Tracking Photovoltaic System

Rensselaer has installed a photovoltaic (PV) system with 32 PV panels next to the Voorhees Computing Center, providing about 2 kw of electricity to the VCC (enough to run about 25 computers). The innovative system moves with the sun, both throughout the day and with the seasons. (You can view a computer animation of its motion).[1] During the 2001 summer heat wave, when Rensselaer's cost for electricity was five to ten times higher than normal, the PV system was continuously reducing the Institute's electric demand.

Although the electricity provided is tiny compared to the total campus energy use, the main purpose of the system is for education and research. The PV system will be used in the Solar Devices course, and will likely be of interest to those in the Embedded Controls course, since the tracking device uses the same 68HC11 computer as the class. It will also be used for research on newer and more sophisticated PV cells and provides opportunities for a variety of student projects and research.

How it Came About

Rensselaer's photovoltaic tracking system owes its existence to the combined efforts of many departments, organizations, and individuals. The PV tracker project was conceived and directed by David Borton, Adjunct Associate Professor, MANE, who has been active in the field of solar energy for over 25 years and teaches the course "Solar Devices and Energy." Dr. Borton collaborated with engineers Peter Kask, Ashwin Shah, Peter Nurse, and Rebecca Hefner ('97) of Solar Age Technologies, Inc. (SolarTech), to design and fabricate the system, which they then proposed to donate to Rensselaer. Oliver Holmes, Director of Campus Planning and Facilities Design, always interested in energy research and in ways to reduce energy costs, was enthusiastic about bringing the PV system to the campus. Through his efforts, Rensselaer agreed to provide funds for the installation on campus, and CP&FD engineer Steve Moise and student Jeremy Magliaro coordinated the process of erecting the PV tracker. Many volunteers from both Rensselaer and SolarTech worked long hours to provide the labor for the installation. As the result of a proposal submitted by Anna Dyson, Assistant Professor of Architecture, the material cost of the PV system was partially offset by a $5 per watt rebate given by New York State Energy Research and Development Authority (NYSERDA) to encourage electric demand reduction.

How it works

Rensselaer's PV array is made up of panels, each containing 36 silicon wafers in series which can produce about 75 watts of electric power in bright sunlight. (For more detailed information on how silicon solar cells work, see David Borton's talk at the Sunshine Festival, October 2001.) There are 32 panels in the array connected in 8 groups of four panels in series. The DC power is changed into AC power by an inverter in the VCC basement and fed into the building's grid.

Rensselaer's array is uncommon in that it is mechanically set up to track the sun across the sky. By doing this more direct sunlight is captured and the array produces about 25% more energy than a stationary array. Even less common is the feature that tracks the sun in two axes, as the sun moves throughout the day and through the seasons. Rensselaer's array uses the two axes that best describe and follow the sun's motion: a polar axis parallel to the earth's rotational axis and a perpendicular declination axis that follows the seasons. The polar axis rotates at an approximately constant 15 degrees per hour to follow the sun throughout the day. This rotation is provided by a gearmotor, while a hydraulic cylinder moves the declination axis. A small 68HC11 computer calculates the sun's position and controls the motion of the array by actuating the gearmotor and the hydraulic cylinder. [1]

[1]Although the system has been providing electricity to the VCC since July 2001, the computer controls for the automatic tracking mechanism were not completed until May 2002. During June and early July 2002, a problem with the gear motor that drives the daily tracking temporarily disabled the tracking feature.

Future Plans

Tracking in two axes allows for the use of PV cells that can use concentrated sunlight. PV cells for solar concentrators with concentration ratios of about 500 solve a variety of problems with flat PV cells. First, getting the same energy from less silicon reduces both the dollar cost and the embodied energy cost by a factor of about 500. Second, tracking increases the amount of solar energy converted to electricity. Third, the efficiency of the cell is increased by the increased density of charge carriers. Because of these advantages, concentrating PV systems can add significant renewable energy resources for our use. The Rensselaer tracker will be used to test a novel PV concentrator cell that has dozens of diode junctions and produces high voltage electricity. This new cell provides an opportunity for scientists and engineers to produce a tracking concentrator system which is inexpensive enough to commercialize PV in direct competition with fossil fuels.

Sunshine Festival, October 15, 2001

The PV system was the focal point of the first annual Sunshine Festival, held on October 15th, from 11 to 1:00 PM on the green between the Voorhees Computing Center and the Cogswell building. According to the students of Ecologic, the goal of this event is to provide information about, and lend support to, solar and other forms of renewable energy (i.e. wind power, fuel cells, etc.). It is intended to guide RPI in further initiatives supporting renewable energy.

The festival included demonstrations of a variety of solar equipment, tables with informational handouts, and informal presentations given by experts on topics such as solar concentrators, energy usage, social change associated with renewable energy, and global climate change. Food and drink was served, and the experts were on hand to answer questions.

 



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