New Materials, Efficiency Upgrades Advancing Solar Technologies

The growth curve for solar power continues its upward trajectory, thanks to technologies such as perovskites, heterojunction solar cells, and energy storage systems designed to complement rooftop solar. New inverters and racking solutions are making solar installations more efficient, helping extract more energy from the sun's rays.

There is no shortage of technological advancements in the solar power sector, contributing to renewed growth in the industry after the coronavirus pandemic paused several projects. More countries are turning to solar as they establish decarbonization targets, more companies are setting sustainability goals, and the push for home improvements-an offshoot of the pandemic-has supported residential rooftop solar.

Researchers have continued to develop more-efficient solar energy equipment, and the market is supporting innovation. A two-year extension of the 26% Investment Tax Credit (ITC) for solar power, passed by the U.S. Congress earlier this year, has provided more impetus for homeowners and businesses interested in adding solar.

And an important factor in the growth of solar power may not be a better solar panel or inverter, but rather deployment of energy storage to support solar development. The most pressing technology issue for the solar industry at this time is securing safe, reliable, and low-cost storage," said Suvi Sharma, founder of Solaria, a California-based solar technology and installation company. Solar has become very economically viable in most parts of the country. It's ramping up and getting installed in virtually every state. It's competitive with the grid. But the one thing solar cannot do is produce energy at all times of the day. To get the maximum value out of solar, we need to store the power that solar systems generate, whether the system deployed is residential, commercial, or utility-scale."

Extending the ITC also benefited energy storage systems. If these systems get at least 75% of their charge from an onsite renewable energy system, they are eligible for the tax credit as well. And government officials are well aware of the need to bring down costs to support more solar deployment; the U.S. Department of Energy (DOE) earlier this year set a target of reducing the cost of solar energy by 60% within the next decade, and pledging millions of dollars to support new solar power technologies.

Collaborative Efforts

Partnerships and collaborations are supporting the rapid pace of technology advancements in the solar sector. Researchers at the National Renewable Energy Laboratory (NREL) and Colorado School of Mines in October announced they are applying a new technique to identify defects in silicon solar cells that reduce efficiency. The groups said the lessons learned from their research could lead to improvements in the way manufacturers strengthen their products against what is known as light-induced degradation [LID]."

The groups said LID reduces the efficiency of silicon solar cells by about 2%, adding up to a significant drop in power output over the 30- to 40-year lifespan of the technology deployed in the field." Silicon solar cells make up more than 96% of the current global market. The most common semiconductor used to manufacture these cells is made from boron-doped silicon, which is susceptible to LID, so manufacturers have looked for ways to stabilize the solar modules. NREL researchers said that without an understanding of the defects at the atomic level, it's impossible to predict the stability of those modules.

Some of the modules are stabilized completely. Some of them are only half-stabilized," said Abigail Meyer, a Ph.D. candidate at Mines and a researcher at NREL. Meyer is lead author of a paper about efforts to determine the source of the LID phenomenon. Her co-authors include researchers from both Mines and NREL, among them Paul Stradins, a principal scientist and a project leader in silicon photovoltaic research at NREL. Stradins said the problem of LID has been studied for decades, but the exact microscopic nature of what causes the degradation has not been determined. Researchers have concluded, through indirect experimentation and theory, that the problem decreases when less boron is used or when less oxygen is present in the silicon.

The collaboration between NREL and Mines, with research funded by the Solar Energy Technologies Office within the DOE, relied on electron paramagnetic resonance (EPR) to identify defects responsible for the LID. The microscopic examination revealed a distinct defect signature as the sample solar cells became more degraded by light. The defect signature disappeared when the scientists applied the empirical regeneration" process to cure the LID that industry has adopted. The researchers also found a second, broad" EPR signature affected by light exposure, involving many more dopant atoms than there are LID defects. They hypothesized that not all atomic changes induced by light lead to the LID. The researchers said the techniques developed to study LID can be extended to reveal other types of degrading defects in silicon solar cells, and also in other semiconductor materials used in photovoltaics including cadmium telluride and perovskites.

Maximizing Panel Efficiency

Solar cell and module developers continue to look for ways to maximize photovoltaic (PV) panel efficiency. JinkoSolar and LONGi, two Chinese manufacturers, have surpassed solar conversion efficiencies of 25% for their crystalline silicon technologies. Australian researchers have developed a bifacial silicon solar cell with an efficiency of 24.3% on the front and 23.4% on the rear, for an effective output of about 29%. UK-based Oxford PV this past year announced a new efficiency record for its perovskite solar cells at 29.52%. Oxford PV completed construction of the manufacturing site for its perovskite-on-silicon tandem solar cells in July, and expects to begin full commercial production in 2022.

Solliance Solar Research, a consortium based in the Netherlands, in late October said researchers from three of its partners had achieved a 29.2% power conversion efficiency on a transparent bifacial perovskite solar cell combined with a crystalline silicon solar cell in a four-terminal tandem configuration. The group said the cell is based on a highly near-infrared transparent perovskite cell built by the Netherlands Organisation for Applied Scientific Research (better known as TNO) and Belgian laboratory EnergyVille, along with an 11.4%-efficient c-Si interdigitated back contact silicon heterojunction cell developed by Panasonic. EnergyVille has touted its work on tandem configurations, saying, By combining two (or more) different solar cells with carefully selected material properties on top of each other in so-called tandem configuration, we can convert a wider part of the light spectrum into electrical energy. In this way we surpass the physical limitations of single solar cells." That is, by combining a perovskite top cell on a silicon bottom cell, EnergyVille is aiming at +30% tandem energy conversion efficiency, which is larger than the theoretical maximum of silicon solar cells of about 28%.

Crystalline silicon technology accounts for the vast majority of the solar power market. In the U.S., though, supply chain issues and trade restrictions on imports from China-including concerns about the production of polysilicon in Xinjiang-have opened doors for thin-film producers. Arizona-based First Solar, which produces cadmium-telluride (CdTe) solar modules and panels, this summer said it is investing nearly $700 million to build a third U.S. manufacturing plant, which will expand its domestic production capacity by 3.3 GW. The company also announced construction of a similar 3.3-GW plant in India.

Chinese manufacturer China National Building Materials, which produces thin-film copper indium gallium diselenide panels, recently said it was expanding its production, adding about 1 GW of capacity for CdTe modules.

New Solar Cells

Sharma told POWER, The most important technology development for solar panels and systems is the emergence of n-type solar cells." Sharma said the two most common n-type solar cells are TOPCon (passivated contact) and heterojunction. A hetereojunction solar cell combines two different technologies into one cell: a crystalline silicon cell set between two layers of amorphous thin-film silicon. The technologies used together allow more energy to be harvested compared to using either technology alone.

N-type solar cells are made from a different chemical composition of wafer," said Sharma. There's going to be a significant evolution over the next three to five years of cell manufacturing. What's predominantly produced now is p-type mono PERC cells [monocrystalline silicon cells]... these will all start migrating to n-type TOPCon and heterojunction cells."

Sharma said, Manufacturing of n-type cells is not made for a specific niche. It's not a specific application; these new cells will improve the efficiency of all solar panels and all applications. This advancement is something that will have a significant impact on the entire industry and all solar deployments-by improving the energy efficiency of all different types of PV panels."

Solaria is set to launch its new PowerXT 430R-PL (430 watt) solar panel in March 2022. The panel will be optimized for next-generation module level power electronics (MLPE), which are devices that can be incorporated into a solar PV system to improve its performance in certain conditions, such as in shade. MLPE devices include microinverters and direct-current (DC) power optimizers-all designed to improve the energy production of the solar power system.

Tracking the Sun

New racking systems also are increasing the efficiency of solar arrays. Solar FlexRack in October announced that its solar trackers have now been installed in more than 80 solar projects on California farms, including at a 2.82-MW project for Danell Brothers Dairy (Figure 1), south of Hanford. The array was installed by Renewable Solar Inc., which installs commercial and agricultural solar projects in California.

1.Solar FlexRack has installed its solar tracking systems at projects across California, including at agricultural sites, such as this installation at the Danell Brothers Dairy. Courtesy: Solar FlexRack

More than 150 of California's dairy farms are now generating solar energy, as more and more such energy-intensive operations are opting for solar energy to reduce operational costs. We're proud to have been able to partner with Renewable Solar Inc. to deliver high-quality clean energy systems and associated cost savings for California farm owners over the years," said Steve Daniel, executive vice president of Solar FlexRack. We look forward to working together further with Renewable Solar Inc. on additional agricultural solar projects in support of California's nation-leading renewable portfolio standard."

Solar FlexRack's Series G racking is offered in both landscape and portrait orientations, to maximize energy production depending on location. The rack features lateral bracing, to stabilize and square the racking system for easier installation. The horizontal rail bracket allows the horizontal rail to be set in place without the need for bolts, which reduces installation time. The rack can accommodate up to a 20% east-west slope, again to maximize energy production.

2. Solar power producers are constantly seeking ways to optimize energy production from their equipment, including trackers, which follow the sun and can enable higher output in both the early morning and late afternoon. Courtesy: Nextracker

California-based Nextracker in early November said it was the first solar tracker (Figure 2) equipment and software provider to surpass 50 GW in global shipments. The company said its equipment is used in major solar power plants in 40 countries.

Nextracker's technology advancements include its NX Horizon solar tracker, which features a balanced mechanical design that delivers bifacial energy production. The company's signature TrueCapture smart control software is helping utility-scale solar power plants mitigate drops in power output triggered by cloud cover, or when one row of panels casts a shadow over panels in neighboring rows.

The company said its latest advancement is the Split Boost algorithm, which optimizes energy yield for split-cell silicon PV modules. Defne Gun, a technical sales engineer for the company, wrote on the company's website: We model Split Boost with our internal raytracing-based backtracking software where the shade tolerance of the module, as well as Split Boost operating mode, are baked into our row-to-row energy gain algorithm, so we can accurately estimate gains. By using the algorithms in simulation mode' before deployment to a solar plant, we can estimate TrueCapture performance at a given site with that site's specific energy model, tracker geometry, and terrain."

Tigo Energy, known for its Flex MLPE systems, in September said its Energy Intelligence (EI) inverter and battery product lines were now available to U.S. residential solar installers. The company said the new inverter and battery products support native integrations of the company's solar and storage components, and are an extension of the Tigo Enhanced commercial and industrial solar partnership program into the residential market.

The new EI Battery and Inverter products provide a very simple installation and commissioning process as well as powerful fleet management features. The end customer, in turn, will benefit from access to an abundance of resilient, renewable, and safe energy with a system that can be precisely tailored for price and performance," said Zvi Alon, Tigo Energy CEO, in a news release.

Storage Key to Growth

Sharma reiterated that developing storage solutions hand-in-hand with solar power will be key to supporting industry growth. Energy storage is still relatively expensive," Sharma told POWER. And it doesn't make economic sense yet in many applications. [But] in residential applications, energy storage makes sense: for resiliency, for security, and for powering through blackouts. It's playing a very important role, especially as we experience increases in incidences of extreme weather. But to really unlock the next phase of solar, we need lower-cost storage across all applications."

Companies already are marketing products designed to support residential solar and storage. Tesla's Powerwall is among the best known; the Powerwall stores solar energy to provide backup power when the grid goes down. Generac Grid Services, a sponsor of POWER's Distributed Energy Conference this past October, recently launched its PWRgenerator, a new type of DC generator designed to rapidly recharge Generac's PWRcell Battery. The DC-coupled PWRgenerator can enable the PWRcell Battery to keep a home powered for a longer period of time during outages.

The PWRgenerator connects directly to the PWRcell inverter; Generac said this essentially creates a residential nano-grid allowing a home to be fully energy independent." A home's solar panels provide power to the home during the day, with excess power charging the battery. At night, the battery discharges, and if the state of charge reaches 30%, PWRgenerator-which can run on either natural gas or propane-will turn on and fully charge the battery in about an hour.

Other solar-plus-storage residential systems include Panasonic's EverVolt; LG's Home Battery RESU (Residential Energy Storage Unit); and smaller systems such as Jackery's Solar Generator line, and Goal Zero's 6000X portable battery model. And technology advancements for solar energy are making headway elsewhere, in areas such as automotive and building applied PV; a host of consumer electronics with solar-charging capability; and wearable mobile power.

Our industry is experiencing an acceleration in innovations that improve solar [performance]," said Sharma, who added there's an increasingly greater focus" on efficiency and aesthetics. That's where we're going to continue to see greater progress and advancements."

-Darrell Proctor is a senior associate editor for POWER (@POWERmagazine).

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