Solar power is a promising route in the ongoing search for an alternative to coal and fossil fuels and photovoltaic panels are leading the race for a sustainable, affordable, and renewable energy source. Photovoltaics involve the conversion of light into electricity (or photons into voltage), better known as the PV effect.
Similar to the microelectronics industry, solar cells are based around semiconductor wafers with a positive and negative side. The semiconductor is surrounded by two contacts with an anti-reflective coating on the one facing the light source. Energy is produced when light strikes the contact and knocks atoms loose within the semiconductor that is connected to electrical conductors. These conductors capture the electrons and convert them into an electrical direct current, which in turn can be used to power a variety of systems.
The PV effect was discovered by Edmund Bequerel, a French physicist, in 1839. He found that when certain elements are exposed to light, such as silicon, they produce a small electric charge. Albert Einstein progressed this approach in 1905 and made legitimate breakthroughs in the explanation of the nature of light and the photoelectric effect.
The technology of photovoltaics is based upon Einstein’s discoveries and lead to production of the first working module by Bell Laboratories in 1954, known then as a solar battery. Initially, photovoltaic collectors were far too expensive to allow commercial adoption. Price has slowly become more affordable since the space boom of the 1960s. Drastic decline is visible in the current market as society searches for a dependable, sustainable energy source.
Performance of PV cells are represented by the current-to-voltage characteristic curve which, in turn, is affected by the device’s material properties and amount of total sunlight absorption. These are able to be shown in a one or two-diode model. Single diode equations assume a constant value for ideality factor of n. However, the ideality factor is a function of voltage across the system. High voltages yield an ideality of factor near one, while low voltages produce a factor closer to two. Figures 2 and 3 below illustrate single and double-diode models:
Figure 1: Single diode model
Figure 2: Double diode model
Structures of individual photovoltaic cells are known as modules and generally hold 40 cells. Modules produce electricity at a specific voltage, typically at the standard 12 volts, but overall output depends on the total amount of sunlight that reaches the cells. Residential homes using solar energy have a range of 10 to 20 panels to provide sufficient power. A multitude of modules combined into one system is called an array. Arrays are used to power large-scale industrial applications.
Solar panel efficiency is defined by the output per given area, affected by a variety of factors such as panel construction, positioning, temperature, and shading. There are three types of solar efficiency: module, area (density) and cell. Module efficiency is a measurement of how well a panel converts sun energy into usable energy. If the sun shines 200 Watts worth of energy onto a module and 30 watts of electricity are generated, then a panel has 15% module efficiency.
Area (density) efficiency is the calculation of the amount of usable energy per area in Watts per foot squared. If a module produces 420 Watts of energy with a 30 square foot panel, its density is 14 Watts per square foot. This measurement of efficiency is the most important to look at if one is working with a limited amount of roof space.
Cell efficiency is the final option when gauging the effectiveness of a solar module. It is the same as module efficiency but limited to a single cell on the panel. While many media outlets report this statistic, it is generally unreliable for a consumer looking to purchase photovoltaics because it is an inaccurate measurement of the total kWh production. Individual cells currently average around 15% output are typically single junction with one layer of silicone per cell. This means that of the total sunlight captured for each cell in a panel, only 15% of the sun’s energy is retained and converted into usable electricity.
Certain cells have been engineered with multiple layers of silicone, known simply as multi junction cells, to produce upwards of 40% output. Each level of silicone is arranged to capture a different frequency of light. However, such solar cells remain very expensive and are used almost exclusively in space and satellite technology. Record cell efficiency stands at 44% as of late 2012 with the improving technology of multi-band 3 junction solar cells. The experiment named SJ3 converted 43.5% of the energy in sunlight into electrical energy — a rate that has stimulated demand for the cell to be used in concentrator photovoltaic (CPV) arrays for utility-scale energy production. CPV systems are thought by many to be the future of photovoltaics because they use less cell material which tends to be the most expensive part of an array. The concentrator efficiently uses inexpensive materials like plastic or metal to capture sunlight on a large area and focus it into a smaller area that houses the solar cell(s). The diagram below shows how sunlight is magnified by the concentrators, allowing the record breaking cell efficiency as stated above:
Figure 3: CPV cell
Panel construction is limited in the sense that most consist of a glass housing for the semiconductor wafer, contacts, and cells. Some involve an adjustable scope that reduces the reflection of any light not striking the panel at a perfect 90º angle. The positioning of panels in adequate sunlight is crucial to generating a high output. Many systems are wired together in a continuous series, similar to a set of decorative Christmas lights. If one of a panel’s cells are affected by partial shading, the entire panel will see a drop in efficiency.
Figure 4 below illustrates a standardized comparison between popular photovoltaic solar panel options as of 2012 (4). All models are rated at 200 W and organized by density and module efficiency from high to low:
Figure 4: Photovoltaic panel comparisons
This next comparison factors in price and PTC rating, or the panel’s output tested in real world conditions at PVUSA in Davis, California. Ratings are usually 10-15% lower than the STC rating found in a lab or given to consumers by the manufacturers. While Figure 5, shown below, is more informative, it also lacks true consistency due to absence of a standardized wattage rating.
Figure 5: PTC photovoltaic panel comparisons
The outlook for the future of photovoltaics is extremely promising. According to data collected from Robert Margolis of the National Renewable Energy Laboratory, photovoltaic module pricing (not including installation) has shown an incredible drop in pricing since 1980 (8). Figure 6 below shows a decline of nearly half every 10.5 years, which equals an estimated 6% drop per year.
Figure 6: Decline of PV price from 1980-2025
The break-even cost of PV systems is the point where PV-generated electricity equals out to the same price as grid electricity and is typically expressed in $/W of a system. Margolis has estimated that 2015 will be the “magic year” for solar panels to actually cost less than grid electricity under the following assumptions: 1.) Photovoltaic installation prices will be twice that of the modules, 2.) the price decline of modules will follow the same curve as above, 3.) today’s average grid power of $0.10/kWh, 4.) the inflation rate for grid power will remain at a 3% increase over the market inflation rate, 5.) one watt of photovoltaic capacity one kWh of electricity per year.
Are photovoltaic technologies and solar energy the solution to the question of “Where will we get our future energy from?” Possibly. Are they a step in the right direction for fossil fuel independence? Absolutely.