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This has meant that reserve power supplies must be available at short notice
to fill the gaps - when the wind slows, or when clouds shade the solar
collectors. Some reserve power can be gas or oil-fired turbines ready to
start, but some must also be 'spinning reserve':
turbines powered by fossil fuels kept running, not generating electricity,
but ready to do so at very short notice. This is inefficient, and increases
the effective cost of renewable electricity.
Price responsive load (PRL)Supply Responsive Electrical Consumption is another name for the same concept, Demand Side Management is a similar concept.There is an alternative, the variability in electrical generation could be matched by a corresponding variable rate of electrical consumption.At present (at least in Australia where I am writing) generators are paid a variable price, depending on the demand at the time. Should the retail price of electricity also vary depending on supply and demand? Consumers could choose to link some of their appliances with variable price electricity. |
Variable price electricity, (VPE)Vary the instantaneous retail price of electricity according to supply and demand and allow consumers to decide the price that they are willing to pay. Supply would only happen when the instantaneous selling price was below the consumer's agreed buying price. The price could be transmitted over the power lines by an AM or FM signal, or the Internet could easily be used; appliances would be switched on and off automatically. The variable pricing will cause a close linkage between the variable rate of generation and the rate of consumption.The VPE concept has a lot in common with the price-responsive-load and supply-responsive-electrical-consumption concepts. |
This would also avoid the problem with the present system where there is a sudden big increase in consumption at the time all the electrically boosted water heaters switch on. With VPE the price would change in small increments and optional consumption would also increase and decrease by small degrees. The VPE price would rise and fall depending on demand and supply - fully market driven, and appliances would automatically switch on and off depending on the maximum price that their operator agreed to pay. Of course some method would be needed to 'inform' the appliances of the current VPE. This could be done, I believe, by sending a signal (AM or FM) through the power lines. I understand that this system is in use in some places for reading power meters from a central location. Alternatively the Internet might be used, or it could be incorporated with digital television broadcasts; such a small amount of bandwidth would be needed that it would not be a problem. This page uses several technical units. Energy units, definitions and conversions are available on an additional page. |
| The price that retail electricity customers currently pay for electricity actually includes two components: (1) the electricity commodity; and (2) the insurance premium that protects customers from price variations. Most consumers are unaware of this second component - the risk premium - associated with traditional electricitty pricing. A system based on price-responsive-load would be able to remove this insurance component from at least some of the electricity supply and thus provide lower prices to consumers without reducing income to generators. |
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Price responsive load: conceptIntroductory remarks
Some electricity consuming processes that could use low-priced
VPE are listed below. These electrical loads could be switched
on and off to take advantage of lower electricity prices at
times of excess capacity.
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Switching periodsDifferent devices and appliances have different minimum and maximum periods for which they can be switched off. Some examples:
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If loads were switched on and off at one price, eg. many water heaters set to
switch on when the price of electricity dropped below $0.10 and also set to
switch off when the price rose above $0.10, then it could cause 'shunting' of
the price. For example, the price drops from $0.101 to $0.10, many
water heaters switch on, the load increases substantially, and in consequence
the price rises to $0.102. Now all these water heaters switch off, the
load decreases and the price drops. The cycle continues.
To stop this from happening there would need to be one 'on price' and a different 'off price', probably with a preset and compulsory difference between the two. For example, water heaters could be set to switch on when the price fell to $0.10, but then they would no switch off until the price rose to, say, $0.11; there would be a compulsory one cent difference between the on and off prices. The same would apply to electric car rechargers and many other power consuming devices. Refrigerators would require a more complicated, but still very achievable, switching system. They would need switching based on a mathematical function relating their internal temperature to the power price. As the temperature of the refrigerator rose so the buying price for power would also rise. |
| Before such a system could be put in place careful and thorough computer modelling of how all the component parts would interact would be necessary. |
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Price responsive load: detailMore detailed remarks on some aspects of the systemFollowing the main thesis of this page: that an electricity grid would be more compatible with sustainably generated electricity if the retail electricity price could be continuously variable, I detail below some of the forms of consumption suited to such variable-price-electricity.A list of electricity consuming appliances or processes is given in Controlled electricity consumption. This section goes into more detail on this subject. |
| Mainly domestic |
| Refrigerators and freezers switch on and off periodically depending on the temperature of their contents and the setting of their thermostats. The precise timing of the switching could be adjusted to take advantage of VPE; this would require some 'smart' switching electronics." |
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As I write, in 2005, there are few electric cars on the world's
roads. However, as petroleum becomes scarcer, and as
governments take the greenhouse problem more seriously, electric
cars will become more common, especially for short-run travelling.
Electric cars would be very well suited to a power grid having variable-price-electricity. A car owner could plug in his/her car, set the electricity price that he was willing to pay, and go away. When (and if) the price fell below that preset point, the car's batteries would be charged. If the car owner wanted the batteries topped up quickly, he would have to be prepared to pay a higher price; if he was willing to wait a longer period, probably overnight, then he could be pretty confident of getting away with offering a considerably lower price. It would also be possible for the battery charging system to be set up so that power could be sold from the battery back into the grid when electricity prices were very high - to the profit of the battery owner. |
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Heat is more easily and cheaply stored than is electricity.
In a system that prices electricity according to how abundantly available
it is, water could be heated when electricity prices are low.
This is similar to the present system of heating water electrically
in off-peak periods.
In winter the relatively low priced electricity (at times of abundant electricity generation) could be used to heat large tanks of water. Heat could later be taken, as required, from this water for space heating. The tanks would probably be several kilolitres domestically and several tens of kilolitres in commercial buildings. The heat could be stored efficiently for periods of up to several days. It would be wise to also use solar water heating panels as far as possible for heating this water, however the efficiency of solar water heating is much reduced when the sun is not shining, as is often the case when heat is most needed. Using electrically powered heat pumps to heat the water, rather than using the simpler but more direct heating effect from electrical resistance could increase the amount of heat per kWh of electricity up to three-fold. |
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The air conditioning electrical load on the power grid on hot summer days is
the single largest cause of troublesome power-consumption peaks.
It would be
quite possible to build refrigerated air conditioners capable of producing
ice instead of, or in addition to, cooling a home.
The store of ice could
then be fallen back on when the grid power supply was particularly stressed,
with the home being cooled by the ice rather than by the
refrigerated air conditioner.
The air conditioner could be turned on before it was needed for home cooling: in anticipation of a forecast hot day. The ice would be produced when electricity was plentiful and cheap, and stored for use later in the day when the power supply became stressed and the retail electricity price rose. |
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| Mainly industrial |
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Many crops require irrigation and a large proportion of irrigation requires
a pump. Electrical consumption by irrigation pumps can be quite high when
either very large volumes of water are concerned, or when the water must be
pumped from a deep well. Timing of irrigation is not critical, the farmer has
a window of at least several days when his plants could be irrigated.
A 'smart', computerised, irrigation system could offer a gradually increasing price for VPE until the going price dropped enough for the irrigation to happen, or, of course, the farmer could adjust his offering price manually. |
| Water pumping is a large part of the cost of running a municipal water supply. The water is usually pumped into large tanks from which it gravitates to consumers. The pumps could easily be switched on and off to make use of VPE. |
| Desalination is an energy intensive process that should be well suited to consuming power when it is cheap. The produced fresh water is relatively cheap to store, certainly over a period of days. This is discussed in relation to a specific project in Eyre Peninsula water supply and in more depth as a way to use the variable electrical output of wind farms in Wind-electricity-desalination. |
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The Cooperative Research Centre for Clean Power from Lignite (Victoria) has
found that by drying coal using a combination of mild heating and pressing,
about 70% of the water can be removed, making it much more energy efficient
as a fuel, and thereby reducing greenhouse gas emissions.
Coal drying processes such as this could be adapted to the use of grid electricity when it is abundant and cheep. |
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Home water heating systems
Refrigeration; domestic
Desalination
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Storing electricityIf market forces caused the higher electricity prices to be much greater than the lower prices then it would be profitable for third parties to set up electricity storing devices. These people would buy power when the price was low and sell when the price was high, thus tending to lower the highest prices and increase the lowest prices.Several methods are given below. |
Batteries of electric cars
Pumped water energy storage
Ammonia for storing energyDissociation of ammonia into hydrogen and nitrogen
Flywheel energy storage
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US Department of Energy, Distributed Energy Program - Technologies, discusses
Electricity load as a reliability resource.
The Consortium for Electric Reliability Technology Solutions discussed a study supposedly examining the performance of "real time pricing". However the prices were advertised a day before the consumption so it was not truly 'real time pricing'. |
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In 2005 the wholesale price of electricity
from fossil fuel fired power stations is around Aus$40 per
megawatt-hour (MWh) (however, this is effectively subsidised
because the people who operate these polluting power stations
pay nothing for the damage they do to the atmosphere, see
Level playing field).
The wholesale cost of electricity from wind farms, for comparison, is about $80/MWh. As an example of the actual price paid for electricity by a retail consumer, my bill for the three months ending in January 2005 was a total of Aus$161 for 692kWh, equal to Aus$232/MWh. Note that some of this is called 'supply charge' and 'goods and services tax' on the bill; however, it is still what I, a retail customer, pay for my electricity. Not that even overlooking the effective pollution subsidy given to the fossil fuel generators, the retail price of electricity would not need to rise very greatly to change to sustainable electricity - it would go from $232/MWh to $272/MWh. If the pollution subsidy was removed, then sustainable power would be no more expensive. |
In a rational market electricity generators would be paid a
premium if their electricity supply could be relied upon. So,
how could environmentally friendly power supply be made more
reliable? Some suggestions:
As the very large blades of wind turbines spin with a tip velocity of above 200km/hr they must have a substantial rotational momentum, and thus serve the same purpose as a heavy flywheel. However, a flywheel system could smooth the output from a large solar array. |
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There are several challenges in modelling a fully price-responsive load
electricity supply system. First is the greatly differing time periods on
which differing generating and consuming equipment typically operates, from
minutes for refrigerators and freezers to many hours for a typical good
blow at a wind farm.
The scale of the system is huge. There will be tens or hundreds of thousands of domestic water heaters, and hundreds of wind turbines. Clever work will be needed to condense this while still maintaining validity. |
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The retail price of firewood is, by a considerable margin, lower than that of other common sources of heat. Why not use firewood to power electrical generators connected to the grid?I should qualify the statement in the first sentence above; my calculations indicate that it is true in South Australia and in terms of the amount of energy per unit mass of fuel. The comparative prices of fuels are detailed on my Energy calculator. Firewood can be a sustainable fuel, unlike fossil fuels. Burning firewood does not lead to a net increase in greenhouse gasses. The world's petroleum supply is starting to run out; the prices of liquid and gaseous fossil fuels will rise more or less steadily over the next few decades. The world must turn away from burning coal.
One problem with this proposal is that major plantations will be required to
produce sufficient wood; the lead time for getting a plantation to the point
where it is ready for harvesting is around fifteen years. If society leaves
it until petroleum has become prohibitively expensive there will be a very
lean period before wood-fired electricity generation can become significant.
The limits to firewood-generated electricity?Firewood-generated electricity (FGE) cannot possibly replace all the forms of electrical generation in use today, there simply is not enough space on the surface of the earth to grow that much firewood; and it would be immoral to take land away from growing food and use it for producing energy while that food is necessary for the world's billions of people.When would one use firewood-generated electricity?Considering the limits to FGE, its use would have to be judicious. Other forms of sustainable electricity should be used when they are available (for example, wind and solar), FGE should be used when the wind is not blowing or the sun is not shining.Where would one use firewood-generated electricity?In the first instance it would make sense to try to place the wood-fired power stations close to where the wood can be grown and close to major power lines. |
| Energy obtained from burning a tonne of air-dry firewood | 16GJ |
| 1 Watt (W) = | 1 Joule per second |
| Therefore 50MW would be obtained from burning | one tonne of firewood each 320 seconds, about 0.2 tonnes per minute. If the power station was 40% efficient it would require 30 tonnes per hour or 250 000 tonnes per year. |
| With an annual rainfall of 500mm, 5 tonnes of firewood can be grown per hectare each year | |
| 250 000 tonnes of firewood per year could be grown on | about 50 000ha, or 500 square kilometres of land |
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What this exercise seems to demonstrate is two things: Western society is extremely profligate with energy; and firewood can do no more than provide a small supplement to electricity supply in any Western nation. Electricity generated from hot dry rocks would seem to be a far more practicable option for base-load supply in the longer term, and wind-generated electricity combined with variable-price-electricity in both the short and long term. |
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