There are two ways of dealing with the projected growth in peak power demand. There is the lazy, business-as-usual approach that simply upgrades the power supply system to meet projected demands. Then there are approaches that actively look for ways of reducing peak demand.
This article looks at the potential benefits and problems associated with reducing peak demand by switching enough air conditioners from “on demand” to off-peak power.
Switching air conditioners to off-peak would not be acceptable to the community if the result is a reduction in comfort levels. However, this comfort problem can be avoided by storing enough cold (or heat) to allow room temperatures to be controlled while the air conditioner heat pump is not running. A number of systems are commercially available which could be used to provide this storage.
Avoiding the power price required for major power-system upgrades is not the only potential benefit of this move to off-peak power. Consumers may benefit not only from the use of cheaper off-peak power, but also from lower power consumption. In addition, the switch to off-peak can be made in very small steps where and when required.
How much might peak demand be reduced by moving air conditioners to off-peak power?
The following graph from Energy Action shows that the average peak power consumption for the three hottest NSW days during 2011 was a massive 45% higher than the average daily peak for the year.
Most of this increased demand on very hot days will come from air conditioner heat pumps. However, heat pumps on refrigerators, cool rooms and freezers will also consume more power on hot days. The graph also shows the potential for using off-peak for reducing peak demand.
In winter, power required for heating will contribute to peak demand. For 2009 to 2011 NSW annual summer peaks ranged from 13,765 to 14,580 kW. Annual winter peaks ranged from 12,908 to 14, 274 kW.
Air conditioners have such a large effect on peak power because their average power consumption increases rapidly as the difference between inside and outside temperature rises.
How might enough cold be stored to avoid loss of comfort when air conditioners go off-peak?
There is growing interest in the use of Phase Change Materials (PCMs) to store both cold and heat. The commercial PCMs that might be used for air conditioning applications behave like “ice with a different melting point”. Like ice, they can store a large amount of cold in a small volume and will release this cold over a narrow temperature range. Unlike ice, a PCM may be selected with a melting point closer to the desired room temperature. For example, the PC25 product produced by Australian manufacturer PCP Australia melts at 25 deg C and requires only 15 litres to store one kWh. The heat required to melt PC25 is enough to raise the temperature of the melted PCM by 55 deg C. (Less energy is wasted the closer the storage temperature is to the desired room temperature.)
The simplest alternative avoids the need to change the air conditioning system by using PCM products such as Dupont Energain Thermal Mass Panels to store cold within the room. One disadvantage is that stored cold can be wasted because the panels will continue to cool the room when this cooling is not needed. The room may also need to be kept at uncomfortable temperatures while cold is being stored.
The preferred alternative places the PCM in an insulated storage tank outside of the room. The system is set up so that the air conditioner system cools the tank. Cold water from the tank is then pumped to the rooms for temperature control when required. This PCB Australia circuit shows one way that this could be done. Separating cold storage from the room avoids the waste of cold when the room doesn’t need to be kept cool as well as providing more flexibility in the choice of PCM melting point. The PCM and water in the storage tank have to be separated to avoid contamination of the PCM.
Power can be saved for both alternatives if cold storage can be scheduled for cooler times of the day when the difference between storage and outside temperature is lower.
Either of the above alternatives could be used to reduce peak power while keeping rooms warm during winter. PCM based cold storage can also be used to allow the heat pumps on cold stores, freezer rooms etc. to be switched to off-peak power.
Would all air conditioners need to be converted for off-peak?
No. Air conditioners would only need to be converted for off-peak when and where the need arises. Some consumers may decide to convert their air conditioners to off-peak early to take advantage of lower electricity bills.
Who should pay for the move to off-peak?
All power consumers should benefit from lower power prices because the need to spend billions on power system upgrades disappears. It doesn’t seem unreasonable that part or all of the cost of converting for off-peak should be shared by all consumers.
Are there better alternatives?
A number of alternatives have been suggested. These include:
1. Using market forces to increase the price of power during peak demand periods: Hard to imagine many people turning off air conditioners during a heat wave just because the power price is higher.
2. Using solar PV: Problem is that NSW summer peaks tended to occur mid to late afternoon when solar PV output is dropping. Winter peaks tended to occur after the sun has set. Solar PV and off-peak air conditioning with cold/heat storage may be a good combination.
3. Moving something else to off-peak first: It may be more cost effective to move things like dishwashers, clothes dryers and aluminum smelters to off-peak before starting on air conditioners.
Conclusion: Moving air conditioning to off-peak power as an alternative to power supply upgrades warrants serious consideration.
John Davidson is a retired process engineer with an interest in climate action. He has no association with PCP Australia, Dupont or any other company that is involved in heat or cold storage systems.
 Only the air conditioner heat pump would be on off-peak. Controls and some fans would still need to use on demand power.
 Both the flow of heat into a building and the power required to pump a unit of heat out of a building are proportional to the difference between inside and outside temperature. Doubling this temperature difference would increase average heat pump power consumption by a factor of four.
 For background notes on PCMs and their applications see here.
 Doubling the difference between storage and outside temperature doubles the energy required to store a unit of heat or cold.