Trends in photovoltaic energy conversion

Photovoltaic (PV) energy conversion (solar cells) is a rapidly growing and changing market. This article will dig into various trends that propel the increasing use of solar cells in applications of all sizes, from small portable devices to MW-sized utility-scale installations. These trends include the continued improvement in cost and efficiency for a variety of PV materials and emerging PV materials. Plus, the emergence of bifacial PV modules and concentrator PV (CPV) technology.

When photons strike a PV cell, they may reflect off the cell, pass through the cell, or be absorbed by the semiconductor material. Only the absorbed photons provide energy to generate electricity. When the semiconductor material absorbs enough sunlight, electrons are dislodged from the material’s atoms. Special treatment of the material surface during manufacturing makes the cell’s front surface more receptive to the dislodged, or free, electrons so that the electrons migrate to the cell’s surface.

The movement of electrons toward the cell’s front surface creates an imbalance of electrical charge between the cell’s front and back surfaces, which creates a voltage potential. Electrical conductors absorb the electrons on the front surface of the cell. When the conductors on the front and back surfaces are connected in an electrical circuit to an external load, such as an inverter or battery, electricity flows.

Solar energy conversion systems typically consist of a series of solar cells assembled into solar modules, which are connected into panels, and the panels are used to build arrays. Solar cells are the basic building block. They are assembled into mechanically and environmentally sealed solar modules.

The number of series cells determines the module’s voltage, while the number of parallel cells determines the current. If many cells are connected in series, shading of individual cells can lead to the destruction of the shaded cell or of the lamination material. Bypass diodes are connected anti-parallel to the solar cells to protect against this possibility. As a consequence, larger voltage differences cannot arise in the reverse-current direction of the solar cells. In general, there is one bypass diode for every 15 to 20 cells. Photovoltaic panels include one or more PV modules assembled as a pre-wired, field-installable unit. Photovoltaic panels are assembled into solar arrays.

Solar cell technologies

Commercial solar cells are available in three primary technologies; monocrystalline, polycrystalline, and amorphous thin-film. Both monocrystalline and polycrystalline solar cells are made with silicon. Thin-film solar cells are made from various materials, the most common being; amorphous silicon, copper-indium-gallium-diselenide (CIGS), and cadmium telluride (CdTe).

Polycrystalline and monocrystalline are most efficient when they are oriented perpendicular to the sun’s rays. Both polycrystalline and monocrystalline experience decreased output if the sun’s rays are not perpendicular. They also lose efficiency due to partial shading or shadows from clouds, leaves, adjacent buildings, etc. And they lose efficiency in increasing temperatures. Monocrystalline solar cells have efficiencies from 15 to 22%, in some cases higher. Efficiencies of polycrystalline cells typically range from 13 to 16%. As a result, even though monocrystalline cells cost more, they are often used in space-constrained installations such as residential rooftops. Polycrystalline cells are often selected as a lower-cost alternative to monocrystalline cells.

Thin-film panels require more surface area to produce the same amount of energy as monocrystalline cells. With efficiencies in the 13 to 16% range, they can offer advantages over polycrystalline silicone in some applications. They produce energy in a wider range of conditions, and they continue producing energy when it’s cloudy or partially shaded. They are flexible and lightweight. Thin-film solar cells can also withstand the summer heat, where mono or polycrystalline panels lose efficiency in the same ambient temperatures. Finally, amorphous thin-film panels absorb a wider spectrum of visible light, giving them another advantage over mono or polycrystalline panels. In a direct comparison with mono or polycrystalline panels, amorphous cells outperform them in low light conditions.

In addition to the three primary technologies, solar cells can be fabricated from various organic materials (most organic solar cells are made with polymers or plastics) and perovskite, a calcium titanium oxide mineral composed of calcium titanate (CaTiO3). Both organic and perovskite solar cells are still in development and not ready for wide-spread commercialization. Polymer solar cells have inefficiency and stability problems. Current perovskite solar cells suffer from similar problems, and the lead content in perovskite solar cells adds an environmental concern.

Solar cell and module efficiency

With typical efficiencies under 25%, it is easy to see that most of the light striking a solar cell does not contribute to electricity generation. Multiple solar cell design factors play roles in limiting a cell’s ability to convert the sunlight it receives.

Wavelength — only photons with a specific energy level (wavelength) can dislodge electrons in a solar cell and generate electricity. The majority of photons are reflected or pass through the cell or generate heat.

Recombination — One way for electric current to flow in a semiconductor is for a “charge carrier,” such as a negatively-charged electron, to flow across the material. Another such charge carrier is known as a “hole,” representing the absence of an electron within the material and acts as a positive charge carrier. When an electron encounters a hole, it may recombine and cancel out its contributions to the electrical current. Direct recombination, in which light-generated electrons and holes encounter each other, recombine, and emit a photon, reverses the process from which electricity is generated in a solar cell. It is one of the fundamental factors that limit efficiency, and limiting recombination is a key goal of solar cell designers.

Temperature — Solar cells generally work best at low temperatures. Higher temperatures cause the semiconductor properties to shift, resulting in a slight increase in current, but a much larger decrease in voltage, reducing overall power output. Solar cell efficiency is typically measured at 25°C or 45°C, depending on the standard being used.

Reflection — A cell’s efficiency can be increased by minimizing the amount of light reflected away from its surface. For example, untreated silicon reflects more than 30% of incident light. Anti-reflection coatings and textured surfaces help decrease reflection. A high-efficiency cell will appear dark blue or black as a result of the anti-reflection coating.

As a result of improvements in solar cell materials and manufacturing processes, solar cells’ efficiency has continuously increased over time.

When a load resistance is connected to a cell or module’s two terminals, the current and voltage being produced will change as the load changes. Efficiency measurements are obtained by exposing the cell to a constant, standard level of light, typically 1000 Watts/m², while maintaining a constant cell temperature and measuring the current and voltage produced for different load resistances. The results are efficiency curves that measure either power versus output voltage (P-V curve) or current versus output voltage (I-V curve). These curves are nonlinear, and their specific shapes have important implications for the design of inverters for PV power systems. Those implications will be one of the topics of the third FAQ in this series, “Under the hood of PV inverters.”

For applications such as spacecraft that demand the highest possible efficiency, multi-junction solar cells are often used. Multi-junction solar cells have multiple p–n junctions made of different semiconductor materials. Each material’s p-n junction will produce an electric current in response to different wavelengths of light. The use of multiple semiconducting materials allows the absorbance of a broader range of wavelengths, improving the cell’s sunlight to electrical energy conversion efficiency. This increased fabrication complexity makes multi-junction solar cells too expensive for most terrestrial-based applications.

Photovoltaics learning curve

In addition to efficiency improvements over time, various solar cell technologies are coming down in cost. Learning curves, also called experience curves, are graphical representations of the relationship between cumulative production experience and the cost to produce a given device or product. Swanson’s law is a learning curve for photovoltaic modules. The observation that photovoltaic modules’ price tends to drop 20 percent for every doubling of cumulative shipped volume. At current rates, costs go down 75% about every 10 years. It is named after Richard Swanson, the founder of SunPower Corporation, a solar panel manufacturer.

Swanson’s law has been compared to Moore’s law, which predicts the growing computing power of processors over time. Crystalline silicon photovoltaic cell prices have fallen from $76.67 per watt in 1977 to $0.36 per watt in 2014. Plotting the module price (in $/Wp) versus time shows a dropping by 10% per year.

Emerging PV technologies

Two important emerging PV technologies are bifacial PV modules and concentrator PV (CPV) technology. Bifacial modules can collect light on both sides of the PV cells, improving electricity generation, depending on environmental conditions. Ground-mounted bifacial modules provide about a 10% gain in annual electricity yields compared to the monofacial counterparts for a ground albedo (percentage of the incident light reflected by a surface) coefficient of 25% (typical for concrete and vegetation groundcovers).

The gain can be increased to about 30% by elevating the module one meter above the ground and enhancing the ground albedo coefficient to 50%. High albedos can be obtained using white concrete, highly reflective roof foils, or snow and ice. The International Technology Roadmap for Photovoltaics (ITRPV) predicts that bifacial technology’s global market share will expand from less than 5% in 2016 to 30% in 2027.

CPV utilizes low-cost optics to concentrate light onto a small solar cell. By reducing the area of PV cells needed, more resources can be focused on high-efficiency cells. Lenses or curved mirrors are used to focus sunlight onto small, highly efficient, multi-junction solar cells. CPV systems typically use solar trackers and can include a cooling system to increase efficiency further.

Systems using high-concentration photovoltaics (HCPV) especially have the potential to become competitive soon. They possess the highest efficiency of all existing PV technologies, and a smaller photovoltaic array also reduces the balance of system costs.  Commercial HCPV systems have reached efficiencies of up to 42% under standard test conditions (with light concentration levels above 400 suns). The International Energy Agency sees potential to increase this technology’s efficiency by 50% by the mid-2020s.

Unfortunately, there are significant limitations to the use of CPV technology. Diffuse light, which occurs in cloudy and overcast conditions, cannot be highly concentrated using conventional optical components. Filtered light, which occurs in hazy or polluted conditions, has spectral variations which produce mismatches between the electrical currents generated within the series-connected junctions of spectrally “tuned” multi-junction photovoltaic cells. These CPV features lead to rapid decreases in the power output when atmospheric conditions are less than ideal.

The next FAQ in this series will consider, “PV module specifications and performance parameters.” The third and final FAQ will look “Under the hood of PV inverters.”

References:

International Technology Roadmap for Photovoltaics, VDMA
Solar cell, Wikipedia
Solar electricity basics, Florida Solar Energy Center
Solar PV – tracking progress, International Energy Agency
Swanson’s law, Wikipedia