In Australia, most electricity is supplied by utilities or electricity corporations from power stations, via power supply networks called grids. These main grids provide power to the majority of Australians using many large coal and gas fired power stations, large hydro generation schemes and more recently some smaller scale wind farms and photovoltaic systems. Remote towns like Port Hedland, Mt Isa and Coober Pedy are not serviced by the main grid and have diesel power stations or combined diesel/wind power stations to provide their power via a mini grid.
For the purpose of this information file, Remote Area Power Supply (RAPS) systems are small scale(<50kW) self-contained units, providing electricity independent of the main electricity grid or mini grid network.
RAPS systems range from small petrol generators, able to power appliances directly, to more complex installations using only renewable energy, with a combination of both also being possible. A RAPS system that has a combination of energy sources is termed a hybrid RAPS system. Figure 1 illustrates a hybrid RAPS system using a combination of a wind generator, solar panels, petrol or diesel generator, battery charge control system, battery storage and inverter. The system chosen is dependant on each user's needs, availability of renewable resources, their preferences and often, most importantly, their budget! energy to separate the positive and negative charges in the material.
The main components can be grouped into 3 categories; components that supply power (generation equipment), components that store energy for later use (energy storage equipment) and components that convert from one form of power to another or control the flow of power in a system (power conditioning and control equipment).
Stand alone generator sets, using petrol or diesel, are typical of most RAPS systems. These generator sets can vary from small portable units to larger units installed in a dedicated power shed. The larger units will often incorporate auxiliary control equipment to
automatically start the generator on demand. Some generator sets will produce DC electricity for charging batteries directly. More commonly a generator will produce AC electricity for running appliances and electrical equipment directly.
Generator units perform best when operated near their rated output. As the load on the generator decreases so does the efficiency of the unit. If a generator set runs for long periods at very low loads significant maintenance problems can occur. In a RAPS application there are often periods of quite low load. The traditional solution to this has been to only run the diesel during high load periods and concentrate all usage of electricity into these periods. However with developments in communication and other technologies it is often desirable to have a 24 hour power supply in remote locations to keep the equipment available and running.
Rising fuel costs and the impracticality of running generators for long periods at low loads has led to the introduction of renewable energy equipment, batteries and inverter technologies, which reduce fuel costs and maintenance and provide a 24-hour power supply. The diesel or petrol generators can still be used as a backup system to meet the load directly or to charge the batteries when there is insufficient sun or wind. The level of generator usage will depend on the size of the load demand and the available renewable energy resources and equipment.
Photovoltaic modules convert solar energy (sunlight) directly into electrical energy. Throughout Australia a reasonable level of solar energy is received year-round and accurate data on solar radiation is available for many areas. The amount of energy received from the sun is quite predictable and can be used to predict the ouptut of solar modules.
A small array, which has four solar modules, can produce up to 300 watts in peak sunlight conditions.
In Australia stand-alone and hybrid RAPS system are used widely in remote areas to provide power for following situtaions: Small holiday homes and shacks, Boats and recreational vehicles, small rural farms (single homes), large station homesteads(with multiple residences), remote aboriginal communities, small mining operations and various telecommunctions applications. The total number of systems in Australia is not accurately known but it has been estimated as being in excess of 10,000 systems.
Renewable energy devices such as PV modules and wind turbines are also being used on a small scale in areas where the electricity grid is available. People who want to make a contribution to the generation of renewable energy usually own these systems, or they are demonstration sites set up by electricity utilities or equipment manufacturers. The output from the renewable energy conversion devices is 'exported' to the grid (after being converted to AC at the correct voltage and sychronised with the grid frequency) during times of excess supply. When there is insufficient renewable energy power is 'imported' from the grid. The home or building owner therefore receives credit for the generated electricity that is offset against the imported power. The rate at which the utility will buy back the renewable energy varies, with some utilities operating a net metering system, where each kWh generated is equivalent to one imported from the grid, whilst other utilities pay a price that is a premium over fossil fuel generated electricity but lower than that charged to domestic customers.