This site uses cookies. By continuing to use this site you agree to our use of cookies. To find out more, see our Privacy and Cookies policy.
Skip to the content

[IOP] A community website from IOP Publishing

Tag Archives: district heating

Wire or Pipe?

The UK’s current energy plan envisages electricity being the main focus, with excess power from offshore wind and nuclear being used to run heat pumps and to charge batteries in electric vehicles. Natural Gas takes a back seat as a heat source, but is used in CCGT plants to supply some electricity, and these gas turbines also act as back-up plants to balance the variable renewables, although gradually some may become biomass fired. All of this will require new grid links, possibly also to the rest of the EU. So that’s the ‘wire’ view.

It’s the dominant one at present. The new DECC Heat Strategy review says ‘electricity is universally available’ and, in well-insulated houses, it says heat pumps can make using it for heating relatively economic.

The rival ‘pipe’ view is that electricity is a poor energy vector, since its transmission is lossy and it can’t easily be stored, except via pumped hydro. Also domestic scale heat pumps are not that reliable, especially in cold weather.

By contrast, gas can be easily stored and transmitted: we actually already have an extensive low loss, high transmission efficiency gas grid, which handles around four times more energy than the electricity grid. Moreover, the gas grid acts as an energy store helping us to cope with variable demand. And we can produce biogas from municipal and farm wastes to provide a carbon neutral replacement for natural gas. In addition to its use for heating, some of this green gas could be used for local electricity generation, where needed, in CCGT or fuel cells. Some could even be used for vehicles, as a better option than mostly imported biofuels.

The weak point in this argument is that there probably won’t be enough biogas to replace all the gas used for heating (National Grid says about 50%, optimistically), much less transport and electricity generation. There are land-use constraints to biomass production and limits to how much waste is available. Moreover, biomass and wastes are, in any case, more suited to local use e.g. in local Combined Heat and Power (CHP) plants, or heat only plants. That would avoid having to shift biomass/wastes in bulk around the country. It’s not quite the same for biogas: although that’s best generated locally from farms and municipal waste, and some could be used locally, some could also be distributed via the gas main, to power electricity generation plants where needed, as well as being use directly for heating in homes, as at present.

However the use of gas by consumers directly for heating may not be the best bet. Natural gas fired plants, and increasingly biomass fired plants, linked to district heating (DH) networks, could be more efficient option and play a major role, as elsewhere in the EU. Indeed solar-fired DH is now moving ahead across the EU, usually linked to heat stores, and in some cases inter-seasonal heat stores. So that’s an extension of the ‘pipe’ view- we can distribute heat as well as gas. Clearly DH only makes sense in urban and perhaps suburban areas, and it also make sense, if we are burning gas, whether natural or bio, to use medium or even large scale high-efficiency combined heat and power plants. Then we also get some electricity, with the ratio of heat to power being adjustable.

The ‘pipe’ view is that this make much more sense, in efficiency terms, than installing micro CHP units in individual homes- with large CHP, the heat and power produced can be better balanced against the varying demands of large numbers of consumers. And big CHP/DH systems with heat stores can also help balance varying power grid inputs from renewables

Even so we may not have enough biomass or biogas to run this system- and any solar input will inevitably need backup. That’s where the next element in the ‘pipe’ argument comes in. We can produce hydrogen gas, using electricity from excess off-peak wind and other variable renewables via electrolysis, store it and then add it to the gas main for distribution, or use it for electricity production when needed. Some also look to syngas production from renewable electricity for vehicle fuel.

There are some quite severe efficiency loss penalties with some of the energy conversion processes required for making, storing and using ‘green hydrogen’, but the technology is improving, with Germany taking a lead: see my next Blog.

So why not consider the pipe option? Certainly the ‘wire’ option looks tricky. It is likely to be hard to get enough electricity generation from renewables, like offshore wind, to meet all (or most) of the UK’s power heat and transport needs. We are talking of perhaps, by 2050, 180-200GW of offshore wind, including floating wind farms further out to sea. Plus maybe some wave and tidal stream. If nothing else, the grid connection costs and problems for renewables on this scale look very serious. But the ‘pipe’ approach also has its limits. If we really are to avoid the biogas limits by producing hydrogen from
offshore wind (etc) for the gas grid, then we would come up against the same problem with getting enough offshore capacity. In both cases, with high costs – and a need for gas plant backup.

In a way then the two approaches are not that different, at least if we are talking, in the pipe version, about large scale generation of green hydrogen: they both rely on having lots of renewable electricity. But they do differ in the main transmission vectors – electricity or gas pipes or wires.

In theory we could have a mix of both: there could be some useful complementarity between the wire and pipe approaches, reducing some of the constraints. But in practice, under present competitive market approaches, there can be conflicts. For example, it’s been said that in Denmark there is a danger that wind energy will drive CHP systems out of business: electricity production from local CHP systems in Denmark went down by 24% from 2000 to 2009. And in the UK, large scale CHP has hardly even started- we
have had so much cheap gas. That may not change, if shale gas turns out to be plentiful and cheap, despite its alleged environmental risks. But in that situation, emissions aside, providing backup for wind would be easier, although the pipe lobby might still argue that gas should be used for heating rather than electricity generation.

Technological advances may help change the picture-if we adopt sensible approaches to infrastructure, so that new supply systems can be plugged on to the heat and power grids. Large scale heat pumps linked to DH systems, or large hydrogen or biogas fired fuel cells for urban CHP, are amongst the options. But cheaper offshore wind would be the key breakthrough. Or for that matter, cheaper wave or tidal power. Some also see PV solar as emerging to cut across much of this debate. Then again there’s Carbon Capture and Storage. If that proves to be viable (and acceptable) on a large scale, then both the Wire and Pipe lobbies benefit, although the gas/pipe lobby might have an edge, since then the combustion of green gas/biomass would be carbon negative.

The DECC Heat Strategy Review looks at some of these options, but basically still comes down in favour of electrification of heat supply, with the role of gas diminishing, although it does also back district heating networks in urban areas. The debate continues!

Posted in Renew your energy | Tagged , , , | Leave a comment | Permalink
View all posts by this author  | View this author's profile

Solar heat – large scale

We’ve got used to the idea that solar heat collectors can be viable in the UK, despite its often poor weather, and when it comes into operation for the domestic sector next year, the Renewables Heat Incentive may lead to a lot more roof top panels being installed on houses. Typically they can halve annual heating bills- depending on location and what heating fuel is usually used .

There are a few larger schemes used for swimming pools or in hotels, but what is less familiar in the UK is large-scale solar, feeding heat to community-scaled district heating networks.

There are some significant advantages to operating at larger scale. For example, then you can have large efficient heat stores; the ratio of their outer surface area to the contained volume decreases with size, and so therefore does heat loss. In addition, instead of having to match the heat demand patterns of an individual household, a large store can serve many houses, so that the individual demand patterns are averaged out.

This approach is sometimes called ‘grouped solar’, with individual housing blocks or terraces sharing a large solar array and large heat store. But to do this on a larger scale you have to have district heating pipe network , and that adds to the capital cost. Although once installed the running is low – and if its solar fed, the heat is free. There are few conventional district heating projects in the UK, but many elsewhere in northern Europe. For example, district heating (DH) networks supply 60% of Denmarks heat at present, much of this from fossil sources, although some from straw burning.

What about using solar? Well there are already some impressive projects, 85MW(th) in all in Denmark, some with solar heat stores. For example see the 5.6MW Braedstrup project and also the 13MW Marstal project – which is shortly to be doubled in size:

And Denmark has some ambitious Solar DH targets:
2015: 1 TWh, 3 % of the DK district heating demand
2030: 2.7 TWh / 10 % of the DK district heating demand
2050: 7 TWh / 40 % of the DK district heating demand

For more:

Austria is also moving ahead in the solar DH field. For example, there is a large district heating network in Graz, with 6.5 MW(th) of solar input . Germany has installed nine research and demonstration solar arrays linked to district heating networks since 1996, including some with inter-seasonal heat stores. Depending on their size, they can meet 40-70% of the annual heating needs of a building.

In most case though, with large DH networks, the solar input is a small to medium additional input alongside other sources, for example gas or biomass fired combustion plants, some of them being ‘co-gen’ Combined Heat and Power (CHP) plants, generating electricity as well as heat. But the proportion of solar seems likely to grow, with some novel ideas emerging. For example, in Copenhagen, a new 280kW demonstration solar plant is being developed to deliver solar heat to its district heating system, with 90 square meter of solar panels, a heat store and a heat pump. The heat pump raises the temperature of the water from the solar panels or the storage tank, before the heat is delivered to the district heating network.,%20Denmark-District%20Energy%20Climate%20Award.pdf

The ‘Heat Plan Denmark’ a study financed by the Danish District Heating Association, claims that district heating combined with CHP and renewable energy is more cost effective than individual solutions based on more investments in the building envelope and/or investments in individual renewable energy solutions. So it argues for the expansion of district heating and energy storage, fed increasingly from large scale solar arrays, biomass and biogas fired CHP, and geothermal sources. More at

Whatever the fuel used, one off the big pluses with CHP/DH systems, especially when coupled to large heat stores, is that it can be used to balance the variable energy inputs from wind turbines. When there is excess wind generated electricity, it can be used to produce extra heat, either for storing or for direct use via the DH network, the CHP plants then throttling back on heat production, or feeding it to the stores. When there is not enough wind, they can increase the proportion of electricity they produce.

A new IEA report on Co-gen/CHP and renewables backs this idea, noting that ‘storing heat is simple but storing electricity is still difficult and expensive.’ info.asp?id=1941

Interseasonal heat stores of course open up even more options- storing energy from winter wind, or summer solar heat, for use at other times.

What is tragic is that the UK does not seem to be taking CHP/DH seriously, much less solar DH. A recent report for the governments Committee on Climate Change produced by consultant Matt MacDonald simply says more work needed to see if it ‘could complement or substitute heat decarbonisation in buildings from heat pumps or resistive electric heating’.

It seems the emphasis is still mainly on electricity, even for heating, given the focus on offshore wind and nuclear. But if nothing else, CHP/DH with heat stores might help match their outputs (24/7 from nuclear, variable from wind) to varying demand. A recent paper from the AECOM Technology Corporation, claims that Combined Heat and Power/ District Heating is the most cost effective solution for grid balancing in terms of carbon saved for a given additional lifecycle cost (Paper C92-EIC_029 to Energy in the City Conference, London South Bank University, June 24th) . And solar heat, along with biomass, could help top up the system.

Posted in Renew your energy | Tagged , | Leave a comment | Permalink
View all posts by this author  | View this author's profile

Community heating – the green options

The UKs ‘zero carbon’ house programme, should see a mix of electricity and heat supplying on-house technologies including photovoltaic (PV) panels, solar heat collectors, biomass-fired CHP units and heat pumps. In addition we now have a Renewable Heat Incentive, which from Oct next year, should also see the wider adoption solar collectors, biomass , heat pumps and so on.

While it is sensible to try to get house energy efficiency up to the maximum possible and to use local energy sources wherever available to meet the needs of individual houses, there are other more collective approaches, supplying groups of houses, whole communities or even cities. The RHI can be used for some community scale projects, so they may prosper, and we might see some innovative ideas.

As I discussed in an earlier Blog, elsewhere in Europe district heating networks are common and some make us of solar and other green energy inputs- and they include systems with inter-seasonal heat stores.—district-heating.html

That idea is now being taken up in the UK, for example, for large school premises. ICAX have developed an Inter-seasonal Heat Transfer system, which includes a ‘Thermal Bank’, used to store heat in a very large volume of earth for a period of months, as distinct from a standard heat store, which can hold a high temperature for a short time in an insulated tank.

The ICAX system involves capturing heat energy from the sun via a collection pipe network just beneath the surface of black tarmac roads , or car parks or school playgrounds, and then storing it in the ground under the foundation of buildings. It is then released to heat the buildings in winter via heat pumps linked to underfloor heating.

ICAX say ‘unlike a normal ground source heat pump which typically starts with an autumn ground temperature of 10°C, the heat pump in an Inter-seasonal Heat Transfer system starts with a temperature of over 25°C from the Thermal Bank. This doubles the Coefficient of Performance of the heat pump and allows a 50% saving of carbon emissions compared to providing heat from a gas boiler’. They add ‘Where it is not practical to create a horizontal Thermal Bank to store energy, ICAX uses a borehole field to perform the same function.’

Interestingly, summertime solar heat storage has also been put forward as a way to heat parts of the tarmac at Heathrow airport in winter- to reduce icing up of the aircraft stands. That, you may recall, was a major issue last winter.

However if the UK summer is not seen as reliable enough for winter heating of buildings or whatever, then how about wind-powered district heating? The Danish District Heating Association says that more than 20 partly wind powered heating element systems, with a total capacity of more than 200 MW, will have been installed in district heating plants by the end of this year. They can be powered using surplus electricity from Danish wind turbines- the energy thus being stored as hot district heating water. Heating elements work like giant immersion heaters, which can automatically heat water when there is surplus power. The system regulates itself based on electricity prices. When wind power is available, the heating elements are switched on, the district heating plant’s own electricity-generating plants are switched off, thus saving on fossil fuel use.

This may be an advanced idea, but District Heating networks in the EU are increasingly supplied using green sources. For example 62% of Danish households are linked to district heating, supplied from gas-fired CHP plants, but also from surplus heat from industrial production, solar heating and waste combustion. And there are plans to increase to solar share to 40% by 2050.

It’s the same in the Sweden. The district heating sector there achieved a market share of 60% during 2008 in the heat market for buildings in the residential and service sector, using a mixture of waste incineration, industrial surplus heat, biomass, with only limited fossil CHP. And in recent years, this fossil CHP has been replaced by biomass CHP. Overall carbon dioxide emissions are now claimed to be more than 80% lower than in other European cities and towns using natural gas and fuel oil to heat buildings. It now supplies over 50TWh p.a. The majority of the plants feeding the DH systems use wood and peat (30TWh p.a). 94% of multi-family houses are connected and 78% of public and commercial premise.

District Heating is cost effective. For example, a paper in Applied Energy 88 (2011) 568-576 puts the marginal capital cost of DH distribution at only 2.1 €/GJ, which means energy at around 4.8p/kWh. And in the Netherlands, CHP/DH was found to be one of the least cost carbon abatement options at 25 EUR per tonne CO2, lower than building insulation, condensing boilers and wind power.

We may yet see it in the UK- the Renewable Heat Incentive does support district and community renewable heating, and the UK Energy Technologies Institute is looking at inter-seasonal heating storage systems, as I noted in an earlier Blog:—district-heating.html

There are some examples already. The University of Warwick has a 4.7 MWe 14MWTh Combined Heat and Power (CHP) system, linked to an extensive district heating network around the campus, which supplies 50% of campus power and reduces overall energy use by up to 34%, compared to separate electrical and heat generation. It also uses thermal energy storage (in a water tank) to meet peak heat demand and also allow for power generation when demand is low, without any heat dumping.

Finally, ‘DH’ may not be the most riveting of subjects graphically, but it’s brought to life in a fun, if simple, Danish animated video:

Posted in Renew your energy | Tagged | Leave a comment | Permalink
View all posts by this author  | View this author's profile

Green heat – district heating and energy storage

District heating networks, using gas, waste heat from power stations or heat from biomass combustion, to heat houses and other buildings collectively, are common across much of continental Europe, especially in the North. There are also some large solar-fed heat grids and many heat stores. There are even some inter-seasonal heat stores, which help to deal with variable supplies over the year, and variable demand for heat, e.g. during winter evenings. See my earlier blog.

More district heating projects are proposed. For example, ‘Heat Plan Denmark’ a study financed by the Danish District Heating Association, argues that District heating is the key technology for implementing a CO2 neutral Danish heating sector in a cost effective way. They claim that the Danish heating sector can be CO2 neutral by 2030 by upgrading and expanding the existing system, with, for example, heat pumps being used to upgrade the heat energy currently supplied and more heat stores being added. At present much of the system still uses gas as the main energy input, but they look to the use of more renewables, and more efficient waste-to-energy Combined Heat and Power (CHP) plants with flue-gas condensation. So the emphasis will shift increasingly to using large-scale solar heating, biomass /biogas CHP, geothermal energy and excess wind energy – and more heat storage.

Overall, they see district heating moving up from 46% to 70% of the market share, and suggest that the remaining heat market can be covered by domestic-scale heat pumps and wood pellet boilers in combination with individual solar heating. However, they claim that district heating combined with CHP plants and larger scale renewable energy is more cost effective than domestic-scale solutions based on more investments in the building envelope and/or in individual renewable energy solutions. More at

A similar conclusion emerged from a study of district heating in Copenhagen, which has the world’s largest heat network, currently fed mostly by 10 CHP plants with a total of 2 GW of heat capacity. About 45% of the fuel is from renewable sources (biomass/wastes), and that proportion is planned to expand. In addition to using geothermal heat, they are testing a demonstration solar plant to deliver solar heat to the district heating system, with a heat pump being used to raise the temperature of the water from the solar panels, or a linked heat storage tank, before the heat is delivered to the district heating network.

By contrast, we have a long way to go in the UK. Heat accounts for about 44% of UK energy consumption, mostly for heating homes and providing hot water, using individual domestic boilers – 84% of UK homes are heated by gas. This may change as and when the Renewable Heat Incentive (RHI) and the Zero Carbon Houses programmes kick in and domestic-scale solar, biomass micro-CHP and so on are taken up. But what about the larger scale and all of the waste heat from power stations?

The UK’s total demand for heat is about 800 TWh p.a., about the same as that released by all power generation/industrial processes as waste. So far, with gas being relatively cheap, Combined Heat and Power, which can reclaim much of this waste, has not lifted off very significantly in the UK, a few biomass-fired plants aside. Neither has district heating or heat storage. But with gas prices rising and concerns about emissions growing, heat reclamation and storage ideas are now being explored-borrowing from what’s happening elsewhere in the EU. For example, the Energy Technologies Institute is looking at waste heat collection and storage on a large scale. As they note ‘It is technically possible to store very large quantities of heat energy below ground in geological structures such as saline aquifers or disused mines. The heat could even be accumulated through the summer to be used during the winter. Many of the potential heat sources and storage areas are close to centres of population and could be used to support large-scale district heating schemes.

And the recent DECC Microgeneration Strategy Consultation also looks at energy storage, and in particular at Underground Thermal Energy Storage (UTES). It includes a mini case-study of a UTES installation in Sweden, which paid back the additional installation cost (compared to an oil fired system) in under four years and continues to save money, energy and carbon year on year. It reported that, as well a tapping heat from power stations, this approach “can be particularly effective to create energy clusters where excess heat from buildings with a net cooling load can be utilised as a source of heat for others nearby with a net heating load to save carbon and reduce energy consumption”.

However, DECC’s focus seems to be on the smaller scale. It added: “Work is on-going between Sweden and the UK to use similar underground thermal energy storage techniques on a domestic scale.” It commented that in the UK: “In the domestic sector there is scope to store hot water generated by renewable energy through the wider deployment of hot water cylinders.” But, it said: “74% of the circa 1.5 million boilers fitted annually are ‘combination’ boilers, so the opportunity to future-proof homes for renewable-heating technologies, through the provision of hot water cylinders, is limited.” And it noted that: “There are no plans to set mandatory requirements for the provision of hot water cylinders – there is a trade-off between the benefits of large water volumes needed to bank/smooth renewable-energy supplies and the higher standing losses from large volumes at elevated temperatures. There are also design issues to consider. Larger cylinders weigh more and take up more space and there is a greater risk of stratification. There may also be an increased threat of legionella from water storage facilities, unless the appropriate elimination steps are taken.”

So, as seem to be the norm, we are taking it very slowly and cautiously, and focusing mainly (the ETI project apart) on the domestic scale. Ideas like large-scale solar district heating are still evidently heretical. Someone had better tell the citizens of Graz, a city in NE Austria, which has a District Heating network with 6.5 MW of solar thermal input.

Posted in Renew your energy | Tagged , , | Leave a comment | Permalink
View all posts by this author  | View this author's profile