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Comment by someuser2345 | Hacker News Reader

Most electricity is generated by using steam to turn turbines, right? So it definitely seems more efficient to just use the steam directly, instead of converting it to electrical energy and then converting it back to heat.

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That reasoning isn't enough on its own, because on the clients' end, electric heat pumps can be more efficient than direct steam heating. Actually their "efficiency" is far above 100%,

https://en.wikipedia.org/wiki/Coefficient_of_performance

“Fifth generation” district heating uses heat pumps to extract heat from a network of lukewarm water.

Yep and IIRC you can even dump waste heat back to the loop at what would normally be just consumers - eq. to cool a hockey stadium or for a big building AC system.

And their efficiency plummets starting at 40F and below.

The efficiency of modern cold-climate heat pumps plummets to 200-300% at 40F and perhaps 175% at 5F.

So at -20 and below they are basically very, very expensive resistive heaters. Good for medium climates, bad further north.

Also either need very expensive oversizing or backup heat sources to compensate reduced heat output, even if you get occasional cold spikes.

Unfortunately nothing in life is as simple as it first seems.

Here is a great Technology Connections video which lays out why the sizing issue might not be as bad as you're thinking. The gist is that many people are steered away from heat pumps (or sold very expensive oversized units) by outdated industry thinking which vastly overestimates load calculations.

I won't try to make his case here, but he is pretty convincing that a reasonably sized heat pump system can be sufficient even in very cold climates (by US standards).

https://youtu.be/DTsQjiPlksA

It's all theoretical. In real life, retrofitting heat pumps into exisinting homes is a bit different, as those homes are neither made for air heat sources (at least in my region), nor they are insullated well. You might say, dough, insulate it, but it is easier said than done, as a lot of those old homes relied on their leakage for ventilation, not to mention that is very expensive investment (much more expensive than the heat pump itself).

I would reccommend keeping backup heat source when installing heat pumps into older homes. A simple wood stove, used during cold spikes, could be enough.

My country has subsidies for heat pump installations into older homes and it had requirement to get rid of chimneys to get that. After the whole europe energy crisis thing, this requirement is gone now. Turns out having alternatives is a good thing :)

This is not theoretical: my heat pump purchased in 2021 worked well for four straight days of below 0F. Kept our temp at 68 inside. We live in an old home from 1917 with poor insulation between the brick exterior walls and drywall.

I live in Canada in an older home with terrible insulation.

Out two tonne heat pump works just fine even at -25C this past winter.

We still have a natural gas furnace but our gas usage is down 65% despite this winter being colder than last year’s.

Tell me again how heat pumps don’t work in colder climates?

This is why I opposed historical designations. Most old buildings are obsolete and need to be destroyed. There are only a tiny number of buildings that are really historic, sure save them, but the vast majority are just old and need to be knocked down for something better. If it isn't taught in history class then it isn't historic.

It is easy to see the loss of something old. However the opportunity cost from not building new is something that is very hard to get people to understand.

You have to go quite far north for -20F to become frequent.

If it EVER gets to -20 that is what matters. You need to prepare for the worst case weather, not the common ones.

It will still work, just not as efficently as in normal temperatures. So you're using 50kWh for the 3 days a year it's that cold and 10kWh for the other 362 - better than using 20kW every day.

Only if there is enough power generation

And a good 24,000 BTU rated heat pump will produce just under 20,000 BTU at -13 F, about 18,000 BTU at -22 F (-30 C).

Fortunately resistive heating is just about the cheapest possible heating in capital costs and trivial to retrofit as a backup.

How often does it get to -20f in New York City?

Ahh damn, I forgot unit, I meant -20C, so -4F

But 5f is still high.

This is actually terrible and supports their point. Heat pumps are not sized as if they were running at a mere 100% or less, they are sized for the btus they normally produce, and aim to run close to 100% of the time, meaning they will be too small and not be able to do the job when they drop below 200-300% efficiency, unless they are grossly oversized. That leads to short cycles which makes condensation and moisture in the summer and wear and tear all year and less electrical efficiency because starting a pump and blower costs more than running it.

Current inverter types that can run slow do allow them to be oversized but it's still not great. Those are more expensive and have more failure points in electronics too.

1990s called, they want their efficiency back ;)

Sure, you can still find good weather heat pump that crap their pants near or under freezing. However, the majority sold here in Europe still stay well of 200% down to -15C, some even -25C.

75% of rated capacity at -22 F (-30 C), over here.

I believe in the NYC case, the steam is also not right off the reactor— it's waste steam that has already been through a turbine and so is still hot and under some amount of pressure, but is no longer economically valuable in a power plant context. So it's run into the city as an alternative to just venting it into the atmosphere.

Except you can ONLY use steam for heating and transporting it over distance is not efficient. If you turn steam into electricity you can use it for ANYTHING and transport it over long distances efficiently.

It also means that at the local scale it might be better to distribute steam. Think about future residential nuclear plants. It might not be worth investing into steam distribution on top of electrical as electrical is more flexible, but there might be steam that's residue otherwise that could be used on heating.

> future residential nuclear plants

What does this mean? Are you imagining a future where individual houses have their own nuclear plants? Or where nuclear plant workers live on-site?

I'm get the argument that we should have more nuclear power, and that some future reactors may be small, but I don't understand the expectation that they would be close enough to residences to pipe steam.

There has been a couple small scale nuclear power projects for a while now,

- https://newatlas.com/energy/oklo-aurora-nuclear-microreactor... - https://www.popularmechanics.com/science/a33896110/tiny-nucl... - https://www.nuscalepower.com/products/nuscale-power-module

They target from home to residential scale. I hope they are eventually a thing.

In many places in the world people don't live on ranches or in 1000 m² houses, but in 80-150 m² twin houses or even flats.

In such tighter conditions, you can easily have enough inhabitants per km² that it would justify building small mobile nuclear reactors.

Who would like to live next to a reactor if it can melt down? Well, you have reactors using fuel pellets locked in marbles that due to their diameter can never meltdown.

Steam in NYC is also used for cooling https://en.wikipedia.org/wiki/New_York_City_steam_system

There's no single efficienty number for "steam". Steam turbines in large power plants for example are >90% efficient when considering just the thermodynamics. There are a hundred other variables involved.

I'll note that powerplant steam turbines are often pretty cutting edge & supported by essentially a special plant to achieve this levels of efficiency and reliability. Stuff like hydrogen cooling the generator bit:

https://en.wikipedia.org/wiki/Hydrogen-cooled_turbo_generato...

The inefficiencies come from infrastructure. Transferring electricity is so goddamn cheap, that city planners and developers hardly consider it a footnote. Water, sewage and drainage on the other hand are much, much more expensive. They require much more space, and oft times need to be pressurized. Imagine that with steam pipes where the steam on top of all that, needs to be heated too. Sure, hooking up to an existing steam main probably doesn't cost too much, but building new mains can't be worth it, even in the long run, as infrastructure constantly needs to be upgraded.

Where I live most electricity is generated from wind. Solar also has a large share of some grids. Your location may be different but it should have a significant renewable share by now.

That's not true. Except for nuclear and solar collectors, all renewables, especially hydro, have nothing to do with steam.

What isn't true?

> Most electricity is generated by using steam to turn turbines

That is true. It is also true that renewables usually do not use steam. But they don't answer for "most electricity". That's natural gas, coal and other types of thermal plants, including nuclear.

Renewable power is 30% of the world grid.

Gas combined cycle power plants get part of their power from gas in the turbine directly, and part of it from steam cycles. I can't figure out what % over of power comes from which part, and not all gas power plants are combined cycled.

It seems reasonable to say that most electricity is not generated by steam turbines. However I'm not sure how to find the real data to verify this. (and pedantically the gas in a gas turbine is mostly steam)

Geothermal plants also use steam (not to be confused with geothermal heating/cooling, which is just a heat pump with the heat exchanger underground rather than in the air).

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