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100% of all energy from renewables?

17 Feb 2018 Dave Elliott

Obtaining 100% of all electricity from renewables by 2050 no longer looks impossible. But what about heating and transport? It will be hard, but it might be possible, with hydrogen and heat storage, along with interconnectors, playing balancing roles, says Dave Elliott

The energy scenario now offered by BEIS implies that renewables might be supplying around 50% of UK electricity by 2035, with 45 GW expected to be in place by then, mostly wind and solar PV. There are more ambitious scenarios, like the one produced for the UK/Ireland by Finland’s LUT and the German Energy Watch Group as a subset of their global 100% renewables scenario. That has renewables supplying all the electricity used in the UK/Ireland by around 2040:  That may be ambitious, but near 100% by 2050 certainly now seem credible for electricity, given the political will. Scotland is already at over 60%. But what about heat and transport?

The UK as a whole has managed to get to around 25% of electricity from renewables so far, led by wind (and helped by Scotland), but has not done so well in the heat and transport energy sectors. The current plan is to decarbonise them with green electricity as that expands – and also with nuclear, if it expands. This electricity would be used to run heat pumps and for Electric Vehicle (EV) charging. But that is not the only option. Indeed, it may not be the best one. Although sufficient generation capacity could be available, it would be hard for the existing electricity grid to deliver the power needed for all UK heating – around 40% of UK energy demand – as well as for EV charging, which some say might require 20%.  The UK gas grid carries around four times more energy than the power grid. The existing power grid could not deliver all the energy needed for heating, as well as the extra needed for EVs, especially since demand for heat and for EV charging is likely to peak in the evenings.

It might be possible to impose “time of use” tariffs to ensure that EV charging was done later in the night, when other energy demands were lower. Or consumers might store electricity that they produced in the daytime from their own rooftop PV arrays in batteries, ready for EV charging at night – although they might have other uses for that stored power, including for heating. That would depend on what was available via the grid.

An alternative approach is to carry on using the gas grid for heating, and existing central heating boilers, but to decarbonize the gas. One way to do that is to convert fossil gas into hydrogen (by steam reformation) and capture the CO2 produced in this conversion process, with the hydrogen gas then being used for heating, delivered by the gas pipes: most have now been upgraded (to plastic) so that this would be possible. However, the conversion losses would be quite high, with as a result, the net emissions saved, even with CCS, only being 59% compared with the conventional gas route, in the case of the H21 scheme being considered for Leeds. Moreover, carbon capture and storage is very uncertain – the BEIS scenario only has 1 GW in use even by 2035.

An arguably better 100% green option, avoiding the need to store CO2, would be to produce hydrogen gas by electrolysis, using electricity from wind and solar, for direct injection into the gas mains or for conversion, using captured CO2, into methane, for use in the gas main. Although, in the later case, CO2 would be produced again when the methane is burnt, it would be balanced by that which had been captured.  The multi-stage conversion processes do involve losses, but the latest PEM electrolysis cells claim to have an 86% conversion efficiency, if heat recovery is included.

This “power to gas” (P2G) approach has some interesting implications. If there was a large amount of renewable capacity in place, sized to be enough, when able to run at full power, to meet peak power demand, then, although there might occasionally be shortfalls, there would also, at times, be a large surplus of electricity produced, which could feed into P2G conversion process. Some of the hydrogen or methane produced could be stored ready to make electricity again, in a gas turbine or fuel cell, when there was a lull in renewable availability and/or large peaks in power demand. The rest could go for heating via the gas main and/or for use as vehicle fuel.

So, on this model, surplus renewable power provides its own balancing and also heat and transport fuel. There might also be enough to export. That would depend on the total installed renewable capacity- and its cost.  Or putting it more positively, on how much income could be obtained from exporting the excess. The Pugwash study I worked on suggested that the UK, with its huge wind potential, could earn up to £15bn p.a. this way from around 100 GW of wind capacity plus 75 GW of other renewables, depending on how much of its output was exported.

That would certainly be far more sensible than curtailing excess wind output as is done at present. Extra interconnectors would have to be build, but they would help with grid balancing via imports, when there were wind/solar lulls or large demand peaks in the UK.  There might of course be times when there was not enough green power available from across Europe to import, and times when UK exports were not needed, but modelling suggests that, given a wide cross-continent footprint, this would not be a major problem.

All the above has been based on wind and solar PV, used directly for EVs and heat pumps,  or indirectly as a source of hydrogen for heating and vehicle synfuel, plus balancing. In reality these two pathways are not mutually exclusive, we could have a mix, and it could be topped up with power from other renewable options, for example tidal lagoons and tidal current turbines and also, possibly, wave energy. Geothermal power too. In addition, there are also non-electrical renewables which can play a direct role in heating and transport. Solar energy is already quite widely used for heating: see my next post. It can be cost effective, especially if done on a community scale, with large efficient heat stores feeding district heating networks. Denmark has several, including some storing summer heat for winter use. District heating systems fed from biomass/biogas plants are also an option, with combined-heat-and-power operation being very efficient and offering flexible grid balancing options. Geothermal heat can also be used in this way.

On the transport side, biomass conversion to liquid and gaseous fuels is an option for vehicles, although there are land use and carbon sink implications associated with large scale biomass/energy crop growing. However, the use of farm and domestic bio-waste, including food waste, to make biogas, is not constrained in that way. It would be particularly useful for heavy vehicles like tractors and trucks, as well as for injection into the gas main for heating. It has been claimed that the biogas and low carbon synfuels could play a major role for both heating and transport. However, clearly, there are limits, to biomass especially. We need to reduce our energy wastage in buildings and elsewhere as a first priority, and also rethink our approach to transport – including, moving away from cars, however fuelled, to more energy efficient forms of mobility.

Even so, at the purely technical level, as can be seen, there are multiple options for low carbon heating and transport fuel. Transport is the hardest nut to crack (see my earlier posts), aircraft especially, but, although flying may have to become more expensive, there is a lot of biofuel work going on and Easy Jet says it will be flying electric planes within a decade. It will not be easy, but there are scenarios in which 100% of all energy globally, including transport, is met from renewables sources by 2050, as Jacobson et al have suggested is possible.

That may be pushing it: road transport demand seems to rise inexorably. However, the options do seem to be there to limit the impact of that. And some countries are trying hard: for example, Denmark plans to be entirely carbon free by 2050. And looking globally, Jacobson and his team, working with Aalborg University in Denmark, have recently produced an update which claims to have demonstrated that it really is possible to have a fully balanced 100% renewables based energy system worldwide.

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