A Solar & Wind Powered Outdoor
Lighting Solution
The Test Results
The system was installed in an open space on the Rochester Institute of Technology campus on Wednesday, April 27th, 2005. Following installation, data was taken to establish system functionality. The test data discussed was recorded on May 3rd, 2005.
Over a 30 minute period, the voltage and current emitted by the solar panel was measured and recorded. The measurements are plotted in Figure 6.
7: Plot of solar power achieved against time.
For much of the plot, the amount of power produced by the panel exceeds the 10 watt rating of the manufacturer. Solar panels function best when new. The manufacturer rating may have been derived from system performance after extended usage. Future data may show performance degradation.
Data was taken for the generator powered by the turbine rotor as well.
8: Generator voltage plotted against wind speed.
The data for the generator is presented as the voltage and not as electrical power. Preliminary current measurements for the initial electrical setup between the generator and the battery were poor.
The data for the generator has many facets. Data points clustered on the Y axis come from wind speeds below the anemometer start speed. The rotor was able to spin at wind speeds lower than the anemometer was able to measure. The general diffuseness of the points can be attributed to the variation in physical properties of the anemometer and wind turbine rotor. The response to increased wind speed was faster in the small anemometer than the larger rotor. These inertial effects can cause the rotor to lag behind the anemometer. The overall trend was logically for an increase in rotational speed, and with that, generator voltage, as the wind speed increased.
Using the generator calibration data and the generator voltage during testing, a rotational speed for the generator was calculated. This speed, given the 1:1 ratio chain drive, is then identical to the rotational speed of the rotor. Given the 24” radius of the turbine, a tangential velocity at the rotor tip in miles per hour is easily calculated. The ratio of this tangential speed to the wind velocity given by the anemometer is a key operating parameter of a wind turbine. Data given in Blackwell, et al [3] showed the most desirable tip speed ratio in a simple Savonius turbine to be in the vicinity of .7 to .9.
: A plot of
tip speed ratio against wind speed.
As wind speed data increases, the tip speed ratio converges to values between .6 and .8. Altering the electrical performance of the generator can alter the tip speed performance, but a custom built generator was not a feature allotted for in the budget. To gain a true idea of the steady state tip speed ratio, a controlled wind source as those found in a large wind tunnel and used in Blackwell, et al, needs to be applied.
Structurally, the original design had some weaknesses. First, a horizontally mounted chain drive is very sensitive to alignment. Over night between the first and second day of testing, the adapter attaching the driven cog to the generator shaft came loose and the chain came off. Enough force was generated before the chain came off to grind the adapter against mounting bolts wearing a shoulder into it. A new adaptor was machined to tighter tolerances and a chain tensioner was added before additional data was taken.
The rotor top and bottom plates, composed of fiberglass sheets only .030” thick, tore where they were not supported by blades. While these nonfunctional parts did not alter rotor performance, the vibrations they caused before being removed loosened bolts throughout the structure. Bolts were retightened with a thread locking compound.
Gusts during testing ran upwards of 20 miles an hour. Additionally, it rained during extended periods of time. Neither damaged the structure with the exception of the areas mentioned. Plastic cases and conduit used to conceal electrical components performed well and the concrete foundation and guy wires held the post well.
CONCLUSIONS
The Wind Solar Project Team designed an outdoor area light powered by a hybrid electrical system of wind and solar power. The prototype design was fabricated and installed.
Following installation, preliminary data was taken to observe system performance and characteristics. Solar panel data was in agreement with expectations, showing a power delivery of 9.5 to 12 watts. Wind power data was less conclusive. The generator voltage showed an increase with wind speed, indicating an increase in rotor speed with wind speed. Inertial effects and other non-ideal system properties may have contributed to disperse the data. Similar spread was seen in the tip speed ratio, a parameter often used to characterize rotor performance. As wind speed increased, data points converged to more reasonable values.
The chief shortcomings found in the initial testing were the inconsistent current measurements in the generator circuit and the structural integrity of the rotor. Despite the damage to the rotor, the functional surfaces remained intact. Problems with the chain drive were discovered but remedied.
FUTURE WORK
Given the iterative nature of complex system design, the 2004-2005 Wind Solar Project Team has evaluated the current prototype to target the largest areas for improvement and opportunities for additional research.
To improve the functionality of the system, a few key elements should be re-examined. Paramount in these is the generator to battery charger circuitry, which performed sporadically at best. The voltage step up became necessary when a mechanical step up, i.e. a gear ratio on the order of 20:1, was inappropriate for the generator utilized. The generator used was a donated model. A generator customized to this particular application would improve performance.
The system was designed to allow for the interchangeability of components, namely the rotor. Initially, simple Savonius designs of varying height to diameters can be tested and evaluated. Vertical axis geometries of vastly different design can then also be applied and compared.
