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Comment by hammock | Hacker News Reader

Bittersweet. Because the reason all those dams are able to be sold is because the textile industry and paper industry that used to rely on them has all moved to Asia and Canada, decades ago, and now these mill towns are depressed drug dens

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We also shouldn't really be removing hydropower at this point in time from a net zero/global warming perspective. It is orders of magnitude less environmentally harmful than generating electricity with fossil fuels, and one of two carbon-neutral sources of significant amounts of baseload power at scale along with fission nuclear.

absolutely agree. hydropower is god-tier technology.

Most of those small and medium hydropower plants generate near insignificant amounts of electricity, even if you damn all the non major rivers you can (and destroy all the surrounding ecosystems) it would be hardly noticeable.

My napkin math disagrees. Small/medium hydro (<100 MW) appears to account for ~1.7% of annual US grid energy output. If we replace that with fossil fuel generation and account for the cost of dam decommissioning, account for estimated benefit of ecosystem restoration in terms of quantified dollars willing to pay by benefiting citizen, and net annual CO2 cost I come up with an annual cost to society of removal of small/medium hydro dams between $1.7B/year on the low and $41.7B/year on the high end. Happy to show detailed assumptions and calculations if desired. But tl;dr fossil fuel generation is very, very bad and replacing even comparatively small amounts of carbon-neutral generation with it results in a significant societal net negative.

a ~5HW hydropower is insignificant and can be replaced by a handful of wind turbines and solar panels. Not sure if it's worth sacrificing local biodiversity for that.

I did some quick napkin math on the net economic benefit, including carbon pollution and local economic benefit, of 3 scenarios:

1. Keep dam: -$2M to -$10M

2. Dam removal with 5 MW capacity replaced/absorbed by natural gas plants (no solar/wind/BESS): -$28.3M to -$249.8M, depending on whether you use the EPA's $190/tCO2 cost of damage or $1000/tCO2 DAC remediation.

3. Dam removal with 5 MW solar/wind/BESS replacement: -$3.5 to -$25M

In other words, removing the dam without equivalent solar/wind/BESS immediately in place is a net negative for society (and even then, keeping the dam might be neutral or still better).

> net negative for society

You did not calculate the impact on the salmon population since dams were added to the Kennebec since 1837. Both the impact on the salmon themselves as well as all of the other animals and people who relied on that salmon fishery.

It's certainly a lot larger than the benefits of the dams in my calculation.

What do you calculate it as?

I would flip the question on its head and ask why the response to uncertainty is to default to an assumption of zero. Environmental benefits are notoriously hard to calculate and it's unfair to weaponize that difficulty as if it confirms the default assumption should be zero. I also don't think the calculation needs to hinge on one-for-one timed tradeoffs in the form of "this windmill exists to replace power from that dam" or assume that backfill is 100% fossil fueled, given that solar is now arguably the most rapidly deployed form of new generation.

It's not that the cost never matters but I think if the cost is really just $25MM for a transformatively beneficial environmental impact that has equivalent no alternative, in exchange for scaling back a fungible resource, is a no-brainer.

In reply to https://news.ycombinator.com/item?id=50146584:

Ok, I found a study estimating a net willingness to pay per citizen for this type of ecological restoration at ~$72.31/per person/year. If we assume 100,000 people derive this level of benefit over a 30-year time horizon (same as I used for the previous numbers) that yields $21,693,000 which brings the removal WITH immediate solar/wind/BESS replacement scenario on par or marginally better than keeping the dam as is.

But even then, I believe that is being overly generous to the dam removal because electricity demand has been rising steeply recently and is projected to continue doing so. Until we are meeting 100% of current and future demand with new renewable energy, every existing carbon-neutral source that gets removed is creating a gap in future demand that will be partially filled by new fossil fuel generation and therefore incurring many of the same associated negative societal costs.

These dams are in the north east, those industries moved to the south long before moving to Asia. My grandmother worked in one of those mills that moved south during WWII. Many moved even deeper south after legally enforced segregation ended.

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