environmentalresearchweb blog

« Biomass limits (Part 2) | Main | Urban freeways zombie awakening in Berlin »

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: www.solarmarstal.dk

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: www.solar-district-heating.eu

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. www.copenhagenenergysummit.org/applications/Copenhagen,%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 www.danskfjernvarme.dk

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.’ www.iea.org/index_ 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 by Dave Elliott on October 1, 2011 12:42 PM | Permalink