DECEMBER 2011 - Electrical Business Magazine

The n- and p-types of silicon are created by
adding minute amounts of other substances—a
process called doping. This produces radical
changes in the electrical properties of the
overall structure. Specifically, an n-type
semiconductor is created by doping the
silicon with a material that has five valence
electrons, commonly phosphorus. A p-type
semiconductor is created by doping the
silicon with a material that has three valence
electrons, commonly boron.
As mentioned, to extract usable energy from
the PV cell, it is necessary to install electrodes
on the cell and connect them to an external
load. The back-side contact, away from
the incoming light, is relatively simple. An
aluminum or molybdenum plate is attached.
But the front side, facing the light source, is
more of a challenge because a solid plate, like
on the back side, would block the light. So the
front-side electrode, by necessity, consists of
some sort of grid that spreads across the face
of the cell and collects the electrical current.
This grid must spread to all parts of the cell
since the silicon has a relatively high resistance
and current would not readily flow through it.
If the ‘fingers’ that form the grid are too
large, they will block the incoming light,
resulting in a loss of energy. If they are too
small, there will be an ampacity problem—also
resulting in energy loss. Various solutions
are possible, such as making these front-side
electrodes deep, rather than wide, and choosing
a low-resistance metal. Top surface grids can be
made by depositing metal vapours through a
mask, or using a screen-printing procedure.
An efficient front-side grid shades only
about 3% to 5% of the cell’s surface, and
minimizes resistance loss.
Another approach has been to employ a
transparent conducting oxide layer. Tin oxide
has been used successfully.
The output of a solar cell is direct current
(DC), and this is fine for many stand-alone
scenarios such as keeping batteries charged to
power electronic equipment in remote locations
where a utility connection is not available.
Isolated homesteads use such a DC output to
good effect for lighting and light-duty usage.
On a somewhat larger scale, individual
homes and businesses with substantial solar
arrays may co-generate along with a utility.
During a sunny day when electrical demand
is low, excess energy is fed back into the grid,
and this fact is reflected in the metering. For
this interaction to take place, the DC has to
be processed by a synchronous inverter. Direct
current is changed to alternating current,
utility frequency is matched, and the waveform
is synchronized, or locked in phase, so that
a useful power transfer can occur. It is also
necessary to provide transfer switch capability
so that in an outage utility workers will not be
exposed to danger.
Constant innovation
Solar technology is characterized by constant
innovation. One of the big stories is the (price)
war between thin-film and crystal factions.
Thin-film solar cells first appeared as small
strips used to power hand-held calculators, and
stand in opposition to wafer and bulk silicon.
A thin-film solar cell is created by vapour
deposition. The silicon is deposited on metal,
glass or plastic, then coated with transparent
conducting oxide to provide a top-side
electrode with good characteristics for the
collection of electrical current. Thin-film solar
panels are being successfully used as integrated
building materials, such as photovoltaic
roofing shingles and building siding. We can
visualize a time when commercial buildings
and residences will come equipped with the
capability to harvest their own energy supply
from the solar radiation that strikes their
outside finish surfaces. (The capability exists
at present, but economic barriers still exist
before this potential can become a reality.)
Thin-film has been less efficient than silicon
crystal cells but, since it is generally cheaper
to manufacture, it has made significant inroads
prior to 2010. Then, without warning, the
price of crystal silicon went into free fall. As a
result, the market share of thin film declined
by 30% that year.
(Some players remain undaunted. First Solar,
the largest manufacturer of thin-film solar
panels, offers turn-key power plants. In Canada
and the United States, First Solar provides
utility customers and independent power
producers with complete solar PV system
solutions, including project development,
financing, engineering, procurement and
construction, as well as operations and
maintenance for the life of the project.)
Market possibilities
It has been suggested that solar photovoltaic
power generation may not be appropriate to
Canada’s more northern regions, where there
are colder temperatures and less direct sunlight.
Also, the presence of abundant hydropower
has been seen as an economic obstacle to the
development of a solar infrastructure.
These observations are not altogether valid,
for several reasons. In the first place, colder
ambient temperatures actually equate to higher
electrical output on the part of a solar cell. The
available power is a function of the temperature
differential between the two sides of the p-n
junction. As a matter of fact, electrical codes
typically require correction factors for ambient
temperatures below 25ºC. In areas under
National Electrical Code (NEC) jurisdiction,
the open-circuit voltage must be multiplied
by a factor of 1.25; for example, when the
temperature drops below -36ºC.
Secondly, direct sunlight is not absolutely
required for PV generation of electrical power.
Moreover, solar power is highly compatible
with hydro since it contributes diversity to
system capability. In a time of drought, when
hydro production diminishes, solar output
will increase dramatically, making a far better
situation for the grid.
It is a fact that solar development in Canada
has been extensive. For a period of time
in 2010, Canada had the largest solar PV
power station in the world: the 97 megawatt
Sarnia PV power plant in Ontario. Far larger
solar utility installations are now being built
in many parts of the world. Government
subsidies play a decisive role but, when they
are removed as a result of shifting political
and/or economic winds, it is possible for solar
development to experience a period of decline.
CanSIA (Canadian Solar Industries
Association) is a national trade association that
represents about 650 solar energy companies
throughout Canada. The organization
predicts that, by 2025, solar energy will be
widely deployed throughout Canada and that
the solar industry will support over 35,000
jobs and displace 15 to 31 million tons of
greenhouse gas emissions per year.
No one know for sure what the future may
bring for Canada, the North American grid,
or the world as a whole, to say nothing of
outer space. However, a few generalizations
are possible.
Fossil fuels are not going to be around forever.
Nuclear power will always be problematic,
unless that highly elusive goal of cold fusion is
achieved. Biomass and similar solutions may be
helpful, but will never be a complete answer. We
probably need to moderate our need for energy,
but it is hard to conceive that we will return to a
pre-industrial existence.
It is likely, therefore, that wind and solar
power will play increasingly significant roles.
Both of these are abundant resources. The only
impediment to full usage is economics, but that
seems to diminish with the passage of time.
As professionals in the electrical industry,
it appears that the best course of action is to
be prepared for the altered reality that wind
and solar will bring to our field of operations.
Already, building and electrical codes are
taking increasing notice of these sources of
energy, and the ways in which they need to
be harnessed so that they are safe for those
who work with them, as well as the end
users. Increasing our knowledge of applicable
mandates and of the underlying electronics
is certainly an appropriate response to our
changing reality.
A regular contributor to Electrical Business,
David Herres is a Master electrician and
author of nearly 40 articles on electrical
and telecom wiring. He recently authored
“2011 National Electrical Code: Chapter-by-
Chapter”, published by McGraw-Hill
and available at Amazon.com.
42 • December 2011 • www.EBMag.com