Grid connected Photovoltaics

Grid-connected rooftop system on Warf Theatre, Sydney

Photovoltaic (PV) cells are a clean renewable source of energy that have been used in stand-alone applications for many years. With the growing concern over greenhouse gas emissions and other environmental issues, renewable energy sources such as PV are increasingly being connected to the electricity network. Europe and Japan are at the forefront of grid-connected PV development, and systems in Australia have grown rapidly in recent years.

GRID-CONNECTED SYSTEMS
Grid-connected PV systems can vary greatly in size, but all consist of solar modules, inverters (which convert the DC output of the solar modules into AC electricity), and other components such as wiring and module mounting structures. Some of the first grid-connected systems consisted of several hundred kilowatts of PV modules laid out in a large centralised array, which fed power into the local high voltage electricity network in much the same way as a large thermal generator. In recent years, it has become increasingly common for home owners to install a small PV system on their roofs to supply some or all of their electricity needs.

For a small grid-connected rooftop PV system, the power produced by the array during the day can be used to supply local loads, and the excess energy is fed into the local grid for other customers to use. At night, the local loads are simply supplied by the grid. If the PV system is large enough, it can supply more energy into the grid than is used by local loads. Instead of receiving a bill every month, the customer would then receive a cheque from their utility for generating this electricity.

Grid-connected rooftop photovoltaic system

Distributed grid-connected PV systems offer many benefits to both the owner of the system and the utility network. For many owners, the main attractions are self sufficiency and the environmental benefits of using renewable energy. The simplicity of the system also means the owner does not need energy storage in the form of batteries – essentially the grid is acting as a storage device. Being a modular system, it can also expand easily as requirements or available capital grow.

The modularity of PV systems offers further benefits. The production costs for some PV system components are related to volume of production, meaning that a large number of small identical components can be cheaper to make than one big component. This means that a small PV system can be as cheap or in some cases less expensive than a large system. Also, many small systems can be distributed throughout an electricity network rather than being centralised in one location. This allows the electricity utility to take advantage of locations where the value of electricity is greater, such as at the end of a long and inefficient transmission line.

On many electricity networks, peak loads coincide with peak solar power production. In regions of California, for example, peak loads occur on summer afternoons because air conditioners are used extensively. Peak loads are more expensive to satisfy than other loads, and thus the electricity generated by a PV system during a peak load is of greater value than that generated at times of low demand.

A grid-connected PV system offers other potential cost advantages when placed at the end of a transmission line, since it reduces transmission and distribution losses and helps stabilise line voltage. PV systems can also be used to improve the quality of supply by reducing 'noise' or providing reactive power conditioning on a transmission line. When all these advantages are considered, well-positioned grid-connected PV systems are already economically viable, even though further cost reductions are required to make PV systems economic over the entire electricity network.

Many utilities are developing buy-back policies that ensure private generators are paid fairly for the electricity they sell, with some opting for rate-based incentive schemes, while other bodies prefer net metering or avoided cost payments. Integral Energy in NSW has initiated one of the more promising energy buy-back schemes in Australia. It uses the net metering process, but if the PV system produces more electricity than required by the site, Integral Energy will buy back the excess at a rate marginally lower than the standard electricity retail rate.

The availability of low-cost high-quality inverters has made grid connection of small PV systems feasible. These inverters convert the DC electricity generated by the PV system into AC grid electricity. Recent developments mean that even smaller low-cost units can be individually incorporated into PV modules. Built-in electronics would then allow such 'AC modules' to be interconnected and grid connected with minimal costly external circuitry or protection equipment.

A variety of grid-connected PV systems have been installed throughout the world. In 1990, Germany began its '1000 Rooftops Program' where 1-4 kW PV systems were installed on 2250 residences. In 1997, Japan installed 3 kW systems on 9400 rooftops, while the USA planned tPV systems on 1,000,000 rooftops. PV cells are being incorporated into roofing materials, cladding and windows. System costs can be further reduced in this way by offsetting them against the cost of building materials.

20 kW grid-connected photovoltaic system at Kalbarri in Western Australia

The use of grid-connected PV systems is rapidly growing in Australia, as some home owners install their own systems and electricity distributors offer customers 'green electricity' generated by renewable energy sources. EnergyAustralia is at the forefront of these developments. It commissioned a 400 kW-capacity solar farm near Singleton in 1998, which provides approximately 550,000 kW hours of electricity. As PV prices continue to decrease and inverter technology develops further, grid-connected PV systems look set to play a substantial role in satisfying our future electricity needs .