E NE RG Y MAT TER S
FACT SHEET 16:
DOMESTIC RENEWABLE TECHNOLOGIES
When one thinks of renewable energy, the
immediate image is of “farms” of large
wind turbines, dams with hydroelectric
power generators and similar large scale
operations. However, they can be brought
down to domestic and light commercial
scale – after all, think of the wind and
water mills that have been and still are
used in many locations around the world.
The newer versions differ mainly in that
they use the movement (kinetic) energy to
produce electricity, rather than do work
mechanical work (eg grinding wheat)
directly.
The most obvious use of renewable
energy in the domestic and light
commercial areas is what was discussed in
the previous section – solar hot water
systems, which use the sun’s heat directly
for heating. In this section, we will look at
the renewable energy technologies that
produce electricity. In Australian homes,
the three most common are:
• photovoltaics – convert sunlight into
electricity
• wind turbines – convert air
movement into electricity
• micro hydro generators – convert
water movement into electricity
Photovoltaics
Silicon-based semiconductor materials,
known as photovoltaics (PVs), are
capable of absorbing the ultraviolet and
visible radiation from the sun, and
converting it into electricity energy. PVs
come in two distinct categories: crystalline
silicon and amorphous silicon thin film.
Both amorphous and crystalline
technologies are commonly used in
efficient grid-connected and stand-alone
installations.
Crystalline modules usually have a
number of cells in a matrix connected in
series to provide an output voltage suitable
for battery charging. A typical module will
provide a peak power output voltage of 17V
and output current of 4.7A under optimum
conditions, giving a rating of 80 Watts
peak (Wp). Modules can be connected in
series or parallel to form an array provide
higher voltage and current outputs as
required.
Crystalline solar modules are covered
with tempered glass on top and a tough
plastic material at the back. The glass and
backing material protect the solar cells
from moisture. Crystalline modules need
to be cool, so efficient ventilation is
required at the back of modules. Exposure
to cool breezes when siting modules is an
important consideration.
Amorphous silicon is one of a number
of thin film technologies. This type of solar
cell can be applied as a film to low cost
substrates such as glass or plastic in a
variety of module sizes. Advantages of
thin film cells include:
• easier deposition and assembly
• low cost of substrates or building
materials
• ease of production
• suitability to large applications
Efficiency of thin film modules is lower
than that of crystalline modules but all the
types of modules are price-competitive.
Those currently on the market degrade in
output by up to 10 percent when first
exposed to sunlight but quickly stabilise to
their rated output.
Thin film modules have various (often
flexible) coating and mounting systems.
Some are less susceptible to damage from
hail and other impacts than those covered
in glass. Solar modules can be supplied
with a frame, usually constructed of
anodised aluminium, or as an unframed
laminate.
More solar modules are being fabricated as
building materials so that they can be
integrated into the building fabric. They
include solar roof tiles, wall materials and
semi-transparent roof material for atriums
and skylights. It is anticipated that further
development of thin film technology will
lead to a proliferation of cost effective, PV
coated building materials that can be
integrated with the building fabric to
reduce costs.
All PV modules need to be cleaned
periodically to maintain their efficiency.
Solar modules produce most power
when they are pointed directly at the sun.
It is important to install them so that they
receive maximum sunlight. Ideally they
should be in full sun from 9 am to 3 pm in
mid winter.
There is also an issue of the angle at
which they are mounted which varies
depending on the latitude eg for Sydney’s
latitude this is 22°, a common roof pitch.
Micro-hydro
Domestic micro hydro generators used in
stand-alone power systems can be DC
units, designed to charge a battery bank,
or AC units designed to supply the
household loads directly.
In micro hydro systems water turns a
wheel or a runner (like a propeller) to
rotate a turbine and produce electricity.
The wheels come in different shapes and
sizes depending on the site and the type of
turbine.
Micro hydro may be the most cost
effective form of renewable electricity. For
an AC unit, the cost of electricity produced
over the lifetime of the unit should be
cheaper than extending the mains power
grid or installing other stand alone
systems. Unfortunately, as Australia has
only a small number of areas where micro
hydro is suitable, it is estimated that micro
hydro is installed in only about two
percent of stand-alone power systems.
The other problem is that it may be illegal
to interfere with a watercourse without
prior approval.
Micro hydro power is best where
water supply is continuously available.
Where supply is seasonal it may still be
cost effective to install micro hydro as a
stand-alone system. This will depend on
whether the cost of installing the system is
Energy Matters 16. Domestic Renewable Technologies
offset by the savings made during the
period when the creek is flowing. Another
renewable system, or a generator, will be
required when water is not available.
The amount of energy in water is
dependent on the distance of fall of the
water (known as the head), the flow rate
and physical factors associated with the
pipes.
The smaller the diameter of the pipes,
the higher the friction between the water
and the walls, and the greater the energy
loss. The physical length of the pipes
contributes to the frictional loss. To
minimise head losses the pipe diameter
should be as large as possible, and the
distance the water must flow through the
pipes as short as possible.
While a micro hydro unit can operate
with as little as two metres of head, most
units used in domestic situations will
require at least ten metres head.
The efficiency with which generators
convert water power to electrical power
can range from 30-70%.
Wind
Domestic wind turbines typically generate
somewhere between 300 W to 5 kW and
stand 10-20 m high. This size obviously
prevents their use in urban environments,
making them only feasible in rural
situations.
The main body of the wind generator
comprises a set of blades, the alternator
and the tail section. The power of the wind
makes the blades turn. The blades are
connected to the rotor inside the
alternator which turns and generates
electrical power. The tail ensures that the
wind generator is facing directly into the
wind.
The output from a wind turbine
increases with increasing wind velocity up
to a certain point where higher wind
velocities achieve no further power output
as the capacity of the turbine is exceeded.
It should of course be noted that turbines
are designed to shut down in very high
winds to avoid damage (known as the
furling speed).
Wind generators need “clean” wind to
operate. Clean wind is where the wind is
constant from the one direction and is not
being made turbulent by near-by
obstacles. The clean wind is required to
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overcome the starting torque (that is the
starting resistance) of the wind generator.
As a rule of thumb, a wind generator
should be installed no closer to an
obstacle than at least ten times its
height, and on the downwind side.
The preferred distance is twenty times the
height.
Wind can be affected by terrain like
hills, trees and nearby buildings or
structures. Some areas of Australia receive
seasonal wind and may only receive winds
in winter while in coastal regions on the
east and west coasts the prevailing wind
will be summer sea breezes.
Most manufacturers will provide
figures on the cut-in speed. This is the
speed of the wind at which the starting
torque is overcome and the wind generator
begins to turn and generate power. In
areas with frequent light winds, a low cutin speed is an important feature for
maximum output. Manufacturers provide
a rated output of a wind generator at a
specified wind speed. Not all
manufacturers rate their units at the same
wind speed.
Wind generators can be noisy when
running in high winds. The noise can come
from the blades, gear-box, brush gear or
wind whistling past the tower, pole or guy
wires. The noise may not be loud but may
be noticeable to you or close neighbours.
Beyond a couple of hundred metres, the
background noise of the wind itself usually
covers the sound of the blades.
Grid vs storage
Most renewable systems are unable to
provide electricity at all times as there may
be insufficient sunlight, wind or water
available. To fill the gaps, electricity can be
supplied from storage batteries or
generators in stand-alone systems or
from the electricity grid in gridconnected systems. Figures 1 & 2 show
the two configurations.
Grid connected systems interact with
the electricity supply grid. Grid connected
systems are generally located in urban
areas and PVs are the usual energy source.
The main components of the system are
the renewable energy source and a grid
interactive inverter.
Energy Matters 16. Domestic Renewable Technologies
Renewable
Energy Source
Inverter
Switchboard
Home
Grid
FIGURE 1 Grid-connected
Renewable
Energy Source
Regulator
Inverter
Batteries
Home
FIGURE 2 Stand-alone (storage)
The inverter converts the low DC voltage
generated by the system to the normal
240V AC household supply. It also
monitors the operation of the system to
control how much electricity is drawn
from or fed to the grid. If the system is
supplying more energy than is needed, the
excess is fed into the grid. Often the meter
just "runs backwards" when electricity is
going into the grid, so the household only
pays for the difference between what is
imported and what is exported. Different
suppliers have different buy-back rates
and metering arrangements.
In remote areas, this may not be feasible
because of the distance to the main grid.
In these situations, systems known as
Remote Area Power Supplies (RAPS) are
used which are self-sufficient through the
incorporation of large storage batteries.
The main components of a standalone system include:
• a renewable energy source
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•
•
•
•
control equipment for battery
charging and backup power source
operation
storage batteries
an inverter – to convert the power to
240V AC
a generator – for emergency backup
and for charging the batteries
RAPS systems are usually installed where
electricity supply is not available or
connection costs are high. However, some
people install RAPS to be independent
from the mains supply or to have reliable
power in areas where blackouts are
common.
RAPS rely on storing the power
generated by the PVs, hydro or wind in
banks of batteries, which are generally
larger versions of the lead-acid batteries
used in cars. They can be 12V or 24 V, and
a house will need perhaps 20-30 batteries
to store sufficient power to run the needs
of the home.
The batteries have a typical life of 10
years, though if they are significantly
discharged each day, then the lifetime will
be less. Nickel-cadmium battery banks are
also used, but are much more expensive,
though they have a longer lifetime. Each
poses an environmental problem after they
are no longer useful.
Energy Matters 16. Domestic Renewable Technologies
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