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20 December 2009
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Variable renewables
Wind power
Solar Photovoltaic
Tidal and wave
generation
Constant
renewables
Hydroelectricity
Biomass
electricity
Geothermal
electricity
Biomass gas
Enhancing the
value of waste
Conclusion
Further reading
To date, renewable energy has not been given a
high priority in some countries, although Northern and mid- Europe has a
higher implementation of both wind and solar than most other places.
California has also made good strides. Of course, hydroelectricity provides
a good proportion in the Alpine arc, the Rockies, China and Norway.
These are defined as energy that cannot be
generated at a constant level. Some examples are tidal, solar and wind
power. These are sometimes called intermittent sources, but I prefer the
term "variable" over "intermittent", because the latter implies an "all or
nothing" energy generation, whereas the output more commonly varies from
nothing to full output with every intermediate level possible. It should be
noted that it is essential that constant power supplies must be available to
cover the maximum demands, so that variable renewables can only serve to
allow conventional power stations to be "eased off", thereby reducing fuel
consumption. Good weather forecasting is a sine qua non of useful
exploitation of variable supplies, so that the constant requirements can be
foreseen and the plant brought up to speed accordingly.
It must be noted that there is a limit to the
amount of variable energy that a power grid can handle at any one time.
Above that limit of 18 - 20 per cent, experience in other countries has
shown that the whole grid system may become unstable, leading to black-outs.
In some countries in higher latitudes, wind
generation, both offshore and onshore, has been well implemented with
further expansion planned.
The graph shows the
empirically measured output from a typical wind generator in the
megawatt range. It is clearly non-linear and gives full output
only over a limited range of sustained high wind speeds near to
full gale force of, say, 15-20 m/s. If the wind drops to an
average of 10 m/s, the output drops to only about 40% of the
turbine's rated output, yet 10 m/s is still quite a powerful
wind (22 mph, 35 km/h, Fresh Breeze on the Beaufort scale).
Above about 20 m/s, the blades are feathered and brakes applied
to prevent damage.
This graph shows the
yearly average wind speed at a land-based location, considered
as very favourable for the exploitation of wind energy
(North-east Scotland). If we integrate the two charts above, we
obtain a total weighted turbine output of about 37% of the rated
turbine output:
It is emphasised
that the location in the above example is considered as very
favourable with strong sustained winds. Strong gusts with a low
average wind speed is more common in many locations and is not
so favourable. To illustrate this point, this is a random 24
hour wind speed chart from my weather station, in a location
sheltered by surrounding hills:
The red line is the
gust speed in m/s, the blue line is the 2-minute average and the
green line is the minimum. The blue line is missing between
about 1930 and 0820 (computer switched off, average not
calculated). The point I wish to make is that, if we look to the
extreme right of the graph, we can see the wind is gusting up to
10 m/s, but frequently drops to zero, with low averages. This
gustiness is useless for wind generation. On the other hand,
between 0500 and 0700, the gusts are lower, up to about 5-6 m/s,
but the minimum wind speed is between 2 to 4 m/s. If this were
in a more exposed location with, say, double these wind speeds,
the latter period would be reasonable for electricity
generation, but it would be difficult to amortise a turbine
under these conditions with a couple of hours generation at less
than half capacity in a day!.
Very few locations
will give a time-weighted turbine output greater than 30% of the
rated capacity of the turbine. Most places with a reasonably
windy reputation and an exposed location will not give better
than 20-25%. The 37% in the above example is exceptional because
it is in the zone of the Westerlies, between 50° and 55° N. This
wind is also very favourable to the southern Scandinavian
peninsula and NW Denmark. In the southern hemisphere, the
equivalent wind is at a lower latitude, known as the Roaring
Forties. Lower latitudes have generally decreasingly useful
winds, although very exposed locations can sometimes be found
where the local topography increases wind speed by the venturi
effect.
According to a letter by Bill Hyde,
published in Engineering & Technology magazine (Vol 3, no. 20, November
2008), nothing is generated at wind speeds below 4 m.s-1 and
he goes on to cite that Germany has 23,044 MW capacity of wind turbines
installed. Between 2100 on 3 November 2008 and 2359 on 5 November, it
produced less than 1000 MW (<4.35 of capacity) with a low 20 MW (0.09%)
at the peak consumption time of 1200 on 4 November. This illustrates the
need for wind (and solar) to have 100% fixed backup available,
considering that Germany is a much windier country than Cyprus. He
finishes "Customers don't like blackouts - especially the home dialysis
people". In fact, he points out a source of data on the German
electricity output from wind at
this site,
with graphs of weekly production. The sample below shows that periods of
day at <10% of capacity are not uncommon and the peak production is at
about 42% of capacity. I haven't integrated the curve, but it would see,
by eye, that the average is less than 20% of capacity over that week.
Other data shows typical output over the summer months at about 10%
capacity.
It would be possible
to design turbines to give full output at lower wind speeds; the
problem is that such machines would not withstand the
exceptional gale. The following video shows what could happen,
although this shows a 10-year old turbine whose gale brakes
failed.
This is perhaps the ideal variable renewable
energy source for sunny climates with well over 2,500 hours of "useful"
sunshine per year, except for its capital outlay. A 3 - 4 kW system
(size which will fit on a typical south-facing villa roof) will generate a
theoretical 7.5 - 10 MWh. However, be warned, you will never reach this
theoretical limit because the efficiency of the solar panels drops at
temperatures above 25°C; it would be wise to budget for 5 - 7.5 MWh
respectively. If one were to buy a PV system at its full price and not sell
any surplus electricity generated, the payback period would probably exceed
the lifetime of the system. With the subsidies and pay-backs available in
many countries, it may be possible to amortise the capital cost in, say, 10
years, depending on the conditions.
The real cost of generating solar
PV electricity is very high, typically 35 - 50 c/kWh. However,
it can make a real contribution to smoothing out peak demands
because it will be reasonably productive at the time when
air-conditioning units and chillers would be working hardest.
This alone makes it interesting, despite the high cost and the
surtax burden on the ordinary electricity consumer.
It has been said that one form of
tidal electricity generation is like wind generation under
water. Where this analogy fails is that tides are largely
predictable, wind is not. However, it should be stated that
there are four periods per day when tide generation does not and
can not work; as the tide turns, hence it being classed as
variable, even though it is predictable.
Another form uses a barrage across
a tidal estuary or bay, while a third type uses the pressure
differentials in a concrete caisson.
As a rule, tidal generation is
likely to be useful only where the tidal range is several
metres, even at neap tide.
Wave generation is a possibility
when the average height of the waves (crest to dip) exceeds 1
metre. I have not studied this possibility. The "best" waves for this are oceanic swells, but local
wind-driven waves would also work. However, as these are
dependent on wind, it is probable that the same conditions as in
the paragraph on wind generation would probably apply. A
Scottish company has constructed a prototype system for
Portugal.
Hydroelectric generation is the mainstay in
countries, like Norway, Switzerland and Austria, where there are large
glaciers. The water from the summer melt-off is collected in large dams, at
altitude, and penstocks lead the water to pressure-operated turbines in the
valleys. Alternatively, large dams across rivers, such as the Three Gorges
Dam in China, can turn flow-operated turbines. Both types are
environmentally disputed for several reasons and large projects are nowadays
very severely criticised. Both types are also potentially dangerous to
downstream life if, for any reason, the dam should burst and this does
occasionally happen, despite the best efforts of civil engineers.
There is a variant of hydroelectric
generation which could possibly have some future relevance to
counter the undesirable effects of variable renewables. Imagine two lakes of equal size, say,
similar to that of one of the larger dams, but separated in altitude by 300
- 500 m. During the night, when there is a surplus of power generating
capacity, water is pumped up from the lower to the higher. At peak demand
time or when variable renewables have a low output (e.g., no wind and a
cloudy day), the water in the upper reservoir is made to flow down to the
lower lake, generating hydroelectricity. This method is the only useful way
of "storing electricity" for later use on a reasonable scale with today's
technologies.
This consists of growing some form of crop,
usually wood from quick-growing trees, for gasification or chipping and
burning in a thermal power station.
This method is ideal in places like Iceland but
requires volcanic rock strata at a constant temperature of about 200°C.
Medium to large-scale poultry, cattle and pig
farming produces large amounts of
excrement and other organic waste. If this is placed in a large anaerobic digester, the gas produced
by fermentation in the first 48 hours can be collected and large quantities
of methane (or natural gas) can be easily separated. This gas is
indistinguishable from fossil natural gas and can be used for any similar
purpose. If transported to power stations, it could complement other fuels,
providing a small percentage of the local power supply.
In a number of countries, up to ten percent of
electricity requirements are being supplied by enhancing the value of
household garbage and other combustible waste. This is .most practical in
regions of high population density, such as large cities. This would also reduce the need for
the many, unsightly, polluting, insanitary landfills, a few of which would
be used only to dispose the sterile cinders. Such power stations are not
cheap to construct, as the exhaust gases have to be scrubbed to eliminate
harmful pollutants, but they do make a useful contribution to the
environment. A sister website to
this one treats this subject in detail.
EU Directive 2001/77/EC
Οδηγία 2001/77/ΕΚ
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