Comment by owenversteeg | Hacker News Reader

Comment by owenversteeg | Hacker News Reader

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Something I forgot to mention - because of the low temperatures involved, district heating people love to talk about the potential use of low-grade heat, but the overwhelming majority of district heating is sourced from high-grade heat. By far the largest installation of district heating is across the former USSR, where the source is mostly nuclear. The USSR used nuclear not for want of industrial processes or black paint, but because the engineering involved is far more practical when you start with high grade heat. From an old report on Soviet district heating: at -23C, buildings were supplied with water around 80C and returned it around 57C. That may sound like it is within the ballpark of low-grade heat, but it required a plant sendout temperature in excess of 150C to get that 80C building supply. And, of course, your heat source has to be higher than your plant sendout temp... and now you're at fairly high temperatures. At that point, you might as well build a nuclear combined heat and power plant for maximum efficiency, which is in fact what the Soviets did.

If you want to do practical district heating, you should look to the Soviets. Unfortunately, most of this information is available in Russian, on physical paper.

[0] http://waterworkshistory.us/DH/1967USSRreport.pdf page 18, figure 2

Another country with highly developed district heating system is Denmark. They heavily invested in this technology since the 1970s, and currently around 2/3 of all households have district heating, in the capital, Copenhagen, 98%. Moreover, more than 60% of the energy used for these systems comes from renewables. No nuclear power involved (which hast been banned in the country since 1985). A few sources: - https://dbdh.org/all-about-district-energy/district-heating-... - https://stateofgreen.com/en/news/new-plans-to-expand-the-dan... - https://www.sciencedirect.com/science/article/pii/S136403212...

It is true that Denmark has a large district heating system - their national system is nearly as big as that of, say, St. Petersburg. But I think the Danes should be honest with themselves about the reality. The heat comes from burning stuff (wood/biomass, trash, and fossil fuels), the overwhelming majority of biomass is imported - and about 40% of those imports are from non-EU countries. If Denmark burns a tree, that is considered to be zero emissions, because - they argue - those emissions have already been accounted for in a change in _Danish_ land use and forestry. If you are deforesting a foreign country to warm your house, that’s a) not accounted for in your national emissions statistics and b) often not accounted for in the foreign country’s emissions either. So you get heat for free. Of course, if you forget to replant the trees, or they fail to grow back, then the emissions are worse than coal, because coal is underground, trees are not, and thus the surface albedo changes are significant.

The steel-man case for Denmark is that trees usually do grow back, and indeed, global _tree_ cover is substantially higher than it was 35 years ago. But global _forest_ cover is still shrinking, and much of the wood entering Denmark comes from forests. Forest cover is more important than tree cover - for the aforementioned reason of albedo changes, but also biodiversity.

Where things really get absurd is when you begin to calculate the area of land you have to turn into a tree farm to keep your homes warm. Or when you calculate the market effects of buying a huge amount of trees - even if all _your_ trees were good ethical trees from a farm, you have just consumed a large amount of the supply and thus financially incentivized cutting down old-growth forests elsewhere - for example look at the history of Ikea’s wood use. If there were more Denmarks, and it was not just a small country with the population of the Chicago area, there would be massive worldwide devastation.

Don’t get me wrong, I am not an absolute opponent of biomass. It is true that burning things is a great source of high-grade heat. It can play a small role in a sustainable energy mix. But it is not the free lunch Danes think it is, and certainly not at the scale they consume it.

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