Grid Connected Systems

The solar arrays at the University of Queensland’s St Lucia Campus are grid connected in that they feed power back into the university’s low voltage (415 volts) network. If a particular array generates more power than its host building can consume at a given time, then the surplus can be exported to other buildings.

Special grid tie inverters are used in a grid connected system as their AC output has to be synchronized with the frequency and voltage of the grid. Grid connected systems often have one or more meters that are able to measure the amount of electricity fed back into the grid. In Queensland, a feed-in tariff provides households with a credit for electricity exported to the grid when a surplus over household needs is generated by the home’s PV panels.

The University of Queensland St Lucia Campus is quite extensive and its electricity demand is large. Thus, it is highly unlikely that enough energy would be generated to enable feed back to the surrounding off-campus utility grid. Nonetheless, the output from the solar arrays is metered for energy management and reporting reasons, and this metering forms the basis of the PV Data Display.

During hot summer afternoons, the PV arrays at St. Lucia, for example, will reduce the campus peak demand by a considerable 6%. This reduces the pressure on the neighboring utility network during a period of high demand, thus extending the  UQ's PV arrays to the wider community.

Although this is not the norm, grid connected PV systems can incorporate batteries or other forms of energy storage as pressure grows to reduce peak demand and shift loads to off-peak periods. Presently, domestic scale PV systems usually supply energy to the utility grid during the day (when few people are home) and then draw power from the utility grid at night when the PV system cannot generates electricity. In a sense, the utility grid can be thought of as a "battery". For research purposes, the University of Queensland has been given a "RedFlow" grid connected zinc-bromine flowing electrolyte battery. This battery takes and stores electricity from the Multi Storey Car Park 2 PV Array. Currently, it is operating at partial capacity. Ultimately, it will have a storage capacity of around 400kWh. This battery is being used to research the issues involved in integrating large batteries and PV installations.

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