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Enthalpy

Iceberg to Capetown

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Hello dear friends!

John Isaacs proposed it in the 40s for California. Peter Wadhams, Olav Orheim and Georges Mougin made studies for Saudi Arabia in the 70s and the Canarias and the UAE in 2010s. Together with Nick Sloane, they now have plans to tow icebergs from Antarctica to Australia or South Africa
bbc.com

Antarctica provides bigger, flatter and more sturdy sweetwater icebergs than the Arctic region do. Australia and South Africa are accessible by pushing the icebeg a bit North and letting cold oceanic currents do the rest
wikimedia.org gratefully pasted here
antarctica.gov.au

CorrientesOceanicasWiki.png.3f5dd047e1da1e49c64c5035d56f1f25.png

This makes the current plans less difficult than targeting the Canarias or the Persian Gulf.

These plans are far-fetched and nicely megalomaniac, so I perceived a golden opportunity to add my own frenzy. Not only for Australia or South Africa: comparable dry locations with cold currents linked with the circumpolar one are Namibia, the Chilean and Peruvian coast including the Atacama and Namib deserts.

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Critics reproached the 4 000 t fuel consumed to deliver 4 000 000 t freshwater to Canarias, or 1kg/t. But this consumption is litte!

Compare with a train. It picks 70t*50 freshwater from a source, and a 40% efficient 4MW engine pulls it in 10h over 500km to the users. It consumes 360GJ heat from 8t fuel to deliver 3500t, or 2.3kg/t: a bit worse, and it needs a nearby source connected by railway.

Compare with seawater desalination by the most efficient process, reverse osmosis. It needs about 60bar to work
wikipedia
and the engine and pump may be 30% efficient, while the brine's pressure can be recycled: 4 000 000 t freshwater need 80TJ heat from 1800t fuel, or 0.45kg/t, slightly better.

==================== Is the new route better?

Antarctica to Capetown is easier than Newfoundland to Canarias. The authors mention a >20 000 hp tanker and three (6000 hp?) tugs, say mean 20MW combined shaft power over 90 days. At 50% efficiency, that's 310TJ heat from 7 000 t fuel for 60 000 000 t freshwater now, or 0.12kg/t, which outperforms reverse osmosis.

==================== Are there better energy sources?

Fresh meltwater and seawater can produce energy as they mix. This osmotic power is worth some 120m water height.
wikipedia
So if a 90 000 000 t iceberg melts to 60 000 000 t, the maximum available energy is 35TJ. Too little to supply the tugs - and osmotic power has negligible efficiency at present prototypes.

Meltwater is 0°C cold and the surrounding Ocean maybe +8°C or even less. Make a thermal engine? At 3% Carnot-limited efficiency, 30 000 000 t water bring 58TJ work as an absolute maximum. Too little again - and existing oceanic gradient power plants are far from Carnot's limit.

Let's forget about solar energy on the Southern Ocean.

Wind energy sounds better in the roaring forties, furious fifties and screaming sixties. 30MW peak tug power can depend on the peak power of five 6MW turbines on the iceberg. Their D=154m rotors cumulate 9hm2, while a 700m*700m*178m iceberg (80 000 000 t at 917kg/m3 in 1013kg/m3 seawater) exposes 17m to the air, or 1.2hm2. In 25m/s wind, the 39MN drag bring the drift to 0.8m/s versus the Ocean, whose current is already faster
wikipedia
Looks feasible. As the turbines cost a few times more than the fuel they save in 90 days, they should serve for several icebergs.

Sails convert more directly wind speed to vehicle speed. Tugs were to achieve 0.5m/s. To produce 14NM to the North and 7MN to the East in 20m/s westwind, sails must total 6.4hm2. Masts 50m high, big sails and jibs 20m long bring 0.1hm2, so it take 64 of them. Not cheap.

Darrieus rotors can combine both. Sails are as big as the wind area they exploit, but turbines move faster than the wind so they can be slimmer than the swept width
wikipedia (picture gratefully pinched there)

DarrieusWindmill.Enwiki.jpg.90a98f8b1f050dce90801cc07b1128d2.jpg

and the Darrieus rotor can, if driven accordingly, produce a force strongly angled to the wind instead of power at the shaft. This rotor isn't so frequent but real-size demonstrators were operated in Canada.

Marc Schaefer, aka Enthalpy

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6 minutes ago, Enthalpy said:

Critics reproached the 4 000 t fuel consumed to deliver 4 000 000 t freshwater to Canarias, or 1kg/t. But this consumption is litte!

Compare with a train. It picks 70t*50 freshwater from a source, and a 40% efficient 4MW engine pulls it in 10h over 500km to the users. It consumes 360GJ heat from 8t fuel to deliver 3500t, or 2.3kg/t: a bit worse, and it needs a nearby source connected by railway.

Who delivers freshwater by train? Is that commonplace?

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Hi Swansont, thanks for your interest!

I don't know of usual examples of water delivery by train. I take it as an example of transport that saves energy.

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"better than a train" is irrelevant.

However "better than reverse osmosis" is interesting.

How much does "not wasting so much fresh water in the first place" cost?

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36 minutes ago, John Cuthber said:

How much does "not wasting so much fresh water in the first place" cost?

It's probably not as simple as that. Waste is at it's highest in places that have lots of water. You wouldn't be taking an iceberg there.

In places where you would take an iceberg, water is probably very scarce, and waste will be already minimised due to cost.

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7 hours ago, Enthalpy said:

Hi Swansont, thanks for your interest!

I don't know of usual examples of water delivery by train. I take it as an example of transport that saves energy.

You can't say it saves energy if it's referenced to a system that isn't used. That's fiction, not an engineering argument.

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Trying to reconcile the power, thrust and drag because the Bbc's paper reflects the innovators' natural optimism...

========== Thrust and power

Instead of the undefined "supertanker with over 20 000 horsepower" I took data from the well documented Maersk Triple E 210 000 t container ships
wikipedia
two 32MW engines and D=9.8m propellers push them to 25 knots.

ThrustPower.png.1b366ffb517e7ffae9b50d06cde21e22.png

An optimistic jet diameter and zero losses let deduce 14.2m/s downstream and 2*2.45MN thrust at v~12m/s before the propellers. If (?) the engines provide 20% more torque at slower speed, the thrust too increases to 5.9MN from 6.2m/s downstream and 2*9.1MW at the shafts.

Over 90 days at 60% efficiency, 2*9.1MW consume 5400t fuel, and the tugboats booze a bit too.

By the way, we can deduce Cd=0.072 for the Triple E, impressive.

Due to its smaller propellers, the tugboat Abeille Bourbon pulls only 2MN with 16MW peak shaft power
fr.wikipedia
three such tugboats could double the force, but above all they manoeuvre better than a tanker.

========== Drag

The tabular iceberg shall be 100+10.5m thin (possibly by melting), 1000m long and 790m wide. Broken edges might achieve Cd=0.8. In decent weather and with luck it could be pulled lengthwise, so at the lower 0.4m/s the drag is 5.1MN. 5.9MN+3*0.7MN with the tugboats achieve 0.5m/s. A 1.5km long iceberg, narrower or thinner, would attain 0.6m/s.

This speed doesn't look critical. 0.4m/s cover 3000km over 90 days, apparently more than is needed. Maybe the speed is necessary for few days only.

========== Updates to wind propulsion

21MW including the tugboats need four 6MW wind turbines.

8MN need fewer Darrieus rotors than 14MN.

3 hours ago, swansont said:

You can't say it saves energy if it's referenced to a system that isn't used. That's fiction, not an engineering argument.

Thanks for you opinion!

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The free jet downward a propeller makes a conus with small divergence. If the jet impinges on the iceberg, 790m wide and 100m deep, the thrust is annihilated. The ice may also melt faster, or the protection geotextile damaged.

Several towing boats could be linked with a transverse mooring and pull a bit to the sides. If they are 100m apart, >1300m forward, and pull 20° outwards, their jets flow past the iceberg's sides. The forward component drops as cos(20°)=0.94. The jets must cross at different depths.

Or the jets can be 5° downwards, as is often the case, and the boats >1200m forward, so the jets pass below the iceberg. Limits: the jets diverge, warmer surface water isn't desired under the iceberg, and surface water may buoy too strongly.

The boats could have deflectors added behind the propellers to direct the jet down or by halves to each side. Sidewise cos(25°)=0.91 and 850m forward suffice, or downwards cos(15°)=0.97 and 400m forward. Affordable modification. If the boat has several propellers with one rudder blade each, decoupling the blades adds fewer parts and keeps the manoeuvrability.

The simpler way is to push the iceberg instead. Upstream a propeller, the flow converges and accelerates steeply, so an obstacle doesn't annihilate the thrust.

========== Adapted propellers

As seen on the previous sketch, a bigger propeller with V~v takes less power for the same thrust. That's why subsonic airliners have wide double-flux turbofans and helicopters a huge main rotor. The limit is when downstream V is but bigger than upstream v and P~F*v. At 25 knots, the Maersk Triple E nears it, but at 1 knot it's inefficient. So can the propellers grow?

Few m/s downstream are far from the cavitation limit, so a special propeller's blades could be narrow like at aeroplanes rather than very wide and overlapping. This saves material but demands precise machining. The blades must be thick to resist bending and rather of steel. Whether existing boats can accommodate bigger propellers? Such adaptations look expensive.

Build a big marine wing or centreboard that makes the big force by lift, let a boat or propeller move this wing sidewise. Beginning at 10m depth, it can be about 10m/s fast and pull ~0.3bar, so 60m*6m create 11MN. If L/D=10 it consumes 11MW, saved one half. Can the wing push the iceberg, or only pull? Have a wing at each iceberg's side?

Add a big nozzle around the propellers. Without cavitation, a convergent increases the suction force. A divergent makes a back force but the convergent compensates it, and at the throat the propeller speed is better for it and the motor. No miracle, but the adaptation seems affordable. Combines with the jet deflectors.

Build (strong) marine parachutes, pull them from the sides of the iceberg. Boats then pull them forward again by their centre so they collapse and move easily. To achieve 2*5MN 80% of the time, two D=100m parachutes drift at 1.1m/s or 1.5m/s versus the iceberg so the winches consume mean 12MW together, saved one half.

========== Fuel cost

Some 7000t fuel for the tanker and tugboats unoptimized option cost 2M€ only. That's a small part of the operation. Saving on that line is more an environmental argument.

Marc Schaefer, aka Enthalpy

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   I'm sure Marc has already thought this through because he's pretty detail oriented, but it would seem that just converting the ice to water and pumping it into a tanker at the source and then pumping it out at the destination would be the most efficient process due to several factors. Towing the berg involves losing a rather large percentage due to melt, not only during transport but also at the destination when it must be processed quickly to prevent continual loss. Towing an ice berg is really no different then putting it in a tanker that has very large holes in its tanks allowing spillage of its cargo in route. And of course the tanker is most efficient in terms hydrodynamics. 

  Another problem is the bergs will periodically roll to adjust as the ice below surface melts quicker then the ice above the water line. Having tow lines pulled over or under the burg could be quite exciting for the crews involved and would likely lead to their disconnection by breakage or intervention that would then result in considerable delays for reattachment, and putting crews up close to reattach the lines would be almost criminally irresponsible if you could find anyone brave enough to get that close to an unstable massive block of ice of those proportions.

An interesting article; https://en.wikipedia.org/wiki/Ice_trade

Edited by arc
Wiki link

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Getting the ice to a suitable bay or port isn't the end of the story. How do you get the iceberg water from it's mooring to where it is to be used? Significant infrastructure at the port and pumps and pipes to suitable land based storage seems necessary.

I'm not convinced this is going to be a cost effective means of supplying water - or that supply of fresh water is the critical element to enable agriculture in places like arid Australia. Greenhouse based cropping, with solar power and thermal desalination of sea water appears to be working effectively in South Australia. Very efficient water use and solar powered climate control appears integral to the success of Sundrop Farms. I doubt the desalination is the biggest issue or expense. The same company has - or plans - more of these, in the USA and Portugal as well as more in Australia -

 

Sundrop.jpg

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Hi Arc, Ken Fabian and the others, thanks for your interest!

On 9/23/2018 at 6:03 PM, arc said:

[...] converting the ice to water and pumping it into a tanker at the source and then pumping it out at the destination would be the most efficient process [...]

The biggest supertankers weigh(ed) 1 million tons. The iceberg pushed from Antarctica to Australia or South Africa is to arrive with 60 million tons. I had wondered if the tanker was to collect just the molten sweetwater during the trip, but even for that it's too small.

A tanker would move 20* faster than 1knot, so trying a fair comparison: 60 million tons delivered need 200 trips by a 0.3 million tons tanker(s), or 10* more travel time. At 20 knots, the tanker consumes also more oil per hour than when pushing strongly at 1 knot.

An other difficulty: tankers are dirty. They carry crude oil and seawater. Sweetwater would rather need a new design, capable of disinfection, and with a nicely profiled hull - but then it costs 100Musd for 0.2 million tons. Maybe this will exist if we transport sweetwater regularly in some future, but for a demonstration it's too expensive. The team considers probably leasing an old tanker.

I made long comparisons about the fuel consumption, but fuel is rather cheap in this enterprise.

On 9/23/2018 at 6:03 PM, arc said:

[...] the bergs will periodically roll to adjust as the ice below surface melts quicker then the ice above the water line. Having tow lines pulled over or under the burg could be quite exciting for the crews involved [...]

Here the iceberg would be tabular, a shape usual in the Antarctica. I suppose they result from snow falling on the Ocean rather than from glaciers flowing to the sea. A 1km*1km*100m iceberg won't capsize. However, it may very well break in two big parts or lose some side fragments. I ignore what the team has foreseen. Pushing the berg means being close to it, which is pretty dangerous.

Moore lines with several MN tension are always damn dangerous too. Maybe some parts built weaker by design, for instance at the anchors or at the tugboats, make the operation less dangerous. And some moore line materials don't fly around when they break - at least the supplier claims so.

On 9/23/2018 at 6:03 PM, arc said:

[...] An interesting article; https://en.wikipedia.org/wiki/Ice_trade [...]

Very interesting indeed! I didn't know that ice trade had become international at some time.

This could be a commercially more viable option: sell ice for cold, not for sweetwater. In Australia they could save electricity wasted in air conditioners. Certainly more value than water, and one century back it made sense with less efficient methods.

Whether this trade pays for insulated boats? It needs investment to equip customers with ice-loaded air conditioners.

18 hours ago, Ken Fabian said:

[...] How do you get the iceberg water from it's mooring to where it is to be used? [...]

The team considers anchoring the iceberg off the coast. Remember it's about 1km*1km, and far too deep for a port. It would be "classically mined" (but how cleanly??) and the molten water brought by tankers somewhere - to the port or to the end of a water pipe, which can be some construction at sea. I'd rather imagine to extend the pipe to the iceberg.

The water must be fed in the existing distribution network, which has some storage capacity anyway, but may need to pump.

Caring inhabitants of a dry country may use 100L/day, so a 1M city uses 0.1M tons a day, which is less than the iceberg's melt rate. 800 days consumption may well exceed the existing storage capacity. Or should the geotextile envelope be closed, so the meltwater is stored at the iceberg and brought to the consumers as needed?

I've seen no details, and even less estimated costs, for this part of the operation.

18 hours ago, Ken Fabian said:

[...] I'm not convinced this is going to be a cost effective means of supplying water [...]

Neither am I. But I'm sure that problems are solved only by tackling them. The comparison with reverse osmosis wakened my interest.

When speaking of water price, we should distinguish the use. For drinking water we can pay 1€/L. For general house water it's few €/m3. Agriculture as I know it presently in Europe needs water much cheaper. In the case of Cape Town, house water was getting scarce recently, with a risk of supply breaks.

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The tide of history is against this idea. 

It's not yet being done. Desalination is currently a big and growing industry. 

Iceberg towing will get more expensive over time. Desalination will get cheaper as new technology gets developed.

Desalinated water is ready to use. An iceberg needs handling to get the water to where it's wanted. 

 

I like the idea of the saltwater greenhouse. I actually thought of it independently, and researched it, and found that someone was already doing it.

I have my own variation in my head, where you pump up salt water into greenhouses, and you grow mangroves in the greenhouses. Mangroves can grow in seawater, you can cultivate mangrove oysters among the roots, use the leaves as fodder, use the wood as fuel, and catch the condensation as fresh water.

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19 hours ago, Ken Fabian said:

[...] Greenhouse based cropping, with solar power and thermal desalination of sea water appears to be working effectively in South Australia. Very efficient water use and solar powered climate control appears integral to the success of Sundrop Farms. I doubt the desalination is the biggest issue or expense. [...]

If wasting water like European agriculture does, desalination is prohibitively expensive, even by reverse osmosis. Up to now, desalination can feed houses where water is too scarce, typically in some Spanish touristic cities.

Water-saving growing methods in greenhouses may afford desalinated water. But can it grow wheat, or even maize and soya? In all these projects I see tomatoes, more expensive per kg than cattle feed. This may make the difference.

I'm surprised that they use concentrated heat for desalination. Reverse osmosis is so much more energy-efficient that it pays for solar cells.

Not much information
https://en.wikipedia.org/wiki/Sundrop_Farms
http://www.sundropfarms.com/

2 minutes ago, mistermack said:

The tide of history is against this idea. 

It's not yet being done.

Well, history tells me that new technologies emerge.

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3 minutes ago, Enthalpy said:

Well, history tells me that new technologies emerge.

Yes, but I'm just pointing out how much of a start desalination has, on towing icebergs.

Also, desalination has a limitless supply of water. Icebergs are a finite resource, and removing them could possibly damage the local environment.

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3 minutes ago, mistermack said:

Iceberg towing will get more expensive over time. Desalination will get cheaper as new technology gets developed.

Why should moving icebergs get more expensive over time?

Technology gets cheaper as people make efforts for that.

Never tell "impossible for humans" and even less "because I don't see how".

A car for each family, a mobile phone that fits in your hand... all this was impossible before people made it possible. Even desalination was completely unthinkable on a commercial basis before people invented reverse osmosis.

2 minutes ago, mistermack said:

Icebergs are a finite resource

I wish all resources were as little finite as that one!

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4 minutes ago, Enthalpy said:

Why should moving icebergs get more expensive over time?

Because the main cost is fossil fuel. And there's not much that improved technology can do about that. As fossil fuels get scarcer, the cost will go up.

Edited by mistermack

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You meant fossil energies get scarcer and more expensive, I suppose?

I've already suggested using wind to move the icebergs. Renewable energies get cheaper over time, very much so. Presently they're cheaper than nuclear electricity, and for the iceberg they make sense over a few trips.

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The technology isn't there, to move icebergs using renewables. And even if you managed to get a giant iceberg to the coast of Australia, how do you get the water onshore, and where is the market for such a sudden glut of fresh water? There are an awful lost of unsolved problems before you get the water to a customer.

Desalination plants can be designed and sized to supply an existing market, and can supply water continuously, rather than intermittently.

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I've already described the use of Darrieus rotors. They have run for decades in Québec.

I've already suggested how to get the water onshore.

The market for water in Capetown is to avoid a shortage. 3 years supply with 4 months reaction time is a permanent solution.

Again, "there are problems" does not mean "it cannot work". I should like to remind that desalination was absolutely unrealistic two decades ago.

And that not conceiving or reading all the solutions now does not make a project unrealistic. All the items around you are full of problems for which you ignore the solutions.

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12 minutes ago, Enthalpy said:

I've already suggested how to get the water onshore

Mining, melting and pumping are all expensive processes, involving a lot of infrastructure. Mining would be full of problems, as you would end up wasting a lot of the berg, over time. And if you mined it inside out, it would weaken as it got smaller. And you would need to melt it, before you could pump it. 

And once you get it onshore, you have to store it, so you need more infrastructure. 

Maybe, you could enclose the berg in a gigantic plastic sack, so that as you tow it northwards, you don't lose water, and it gradually melts. So that by the time you got to the destination, all you need to do is pump the melted water out of the sack, as it is required.

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4 hours ago, Enthalpy said:

Hi Arc, Ken Fabian and the others, thanks for your interest!

Pleasure is mine, I love this kind of stuff. Much of my childhood was spent reading old Popular Science mags from the decades before I was born, they were full of this kind of grand, over the top "soon we will be (fill in blank) extravaganzas. 

     One Idea that popped into my head and then discarded; was to build an underwater pipeline, could the pressure on the outside counter the pressure needed on the inside?  How many relay pumps would be needed at depth? OK, too complex.  

     How about using geothermal heat at the source to generate the electric power for the processing and melting of the ice that would then be loaded into massive bladders that floated at almost neutral buoyancy.  They would be sent off with autonomous guidance systems operating electric propulsion systems powered from solar voltaic receptors integrated into the bladders surface. They would be monitored from satellite and make up for their smaller volume and slow progress by being very low cost to operate and one of many in a vast fleet that would regularly arrive and then act as the destination's storage facility, no need to pump it into tanks on shore. They would just need to hook up a hose pipe and draw directly into the distribution system.  The bladders would then be stacked on autonomous barges to be driven back to the source. 

Hey, you think I and my investors are going to let you have this whole market for yourself?!!!

 

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the bergs will periodically roll to adjust as the ice below surface melts quicker then the ice above the water line. Having tow lines pulled over or under the burg could be quite exciting for the crews involved and would likely lead to their disconnection by breakage or intervention that would then result in considerable delays for reattachment, and putting crews up close to reattach the lines would be almost criminally irresponsible if you could find anyone brave enough to get that close to an unstable massive block of ice of those proportions.

Kevin Mulisa Issaya

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6 hours ago, Enthalpy said:

Water-saving growing methods in greenhouses may afford desalinated water. But can it grow wheat, or even maize and soya?

 

I agree that Greenhouse agriculture doesn't suit all crops and crops like wheat are unlikely to be grown that way. But where wheat or other broad-scale crops are grown using irrigation - most wheat isn't - it is done because the availability of low cost, bulk water makes irrigation possible. I'd expect iceberg water to only be for high need, high value situations - municipal water, industrial processes or intensive, water efficient agriculture. Which competes with options like desalination.

The problems look significant. Much municipal water infrastructure will be unsuited to repurposing for extraction and storage of iceberg water. I would probably count on having to invest in significant, dedicated infrastructure. If I were really serious, I'd consider some kind of sea level pondage in the style of a ship's dry dock - float the iceberg in, close the water gates, pump out the sea water and let it melt in place.

 

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Ice has more value in air conditioners than molten at a tap. It might also supply +5°C to fridges, but uneasily -10°C to freezers. The former iceberg is too big for that use, and the main cost is to cross the last km to the customer, not the Southern Ocean.

de.wikipedia metric, en.wikipedia exotic

Every 1000kg=1.09m3 provides 334MJ cold at 0°C, and I neglect the cold liquid's 30MJ. To provide the same, a 350% efficient air conditioner would consume 26kWh costing 5.3€ at 0.2€/kWh. A 40% efficient generator would consume 5.4kg fuel to make the 26kWh - compare with 0.12kg fuel per ton to transport the iceberg from the South.

The previous iceberg arriving with 60 000 000 t would be worth 318M€ electricity at retail price and make routine transport very profitable, but it's slightly too big. A rich hot city with 1M inhabitants may have 300 000 buildings with air conditioners that provide each mean 2kWth for 8h a day, 150 days a year: the city consumes 2.6PJth/year. 7 800 000 t ice suffice and save 41M€ electricity. That's an 80m thick, 400m long and 270m wide berg yearly.

Bringing net cold to a hot city is more rasonable than dumping net heat to cool the buildings. Over 40km2 at mean 250W/m2, the city receives 130PJth from the Sun over the 150 days. 2K difference?

I suppose that a floating bucket wheel can exploit the iceberg anchored at the cost and coveyor belts bring the ice blocks to land storage. Unclear to me. This hardware can serve for several cities and years.

Storage would be on the ground, within 2m sand between geotextiles: 1000m long, 90m high, 190m base expose 0.45km2. <1W/m/K and 25K leak <5.6MW. Over 150 days, 220 000 t melt, or 3%. This would apply also if using the iceberg as sweetwater.

Every day, a house with air conditioners using mean 2kWth needs 58MJth from 173kg ice. Storage for a week means 1200kg that occupy <3m3 and are only 6.4€ worth of electricity. Carrying 14.4t in the city, a truck should make a rotation for <<77€, not easy. The storage must be accessible from the truck, not by foot over a lift. A D=1.5m H=1.7m tank insulated by 0.2m foam leaks 43W, that's 78kg lost in a week, or 7%. Bigger buildings don't ease much, since trucks have a limited size.

A heat exhanger at the tank and few air pipes and fans replace the expensive air conditioner.

Marc Schaefer, aka Enthalpy

==========

Hi everybody, nice to see that the topic catches the imagination!

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