WHAT ARE PHOTOVOLTAICS?
Photovoltaic cells convert sunlight directly into electrical energy. The electricity
they produce is DC (direct current) and can either be:
• used directly as DC power
• converted to AC (alternating current) power, or
• stored for later use.
The basic element of a photovoltaic system is the solar cell that is made of a
semiconductor material, typically silicon. There are no moving parts in a solar
cell, its operation is environmentally benign and, if the device is correctly
encapsulated against the environment, there is nothing 
that will wear out.
Because sunlight is universally available, photovoltaic devices can provide
electricity wherever it is needed. Since the power source will last for hundreds
of thousands of years, and it is very hard to interfere with its delivery,
photovoltaic (PV) is widely expected to become a major source of power
worldwide in the long term.
Photovoltaic systems are modular and so their electrical power output can
be engineered for virtually any application, from low-powered wristwatches,
calculators, remote telecommunications systems and small battery-chargers
to huge centralised power stations generating energy only from the sun. PV
systems can be incrementally built up with successive additions of panels
easily accommodated, unlike more conventional approaches to generating
energy such as fossil or nuclear fuel stations, which must be multimegawatt
plants to be economically feasible.
PHOTOV
OLTAICS
HOW PV CELLS WORK
Although PV cells come in a variety of forms, the most common structure is a
sandwich of semiconductor materials into which a large-area diode, or p-n junction,
has been formed. In the presence of light an electric charge is generated across
the junction between the two materials to create a charge similar to that between
an anode and a cathode. The fabrication processes for making the cells tend to be
traditional semiconductor processes, the same as those used to make microchips, by ‘doping’ the silicon with different elements using diffusion and ion implantation
of the elements into the silicon. The electrical current is transferred from the cell
through a grid of metal contacts on the front of the cell that does not impede the
sunlight from
entering the silicon of the cell. A contact on the back of the cell completes
the circuit and an anti-reflection coating minimises the amount of sunlight
reflected back out from the silicon, so maximising the light used to generate electricity,
as shown in Figure 8.1. See 21ADPV for a more detailed account of how a
cell works.
Photovoltaic panels have been commercially available since the mid-
1970s and were initially used to power some early demonstration buildings,
such as those that are still working at the Centre for Alternative Technology
in Wales. However, it was the 1990s that saw the first great boom in PV
buildings around the world. Germany and Japan lead the way with Japan
installing 110 MWp in 2001, Germany installing 77 MWp and USA installing
18 MWp. These three programmes accounted for over half the world PV production
in 2001. The Netherlands and Spain came next in the table of installations.
Some countries are way behind in the solar race. Britain installed
around 300 KWp in 2001.
WHAT IS A PV SYSTEM?
PV cells are typically grouped together in a module for ease of use. A PV
system consists of one or more PV modules, which convert sunlight directly
into electricity, and a range of other system components that may include an
AC/DC inverter, back-up source of energy, battery to store the electricity until
it is needed, battery charger, control centre, mounting structures and miscellaneous
wires and fuses.
WARNING
Direct current (DC) electricity is much more dangerous to handle than alternating
current (AC) electricity, which is typically used for all household appliances. This is
because there is no break in the flow of a DC current and, if you grab hold of an
exposed DC wire, the muscles contract and it is very difficult to let the wire go
again. Great care should always be taken when dealing with DC electricity.
WHY PV IN BUILDINGS?
Even in cloudy, northern latitudes, PV panels can generate sufficient power to
meet all, or part of, the electricity demand of a building. The Oxford Ecohouse
(see page 330), for example, incorporates 48 PV panels on the roof that generate
enough energy to lower the household electricity bills by 70 per cent.
The flexibility of PV enables its use in many building products, such as solar
roof tiles, curtain walls and decorative screens, which can directly replace conventional
materials in the building fabric. These products serve the same structural
and weather protection purposes as their traditional alternatives but offer
the additional benefit of generating the power to run the house.
WHAT’S GREEN ABOUT PV?
The electricity produced by every square metre of PV can effectively displace
emissions of more than two tonnes of CO2 to the atmosphere over its
lifetime. Few now dispute that CO2 emissions can continue to increase at
current rates without dire consequences, such as global warming. Wider use
of PV power in buildings can help to reduce such environmental impacts of
buildings that are responsible for generating over 50 per cent of all emissions
of greenhouse gases globally.
Let us use the Oxford Ecohouse as an example. In order to calculate the
environmental impacts of the PV system it is necessary to know the UK
energy generation conversion values, the amount of CO2 released into the
atmosphere for every unit of energy delivered to a house. It has been estimated
that an average energy conversion efficiency for thermal electricity
generation plants in the UK is around 37 per cent. This results from an electricity
mix generated from 65 per cent coal, 15 per cent gas, 22 per cent
nuclear and 9 per cent oil. For the PV manufacture assumptions see Energy
Technology Support Unit (1996).
Based on the monitored data, the PV system produces 3093 kWh per
year, that is around 77 000 kWh in its 25-year life cycle.
The Oxford Ecohouse PV system avoids the release of 1.84 tonnes CO2 per
year. These values can be extrapolated to give the avoided emissions in the case
of a massive programme of installing PV on residential building. A system oneeighth
of the size of the Oxford Ecohouse would avoid 230 kg CO2 per annum.
WHAT WILL IT COST TO USE PV IN BUILDINGS?
Solar electric PV systems are now an economic and viable technology in many
parts of the world. More than that, they are a sensible economic investment for
ordinary householders who want to begin to protect themselves from future
changes related to energy and the climate. They should begin to consider
the following:
• Climate change is driving the move towards carbon taxes that will make
energy more expensive.
• Fossil fuel depletion will push up oil and gas prices. We have around 40
years of conventional oil reserves left and around 60 years of gas left. By
2020 oil and gas scarcity will make future energy prices very unpredictable.
• Climate change may well make heating and cooling our houses more
expensive in energy terms as the climate gets warmer or colder.
• Security of energy supply. PV systems can provide electricity during conventionally
produced electricity blackouts resulting from poor supply conditions
or bad weather. There is already a range of uses for which a secure
energy supply should be essential; these include, water pumping, electric
garage doors and gates, lift safety systems, smoke and fire alarms, emergency
lighting and security systems, computer UPS systems and communications
systems.
Investment by people now in their high-earning years in energy efficiency
and renewable energy will pay dividends in, say, 10 years when they retire
and must inevitably face higher energy bills they are less able to afford.
Anyone with a £500-a-year electricity bill would be wise to envisage at least
a doubling of electricity costs in 10–15 years time. Will your pension cover
£1000 a year for one bill?
Costs of installing PV systems today vary significantly according to the
technology used and the application and the efficiency of the system.
Capital costs of PV panels are broadly similar to prestige cladding materials,
ranging from £350 to £750 per m2 depending on the technology and its
detail. Prices are expected to fall significantly over the next decade as
demand grows and the PV industry achieves economies of scale in production.
In parts of Germany and the USA (Sacramento municipality) the cost of
installing one watt of PV power into a home has already fallen to around
£2.75 per watt, which is very low compared with current UK estimates of £6
per watt for an installed system. In those countries, the impact of early
investment in the technology by national and local government bodies has
paid dividends for consumers while people in countries such as the UK have
to suffer because of short-sighted investment policies in this, one of the
most important technologies of the twenty-first century.
Your own investment decision should also take account of the marginal cost
of the PV system (capital cost minus the cost of the alternative material) and
power output. PV systems are not difficult to install and, if maintained properly
(annual washing), have an expected lifetime of around 25 years.
What is certain is that today PVs should be an essential feature of a real
ecohouse, because ecohouses are setting the agenda for building in a changing
climate and helping to prepare society for the ‘post-fossil fuel age’. PVs
have a very important role to play, like solar hot water systems, in the new
agenda for buildings; the earliest PV ‘pioneers’ in the twentieth century often
installed PV systems for ecological reasons rather than economic ones.
However, in some farsighted cities, such as Aachen in Germany, a green tariffon every electricity bill enabled the local utility company to pay every householder
with a PV roof DM2 per kW exported. This enables householders to
pay back the installation costs in around 10 years for systems that will last
for 20 years.
But it is no use placing a PV system on an energy-profligate building and
expecting it to solve the problems wrought by the building designer. This is
just throwing good money after bad. Forget PV for air-conditioned buildings
for the foreseeable future. PVs will work well with low-speed fan-assisted
passive cooling systems, such as earth-coupling and the night cooling of
buildings (see Chapter 5 on ventilation).
To use PVs properly the building electricity loads should be as low as possible,
and only then should the system be designed to meet part or all of
those loads to give you a magic building that generates its own energy.
ADVANTAGES OF PHOTOVOLTAICS AS A DOMESTIC
SOURCE OF ENERGY
• It is a clean green energy source. It does not produce CO2, NOx or SO2
emissions.
• The silicon PV panels are non-toxic in production.
• The energy payback (the time for the PV to produce as much energy as is
required for manufacture) is 2–5 years, while the working life of a PV
panel can be well over 20 years.
• Energy is generated on site so there are very few losses in transport,
unlike remotely generated supplies relying on long supply lines.
• It is reliable. You just put them on the roof and they work. Panel warranties
are now typically for 20 years.
• They are silent.
• They are low maintenance. Once installed they will simply require their
surfaces to be cleaned, especially in dusty environments.
• They can provide power in locations remote from the grid.
• PVs are a transportable technology and can be moved between buildings.
• They can provide power during blackouts.
Subscribe to:
Post Comments (Atom)
Post a Comment