Forty-five scientists and industry leaders envision global PV capacity soaring from 500 GW now to 20 times that amount by 2030, through 30% average annual growth in PV deployments.

But it won’t happen by magic. Given robust PV research programs, PV costs will keep falling, say the authors of the Science journal article “Terawatt-scale photovoltaics: Transform global energy”. Yet maintaining the 30% average growth rate in PV deployments, observed from 2008 to 2018, over the next 10-14 years will involve “opportunities and challenges at the systems level,” they say.

In three compact pages, the authors synthesize expert knowledge on the path ahead.

PV generation will need to be balanced with energy storage and coordinated demand response “to a much greater extent than is used today,” they say, as well as wind power, aided by long-distance transmission. Moreover, PV at increasing levels will need to provide grid services, and at very high levels, as fossil-fueled synchronous generators are retired, “PV systems will need to generate their own voltage reference waveforms and synchronize together.”

To reduce battery storage costs toward a target of $150 per kilowatt-hour, the authors say, research should explore materials with higher energy density, “focusing on nickel-rich, critical-materials-free cathodes and advanced anodes for lithium-ion systems.” With cost reductions, flow batteries could compete with lithium ion batteries. And pumped-storage hydropower has substantial technical potential, “potentially at low cost.”

The “biggest challenge” may be meeting energy needs in winter at high latitudes, says the paper, noting that wind power may be helpful in these regions, and lower population densities than at low latitudes reduces the scale of the challenge.

From 2030 to 2050, the path toward 30 to 70 terawatts of PV globally would involve “major electrification” in heating, transportation, desalination, and industrial sectors, matched with annual PV deployments increasing only 2% per year beyond the level reached by about 2030. In the second graph below, the gray triangle represents increased electrification, and the rising blue lines represent three scenarios for PV growth beyond about 2030.

PV can play a “critical role” in electrifying the transport sector, the authors say. Heating can be electrified using heat pumps with a coefficient of performance of 3 to 4, which would also enable thermal storage that may cost less than battery storage, they add.

“A growing body of research,” says the paper, “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.”

In addition, photovoltaic power-to-gas or power-to-X approaches can be used to store PV power as fuels, such as hydrogen or ammonia, or to produce chemicals used in industrial processes.

PV research, deployment, and manufacturing

As PV researchers pursue roadmaps to increase solar cell efficiency across all established PV technology areas, their progress will help drive costs down “as the market grows to terawatt scale,” say the authors. (Increased cell efficiency means more watts per module, and more megawatts per installation, bringing down costs per watt.)

PV research roadmaps, and recent advances cited in the article, include:

In silicon, with ~95% market share in 2018, “there is a push for developing low-cost ‘passivating contact’ solar cells, with higher efficiency thanks to a reduced carrier recombination at the metal contact areas.” At laboratory scale, 26.7% efficiency has been achieved by a silicon solar cell using an n-type rear interdigitated back contact heterojunction.

Single-junction record efficiencies have been reached in thin-film copper indium gallium diselenide of 23.4%, and gallium arsenide (GaAs) of 29.1%.

For cadmium telluride (CdTe), “increased hole density in polycrystalline films by 2-3 orders of magnitude could provide a viable path to achieving higher efficiency.”

Advances in organic-inorganic hybrid perovskite PV include record efficiencies at the experimental scale from 20.9% to 24.2%. “The combination of high external radiative efficiency, steep absorption edge, and an open-circuit voltage ratio to the bandgap at >90%, in a solution-processable system, continues to attract attention.”

“Mechanically stacked GaInP/GaAs/Si and monolithic perovskite/silicon tandem cells have achieved 35.9% and 28.0% efficiency, respectively.”

To help ensure PV reliability, “increasingly accurate modeling” of PV designs will be needed, “relative to snow, wind, and other climate and application-specific mechanical stresses,” say the authors. That is partly because cost reduction by minimizing materials usage, and efficiency improvement by new solar cell designs, “will likely continue to introduce new failure modes.”

To avoid a potential bottleneck in silver supply by 2030, the authors call for reducing silver usage in PV manufacturing through targeted research and development, perhaps replacing silver with copper, coupled with recycling efforts.

The authors do not see challenges in financing new PV factories, noting that a capital expenditure of $120 million can fund a 1-GW/year factory with ingot-wafer-cell-module production lines for monocrystalline PERC (passivated emitter rear contact) silicon technology.

The authors note California’s growth in PV generation generated in-state from less than 1% in 2010 to about 18% in 2018. As they are writing for a wide audience that may be unfamiliar with recent dramatic PV cost declines, they note that “PV already is or will soon become cost-competitive with conventional electricity generation in many parts of the world.”

The study is a collaboration among experts mostly from the U.S., Germany, and Japan. Led by senior author Nancy Haegel of the National Renewable Energy Laboratory (NREL), the 44 other co-authors work either at NREL, comparable solar energy research institutes in Germany, Japan, and other countries, universities, firms in the PV industry, including U.S.-based firms First Solar, SunPower, Sinton Instruments, and Siva Power, or a government agency.

The study was informed by the 2nd Terawatt Workshop convened last year by the Global Alliance of Solar Energy Research Institutes, encompassing NREL, Germany’s Fraunhofer Institute, and Japan’s National Institute of Advanced Industrial Science and Technology.