Solar energy is used for:
(1)
Home and workplace heating,
(2)
Conversion of solar heat
into electricity, and
(3)
Direct conversion of sunlight
to electricity by using
photovoltaic (PV) cells.
Most of the emphasis
in recent decades
has been on PV cells.
Module (aka; solar panel)
efficiency for PV cells
has improved a lot
since 1975.
Efficiencies
approaching 50%
have been obtained
for very small,
very expensive cells.
But for
large-scale
photovoltaics,
the efficiency
tends to be
about 15%,
and has not
improved much
over time.
The electricity
generated
by PV cells
must be converted
from
direct current (DC),
to
alternating current (AC)
before household use,
resulting in a loss of energy.
PV cell efficiency in 2016
averaged about 15%,
and conversion
to grid AC voltage
was about 80%,
making an
overall efficiency
of only 12%.
Wind turbines
extract
kinetic energy
from the wind.
Wind turbines
rotate slowly,
but the tips
of the blades
move rapidly.
The wing tip speed
is a multiple
of the wind speed,
regardless of the
rotor diameter.
Electricity
generators
work best
at high
rotation rate,
typically
hundreds
to thousands
of revolutions
per minute (RPM).
Gearboxes
are required
to convert
the slow rotation
of turbine rotors
into the high RPM
of the generators.
The power produced
by the wind turbine
is proportional
to the area swept
by the rotors:
If you double
the diameter
of the rotor,
you quadruple
the power.
Unfortunately, the
power generated
by a wind turbine
varies dramatically
with wind speed.
Due to the nature
of wind and sunlight,
wind turbines and
solar PV cells
produce varying,
intermittent power.
Wind turbines
and solar
photovoltaic cells
can't produce
a steady stream
of energy into
an electric grid,
and sometimes
they produce
no power at all
for hours,
days, or
even weeks.
PV cell output drops
when clouds,
rain, or dust
reduce the amount
of sunlight
reaching the
solar panel,
especially
during seasons
that tend to be
cloudy or rainy,
... and every night !
Actual solar
energy production ,
in southeastern
Australia
on a typical
June 2018 day,
had high levels
of variability
in daylight hours,
and zero power
production
at night.
Actual wind
energy production ,
in southeastern
Australia
on a typical
June 2018 day,
was even
more volatile:
Turbines were
constantly ramping
up and down,
from 100% to 0%
often changing a lot
in just minutes.
In many
countries,
calm wind
periods
sometimes
last longer
than a week.
Energy can be stored as:
(1)
Chemical energy in batteries,
(2)
Gravitational potential energy
behind dams, and
(3)
As heat, typically heated water.
Without back-up
supplied by
fossil fuel
power plants,
a 'renewables-only'
energy system
would require
a vast amount
of energy storage.
The only technology
available today is
pumped storage
hydropower.
An installation
consists of
a lower lake,
and an
upper lake,
300 to 800 meters
above the
lower lake.
The two lakes
are connected by
a pipeline, with
a power station
at the lower lake.
The power station
is used to
pump water uphill,
from the lower lake
to the upper lake.
Then the
power station
can generate
electric power
from water
flowing downhill
from the upper lake
to the lower lake.
Conventional pumped
hydro power stations
typically store water
for 6 to 10 hours,
and typically
generate electricity
during the morning
and evening peak
demand periods.
A large quantity
of water has to be
pumped uphill
to the upper lake,
using expensive
wind and solar power,
held there for
hours, days,
weeks or months,
and then released
to flow downhill,
to generate
electricity,
( with an overall
efficiency loss
of about 20%. )