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Price responsive load (PRL)Supply Responsive Electrical Consumption is another name for the same concept, Demand Side Management is a similar concept. The 'smart grid' idea takes the same concepts even further.There is an alternative to inefficient and expensive 'spinning reserve' and other short-startup reserve generation; the variability in electrical generation could be matched by a corresponding, and controllable, variability in electrical consumption.
At the time of writing (2018, and still in 2020, at least in Australia where I am writing) generators are paid a variable price, depending on the demand at the time.
The retail price of electricity could also vary depending on supply and demand.
Then consumers could choose to link some of their appliances with this variable price electricity.
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.
Current retail pricingThe 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 electricity 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. |
| A South Australian wind farm |
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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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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.
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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 |
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. This concept was discussed in an article in the New York Times dated 2013/04/25, written by Matthew L. Wald, see here.
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.
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 |
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.
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 systemsThe table in the box below shows calculations that indicate about 3.5kWh of electricity are consumed to heat the water in a typical home water heater by 30°C.If 10 000 water heaters are converted to using variable priced electricity (VPE), and supposing that the water in the heaters only needed a 15°C boost, then up to 10 000 * 0.5 * 3.5kWh = 17.5MWh could be consumed at selected times each day to help stabilise the electricity distribution grid.
Refrigeration; domesticTypical electrical consumption of fridge or freezer is around 300W. If there are a million refrigerators and freezers in South Australia; 1000 000 units x 300W = 300MW controllable load.DesalinationThe amount of electricity required to desalinate a kilolitre of water varies from 3 to 9kWh depending on the quality of the feed water and the desalination method. Desalination of, say, 1ML of sea water could be expected to consume 7 to 9MWh.More extensive notes on using wind-generated electricity for desalination are in my page on Eyre Peninsula water supply. |
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A small South Australian solar farm
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Peterborough Solar Farm, photo 2018/05/12 |
So far as I have not found any fully price-responsive load electricity supply systems anywhere in the world. One of the challenges of constructing one will come from the varying time-spans that different types of generating machinery operate and different types of electricity consuming equipment need power. For example, periods of stiff breezes will run wind farms typically for a number of hours every few days, while domestic water heaters need significant power at least once a day and refrigerators and freezers need power at least for, say, quarter of an hour every hour. I suspect that, in practice, fridges and freezers will be useful for balancing the short term fluctuations in generation from sustainable sources such as solar photovoltaics affected by intermittent cloud cover while water heating, electric car charging, and municipal water pumping will more nearly match the time spans involved in wind farm generation.
Small petroleum-powered generatorsAlthough it might seem a step backward in relation to sustainable energy, small petrol or diesel-powered generators, privately owned, could be very useful in a system in which the price of electricity is continually varied depending on supply. It would be quite possible to have these set up to automatically come on-line whenever the electricity price goes above a preset level. They could be profitable for their owners, and if they are numerous enough, could 'fill in the small gaps' in the electricity supply and greatly reduce the need for 'spinning reserve' and utility-scale backup power supplies. |
Storing electrical energy
Wholesale electricity prices follow the abundance of electricity generation; low prices when there is plenty of generation from renewable sources, high prices when the wind isn't blowing and the sun isn't shining. So it becomes potentially profitable to buy energy and put it into storage when it is cheap and take it out of storage and sell it back into the grid when wholesale prices are high. There are a number of ways that energy can be stored, as detailed below. See also:
Storing energy as heatWhen the electricity will ultimately be used to produce heat, it is very economically effective to store the energy in the form of heat that can later be used as needed. Electricity can be used to heat an insulated tank of water and then the water can be used to heat a home. An alternative is for the electricity to heat an insulated mass of stone from which the heat can be taken as needed.
Latent heat energy storageAn interesting developing system for storing energy in a mass of silicon is being developed in Adelaide, South Australia."It can provide energy storage on an industrial scale, up to several hundred megawatt hours – enough to power around 7,000 homes for a day; and is also small enough to fit inside a 20-foot shipping container, making it suitable for both on- and off-grid applications."From an article written by Sophie Vorrath and published in RenewEconomy on 2015/10/15. The system relies on the latent heat of silicon (common quartz sand is silicon dioxide).
Battery energy storageOne of the most publicised examples of storing renewably generated electricity in big batteries was when Elon Musk promised to build a big battery in South Australia in 100 days or do it for free. The original battery, in 2017, was 100MW power and 129MWh energy. A few years later this was increased by 50%.Since then many 'big batteries' have been built around the world, and they are getting bigger and bigger, with at least one having a power capacity greater than a gigawatt being proposed in 2023.
These batteries have mostly been based on lithium. The environmental sustainability of lithium-based batteries is very much an open question at the time of writing this (July 2023).
Batteries of electric carsIn late 2008 it apears that electric cars will gradually replace petroleum-fuelled cars. Owners of electric cars will plug their cars into the electricity grid whenever the cars are not in use (this may be at the owners home and it could also be at parking stations). It would be quite possible, and highly desirable, to set up the charging stations so that power from the grid would go into the batteries when the retail electricity price is low and power could be taken out of the batteries when the retail price is unusually high. The stability of the grid would be improved and the battery owners could profit at the same time.Pumped water energy storage (pumped hydro)Water is pumped from a low altitude reservoir to a high altitude reservoir when electricity is abundant. At periods of high electrical demand the flow is reversed, the water flows through turbines, and electricity is generated. The system has been used in the Snowy Mountain Scheme (between Talbingo and Journama storages) and elsewhere as a means of storing electricity for many years.I have discussed the possible use of pumped hydro in my home state of South Australia elsewhere on these pages.
About 270kWh of electricity are consumed in raising one megalitre
(1ML) of water by 100m. (1ML=1000kL = 1 000 000L)
Compressed air energy storageCompressed air energy storage [CAES] should also be considered, the Electricity Storage Association have produced a chart indicating that it is economically competitive.Ammonia for storing energyDissociation of ammonia into hydrogen and nitrogenAmmonia can be split into its constituent hydrogen and nitrogen in an energy consuming reaction. The hydrogen and nitrogen can later be recombined and the energy recovered – refer to A method of storing solar energy elsewhere on this site.Of course hydrogen has long been discussed as a medium for the storage of energy; per kilogram the amount of energy in hydrogen is higher than in any other common fuel. A disadvantage of hydrogen is that it is difficult to efficiently store and transport, but ammonia is easy to store and transport and the hydrogen in the ammonia can be recovered. Apparently separating the hydrogen from the nitrogen has been problematic, but on 2018/02/15 the CSIRO announced progress in development of an efficient method of separating the nitrogen from the hydrogen. The CSIRO article stated that there is an existing global trade in ammonia.
Flywheel energy storageFlywheels have been used successfully to power such things as shunting locomotives. The principle is simple, there is an electric motor/generator on the same shaft as one or more massive (ie. heavy) flywheels. In the case of a shunting locomotive the driver goes to a recharging station, plugs-in to an electricity supply, builds the flywheel up to its maximum velocity, unplugs and then takes power from the flywheel - by using the generator mode of the motor - to power his locomotive.In the case of an electricity supply, when there is an excess of electricity, power is fed to the motor which increases the speed of rotation of the flywheels, and energy is stored as kinetic energy. When power needs to be taken out of the flywheels - because a cloud crosses in front of the sun for example and a solar farm stops producing power for a short while - the motor shifts to generator mode and converts kinetic to electrical energy, feeding it into the grid. Wind farm operators, or solar power producers could use such as system to even out their power production rates and so command a higher price for their electricity. The flywheel energy stores should be capable of maintaining electrical supply long enough for alternative power sources (at present probably fossil fuelled) to power up; avoiding so much need for 'spinning reserve'. Such flywheels would probably use magnetically levitated bearings and run in a vacuum so that very little power would be lost when it was idling. They could be used independently of any generating facility. Its operator would buy power to accelerate his flywheel when power was cheap and wait for the price to rise. Then he would switch his motor into generator mode and sell power back into the grid. In practice this would all be done automatically, under computer control. I imagine that this would be ideal for use in generating power for release into the grid during short duration peaks of very high demand. The flywheel generator could be switched between its three modes (accelerate, idle, generate) in a matter of a few seconds or even less.
Energy storage by raising and lowering heavy weightsThere are at least three variations on this, all of which use energy when it is cheap and abundant to lift something and then, when energy is in higher demand, allow gravity to take the weight down while powering a generator. The variations I've come across are:
References can be found by a Web search. |
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Retail cost of electricityIn 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.
How could generators of sustainable electricity command a higher selling price?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.
Computer modelling of a price-responsive load gridThere 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.
Electricity from firewoodThe 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. So long as trees are grown at the same rate as they are harvested 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.Firewood power station?A small power station (by South Australian standards) is 50MW. South Australia has a population of about 1.3 million; its power consumption at periods of minimum load is about 1000MW. How much land would you need to supply the firewood for a 50MW power station?
Also see
Energy units and conversions; another page on this site.
Electricity from firewood - conclusionWhat 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. | ||||||||||||||||
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Related pagesExternal pagesWikipedia has an article under the title Smart grid. As usual, there is a good listing of links at the end of the page.Independent Review into the Future Security of the National Electricity Market, (Australia); Preliminary Report, December 2016, Dr Alan Finkel AO, Chief Scientist, Chair of the Expert Panel; Ms Karen Moses FAICD, Ms Chloe Munro, Mr Terry Effeney, Professor Mary O'Kane AC 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'. Price Responsive Load Coalition. "The PRLC's Mission is to promote the ability of electric customers to respond to market signals through load reduction, curtailment, fuel switching, generation, energy-efficiency, and other technologies." US Department of Energy, Distributed Energy Program - Technologies, discusses Electricity load as a reliability resource. Prospects for Large-Scale Energy Storage in Decarbonised Power Grids, International Energy Agency, Shin-ichi Inage, 2009. |
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