In considering energy storage it is essential to consider both of the forms in which we use energy for buildings - electricity and thermal. Thermal energy includes heat for space heating, cold for cooling, and DHW. The two forms of energy are highly interactive - we use electricity for both cooling and heating, for example - so if we store either heat or cold we can reduce our consumption of electricity.
In considering electricity storage it is common practice to assume that stored electricity must be returned as electricity. It is often more practical to return the accumulated electricity in the form of demand reduction. Reducing the demands on the grid accomplishes the same end as returning electricity to the grid, with the added advantage that the costs and losses associated with power transmission are avoided.
The articles on storage that REW has been publishing on the subject of energy storage have mostly failed to take either of these basic factors into consideration.
The EPA uses a Global Warming Potential (GWP) of 21. The latest IPCC value for the GWP of methane is 72 (20 year). The implication is that natural gas is NOT better than coal in terms of its GHG emissions even if you use the EPA's questionable low estimates for fugitive methane. As the author observes, we should not be using either coal or natural gas. There are better alternatives.
In Alberta solar thermal collectors are more cost effective than solar PV systems. The reason stems from the need to supply a lot of heat in cities that have very cold winters. Solar heat plus heat extracted from the summer air and stored in the ground can meet that need for heat (plus the need for cooling and DHW). That could eliminate the dependence on fossil fuels for heating and it also reduces the consumption of electricity since about one third of the present power consumption is used for thermal applications. The potential for power demand reduction is considerably greater than the potential for generation via PV systems, and the collectors are smaller as well. See http://kanata-forum.ca/kegs.pdf
Gordon If you check the reference that I cited you will see that the concerns that you raised about exergy storage systems are not valid. The periphery of the heat store is always at the ambient ground temperature so no heat is lost in the radial directions, and almost none is lost from the ends. The efficiency of the recovery of heat delivered by the solar thermal collectors is almost 100%, and because of this high efficiency the total collector area can be relatively small (but remember that most of the heat is actually being extracted from the air). The storage of the solar heat is particularly effective in the winter because the heat is trapped so that it cannot flow away from the boreholes. In Alberta wind power would be used for the exergy pumps rather than the run-of-the-river hydro used in Eastern Ontario. Because the energy is stored intermittent sources will work well and there is no power demand at all during the peak grid power demand periods.
The Chaudiere Falls generator (1881) is a run-of-the-river generator that lacks the ability to store the river's energy when there is a low demand for power. The energy output from such facilities can be greatly increased if the surplus energy can be stored in an external facility. One way of doing that is to use the excess power to boost the exergy of a heat store that is extracting heat from a low temperature source. The power can be used to drive a heat pump that moves the heat into a smaller volume where it is concentrated and at a higher temperature, i.e. its exergy has been boosted. The heat can then be used for space heating without needing a heat pump for that stage. Such systems can also provide for cooling, again without needing a heat pump for the output.
In Canada the power systems have two large demand peaks, one in the winter created by the power used for heating and the other in the summer from the cooling demand. If stored heat and cold are used to meet those seasonal demands then the power demand peaks can be flattened and the need for using fossil fuels for heating and peak power generation can be eliminated.
The notion that you need to build a far flung transmission grid to handle fluctuating electricity sources like wind and solar power is fundamentally flawed. It is much easier and cheaper to employ storage instead, but to do that you need to jointly consider the needs for both electricity and heating/cooling. A single store can store both electricity (in the form of exergy) and heat and those two functions can be handled almost independently providing the power companies and the consumers (building owners) can agree on annual quotas and on sharing the costs. Such systems shift much of the grid load from daytime to nighttime and from the peak seasons to off peak periods so they do not need to convert the stored exergy back into electricity - they accomplish the same objective via peak demand reduction.
Dennis, Storage can do a lot more than just shift the peaks. It can handle both variations in supply and in demand. It can accumulate energy at a trickle rate but deliver it at a high rate to meet peak demands. It can accumulate energy in one form, such as electricity, and then deliver it in another form, such as heat, if that is what is needed. Some storage systems can handle very large amounts of energy for long periods of time, including between opposite seasons. The potential has hardly been scratched, but to begin to understand it you need to distinguish between storing energy, storing electricity, and storing exergy. (If you are not familiar with exergy storage do a Google search for "storing exergy in the ground").
Re. your Question 2, exergy storage systems can store very large amounts of both heat and electricity. That provides an affordable solution to the use of intermittent energy sources and perhaps more importantly it means that peak loads can be met from storage instead of boosting the generation capacity to meet those loads. A set of slides illustrating the principles and showing how they could be applied in the UK is available at http://kanata-forum.ca/decc1.pdf
There are two variants of this type of storage system: 1) Atmospheric Energy systems, which extract heat from the air and store the heat in the ground for later use (Mark is familiar with this variant), and 2) the Exergy Storage systems which use the same storage concept but add a heat pump to boost the exergy of the stored heat. That pump is controlled by the power grid operator so it uses electricity only when surplus power is available, and later returns that energy in thermal form along with a power demand reduction, so the system performs like a giant battery from the grid operator's point of view, and concurrently provides heating and cooling to the buildings.
It is much cheaper to store energy in the form of heat. An exergy storage (ExS) system uses a heat pump to move heat from one zone in the ground to another. That doesn't significantly change the total amount of energy in the two zones but it raises both the exergy of the pair and the temperature of the hot zone, making it capable of doing work. It also chills the cold zone, making it easy to replace the extracted heat using heat from the air. That air-heat can subsequently be pumped into the hot zone, building up the storage of energy, most of which is coming from the air, not the electricity that was used to drive the heat pump. Such systems have been designed to deliver outputs at 4 degrees C (for space cooling), 40 degrees (for space heating) and 60 degrees (for domestic hot water) without the need for using heat pumps at the output stage. The power demand reduction in the output operations is much larger than the MWh of electricity used to drive the exergy pump at the input. A variant of this concept has been used in Toronto for many years to provide cooling for that city's largest buildings.
In Ottawa the temperature can drop to as low as -40 degrees C. A home with normal insulation needs a considerable amount of heat storage (well over 10MWh) to handle that. My interest has been in systems that can be retrofitted to existing buildings so I haven't looked at variants intended for super-insulated buildings. For storing heat for very light loads like the one that you describe it is probably more practical to use water tanks rather than ground storage, which is what I described, but I have not looked into it.
For housing development projects, I would suggest that the least expensive option is likely to be to build homes that meet the Building Code for insulation and that use a single heat store for each city block of homes. If the homes need 12 kW of heat the borehole length works out to about 60 metres per home, which is pretty cost competitive. However, if the store is an ExS type of store that is capable of storing electricity (in the form of exergy) then the power companies should pay for it because it is much cheaper for them to employ storage to flatten the power loads than to generate power to meet the peak demands. With such a system the homeowner does not need a heat pump for heating or cooling, so if the capital cost of the ground store is covered by the power company the only cost is for the circulation system. The energy is mostly coming from the air, which effectively has an unlimited supply capacity.