System Types Explained
Although the principle is the same, yielding electricity from the sun, there are many ways that a PV installation can be installed to best suit the customer.
The Types Of System And Applications Off-Grid
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 has to 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 attractive especially for households with family at home during the day.
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:
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. 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:
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. A typical daily load profile for a residential property is shown below; With the above example 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. In the example above there is about 8kWh of surplus energy with 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.
Many electricity users in South Africa, especially industrial, pay a very high tariff when they use higher amounts of electricity than normal during peak times. Some residential users are on tariffs which are higher during peak times of the day 5:00PM – 7:00PM or 06:00AM – 09:00AM for example or more for energy usage at any time above a certain threshold, e.g. 600kWh per month. In these cases a PV system may be used to limit the amount of higher cost electricity consumed by storing energy during the hours of sunlight and releasing it during the high cost periods. This may require additional timing controls to limit the time of usage of stored energy to these higher charge periods. The type and size of the battery is essential to any grid-backup or enhanced self-consumption system with many factors to take into account.