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Comment by coffeebeqn | Hacker News Reader

I wonder why they used tap water instead of seawater

Seawater is a corrosion nightmare. Even tap water is going to soak up minerals from the rock eventually and be a problem, but it will still be less of a problem than seawater will from the very start.

The only advantage seawater usually has is it’s ’free’ if you’re on the coast. But in many habitable climates, freshwater is ‘free’ too. Like most of Finland.

Why they should use seawater? Fresh water is not in shortage in this region - yearly precipitation significantly exceeds evaporation.

Please, take a look at the map. Even cursory inspection of it will reveal to you that Finland is the land of lakes.

Finland has vast stores of fresh water and annual precipitation has nothing to do with it.

They have a massive desalination and pipeline operation to create lakes throughout the country?

Or is it the the sun and rain that created the lakes?

https://www.geographyrealm.com/finland-many-lakes/

Thanks to the last ice age and retreating glaciers...

Ice age and retreating glaciers created holes. And those holes filled up by the melt water of the glaciers.

The fact that they are still full of water is due to them being replenished by precipitation more than they lose water. If they would lose more water then what flows into them we would be speaking about the "land of many bone dry holes" in the famous Finnish desert.

To a first approximation, Finland is on average a swamp. Thousands and thousands of lakes everywhere.

The Finnish name for Finland, Suomi, literally means "Land of Swamps". And it is awesome here.

That is one proposed etymology of "Suomi".

It makes sense that Finns love rally so much then, if all the roads are weaving around swamps!

Slightly higher boiling point? Fewer antimicrobial additives required?

Seawater erodes everything it touches. It isn’t impossible to build hardware that touches it, but it’s more expensive and needs more maintenance.

Fresh water makes the entire project much cheaper.

(Edit: this is wrong – see comments)

And this water is supposed to be fed directly into the homes of hundreds of thousands of customers through the existing district heating network which obviously isn't built with seawater in mind. A seawater storage facility would at the very least require a heat exchanger which would decrease efficiency.

I don't think they'll be pumping that water directly into the heat distribution system. My understanding is that it'll be used as a heat reservoir and there will be some sort of heat exchanger to get the heat out of it.

It would be more efficient without the heat exchanger, but you’re almost certainly right - the reservoir water would probably be too oxygenated/mineralized for direct use if being stored in contact with the rock, at least without a lot of additional treatment.

Don’t want to kill all those underground pipes with mineral buildup as things precipitate out as it cools.

> It would be more efficient without the heat exchanger,

No, the only cost really is the cost of heat exchanger and maybe energy to pump water through the heat exchanger. Heat exchange approaches 100% thermal efficiency with large enough heat exchanger.

> It would be more efficient without the heat exchanger

Not really, heat pumps as a transfer method can achieve better total efficiency and make distribution of the heat easier.

That is an interesting thought, but seems like the capex and ongoing electrical costs of the heat pump would be too high?

They’re looking to make cheap use of the existing waste heat, after all, which while ‘low quality’ is high enough quality for district heating already.

Counterflow heat exchangers easily approach 100% efficiency. It needs to be large enough, and needs sensors to control the flow, but both of those are trivial at this scale.

They’ll need to move 2x the volume of water, and have more infrastructure to maintain - even if it was actually 100% thermally efficient. Not counting capital costs. Which I’d count as efficiency issues.

The water pumping and maintenance costs are likely going to be their largest ongoing costs, no?

Ah, on second thought you're probably right.

There is a heat exchanger.

https://www.slideshare.net/HelsinginEnergia/innovation-and-s... (slide 21)

I don't think it is this water that is going to be fed directly to homes. More likely there is going to be systems of heat exchangers.

They mention they are recovering heat from waste water. Obviously, they aren't pumping waste water into the system but rather use heat exchange to extract energy.

Sibling comments are on to this too, but I think an important detail here is that the water in lakes, river and ground in the nordics is so clean that the extra cleaning needed to make it "tap water" isn't really as big of a deal as it might be in other parts of the world.

The little town I live in (about 40 km south of Oslo) is supplied from a lake in the granite hills. They filter out the bugs and precipitate the solids with a little aluminium sulphate and that's it. It's above all the agriculture so no chlorine is needed and there is no chalk so no problems with deposits on the insides of kettles and water heaters.

Does it taste good?

My favorite so far has been Vienna - I would describe it as similar to Evian.

It has been crowned best water in the country. Yes it tastes good, I'm quite happy to put it in my Laphroiag.

Finland has a small number of lakes that are basically dead from pollution from pulp & paper plants. The rest are quite clean. Helsinki's tap water comes from Päijänne, the large lake that basically runs from Lahti northward to Jyväskylä.

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