PV Energy Payback

by Justine Sanchez

Photovoltaic technology is a fantastic miracle of science that silently converts sunlight into streaming electrons that can be used to do work. While sunlight magically falls from the sky, PV modules and their associated components do not—each consumes energy and resources along every step of the production process, from material harvesting to manufacturing to assembly and shipping.

A common myth about PV technology is that it takes more energy to produce a PV system than the system will produce in its lifetime. Thankfully, this is not the case. Recent studies of energy payback time (EPBT) estimate that it takes a PV system one to three years to produce the same amount of energy that it took to manufacture it. Given that a PV system will continue to produce electricity for 30 years or more, a PV system's lifetime production will far exceed the energy it took to produce it. Here's an in-depth look at the embodied energy along the way.

Module Manufacturing Methods

A batteryless grid-tied PV system has many parts—modules and mounts, inverter(s), and wiring components (including conduit, fittings, electrical boxes, wire, and overcurrent protection). Each part of a PV system takes energy to both produce and transport (embodied energy), but of all of them, the modules require the most energy to manufacture—about 93% of the entire system.

Single-crystal (monocrystalline) PV cells are commonly manufactured using the Czochralski (CZ) method, where a "seed" silicon crystal is dipped into purified molten silicon and slowly raised out of the pot. As the seed crystal is raised, the molten silicon cools and solidifies into a single cylindrical crystal around and beneath the seed crystal. This process is referred to as "pulling" or "growing" an ingot. Thin slices—about 200 microns (0.008 inches) thick—are cut from the ingot and, with the addition of an antireflection coating and a wire grid to collect the electrons, become individual PV cells. To create a module, several cells are laid out and joined together electrically. Finally, the module is given a protective backing, topped with a glass covering, and then sealed and framed with extruded aluminum.

Each step and material used in this manufacturing process requires energy. Purifying and melting the silicon uses a lot of heat energy. There is also a fair amount of energy used to make the aluminum module frame, as well as the coatings and glass. Purifying and growing the silicon crystal, along with the embodied energy of an aluminum frame, make up the lion's share of energy involved in producing a single-crystal PV module.

Multicrystalline PV cells are generally made using a casting process, where molten silicon is poured into a square mold and left to solidify. This process creates many crystals within an ingot.

The ingot is sliced into thin square wafers to produce PV cells in the same way as the single-crystal process. Once the cells are created, the manufacturing proceeds as for monocrystalline modules. You can spot a multicrystalline PV module by its varied, glittering crystal surface, compared to very uniform-looking single-crystal silicon cells.

Multicrystalline PV modules do require less energy to produce than CZ-produced monocrystalline PV modules partly because the cooling process for the cast ingot uses less energy. The energy payback times for multicrystalline PV systems are about 15% less than for monocrystalline PV systems.

String Ribbon Silicon. Another way to produce a crystalline PV module is to grow thin ribbons of silicon that can be cut into individual cells. One method that produces these types of cells pulls two parallel wires out of a vat of molten silicon. As the wires are pulled up, a thin sheet of silicon the width of a finished cell stretches and hardens between them, much like soapy film stretches between the sides of a child's bubble wand. Because the ribbon silicon sheet is so thin, it does not need to be sliced as ingots do, but is sectioned into cell-sized lengths to make individual PV cells. The ribbon silicon cells are formed into modules the same way as the monocrystalline and multicrystalline PV technologies.

The ribbon technique reduces the energy and silicon crystal waste associated with sawing the wafers (kerf loss) from a crystalline ingot, reducing the energy payback time compared to monocrystalline by about 25% and multicrystalline PV by approximately 12%.

Thin-film PV modules use a deposition process, in which different layers of the PV cell are sprayed directly onto a

The energy embodied in a PV module includes not only the energy to produce its basic materials, but the energy of the manufacturing process as well.

substrate. Since there are no individual crystals to break, this substrate can be flexible and virtually any shape or size. The PV cells are completed after all the layers of the semiconductor material have been applied to the substrate by scribing the entire module into individual cells with lasers. Thin-film modules use a transparent conducting oxide (also applied as a layer in the deposition process) for electrical contacts, instead of an unbendable metal grid as crystalline cells.

Crystalline vs. Thin Film Cell Thickness

The thin-film deposition process uses less energy—and less energy-intensive silicon—than a crystalline PV cell.

The thin-film deposition process uses less energy—and less energy-intensive silicon—than a crystalline PV cell.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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