1,000,000 kW

Energy (E) is the power (P) consumed by electrical equipment or produced by a generator over a period of time (t):

  • The unit of energy is the watt-hour (Wh).
  • Energy (watt-hours) = power (watts) × time (hours) or E = P × t
  • 1 kilowatt-hour (kWh) = 1,000 watts-hours = 1 kWh

Example 1

A 300 W machine is on for 8 hours.

How much energy will the machine consume?

Energy consumed = 300 W × 8 h = 2,400 Wh = 2.4 kWh.

Example 2

A 100 W computer is on for 8 hours.

How much energy will the computer consume?

Energy consumed = 100 W × 8 h = 800 Wh = 0.8 kWh.

Intro
January 17, 2019 No Comments

There are many reasons to install a PV system.

Main reasons for installing a PV system.

The main reasons are:

  • Economic:
    • It may be cheaper to generate one’s own electricity than purchase it from the grid.
    • To reduce electricity bills.
    • To sell the electricity generated by the PV system.
    • It may be less costly than using a diesel generator.
  • Energy security and access:
    • To have electricity if the electric grid shuts down.
    • To have electricity where there is no grid supply.
  • Ease of operation in comparison with diesel generators:
    • Diesel generators require a regular supply of fuel and regular maintenance.
    • PV systems require no fuel and little maintenance.

Environmental-protection reasons include:

  • Reduction of carbon-dioxide emissions from the burning of fossil fuels.
  • Reduced water usage associated with the cooling of conventional power stations.
  • When solar electric replaces or reduces the use of diesel and petrol generators: noise reduction, improvement of local air quality, avoidance of soil/ground/water pollution caused by fuel spillages, and reduction of environmental impact caused by the transport of large quantities of fuel.

However, reasons for installing a system differ according to the type of system which is installed; whether it is grid-connected or off-grid; whether there are batteries or a back-up generator; the size and location of the system; and other factors. When selecting, designing and sizing a grid-connected solar electric system, it is important that the electricity tariff structure of the utility electric grid is taken into consideration to ensure the system makes economic sense.

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