The rate of solar insolation in the UK is less than 120 W/sq. m and the country is legendary for its long gaps between sunny periods. That does not imply that solar energy is useless in the UK but it does suggest that its role should be niche applications that take advantage of the potential to store solar thermal energy.
Neil: I assume that you have dropped a few zeroes in citing 4 kWh/year as your output. A solar thermal collector (with storage) for a house would be only one quarter the size of a PV collector and in spite of that small size the storage system would deliver about five times as much energy. Most of that energy would be extracted from the air, but air-source systems have difficulty delivering the temperature that is needed for DHW and in handling peak demands. The solar input fixes both problems. Homes that use such systems burn no fuels at all and they reduce the peak demands for electricity. A given reduction in peak demand is more useful than the equivalent amount of power generation because it does not have to be transmitted.
Solar thermal collectors are five times more efficient than solar PV collectors and heat storage is about 1000 times cheaper than batteries. Used together they can shift the power demand from daytime to the night, and in the north from winter to summer (see exergy storage). The reduction in peak power demands is just as useful to a power grid as the generation of the same amount of electricity, especially as it also reduces the need for power distribution.
Stored heat does not have to be converted back into electricity. The primary energy needs for most buildings are thermal, not electric (for heating, cooling and hot water). An exergy store can provide heat at 60 degrees C (for hot water), 40 degrees (a viable temperature for space heating) and 4 degrees for cooling, all without needing any power at the time of extraction. The resulting reduction in power demand during the peak demand periods is more useful than the equivalent amount of power generation. Exergy storage systems both meet the needs for thermal energy and also shift the time of their power demand from the peak demand periods to times when there is ample power. From the point of view of the grid operator they act like giant batteries.
At the present time there are 40 million square feet of building space in Toronto that are cooled by cold that is extracted from the winter air, stored at 4 degrees C, and is then used to air condition the buildings without needing heat pumps. That means they are not taking up space, they are not adding to the costs, and they are not using power.
Many millions of dollars are currently being spent on supply-side storage, such as batteries. It would be about 1000 times cheaper to use consumer-side storage for that task and consumer-side storage like exergy stores can also deliver most of the energy the buildings need. That has a down side - the power generators and distributors expect the building owners to pay for the stores, which would leave the building owners with all of the expense while the generators reap most of the economic benefits.
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.