ISIS Report 09/10/13
Sunny Prospects for Solar Photovoltaic
better and cheaper all the time, and even more promising varieties on the
horizon; it will keep the lights on instead of nuclear power Prof Peter Saunders
A fully referenced version of this article is posted on ISIS members
website and is otherwise available for download here
photovoltaic price drop typical of semiconductor devices
solar cells were first developed, just over half a century ago, they were
expensive and not very efficient at converting light into electricity. Their
most significant application was in satellites, for which they were the only
feasible energy source. Since then, the price per watt has dropped so much
that solar photovoltaic (PV) is very close to achieving grid parity, i.e.
producing energy at the same cost that utility companies pay for their current
mix of sources. Furthermore, this price drop shows every sign of continuing;
the cost is now only 10% of what it was five years ago . Countries like
Germany and the UK, which have been offering subsidies to help solar power get
off the ground, are now phasing them out as the need has disappeared.
not surprising that this is happening. Solar cells are based on semiconductors
and the physics that underpins them, and every year, just about all such
devices become better, smaller and cheaper. If you own a smart phone, you are
carrying in your pocket or handbag the computing power that once would have
occupied a large room and cost millions of dollars. As with other semiconductor-based
devices, we can be confident there is more to come.
solar cells work
a photon hits an atom, it can raise the energy of one of the electrons and
allow it to become free, i.e. no longer attached to that atom. By itself, this
isn’t very useful because the electron soon gives up its extra energy (as heat
or light), falls back into orbit around a nucleus, and we are back to where we
started. To get useful energy out of a system, we need an electric current,
i.e. there must be some way to make the free
electrons move off in the same direction.
The most common solar cells in use today consist essentially of two thin
crystals of silicon, one doped with phosphorus (i.e. one with some of the
silicon atoms replaced by phosphorus) and the other doped with boron. A silicon
atom has four electrons in its outer shell whereas phosphorus has five and
boron only three. This difference results in an electric field being
established at the interface between the two pieces, and this drives the electrons
in the direction of the phosphorus-doped wafer.
the circuit is completed by a wire connecting the two sides of the cell, a
current will flow from the phosphorus-doped wafer to the boron-doped wafer and energy
can be extracted from the system.
bright cells on the horizon
cells have proved their worth and continue to improve, but they have some
drawbacks. Although silicon is easily obtained because it is a constituent of
common types of sand, to be used in solar cells it has to be refined to a
purity of 99.9999%. Silicon crystals are hard to work with and cells made of
them are relatively bulky.
even though the tremendous improvement in solar PV has almost all been accomplished
using silicon wafers, there is a great deal of research into thin film solar
cells made of other materials. The layers are typically about 1μ (one micron,
i.e one thousandth of a millimetre) thick whereas silicon wafers are about
thin film cells you are most likely to have seen are made from amorphous (i.e.
non-crystalline) silicon and work in much the same way as the more common
silicon wafer cells. Amorphous silicon is not very efficient at converting
solar energy and is mostly found in devices such as calculators where size is
an issue and only a very small amount of energy is required.
other common types are based on cadmium telluride (CdTe) and copper indium
gallium selenide (CIGS). These can reach about the same efficiency as silicon
wafers, but present technical challenges in design and manufacture, though
these are being overcome at an impressive rate. There are also problems with
the raw materials they require. Cadmium is highly toxic and so CdTe solar cells
must be strictly regulated both in use and in recycling (see  How Green is Solar? SiS
49). Tellurium is common on some undersea ridges and it is relatively abundant
in the universe as a whole, but few accessible sources are known, though that
may be simply because it is only recently that it has become commercially
important and geologists have started looking for it  .
which is also used in screens for televisions and mobile phones, is expensive
and in relatively short supply. It may be possible to increase the amount
obtained as a by-product of the refining of zinc, with which it generally
occurs. Alternatively, a group at Oxford University are developing cells in
which indium tin oxide is replaced by silicon doped zinc, which has a lower
conductivity but is much cheaper and easier to source .
Dye sensitive solar cells (DSSCs) are also thin film but they
operate differently [5, 6]. The basic idea is actually quite simple; you can
even find on the web instructions for making one on the kitchen table . A
thin layer of an organic dye (blackberry juice will do) is immersed in an
electrolyte, i.e. a liquid conductor, above which is a transparent electrode.
The electrolyte is a solution with a lower redox potential than the dye, which
means that electrons that are freed by photons striking the dye will move into
the titanium dioxide rather than into the electrolyte. Again, connecting the
two sides of the cell with a wire will cause a current to flow, in this case
from the titanium dioxide to the upper electrode.
As this example shows, DSSCs can be made from materials that are neither
rare nor toxic. The materials do not have to be highly purified and the actual
manufacture is quite straightforward: it can be done commercially by printing
onto foil. The challenge is the practical one of designing DSSCs that are
efficient enough to be competitive with other solar cells while retaining the
advantages of relative ease of manufacture and low cost of materials .
A big step forward has been the discovery that DSSCs can be made
using perovskite, a mineral that is much cheaper and more abundant than the
rare earths used in other thin film cells. Perovskite was first identified in
1839, but it is only recently that Michael Grätzel and colleagues in Lausanne began
experimenting with it .
first perovskite cells were unpromising because the material was rapidly
degraded by the electrolyte. Even while they lasted, they were very inefficient,
converting only about 3.5 % of the energy from sunlight into electricity as
opposed to about 15 % from silicon wafers . But researchers have since
discovered how to replace the liquid electrolyte by a solid material and this
solved the problem of rapid degradation. The efficiency has now been raised to
15% and from their successes so far and their understanding of the physical
processes involved, researchers expect it could equal that of silicon wafers,
which is over 20 % under laboratory conditions.
possibility is to use graphene, either in all-carbon cells or in combination
with silicon. These are still in the experimental stage, but they may prove to
have high efficiencies because while most cells only operate over a
comparatively narrow range of frequencies (i.e. they collect energy only from
light within a small part of the spectrum), these are not limited to the same
extent. (. Graphene
and Solar Power for the Masses SiS 59).
the meantime, improvements continue to be made in silicon wafer cells, and
because they are the established technology, they may well remain the most
widely used, at least for the next while. It has also been suggested that
perovskite could be used to augment silicon cells rather than independently .
How much energy can PV provide?
the improvement in solar cells over the past decades has been an impressive
achievement, it doesn’t tell us what we can expect from them. How much of our energy
will PV be able to provide and when, and what are the issues involved in
reaching that proportion? In 2012, for example, the UK total electricity production
was 363 TWh, approximately 6 000 kWh for every person in the country, so we are
talking about very large amounts.
can get an idea of how much solar energy is available from a simple calculation.
The sun provides about 10 000 times the total commercial energy that humans use
. With an efficiency of 20%, we would need to cover 0.05% of the Earth with
solar panels. The surface area is roughly 5× 108 km2, so
that would mean about 250 000 km2, roughly twelve times the size of
In principle, therefore, we could actually get all the energy we
need from PV. That’s not really practicable, of course. There is a limit to how
much of the Earth’s surface we want to cover with solar panels, not forgetting
that less than 30 % of the area is land and much of that is at relatively high
northern latitudes. And even if we had solar cells with efficiencies of 20 %,
that wouldn’t mean we could harvest a fifth of all the sunlight that fell on
the panels. All the same, PV can clearly make a significant contribution.
European Photovoltaic Industry Association estimates that solar PV could be
providing as much as a quarter of the EU’s electricity demand (electricity is
about a fifth of the total energy) by 2030, with the obstacles they envisage
being political and organisational more than technological . Manufacturers’
organisations have a tendency to be a bit optimistic about what their industry
can achieve, but the above calculation tells us they are at least in the right
ball park. And considering how much solar cells have improved over the past 10
or 20 years, this is one area where a bit of optimism may be justified.
cells generate electricity only during daylight hours and more in the summer when
the sun is high in the sky than in the winter. There therefore has to be
either some means of storing energy or an alternative source, or both.
progress has been made, the problem of storage has not yet been solved. Batteries
are improving, but they are still too expensive and too big for the amount of
electricity they can store; this is of course also a major obstacle to the
electric car. There are other ways of storing energy, for example by pumping
water uphill when more electricity is being generated than is needed and
letting it flow down again and drive turbines when there is a shortfall. We can
heat molten salts or even water and recover the energy when it is required. We
can even store energy in heavy flywheels. All of these are already in use, some
in connection with solar heating rather than PV, but mostly at relatively small
alternative is to use the energy of sunlight to split water into hydrogen and
oxygen, replicating the natural process of photosynthesis ([13, 14] Splitting Water with Ease,
Energy from Sunlight with Artificial Photosynthesis, SiS 43 ). The hydrogen can then be used whenever and wherever it
is required. There has been research in this area since the 1974 oil crisis,
but there are serious technical problems that have not yet been overcome.
For the foreseeable future, the sun will not be the only source of
electricity for most consumers, and the obvious solution will be to use the
alternatives when more energy is needed. In developed countries, users will
continue to remain connected to a grid, feeding electricity in or drawing it out as
The sort of grid that will be needed will be quite different from
the present ones; in particular there will be far less electricity being
transmitted over long distances, with the large loss of energy that entails 
Ousting Fossil Energy, SiS 60), . This is already creating
problems for grid operators not only in areas of high insolation like the
southwestern US but even in countries as far north as Germany. Moody’s
investors’ service states : "Large increases in
renewables have had a profound negative impact on power prices and the
competitiveness of thermal generation companies in Europe.” A problem for those companies but an advantage for almost everyone else is that
both PV generation and the demand for power peak at midday.
team at Imperial College London has estimated the extra costs involved in
integrating PV into the electricity supply system in Europe. They showed that
to install up to 480 GW by 2030 is both technically feasible and not very expensive. The largest extra cost would
be back-up capacity which they estimate as €14.5/MWh
in northern Europe though considerably less in the south. Most of the extra costs
would be relatively smaller when PV is providing less than 10 % of the
electricity supply. This means there is a relatively small expenditure in the
early stages and consequently little wasted investment if the technology
improves over the next 15 or 20 years  -- yet another contrast with nuclear
energy where we are locked into yesterday’s technology by the very high up-front
parts of Africa, many communities are not connected to a grid at all and much
of the electricity is provided by small generators, often using kerosene as
fuel. When solar panels are installed, the generators typically remain in
place, but are now required only for backup. In these areas there may
eventually be local grids but none on a regional scale, just as there is no
need to go the great expense of building a national landline telephone network
when so many people are already using mobile phones. Artificial photosynthesis
could be especially important in these areas, which also have the greatest
insolation and consequently the greatest capacity for solar power of all kinds.
Other renewable sources, such as anaerobic digesters, could be also used to
supplement solar PV.
Keeping the lights on without nuclear
the 2008 White Paper on nuclear energy , the UK government pointed out that
almost all the existing nuclear power stations are scheduled to stop producing
electricity by 2023. Nuclear currently provides about
a sixth of the UK’s electricity . About a third of the coal fired stations
are due to close in the next few years as well. Much of this capacity will have
to be replaced, and building a new fleet of nuclear power stations will enable
us to do this without increasing CO2 emissions.
Chernobyl and Fukushima have demonstrated clearly the hazards of nuclear power
(See e.g.  The
Truth about Chernobyl;  Lessons
of Fukushima and Chernobyl;  The
World Must Take Charge at Fukushima). It is also not an economical option;
the government is expected to guarantee a price of just under £100/MWh for the
next 40 years and take over responsibility for the long term disposal of the
waste and almost all the liability if there is a serious accident.
if we were willing to accept the risk of a serious accident and to pay such a
high price for electricity (and commit our children and grandchildren to doing
the same), there is another issue to consider. Nuclear plants take a very long
time to build. The earliest the first new one could be in operation is 2020,
and it would probably be later than that. A new nuclear fleet could contribute
nothing towards keeping the lights on in the short term and not very much in
the medium term.
contrast, solar PV can make a major contribution and it can do it now. The Department
of Energy and Climate Change (DECC) estimates that 11.3 GW or more of solar PV
could be available in the UK by 2017; even more could be available, but the
present grid would have great difficulty coping . The problem therefore
appears to primarily one of management and storage rather than generation; if
those can be overcome, as much as 22GW could be up and running before the first
1.6 GW of nuclear could come into operation.
There are 3 comments on this article so far. Add your comment
|James Cooley Comment left 9th October 2013 18:06:28|
"For the foreseeable future, the sun will not be the only source of electricity for most consumers"
Of course the sun is the source of almost all electricity used by consumers today, either from fossil fuels such as coal and oil, falling water from solar evaporation, wind from solar derived temperature gradients, or nuclear fuel from solar fusion in the distant past. Even Geo-thermal energy is derived from solar heat trapped in the earth's core, supplemented by radioactive decay of solar forged fusion elements such as uranium and radium. Our electricity is almost entirely solar based, but it's not very renewable with a few important exceptions.
|Alan Bangay Comment left 16th October 2013 08:08:24|
As far back as the 1960;s Thorium was seriously considered as the fuel for so-called nuclear power plants. I suspect that we went for enriched uranium instead because it can produce weapons grade forms of the original uranium as the nuclear plant is functioning. Apparently if we were to use thorium as fuel we would have very safe plants with no risk of meltdown. I do not know how the cost of oonstruction and decommisioning actually compares with enriched uranium plants but it is bound to be considerably less and a whole lot safer. It might be a better option than solar because it could be done on a very small scale - for use in cars and would work 24 hours a day with little need for batteries or sunshine. Food for thought.
|Peter Saunders Comment left 30th October 2013 17:05:08|
We discussed thorium reactors a couple of years ago, in SiS 52. They do seem to be safer than pressurised water reactors (PWRs). They would probably produce electricity at about the same cost as PWRs, but that's still a lot more than we expect to pay for energy from renewable sources by the time any thorium reactors could be in operation.
Neither the Liquid Fluorine Thermal Reactor (LIFTR) nor the Accelerator-driven System (ADS) is really suitable for small scale use. Actually, one of the reasons the Americans went with PWRs was that they were considered better for powering nuclear submarines. I certainly wouldn't want any kind of nuclear reactor in something that's going to be driven on public roads!