Saturday, September 13, 2014

Wires or pipes?

There is a fascinating debate over whether long distance High Voltage Direct Current supergrids are the right way ahead for energy transmission. In theory they allow variable local supplies  and demands to be balanced across wide geographical areas and are much more efficient, in terms of energy losses over long distance transmission, than conventional AC links.

However there are problems.  It is much harder to shift power up and down from HVDC to and from local AC- expensive converter transformers are needed. That’s why HVDC is usually seen as best for very long distances, with transformers just at each end. Another approach, a bit of a compromise, is an HVDV ring, with tap offs along the way. Others argue that a HVAC overlay grid may be best for regional distribution.   It can get quite technical.  AC allows for easier local, regional or even national frequency balancing, whereas a fully optimised HVDV system would require central control of all the resources.  Some say that we ought to simply interlink national AC systems, so as to retain local autonomy, although some long distance HVDC could also be added as a feed in (and out) to and from strong points in the systems. At the extreme is the idea of local ‘island generation’, with each region operating more or less independently on AC, but linked to others for trade and balancing. That raises all sorts of grid stability and reliability issues as the experience in the fragmented USA shows.

Germany is planning a major series of HVDC ‘corridors’ 660 km long in an €10 bn project to shift electricity from the north, which is where most of the wind resource are located, to the south, which is where there are some large cities. and

HVDC is of course already widely used for undersea links, and China has used it for bringing electricity from its huge but remote hydro projects to cities on the coast. A series of High Voltage Direct Current links have been built to East and South China, over distances of around 1,000 km, to transfer electricity from the Three Gorges hydro plant. The total capacity of the HVDC links is 7,200 MW, with line losses put at about 3%.

Another idea entirely is to down-play electricity transmission and make much more use of gas and gas transmission.  Energy losses are even lower and buried gas mains are much less invasive, once installed, than power grid towers and cables.  The gas grid already handles four time more energy than the electricity grid in the UK, and in effect acts as a buffer store, helping to deal with variable demand: demand for heat varies much more, both daily and over the year, than demand for electricity.  That approach can be expanded with large gas stores (gas is easier to store) and we could switch to green gas, methane from bioenergy sources (e.g. AD biomass using farm and food wastes) and from wind-to-gas electrolytic conversion, making use of the excess energy produced from wind at times of low demand. Some of the gas could of course be used to make electricity locally where needed.  Some of the gas can also be exported and imported. So we don't need electricity supergrids. Green gas could provide a cheaper and more flexible balancing and transmission option.  And if the combustion of green gas is combined with carbon capture and storage, then you get negative carbon emissions.  Sounds like a winner! Pipes not wires!

Some of these ideas are already being explored in Germany. Biogas is being added to the gas mains and 'wind to gas' projects are spreading, with some developing synfuel production using captured CO2 and electrolytically produced hydrogen: and

Heat production, transport and storage is another possible winner –storing energy as heat is even more efficiently than storing it as gas, and although heat transmission is less efficient it can be sent quite long distances with low losses- the longest example so far is 65km from a rural waste to energy power plant to the city of Prague, linked with a 200MW capacity pipe.,%20Czech%20Rep-District%20Energy%20Climate%20Award.pdf

So rather than distributing electricity, or for that matter fossil or green gas, to individual domestic consumers for heating, wherever possible, heat could be supplied via district heating (DH) networks, fed from high-efficiency community-scaled green energy fired Combined Heat and Power (CHP) plants. While heat can be sent long distances, in economic terms, building local DH distribution networks only makes sense in urban and perhaps suburban areas where there are good heat loads. The heat can be from a range of sources.  Biomass and solar-fired DH is now moving ahead across the EU, usually linked to heat stores, and in some cases inter-seasonal heat stores. Most of these will be sited to meet local loads, but in some case long distance transmission might be appropriate. For example, Oslo’s district heating network is fed via a 12.3 km pipe from a waste burning plant in the city outskirts. In Denmark there is a 17km link from a CHP plant to the city of Aarhus. Helsinki has a CHP/DH system, supplying over 93% of Helsinki’s heat, including a plant linked in via a 30km pipe in a tunnel.  So that is an extension of the ‘pipe’ rather than ‘wire’ approach, with piped heat as well as piped gas.

Probably though a mixed system would be best, capitalising on the strengths of each. However the best balance between heat, gas and electricity and which will, or should, dominate in future, is unclear. It will be influenced by the location of the sources and the demand. For example, access for pipes may be hard in some locations. Technological change could also tip the balance of advantage between these vectors. The wind to gas route may prove too expensive, whereas the availability of cheap storage of electricity might make electricity more attractive. The debate continues.
One way follow it is via the Claverton Energy Group e-conferences:

*This post was delayed from its usual start of the month slot since I was away on holiday!

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