By Dave Elliott
This helpful paper from a team at Sheffield University, UK, entitled ‘Great Britain’s Energy Vectors and Transmission Level Energy Storage’, suggests that ‘power to gas’ conversion systems could supply synthetic gas (syngas), made using renewable electricity, for storage in the gas pipe network, so as to balance variable renewables, this being a substantially larger storage option for the UK than pumped hydro.
The paper looks first at the way in which energy is currently stored, which is mostly as high energy density fossil fuel (coal, oil and gas), before it is burnt. It says that, ‘in stark contrast with fossil-fuel supply chains, there is NO ability to store wind or solar energy before it is harvested; leaving the only option for a store of energy to be provided after energy has been harvested. If a technology transforms wind or solar energy into electricity (rather than heat), the drawback of this fantastically useful form of energy is its difficulty to be stored in an economic manner.’ It is ‘technically possible to store electrical energy directly in capacitors, but their costs and the meagre energy density of such devices preclude their use for storing an appreciable amount of energy’. So ‘if electrical energy is to be stored, it is therefore converted into a form of energy that is easier, safer and cheaper to store, and then transformed back to electrical energy when demand requires.’
However, ‘there is an efficiency loss in conversion to the desired form of energy i.e. the energy penalty associated with transformation’ so that ‘electrical energy storage may therefore increase the overall energy requirements of the system, implying that other benefits to the system are required to overcome this obvious drawback. Fortunately, using storage to decouple supply and demand provides an energy system with flexibility, which can fundamentally change its operation, which is especially desirable when greater amounts of primary energy are likely to be from low-carbon energy sources.’
So storage is in with a chance! But what sort of storage is best? The paper points out that ‘the services provided by capacitors, BES, FBES, CAES or PHES are commonly classified as ‘Energy Storage’, with the unfortunate implication that they are readily interchangeable. In reality, there may be areas of competing technologies, but there are also distinct groups of technologies that are best suited to provide particular storage services.’ It adds that ‘some are irreversible in the sense that the conversion to heat or electricity is one-way only, although the store of energy itself can be physically replenished, whereas others can transform energy to and from a readily storable form. This distinction is not necessarily driven by the technical ability to transform to and from a store,…more by the financial and energetic implications of doing so.’ The distinction between rechargeable and refuelable is important ‘as there tends to be a natural focus on rechargeable systems despite those with refuelable stores accounting for the over whelming majority of stored energy’, e.g. gas.
Having set out the ground rules, the paper looks at pumped hydro – currently the largest non-fossil storage option. But at 28 GWh now, and maybe 100 GWh with all possible UK sites developed, it’s tiny compared even with just the ‘linepack’ storage offered by gas in the UK gas transmission pipeline- around 5000 GWh, much less than the ~40,000GWh in actual gas stores. We may not want to rely on fossil gas in future, but the paper suggests that it can be replaced progressively with electrolytic hydrogen made using renewable electricity, but more likely converted to synthetic natural gas (SNG), using captured CO2, for grid injection/storage – the ‘power to gas’ idea. This is being tested in Germany with, significantly, technology from Sheffield-based ITM Power. In time, the paper says, it could offer a major UK green energy storage option – maybe 500 GWh just from variable SNG transmission linepack storage, via pressure adjustments, to balance variable renewables. And maybe the same from the local gas distribution network. Overall a flexible buffer system, enabling surplus output from wind and solar to be stored for later use, with the power-to-gas conversion and gas network storage option allowing ‘SNG production to be driven by times of possible curtailment on the electrical network rather than driven by demand from the gas network’.
The gas grid/storage approach could be an easier option than seeking to balance electricity via pumped hydro and similar storage systems, using the power grid/electricity as the transmission vector. The electricity grid will in fact find it increasingly hard to deal with expanded renewable energy use for heating and electric vehicle charging, as well as power supply: ‘Even allowing for future improvements in domestic energy efficiency and electric heating technologies (such as heat pumps), the UK’s ageing electrical system is likely to see a significant rise in daily energy flows if heat and transport demands are shifted over from the gas and liquid fuel networks, which would result in significant upgrading costs to cope with increased peak flows’. By contrast the gas grid already deals with much larger energy flows than the power grid and also with much larger daily and seasonal variations in demand (for heat). With power-to-gas conversion and storage, it can also deal with the variations in inputs from renewables.
What are the problems? When the SNG is burnt, it will produce CO2, but that is offset by the use of captured CO2 (and zero carbon renewable electricity) in its initial production. Moreover, the paper says, ‘if the CO2 element of the SNG can be captured and used again and again, or even sequestered, then there may be additional benefits in the reduction of CO2 emissions’. That is inevitably speculative: Carbon Capture and Utilization may prove to be too expensive. That was the conclusion of a more recent study looking at CCU for transport fuel production: http://pubs.rsc.org/en/content/articlelanding/2015/ee/c4ee04117h#!divAbstract
The approach proposed in this paper is of course different: it is more about system balancing than fuel production. However there will be round trip efficiency losses in the power-to-gas and then gas-to-power conversion process. That point has been picked up in a more recent US/UK paper, which however concludes that, even with only a 30% round-trip efficiency, a regenerative electrolyser/hydrogen fuel cell system, with interim hydrogen storage, can achieve the same Energy Return on Energy Invested (EROEI) as Li-ion batteries, when storing over-generation (i.e. surplus output) from wind turbines: the materials needed for pressurised storage of hydrogen are much less energy intense than those for battery storage and the high EROEI of wind turbines offsets the low round-trip efficiency. http://pubs.rsc.org/en/content/articlehtml/2015/ee/c4ee04041d
The original Sheffield paper takes an even wider view of overall efficiency and, in nice aside, looks at the value and impact of the use of stored v real-time solar energy. It notes that ‘fossil fuels are stored solar energy, and the energy released by the combustion of fossil fuels suggests an efficiency from primary solar energy to a fossil-fuel store to be well under 1%, which is diminished further when heat from combustion is converted to electrical energy. In comparison, the best commercially available solar PV technology coupled with a battery, where electrical energy is generated from primary solar energy, stored and then released is estimated to provide an efficiency above 20%. Fossil-fuels are the outcome of natural energy harvesting over millions of years and have accumulated their vast energy content due to the length of time over which the solar energy was stored, which more than compensates for the low efficiency of the conversion. However, it is painfully obvious that humanity is literally burning its way through this wonderful resource at a rate that is not sustainable.’
Clearly then it sees a switch to non-fossil energy as being both vital and realistic, and SNG, derived from renewable sources, as a key option for the future, offering, with the gas grid, major storage and transmission capacity. An interesting new energy vector – gas in pipes not electricity in wires, for renewable balancing. www.sciencedirect.com/science/article/pii/S1876610214034560