Abaokoro, North Tarawa, Republic of Kiribati [an island nation in the central tropical Pacific Ocean]
The client’s brief was to design a system to replace the existing failing 24v Stand Alone Power System [SAPS] with an energy system that would be able to supply the existing 9.0kwh daily maximum demand and future needs without generator backup, and site adult occupancy varying between 3 and 12 for extended periods.
The site had a fixed occupancy of 3-4 persons, which regularly trebled, servicing a preschool, providing laser photo copy service to a local school with no power services, and a local family of 5.
Due to transport considerations – open canoe and tidal beach landing – the maximum weight for any single item had to be 100 – 110kg, and all items had to be sourced and shipped from Australia.
To add to the complexity, the design had to be completed without a site visit.
Continuous communication with client and others who had been to the site helped to determine the maximum demand and optimum architectural layout.
This resulted in a 20kwp ground mount array supplying a hybrid DC/AC coupled using x2 DC MPPT Charge Controllers and 3 x Grid Tie Inverters supplying a 48v 600ah Lithium Phosphate [LiFePo4] battery Energy Storage System [ESS] controlled by a Positronic Solar Battery Management System [BMS].
The energy system is capable of delivering 23kwh to 20% depth of Discharge [DOD] but designed to deliver to 45% DOD via computer controlled load management. This excluded the “daylight AC coupled generation capacity”.
A remote monitoring protocol also allows offshore system supervision. When local satellite conditions improve, this monitoring will be in real time.
A Stand Alone Power System [SAPS], usually fully recharged within 2.5 – 3.0 hours of sunrise.
The AC coupled component providing approximately 100% of the daytime load, with a nighttime battery depth of discharge of 15-20%.
This has allowed the client use of “regular” kitchen appliances: microwave, electric kettle, toaster, sandwich toaster and mixers, in addition to more regular use of the washing machine; something they had not been able to do with their previous system set up.
Some client comments:
- “9.00am – fully charged”,
- “Started using the washing machine at 8.30am”,
- “Overcast all day 99% by lunch time”,
- “Rained all day: 70mm, 100% charged by lunch time”,
- “Had toasted sandwiches for lunch!”