Solar Photovoltaic Energy Systems Explained — Solar PV Panels, Cells, Modules, & Science Of The Photovoltaic Effect
Solar photovoltaic (PV) technologies are becoming ubiquitous in many parts of the so-called developed and developing worlds it seems. Even in places as seemingly different as Australia, India, Chile, the US, and China, various solar energy technologies have begun to be deployed on the mass level — whether residentially, commercially, or on the industrial-scale (utility-scale power plants).
Who in the urbanized portions of the modern world hasn’t seen a residential solar PV system on the roof of a house? Or an image of a solar PV system connected to a satellite in the far upper-atmosphere? Or an image of a huge industrial-scale solar PV power plant sitting in the desert?
A tipping point has seemingly been reached, wherein the technology has become a notable part of the public consciousness — rather than simply a concern of those in specialized fields of engineering or physical science.
With that in mind, I put together the article below to present a basic overview of current solar PV technologies, and of the science behind the photovoltaic effect itself. Enjoy.
Solar Photovoltaic Energy Systems Explained — Solar PV Panels, Cells, Modules, Efficiency, The Science Of The Photovoltaic Effect, Space Applications, & Building-integrated Photovoltaics
Solar PV Cells
A solar cell, alternatively known as a photovoltaic cell, is, to put it simply, an electrical device that converts some of the energy in light into electricity — through the physical and chemical phenomenon known as the “photovoltaic effect.”
Such solar cells are in practical-use combined in high numbers, and with other components, to produce solar photovoltaic panels and modules. These solar PV panels and modules are simply a collection of various numbers of solar cells, with associated components.
Here’s a simplified run-down of how solar cells work:
- When photons hit pieces of purified silicon a number of things can happen, most notably: the photon can pass right through the silicon; the photon can be reflected off the surface of the silicon; or the photon can be absorbed by the silicon (if photon energy eclipses the silicon band-gap value).
- If the photon is absorbed then this leads to the generation of an “electron-hole pair” and heat as well, depending on the band-gap present.
- The energy of the photon is transferred to an electron in the crystal lattice of the silicon. This electron usually remains in the valence band, and is bound in covalent bonds with bordering atoms, and thus doesn’t move far.
- The now “excited” electron moves into the conduction band thanks to the new energy and moves around within the semiconductor.
- With this excitation and movement, the network of covalent bonds that the electron had previously been a part of is now diminished by one electron. In other words, a hole has been created.
- The new creation of a missing covalent bond then allows the bonded electrons of neighboring atoms to move into the “hole,” leaving another hole behind, thus propagating holes throughout the lattice. It can be said that photons absorbed in the semiconductor create electron-hole pairs.
- Thusly these excited electrons diffuse, and many reach the rectifying junction (usually a p-n junction) where they are then accelerated into a different material by a built-in potential (Galvani potential).
- “A photon only needs to have energy greater than that of the band gap in order to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at about 5,800 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations — called phonons) rather than into usable electrical energy. The photovoltaic effect can also occur when two photons are absorbed simultaneously in a process called two-photon photovoltaic effect. However, high optical intensities are required for this nonlinear process.”
- “The most commonly known solar cell is configured as a large-area p-n junction made from silicon. As a simplification, one can imagine bringing a layer of n-type silicon into direct contact with a layer of p-type silicon. In practice, p-n junctions of silicon solar cells are not made in this way, but rather by diffusing an n-type dopant into one side of a p-type wafer (or vice versa).”
Individual solar cell solar conversion efficiency, along with the number of solar cells present, is a large part of what determines total solar PV system or installation output — with solar insolation levels (the amount of sun that a region receives based on latitude, climate, etc), dust, heat, and weather modulating output as well.
To provide an example here of what this means in actual practice:
For example, a solar panel with 20% efficiency and an area of 1 m² will produce 200 W at Standard Test Conditions, but it can produce more when the sun is high in the sky and will produce less in cloudy conditions and when the sun is low in the sky. In central Colorado, which receives annual insolation of 5.5 kWh/m²/day, such a panel can be expected to produce 440 kilowatt-hours (kWh) of energy per year. However, in Michigan, which receives only 3.8kWh/m²/day, annual energy yield will drop to 280 kWh for the same panel. At more northerly European latitudes, yields are significantly lower: 175 kWh annual energy yield in southern England.
There’s a lot of technical terminology that could be used to explain the determining factors behind solar cell solar conversion efficiency, but the basic takeaway would be to say that the better a solar cell is at absorbing a wide-portion of the energy contained in the solar spectrum and generating “electron-hole pairs”, the higher the solar conversion efficiency will be. The energy present in sunlight of course encompasses a wide-range of different frequencies — not all of it being what humans experience as light or heat.
Here’s a bit of an elaboration on that:
Photons with an energy below the band gap of the absorber material cannot generate an electron-hole pair, so their energy is not converted to useful output, and only generates heat if absorbed. For photons with an energy above the band gap energy, only a fraction of the energy above the band gap can be converted to useful output. When a photon of greater energy is absorbed, the excess energy above the band gap is converted to kinetic energy of the carrier combination. The excess kinetic energy is converted to heat through phonon interactions as the kinetic energy of the carriers slows to equilibrium velocity. Solar cells with multiple band gap absorber materials improve efficiency by dividing the solar spectrum into smaller bins where the thermodynamic efficiency limit is higher for each bin.
Here’s some further information on electron-hole pairs and “Quantum efficiency”:
As described above, when a photon is absorbed by a solar cell it can produce an electron-hole pair. One of the carriers may reach the p-n junction and contribute to the current produced by the solar cell; such a carrier is said to be collected. Or, the carriers recombine with no net contribution to cell current. Quantum efficiency refers to the percentage of photons that are converted to electric current (ie, collected carriers) when the cell is operated under short circuit conditions. The “external” quantum efficiency of a silicon solar cell includes the effect of optical losses such as transmission and reflection.
In particular, some measurement can be taken to reduce these losses. The reflection losses, which can account for up to 10% of the total incident energy, can be dramatically decrease using a technique called texturization, a light trapping method that modifies the average light path. Quantum efficiency is most usefully expressed as a spectral measurement (that is, as a function of photon wavelength or energy). Since some wavelengths are absorbed more effectively than others, spectral measurements of quantum efficiency can yield valuable information about the quality of the semiconductor bulk and surfaces. Quantum efficiency alone is not the same as overall energy conversion efficiency, as it does not convey information about the fraction of power that is converted by the solar cell.
And a brief overview of the “maximum power point” can be found here:
“A solar cell may operate over a wide range of voltages (V) and currents (I). By increasing the resistive load on an irradiated cell continuously from zero (a short circuit) to a very high value (an open circuit) one can determine the maximum power point, the point that maximizes V×I; that is, the load for which the cell can deliver maximum electrical power at that level of irradiation. (The output power is zero in both the short circuit and open circuit extremes). A high quality, monocrystalline silicon solar cell, at 25 °C cell temperature, may produce 0.60 V open-circuit (VOC). The cell temperature in full sunlight, even with 25 °C air temperature, will probably be close to 45 °C, reducing the open-circuit voltage to 0.55 V per cell. The voltage drops modestly, with this type of cell, until the short-circuit current is approached (ISC). Maximum power (with 45 °C cell temperature) is typically produced with 75% to 80% of the open-circuit voltage (0.43 V in this case) and 90% of the short-circuit current. This output can be up to 70% of the VOC x ISC product. The short-circuit current (ISC) from a cell is nearly proportional to the illumination, while the open-circuit voltage (VOC) may drop only 10% with an 80% drop in illumination. Lower-quality cells have a more rapid drop in voltage with increasing current and could produce only 1/2 VOC at 1/2 ISC. The usable power output could thus drop from 70% of the VOC x ISC product to 50% or even as little as 25%. Vendors who rate their solar cell “power” only as VOC x ISC, without giving load curves, can be seriously distorting their actual performance.”
Solar PV Modules, Panels, And Systems
Solar photovoltaic (PV) modules and panels are packaged, integrated assemblies of solar cells. They are systems designed to exploit the photovoltaic effect for the generation of electric current (electricity).
Solar PV modules and panels are often used in the creation of home solar energy systems, commercial solar installations, and utility-scale solar energy power plants.
Solar PV modules are generally rated based on DC (direct current) output power when used in specific testing conditions meant to approximate average-use. These ratings typically vary between 100-400 watts (W). The higher the solar conversion efficiency of the solar cells used, the higher the solar PV modules output will be when all other things are equal (size). Solar PV modules utilizing higher efficiency cells are thus often better suited for use in applications calling for economy of space than lower efficiency modules are. Lower efficiency modules, though, can in turn be more economical if installation space is no issue.
Applications calling for high-efficiency modules (or cells) include: space/satellite applications, solar-powered vehicles, and small buildings without usable adjoining land.
Solar PV systems are generally composed of multiple (or up to many hundreds of thousands, or more) modules or panels, an inverter or inverters (for the conversion of DC to AC), wiring, and often battery or energy storage systems as well. Solar trackers are sometimes used as well, as a means of maximizing solar PV panel exposure to the sun. Most solar PV panels feature a sheet of glass over the semiconductor wafers in order to limit exposure to the elements, and prolong the working-life.
Depending upon the needs and preferences of the owner or developer, solar PV systems can be either off-grid or grid-connected.
“Solar cells are usually connected in series and parallel circuits or series in modules, creating an additive voltage. Connecting cells in parallel yields a higher current; however, problems such as shadow effects can shut down the weaker (less illuminated) parallel string (a number of series connected cells) causing substantial power loss and possible damage because of the reverse bias applied to the shadowed cells by their illuminated partners. Strings of series cells are usually handled independently and not connected in parallel, though as of 2014, individual power boxes are often supplied for each module, and are connected in parallel. Although modules can be interconnected to create an array with the desired peak DC voltage and loading current capacity, using independent MPPTs (maximum power point trackers) is preferable. Otherwise, shunt diodes can reduce shadowing power loss in arrays with series/parallel connected cells.”
Photovoltaics And The Photovoltaic Effect
Photovoltaics are a field of study and engineering based around the conversion of solar energy into direct current (DC) electricity through the use of semiconducting materials that showcase the phenomenon known as the “photovoltaic effect.”
This phenomenon is both physical and chemical in nature — with the photoelectric effect kicking things off, to be followed by an electrochemical process takes involving crystallized atoms being ionized in a wave-series, culminating in the generation of an electric current.
With current production technologies, most photovoltaics recoup the energy used in their manufacture within only 1.5-2.5 years in Europe. Note here that that’s a recouping of the energy costs of manufacture not of the resource costs.
Following behind only hydroelectric and wind energy technologies, solar PV is now the third largest renewable energy generation source in the world. As of 2014 there were over 177 gigawatts (GW) of solar PV nameplate generation capacity installed worldwide — equal to roughly 1% of global electricity demand that year.
The top solar PV markets in the world currently are: China, the US, Japan, and the EU.
The Photovoltaic Effect
As discussed at the start of this article, the photovoltaic effect is a physical and electrochemical phenomenon that allows for the creation of electrical current or voltage in certain materials when exposure to light occurs.
“The photovoltaic effect is closely related to the photoelectric effect. In either case, light is absorbed, causing excitation of an electron or other charge carrier to a higher-energy state. The main distinction is that the term photoelectric effect is now usually used when the electron is ejected out of the material (usually into a vacuum) and photovoltaic effect used when the excited charge carrier is still contained within the material. In either case, an electric potential (or voltage) is produced by the separation of charges, and the light has to have a sufficient energy to overcome the potential barrier for excitation. The physical essence of the difference is usually that photoelectric emission separates the charges by ballistic conduction and photovoltaic emission separates them by diffusion, but one should note that some ‘hot carrier’ photovoltaic device concepts blur even this line of distinction.”
Here’s an overview of the timeline of the scientific theory of the photovoltaic effect:
- The first scientific observation of the photovoltaic effect occurred in 1839. The 19-year-old French physicist AE Becquerel detailed his “discovery” in Les Comptes Rendus de l’Academie des Sciences. The title of that paper was: “The production of an electric current when two plates of platinum or gold immersed in an acid, neutral, or alkaline solution are exposed in an uneven way to solar radiation.”
- Awhile later, in 1873, Willoughby Smith published a paper titled “Effect of Light on Selenium during the passage of an Electric Current” in the journal Nature.
- This was followed by Charles Fritts building a solid-state photovoltaic cell — by coating the semiconductor selenium with a thin layer of gold to form the junctions — in 1883. This PV cell possessed a solar conversion efficiency of around 1%.
- A few years later in 1888 the Russian physicist Aleksandr Stoletov created a cell that was based on the “outer photoelectric effect” detailed by Heinrich Hertz the previous year, in 1887.
- Two decades later, in 1905, Albert Einstein revealed a new quantum theory of light and presented a new explanation of the photoelectric effect. He won the Nobel Prize in Physics for that paper, in 1921.
- Vadim Lashkaryov created p-n-junctions in CuO and silver sulphide protocells in 1941.
- The modern junction semiconductor solar cell was patented in 1946 Russell Ohl.
- Practical photovoltaic cells were publicly demonstrated on 25 April 1954 at Bell Laboratories. The researchers begins the cells were Daryl Chapin, Calvin Souther Fuller, and Gerald Pearson.
- Solar cells were included on the 1958 Vanguard I satellite.
Solar PV Power Plants, Utility-Scale Solar PV Projects
In addition to residential and commercial applications, solar PV pants can be deployed in the mass-scale — for the creation of industrial (utility-scale) power plants. Such facilities are generally designed to feed large amounts of electricity into regional or national grids — often replacing coal- or gas-fired power plants.
Such facilities are often also referred to as “solar parks” or “solar farms.”
Notably, such projects are distinct from concentrating solar power (CSP) plants — which utilize mirrors and/or parabolic troughs to focus light and heat on specific points, and thus drive turbines, melt salt, etc.
“In some countries, the nameplate capacity of a photovoltaic power stations is rated in megawatt-peak (MWp), which refers to the solar array’s DC power output. However, Canada, Japan, Spain and some parts of the United States often specify using the converted lower nominal power output in MWAC; a measure directly comparable to other forms of power generation. A third and less common rating is the mega volt-amperes (MVA). Most solar parks are developed at a scale of at least 1 MWp. As of 2015, the world’s largest operating photovoltaic power stations have capacities of close to 600 megawatts and projects up to 1 gigawatt are planned. As at the end of 2015, about 3,400 projects with a combined capacity of 60 GWAC were solar farms larger than 4 MW.”
Space-Based Solar PV Applications
Space and satellite-based application of solar PV cells has been more or less steady since 1958, when solar PV cells were featured on the Vanguard satellite as an alternative to solely battery power.
This was followed a year later in 1959 with solar PV cell inclusion on Explorer 6 — the wing-shaped solar arrays featured on the Explorer 6 (composed of 9600 Hoffman solar cells) became something of a trendsetter. Such arrays are now very common on satellites.
Beginning in early 1990s solar cells intended for use in space began to diverge from ones intended for ground-use — with a shift away from silicon, and towards gallium arsenide-based III-V semiconductor materials. Nowadays III-V multijunction photovoltaic cells are the standard on most spacecraft.
Building-Integrated Solar PV
While ground or roof based solar PV systems are the most common type of installation, another option is to implement solar PV systems directly into buildings — with solar windows, skylights, and facades, taking the place of conventional installations.
A number of prominent buildings constructed in recent years in some parts of the world have featured building-integrated solar PV systems. While such an approach has been becoming seemingly more and more common in recent years, uptake has been considerably slower than some early proponents expected. Barriers remain with regard to economic competitiveness and ease of construction.
Image Credits: Public Domain; NASA; NREL