System Design  

The driving force behind the design of the energy production system was the driven electrical load.  The light source chosen was a high intensity 120 white LED array designed to operate on 120V AC power [2].  The power draw of bulb was 10 watts.  Based on an electrical system efficiency of 50%, a power generation system capable of 20 watts was targeted.

     Solar panels are available commercially in a multitude of sizes.  Their area relates directly to their power capacity as panels are composed of strings of small photovoltaic cells.  It figures that any expected contribution from the solar panel would directly reduce the required contribution from the wind turbine.

     The wind turbine was a major focus of the design effort.  Wind power available varies cubicly with wind speed, and wind power harnessed varies directly with the swept area of the turbine rotor.  A minimum efficiency for the mechanical power extracted from the wind against the total power available was set at 15%.  This efficiency was the result of an examination of available commercial designs as well as theoretical mathematical limitations on performance such as the Betz limit.  Test data published by Blackwell, et al., provided a source of efficiencies for varying Savonius rotors [3].  The Betz limit states that, because slowing the velocity of the wind during the power extraction process decreases the mass flow of air through the system, a optimum value exists where the mass flow and energy extraction are balanced to achieve the greatest possible power.  This value is proven in Wortman to be 59.3% of the available power in the wind [4].

     A design space was developed to aid in sizing a rotor size.  The power capacity of the solar panel was used as the independent factor and the resulting rotor swept area was determined.  The average wind speed for any given month in the local Rochester environment as well as the equivalent amount of sunlight available and its duration was supplied by the National Oceanic and Atmospheric Administration database (NOAA.org) [5].

Figure :  Design space determining required rotor swept area from solar panel capacity.

Readily available commercial solar panels were considered.  A 10 watt panel was selected.  A minimum swept area for the turbine rotor of 1.17 m² was found.