Utility-scale electricity generation plants using renewable sources in the United States depend on only a few methods for energy capture. For solar energy, photovoltaic (PV) cells and, to a lesser extent, concentrated solar power (CSP) are the technologies of choice. For wind energy and hydroelectric energy, turbines installed in dams and atop hills are used to capture energy of flowing water and air. Biomass is used as an electricity source by first converting it to methane. Lastly, geothermal energy is captured through underground heat exchangers.
Renewables make up 13% of electricity generation in the United States, with about 65% of renewable generation from hydroelectric dams. Homeowners and businesses are also generating energy with renewables: solar PV cells are popular with homeowners due to tax incentives, and industries that need to treat high biomass waste streams find benefits through energy generation. By any assessment, renewables could power much more of the United States via more installations, installations with more efficient technology, and tapping of new renewable sources. Fundamental research and development is constantly producing new ideas for improving the efficiency of existing technologies. Here are a few of these ideas that can upgrade solar, wind, and hydroelectric power.
Improvements in solar technology lie primarily in the materials used in solar cells. Current materials have limited efficiency and raw materials have high environmental footprints.
Thin film technology. While crystalline silicon is the most common photovoltaic material in existing solar cells, amorphous silicon has the potential to be more efficient than crystalline silicon, owing to its ability to be rolled out into thin sheets. The energy captured per mass of photovoltaic material is higher for thin film panels. Cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) can also be formed into thin sheets.
Hybrid photovoltaic-thermal technology. The two methods of solar capture are photovoltaic and thermal. A hybrid device uses thermal energy capture technology to harness the excess energy not captured by a PV cell. The combination also keeps PV cells cooler and thus operating at higher efficiency.
Perovskites as photovoltaic materials. A class of compounds that possess a certain crystal structure has been found to be useful as a photovoltaic material. Cells made with these crystals, known as perovskites, were able to perform equally well compared to existing cells. However, since perovskites can absorb some wavelengths more efficiently than conventional materials, a combined solar cell has better efficiency than either material alone.
The biggest hindrance to wind technology is the intermittent nature of wind as a resource. Unlike solar, which has a predictable energy input rate, wind can change direction and intensity nearly instantly. Technologies that can handle these interruptions will have improved efficiencies.
Energy storage technology. One solution to the problem of intermittent power is to channel any captured energy into a storage device, such as batteries or pumped water, for later discharge at a constant rate. Arguably, wind factories are currently capable of connecting to the grid without storage devices; if wind is to become a major component of an energy portfolio, energy storage will be the only way to seamlessly integrate a large number of intermittent inputs into our grid. Innovations in battery technology, such as the scaling of lithium-ion batteries to utility-scale applications, further increase energy harvesting efficiency.
Hydropower is the oldest form of harvested energy. The efficiency of turbines used for large-scale hydroelectric plants is already quite high, sometimes as efficient as 95%. The main area of innovation for hydroelectric power is accessing currently untapped forms of hydropower.
Low-head hydroelectric technology. In addition to dams currently used for hydroelectric power generation, there are a great number of dams with changes in elevation too low to run conventional hydroelectric turbines. A hydroelectric screw turbine is one solution to harnessing low potential water flows. Innovations to access low-head sources would greatly increase the potential capacity of hydropower.
Hydrokinetic power. The ocean contains a vast amount of kinetic and potential energy derived from the gravitational pull of the moon. The ever-moving ocean, with its tides, currents, and waves, can potentially provide a large amount of energy to coastal cities. Generators can harvest kinetic energy from currents, especially those resulting from coming and going tides since tides move large volumes of water to and from the shore. By placing a turbine horizontally in the tidal flow path, the energy of this water movement can be captured. Wave power can be captured similarly with generators placed near the water surface. Tidal and wave power has not seen widespread adoption at the utility scale, owing primarily to concerns regarding effects on marine ecosystems, as well as a high potential for organic and inorganic fouling of machine components.
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