https://cleantechnica.com/2017/08/15/efficient-will-solar-pv-future-10-year-predictions-industry/
[links and images in on-line article]
How Efficient Will Solar PV Be In The Future? 10-Year Predictions For
The Industry
August 15th, 2017 by Ricky Dunbar
If you were to walk into a solar store and purchase some of their
best-selling PV panels, it is likely that their solar
irradiance-to-electricity conversion efficiency would be around 17%.
This is the typical efficiency (Fraunhofer ISE Photovoltaics Report,
2017) of the top-selling PV product, a multi-crystalline silicon panel.
This means that for a typical panel, 17% of all incident solar energy is
converted directly to usable electricity. This is quite impressive for a
device that has no moving parts and can generate power at the location
where the electricity is required (no transmission losses). It is no
wonder that PV is already one of the cheapest power technologies available.
However, the question arises: more than 60 years after the first
demonstration of a practical solar cell by Bell Labs, have PV
researchers already reached the practical efficiency limit for the
technology? According to predictions made in a recent publication (2017
International Technology Roadmap for PV), the answer seems to be very
much a resounding “No.” Various manufacturing trends that will lead to
further major improvements in efficiency over the next decade are
already being implemented by industry. In this article, we explore these
trends, and summarize with predictions as to how efficient PV panels
will be in 10 years’ time.
[Side note: Here, we’ll just be looking at the conversion efficiency of
the PV panels themselves (i.e., we will not be addressing system losses
such as those associated with the inverter). In addition, as many
CleanTechnica readers would know, there are many different classes of PV
panels, but in this article we’ll focus on just the two leading sellers:
multi- and mono-crystalline silicon, which account for more than 90% of
the world market. Both multi- and mono- panels are constructed with
individual silicon wafer cells. The primary distinction is the crystal
structure of the silicon wafers themselves, with mono-crystalline having
a higher quality crystal structure than multi-crystalline.]
When attempting to predict the efficiency (and cost) of future “factory
standard” panels, looking at the current world record for device
efficiency can be a good place to start. These records, which are
maintained by the PV research community, usually pertain to devices that
are specifically fabricated or selected for a world record attempt
(certified measurement at an independent test center). In other words,
they do not represent the products that are churned out of PV factories
under regular operation. However, it goes without saying that as soon as
a PV company achieves a world (or internal) record with a particular
fabrication method, they will probably be motivated to accordingly
modify their standard fabrication to increase the value of their
mass-produced products.
The current world-record efficiencies for modules (the term “module” is
interchangeable with “panel” – the product sold by PV manufacturers) and
cells (the constituent building blocks of a module) are shown in Figure
1. We see that the world record efficiencies for mono-crystalline
silicon devices are 25.6% and 21.3% — for cells and modules,
respectively — and for multi-crystalline silicon, the corresponding
values are 23.8% and 19.5%. Interestingly, even though this plot is less
than 1 year old, it is already out of date: the record for
mono-crystalline solar cells is now 26.6%!
The difference observed in Figure 1 between module and cell efficiency
is not only true for record devices: modules are – at least for now –
typically less efficient than individual cells. This can be due to a
number of reasons including i) the fact that modules typically contain
so-called dead space – areas that don’t produce power – such as the
module frame and space between cells and ii) the fact that electrical
losses occur along the connections joining individual cells.
So how much higher are these records than the present typical panels
mass-produced by PV factories? As mentioned at the start of this
article, the typical efficiency of the most commonly installed panels
(multi-crystalline silicon) is around 17%. For the high end of the
market (premium mono-crystalline), the value is around 21%. In other
words, the efficiency of typical panels is lower than the records shown
in Figure 1 by a margin of a couple of percentage points. So, without
any major further innovation, it can be expected that these typical
values will creep up towards the present records. But can we expect
further innovations to emerge, which will raise the record efficiencies
and drag the typical values along with them?
According to the 2017 International Technology Roadmap for
Photovoltaics, there are some key developments that will do exactly
that. In the following, we’ll look at three of them in detail, and for
each innovation, we’ll present the accompanying industry market
projections. We will then finish with their overall predictions of the
typical panel efficiency we can expect in 10 years’ time!
More Sophisticated Cell Structures
The first innovation we will examine is the transition of the standard
cell architecture towards higher efficiency designs. The
industry-standard design (known as aluminum back surface field, Al-BSF)
currently accounts for 80% of silicon cells manufactured, but its share
is predicted to shrink in the next few years. A more sophisticated
design, the PERC cell (passivated emitter and rear cell), is expected to
become dominant. PERC cells can achieve higher efficiency via the
inclusion of a non-continuous dielectric (insulating) layer at the back
side. By virtue of its optical properties, this layer improves the
reflection at the aluminum electrode, which has the consequence that
more light gets reflected back into the cell for a second chance at
absorption. An additional benefit is a reduction in electrical losses at
the back (Al) side. Shown in Figure 2 are schematics of an Al-BSF cell
and a member of the PERC family: a PERL cell (passivated emitter, rear
locally-doped). It was the PERL design that enabled researchers to reach
the 25% efficiency landmark for silicon solar cells for the first time
in 2008.
However, even as the industry embraces the PERC structure, large-scale
manufacture of even higher efficiency devices is growing at the same
time (predictions for manufacturing trends for the next 10 years are
shown in Figure 3). These include the heterojunction (HJ) cell, which
uses layers of amorphous (non-crystalline) silicon to reduce electrical
losses at the edges of the crystalline silicon wafer, and the
inter-digitated back contact cell, which we will introduce shortly. In
fact, the world record solar cell, as fabricated by the Kaneka
Corporation this year, effectively combined both design concepts to
achieve an efficiency of 26.6%.
Improving Cell Contact Structures
The second innovation we will look at relates to how the current is
extracted from within the silicon wafer, where it is generated, to the
external circuit. For the most common c-Si solar cell types (including
Al-BSF and PERC), current flows axially in the wafer between the top and
bottom sides and then laterally along the contacts. The top (sun-facing)
contact takes the form of the familiar conducting grids, which aid
lateral transport. The grid is comprised of fingers (thin strips of tens
of micrometers in width) and busbars (somewhat thicker strips, of the
order of millimeters). The fingers can be thought of as arterial roads
and the busbars as highways for current. Usually, there are either 3 or
4 busbars, and hundreds of fingers. A schematic indicating the function
of the grid, and an accompanying photo of actual cells is shown in Figure 4.
Without the grid to assist lateral current flow, the electrical
resistance losses would significantly drag down the overall device
efficiency. However, the grid also causes shading, as no power is
produced in the area under the grid. Therefore, a trade-off arises: too
much coverage leads to good electrical properties but large shading,
whereas an overly sparse coverage allows more light absorption and hence
current generation, but the electrical losses when extracting that
current become excessive. (On the back side of the cell on the other
hand, there is no such shading concern, so the back side is typically
largely covered by a conducting, opaque material.)
So how do PV companies intend to innovate to achieve further
improvements in this space? Interestingly, two diverging industrial
trends are emerging: increase the number of busbars or remove them from
the front surface altogether! Manufacturers following the first approach
are doing so because the improved electrical properties of 5 (modern)
busbars instead of 3 or 4 can outweigh the increased shading losses. The
second approach eliminates the busbars on the top of the cell by
introducing an equivalent structure on the back side of the cell. Space
for each of the contacts — recall that the other contact is already on
the back side — is achieved by endowing both contacts with an
inter-digitated pattern — this is where this particular cell design, the
inter-digitated back contact (IBC) cell, gets its name. This allows the
electrical current to enter and leave the cell on the back side, leaving
the front side free of shading! Ten-year projections for the market
share expected for these contact structures is shown in Figure 5.
Exploiting the Module
There are many more innovations driving the efficiency of solar panels
upwards, but we’ll focus on just one more. Whereas the first two trends
related to the cells themselves, we’ll now look at one that addresses
the losses associated with assembling cells together to form modules.
Scientists often use a metric to quantify the losses, known as the
cell-to-module power ratio (CTM).
Currently, CTM ratios are typically less than 100%, meaning that there
is an overall net loss of efficiency when assembling the module.
However, this is set to change, such that it will soon be the normal for
there to be a net gain! Earlier, we mentioned some factors that act to
decrease the CTM (electrical losses along cell connections and module
dead space), but at the same time, there are also factors that can
increase the CTM.
One example is that optimized modules can be an optically favorable
environment for the cell. Incident light that reflected from the surface
of the cells can be more likely to undergo internal reflections within
the module and hence the amount of light absorbed by the cells can be
increased. As the industry learns to increasingly exploit effects such
as this, and simultaneously reduces losses, such as by transitioning to
larger and/or frameless modules with better cell packing (less relative
dead space), it is expected that CTMs of greater than 100% will the norm
from 2021, as shown in Figure 6.
In this article, we have discussed some major trends that are likely to
drive up the efficiency for commercial modules. The obvious question at
this point is: how will all of these factors combine to determine the
efficiency we can expect from PV modules in 2027? The 2017 ITRPV
predictions, shown in Figure 7, take into account the trends discussed
above, as well as many others, to reach overall predictions specific for
each variant of crystalline silicon PV panel.
In this image, we can see individual predictions for many of the
technologies we have introduced above. These predictions are made for a
fixed module size (60 cells with dimensions of 156 mm x 156 mm). In
terms of efficiency, this is equivalent to:
Approximately 20% for the low-end technologies (such as p-type
multi-crystalline Al-BSF), up from about 17% today.
Approximately 26% for the high-end technologies (such as n-type
monocrystalline IBC), up from about 21% today.
This means that in 2027, an average premium module will be able to
convert more than a quarter of all incident solar energy to electricity.
If these predictions prove to be correct, the result will be a very
significant increase (roughly 20%, relative) on today’s product. This
will be a remarkable example of continued innovation for an industry,
which, already more than 60 years old, is already very mature.