N-Type Cells: Negatively Doped and More Efficient Than
the PERC Cells They Can Replace
According to
Chris Deline, a research engineer who leads the National Renewable Energy
Laboratory’s photovoltaic field performance group, over the past five years the
solar industry ‘entirely switched over’ from aluminum back surface field solar,
or Al-BSF, cells to passivated emitter rear contact, or PERC, cells. Now the
industry, he says, is poised to make another major switch to n-type cells from “p-type
PERC to n-type tunnel oxide passivated contact, or TOPCon, cells. N-type cells
have their wafers doped negatively using chemicals like phosphorus, while
p-type cells are doped positively. Doping is the process of adding an impurity
to the semiconductor to increase its ability to conduct electricity.” PERC
cells still dominate but predictions are that the more efficient n-type cells
will gain market share from 10% in 2022 to 60% within the next decade.
Perovskite-Based Tandem Junction Cells: More Efficient
but There Are Chemical Stability Challenges
Perovskite is
a calcium titanium oxide mineral (CaTiO3) discovered by a German in 1839 in the
Ural Mountains of Russia and was named after Russian mineralogist Lev von
Perovski. Perovskite-based tandem cell technology is being produced and its use
expanded. Tandem-junction cells have higher modular efficiency than
single-junction cells. Utility Dive reported in July that “NREL announced a
single-junction efficiency breakthrough on Oct. 25, reaching an efficiency of
27% with a gallium arsenide cell. In a release, the lab said researchers
optimized the doping and structure of the top layer of the cell to minimize the
negative impacts of defects.” Thus, improvements in solar efficiency are
still occurring in single-junction cells as well.
Tandem-junction cells or tandem solar cells are basically solar cells
that are stacked. Perovskite cells are stacked above silicon cells, and each
absorbs a specific range of wavelengths from the light spectrum.
Single-junction silicon cells are subject to an efficiency limit known as the “Shockley-Queisser
limit”. Scientists estimate that the theoretical efficiency limit for
perovskite cells is 44% (double the average output of panels today) so if more
improvements can be made this could eventually be a huge boon to the solar
industry.
First Solar
and Hanwha Qcell are both investing in perovskite-based tandem-junction cells
with Qcell announcing a $100 million investment in a perovskite-based tandem-junction
cell production line. They plan to begin limited production by the end of the
year and begin volume production in 2026. Hanwha Qcell describes the upsides
and downsides of perovskite cells as follows:
“Perovskite
cells are created through a technique known as solution processing, an approach
that not only makes perovskite manufacturing scalable but also holds the
potential for remarkably low production costs. These thin-film panels aren't
just efficient; they're also highly flexible, lightweight, and even
semi-transparent, opening up innovative applications beyond conventional solar
panels. With fewer materials needed for production and exceptional light
absorption, they offer a cost-effective and compelling value proposition for
companies in the solar industry.”
“So, why
doesn’t the industry just produce perovskite cells? The material, while highly
efficient, faces issues related to stability and durability over time. They are
more susceptible to environmental factors, like moisture and heat, which can
degrade their performance and lifespan. This limited stability hinders their
long-term reliability and commercial viability.”
“Tandem
cells, on the other hand, combine perovskite with traditional silicon cells in
a way that leverages the strengths of both materials. By stacking different
solar cells together, tandem cells broaden the captured spectrum of sunlight.
Tandem cells typically consist of a perovskite layer on top, which absorbs
short-wavelength light, including visible light and ultraviolet rays. At the
same time, the silicon layer beneath it captures long-wavelength light, such as
infrared rays. This dual-layer approach not only boosts efficiency and
electricity generation — it also charges the future of the solar industry with
new possibilities.”
Hanwha Qcells claims a maximum of 29.9% efficiency for
their perovskite-based tandem-junction cells. The video below gives some great
info about their future product.
Bifaciality: Higher Cost for Higher Efficiency.
Perovskite Bifacial Panels Outperform Monofacial Panels for the First Time in
the Lab
Bifacial solar cells capture sunlight on both sides of the panel. Reflected and diffused sunlight is captured on the back-facing side. Bifacial cells are able to harness albedo radiation from the reflected sunlight. The challenge of bifaciality is that as back-side efficiency goes up there is some loss of front-side efficiency. Monofacial panels generally max out at 26% efficiency, which is great compared to the 17-18% max efficiency panels on my house. The NREL reported on research in July in the journal Joule that for perovskite single-junction bifacial cells the front face reached 23% and the back face reached up to 21% which could lead them to make 10-20% more power than monofacial cells. Perovskite cells and bifacial cells cost more so the efficiency and power output increases would have to overcome the cost to manufacture and deploy but that looks probable. According to the researchers: “simulations guided the design of the bifacial cell, and without that assistance the researchers would have had to experimentally produce cell after cell to determine the ideal thickness. They found the ideal thickness for a perovskite layer is around 850 nanometers. By comparison, a human hair is approximately 70,000 nanometers.” Thus, simulations, or modeling, guided the process of discovering the ideal thickness. In this case, one might say that modeling can lead to optimization. The graphic below is from the paper in Joule that describes the research on the promise of perovskite-based bifacial solar cells. The researchers note that a perovskite solar cell (PSC) “is uniquely suited to a bifacial structure, owing to its high absorption coefficients, long carrier lifetimes, benign and readily passivated surfaces, and proper band alignment with charge selective contacts.”
Source: Highly efficient bifacial single-junction perovskite solar cells. Qi Jiang, Zhaoning Song, Rosemary C. Bramante, Paul F. Ndione, Robert Tirawat, Joseph J. Berry, Yanfa Yan, and Kai Zhu. Joule. Volume 7, Issue 7, 19 July 2023, Pages 1543-1555. Highly efficient bifacial single-junction perovskite solar cells - ScienceDirect
Now that perovskite-based bifacial cells have been proven to be competitive at
lab scale, the next step is to scale up. Utility Dive’s Diane DiGangi sums it
up: “Silicon cells remain slightly more efficient overall, with a record
efficiency of 26.7% compared to monofacial perovskite cells’ record efficiency
of 26%. Bifacial perovskite cells could allow the material to surge past
silicon in terms of efficiency.” That should be considered amazing since
perovskite cell designs only reached a meager 3% efficiency in 2006. In 2021
about 50% of solar cells on the market were bifacial. According to forecasts from
2020, this was not expected until 2024 as the graph below shows.
Source: Wikipedia
Inverter Design Optimization
Inverters are a
necessary part of a solar PV system that converts DC energy into AC energy. A
2012 study found that most service and maintenance calls for utility solar
systems were about failing inverters. Some industry vets say this is because
inverter development has outpaced inverter testing and standardization. There
are different types of inverters and two types in particular have dominated:
central inverters and string inverters and now a third type, hybrid inverters
is growing in deployment. The central inverter model uses a single large
inverter while a string inverter design uses many smaller inverters that work
for a ‘string’ of panels. CapEx favors the central inverter design but OpEx
favors the string inverter design. The installation cost of multiple string
inverters is much higher than the cost of a single central inverter, but
downtime costs are way higher with a central inverter. If a central inverter
goes down, the whole plant goes down but if a string inverter goes down only a
part of the plant goes down. With today’s supply chain issues downtime for
maintenance can be more extensive. Hybrid inverter designs try to bridge the
best of both central and string inverters and take advantage of modularity. Modularity
can make maintenance easier and faster, which saves money and time. Solar
company Sungrow makes inverters and touts its hybrid modular inverter design: “Sungrow
designed its hybrid inverter with discrete functional modules to make
troubleshooting and repairs faster and more cost-effective.” The company’s
inverter design “also enabled the company to automate manufacturing for
ramped up production, lower costs and a reduction of human errors in assembly.
With today’s supply chain issues and increasing demand driven by the Inflation
Reduction Act (IRA), the renewable energy industry needs strong suppliers.”
Sungrow is a major supplier of PV solar inverters. Inverters are also used for
battery energy storage systems. They shipped 47GW of inverters in 2021.
Floating Solar
Floating solar
has advantages and disadvantages. A major advantage is that it does not take up
land space and it is cheaper to lease water space than land space for
deployment. A disadvantage is that the solar panels are far from optimally
tilted toward the sun. Floating solar is more common in Europe and Southeast
Asia than in the U.S. There is an ongoing project to deploy floating solar in
reservoirs controlled by the U.S. Army Corp of Engineers, the Bureau of
Reclamation, and the Federal Energy Regulatory Commission (FERC). Regulatory
and permitting issues are being worked out. One proposed project combines
pumped hydro with floating solar in the upper reservoir, presumably to power or
partially power the pumping of water back up to the upper reservoir. As the
video below shows, researchers at Cornell are investigating what kinds of ecological
effects floating solar could have on aquatic ecosystems.
Solar Canals to Reduce Evaporation in Drought-Prone
Areas
Another type
of solar deployment over water is making solar canals in drought-prone areas
where solar panels are placed above canals to both generate energy and slow the evaporation of water in arid areas. Unlike floating solar, these solar
canal coverings can orient the panels to optimize orientation for sunlight. A
first-of-kind project in Arizona involving the Gila River Indian Community and
the Army Corp of Engineers aims to be the first solar-over-canal project in
operation. Another similar project is in the planning stages in California. Solar
canals even have other benefits. In India, solar canals have managed to help
control weeds that were choking off canals. This can also lower herbicide costs
and the effects of herbicides on the environment. Cooler microclimates below
the panels have resulted in better solar panel performance over canals. A 2021
paper in Nature Sustainability notes the potential of solar canals in
California. From the abstract:
“Here we use regional hydrologic and techno-economic
simulations of solar photovoltaic panels covering California’s 6,350 km canal
network, which is the world’s largest conveyance system and covers a wide range
of climates, insolation rates and water costs. We find that over-canal solar
could reduce annual evaporation by an average of 39 ± 12 thousand m3 per km of
canal. Furthermore, the financial benefits from shading the canals outweigh the
added costs of the cable-support structures required to span the canals. The
net present value of over-canal solar exceeds conventional overground solar by
20–50%, challenging the convention of leaving canals uncovered and calling into
question our understanding of the most economic locations for solar power.”
Agrovoltaics: Dual Use Solar Generation with Crop Enhancement
There are
three basic types of agrovoltaics: 1) Solar arrays with space between for
crops, 2) Stilted solar arrays above crops, and 3) Greenhouse solar arrays. Other
activities such as grazing sheep to control grasses and weeds are called agrovoltiacs
as well. According to Wikipedia: “Agrivoltaics, agrophotovoltaics,
agrisolar, or dual-use solar is the simultaneous use of areas of land for both
solar panels and agriculture. Because solar panels and crops must share the
sunlight, the design of agrivoltaic facilities may require trading off such
objectives as optimizing crop yield, crop quality, and energy production. In
some cases crop yield increases due to the shade of the solar panels mitigating
some of the stress on plants caused by high temperatures and UV damage.”
“The technique was originally conceived by Adolf
Goetzberger and Armin Zastrow in 1981,[4] Agrivoltaics can refer to different
methods of combining crops with solar panels, from conventional solar panels
placed on top of crops, to greenhouses made of semi-transparent PV panels.”
It seems that finding synergies is the key to successful
agrivoltaics. Modeling should be compared for different crops and regions. Moisture
retention, cooling, and lowering UV exposure are some crop benefits of
agrivoltaics as a result of shading. Tilt angles of the panels, panel height,
and space between panels and panel banks can be optimized. One design is simply
solar panels on greenhouses. Another design uses vertical bifacial cells as shown
below (without crops).
Source: Wikipedia
A 2022 paper
in the Journal of Cleaner Production macro-modeled agrivoltaics in Japan, and
found that certain areas were more suitable, particularly the rice paddy areas
near to high energy consumption areas. Of course, like most solar PV modeling,
the study also confirmed that concurrent battery storage and expanded electrical
transmission would be required to avoid issues like curtailment and availability
in low or no generation times.
Decoupled Photovoltaic Thermal Systems
Decoupled
photovoltaic thermal systems use a liquid to filter out excess heat and light. They
draw away ultraviolet rays that would overheat the solar cells. It operates
effectively as a cooling system. The liquid filter keeps solar cells cool while
storing the heat away for later use. Water or nanoparticle solutions were
common in the past as the liquid filters but are not very good at filtering
harmful UV rays. Researchers at the Korea Maritime & Ocean University
(KMOU) “found that fish oil excelled at filtering out excess light. While
most water-based decoupled systems operate at 79.3% efficiency, the KMOU team’s
fish oil-based system achieved 84.4% efficiency. For comparison, the team
measured standalone solar cells as operating at 18% efficiency and standalone
solar thermal systems at 70.9% efficiency.” The goal of decoupled PV
thermal systems is to avoid accumulation of heat on the solar cells which lowers
their operating efficiency. Waste-heat recovery via a small thermoelectric
generator is the main model of these systems. The effect is similar to a
combined-heat-and-power system.
An October
2022 paper in Renewable and Sustainable Energy Reviews describes the state of
the new technology of decoupled PV thermal systems as follows: “Thermal
decoupling of the photovoltaic (PV) and photothermal (PT) modules has been
successfully implemented by employing the spectral separation technique, which
is conducive to the thermal damage prevention of the PV cells caused by the
absorption of unavailable photons. Moreover, the thermally decoupled spectral
splitter-assembled PV/T (SPV/T) system has also achieved high-grade output heat
while utilizing the full spectrum of solar energy. It has been widely reported
that recovering the output thermal energy from the SPV/T system is an effective
approach to improving the overall solar energy utilization efficiency. However,
due to the limitations of optical performances of spectral splitter and heat
transfer coefficients of the heat sink, the up-to-date solar energy utilization
efficiency is still lower than the theoretical expectation.” Research is
ongoing for thermal energy recovery in these systems.
Research in 2022 revealed that by taking advantage of radiative cooling solar panels could be modified to capture some energy at night. The new design uses a thermal electric generator and a heat sink to capture energy from the temperature difference of the solar panels and the ambient air, as shown below.
Other Solar Innovations
Other solar
innovations include bigger panels up to 650W in output, panels with better
bifacial solar capture, “different ingot and wafer types, wafering techniques
and cell structures, or module architectures, along with cheaper, simpler and
more resilient racking systems.” Solar materials recycling is also advancing,
with particular focus on recycling the tellurium and cadmium from CdTe panels. Tellurium
availability is dependent on China. Improvements in tellurium recovery are
being implemented.
References:
More
powerful, resilient and versatile: The next generation of solar tech is
emerging. Diana DiGangi. Utility Dive. November 16, 2023. More powerful, resilient and
versatile: The next generation of solar tech is emerging | Utility Dive
How
inverter design contributes to long-term profitability of solar and storage.
Utility Dive. Feb. 6, 2023. How inverter design contributes to
long-term profitability of solar and storage | Utility Dive
‘Bifacial’
perovskite solar cells could produce more energy at lower costs, NREL finds.
Diane DiGangi. Utility Dive. July 20, 2023. ‘Bifacial’ perovskite solar cells
could produce more energy at lower costs, NREL finds | Utility Dive
Hybrid
Tandem Solar Cells. National Renewable Energy Laboratory. Hybrid Tandem Solar Cells |
Photovoltaic Research | NREL
Arizona's
solar-over-canal project will tackle its major drought issue. Can Emir. Interesting
Engineering. November 24, 2023. Arizona's solar-over-canal project
will tackle its major drought issue (msn.com)
First
solar canal project is a win for water, energy, air and climate in California.
Roger Bales. The Conversation. February 22, 2022. First solar canal project is a win
for water, energy, air and climate in California (theconversation.com)
These
cutting-edge solar panels can even generate electricity at night — here’s how
they work. Ben Stern. The Cool Down. November 21, 2023. These cutting-edge solar panels can
even generate electricity at night — here’s how they work (msn.com)
News
Release: Bifacial Perovskite Solar Cells Point to Higher Efficiency. National
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Solar Cells Point to Higher Efficiency | News | NREL
Highly
efficient bifacial single-junction perovskite solar cells. Qi Jiang, Zhaoning
Song, Rosemary C. Bramante, Paul F. Ndione, Robert Tirawat, Joseph J. Berry,
Yanfa Yan, and Kai Zhu. Joule. Volume 7, Issue 7, 19 July 2023, Pages 1543-1555.
Highly efficient bifacial
single-junction perovskite solar cells - ScienceDirect
Perovskite
Tandem Cells: Shedding Light on the Future of Solar Energy. Hanwha Qcells.
How Perovskite-Based Tandem Cells Can
Scale Up Solar Energy (hanwha.com)
Tandem
Solar Cells. University of Edinburgh. The Solar Spark. Tandem Solar Cells | The Solar Spark
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Energy
and water co-benefits from covering canals with solar panels. Brandi McKuin,
Andrew Zumkehr, Jenny Ta, Roger Bales, Joshua H. Viers, Tapan Pathak & J.
Elliott Campbell. Nature Sustainability volume 4, pages609–617 (2021). Energy and water co-benefits from
covering canals with solar panels | Nature Sustainability
Nighttime
electric power generation at a density of 50 mW/m2 via radiative cooling of a
photovoltaic cell. Sid Assawaworrarit, Zunaid Omair, and Shanhui Fan. Applied
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Agrivoltaics.
Wikipedia. Agrivoltaics - Wikipedia
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Researchers
discover unexpected material can boost solar panel efficiency: ‘Effectively
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material can boost solar panel efficiency: ‘Effectively absorbs ultraviolet …
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Bifacial
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