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District heating networks may not be as efficient as hoped

Posted by Tasha Kosviner on 10 January 2014 at 9:30 am

The difference between the actual and assumed efficiency of district heating networks has come under the spotlight from a number of different sources. 

A YouGen reader, Adrian Turner, writes that his system linking three holiday cottages, runs at a disappointing 37 per cent. Bruno Prior, of wood pellet company Forever Fuels, has suggested in his blog, that his system linking three homes runs at an average 61 per cent efficiency, despite being a 90 per cent efficient boiler on paper. 

This got us thinking. Is this the standard you can expect from small scale district heating networks? If so, are they really worth all the government support they get, not least through the renewable heat incentive (RHI)?

A district heating network comprises a series of insulated pipes used to deliver heat, in the form of hot water or steam, to a number of different locations or dwellings. They range from small, providing heat to a house and a couple of holiday cottages for example, to large scale systems such as those in Stockholm or Flensburg in Germany.

Most commonly powered by biomass in this country, heat networks currently provide less than two per cent of the UK’s heat demand. The Department of Energy and Climate Change (DECC) is keen to increase this figure. It offers support to local authorities to develop heat networks and is exploring how such schemes could benefit more from renewable heat incentives in 2014.

However, as Bruno Prior reports, when it comes to small scale networks as least, users need to be aware that there is a difference between theoretical and real-world system efficiencies.

“it’s obvious to me that if my boiler is running at 90 per cent efficiency, but only 45-60 per cent of heat is actually reaching the houses, then there is something wrong somewhere.”

Bruno's findings are rather worrying given the way in which district heating networks are incentivised by the renewable heat incentive. If RHI is designed to encourage the move to low carbon heating, it is important to ensure that the technologies supported are indeed low carbon. Even if it powered by biomass, a district heating system that is only 45 per cent efficient may be far more carbon intensive than the RHI models assume.

Because they are classed as community projects, they are already receiving payments via the commercial RHI scheme (as opposed to the domestic scheme which will begin to come into effect in spring 2014).

Moreover, since non-domestic RHI rates incentivise larger boilers, the scheme is designed in such as way as to encourage people to choose a larger boiler and link it up to multiple properties to qualify for the higher rates, rather than settling for a smaller boiler that heats only one property and nets a lower incentive.

The heat loss issue is something which, Bruno argues, DECC and Ofgem should already know about. 

“Every RHI site with more than one heat source or where more than one building is supplied by the heat source, is termed a ‘complex’ installation,” says Bruno. “The RHI rules require a heat meter on every heat source and in every building for complex installations. Ofgem could therefore analyse the meter data for all complex installations to caluculate the distribution losses that are occurring. DECC should be requiring that and Ofgem should be volunteering it. Establishing how common and serious a problem this is would be the place to start.”

While Ofgem confirm that while they do have the data, they only use to it ensure eligibility requirements of RHI are met.

"As administrator of the scheme, Ofgem does gather data on efficiency," a spokeswoman tells me. "This is analysed for payment purposes and to ensure eligibilty requirements of the scheme are met. We also provide data to DECC, and they may use this as part of their role in setting the wider policy on this topic."

Over at DECC it appears that analysis of data is not ongoing.

"We model heat outputs for all heating systems includng small district heating systems," a spokeswoman tells me. 

However, she failed to confirm that the field data collected by Ofgem is examined, nor whether there were any plans to do so.  

Rather she issued a confirmation that the government believes that district heating networks on all scales, are A Good Thing. 

"Heat networks can offer a low carbon, secure and lower cost method of heating and cooling. The emissions saving potential depends on the heat generation technology... A key advantage of heat networks is their ability to facilitate transition to lower carbon fuel alternatives such as sustainable biomass, waste heat from industry, geothermal and energy from waste; changing one large heat source to a lower carbon alternative is easier than replacing the equivalent number of individual boilers.

"Heat networks also have the benefits of increased energy security and resilience, cost savings and resource efficiency. In communities they can support urban regeneration, impact on fuel poverty and benefit customers as they require minimal maintenance or servicing. Heat networks also have potential in wider energy architecture for energy storage and grid balancing."

RHI aside, why is so much heat apparently being lost through small scale networks such as Bruno's and what can owners of underperforming networks do? Bruno says that attributing the discrepancy in efficiency rates to heat lost during transfer through the underground pipes between buildings is only part of the story. 

“For the purposes of RHI, the assumption is made that all the heat transferred from a boiler is then used at its destination. In reality though, that may only be the case in the depths of winter. In summer, when you’re only using the heating for your hot water, that heat may be circulated three or four times through the system, or simply held in the pipes until needed, and cooling down all the time.”

So what can be done? Installing an intelligent but expensive ‘weather compensation’ control, and an immersion heater or buffer tank at each property would help with the heat loss issue, but the real solution is, Bruno feels, rather starker. 

“From a sustainability perspective, several discrete boilers are clearly better than a larger system with higher losses, because losses will reduce the carbon avoided by the renewable fuel, and increase the strain on the biomass resource for a given level of delivered heat and fossil-fuel displacement,” he writes. In other words, in inefficient networks, heating with wood is generating more carbon than heating with oil. 

“Through RHI," he tells YouGen, "the government is still pushing people towards clubbing together with their communities or neighbours, to create these small district heating networks. But small district heating networks don’t save carbon – they should either be large scale, or not at all.”

More information

YouGen guide  to the renewable heat incentive

YouGen guide to biomass boilers

From the blog   Domestic or not: where's the divide for RHI? (Nov 2012)   Is biomass suitable for urban flats? (Feb 2013)  

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

8 comments - read them below or add one

Casey Cole

Casey ColeComment left on: 24 October 2014 at 11:11 am

@Twalker,

It looks like your calculation should give you the total system efficiency including a) conversion losses in the plant room and b) heat losses on the network. Assuming just gas boilers (i.e. no CHP or biomass) then efficiency is:

efficiency = (kWh heat used in flats) / (kWh of gas consumed)

If that's all you're interested in, then you're there already - no need to make an extra allowance for heat losses.

But given the meters you've got in place, you should be able to be more specific. I'll refer to the heat meter at the boundary of the plant as "bulk heat meter." Then you could get your plant room efficiency this way:

efficiency of plant room = (kWh measured on bulk heat meter) / (kWh gas consumed)

Similarly, you could quantify the heat losses on your whole network in terms of % efficiency as:

efficiency of network = (kWh heat used in flats) / (kWh measured on bulk heat meter)

And in a similar way, you could break down the network losses by branch using the bulk meters in each block of flats.

Hope that helps?

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Cathy Debenham

Cathy DebenhamComment left on: 24 October 2014 at 11:05 am

Hi twalker

district heating systems aren't really our area of expertise, but a put a question out on Twitter, and this is what came back:

Tim Rotheray ‏@trotheray  1h1 hour ago

1/2 @YouGenUK Hi, DH pipe losses can vary a lot but good systems should be able to achieve 10%. @carbonltd may be able to help further

 

Tim Rotheray ‏@trotheray  1h1 hour ago

2/2 @YouGenUK our heat blog just featured the issue of metering, losses and performance http://www.heatconference.co.uk/index.php/blog/entry/unlikely-heroes-how-the-humble-meter-could-hold-the-key-to-better-network-performance …

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twalker

twalkerComment left on: 23 October 2014 at 12:04 pm

Hi all,

New to this but you look like you know what you are talking about. 

Issue, we run a district heating system, boiler house and 2 blocks of flats, There is a heat meter in the boiler house and 1 in each block of flats after the heat exchanger. How would i report on the efficiency of this system. At the moment we are taking the reading of gas used in kw/h and the total usage of the flats in kw/h, deviding the flats usage by the gas usage to give us a %. Seams a bit strange to me also should we not be factoring a % loss for combustion on mains loss? is there a standard % loss say 15% ?

 

Any Idea ??

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Ian Smith

Ian SmithComment left on: 30 January 2014 at 5:21 pm

I came across one district heating system with high losses recently and this was due to the circulator on the DH pipework running 24/7.  It was not connected into a boiler demand signal.  When contacted the installer insisted that the installation complied with the boiler maker's (a very well known make) recommendations.   Doing this can reduce the number of hours that the heat main is at temperature and contributing to heat losses substantially.

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Prescient Power Ltd

Prescient Power LtdComment left on: 15 January 2014 at 5:29 pm

Thanks Tasha and Bruno.

It's difficult to give a more complete response without the details of the complete system (including occupancy rates, DHW configuration and usage etc), which we'd be happy to analyse if only for the fun of an intellectual challenge.

However I would still assert you have an insulation problem.  If your DHP and thermal stroes were perfect insulators, your only losses would be electrical as the system cycled without appropriate controls.

Picking up on the final point of the article that "poor performing biomass is more carbon intensive than oil" really depends if you buy-into the biomas argument that it is a carbon netural fuel (perhaps one for another blog Tasha!). If so, then you could burn as much as you want through a hopelessly inefficient system and there would be no carbon burden- only one of cost.

Carl Benfield

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Tasha Kosviner

Tasha KosvinerComment left on: 15 January 2014 at 1:25 pm

Here's what Bruno has to say:

"I had a number of comments like Prescient Power's. None of them paid attention to the key statistic that shows it isn't primarily the insulation. My heat losses are higher in summer than winter. If the problem were the insulation, my heat losses would be higher in winter than summer. 

The fact that it's higher in summer than winter shows that the problem is primarily with recirculation and dissipation, not with the level of insulation. It's a well-known phenomenon in countries that do a lot of district heating, which is why DH tends to be done (a) with large-bore pipes in municipal-scale installations, and (b) with waste heat, where the costs of factoring in 20-30% losses are less significant than if you are wasting heat produced from an expensive fuel. 

The DH lobby adopts a three-wise-monkeys approach to this. They are doing no one any favours, including themselves, because the reality will out, and discredit DH generally. If they recognised the problem, they could differentiate between good cases and bad cases for DH and preserve its reputation."

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Tasha Kosviner

Tasha KosvinerComment left on: 15 January 2014 at 11:03 am

Thanks for your comments, Prescient Power Ltd! I'll pass them on to Bruno and see what he says. Tasha

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Prescient Power Ltd

Prescient Power LtdComment left on: 10 January 2014 at 10:47 pm

Another great blog Cathy. Looking at the photo and the text, my bet would be that the system(s) shown used open cell rather than closed cell insulation. The former is cheaper, but if the integrity of the protective skin is compromised by poor connections, lack of sand blinding, poor backfilling etc, then water will ingress and track along the system leading to increased heat losses. The extra cost of good quality closed cell insulation is well worth it. The heat loss may also be exacerbated by a shallow trench.

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