Five-hundred gigawatts (GW) of solar power capacity had been installed globally as of year-end 2018 and another 500 GW is expected to be installed by 2022–2023, ushering in an era of terawatt-scale solar power, according to an international group of solar energy scientists .

The pace of reductions in the cost of solar power and increases in the scale of manufacturing caught the group of scientists by surprise, leading them to revise upwards the forecast they made two years ago. At that time, Science published an article in which the scientists focused on the challenges of achieving 3–10 terawatts (TW) of solar power capacity by 2030. In a follow-up article, they now envision and come to grips with the challenges associated with ~10 terawatts (TW) of solar power being deployed worldwide by 2030 and as much as 70 TW by 2050, which would make solar the largest energy resource globally.

Alluding to the electrification of energy across entire economies and societies,

The very rapid decline in PV pricing realized to date—the price of PV has declined more than two orders of magnitude in the last 40 years—and the prospect for continued declines at both the PV module and system levels creates an opportunity for PV to serve as a central contributor to all segments of the global energy system in a cost-effective and environmentally sound manner.

—Robert Margolis , report co-author and group manager of the U.S. Department of Energy’s National Renewable Energy Laboratory’s (NREL) Washington D.C. Office in the Strategic Energy Analysis Center, told Solar Magazine.

Decline in solar energy costs, increase in manufacturing capacity catches scientists by surprise

The confluence of several factors is fueling solar PV growth worldwide and at a pace that has surprised many, including the scientists that authored the article, Margolis explained. Prominent among them, “As the PV industry has scaled production, it has developed a dedicated supply chain for both manufacturing equipment and materials, which has brought down cost dramatically. The industry is also in a period of fierce competition, which has pushed companies to take out cost where ever they can and to push innovation to increase efficiency of PV technology as well as efficiency of their manufacturing processes,” he said in an interview.

Margolis is one of an international group of leading PV scientists assembled by the Global Alliance for Solar Energy Research Institutes (GA-SERI) who collaborated in writing the paper. GA-SERI was created by and is made up of the Fraunhofer Institute for Solar Energy Systems ISE (Germany), the National Institute of Advanced Industrial Science and Technology AIST (Japan) and the National Renewable Energy Laboratory NREL. GA-SERI’s member scientists, joined by researchers from other organizations and countries, have been discussing the opportunities and challenges associated with achieving terawatt-scale solar generation capacity since 2016.

In their updated article, the group summarizes what’s needed with regard to PV performance research, reliability, manufacturing and recycling in order to realize their updated forecast, as well the ramifications and challenges for complementary technologies, such as energy storage, power-to-gas and liquid fuels and chemicals, grid integration and electrification across multiple economic sectors.

Whither from here for solar energy growth?

The costs of deploying solar energy generation have dropped faster than the GA-SERI group expected two years ago. At that time, they pointed out that, given ongoing declines along the historical learning curve, PV module prices would decline to USD0.50 per Watt (W) and USD0.25/W at a cumulative deployed capacity of 1 TW and 8 TW, respectively.

As it turned out, average costs dropped below USD0.25 with just 500 GW of PV capacity installed globally by the end of 2018. The costs of dominant fossil-fuel and nuclear power generation in Germany, Japan and the U.S., in contrast, have remained relatively constant over the period.

Rapid growth of solar power penetration has raised questions about the growth path solar energy will take in coming years. The article’s authors point out that some research has suggested that the value of solar PV will decrease as penetration increases given current electricity generation operation practices. More recently, researchers have identified ways in which much higher levels of solar energy could be incorporated in electricity generation grids given changes in those operational practices.

“California is already implementing some of those operational practices, enabling annualized utility-scale PV plant curtailment to stabilize around 1 to 2%,” according to the research group.

However, if renewable electricity generation continues to increase rapidly without substantial storage and/or load shifting, then curtailment could increase. The challenge is to develop low-cost operational strategies and complementary technologies to accommodate the growing fraction of renewable generation.

Targeting the entire energy economy

More broadly, the scientists highlight that electricity demand could be increased via electrification across entire economies and societies. That includes heating, transportation, desalination and industrial sectors.

“A growing body of research concludes that decarbonization of electricity followed by electrification of almost all parts of the energy system is a least-cost pathway for a low-carbon sustainable energy system, with many possible scenarios for PV growth,” they state.

Multiple prospective pathways leading to the transformation of the global energy system have emerged, however. Opportunities and challenges present themselves at the systems level and regarding technological research in related sectors when it comes to solar providing a majority share of the total energy economy, they highlight. Multiple prospective pathways leading to the transformation of the global energy system have emerged, however. Opportunities and challenges present themselves at the systems level and regarding technological research in related sectors when it comes to solar providing a majority share of the total energy economy, they highlight.

The scientists zoom in on the use of utility-scale solar power plants in their paper. “In many ways, we are just at the beginning of thinking about how large PV plants can be operated to support the grid more broadly,” Margolis said.

Transforming the way power grids and networks are designed and operate

Making use of electricity generated by solar PV to provide spinning reserve capacity and using a combination of day-ahead forecasting and planning curtailment so that PV systems produce electricity below the maximum possible are two examples. “With this type of operational strategy, it has been demonstrated that PV can ramp up or down to provide spinning reserves, and because other plants are not kept online to provide these spinning reserves it can actually reduce the amount of curtailment required by PV. In short, planning curtailment, along with good forecasting, can reduce curtailment and provide a required service to the system,” Margolis said.

The key to reducing curtailment as PV penetration increases is to increase the flexibility of the system. This can be done by shifting PV output, through storage, or shifting demand, through load shifting.

—according to Margolis.

Margolis offered the demand for hot water in a home as an example. “If one thinks about hot water demand in a home, what people want is hot water when they want it. They don’t really care when the water is heated.”

A solar homeowner has three choices if they’re generating excess electricity during the day and the grid won’t take it, Margolis continued. They could:

Curtail it—throw it away; Store it in a battery, given batteries are cheap enough to warrant homeowners to purchase them for this or other purposes; or Use it to preheat their hot water. “There will be some losses, but basically a hot water heater is like a big battery, which is how we model it,” Margolis elaborated.

Addressing two key challenges to multi-terawatt solar power generation

More broadly, the scientists identify two wide-ranging areas that pose key challenges for reaching the multi-terawatt levels of solar energy capacity they envision and highlight ways they can and are being addressed:

Developing and deploying complementary technologies, for example, energy storage, power-to-gas and liquid fuels and chemicals, grid integration, and multiple-sector electrification; and

Continuing R&D advances in PV, e.g. performance, reliability, manufacturing, and recycling.

Taking power-to-gas and liquid fuels and chemicals as an example, the scientists highlight that industrial production of cement, iron and steel, aluminum, pulp and paper and chemicals and the like consumes about 27% of the fossil-fuel portion of total fuel consumption. “Very low-cost solar could be used to produce hydrogen and ammonia, which could provide a pathway to substantially reduce greenhouse gas emissions associated with the iron and steel and fertilizer industries and provide precursors for chemical and materials industries,” they write.

Similarly, electricity from very low-cost PV and wind energy resources can be used to produce hydrogen, methane or more complex hydrocarbons. “Power-to-gas (PtG) or power-to-X (PtX) approaches could use many TWs of installed capacity of solar and wind generation,” they explain.

In addition, they highlight that electrolysis can be used to store electricity in chemical fuels, such as hydrogen. That would enable solar energy to have an impact across many industrial uses of fossil-fuel energy, steam and heat, such as metals refining, biofuels upgrading, ammonia synthesis for fertilizer production, and synthetic fuel generation. “Such fuels can be synthesized from hydrogen by combining with CO 2 through the industrial-scale reverse water gas shift and Fischer-Tropsch reactions,” they explain.

Furthermore, solar energy could be used to produce ammonia on a cost-competitive basis, below USD400 per metric ton, and hydrogen below USD2 per kilogram in regions where solar production of electricity comes in below USD30 per megawatt-hour (MWh). The capital cost of electrolyzers needs to come down in order for that to happen, however.

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Solar energy R&D, by and large, was originally driven by the need to power the first generation of satellites and a new era of space exploration, the scientists recount. The new wave of solar energy R&D underway today, fueled by investments on the part of leading countries such as the U.S., Germany, Japan, Australia and China, “has brought the PV industry to a point of having greater than USD100 billion/year in revenue,” they wrote.

Research spanning materials science, module design, systems reliability, product integration, and manufacturing will be required to address the challenges related to multi-TW-scale PV deployment. Addressing these challenges could enable PV to play a critical role in transforming the global energy system.

—they conclude.