by Jaro Franta (reblogged from a Facebook page)
Jacobson’s “100% WWS” scheme (“The Solutions Project”) relies heavily on Underground thermal energy storage (UTES).
The 2015 PNAS paper references a small UTES project in Canada, the Drake Landing Solar Community (DLSC) project (a master-planned neighbourhood of 52 homes in the Town of Okotoks, Alberta).
Jacobson scales it up to the continental US, with a UTES heat capacity of 715 GW and 515 TWh of storage.
The DLSC project demonstrated a UTES efficiency of about 50%, but Jacobson assumes 56%.
Aside from the optimistic efficiency assumption, there is the larger question of the overall impact of Jacobson’s scheme.
The whole point of “100% WWS” is to de-carbonise all energy sectors, in order to reduce GHG emissions and global warming.
GHGs absorb solar radiation, converting it to heat.
Different gases have different GHG potential, depending on how effectively they absorb different frequencies of solar radiation across the spectrum, and convert it to heat (infrared radiation).
If we compare large-scale use of UTES to the effects of GHG gases, it appears that UTES would fall at the high end of environmental warming effectiveness, well above that of carbon dioxide for example (CO2) – at least in the short term.
The solar heat collectors have a very low albedo – about 5% of incident light reflected – but their efficiency in heat trapping and transfer to UTES is only about 33%, meaning that about 67% of the collected heat is lost before it gets to storage (most of that heat loss is from the large surface area of the solar panels – by conduction, convection and infrared radiation).
Of the 33% transferred to UTES only half is returned – for a net 17% usable heat.
The rest – 83% -- is heat lost to the environment, as the losses from UTES eventually pass through the surface to the atmosphere.
Of course the 17% of usable heat also winds up in the environment, as the homes and other buildings lose heat through their imperfect insulation.
But it’s a small fraction of the trapped solar heat, compared to, say, heat from burning gas or oil to heat a house.
The big difference being of course that fossil fuels emit large quantities of GHGs in the process, which contribute to the global inventory, which has a very long half-life in the atmosphere.
Nevertheless, it seems that large-scale deployment of a technology that releases 83% of trapped energy into the environment without serving a useful purpose, would figure significantly on a global heat budget.
On top of that, include the large solar collector areas where trees cannot be planted, because they would shade the equipment – trees not available to absorb CO2 from the atmosphere.
Plus of course all the energy used in manufacturing the equipment and drilling millions of boreholes for large-scale UTES.
Compared to the 83% thermal inefficiency of the solar-UTES system, the ~65% inefficiency of existing Gen-III nuclear power plants doesn’t seem so bad !
And there is potential for considerable improvement in the future – to about 30% inefficiency, with high-temperature reactors running on combined Brayton + Rankine conversion cycles (or 50% with high-temperature Brayton alone).
Those figures are of course for thermodynamic conversion of heat to electricity – which would then be distributed to homes for electric heating & A/C.
If Small Modular Reactors (SMRs) could be built locally, and their heat transmitted directly via a network of pipes carrying hot antifreeze (like the solar heating units), then losses could be much lower still, because the system is “dense” in the sense that it does not require large areas of heat-losing panels like the solar-UTES system.
Something close 5% overall heat loss should be feasible (compared to 83% for solar-UTES).
So from this perspective at least, it seems that nuclear is a much better option than Jacobson’s “100% WWS” – even ignoring all the other insane requirements of that scheme, such as the two-trillion-dollar construction of HVDC transmission lines all over the US, to compensate for regional solar and wind energy variation.
Long-term energy storage for winter!
The challenge is to store summer heat for winter use: (BTES stands for Borehole Thermal Energy Storage -- in other words Jacobsonès UTES)
The BTES takes about 5 years just to build up enough ground heat capacity to heat the homes through winter. For cooling, you would need a separate system, in a different location.