It's an idea that has been around for well over a century.

In 1839, French scientist Edmond Becquerel discovered that certain materials would give off sparks of electricity when struck with sunlight. Researchers soon discovered that this property, called the photoelectric effect, could be harnessed; the first photovoltaic (PV) cells, made of selenium, were created in the late 1800s. In the 1950s, scientists at Bell Labs revisited the technology and, using silicon, produced PV cells that could convert four percent of the energy in sunlight directly to electricity.

The components of a PV cell

The most important components of a PV cell are two layers of semiconductor material commonly composed of silicon crystals. On its own, crystallized silicon is not a very good conductor of electricity, but when impurities are intentionally added—a process called doping—the stage is set for creating an electric current.

The bottom layer of the PV cell is usually doped with boron, which bonds with the silicon to facilitate a positive charge (P), while the top layer is doped with phosphorus, which bonds with the silicon to facilitate a negative charge (N).

The surface between the resulting "p-type" and "n-type" semiconductors is called the P-N junction (see diagram below). Electron movement at this surface produces an electric field that allows electrons to flow only from the p-type layer to the n-type layer.

Photo: pedrosala/Shutterstock

When sunlight enters the cell, its energy knocks electrons loose in both layers. Because of the opposite charges of the layers, the electrons want to flow from the n-type layer to the p-type layer. But the electric field at the P-N junction prevents this from happening.

The presence of an external circuit, however, provides the necessary path for electrons in the n-type layer to travel to the p-type layer. The electrons flowing through this circuit—typically thin wires running along the top of the n-type layer—provide the cell's owner with a supply of electricity.

Most PV systems are based on individual square cells a few inches on a side. Alone, each cell generates very little power (a few watts), so they are grouped together as modules or panels. The panels are then either used as separate units or grouped into larger arrays.

There are three basic types of solar cells:

- Single-crystal cells are made in long cylinders and sliced into thin wafers. While this process is energy-intensive and uses more materials, it produces the highest-efficiency cells, those able to convert the most incoming sunlight to electricity. Modules made from single-crystal cells can have efficiencies of up to 23 percent in some laboratory tests. Single-crystal accounts for a little over one third of the global market for PV [1].

- Polycrystalline cells are made of molten silicon cast into ingots then sliced into squares. While production costs are lower, the efficiency of the cells is lower too—with top module efficiencies close to 20 percent. Polycrystalline cells make up around half of the global PV market [2].

- Thin film cells involve spraying or depositing materials (amorphous silicon, cadmium-telluride, or other) onto glass or metal surfaces in thin films, making the whole module at one time instead of assembling individual cells. This approach results in lower efficiencies, but can be lower cost. Thin film cells are around ten percent of the global PV market [3].

Historically, most PV panels were used for off-grid purposes, powering homes in remote locations, cell phone towers, road signs, and water pumps. In recent years, however, solar power has experienced remarkable growth in countries for applications where the power feeds into the electricity grid. Such grid-connected PV applications now account for more than 99 percent of the global solar market [4].

How solar power is integrated into the electricity grid

How solar power is integrated into the electricity gridThe transition to an electricity system with a larger amount of solar power provides many benefits. The range of technologies, including small-scale distributed solar (mostly rooftop systems) and large-scale PV systems—come with different advantages for home owners, businesses, and utilities.

The electricity generated by rooftop solar panels first supplies on-site needs, with the grid supplying additional electricity as needed. When the home or business generates more electricity than it consumes, the electricity is fed back into the grid.

One of the biggest benefits that rooftop solar provides to the grid is that it often produces electricity when—and where—that power is most valuable. For example, in many regions demand on the electricity system peaks in the afternoon on hot, sunny days, when air conditioning use is high and when rooftop solar is performing strongly. Such systems therefore help utilities meet peak demand without firing up seldom-used power plants that are both expensive and more polluting than most other options [5].

Rooftop systems also reduce strain on electricity distribution and transmission equipment by allowing homes and businesses to first draw power on-site instead of relying completely on the electricity grid.

The benefits are twofold: the use of on-site power avoids the inefficiencies of transporting electricity over long distances, and on-site systems potentially allow the utility to postpone expensive upgrades to its infrastructure [6].

Large-scale solar systems, unlike rooftop solar, feed their electricity directly into the high-voltage electricity grid.

Large-scale PV, like rooftop systems, has the benefit of often operating at highest capacity when demand is also the greatest. In addition, the inherently modular nature of PV technology helps to make PV systems more resilient to extreme weather than traditional power plants that they replace. Large coal, natural gas, and nuclear plants are prone to cascading failures when part of a system is damaged. With large-scale PV, even if a section of a solar project is damaged, most of the system is likely to continue working.

And while large-scale solar systems depend on transmission lines that may be affected by extreme weather, the projects themselves are frequently back in service soon after the events.

Solutions for high levels of solar power

Getting to high levels of PV usage is desirable, given all the benefits that solar offers, but it also presents challenges. Those challenges are not insurmountable, however; upgrades to technology and updates to how electricity is bought and sold can help make increasing levels of solar penetration possible.

One challenge for rooftop solar is that having power flowing from customers, instead of to them, is a relatively new situation for utilities. Neighborhoods where many homes have adopted solar can approach a point at which the rooftop systems can produce more than the neighborhood can use during the day. Yet “feeder” lines that serve such neighborhoods customers may not be ready to handle flows of electricity in the opposite direction.

Large-scale PV projects face their own challenges in that they can be located far away from urban centers, often requiring transmission lines to carry the electricity to where it will actually be used. This requires investment in building the lines themselves and results in “line losses” as some of the energy is converted into heat and lost.

The variability of solar generation associated with PV at both scales presents new challenges because grid operators cannot control the output of these systems with the flip of a switch like they can with many non-renewable power plants. The amount of generation from PV systems depends on the amount of sunshine at any given time. When clouds block the sun, generation from a solar array can drop suddenly.

Conversely, on particularly sunny days with high amounts of solar on the grid, if the output from non-renewable energy power plants is not reduced to allow for the solar generation, electricity supplies could exceed demand. Both situations can lead to instability on the grid.

But the issues associated with adding more PV to the grid are eminently solvable. Fixes to the transmission and feeder issues are largely economic, not technical. And variability challenges are well understood in part because grid operators already manage fluctuations caused by constantly changing electricity demand and drops in electricity supplies when large power plants or transmission lines unexpectedly fail.

Much of the variability inherent in solar generation is also predictable and manageable, and can be handled in several ways including:

- Using better forecasting tools to allow for more accurate predictions of when solar generation might decline

- Installing solar across a large geographic area to minimize any impact of generation variability due to local cloud cover

- Shifting electricity supply and storing excess energy for later use

- Shifting electricity demand by encouraging customers to use electricity when it is more readily available

- Collaborating with neighboring regions to expand electricity import/export capabilities and share resources

Overall, renewable energy sources including solar help to stabilize and make the electricity system more resilient, both economically and environmentally.

References:

[1,2,3] Fraunhofer Institute. 2015. Photovoltaics report.

[4] International Energy Agency (IEA). 2014. Technology roadmap: Concentrating solar power. Paris, France.

[5] Burger, B. 2011. Solar power plants deliver peak load. Freiburg, Germany: Fraunhofer Institute for Solar Energy Systems ISE.

[6,7] Bird, L., J. McLaren, J. Heeter, C. Linvill, J. Shenot, R. Sedano, and J. Migden-Ostrander. 2013. Regulatory considerations associated with the expanded adoption of distributed solar. Golden, CO: National Renewable Energy Laboratory.

[Union of Concerned Scientists,ucsusa.org]