Electricity From Sunshine Is Magic

Sunshine is magical. It takes knowledge of nuclear physics and the famous E=Mc2 to begin to understand the source of sunshine in the friendly nuclear reactor 93 million miles away. A lot of energy processes happen in the sun before light starts in our direction.

Part of the magic is that all of our thoughts and actions are powered by chemical reactions in our bodies. All of our food is stored solar energy that biological organisms have captured and transformed into accessible energy meals.

When winter comes, the sun heats the passive solar home I live in, and our wood stove uses solar energy recently stored in trees. Even fossil fuels represent millions of years of stored solar energy.

Electricity is another form of magic, energy we have learned to use in the last 100 years. We have AC in the walls, DC in our cars, and batteries in our flashlights and computers. Part of the magic of electricity is that it is basically pure available energy governed by the Second Law of Thermodynamics. Electricity has helped provide the knowledge and information that surround us, especially at our universities. Part of what we know explains energy conductors like the pot we put on the stove to cook our food and the insulator that makes the handle of the pot so we can pick it up.

The magic of electronics is in semiconductors that are halfway between conductors and insulators. Materials like silicon with four valence electrons are insulators at absolute zero and become conducting as increasing temperature elevates some electrons from the valence band into the conduction band. Speaking more technically, semiconducting properties can be changed by doping the silicon with periodic chart group 3 or group 5 elements to make p-type or n-type semiconductors with holes or weakly bound electrons, respectively, as charge carriers. A semiconductor diode is a junction between p-type and n-type semiconductors, which generates a potential difference as the chemical potential of the two materials seeks a common level.

A silicon solar cell, or photovoltaic cell (PV), is a flat wafer of diode. On the side facing the sun, a fine grid of wires connects the n-type material to an external wire. The back side p-type material is likewise connected to a wire to conduct electricity to some application. A photon, the quantum unit of light energy, can, if it is energetic enough, create an additional valence electron-hole pair by unbinding the electron from its atom. The electron and hole can move under the influence of the electric field creating power in the external circuit. It's a bit magical!

Rensselaer's 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. 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 computer center basement and fed into the building's grid. This magic is expensive compared to current fossil fuel power, but is not uncommon for small applications. You might even have a PV powered calculator.

The material expense of the PV panels is not the only drawback to large-scale use. It requires about 5 years of operation to pay back the energy used to make the cells. Additionally, the wafers used for PV are reject scrap from the electronics industry, and therefore the quantity of this material is not sufficient to make a dent in the energy requirements of this country.

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 which was made in India. 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.

Twenty five years ago a slight shortage in oil supply sparked interest in renewable energy sources. That interest in the form of research and development, some of it here at Rensselaer, proved the feasibility of solar concentrator technologies. Concentrating the sunlight with lenses or mirrors produces power densities that can melt metals, perform chemical processing, or make electricity through steam or other thermodynamic cycles.

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.

It takes a large area of PV to collect energy on a commercial scale. Making large renewable energy systems requires about 3 times the labor input per unit energy output as fossil fuel technologies. The cost of the labor in a PV concentrator systems is high in the developed world. However, it is not high in developing countries where there is a large supply of inexpensive labor. And developing countries are often sunny places. Solar electricity is an international priority for meeting people's needs and reducing greenhouse gas emissions.

This world, this campus, and all of us as individuals rely on the sun for many energy services. We have not yet effectively or efficiently tapped the energy the sun. Electricity from sunshine provides Rensselaer with many opportunities for education and research. Electricity from sunshine is real magic.