Photovoltaics

The electricity-generating process outlined in this article is called photovoltaics, pronounced photo-volt-ā-icks or fo-toe-vole-tay-icks, or PV for short. The word comes from the combination of two Greek words: phos, which means “from light”, and volt, which is the unit for electrical pressure. Simply put, photovoltaic means voltage from light.

While research into new, more efficient, and less expensive types of photovoltaic materials continues, currently the majority of PV modules that are installed are made from silicon (Si), with other technologies also having considerable market share. Often categorized as polycrystalline or monocrystalline and abbreviated as cSi, the base component is essentially sand or some other form of silicate, which is highly processed into Silicon, the same stuff computer chips are made of.

The key here is that photovoltaics makes electricity with sunlight as the only fuel source! A solar module or array produces electricity whenever sunlight hits it; a process as reliable and predictable as the sun rising and setting each day. While solar cells and modules can get hot, electricity generation is not based on heat.

How Photovoltaics work

While the explanation outlined above may be all a homeowner who wants solar needs to know about the theory behind the photovoltaic process, lets dive a little deeper into how this all works.

Silicon solar cells, the smallest building block of a PV system, consists of two or more layers that are doped with special chemicals so that they have opposite charges (positive and negative). From a physics perspective, when photons of sunlight strike the surface of the cell, electrons in the silicon are excited and knocked free. Wanting to return to their source (i.e. back to the PV cell), a wire grid on the surface of the cell allows them to do so freely. This flow of captured electrons is electricity in the form of direct current (DC), and unlike most electricity generators, no fuel is used in the PV process.

Solar cells are then made into modules by wiring the cells together and typically sandwiching them between a glass front and plastic back sheet. Put a frame around it (usually), add wires or terminals, and you’ve got a PV module (modules are also typically referred to as solar panels). Multiple solar modules wired together become a solar array. Solar arrays often get connected to some type of inverter. Since solar panels produce direct current (DC), and most homes and businesses use alternating current (AC), an inverter is necessary for converting from DC to useable AC power, providing electricity for AC-powered equipment such as lights, appliances, fans, and beyond.

Physics and PV components aside, the important concept to understand is that the photovoltaic process is dependable.

A brief history of PV

In 1873, British scientist Willoughby Smith noticed that selenium conducted more electricity when it was exposed to light. Then in 1880, Charles Fritts developed the first selenium-based solar electric cell, which produced electricity without consuming any fuel or generating waste heat. At this point PV was born, though it would be many years before it was very effective or practical. In the 1950s, scientists at Bell Laboratories began using silicon, the earth’s second most abundant element, to develop far more efficient, less expensive photovoltaic cells. Still, early silicon cells were only used in exotic places where power was extremely valuable and otherwise unavailable – like a spacecraft in orbit.

Other early applications for PV modules were in locations where it was difficult to get power from an electric utility, or where running generators was too expensive, too hard to maintain, or just flat out undesirable. Many of these applications are still big PV markets to this day, providing power to things such as lighthouses, highway signs, railroad crossings, telecommunication systems, sailboats, RVs, billboards, and agricultural uses (such as water pumping).

Some of these systems have energy storage, and the very function of the PV system is to charge the batteries. In other cases, such as water pumping or ventilation fans, the load may only run during the day, and only when the sun is shining. These two applications have very different system designs, even though they have a similar goal.

What can photovoltaics power?

The answer is anything at all, from calculators to buildings to entire towns, as long as it is designed/sized, installed and maintained properly. Although a single PV cell produces a relatively small amount of direct current (DC) electricity, with cells typically wired together with other cells to form PV modules, the sky is the limit. Most of the systems on rooftops in the U.S. have multiple modules connected in series strings to reach higher voltage. Once the proper voltage is achieved, additional series strings can be wired in parallel to increase current. In fact, PV can be configured to provide any given voltage, at any given current, to power a desired electrical load.

Types of PV Systems

Grid-direct
Grid-direct PV systems are the most common and most rapidly growing segment of the PV industry. With these systems, the PV array is connected to an inverter, which synchronizes with utility grid power and changes the DC electricity produced by the array into AC electricity. Electricity flows from the inverter to a point of connection with the utility. Grid-direct systems have no added storage components such as batteries or charge controllers. If the utility goes down, the inverter will stop operating and the energy production from the system’s solar panels cannot be employed until the utility is brought back online. There are two types of grid-direct PV system applications:

Residential
In the case of a residential grid-direct system, the electricity either powers loads in the building that they are on or, if there is a surplus beyond what is being consumed on the site, it flows onto the utility grid, amassing a credit for the user, called net-metering by utilities. 

Utility-scale
There has been a huge increase in very large PV systems, often called “utility-scale” or solar farms. They can cover many hundreds of acres, and are connected directly to the utility grid, making up a small, but rapidly growing amount of the generating resources that operate together to energize utility grids.

Off-grid
This is a stand-alone PV system operates as a totally independent energy system with no connection to utility power. All electricity is produced, processed, and consumed on site. In many cases there is also storage, in the form of batteries, which often make off-grid systems more complex and involved than grid-direct systems.

Remote homes and cabins were the first residential use of PV in the United States. In many remote areas, utility power is unavailable or very expensive to bring to the site. Fossil-fuel generators were previously the only option for remote electricity generation, but the noise, emissions, and ever-increasing fuel and maintenance costs left many looking for an alternative. Stand-alone PV systems with battery storage were able to fill this need.

Utility-interactive with batteries
If there is a need or desire for back-up electricity when the grid goes down, a system must be installed with batteries and an inverter that incorporates the functionality of both a grid-direct inverter and a battery-based inverter.

These systems are complex when compared to grid-direct systems, they include the added cost of a charge controller, batteries and an inverter that can work in both a grid-direct and stand-alone mode. With the added components, this also adds inefficiencies to the system. On top of this, batteries may also require maintenance and will need to be replaced (they will not last as long as the other system components). For these reasons, they are not as widely used, but are appropriate for certain applications. These types of systems can be configured in different ways, depending on customer desires or utility requirements. PV systems with some sort of storage component are all the talk of the industry now – it is an exciting time to be involved!

The take-away here is that you can have a system that is both connected to the utility and have backup power when there is a utility outage…but it can become cost-prohibitive.