How do we get more energy from the sun?

So how can we squeeze more energy from the sun? That is a question driving tremendous amounts of research and innovation since the sun sends down an enormous amount of energy every hour — enough to power the Earth for a year. Our problem isn’t lack of solar power; it is how well we capture, convert and store it. Most of the advancement in solar energy will come from improvements in current photovoltaic technologies and discovery of new materials to make those technologies. Another way to advance solar energy is to develop smarter ways to install and manage solar systems.

Photovoltaic evolution: Today’s technologies

Most people associate solar energy with photovoltaic (pv) cells, i.e., those shiny panels you see on top of houses and in desert fields. Those pv cells take sunlight and turn it directly into electrical power using the photovoltaic effect. At present, the most prevalent form of photovoltaic technology is based on silicon.

Silicon-based solar panels: Crystalline Silicon

At present, the bulk of the world’s solar panels are constructed using Crystalline Silicon. There are two types of Crystalline Silicon panels — monocrystalline and polycrystalline. Monocrystalline silicon panels are recognized by their uniform black color, because they are produced from a single, pure silicon crystal. Commercial monocrystalline silicon panels achieve efficiencies ranging from 18-22 percent. Laboratory demonstrations show significantly higher efficiencies. Polycrystalline silicon panels are produced from many silicon crystals fragmented together. As such, they are slightly less efficient than monocrystalline silicon (commonly 15-17%), but are much cheaper to manufacture. Even though Crystalline Silicon has been around for a while, researchers continue to investigate ways to perfect manufacturing techniques so that defect-free modules can be made at reasonable prices and reach theoretical Limits of efficiency. Companies like SunPower regularly exceed commercial efficiencies using innovative cell architectures.

Flexible thin-film solar cells

In addition to Crystalline Silicon, there is another important group of solar cells called thin-film solar cells. Thin-films are layers of semiconductor material deposited onto a base material like glass, plastic or metal. The most common thin-film materials are cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and amorphous silicon (a-si). Generally, thin-film solar cells are less efficient than Crystalline Silicon (usually 10-14%), but they tend to be lighter, more flexible and perform better under low light conditions or at high temperatures. An example of a major manufacturer of CdTe thin-film panels is First Solar. Research continues to improve the quality of the extremely thin films used in these devices and find inexpensive alternatives to elements such as tellurium and indium.

Advancing photovoltaics beyond silicon

There are other families of materials being developed with the purpose of creating even more powerful solar energy sources. Many times, these newer materials promise improved efficiencies, reduced costs, or different applications.

Perovskite solar cells: the newcomer

Perovskite solar cells may represent one of the biggest developments in the last few years. Perovskite materials (often organic-inorganic hybrid compounds) exhibit surprisingly rapid increases in efficiency. In some cases, perovskite solar cells have demonstrated efficiencies comparable to that of traditional Crystalline Silicon cells (sometimes >25%). One of the perovskite advantages is low cost of production, flexibility and tunability (meaning they can be engineered to absorb a variety of frequencies within the solar spectrum). Although perovskite solar cells demonstrate great potential, there are still several challenges that need to be overcome before commercial viability is realized including long term stability and scalable production without toxic substances. Researchers at Oxford University and many others are leading efforts in this area as they continually improve the chemistry.

Multi-junction solar cells: layered approach

For applications requiring maximum efficiency where space is limited — like spacecraft or Concentrated Solar Power (CSP) systems — multi-junction solar cells provide substantial advantages. Multi-junction solar cells consist of stacked layers of different semiconductor materials optimized to capture various portions of the solar spectrum. When combined, these layers achieve significantly higher efficiencies — frequently above 40%, and occasionally >47% in experiments — than single layer silicon cells. However, because of their complexity and associated expense, multi-junction solar cells are rarely deployed on Earth. Efforts are underway to reduce their cost while preserving their superior performance characteristics.

Emerging materials: Quantum Dots & organic pv

Additional emerging technologies include quantum dot solar cells, which utilize tiny semiconductor nanoparticles that can be tailored to absorb selected wavelengths of light, and organic photovoltaic (OPV) cells, which employ conducting polymers and small molecules. Each offers potentially low cost and/or flexibility. However, their efficiencies are generally lower than those exhibited by silicon and perovskites. Niche markets like integrated electronics and transparent solar windows are good candidates for both opvs and qds.

Beyond the module: optimizing system performance

While all advances help maximize energy generation from modules themselves, equally important is how we install, manage, and coordinate these systems to obtain maximum benefit from available energy.

Concentrating solar power: focusing heat

Unlike traditional photovoltaic technology that converts sunlight directly into electrical power, concentrating solar power (CSP) uses mirrors or lenses to concentrate sunlight onto a smaller area generating heat, similar to a steam engine. The resulting heat is used to drive a conventional steam turbine that produces electricity in much the same manner as traditional power plants. Some CSP systems also contain thermal energy storage — commonly molten salt — enabling them to produce electricity even after sunset. Examples of large-scale CSP installations include projects like the Ivanpah Solar Electric Generating System in California. Potential areas for further optimization exist with respect to mirror design, heat transfer fluids, and turbine design.

Following the sun: solar tracker technology

Static solar panels only work best for a portion of each day, when the sun is overhead. Solar tracker systems dynamically position solar panels so that the face of each module is perpendicular to the sun’s ray direction at each point in time throughout each day. Single-axis trackers move the panels east-west, while dual-axis trackers adjust both horizontally and vertically seasonally. Depending on the location of installation, tracker systems have been reported to increase energy yields by 15-30% relative to stationary arrays.

Increasing reliability: energy storage

As we know now, one major drawback of solar energy is that it is an intermittent resource that only works when the sun is shining. Therefore, in order to enable more reliable utilization of solar energy for electric loads throughout the day/night cycle, it is essential that cost-effective and efficient energy storage methods be developed and implemented. Batteries, primarily lithium-ion, are currently the most popular battery technology used in solar energy storage applications. However, considerable effort exists for development of flow batteries, solid state batteries, as well as hydrogen production through electrolysis driven by solar energy. Ideally, excess solar-generated electricity will be stored during periods of optimal sunlight hours for later use during nighttime or cloud covered hours.

Increased efficiency using Smart Grids & artificial intelligence: smarter use of available energy

A second approach to increasing the overall utilization of available solar energy includes developing Smart Grids capable of optimizing distribution and use of solar energy. Additionally integrating artificial intelligence (ai) and machine learning capabilities enables prediction of solar irradiance levels based upon weather forecasts as well as intelligent control over charging/discharging cycles for energy storage systems. Overall, ai integration enables better utilization of available renewable energy resources (like solar), reduces waste in the grid system and supports stable operation of the grid during periods when available energy is fluctuating greatly. For instance, the National Renewable Energy Laboratory (NREL) is actively engaged in researching modernization of the nation’s grid.

Future outlook: brighter skies ahead

The road to obtaining additional energy from our star involves scientific research and technological innovations in multiple disciplines including materials science, engineering and system management. Whether through enhancements in conventional silicon panels or exploration of novel technologies like perovskites or creation of more intelligent grids for managing solar energy systems — each incremental improvement brings us closer to a future with most of our planet’s energy needs supplied by our star.