by Dave Elliott
‘Heat is very difficult to decarbonise and no consensus is yet reached on the mix needed for the long term and you will have seen that from the various different reports on the subject.’ So said the then UK Minister of State for Energy, Baroness Neville-Rolfe, at the Heat Summit last December, with the next phase of the Renewable Heat Incentive (RHI) central to the agenda. There certainly are some competing options, including community-wide heat networks, green gas supply networks, biomass and solar home heating and domestic heat pumps powered by electricity.
By Dave Elliott
Heat supply is one of the key weak links in the UK government’s attempt to meet the EU-imposed 15% by 2020 renewable energy target. That target still applies – until the UK finally leaves the EU, if it ever does fully. Although there is talk of green heat networks, for the moment the focus is mostly on direct green heat supply for business and private consumers, and there are some changes underway. The UK’s Renewable Heat Incentive (RHI) has escaped cuts so far. Indeed it is set to expand, but the government wants to restructure it to keep energy costs down for consumers and get better value for money. So, concerned also about impact on food growing, it wants to support the use of food and farm waste-based biomass feedstock rather than crop-based feedstocks for biogas production in Anaerobic Digestion (AD) plants. It has also proposed cutting support for solar heating since it is not seen as good value for taxpayer support.
By Dave Elliott
The current decarbonisation plan in the UK is to replace the use of fossil gas for domestic heating by the use of electricity. That may have been slowed by the abolition of the Zero Carbon Homes policy for new build, with its 2016 deadline, but there is still a strong push in that direction, with a ‘Near Zero Energy in Buildings’ concept and specific programmes for roll-out of technologies like electric heat pumps, apparently one of the government’s favoured heat supply options, along of course with insulation and improved building design to reduce demand. The latter makes sense, the former, well that is debatable.
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!