District heating vs. electricity generation

[TSN79]

My city has large district heating system, where the heat is generated at an incineration plant. I'm personally not a fan of district heating, and I would rather see the plant generate electricity through steam instead. I don't know if that was ever considered when the plant was built. Are there facets with the electricity-idea that makes district heating the absolute way to go?

 

[anorlunda]

See these from earlier threads

https://www.physicsforums.com/threads/technology-of-west-and-east.978305/post-6241093

https://www.physicsforums.com/threa...cs-of-pebble-bed-reactors.978118/post-6240721

 

[OmCheeto]

... Are there facets with the electricity-idea that makes district heating the absolute way to go?

According to this article, Norway is already exporting 10% of its generated electricity. Perhaps those in charge didn't see a need for more? Perhaps the cost of a steam turbine was too much? Perhaps there's no guarantee that garbage is going to be so plentiful in the future, and a turbine would have been a waste of money?

Btw, I just ran across an article dated July 2017, and they say Norway is to ban oil heating on Jan 1, 2020. That's a month away! I can't find any articles less that 1½ years old. Is this ban still taking effect?

 

[russ_watters]

My city has large district heating system, where the heat is generated at an incineration plant. I'm personally not a fan of district heating, and I would rather see the plant generate electricity through steam instead. I don't know if that was ever considered when the plant was built. Are there facets with the electricity-idea that makes district heating the absolute way to go?

Yes: district heating is much more efficient then electricity generation (around double).

Even better is Cogen.

 

[Keith_McClary]

Efficient steam electric generation requires a high temperature. If they are just burning trash it may not get hot enough.
Since the efficiency of the plant is fundamentally limited by the ratio of the absolute temperatures of the steam at turbine input and output, efficiency improvements require use of higher temperature, and therefore higher pressure, steam.

 

[OmCheeto]

Efficient steam electric generation requires a high temperature. If they are just burning trash it may not get hot enough.
Since the efficiency of the plant is fundamentally limited by the ratio of the absolute temperatures of the steam at turbine input and output, efficiency improvements require use of higher temperature, and therefore higher pressure, steam.

I was under the impression that garbage burning plants operated at really high temperatures, as there's lots of potentially nasty stuff in there, and only high temps will break it down. I'm pretty sure these plants aren't just some guys throwing garbage into 55 gallon drums.

 

[pbuk]

Yes: district heating is much more efficient then electricity generation (around double).

Measured how? Do you have a source?

 

[russ_watters]

Measured how? Do you have a source?

Thermodynamic efficiency, and this is pretty fundamental. Think of it this way: if you are making steam either way, you can either use it as-is for heat at near 100% efficiency (if you return the condensate) or you can run it through a thermodynamic cycle at 30-50% efficiency to make electricity. Heck, you can burn the gas in a gas turbine to make electricity and still get more heat than electricity from it. Here's an example, offering 65KW of electricity and 150kW of heat (model c65):
https://www.capstoneturbine.com/solutions/energy-efficiency

 

[pbuk]

Ah, so you are measuring at the point of generation. Unfortunately much of this gain can be lost in distribution in a district heating system and also in excess supply.

Agree that co-generation is the right way to go though in most situations.

 

[russ_watters]

Ah, so you are measuring at the point of generation. Unfortunately much of this gain can be lost in distribution in a district heating system and also in excess supply.

Yes, I would expect that heat loss is a lot higher than electricity loss in distribution, but I don't know how much. But I doubt it is half of the heat.

 

[Keith_McClary]

Agree that co-generation is the right way to go though in most situations.

In the Arctic, but not so much in Florida.

My understanding is that for maximum efficiency you need a more expensive multistage turbine (for physics/thermodynamics reasons).
https://upload.wikimedia.org/wikipedia/commons/1/12/Feedwater-heating.png
This expense can be avoided if you have a use for the low grade heat that a single turbine doesn't convert to electricity.

 

[essenmein]

District heating is also a great way to use the low grade waste heat from a power station instead of throwing it away in a body of water or a cooling tower.

(I know that's not what this plant specifically is doing, the point is simply you could generate electricity and still have district heat, which is a win win IMO)

 

[pbuk]

Yes, I would expect that heat loss is a lot higher than electricity loss in distribution, but I don't know how much. But I doubt it is half of the heat.

In this report the average was exactly half, with the maximum in 2 cases out of 11 being 66%. Even taking an optimistic 25% gives a net 2.0 x 0.75 = 1.5 = 50% gain over electricity rather than the 100% you quote. And that's before taking surplus supply factors into account (what are you going to do with all that heat in the summer?), as well as excess demand issues (how do you meet demand in a cold snap, burn fossil fuels?)

 

[russ_watters]

In this report the average was exactly half...

That paper is hard to read (there isn't much detail on the systems), but it appears to be talking about hot water as a distribution medium, not steam. It isn't all that surprising to me that the loss is that high for a hot water distribution system, where the quality of the heat is much lower than steam. I would hope steam distribution is much better.

And that's before taking surplus supply factors into account (what are you going to do with all that heat in the summer?)

Just turn it off. You end up with the default for the power plant, which is just dumping the heat into the atmosphere. Unless you can use the steam to run absorption chillers.

...as well as excess demand issues (how do you meet demand in a cold snap, burn fossil fuels?)

It would depend on the type of system. If it's primarily a cogen system, yes, you could supplement it with natural gas fired boilers on top of the natural gas fired generators. The overall generation efficiency would drop a little, but not much. And the distribution loss will go down as load goes up.

 

[Rive]

Are there facets with the electricity-idea that makes district heating the absolute way to go?

Depending on the local weather (all year) and electricity prices heat supply alone might be economically better than cogeneration or electricity generation.

Norway, right? I would expect a high demand for heat and there is already some surplus of electricity.

 

[pbuk]

... I would hope steam distribution is much better.

... The overall generation efficiency would drop a little, but not much.

I have seen you post on a number of topics Russ, and generally have a great deal of respect for what you say and the way you say it. So why the hand-wavy stuff here?

In the UK at least, district heating has a very bad reputation which, at least in part, is due to decisions being taken on the basis of theory, hand waving and idealism without considering empirical evidence and real-world needs and behaviours. In an engineering topic in these forums we should be better than that.

 

[anorlunda]

So why the hand-wavy stuff here?

We don't need the adversarial attitude please.

The devil is in the details. That's very true for thermal systems. District heating is a very broad term. Without reference to a specific system's details, any discussion of something so vague is necessarily hand wavy.

In the UK at least, district heating has a very bad reputation

That is also hand wavy, and cites no source, nor is it specific enough to fact check.

 

[pbuk]

We don't need the adversarial attitude please.

...

That is also hand wavy, and cites no source, nor is it specific enough to fact check.

You are right, on both counts. @russ_watters please accept my apologies.