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Optimising Self Consumption System for Zero Export Installations

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With increasing proportions of renewable generated electricity feeding into the distribution network, contributing to grid instability and with high voltage grids approaching their capacity in some regions, feed-in management is the topic currently surrounding larger commercial installations. It was certainly one of the considerations for installers sizing up a 1.2 MW installation at Alan Bartlett and Sons’ vegetable processing and packing facility in Chatteris.

Recognising the importance of sustainability and the desire to reduce their carbon footprint, Alan Bartlett and Sons decided to invest £1 million in a PV system consisting of 4,000 modules and 9 FroniusAgilo 100kw central inverters. With the system now running for just over a year, the company benefits two-fold from the power of the sun – generating a whopping yield of 700 MWh a year and saving approximately 371 tonnes of CO2 annually. One hundred percent of the electricity generated is consumed by the factory, which is thought to be around 25% of their annual usage.

With zero export allowed at the time of installation (due to a large wind farm next door consuming grid capacity), the PV system was optimised to ensure no electricity can be sent to the grid. This was achieved by using current transformers and AC contactors, which mechanically isolate unwanted power on detection of export. One by one, the system switches off the inverters until electricity generation equals consumption.

This solution works well for the organisation, particularly due to  the long hours of consumption required in the summer months for their refrigeration units.

The introduction of the Fronius smart meter creates the possibility of an alternative set up to that above.  Suitable for both three-phase and single-phase measurement and more accurate than the S0 terminals of an export meter, the Fronius Smart meter is a bidirectional meter which optimises self-consumption and records  a load curve. Its rapid communication via Modbus RTU provides perfectly coordinated management of energy flows, optimising overall energy. In systems like that at Alan Bartlett and Sons this means that instead of turning off a whole 100kW inverter to prevent what may only be 1kW of export, the Smart meter monitors for any export and simply removes only the exported amount by moving the MPP point of the inverter. They could then continue making 99kW out of the inverter which would have been isolated for exporting before. The Smart Meter is preconfigured for systems with Fronius products making it a simple plug and play device. Together with Solar.web, the Fronius Smart meter makes it possible to graphically visualise the self-consumption of power.

The same setup can be used in a domestic scenario, where a 3.68 kW limit applies, but a large household with several outbuildings could accommodate a 6-10 kW system, for example.

As the costs of producing storage systems such as battery packs are reduced and become a more feasible option for commercial PV systems, installations such as this one can be optimised further to make use of non-consumed energy. Currently at the Bartlett plant, little electricity is generated at the weekend due to low consumption. With a storage solution this electricity could continue to be produced and redirected, using a Fronius Smart Meter, to a storage device.

Source: Fronius

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