you can't compare heat storage to electricity, because you can't directly use heat for anything else other than for heating, where as electricity can be used to perform motion.

If you used heat storage as a battery, there's an additional loss when converting to electricity.

However, if the heat is cheap/free during summer, storing it for winter is a no brainer.

To add to that, what is the energy expenditure of building the battery compared to sand containment plus heat exchanger and turbine - i.e. mining, refining, transport, manufacture, delivery?

I'd guess 20%

The report says that the sand will be heated up to 1200 Celsius degrees.

This is much higher than the maximum temperature for steam turbines and equal to the temperature of the gas in the best gas turbines.

Therefore they will use a combined cycle, first a gas turbine will use hot air passed through the sand and the exhaust from the gas turbine will produce steam for a chain of steam turbines with decreasing working temperatures.

There should have been no problem in reaching a 65% efficiency for the conversion from heat to electricity, except that between hot sand and a gas burner there is the same difference as between an electric capacitor and a battery, while heat is extracted from the sand, it cools down.

Presumably, when the sand becomes too cold, the gas turbine is bypassed and the hot air just produces steam. When it becomes even colder, I suppose that the first steam turbine is also bypassed and only the low-temperature steam turbines are used.

This will lower the average efficiency, probably to around 50%. If the residual heat (after the steam turbines) had been used for heating or for cooling (i.e. heat-powered air conditioning), the efficiency could have been higher, e.g. over 80% in the beginning, while the sand is still very hot.

I have seen another proposal where instead of sand the material would be graphite, which can be heated to more than 2000 °C. Graphite also has a huge heat conductivity. But it's more expensive.

Plenty of times+places where you only need heating at night and still have some net electrical draw (because the sun isn’t shining at night).

Also, cold fronts move in with a lot of wind, but then it can stay cold a few days with calm winds until a warm-front moves in.

Maybe today. But NASA has some interesting metal tires [0][1] which might change your mind for the future.

[0]: NASA info https://technology.nasa.gov/patent/LEW-TOPS-99

[1]: Neat youtube vid: https://www.youtube.com/watch?v=vSNtifE0Z2Q

You can use heat to generate steam that can spin turbines to perform work. But, I am not sure if it’s practical to generate steam from a sand battery, my background is in electrical construction. I’m guessing the sand battery isn’t nearly as hot as a natural gas steam boiler’s combustion chamber.

https://www.johnsoncontrols.com/en_sg/hvac-equipment/chiller...

I believe it largely depends on the application. If you have a lot of waste heat, it's potentially a way to get "free" refrigeration. (e.g., a paper plant that uses a lot of steam can use absorption chillers to make use of waste heat.) If fuel is much cheaper than electricity, it can be economically viable. Peak shaving can save lots of money. etc. But it's probably not competitive purely in terms of energy efficiency or GHG emissions.

Typical refrigeration COP (Coefficients of Performance):

Absorption refrigerator: 0.6-1.2

Compressor refrigerator: 1.5-4.0

Estimated TES economic advantage: 1.1-2.5x

Conclusion: yes, absorption refrigeration is probably inefficient enough to make it a long shot in this application. The only way I can see it becoming viable is if extremely hot TES can completely change the efficiency game, and then only just.

"Off the shelf" isn't a constraint here, sort of the opposite: I'm trying to imagine the space of things that are physically possible but not yet commercially mature. Theoretically, higher temperatures mean more possible efficiency. That's the one ray of hope in otherwise dismal efficiency figures that are currently only viable, as you point out, if the heat is ~free.

I think you might be conflating a few things here. For a cycle to produce electricity, that's correct. But the mechanism of absorption chillers is fundamentally different. The chemistry of the materials and their phase change temperatures are definitely a constraining factor. Could you, in theory, develop some other absorbent/refrigerant that works at those higher temperatures? I suppose, but I would suspect there are much easier ways to get efficiency gains.

There's Stirling Engines. If the solar collection is pure thermal, and if that collection and the storage can be made dirt cheap, then the 37% or so efficiency of conversion to electricity stops being a problem. But what are currently problems with Stirling Engines -- Hardly any of the industrial optimization has been applied to them Re: Wright's Law. So they are quite costly! Heat pipe solar thermal could be made dirt cheap through economies of scale, and it works very well, even in climates like England's.

I could envision house construction changing to include sub-basements which are just polystyrene insulated boxes filled with sand. By over-provisioning storage by 4X, houses in cold climates could have huge electrical power stores, especially in summer. (Especially if the house uses heat exchangers which can draw directly from the thermal store.)

There are air conditioning systems that are powered by heat, not by electricity.

There are places where the power plants use the residual heat from the generation of electricity not only for heating during the winter, but also for cooling during the summer, by producing chilled water.

How does that work?

Especially since you can use heat to drive a heat pump, which ends up actually achieving above unity efficiency effectively since the heat input is used to move additional heat from the environment. Even if it's inefficient, if the heat would otherwise have been wasted, it's a net benefit.

Did you mean to type "problem"?

It strikes me that heat has the problem that it always loses energy in its “stable state” because the surrounding environment absorbs the heat, and gravity doesn’t have this problem.

Heat sinks meanwhile can be built wherever you have a big rock by drilling some holes. Additionally, gravity definitely does lose stored energy in its "stable state", through evaporation and water entering the water table. Losses depend on geology and local climate, but it's not negligible.

Not to say that pumped hydro is a bad technology, it's just got it's own challenges and uses. It's most applicable in the form of electrical grid storage. But specifically on the scale of keeping towns and cities warm, heatsinks outperform almost across the board.

Look at https://www.whitepinepumpedstorage.com/ and the sizes of the upper and lower reservoirs there. This is to be a 8 GWh, 1 GW facility.

Either way, the details page[1] supports all of my above points. It even comments on the page how rare it is to find a suitable site like the one they've chosen.

[1]https://www.whitepinepumpedstorage.com/project-details

That is big for time-shifting daily energy usage, but way too small for seasonal energy storage

Either way, it just goes to show further that pumped hydro would be more efficient, when and where it is feasible.

Compressed air storage is another one that's pretty good, but it's only particularly good if you can store it in underground caverns or unused mines, so it's also geography dependent.

For longer term storage, producing hydrogen can be a good one.

And batteries are actually fast becoming competitive with some of these options from the other end, generally better for shorter term storage but getting better at longer term. Even shorter term, flywheels can be a good option.

There's room for several different types of grid-scale energy storage, based on how efficient they are at different energy storage periods and numbers of charge-discharge cycles, and also in some cases on local conditions, like the availability of terrain and water sources for pumped hydro.

There's a good paper here which shows what the most efficient energy storage systems are for various combinations of length of storage (hours per discharge) against number of discharges per year, and for each one shows the current cost, and a predicted cost based on trends in technological advancements: https://www.sciencedirect.com/science/article/pii/S254243511...

There's a lot of the chart that is dominated by pumped hydro currently, but plenty of other storage technologies that are more cost effective on different timescales and numbers of discharges. But it looks like it's predicted for prices of battery storage and hydrogen storage to fall relative to the others, causing a different predicted landscape in 20 years.

And some of these, like pumped hydro, are dependent on geographic features, or access to certain resources, so even when one dominates overall, there can be others that dominate in particular geographical regions.

https://en.m.wikipedia.org/wiki/Biot_number#:~:text=The%20Bi....

Did you mean to write the reverse? i.e the ratio of the surface area to volume decreases the larger you make a container.

Or maybe just putting a gigantic Fresnel lens in the desert and pointing it at the ground. I almost wish someone would try this just to see what would happen.

And no, it will not help too much if you use a heat pump before storing the energy in this facility. You still have huge conversion losses, the volume for low-temperature heat storage will be huge. If you want high temperature storage your heat pumps will need more stages and run less efficient.