Thermal Energy Transport - Low Grade - Long Distances

tahanson43206 · 2023-02-17 08:37:45

For SpaceNut ... there was not a pre-existing topic containing these three words.

This topic is offered for those who might wish to have a consolidated repository for posts about a variety of thermal energy transportation concepts.

Over recent times, members have contributed posts about interesting thermal energy concepts that may well turn out to be worth considering for Mars (as well as the Earth).

The purpose of the topic is to provide a repository for insights, links and text about how to move low grade thermal energy over long distances.

An example is geothermal energy. There is a substantial supply of thermal energy in the Earth's mantle, well below the surface.

The traditional way to obtain access to this energy is by use of a liquid (such as water) to absorb thermal energy from the environment, and then deliver it to the surface for productive use.

While use of a liquid for thermal energy transport is well established, I am hoping this topic will provide a place for posts about alternative transport methods.

To my knowledge (necessarily limited) there are NO thermal energy transport alternatives to water.

This topic will (hopefully) provide a gathering point for insights/knowledge/links on how to move low grade thermal energy from sources (such as power plants and the Earth itself) to locations where it can be used which may be many (30+) kilometers away.

(th)

Calliban · 2023-02-17 19:39:35

Water is the most practical heat transfer fluid for low grade heat over any significant distance. It certainly isn't the only heat transfer fluid that is used or could be used in all situations. Water is cheap, dense, non-toxic and has high volumetric heat capacity.

Long distance transportation of low grade heat makes more sense with increasing scale and use of district heating systems. If water can be injected into the ground at 30°C, then that temperature is already hot enough for some space heating applications. Scale is important, because larger diameter pipes will lose less heat per unit volume and frictional energy losses will be lower, allowing higher flow speeds. A 2m diameter pipe, carrying water at 30°C, with flow speed 10m/s, will convey some 2.6GW of thermal power, relative tona background temperature of 10°C. That is the waste heat production of a large nuclear power reactor. Enough heat for about 1 million homes in the UK at least.

Void · 2023-02-18 12:04:50

I hope this will be tolerated, I would like members input on it: http://newmars.com/forums/viewtopic.php … 24#p206224

Basically trying to use a sewer system to store and distribute heat. Low grade, but maybe a bit better than that as well.

Done.

kbd512 · 2023-02-18 13:59:07

Calliban,

If all you needed was a blower motor in your home to circulate the heat, how much of a drop in electrical power demand using waste heat from the nuclear reactors?

There are about 25 million dwellings / domiciles in England, so does that mean operating 25 ~1GWe nuclear reactors to supply the heat?

England has 9 operating commercial power reactors, so you'd need another 18 or so to supply that much low-grade heat?

Do existing nuclear generating stations have the land area required to accommodate the additional reactors, or would new sites have to be built?

RobertDyck recently brought up Helion's new fusion breeder reactors in the Large Ship power / propulsion thread, which perform D-D fusion, and could be used to breed Tritium and Helium-3 for electric power fusion reactors. Could those D-D fusion reactors supply the waste heat while they breed enough Tritium and He3 for economical electric power generation?

If the only thing the D-D reactors have to supply is hot water, and input electrical power to achieve fusion is provided by wind turbines, does that produce enough gain from fusion, in terms of direct thermal output, to make the project worthwhile, and could it be less expensive than building 18 brand new fission reactors?

Helion is claiming that 25t of D2 fuel could supply home energy requirements for 25,000,000 homes for an entire year. There are 34.4g of D2 per cubic meter of sea water, so that means processing a bare minimum of 726,144m^3 of sea water, so 0.001km^3 per year to supply the energy requirements for 25 million homes. There are at least another 142 million homes in the US. If you figure on 250 million homes between the UK, US, and Australia, then we need to process 0.01km^3 of sea water per year. The US alone consumes 4,000km^3 of fresh water per year, so this should be achievable (we're already moving and processing orders of magnitude more water than what's required). Globally, ~31.755km^3 per year are desalinated. We may as well collect some D2 from that while we're at it, but to my knowledge this requires electrolysis or centrifuging. China's electrolysis process is only about 15% efficient, so water that starts off with 145ppm of D2O is depleted down to 120ppm. This seems to imply that wee need to process 0.0484km^3 per year to power 250 million homes.

tahanson43206 · 2023-03-13 05:44:05

Rather than start a new topic, I'll just drop off a note here that I am pursuing the feasibility of using a combination of thermoelectric devices and liquified nitrogen (or similar gas) to harvest thermal energy at the bottom of a 4 kilometer deep well, and then to harvest more energy from the nitrogen gas delivered to the surface due to thermal energy collected by the gas at the bottom of the well.

I am working up a concept for geothermal well energy harvest using solid state devices. The proposal is to deliver Liquid Nitrogen to the bottom of a geothermal well for use as a heat sink for TEG devices. I note that one of your devices has a lower range of -50 Celsius. LN2 is -196 Celsius. The high temperature end is given as 100 Celsius as a planning temperature in a geothermal well. Actual temperature may be higher or lower. The question I have is whether any of your devices can operate for extended periods in the proposed environment, and if they can, what performance might be expected?

Above was submitted to the contact form of a company located in Texas.

Desired output from the system is electricity, and all components below ground level are intended to be solid state.

Non-solid-state subsystems can be installed on the surface, as needed.

Ideally, the entire system would be solid state, but at this point, I am doubtful that is possible.

To summarize in the context of this topic, the intended system would deliver electricity and warm nitrogen gas to the surface.

Thermal energy of the warm nitrogen gas would be harvested at the surface in the most practical way.

Some of the electricity delivered by the harvest system at the hot end of the well would be used to power the topside subsystem.

Because the supply of thermal energy from the Earth's core is reasonably reliable, the system envisioned should last for a reasonable time, with only maintenance required at the surface.

(th)

Calliban · 2023-03-13 07:06:15

TH, why not use a water loop to extract heat from the geothermal well? Most of these wells are filled with ground water anyway. Hot dry rock techniques take advantage of natural or man made fractures in the bed rock. You have a feed well and a withdrawal well and you establish a circuit between them. You can raise steam directly from the hot brine that comes to surface. Or you pump the brine through a steam generator. The second adds a capital cost, but it avoids blasting the turbine and contaminating the condenser with fine salt particles.

If you have liquid nitrogen, then you can use condenser heat to produce a twin cycle plant. The condenser sits under the LP turbine. You pump water out of the condenser at 30°C and use it to heat the evaporator. A bonus of doing this is that you don't have to pay for cooling towers or water treatment facilities (if your cooling water otherwise comes from a river). Your condenser water also operates on a closed loop. That is a big bonus because corrosion is a serious life limiting problem in steam plant condensers. If you can operate a closed loop of deoxygenated water between the condenser and the evaporator, then condenser corrosion can be almost eliminated.

Last edited by Calliban (2023-03-13 07:09:44)

tahanson43206 · 2023-03-13 08:24:50

For Calliban re #6

Thank you for your addition to the topic! Water appears to be the default energy transport mechanism so far. It is good to be reminded of it's practicality in many situations. On Mars, the use of water as an energy transport mechanism may be less attractive, but I agree that ** all ** materials should be evaluated to achieve the optimum solution wherever the plant is situated.

My main goal right now is to establish whether a solid state solution is even possible. The record and practicality of a water based solution is well established, and I would expect any site where water is abundant to use it.

However, there are locations on the Earth where water is not readily available, but thermal energy from the core is ever present, so a solid state solution might prove more attractive to investors.

In the case of SpaceNut's situation in New Hampshire, water is available, so that might prove optimum, so long as the challenges of working with water can be tolerated.

One idea that comes to mind, for the solid state solution, is that water might be deployed to the bottom of the well, to enhance the flow of thermal energy from the surrounding rock. In that case, the material chosen for the exterior of the well needs to be able to withstand the powerful corrosive capability of water.

In any given site, I would expect that some combination of materials and techniques will prove superior to all others, so flexibility in thinking by designers would seem appropriate. The time frame for the investment may be a factor as well. A cheap solution that lasts long enough to pay back the investment and deliver a small profit may be perfect for some situations.

(th)

Calliban · 2023-03-13 09:00:15

Water is overwhelmingly attractive as a heat transfer fluid over a temperature range of 0-323°C. There are lots of reasons why. It is cheap and abundant. It has excellent specific heat capacity. It is a liquid across this temperature range, with varying degrees of pressurisation. It's corrosion properties are not excellent, but are at least well understood. Corrosion is a down side in using water as heat transfer fluid. Another problem with water is higher viscosity compared to some other fluids like liquid sodium. Thermal conductivity of water is poor as well. Fast breeder reactors were able to achieve a really insanely high core power density, thanks to the high thermal conductivity and low viscosity of liquid sodium. This is how fast reactor designs with cores the size of an SUV, could have powered entire cities.

I think a solid state geothermal powerplant is a dubious proposition. Yes, a thermoelectric device could be used to generate electricity (with relatively poor efficiency). We are talking 5-10% efficiency. But you still need a coolant of some kind to remove heat from its cold side. Could you pour liquid air down the bore holes to do this? Yes, I suppose it could work. But your borehole must then convey a pressurised gas rather than saturated water. And the pressures involved may end being much greater than those involved in a comparable steam based system. You also need to consider issues of thermal shock.

tahanson43206 · 2023-03-13 12:10:14

For Calliban re #8

It is good to have your advice and counsel in thinking about the most effective (productive for investment) geothermal well.

Water is (as you point out) wonderful if you have a supply at hand.

However, a significant percentage of the Earth's surface is desert. Some of that desert is adjacent to ocean water, but I deduce that most desert on Earth is NOT near water.

The people who live in desert lands are candidates for a solid state geothermal well, if they can be made to work.

Your figure of 5% efficiency for (high quality) Seebeck devices is interesting but totally irrelevant. The electric output of the device is ALL that matters.

The supply of thermal energy from the Earth's core is effectively unlimited. 5% of Infinite is Infinite.

Where I could definitely use some help, from a qualified, working engineer who understands heat flows in Real Universe machinery, is at the ** top ** of the well.

At the top of the well, we are going to receive warm nitrogen gas that has been heated by the thermal environment all the way up and down the pipe.

The LN2 is going to pull 5% of the thermal flow from the core through the solid state devices, resulting in a flow of electrons to the surface. The 95% of the heat flow absorbed by the LN2 from the thermal environment up and down the well is going to deliver nitrogen gas to the surface. I agree that pressure is involved, but happily human engineers have learned how to deal with gas under pressure.

I'd like to take advantage of the heat content delivered to the surface, but am depending upon highly qualified, working engineering talent to do so.

Some fraction of the thermal energy harvested from the heated gas delivered to the surface must be invested in making LN2, so we can enjoy a continuous cycle of operation. However, whatever process is used to liquefy nitrogen gas will liberate thermal energy, which itself can be harvested.

The electrical productivity of the solid state devices will depend upon how many there are (a) and (b) the quantity of LN2 delivered to their cold sockets.

It seems to me at this point, subject to correction (which I welcome) we are talking about pulling 5% of the available thermal energy from the core directly as electric current, and we are talking about delivering 95% of whatever energy is harvested to the top of the well as warm gas under pressure.

The only place where efficiency becomes an issue is at the surface, where we are dependent upon traditional machinery to collect as much delivered thermal energy as we can. What cannot be collected to make energy can feed into low grade thermal energy applications, such as home heating.

In short, a well designed system (if it can be realized) would deliver electric energy and low grade heat continuously.

(th)

kbd512 · 2023-03-13 16:13:27

I still don't understand why we need a deep bore hole to do this. This bore hole has to be 4km to 4.5km deep in most places in the US, in order to achieve 100C+, outside of some place like Yellowstone. The hydrostatic pressure at the bottom of a 4km well is about 388 atm. It's gonna take a LOT of pumping power to circulate water under that kind of pressure in a closed-loop system. If I had to guess, that's why TH wants to use Nitrogen. Nitrogen is inert, it'll be far easier to circulate, and the corrosion problem is nonexistent so long as the Nitrogen is dry. N2 can embrittle certain kinds of metal at elevated temperatures, but I don't think it'll mess anything up at these temperatures. I would say you use the TEG on the outlet from the power turbine, using the air as your cold sink for the TEG, to add to the overall plant output. Maybe you can gain another 5% to 10% efficiency using some of the newer TEG materials.

tahanson43206 · 2023-03-13 18:11:16

For kbd512 re #10

Your figure of 388 atmosphere 4 km down looks high to me ... there are mines in Africa that go nearly that far down...

3,581 meters
"The deepest mine is a gold mine in South Africa; in 1977 the Western Deep Levels reached a depth of 3,581 meters. Lotsberg, Gunnar. Mines. The Worlds Longest Tunnel Page, 2003. "It has the world's deepest mine, 3585 m below surface at the East Rand mine."

Could it be you are figuring water pressure at that depth? I am thinking of air pressure at that depth.

Here's a documentary on a visit to the bottom of the Gold Mine ...
https://www.cbsnews.com/news/south-afri … 020-02-09/

The article shows folks working at that depth, and there was no mention of special breathing equipment, or of having to pause in the elevator on the way up to deal with the bends.

Update: I asked ChatGPT for an estimate of the air pressure at the bottom of the deepest gold mine (in South Africa) and it came up with:

Converting this to PSI using the same conversion factor as before (1 hPa = 0.0145038 PSI), we get:
1120.4 hPa * 0.0145038 PSI/hPa = 16.27 PSI

However, I don't think the equation shown is correct, but it appears to be identical to one that gave a result for air above the surface of Earth. If there is a forum member who knows how to compute the air pressure at 4 kilometers down, please confirm or falsify ChatGPT's estimate of 16.27 PSI.

(th)

kbd512 · 2023-03-13 19:30:06

tahanson43206,

Yes, I am figuring on water pressure at 4,000m. When you dig a hole that deep, you're probably going to encounter some water along the way, believe it or not. You know there's a difference between a well and a mine, right? There's a very specific way in which that gold mine in South Africa was excavated to prevent it from collapsing.

4,000m = 13,123ft

13,123ft / 33.8995ft per atmosphere (roughly) = 387.1 + (1 atm of air on top) = 388.1 atm

I forgot that this was a closed loop system, so 387.1 atm.

I was off by 1 atm. Happy now?

We're putting something in that hole of equal pressure, or the Earth will fill it in for you. I can guarantee that. I'm going to assume that the instructor who taught mud school knew what he was talking about.

I'm trying to imagine how else we're going to keep this pipe from imploding, or how thick it has to be to hold back that much pressure. That's like half way to the bottom of the Mariana's Trench, far deeper than any ordinary submarine can dive.

tahanson43206 · 2023-03-13 20:02:19

For kbd512 re water invasion of geothermal wells....

The main benefit of your contribution in recent posts is to remind me and any possible readers that water is likely to show up in wells dug in the surface of the Earth. SpaceNut (as just one example) reported that drillers found water about 150 feed down on his property.

The air pressure in the deepest gold mine in South Africa is estimated to be 16.7 PSI, compared to sea level at 14.7 PSI.

The gold mine in South Africa is (pretty obviously) NOT invaded by water. I presume that there is water in the environment, but removal of that would would be a priority for a gold mine, just as it would have been in a coal mine.

Thanks for your persistence in making the point that water is likely to be present in many well sites, if not most.

I tried a Google search to find out how many oil or gas wells are having to deal with water, but didn't find much that was helpful.

I did find a PhD Thesis on dealing with water that invades an oil field after drilling is complete, but I got the impression drilling at ** those ** sites goes through non-water-bearing layers to reach oil in domes under rock.

There ** is ** a distinct advantage if there is water at the deepest part of the geothermal well, because water is (presumably) able to convey thermal energy through the rocky region better than rock. A problem (I gather) with geothermal wells is that they eventually run out of readily available thermal energy, so the well operator has to dig deeper. That might not be as much of a problem if the well is immersed in water.
(th)

kbd512 · 2023-03-13 20:20:03

tahanson43206,

I can only reference people who drill oil, gas, water, and geothermal wells for a living. I have no PhD thesis to draw upon. The Earth is like a layer cake of different materials- sand, rock, salt, water, oil, and various mixtures thereof. There are probably certain places where you can drill through an almost homogeneous material comprising very hard rock, but relying upon that happenstance is not a good strategy.

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Thermal Energy Transport - Low Grade - Long Distances

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