Permenance Movement

Calliban · 2024-10-01 05:22:52

The average cost of electricity in the UK in 2023 was £0.2126/kWh and gas was £0.0526/kWh. If burned in an 80% efficient boiler, the energy cost of gas would be £0.066/kWh.
https://assets.publishing.service.gov.u … e-2023.pdf

Assuming our town heat pump can achieve an overall COP of 12 using grid electricity (when pumping losses and heat losses are accounted for), it will produce heat at an energy cost of £0.0177/kWh, which is a little more than one quarter the cost of gas. The heat pump system will have considerable upfront capital cost, mostly associated with the distribution pipework. But it should provide the town with reduced heating costs in the long run.

In 2018, the UK consumed 394TWh of energy for residential space heating. This accounts for 41% of UK final energy consumption. Suppose we could meet 80% of that demand using town heat pumps with an overall COP of 12?

Q = 0.8 x 394 = 315.2TWh.
W = 315.2 / 12 = 26.3TWh.

That would add about 8% to UK annual electricity demand, but woukd reduce total energy demand by 30% overall. We would be replacing about 300TWh of natural gas with 26TWh of intermittent electricity. Which looks like a bargain.

Last edited by Calliban (2024-10-01 05:51:02)

Calliban · 2024-10-01 09:42:27

Nonadecane (C19H40) melts at 32°C and has density 786kg/m3 at room temperature.
https://en.m.wikipedia.org/wiki/Nonadecane

Long chain hydrocarbons like this have a huge number of isomers, so in reality a wax is a blend of thousands of similar molecular weight hydrocarbons that melt over a narrow temperature range.

Low-melting paraffin wax is a waste product of the oil refining process, as its cetane number is too high to be directly useful as fuel. It is usually cracked into shorter chain hydrocarbons that are more useful as middle distillate diesels. A direct use for this material would be a desirable market for the petrochemical industry. Paraffin wax has a heat of fusion of 200KJ/kg. We would need 5500m3 of this material to store 10MW-days of heat. That would cost about £5m in bulk, which is about £625/resident in our study. This is a significant capital expenditure, although this material once sealed into tanks, would not oxidise or wear out. It could be used over and over again for centuries. The use of paraffin phase change material allows the heat pump to cease operating when grid power is scarce and start when the grid is over supplied. Which is what is needed in a system where energy supply is intemittent.

Tanks of this material could be blended to store heat at different temperatures. This allows flexibility in the design of a district heating plant, as it would allow the pressure ratio across the compressor to vary. Reduced input power would result in reduced compressor rotational speed, and lower pressure and temperature rise across the compressor.

By integrating thermal energy storage at different temperatures into a heat pump system, we could in fact completely do away with the need for electrical inputs and couple the compressor directly to a mechanical drive shaft from a wind turbine. When wind speeds are low, the compressor would result in a temperature rise of only a few degrees. When wind speeds are high, a fluid brake would need to be applied to prevent overspeed of the compressor. Both would generate heat at the wrong temperature for our district heating system. This heat could be stored in phase change material and later input to supply the boiler with heat at temperatures greater than 15°C. This would allow the compressor to generate the same output temperature at a lower pressure ratio.

This is useful, because mechanical energy produced by a directly coupled wind turbine, will always be cheaper than electrical power drawn from the grid. But the mechanical power output of a single wind turbine will vary with wind speed, which means the pressure ratio and temperature rise over the compressor will also vary. To use the mechanical energy efficiently, we need a way of storing and recycling the heat produced at non-optimal pressure ratios. Blended organic phase change materials with different melting points, allow a variable pressure ratio to produce the desired output temperature, even if input power varies unpredictably. But it means more valves, tanks and a more complex system. A higher capital cost buys reduced long-term operating cost.

Last edited by Calliban (2024-10-01 09:47:23)

Calliban · 2024-10-03 08:08:27

My previous plan for district heating in Carnforth may stand for some adjustment. This image shows temperature at 1km depth in mainland UK.

Take from here: https://eurogeologists.eu/european-geol … d-kingdom/

For Carnforth, there is a 34°C temperature at 1km depth. Assuming a near surface ground temperature of 10°C, that implies a 24°C temperature rise per km, or a 2.4°C gradient per 100m. A well depth of 210m would give us an input temperature of 15°C to our heat pump and another 100m would give 17.4°C input temperature.

We only need to start recharging with sea water if we extract heat more rapidly than the local temperature gradient can recharge it. This does suggest to me that the project can be staged. Firstly, we build the heat pump. We expand the network throughout the town gradually over a period of years. We add more boreholes as the distribution network expands. The seawater recharge capability can be built some time into the project. This is useful from a development perspective. It means that a staged approach can be employed, allowing the project to earn money that will at least partially finance the next stage. So we don't need to heat the whole town from day 1. We could even build a smaller heatpump that is then sold to another project somewhere else and replaced with a bigger pump as demand grows. This suggests to me that the district heating idea should be developed at a number of difference sizes that fit local environments.

Last edited by Calliban (2024-10-03 08:22:36)

Terraformer · 2024-10-03 08:18:32

The capacity of a geothermal heating system can be significantly improved by circulating it year round. It should be straightforward to heat the rest of the borehole with the heat during the summer, when we don't have much need for it. Then during the winter we can draw on that stored heat.

tahanson43206 · 2024-10-03 09:22:10

For Terraformer re #54

Please develop your ideas further!

What you see as "straightforward" looks exceedingly expensive and complex to me.

Fortunately, there are members in the forum who can help with both sides of that question.

(th)

Calliban · 2024-10-03 10:01:06

Running the system in reverse won't work particularly well. Whatever insulation we put around distribution pipes to avoid heat losses in winter, would also hinder heat absorption during summer.

But we could potentially use the land itself a solar collector. The annual insolation for Carnforth is about 1000kWh/m2 per year, with about 80% of that delivered in the warmest 6 months of the year. Suppose we put our boreholes in an open piece of land. We run buried pipes about 1' under the soil. When the soil temperature gets to 1°C warmer than the borehole water temperature, a pump activates and water circulates from the borehole, through the pipe network and back again. We could harvest perhaps 500kWh of heat per m2 of land per year. To heat the whole town, we would need to harvest heat from a patch of land some 68,000 square metres or a square 260m aside.

That shouldn't be difficult. The whole built up area of the town is 2.33km2 or 34x greater than the collector area we would need. The land could still be used for grazing animals, but wouldn't be suitable for ploughing obviously. A carpark with a black asphalt surface would work well. The boreholes don't necessarily all have to be in the same place.

Last edited by Calliban (2024-10-03 10:05:13)

Terraformer · 2024-10-03 12:19:07

Oh I wasn't suggesting circulating it through the entire system. Just the borehole. During summer we can move heat from the bottom to (almost) the entire length. That will allow us to extract at a far better rate come winter - - we're using the borehole itself as a thermal battery to make use of it's capacity during the summer.

Re. Costs, I've seen borehole costs from £50-100 per metre to £1000. How deep can we drill before going above £100 per metre? What is the optimum depth to drill to based on cost? A 500m borehole would still give better performance than summer seawater.

tahanson43206 · 2024-10-03 12:38:58

For Terraformer re #57

Your hint about "moving heat" is tantalizing but so far I don't see the substance I'm looking for.

How would you "move heat" from the bottom of the borehole to anywhere else?

I suspect you have a very clear mental picture, but please ** try ** to recreate that vision in other minds.

Talliban offered what sounds like an option, to circulate fluid from the borehole through pipes laid out under ground in fields or parking lots.

(th)

kbd512 · 2024-10-03 15:05:25

Drilling cost is related to the fluid system used (water-based or oil-based), loss rate (controlled by adjusting fluid system properties, such as adding salt, so it doesn't seep out into the rock formation), low gravity solids removal efficiency (LGSRE - how efficiently rock chips / cuttings are removed from the fluid system so the drill bit is not dulled by grinding them up), and rate-of-penetration (RoP). If you have really good LGSRE, your RoP will be very high even with modest power input, so total drilling time goes down, and typically, losses also go down.

If you can swing a water-based mud (we call the drilling fluid system "mud", due to the fact that it looks like mud) because bore hole temps are not so high that they require oil-based mud (almost endlessly recyclable, but much more expensive and all of it has to be sucked out of the well and chemically reconditioned at a liquid mud plant / LMP), then your cost will be drastically lower. We call quick drilling ops with water-based mud "pump-and-dump" operations, because all the chemicals used are food grade products, so we simply dump them when we're done. Water and salt are not particularly toxic.

Water-based mud is mixed onsite, just before drilling. Oil-based mud requires a LMP to mix and recycle, plus more than twice as much transportation cost. Dry chemicals like salt are easy to transport on a flatbed rig. Liquid mud requires specialized tanker trucks. Most oil-based mud requires diesel fuel. Some is even more expensive and exotic synthetic oil for extreme temperature drilling operations. A lot of drillers in the Northeastern US use exotic mud, as well as offshore / deep sea drilling operations. If you're using that to drill onshore, and it's not for oil, then one might question what you're hoping to achieve, because you'd need a good answer to justify the costs.

Brines used to clean out the bore hole following drilling-out of a section, also greatly affect cost. Brine can absolutely explode drilling costs. These are typically at least mildly toxic and must be siphoned out of the well after you're done using them. The exotic and toxic brines would not normally be used for shallow depth onshore drilling.

Cementing / casing costs are very predictable and don't generally add greatly to the cost of a well.

If you used a synthetic oil-based mud, a bromide-based brine, centrifuges to remove rock cuttings, and an exotic high strength concrete, then expect costs to be closer to $1000 per meter drilled. If you used non-toxic water-based "pump-and-dump" mud mixed onsite using onsite water, good quality shakers and screens to remove the cuttings vs much more expensive centrifuges, a non-bromide brine for cleaning, and there were no great losses during drilling, then expect cost to be closer to $100 per meter drilled. About the lowest I saw was around $70 per meter before COVID, but again, all the material / site / resource selections matter so greatly to cost that nobody can simply "throw out" an accurate cost per meter. Some prep work is required to know what you're getting into and what you'll need. Onshore drilling jobs here in America are done and over with inside of 2 weeks or so. The rest of the operation might take another week or two. The onshore rigs move from site to site, drilling top sections, middle sections, and bottom sections. It's cheaper to drill out a dozen or more top sections at different co-located sites, then drill the middle, then the pay zone. The more consistent the operational cadence you establish, the lower the total / aggregate costs. Idle time costs a lot of money and delivers nothing.

I can tell you beyond all doubt that LGSRE and RoP has an outsized effect on the cost of the driilling materials themselves. If you know you're gonna use a lot, then optimize for this. Why use centrifuges if they cost so much? Imagine you're drilling many kilometers under the ocean and it takes an entire day to trip-in or trip-out to replace a very expensive drill bit you dulled. Spending even more money to ensure that doesn't happen is well worth it. For a shallow onshore well? Not so much. If you wish to reduce the cost of all your consumables to the minimum, then efficiently getting rid of rock cuttings so you can drill faster is a very good way to spend your money. Using decent shakers and screens is sufficient. If you're in and out in 2 weeks or less and your mud engineer is ensuring you have enough nut plug and other ingredients to minimize fluid losses, then again, you're going to reduce time and money spent. Beyond that, drilling deeper to get fractionally more efficiency out of the system is probably not a good trade, because all the cost is front-loaded and all the benefits pay out over decades.

Think about what parts of your drilling solution you want to optimize for. I would think that getting jobs done quickly with the lowest cost materials input, with no special handling requirements, would be high on my list of priorities if I need to drill many thousands of shallow onshore wells.

kbd512 · 2024-10-03 15:27:05

For virtually all non-trivial drilling jobs you'll need:
1. A fluid (water or oil) into which you mix the rest of your ingredients
2. A weighting agent (barite or iron oxide; barite is the most common, but iron oxide is sometimes used to reduce cost)
3. salt
4. viscosifiers and surfactants
5. some kind of plugging material to prevent fluid loss (ground up coconut husks or walnut shells)

The barite is ground into the consistency of fine flour at a grinding plant, and it does not "come out of solution" during drilling. I can't speak to how good a weighting agent "rust" happens to be. We very rarely used it. There was a technical reason for that, but I can't recall what I was told about it. I know that it does work, but barite is the preferred weighting agent. I remember that one of our rules was, "never ever run out of barite".

The salt is used to make the fluid's salinity so high that it doesn't interact with layers of salt that you'll inevitably drill through.

The viscosifiers change the fluid viscosity for pumping, because the fluid must be circulated at all times to remove cuttings / rock chips.

The surfactants prevent things from "clumping up" in the fluid.

The nut plug will plug tiny holes in porous rocks that would otherwise cause your fluid system to seep out of the bore hole during drilling operations.

I recall chemicals were also added to control pH, but off the top of my head I cannot remember what was commonly used. I know we added lime to the fluid as well.

I'm probably forgetting some other classes of chemicals commonly added, but you'll need that stuff at a bare minimum.

Calliban · 2024-10-03 16:16:12

Kbd512, many thanks for this useful information.

Most boreholes for water supply go down 60m, in the UK at least. I have seen cost estimates rangeing from £10,000 - £20,000 for boreholes of this approximate depth. I am guessing the lower end was for 4" boreholes. £10,000/60 = £167/m (~$200/m). Which isn't too far from your estimate of $100/m. If we are drilling multiple holes on a single site we can probably get cost per hole down a bit, as a lot of cost will be incurred transporting people and equipment and setting up. So for large scale operations, we can probably zero in on your $100/m figure.

The holes we need for district heating will be shallow by the standards of the oil industry. At most a depth of 500m, likely less than half that. Temperatures will be comfortably below the boiling point of water. So I cannot see the need for exotic drilling muds from what you have described.

Beneath 10-20m, ground temperature no longer varies with seasons. Let's assume a year round average top soil temperature of 10°C and a thermal conductivity of rock soil mixture of 2-3W/m.K. How much heat would conduct through a 10m layer of overburden if we store heat underground?

Q=K x dT/dX = (2-3) × 1/10 = 0.2 - 0.3W per K of temperature difference.

If we are storing heat at 20°C after a hot summer, each m2 of land will be losing 2-3W of heat. Over 3 months, that adds up to 4.3 - 6.5kWh/m2. How much heat is stored under 1m2 of land to a depth of say 100m?

Assumptions: (1) Rock and subsoil density average to 2500kg/m3; (2) Specific heat of rock is 0.8KJ/kg.K; (3) Storage temperature starts at 20°C and drops to 15°C by end of heating season.

Q = 100m x 2500kg/m3 x 0.8KJ/kg.K x 5K = 1GJ (278kWh)

If we store heat at a temperature of 20°C in a borehole 100m deep, we can expect to lose about 2% of the heat we store every 3 months. That is an easily acceptable loss, as in reality, the temperature of the borehole will be declining after the beginning of autumn as we start withdrawing heat.

How many boreholes would a Carnforth system need? I cannot say without heat transfer calculations. One thing I did discover, is thas borehole cost scales roughly linearly with diameter. Given that surface area also scales linearly with diameter, and heat transfer into the borehole is proportional to its surface area, the cost of thermal capacity is independent of borehole diameter. But wider boreholes do result in reduced pumping losses, which is an important cost driver as it effects overall COP. Also, the cross sectional area and water volume contained in a hole increase in proportion to diameter squared. So heat capacity increases with the square of borehole diameter. So wider boreholes are desirable.

If borehole cost increases linearly with depth, it makes sense to drill deeper holes. Borehole heat capacity increases linearly with depth, as surface area increases linearly with depth. If there were no geothermal gradient, the cost of thermal storage capacity would be independant of depth. But the presence of geothermal gradient makes deeper drilling desirable so long as drilling cost remains a linear function of depth. Overall COP is inversely proportional to the necessary temperature rise. A higher COP reduces operational costs, because less electrical energy is needed to pump each kWh of heat is COP is higher. For a drilling depth of 1km, COP becomes effectively infinite, because temperature reaches 34°C without need for the heat pump. The heat mining operation would still consume power for pumping due to fiction with the pipe walls. One problem is that the ground heat is not an infinite resource. This is really a type of mining. But using a relatively shallow borehole to store summer heat for use in winter, is fully renewable.

Last edited by Calliban (2024-10-03 16:24:45)

tahanson43206 · 2024-10-03 18:32:29

For Calliban re #61

Your post certainly seems to support Terraformer's proposition. What I'm not clear on is whether procedure to store summer heat is "straightfoward" as Terraformer predicted.

Terraformer could well be right.

What I'm hoping you can add to the topic is a few details about how heat would be stored in the pipe, and how it would be recovered.

So far, I've not heard/seen ** any ** mention of multiple pipes, as would seem to me mandatory for any heat storage scheme.

(th)

Calliban · 2024-10-04 06:43:01

I found this link: https://www.greenmatch.co.uk/ground-sou … p/borehole

A single borehole, 150mm in diameter and 50-150m deep, is sufficient to provide about 6kW of heat input to a ground source heat pump. It would have been better if they could have told us exactly what depth is needed to provide 6kW, but lets take an average of 100m. Drilling cost is about £10k per borehole, which is close to Kbd512s average of $100/m.

Peak demand in the Carnforth district heating system is 10MW. So we will need 1667x 0.15m x 100m boreholes. That will cost around £16.7million, or just over £2000 per resident. If the cost of drilling deeper is a linear function of depth, then we would be better drilling fewer but deeper boreholes and taking more advantage of natural geothermal heat.

Something that should have occured to me previously is that during summer, we could run the heat pump in reverse. Using summer heat absorbed in the upper layers of soil above the boreholes as the heat source to the heat pump and putting warm water back into the boreholes. The heat pump doesn't have to stop working in the summer.

Last edited by Calliban (2024-10-04 06:52:50)

Terraformer · 2024-10-04 07:20:15

Of course, the town is not uniform... a system for the central area (primarily terraced houses) will cost less per home than one that is designed for detached bungalows. So it doesn't have to be built out all at once -- and the fixed cost of moving in the equipment doesn't seem to be high enough to make the cost per borehole for 100 significantly higher than for 1000. It seems from the link that the machines used are good for 200m, so I think we can get at least to that without much increase in cost per metre? If we could get to 500m, we could be getting heat at 20c.

I think it's important for systems to be incrementally improvable. E.g. start with a basic borehole system, then add in a summer heat collection system, then add more boreholes as streets are added. This country is tightfisted with capital spending, and even if it wasn't, converting the whole country over is still going to take decades (though the older denser housing stock should be done within a decade if we really go for it). I think for Carnforth a starter system covering the terraced streets between the canal and the railway would be an easier sell, and create an energy redoubt for the town

tahanson43206 · 2024-10-04 07:53:26

For Terraformer re #64

Please consider developing your proposal in the forum .... you have a mix of support and skepticism which might match the Real Universe of residents.

Can you start with a single bore hole? Would a single bore hole prove itself?

I'm still hoping someone in the forum will explain how a "straightforward" thermal storage system would work.

(th)

Terraformer · 2024-10-04 08:02:54

A single borehole is already proven. Several boreholes are proven. The reason for multiple boreholes at once is that just getting the equipment in for one represents a major fraction of the cost; it's about amortising that cost over multiple. None of this stuff is new, including using the boreholes for storage, except maybe a butane based heat pump.

Thermal storage is simply reversing the flow of heat. Instead of circulating cold fluid during the winter to extract heat from the borehole, you circulate warm fluid during the summer to put heat in.

kbd512 · 2024-10-04 08:12:54

Calliban,

It makes sense to drill deeper until you start requiring more exotic drilling fluids such as oil-based mud, and then your cost shoots up. If you stay below the boiling point of water at your target depth, then the costs will typically be much lower, because everything required to run the drilling operation costs a lot less.

If I was a local resident, I wouldn't worry too much about the health effects of pump-and-dump mud. It might sound bad, but everything that goes into it is also something that goes into your processed food products. As someone who primarily worked for foodstuffs companies on their forecasting and supply chain management solutions, I recognized all the products we were forecasting for mixing water-based mud. Oil-based mud is something which requires far more care and attention. You don't want oil-based mud anywhere near your water supply.

My perspective on this topic is as someone who worked in supply chain management and forecasting of all the consumable materials used by one of the largest international oil drilling companies. I worked quite often with the equipment and downhole tools groups as well. I don't have much knowledge on cementing, although I've worked with engineers who did. I specifically worked on forecasting of per-meter-drilled projects. I worked extensively with a project engineer who taught mud school, which is where I learned how to perform the calculations project engineers and mud engineers used to estimate materials consumption rates. I took a look at all their spreadsheet tools and then we incorporated the best features into an online tool, as well as a commercial / enterprise level tool. I worked on a near-real-time tool to give high level executives (VPs and VP assistants) visibility to the entire supply and demand picture across the organization. I learned the portions of mud school relevant to using these in-house created software tools for forecasting. I taught forecasting concepts to project engineers. I was sent to supply warehouses / bases, liquid mud plants, and drilling rigs to "see how the sausage is made". The wireline and downhole tools groups have some very cool electronics toys to map the interior of the bore hole and the rock formation around it.

I don't claim to be any kind of expert, because I'm definitely not, but I probably know a little bit more than the average person you'll meet on the street.

Terraformer · 2024-10-04 08:38:17

Below the boiling point of water? Oh okay. Seems like drilling for hot water is a lot cheaper than drilling for electricity then. That's good. I don't think we're talking about much above 30-40c here.

tahanson43206 · 2024-10-04 09:48:11

For Terraformer re bore hole vis heat pump system

A traditional heat pump system employs coils of pipe through which fluid circulates to either deliver thermal energy to the environment, or pull it out.

Your focus on boreholes has omitted any mention of the technology you are planning to use to interact with the environment.

I suspect you have the entire system built and running in a comletely anmated simulation inside your head.

A few words you put into the posts (mostly to Calliban) probably work for Calliban, but I have not seen any references to your "straightforward" technology.

Are you planning to fill your borehole with water, and then drop a loop of pipe down the borehole to exchange thermal energy like a normal heat pump system?

(th)

Terraformer · 2024-10-04 10:56:33

th, if you follow the link Calliban posted it will be explained. Like I said this is well established technology, heat will be exchanged however heat is exchanged in those systems, though the particulars do not matter at this point anymore than you need to understand semiconductors to post here.

Calliban · 2024-10-04 11:58:49

In the meantime, the UK government pledge £22bn to this.
https://x.com/notfarleftatall/status/18 … 47162?s=46

So long as we have utter morons in charge, nothing we do here makes a fig of difference. No amount of planning, knowledge of physics or heat transfer calcs matters a damn when the people running the show are clowns. As Britain's leaders so obviously are. We can plan district heating systems, nuclear reactors, trips to Mars, etc, until we are blue in the face. But without the right people in charge, we are pouring our energy into vacuum. We are run by utter goons. The only thing worth discussing with these people is how we get rid of them. District heating might be a grsat idea, but it won't be a sell because these people are interested in ideology and not practical solutions.

Trying to plan district heating in Carnforth is like rearrangeing deckchairs on the Titanic. It is like trying to discuss economic policy with Chairman Mao. Absolutely nothing positive could happen in China until Mao was removed. Nothing positive can happen in Britain until Starmer and the rest of the left wing mentally ill squad are removed.

Last edited by Calliban (2024-10-04 12:07:59)

Terraformer · 2024-10-04 12:08:23

We replace the Conservatives with Labour and they turn out to be the same. Next up is Reform, who give all indication of being much the same.

Though local politics pretty much go to whoever shows up, so on that level you only need 20% of the populace to support your plans. Idk, maybe a group pushing viable solutions could make inroads there. Other than that, its energy redoubts I guess.

Calliban · 2024-10-04 12:27:30

Terraformer wrote:

We replace the Conservatives with Labour and they turn out to be the same. Next up is Reform, who give all indication of being much the same.
Though local politics pretty much go to whoever shows up, so on that level you only need 20% of the populace to support your plans. Idk, maybe a group pushing viable solutions could make inroads there. Other than that, its energy redoubts I guess.

You are right in what you say above. I should work harder to hide my frustration. But I'm in my mid 40s and have been watching the world around me get worse for as long as I've been old enough to notice. Every year things seem to get worse and the people in charge are doing it to us deliberately. The people who end up running the show seem to hate England and its people and are bent on undermining them. Somehow, they always manage to remain in charge. It is hard to get past that and focus on small changes that might make things marginally better for at least a few people, locally. I will get back on topic tomorrow.

tahanson43206 · 2024-10-04 14:01:03

For Terraformer re scattered data ....

You may well be right that everything that is in your mind is also scattered around the NewMars forum.

What I would like to see at some point is a solid proposal to invest some funds to make something happen.

This forum has been talking about exciting possibilities for 20+ years. SpaceNut is actually building a structure out of pallets. Can our members meet and possibly even exceed that? Can you find an investor who will take the long view, and fund your project for something smaller than an entire town?

Why discuss what various groups of people can or cannot do. Your project needs an investor who wants to build what you are selling.

No one else matters. Just you and the investor.

(th)

kbd512 · 2024-10-04 19:10:16

I still don't quite understand the benefits behind using paraffin wax as the thermal energy storage medium, given that the temperatures are above the freezing point and below the boiling point of water. Water has double the specific heat capacity of paraffin wax over the temperature range in question. Supercritical CO2 has a better specific heat capacity, as compared to paraffin wax, at the low temperatures being considered here, and it's much easier to pump than liquid wax.

The temperature delta across the hot and cold molten salt thermal energy storage tanks doesn't vary by more than 1°C, from "fully charged" to "fully discharged". The simplest and cheapest way to reduce pumping losses is to simply build a very large thermal energy store, and water helps us do that rather cost-effectively at the modes temperatures required.

Have a look:

Nothing else does the job of storing thermal energy any better than water at the temperatures being considered here. The water is in the ground in an enclosed / sealed tank that's being externally heated by both an external heat source (heat extracted from the surface water of a lake or ocean) and the Earth itself (more like regulating temperature). It won't freeze or boil. It cannot catch fire if some is spilled. It cannot adversely affect the drinking water supply if some is accidentally lost downhole due to a seismic event. The sCO2 does the thermal power transfer to another "warm water" loop for space heating. The water in the loop providing the district heating is separated from the thermal storage loop, so a problem outside the storage farm does not affect the storage farm itself.

sCO2 thermal energy transfer fluid won't freeze, boil, or catch fire, and ordinary CO2 is already used as a refrigerant. Water and CO2 are very cheap and easy to come by in enormous quantities. No highly refined hydrocarbon products are required here. The sCO2 stays within the confines of the storage farm and thermal energy collector loop, meaning it's performing a water-to-sCO2-to-water (if the heat source is a lake or ocean) thermal transfer. If the heat source is a much hotter solar thermal collector, then it's still a good choice and may not require any pumping power. We'll need some pressure-regulated or temperature-regulated check valves, but we may not even need active control over the storage farm.

I presume the end use for all this thermal energy is space heating and cooling. That takes care of the lion's share of home energy usage.

Now to inject some reality into the magnitude of the requirement...

Space Heating accounts for 434TWh of 760TWh of the entire heating energy consumption for the UK. We'll assume a 25C to 15C discharge during the winter. During summer, water temps in the wells builds to 25C, and then we discharge over the winter.

4,184J/kg°C * 10C * 2,000m^3 per well = 23,244,444Wh of thermal energy to transfer per well
41,840J * 2,000,000kg = 83,680,000,000J
3,600J = 1Wh
83,680,000,000J / 3,600J = 23,244,444Wh

434,000,000,000,000Wh / 23,244,444Wh = 18,671,128 wells

Maybe half of the energy is wasted during the act of providing it, but that's still a LOT of wells to drill, if they only hold 2,000m^3 per well.

66,970,000 residents of the UK / 18,671,128 wells = 3.58 residents served per well, assuming 100% efficiency

As of May 2024, Texas has drilled 693,909 oil and gas wells since 1993. We drill here like it's going out of style, but we haven't drilled anywhere near 18 million wells.

Solar thermal water heating systems in the UK can heat water to temperatures between 60°C and 80°C. In the summer, solar panels can provide most or all of a household's hot water needs. In the winter, solar panels can still provide 20–30% of a household's hot water needs.

4,184J/kg°C * 80C * 2,000m^3 per well = 185,955,555Wh per well
334,720J * 2,000,000kg = 669,440,000,000J
434,000,000,000,000Wh / 185,955,555Wh = 2,333,891 wells

That's still an awful lot of wells using solar thermal heat injection. What if we built ponds instead of drilling, or used tunnel boring machines?

We really need the storage volume. 2m in diameter is about as large as oil well top sections get. I'm not saying you can't go bigger, but then we're talking about stuff that's not very common, or possibly never used in oil well drilling.

We need to store 4,667,782,000m^3 of water with an 80C temperature delta (15C to 95C). If we had a reservoir 10m deep, then it needs to be 21,605m by 21,605m. That seems huge, because it is, but I think this would be broken up into more manageable construction projects spread across the entire UK.

UK's Top 20 largest cities include the following:
1. London
2. Birmingham
3. Glasgow
4. Liverpool
5. Bristol
6. Manchester
7. Sheffield
8. Leeds
9. Edinburgh
10. Leicester
11. Conventry
12. Bradford
13. Cardiff
14. Nottingham
15. Newcastle upon Tyne
16. Kingston upon Hull
17. Stoke-on-Trent
18. Southampton
19. Plymouth
20. Derby

It's going to be a very big construction job, no doubt about that, but after it's built it's essentially immortal. 1.12 cubic miles is quite a lot of water, so I would recommend using sea water. If you run out of that, you probably have bigger problems. I do think solar thermal input will be required. I don't think you're going to do this using seasonal temperature deltas alone.

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