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tahanson43206

Moderator

Registered: 2018-04-27

Posts: 18,912

Re: Permenance Movement

For kbd512 re #75

First, thank you for your clear presentation in this post....

Since Terraformer is talking about boreholes 100 meters deep or so, I decided to see how many boreholes that deep and 1 meter in diameter would be needed to reach the volume you specified.

A cylinder of the size given would be 78 cubic meters and change.

That number divided into the total you provided would be: 59,843,448 wells, which is ** way ** more than you computed, so I assume your wells are deeper.

Calliban had been suggesting the UK consider taping hot crust at 1 km deep, so I changed the depth to 1000 meters. That reduces the number of wells to:

(785 cubic meters per well) 5,946,228 wells, which is still a ** lot ** of wells.

On the other hand, going that deep insures access to hot crust in much of the UK, so the energy stored from renewable sources would be supplemented by natural heating from the core, with the caveats that Calliban keeps reminding us... The energy flow from the core is slow due to poor thermal conductivity in the absence of water in the crust.

Follow up question: 78 cubic meters of Earth crust might have some use on the surface.  ???

(th)

kbd512

Administrator

Registered: 2015-01-02

Posts: 7,771

Re: Permenance Movement

tahanson43206,

When you use natural temperature deltas for thermal energy storage, the system's volume is enormous out of necessity.  It has to be that way because the energy density is so low.  That is why energy storage materials must be incredibly abundant and not require any energy-intensive conversion process prior to using them.  Water, air, rock, sand, salt, and Iron or Aluminum oxides all immediately come to mind.  Everything else we have is a minor fraction of what we require to live as we presently do.

H2O (84.7bars / 1,246psi of pressure): 4,184J/kg⋅°C * (300°C - 20°C) = 1,171,520J/kg
Very hot highly pressurized H2O (300°C to 20°C):
434,000,000,000,000Wh / 325.42Wh/kg = 1,333,652,007,648kg or 1,333,652,008m^3 or 1.3337km^3

H2O (15.5bars / 228psi of pressure): 4,184J/kg⋅°C * (200°C - 20°C) = 753,120J/kg
Hotter moderately pressurized H2O (200°C to 20°C):
434,000,000,000,000Wh / 209.2Wh/kg = 2,074,569,789,675kg or 2,074,569,790m^3 or 2.0746km^3

H2O (1bar / 14.7psi of pressure: 4,184J/kg⋅°C * 80  * (95°C - 15°C) = 334,720J/kg
Hot to warm unpressurized H2O (95°C to 15°C):
434,000,000,000,000Wh / 92.98Wh/kg = 4,667,782,026,769kg or 4,667,782,027m^3 or 4.6678km^3

To supply 434TWh using diesel fuel:
434,000,000,000,000Wh / 5,350Wh/L (50% thermal efficiency, so half of 10,700Wh/L) = 81,121,495,327L
81,121,495,327L / 1,000L/m^3 = 81,121,495m^3 = 0.0811km^3

To supply 434TWh using U-235 (19,050kg/m^3) fuel:
Uranium 235: 3,900,000,000,000J/kg or 1,083,333,333Wh/kg
434,000,000,000,000Wh / 1,083,333,333Wh/kg = 400,615kg or 21.03m^3

Alternative thermal energy storage materials:
Al2O3: 880J/kg⋅°C * (800°C - 200°C) = 528,000J/kg
Fe2O3: 750J/kg⋅°C * (800°C - 200°C) = 450,000J/kg
"Solar Salt" / KNO3-NaNO3: 1,520J/kg⋅°C * (565°C - 290°C) = 418,000J/kg
"Pure Iron" / Fe: 450J/kg⋅°C * (800°C - 200°C) = 270,000J/kg

4X smaller is a meaningful difference, but requires a lot more technology to store the water as a liquid, namely boiler plate steel.  I'm merely talking about heating up the water to 300C, keeping it pressurized at 1,246psi in a steel or steel-lined concrete pressure vessel so it doesn't flash to steam, and then siphoning off "the heat" using sCO2, and then transferring or "dumping" that heat into a much greater volume of water in a secondary loop which provides district heating.  The input heat into the hot water tanks can be provided by resistive electrical heating elements powered by wind turbines, concentrated solar thermal, nuclear thermal, natural gas, or coal.

A typical commercial electric power reactor is pumping out about 3.75GWth if it's producing 1.25GWe, so we'd need 12 reactors to supply most of the input heat.  If all of its power was dumped into a district heating system, then only 13 thermal power only reactors are required to supply the heat required.  All told, all of UK's wind turbines produce 81.6TWh of electrical power.

UK consumed 357.2TWh of electrical power in 2005 (60.4M people), but only 266TWh in 2023 (68.7M people), because they installed a lot of wind turbines to provide a lot less power for a lot more money.  UK has 9 nuclear reactors spread across 5 locations, with a demonstrated need for about 6 more reactors.  All of the reactors could be engaged in direct heating applications, rather than electric power generation.  That makes them incredibly simple water boilers.  A very funny thing happens when you do this, though.  You no longer have any use for a ridiculously large energy storage system.  You only need pipelines, which are a lot more cost-effective and use a lot less material, but this requires uncommon sense.

That is the real reason we built modern society on hydrocarbon energy.  Without a highly concentrated energy source, we couldn't produce enough materials to use natural energy systems at the scale required until after we harnessed the power of hydrocarbon fuels to do it.  We've had mirrors, pipes, wheels, gears, pistons, and crankshafts for centuries, photovoltaics and batteries since the late 1880s, yet nobody built that first 1MW photovoltaic power plant until 1982.  The ancient Greeks had a steam engine for crying out loud.  All the basic tech was proven to work a century or multiple centuries before, yet manufacturing capabilities hadn't "caught up" to where science took us until 100 years and 1,000X+ greater energy abundance / pervasiveness later.  Maybe 100 years from now we will truly have a workable all-electric solution we can manufacture at the scale required, but in the mean time we still need energy to keep people alive, in order to reach that coveted future state.

The H2O at 200°C vs 95°C only reduces total volume to 44% of the zero pressure system that runs 95°C to 15°C, but it adds quite a bit of cost, complexity, and extreme operating conditions requiring certified boiler pressure vessels.  4 vs 2 cubic kilometers is not a deal breaker, never mind a game changer.  We're still talking about very large systems.  H2O at 300°C is quite a bit smaller, but also requires another level of storage tech, with highly robust boiler pressure vessels being mandatory.

If you're wondering why I didn't use even higher temperatures for the Iron, ask yourself what you're going to use to transfer power and how long it's expected to not completely oxidize and then crack from repeated extreme thermal cycling at temperatures above 800°C.  There are some materials which will work, but they're not cheap or abundant and require quite a lot of energy-intensive processing.  Are you going to keep something super-heated under a hard vacuum so Oxygen in the air doesn't interact with it?  How practical is that likely to be at the scale required?

This is why building energy systems which require either energy input or operating conditions extremes are mostly a waste of time and money, because they simply cannot scale up to the degree required to be truly useful at a human civilization scale.  That's why I've gone so hard after nuclear thermal and solar thermal.  Nothing else scales.

Water below its boiling point provides 80% of the heat energy storage capacity of much more energy intensive materials such as solar salt or various metal oxides would provide over a much greater temperature range.  That means the physical size of your energy storage system doesn't get meaningfully smaller until extreme temperatures or pressures are involved.  High pressures or temperatures are great for mobile applications where weight matters more than cost or complexity, or for generating electrical power, because the thermal efficiency of the turbo-generator goes up substantially, but it's ultimately a losing battle when you somehow have to source enough salt or other materials and process them into usable forms.