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.

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?

> 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.

Ah, on second thought you're probably right.