How Do Solar Panels Function?

With rising power grid demands driven by innovations like electric vehicles, both industries and governments are seeking cleaner and more abundant energy alternatives to traditional coal boilers. The sun, our solar system's central star, stands as the most abundant energy source. Functioning as a massive fusion reactor, it emits heat and plasma and will continue to do so for billions of years.

To convert solar energy into a usable form for powering everyday devices, humanity has developed photovoltaic cells, widely known as solar panels. But how exactly do these solar panels operate?

Originally utilized primarily in space to power satellites' electrical systems as early as 1958, solar technology is now increasingly integrated into everyday applications. It continually emerges in innovative devices, ranging from sunglasses to charging stations for electric vehicles.

The vision of a "solar revolution" has persisted for decades, envisioning a future where free solar electricity from the sun powers our lives. This promise is compelling, as the sun emits roughly 1,000 watts of energy per square meter of the Earth's surface on a clear day. Harnessing this energy could effortlessly supply power to homes and offices, with surplus energy stored for future use.

This article explores how solar panels produce electricity and the mechanics behind their operation. You'll discover why solar energy is becoming more accessible for daily use and why further research is needed to make it a cost-effective solution.

Photovoltaic Solar Panels: Transforming Photons into Electrons

Photovoltaic (PV) panels, also known as solar cells, are the technology behind the solar panels seen on power stations and satellites. As the name suggests (photo meaning "light" and voltaic meaning "electricity"), these panels convert sunlight directly into electrical energy. Multiple panels are grouped into modules, which are then assembled into larger arrays, such as the one at Nellis Air Force Base in Nevada.

Photovoltaic cells are constructed from specialized semiconductor materials, such as silicon, which is the most widely used today. When light hits the panel, part of it is absorbed by the semiconductor. This absorption transfers the light's energy to the material, freeing electrons and enabling them to move freely.

PV solar panels operate using electric fields that direct electrons, released by light absorption, to flow in a specific direction. This electron flow creates a current. By attaching metal contacts to the top and bottom of the PV cell, this current can be harnessed for external use. The combination of this current and the cell's voltage, generated by its internal electric fields, determines the power output of the solar cell.

While that's the fundamental process, there's a lot more complexity involved. Let's delve deeper into a specific type of PV panel: the single-crystal silicon panel.

How Silicon Makes Solar Panels

Silicon possesses unique chemical properties, particularly in its crystalline state. A silicon atom contains 14 electrons distributed across three shells. The first two shells, accommodating two and eight electrons respectively, are fully occupied. The outermost shell, however, is only half-filled, containing just four electrons.

A silicon atom constantly seeks to complete its outermost shell by sharing electrons with four adjacent atoms. This process resembles atoms linking hands with their neighbors, with each atom connecting to four others. This bonding creates the crystalline structure, which is crucial for this type of photovoltaic cell.

The primary issue is that pure crystalline silicon is an inefficient conductor of electricity, as its electrons are not free to move, unlike those in better conductors such as copper.

To solve this problem, impurities are intentionally added to the silicon in solar cells. These impurities, which are other atoms mixed with silicon atoms, alter the material's behavior. While impurities are often seen as undesirable, they are essential for the cell's functionality. For instance, adding a phosphorus atom for every million silicon atoms changes the dynamics significantly.

Phosphorus has five electrons in its outer shell, unlike silicon's four. It bonds with neighboring silicon atoms, but one of its electrons remains unpaired. This electron isn't part of a bond but is held in place by the positive charge of the phosphorus nucleus.

When energy, such as heat, is applied to pure silicon, some electrons break free from their bonds, leaving behind holes. These free electrons, known as free carriers, move randomly through the crystalline structure, seeking holes to fill and generating an electrical current. However, pure silicon has too few free carriers to be effective.

However, silicon doped with phosphorus behaves differently. The extra electrons from phosphorus require much less energy to become free, as they aren't bonded to neighboring atoms. This results in a significantly higher number of free carriers compared to pure silicon.

The intentional addition of impurities is called doping. When silicon is doped with phosphorus, it becomes N-type ("n" for negative) due to the abundance of free electrons. N-type silicon is far more conductive than pure silicon.

The second part of a standard solar cell is doped with boron, an element with three electrons in its outer shell instead of four, creating P-type silicon. Unlike N-type, P-type silicon ("p" for positive) has free holes and carries a positive charge instead of free electrons.

Anatomy of a Solar Panel

Initially, the two silicon pieces are electrically neutral. The magic happens when they are combined. The electric field, essential for the cell's operation, forms at the junction of N-type and P-type silicon. Free electrons from the N side rush to fill the holes on the P side, creating a dynamic flow.

Do all the free electrons occupy every available hole? No, because that would render the system ineffective. Instead, at the junction, they combine to create a barrier, increasing the difficulty for electrons on the N side to move to the P side. Over time, equilibrium is achieved, forming an electric field that separates the two sides.

This electric field functions like a diode, facilitating and even driving electrons to move from the P side to the N side, but not in reverse. Imagine it as a hill — electrons can effortlessly roll down (to the N side) but cannot climb back up (to the P side).

When photons strike the solar panel, their energy disrupts electron-hole pairs. Each sufficiently energetic photon typically liberates one electron, creating a free hole. If this occurs near the electric field or if free electrons and holes enter its influence, the field directs electrons to the N side and holes to the P side.

This disrupts electrical neutrality further. By providing an external current path, electrons flow to the P side, reuniting with holes sent there by the electric field and generating work in the process. The electron movement creates current, while the cell's electric field generates voltage. Together, current and voltage produce power.

A few additional components are necessary for the cell to function effectively. Silicon's reflective nature can cause photons to bounce away before being utilized, so an antireflective coating is applied to minimize such losses.

The last step involves adding a protective layer, often a glass cover, to shield the cell from environmental factors. PV modules are typically constructed by linking multiple solar panels to achieve adequate voltage and current levels, then encasing them in a durable frame equipped with positive and negative terminals.

Unfortunately, the process of converting sunlight into usable energy is not flawless. As of 2023, commercial solar panels operate at less than 30 percent efficiency, meaning over two-thirds of potential solar energy is lost. In laboratory conditions, some researchers have achieved up to 47 percent efficiency, but this required a highly concentrated light beam far more intense than natural sunlight.

Currently, the only viable solution to this efficiency issue is to deploy more solar panels across larger areas, significantly increasing the cost of solar farms in terms of land and resources. Historically, solar power was one of the most expensive energy sources to develop relative to the energy it produced. However, prices have trended downward in recent years, making photovoltaics more competitive with the costs of wind turbine construction.

A viable alternative to photovoltaic systems is the concentrated solar plant. This approach eliminates silicon panels and instead employs a vast array of mirrors to capture sunlight. These mirrors are designed to track the sun's movement and focus the light onto a central tower, known as a receiver. The receiver contains molten salt, a chemical solution that heats up to over 1,000 degrees Fahrenheit (538 degrees Celsius). This heat can then be used to power traditional steam turbines, both during the day and at night.

Concentrated solar systems can generate significantly more power than standard solar arrays, but they require extensive flat land and pose risks due to the extreme heat involved. As a result, these plants are typically situated in remote areas, such as the Mojave Desert.

Energy Loss in a Solar Panel

Visible light is merely one segment of the electromagnetic spectrum. Electromagnetic radiation is not uniform; it consists of a variety of wavelengths, each with different energy levels.

Light can be broken down into distinct wavelengths, visible as a rainbow. The light striking a solar cell contains photons with a wide range of energies. Some photons lack sufficient energy to disrupt electron-hole pairs, passing through the cell unaffected. Others possess excessive energy. Only a specific amount of energy, measured in electron volts (eV) and determined by the cell's material (approximately 1.1 eV for crystalline silicon), is needed to free an electron.

This is referred to as the material's band gap energy. If a photon carries more energy than necessary, the excess is wasted. (An exception occurs if a photon has double the required energy, potentially creating multiple electron-hole pairs, though this effect is minimal.) These two factors alone can lead to the loss of approximately 70 percent of the radiation energy hitting the cell.

Why not select a material with an extremely low band gap to utilize more photons? Unfortunately, the band gap also influences the electric field's strength (voltage). If it's too low, the gain in current (from absorbing more photons) is offset by a reduced voltage. Since power equals voltage multiplied by current, the ideal band gap, balancing these factors, is around 1.4 eV for single-material cells.

Additional losses occur as electrons must travel from one side of the cell to the other via an external circuit. While the bottom can be coated with metal for efficient conduction, completely covering the top blocks photons, eliminating current flow. (Some solar panels use transparent conductors on the top, but not all.) Placing contacts only on the sides forces electrons to travel a much greater distance to reach them.

Recall that silicon is a semiconductor — far less efficient than metals in conducting electricity. Its internal resistance (series resistance) is relatively high, leading to significant energy losses. To reduce these losses, cells are often fitted with a metallic contact grid, minimizing electron travel distance while covering only a small portion of the cell surface. However, this grid can block some photons, and if made too small, its resistance increases.

Now that we understand how a solar cell functions, let's explore what it takes to power a home using this technology.

How Solar Panels Generate Electricity in a House

What steps are needed to power your home with solar energy? While it’s not as simple as placing a few panels on your roof, the process isn’t overly complicated either.

First, not every roof has the ideal orientation or tilt to maximize the sun's energy. In the Northern Hemisphere, non-tracking solar systems should ideally face true south, though east or west orientations can also work, albeit with reduced efficiency.

Solar panels should ideally be tilted at an angle close to the area's latitude to capture the most energy throughout the year. Adjusting the orientation or tilt can optimize energy output for specific times, such as mornings, afternoons, summers, or winters. However, it’s crucial to ensure the panels are never shaded by trees or buildings, as even partial shading can drastically reduce the power output of an entire PV module.

For a house with an unobstructed, south-facing roof, the next step is determining the appropriate system size. This decision is complicated by fluctuating electricity production due to unpredictable weather and varying household energy demands.

Fortunately, these challenges are manageable. Meteorological data provides average monthly sunlight levels for different regions, accounting for factors like rainfall, cloud cover, altitude, and humidity. Designing for the least sunny month ensures consistent electricity generation year-round.

Using this data and your average monthly energy consumption (available on your utility bill), you can calculate the number of PV modules required. Additionally, you’ll need to choose a system voltage, which depends on how many modules are connected in series.

You might have already identified a key issue: what happens when the sun isn’t shining?

Solving Solar Energy Issues

Relying solely on the weather for power might not appeal to most, but there are three primary solutions to ensure uninterrupted electricity. For those living off-grid who doubt their PV panels' reliability, a backup generator can provide power when solar energy is insufficient.

Another standalone solution involves storing energy in batteries to generate electricity. While batteries increase the cost and maintenance of a PV system, they are essential for complete energy independence.

Another option is to connect your home to the utility grid, purchasing power when necessary and selling excess energy back. This approach leverages the grid as a virtually limitless storage system. However, government regulations vary by region and can change, affecting whether your utility company participates and the rates they offer for buyback.

Specialized equipment may also be required to ensure the power you sell to the utility is compatible with their system. Safety is another concern; the utility must prevent your PV system from feeding electricity into power lines during an outage, a hazardous situation known as islanding. This can be avoided with an anti-islanding inverter, which we’ll discuss shortly.

If you opt for batteries, be prepared for maintenance and eventual replacement. While most solar panels last about 30 years (with ongoing research aiming to extend this), batteries have a shorter lifespan. Additionally, batteries in PV systems can be hazardous due to their stored energy and acidic electrolytes, necessitating a well-ventilated, nonmetallic enclosure.

Regardless of the type, all batteries used in PV systems should be deep-cycle batteries. Unlike car batteries, which are shallow-cycle and deliver a large current briefly, deep-cycle batteries can discharge more energy over longer periods without compromising their lifespan.

PV batteries typically discharge a smaller current over extended periods, such as at night or during outages, while being recharged during the day. The most common types are lead-acid (sealed and vented) and nickel-cadmium batteries, each with its own advantages and disadvantages.

Lithium-Ion batteries are increasingly being used in solar applications due to their higher energy storage capacity compared to older designs. However, they come with a higher price tag and carry risks such as overheating or even catching fire if overcharged or exposed to high temperatures.

Finishing Your Solar Energy Setup

Using batteries necessitates the addition of a charge controller. This device prevents overcharging or excessive draining, both of which can shorten battery life. Once the batteries are fully charged, the charge controller stops the flow of current from the PV modules to the batteries.

Similarly, once the batteries reach a specific voltage level, many charge controllers prevent further discharge until the batteries are recharged. Monitoring battery voltage ensures this protection. A charge controller is crucial for maximizing battery lifespan.

Another challenge, apart from energy storage, is that the electricity produced by solar panels (or drawn from batteries) is not in the same form as the power supplied by utilities or used by household appliances.

Solar systems generate direct current (DC), so an inverter is required to convert it to alternating current (AC). Additionally, some inverters include anti-islanding features to protect the grid if your system is connected.

Many large inverters offer automated system control. Some PV modules, known as AC modules, come with built-in inverters, eliminating the need for a central inverter and simplifying wiring.

In addition to mounting hardware, wiring, junction boxes, grounding equipment, overcurrent protection, and disconnects, you’ll need various accessories to complete the system. Compliance with electrical codes (including the PV-specific section of the National Electrical Code) is mandatory, and hiring a licensed electrician experienced in PV systems is highly recommended. Once installed, a PV system requires minimal maintenance (especially without batteries) and can provide clean, quiet electricity for over 20 years.

Developments in Solar Panel Technology

While we’ve discussed how standard PV systems work, ongoing research focuses on improving cost-effectiveness (explored further in the next section) to make solar energy more competitive with conventional energy sources.

For instance, polycrystalline silicon is used in PV panels to reduce manufacturing costs, though it’s less efficient than single-crystal silicon. Second-generation technology includes thin-film solar panels, which offer new possibilities for solar energy.

Although thin-film solar panels often sacrifice some efficiency, they are simpler and more cost-effective to produce, and their efficiency continues to improve. These panels can be manufactured using various materials, such as amorphous silicon (lacking a crystalline structure), gallium arsenide, copper indium diselenide, and cadmium telluride.

Another approach to boosting efficiency involves layering two or more materials with different band gaps. Since materials absorb photons of varying energies, stacking higher band gap materials on top to capture high-energy photons (while lower-energy photons are absorbed by the underlying lower band gap material) can significantly increase efficiency. These multi-junction panels can feature multiple electric fields.

Concentrating photovoltaic technology is another exciting area of innovation. Rather than passively collecting sunlight, these systems use optical devices like lenses and mirrors to focus more solar energy onto highly efficient panels, maximizing electricity generation.

While these systems are generally more expensive to produce, they offer several advantages over traditional solar setups and drive further research and development in the field.

These advancements in solar technology have inspired a wide range of applications, from solar-powered aircraft and space-based power stations to everyday items like PV-powered curtains, clothing, and laptop cases. Even the realm of nanoparticles is being explored, with researchers investigating the potential for organically produced solar panels.

If photovoltaics are such an excellent source of free energy, why hasn’t the entire world transitioned to solar power?

Solar Energy Costs

Some people misunderstand solar energy. Although sunlight is free, the electricity produced by PV systems is not. Several factors determine whether investing in a PV system is financially worthwhile.

First, your location plays a significant role. Those living in sunnier regions have a natural advantage, as their PV systems can generate more electricity. Additionally, local utility costs must be considered. Since electricity rates vary widely, even residents in less sunny areas might find solar power worthwhile if their rates are high.

Next, installation costs are a factor. As we’ve discussed, setting up a household PV system requires substantial hardware. Between 2010 and 2020, the National Renewable Energy Laboratory reported that the average cost per watt for residential solar systems dropped from $7.53 to $2.71. With costs now a fraction of what they were a decade ago, solar panels have become a more attractive option for homeowners. Government tax incentives can further reduce expenses.

Despite the initial expense, there are multiple ways to offset the cost of a PV system for both individuals and businesses. These include federal and state tax incentives, utility rebates, and various financing options.

Additionally, depending on the size and efficiency of the solar setup, it can generate surplus power, helping to recoup costs faster. It’s also worth noting that installing a PV system can significantly increase a home’s value.

While solar energy still faces challenges competing with traditional utilities, costs are declining as technology advances. Advocates believe PV systems will eventually become cost-effective in both urban and remote areas.

One major challenge is the need for large-scale manufacturing to minimize costs. However, such demand for PV systems won’t materialize until prices drop to competitive levels, creating a catch-22 situation. Despite this, as demand grows, module efficiencies improve, prices decline, and global awareness of environmental issues tied to traditional energy sources increases, the future of photovoltaics looks bright.

For additional details on solar panels and related topics, explore the links provided in the next section.