Power Requirements

The power requirements for the system to be installed are shown in the following table

System Design

A hybrid wind/solar/battery system will be used to deal with these power requirements. The design of this power system was done with the assistance of HOMER, a modelling package that allows the evaluation of design options, principally from a financial point of view. We are constrained by practical considerations more than financial and so HOMER was used mostly to test different system designs.

HOMER allows the user to input energy resource data such as wind and solar data. AIMS have wind data recorded every half hour for Davies Reef from 1998 to 2003. HOMER can take wind data either as monthly averages or as hourly averages. I resampled the AIMS half hourly data to get hourly data to input into HOMER. The annual average wind velocity varied between 6.91ms^-1 (2001) and 7.63ms^-1 (1999) for the six year period AIMS has data for.
Daily Solar Radiation data as monthly averages were obtained from NASA's Surface meteorology and Surface Energy website. HOMER calculates hourly data based on these monthly averages. The NASA website also has daily averages for each year from 1983 to 1993. The annual daily average for this period ranged between 5.90 kWh/m²/day and 6.23 kWh/m²/day.

System Optimization
To test various system designs, I used the worst case load scenario of 130 Watts continuous power consumption as well as scenario with a 50% duty cycle for the microwave link. The year 2003 wind data was used (average of 7.12ms^-1) with sensitivity analysis over a range of 6.5 ms^-1 and 7.5 ms^-1 (HOMER scales the data for these sensitivity analyses, maintaining the overall distribution of the data). The monthly average solar data was used with a sensitivity analysis range of 5.9 kWh/m²/day to 6.07 kWh/m²/day.

Power components considered were one or two Southwest Windpower AirX Marine wind turbines, up to four 125 Watt solar panels, and up to 600Ah of battery capacity. Sensitivity of the system to a loss of efficiency of the solar panels as a result of fouling by bird droppings or salt build up was also checked, with efficiencies of 60%, 75% and 90% considered. Scenarios with 0%, 1%, 2% and 5% capacity shortage were analyzed.

In the worst case scenario, with maximum load, minimum wind and solar power (radiation and efficiency), the optimum system has four solar panels, two AirX turbines, and 600 Ah of battery capacity. This system still has 2% (~7days) of unmet demand. When the system is run with the microwave at 50% duty cycle, 100% availability can be achieved with three solar panels, two AirX and 600Ah of battery capacity. Increasing the efficiency of the solar panels to 75% makes a four solar panel, single AirX and 400Ah of battery capacity system the most favourable still with 2% unmet demand at maximum load and 100% availability with the microwave at 50% of its duty cycle. The sensitivity analysis showed that the optimum system has either three solar panels and two wind turbines or four solar panels and one wind turbine, depending on solar panel efficiency, total solar radiation, and average wind speed.

Taking into account ease of installation and maintenance issues, the system to be installed will have four 125 Watt solar panels, a single AirX Marine wind turbine and six 100Ah batteries. This system provides the best compromise for available power, ease of installation and future maintenance.

Solar Power Regulator

A Solar Power Regulator is required to regulate the output of the solar panels to provide optimum charging characteristics for the batteries. A Plasmatronics PL40 will be used. The charge cycle of this device can be fully programmed. It will also allow the output of the solar panels and wind turbine to be monitored as well as keeping track of the condition of the battery bank. The AirX Marine wind turbine has its own inbuilt regulator so the Solar Power Regulator will only be used to monitor the turbine.

Circuit Layout

The circuit for the power supply is relatively simple. It is 12V so the four solar panels and wind turbine are attached in parallel. The six batteries are also set up in parallel. Each battery has its own fuse. Diodes are used to protect the wind turbine from a reverse current. A shunt and monitoring cable from Plasmatronics are used to monitor the wind turbines output. Here is a circuit diagram (and one for the system enclosure).

Recent Changes

We have added smaller circuit breakers (7.5A) to protect the 12V/48V converter and the 12V/12V regulator. These both tripped recently when the battery voltages dropped below 8 Volts. We have also added low voltage protection with a relay controlled by the Plasmatronics PL40. This cuts power to the whole system if battery voltages drop below 11.6 Volts and turns it back on only after the battery voltage is back at 12.7V (I think - will need to check how it was programmed).