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ESS – Energy storage systems
Operating totally off the grid requires a large capacity battery array capable of powering the property during periods of low irradiance in winter and an inverter capable of supplying the maximum load ever required at one time. This requires a significant investment in PV modules, inverters and especially batteries which cannot normally be justified if there is a good quality grid connection available at the property. An off-grid system is well suited to rural areas with little or no grid connection but is unlikely to be a viable solution in a well-connected urban area. Should however fixed connection charges for electricity become more common and higher then disconnecting from the grid may become a more viable option in the future.
A pure grid-tied system with no storage or load management for a user with fixed rate power charges is a viable option for South Africa but the system will need to be significantly under sized to minimise the wasted energy generation as typically no surplus power can be exported. Essentially the PV system must be sized to generate only sufficient power for the base load during the day, i.e. the fridge, freezer, pool pump and other permanently on devices. The low investment cost of a small PV system with a high self-consumption rate should make them quite especially for households with family at home during the day.
If frequent load shedding continues each winter, then there will be continued demand for grid-backup systems that can operate with no grid for prolonged periods of time. Adding a battery inverter or a hybrid inverter along with a battery makes it possible to combine the energy from the PV system with that from the stored battery to power at least the essential loads in the property. The size of the battery required depends on the rating of the essential loads to be driven from it at times of no solar power being available.
Typically, a residential system will generate the most power during the day when household consumption is not at the maximum. Without the ability to export surplus power the only options are to reduce the size of the PV system so that excess energy is minimised, but this also reduces the usable energy or to store the energy until needed. With an example of reducing the size of the PV system by 50% would all but eliminate the surplus but would reduce the useful energy generated by 30%. The storage option could take the form of a battery system or using the surplus power to heat the hot water geyser. For example, the surplus energy could be used to heat the hot water with an insulated tank and/or recharge a battery for use in the evenings. Hot water heating is a very cost-effective option which with a correctly sized PV system could provide the best return on investment. A large battery solution may add significantly to the cost and would have a long financial payback time at current electricity prices. The use of a time switch on a washing machine and/or dishwasher can help to maximise the use of generated energy during the sunshine hours.
Load shedding typically occurs during the evening peak in winter from 5:00PM – 10:00PM so there will normally be little or no solar power available to supplement the battery. Shown below is a typical system layout for a grid-backup system using a Solar PV inverter and a Battery Inverter which gives maximum flexibility in the system design and can be retrofitted to an existing Solar PV system: An alternative solution for new installations is to use a combined PV and battery inverter, commonly referred to as a hybrid inverter. This is a multi-function device that includes all the functions required to configure a grid-backup solution.
An advantage of hybrid systems is that it can control and balance the available sources of energy: