Sustainable Electricity:

How an electricity supply system might be operated to make it compatible with sustainable generation technologies; a layman's thoughts

This page was started 2004/10/27, last edited 2020/05/07
Contact: David K. Clarke – ©


Several forms of sustainable electricity generation have the disadvantage that they are variable: for example, wind-generated electricity is only available while the wind blows, electricity from sunlight is usually available only while the light is bright.

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Reserve power supplies must be available at short notice to fill the gaps – when the wind slows, or when clouds shade the solar collectors. Not that this is a new thing, reserve power supplies have always been needed to allow for unexpected failures or scheduled maintenance shut-downs in conventional power stations. 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.

The energy situation in Australia has changed enormously over the period in which this page has been written and periodically added to. Some sections will be dated.

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 (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.

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.


Level playing field

At present, at least in Australia and the USA, power generators who burn fossil fuels are allowed to pollute the atmosphere with CO2 free of charge. This gives them an unfair cost advantage over non-polluting generators. In a rational, fair, and environmentally sound market, they should either have to dispose of their CO2 sustainably, or pay a fee that is at least as great as the cost of geosequestration of the carbon dioxide that they produce. Note that geosequestration is far from an ideal way of disposing of CO2.

This 'licence to pollute' is effectively a subsidy payed by everyone on earth to the owners of the power stations.

As an example, at present 'off peak' electricity for water heating is supplied at a much lower price than general electricity. 'Off peak' is defined as a time of day when demand is usually low; generators like to have a minimum load and water heating can supply some load when most other consumption has been switched off. Why not have a continually variable price for variable-price-electricity? Instead of switching on at a particular time of day, a water heater could switch on when the VPE price dropped below a preset figure. The consumer could set the price that he/she is willing to pay; a higher price would guarantee having your water heated, setting a price that was too low would mean that some days you might not have your water heated to the temperature that you wanted - because the VPE price didn't fall low enough for long enough.

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 pricing

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.

A South Australian wind farm
Wattle Point Wind Farm, Yorke Peninsula, photo 2016/06/19

Price responsive load: concept

Introductory remarks
What appliances or facilities could use intermittently available electricity?

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.
  1. Heating water, domestic and industrial;
  2. Heat bank room heaters;
  3. Irrigation pumps;
  4. Municipal water pumping;
  5. Refrigeration, domestic and industrial;
  6. In summer, producing slush ice for home cooling;
  7. In winter, producing hot water for home heating;
  8. Recharging electric cars;
  9. Drying coal to be burned in power stations;
  10. Desalination (removing salt from water);
  11. Producing hydrogen and oxygen by the electrolysis of water (the hydrogen could later be used to power clean fuel-cell vehicles, oxygen has many uses);
Price responsive load: detail discusses this subject in more depth elsewhere on this page.

Desalination and electrolysis would probably be viable only on a commercial scale in the near future; in the longer term, who knows?

Several methods could also be used to take power from the grid at times when the price was low and feed it back when the price was higher, and so, to some extent, level the peaks and troughs. See Balancing the electricity grid, below.

Switching periods

Different devices and appliances have different minimum and maximum periods for which they can be switched off. Some examples:
  • Domestic water heaters and electric car chargers could be switched on or off for periods of seconds up to hours;
  • Pumped storage of water would require start-up times and shut-down times of at least several minutes;
  • Irrigation and municipal water pumping would need to run for at least around a half hour - it would depend on individual cases;
  • I would think a desalination unit would need to run at least for an hour at a time, unless it was specially designed;
  • Refrigerators and freezers could be switched off for periods off a few minutes at a time without the stored food moving outside optimal temperatures;

Some details of the switching arrangement of loads

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.

Need for computer modelling

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.

Price responsive load: detail

More detailed remarks on some aspects of the system

Following 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

Refrigeration, domestic and industrial

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."

Recharging electric cars

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. This concept was discussed in an article in the New York Times dated 2013/04/25, written by Matthew L. Wald, see here.

Heat bank water and space heating

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.

Ice production for home cooling

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.

Mainly industrial

Irrigation pumps

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.

Municipal water pumping

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.

Drying coal for power stations

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.

Some calculations of the quantity of electricity that could be controllably consumed

Home water heating systems

The 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.

Conversion of electricity to heat in water (for heating 100L of water from 20°C to 50°C - in a typical home water heater).

From Energy Units we have (approximately) 1 calorie = 4.19 joules (J), 1J = 1 watt second and 1 calorie is sufficient to raise the temperature of 1ml of water by 1 degree Celsius.

Therefore, to raise the temperature of 100L by 30°C requires 100 000 * 30 * 4.19 watt seconds
= 3492 watt hours, or
= 3.492 kilowatt hours (kWh)

Refrigeration; domestic

Typical 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.


The 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.


A small South Australian solar farm
Peterborough Solar Farm, photo 2018/05/12

Balancing the electricity grid


Refrigerators and freezers

Some existing refrigerators are poor at holding a near constant temperature. These would not be suited to load-balancing as this problem would become worse.

If new refrigerators were made with more insulation or more efficient insulation it would be possible to have greater flexibility in the periods during which they run.

An electricity supply system (power grid) has to continually balance supply and demand. Total electricity being generated must, at all times, be matched by total electricity consumption. This is achieved to a limited degree by allowing the voltage in the grid to rise and fall – within bounds. If generation falls behind consumption the voltage in the grid falls and electric motors run a bit slower, electric toasters and jugs run a bit cooler, etc. with the result that less power is consumed. The reverse happens is generation excedes demand. However, if the voltage is allowed to vary too far from its nominal value serious problems arise.

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 generators

Although 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 electricity

If 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.

See also:

Storing energy as heat

When 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.


Batteries of electric cars

In 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 storage

Compressed 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 energy

Dissociation of ammonia into hydrogen and nitrogen

Ammonia 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 storage

Flywheels 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.

Latent heat energy storage

An interesting developing system for storing energy in a mass of silicon is being developed in Adelaide.
"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).

Retail cost of electricity

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.

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:
  1. Accurately forecast the amount of wind power that will be generated in the near future by improving wind forecasting;
  2. Accurately forecast the amount of solar power that will be generated in the near future by improving sunshine forecasting;
  3. Use massive flywheels as a very short-term store of energy and to stabilise the output of a sustainable power generator;
  4. Use large chemical methods of energy storage (batteries)
  5. Use the dissociation of ammonia into nitrogen and hydrogen as an energy store;
  6. Operate the sustainable energy generator in conjunction with a pumped water energy storage, not necessarily at the same place;
Further to point 3 above...
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 grid

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.


Electricity from firewood

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?

Also see Firewood: an environmentally responsible fuel.

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?

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

Also see Energy units and conversions; another page on this site.

Electricity from firewood - conclusion

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.

Related pages

External pages

Wikipedia 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.