By Dave Elliott
There have been reports that the Desertec Industrial Initiative (Dii) had abandoned its plan to help support the development of solar power in the Sahara and the export of some to Europe, since it looked as if the EU could meet most of its green energy needs indigenously, without significant imports. So is the desert CSP/supergrid idea dead? (more…)
As electricity generating renewable energy project spread, there will be an increasing need for new grid links, often across remote areas. But pylons are invasive and there have been objections to some new wind projects on that basis. For example, this issue has come to a head recently in mid Wales. See the consultation at:
National Grid notes that the total generation capacity of the new wind farms proposed in Mid Wales is 874MW, with start up dates in 2015/16. It says that without ‘new significant transmission infrastructure’ it is ‘highly unlikely that the Welsh Assembly Government’s target of 2 GW of onshore wind farm capacity by 2015/2017 will be met’.
It goes on ‘Currently there is no electricity transmission system in the region of the proposed new wind farms in Mid Wales. The nearest points of connection to the existing system are in North Wales, South Wales and the West Midlands’. And it then looks at grid connection options, geographically and technically.
Basically, conventional overhead AC links are cheapest, but are very invasive. Underground AC costs a lot more and High Voltage Direct Current grids cost even more, over relatively short distances – the main cost with HVDC is in the AC-DC converters and their losses at each end; the cable link itself is much cheaper/km, and less lossy, than for AC.
However, Friends of the Earth Cymru has proposed an alternative, potentially significantly cheaper, approach, allowing for underground HVDC, with the capital costs of an under ground link possibly being reduced from around the £600m estimated by National Grid to £300- 390m, depending on revised wind farm capacity, link configuration and whether new energy storage technology was included.
How come? Energy adviser to FOE Cymru, Neil Crumpton, explains ‘The existing Grid regulations require two circuits in the link, each of which could carry the maximum output of all the wind farms, to avoid lost production if there is a fault in one or other of the circuits. Yet, load-duration data from a group of wind farms in southern Scotland indicates that the farms generate around 95 % of their annual electricity production at below 66% of their maximum output. So for the wind regime in that region a 66 MW circuit from a 100 MW wind farm would still transmit around 95 % of annual production. The wind profile in Mid Wales may well be similar. So the proposed 800 MW or so of wind farms may well be served by 2 x 500 MW links rather than 2 x 1,000 MW links with little loss of production during faults of several weeks a year on one or other circuit. If so, the HVDC hardware and undergrounding cost might be nearly halved.’
FOE Cymru says an energy storage facility at or near the upland sub-station could minimise production losses during a fault by delaying transmission until the wind eases and link capacity becomes available. Storage would also ‘bring wider system benefits in terms of routine demand-responsive supply to Grid and power quality improvements’. The group suggest using vanadium flow cell technology or ABB’s battery system which could scale to 50 MW for an hour or more.
Underground HVDC links have been used round the world e.g. in Australia, but usually in larger, longer distance transmission or subsea schemes.
However there may be another, much more radical, approach to energy transmission. Energy transmission by electricity pylon, let alone underground cable, is much more costly than by gas pipeline. Estimates vary, but the costs to transport energy by gas pipeline may be 10 – 100 times lower than electricity pylon and 100 – 150 times lower than underground AC or HVDC cabling. Importantly, as pipelines are underground, they are visually non-intrusive, so route planning is less likely to attract public opposition. Pipelines also have lower transmission losses as no wires are getting hot, and they also offer a degree of energy storage, be it within the pipeline (‘stacking’) or in purpose built stores, at potentially strategic scale.
But what sort of gas and where would it come from? Well a new 48 inch diameter pipeline across south Wales from the new Milford LNG terminals is now supplying the UK with around 200 TWh/y of natural gas from around the world. One idea closer to home is to progressively switch parts of the UK gas network to hydrogen, or a mix of hydrogen and natural gas, to supply decentralised systems.
The hydrogen could come from a number of sources, be it coal or biomass gasification, natural gas or bio-methane reformation. For example large CCS-fitted gasifiers could be located on brown-field sites near port or rail facilities and carbon dioxide pipelines to the sea. The gasifiers could supply hydrogen by new strategic pipelines to dedicated hydrogen distribution networks, or to the existing gas distribution network, to be used locally in fuel cell CHP schemes, domestic micro-CHP boilers, or in urban areas, in larger scale arguably more efficient CHP /District heating projects.
More radically still, some of the peaking output of offshore windfarms could be converted to hydrogen by electrolysers within the turbine or a dedicated electrolyser platform, and then piped long distance.
Much would depend on the relative cost of electrolysers and hydrogen pipelines compared to HVDC converters and cabling capacity. There would be significant energy losses in converting electricity to hydrogen (25 %), though electrolyser efficiencies are improving and sea water electrolysis is being developed. Re-conversion losses to electricity should be minimised (to 10% or less) by utilising the heat in CHP systems. So given the benefits of storage and lower cost transmission infrastructure, piping hydrogen may be worth while.
Piping pure hydrogen would present problems (e.g with pipe embrittlement for non plastic pipes), but modern gas piping has been designed for higher pressures than the old town gas, since the energy density of natural gas is lower, so running with a mix of hydrogen and methane could be viable. Indeed it is already done widely around the world -it’s called hydrane. Some or eventually perhaps all of the natural gas could be replaced by biomethane, produced by AD conversion using biomass wastes. Some biogas is already being added to the gas grid. That way we have a 100% green energy system fed by wind (and possibly also wave and tidal) and biomass. And no unsightly pylons across sensitive areas.
There are plenty of technical and economic details to be considered before this particular ‘pipe dream’ can be taken seriously, but National Grid is currently consulting on grid connection issues, and their web site has much useful information about he details: see http://www.nationalgrid.com/uk/Electricity/UndergroundingConsultation/