Sustainability Journal

April 2013 Issue (files are in PDF format)

Storing electricity: Principles

If you have two insulated heat-storing bodies (A and B) you can use a heat pump to transfer heat from A to B. That does not add any heat to the system except for a small amount from the heat pump's motor but it increases the temperature difference between the two bodies, so they have the capacity to do work (exergy). The heat in the two bodies can be stored and their exergy could be recovered later and converted back to electrical form by using a heat engine such as a Stirling engine. However the conversion from heat to electricity is not thermodynamically efficient unless the temperature difference is very large so it is preferable to make direct use of the stored heat for space heating or hot water, in which case there is no loss of energy. The second illustration shows how that can be done by using a building's heat pump to extract heat from A (the normal connection), which requires long operation cycles because A is cold, then when power is scarce the heat source can be switched to B, which is hot so the operation cycle is shorter and the power consumption is reduced commensurately. The overall result is that the operator can use the electrically driven heat pump to transfer heat from A to B when power is abundant and can recover the electric energy when power is scarce.

At all times of the year the cost of electricity is much lower at night than during the day so normally the exergy boost should be done at night, The process chills A so in the summer the heat that was extracted from A at night can be replaced by heat extracted from the air via the AIR heat exchanger during the day so the system is now storing both exergy and heat. The air around us is an unlimited source of heat and in Canada heat is our primary energy need so this solves two fundamental problems - how to heat our buildings and how to deliver power when the power supply and demand occur at different times. Buildings also need to be cooled in Canada so these processes can be reversed, storing cold from the frigid winter air for recovery in the summer. Large buildings need to be cooled throughout most of the year so a combination of heat stores and cold stores makes it possible to recover electricity at any time of the year.

In a concentric ring heat store body A is wrapped around body B so any heat that escapes laterally from B is captured by A and will eventually be used for heating the building. A little bit escapes from the ends of the cylindrical heat stores but it can be made up by natural ground heat absorbed in the winter so on an annual basis such systems deliver the same amount of heat as was injected into them. They can also collect, store and re-utilize the heat that is currently wasted by air conditioning systems in the summer by utilizing the bidirectional capacity of heat pumps. In that case the AIR heat exchanger's function is to balance the winter and summer heat flows. If the system is serving large buildings the AIR heat exchanger may extract heat rather than injecting it. Such systems are inherently tolerant of annual fluctuations in the heat supply and demand because natural ground heat provides a substantial thermal reserve. (See the March issue of SJ for more details.)

In Canada such systems can generally recover the electricity that was used by the boost pump, but there are exceptions. Although heat flowing from B to A represents a loss in B's exergy, the process of extracting heat from A to run the building's heat pump restores the temperature difference, so the exergy of the pair is maintained even though the stored energy is decreased. The system designer can balance the exergy loss from leakage against the exergy gain as A is cooled. Moreover, there is a strong incentive to run the booster pump every night because the value of the electricity during the day is typically about three times higher than its value at night. Since the booster pump's work is being recovered in the form of reduced power consumption during the day there is no net consumption of electricity but the direct consequence is that a large amount of heat is extracted from B without any net consumption of power. In the cooling season the same process returns "power-free" cooling from a cold store. This can offset much of the increased power consumption that would otherwise result from switching from natural gas to heat pumps, but it depends on the power companies making use of the storage capability. (A large power consumer that pays the spot market price for power could use this same practice to heat and cool their buildings using cheap nighttime power.)

Note that although heat escaping from B to A reduces the ability of the grid operator to recover electricity the resultant rise in the temperature of A reduces the building operator's consumption of electricity so that exergy is still being recovered and it shifts the power load from a deficit period (the winter) to an energy surplus period (the summer). The process has also made it possible to greatly increase the recovery of energy from the summer air, to store the heat in a smaller and less expensive heat store, and it sets up the conditions for achieving diurnal exergy storage.

If the building has a solar thermal collector for hot water then the surplus heat from the collector can be transferred into B, boosting its exergy and reducing the size of the store because the solar thermal collector can add some heat throughout the year. The hot water system, the heating system and the power recovery all benefit from such a connection. Nearly all of the energy still comes from the air but the high temperature of the solar input boosts the exergy significantly even though it may not contribute much heat. The solar input is particularly effective during the winter because none of the solar heat flows from B to A at that time.

If the solar collector has both thermal and electrical (PV) outputs then buildings or city blocks or whole cities could theoretically meet all of their own thermal and electrical needs. To be completely self-sufficient such communities would need batteries for electricity storage but since the system itself is 'storing' electricity the batteries can be relatively small (to be more precise the ground storage reduces the power demand rather than actually supplying electricity). In between those extremes the community could be designed to meet a "net zero" objective or could incorporate sufficient storage to handle emergency situations when the power grid is down. As PV collectors become more efficient these options will become more practical but note that solar collectors that collect both thermal and electrical energy are very efficient now with respect to their total energy collection and that by boosting the stored exergy the solar collectors are doing a dual duty, reducing the power demand and generating electricity.

Game changer

This type of system is a game changer in designing systems that can meet our needs for energy in the forms of both heat and of electricity. It produces no GHG's, can be expanded without limit since it uses an unlimited energy source, and it would reduce the costs of space heating and cooling, DHW and electricity. A very large part of Canada's electricity consumption is used for thermal applications so if we use air heat (and cold) for those applications then our existing electricity generation capacity, based primarily on hydro power, will be sufficient for decades to come, and the distributed power storage makes the power grid simpler, more efficient and much more able to match supply and demand. If in 50 years' time we need more generation capacity we can at that time choose the cleanest, most cost effective means of generating electricity. We do not need to make that decision now.

Numerous efforts are underway to redesign our energy supply systems. Most are extremely intrusive - flooding lands, covering it with wind turbines, rebuilding our national building stock to make it more energy efficient - and many intrude on our life styles, such as using bikes instead of cars, turning down thermostats, boxing us into high rise buildings, etc. In comparison the AE concept is almost totally unobtrusive, and it would also be cheaper and easier to implement.

Some examples of the intrusive approach are outlined in the links below (from Trottier Energy Futures and the David Suzuki Foundation):

Such projections need to be reconsidered.