The UK has exceptional offshore energy resources, with the latest data showing the potential to harvest 2,000TWh each year – about six times greater than the country’s present annual electricity consumption and more than sufficient to meet all current power needs including transport and heating, if that energy could be converted to suitable forms.
One impediment to the exploitation of this immense resource has been cost per unit of energy. Until recently, offshore wind has been estimated to cost £140/MWh, and both wave and tidal energy incur substantially higher costs. Private consumers may not see this as expensive, since few of us enjoy electricity in our homes at less than 14p/kWh. However, when compared with the average price of generated electricity at around £40/MWh, there is clearly some way to go.
The progress being made in reducing costs is good, however. A recent report by the Carbon Trust produced for the Scottish government assessed that floating offshore wind arrangements could reduce costs to £85-£95/MWh. Compare this with the £92.50/MWh that we expect to pay for power from the new nuclear station at Hinckley Point C to see how competitive offshore energy can be. If environmental effects are fully considered, the true costs of generation by burning fossil fuels are estimated by many to be this much or more.
Cost per MWh has been the driving force behind developments in offshore energy in the past but another, equally important, consideration comes into play as the penetration of renewables increases – both onshore and offshore. That issue is flexibility: the wind does not blow all the time, and demand for electrical power does not necessarily exist when the wind does blow. Similar statements obviously apply to the sun, waves and tides.
Contrary to widespread opinion, nuclear power does not solve the flexibility problem. You can choose not to generate power from a nuclear plant just as you can curtail tidal, wave, wind and solar power, but no money is saved by making this choice. Curtailing any one of these generation sources does deliver a degree of flexibility but basically increases the cost per MWh. Curtailment alone cannot provide an affordable solution.
Many of these issues were debated at the recent Offshore Energy and Storage Symposium (OSES2015), which was set up to address aspects of the flexibility problem affecting, or affected by, offshore renewables. Following the inaugural OSES conference in Windsor, Canada, in July 2014, this year’s symposium was run in early July at the University of Edinburgh and attracted delegates from many countries.
Move to decentralised system
Conference chairman and professor of energy storage, Win Rampen of the University of Edinburgh, opened the conference with the observation that our energy system is going through a set of changes that will be more rapid and more sweeping than we have seen for many decades, as power generation moves away from the large, centralised model of the latter half of the 20th century to the new decentralised system, where individual houses with photovoltaics are net generators some of the time.
Flexibility can be defined as the ability to cope with variations between the natural profiles of generation and demand – what each of these would naturally do if the other stayed constant. Some flexibility can be achieved without energy storage, and an address by Alastair Martin, chief executive of Flexitricity, highlighted the flexibility contribution that can be made by aggregation and demand-side response.
Martin was one of several speakers to emphasise that the system flexibility has to be provided over many different timescales. Very fast events, such as the loss of generation from one large power station or the failure of a large (1GW) HVDC interconnector line to France, have historically been compensated for over a few tens of seconds by the large inertia of many spinning generator rotors. But this system inertia is steadily being lost as new generation formats contribute more. ‘Synthetic inertia’ can be achieved with modern power electronics but only if there is stored energy available for use. Wind turbines do provide some stored kinetic energy, but they generally have lower inertia time constants than the large generators of old. Flexibility in the time range from about 30 seconds to 1 hour can be contributed by switching off non-critical loads such as air-conditioning and refrigeration.
A separate keynote lecture was given by Eric Ingersoll, a founding director of the energy storage company General Compression, who discussed the role that energy storage can play. He said that for some technologies, including compressed air storage in salt caverns, the marginal cost can be extremely low, below £1/kWh. In most cases where energy storage is applied, the ratio between the marginal value of increased capacity and the marginal cost decreases monotonically after some initial peak. However, if storage capacity reaches 100h and above, the energy storage plant can offer genuinely firm power and can potentially command a much higher price for it.
Energy storage implemented offshore can deliver functionality not possible with onshore installations. The power transmission lines delivering power from offshore renewable generators to shore comprise a significant part of total project cost and, without energy storage, these lines are typically used to only 40% of their capacity on average. Implementing storage reduces the required transmission capacity, and it can be used to defer transmission line upgrades onshore.
The offshore environment obviously presents challenges for any equipment installed there, but the environment can also provide resources that may have value – such as an infinite thermal source/sink, an inexhaustible source of salt water for electrolysis, and the potential to implement the perfect ‘yaw-bearing’ using a floating platform. In some cases, a stream of concentrated carbon dioxide may also be readily accessible nearby.
OSES2015 was rich on the theme of integration. Professor Tonio Sant from Malta showed that a hydraulic transmission driven from a floating offshore wind turbine could deliver a valuable stream of cold water to shore to assist air-conditioning, at the same time as transmitting power for generation. Professor Seamus Garvey from the University of Nottingham presented a concept for a floating 10MW wind turbine equipped with up to 1GWh (100h) of integrated storage, implemented using a concept similar to pumped thermal energy storage. Dr Ing Franc Mouwen of ParkID in the Netherlands outlined why there may be a compelling case for employing a large tunnel filled with pressurised air for the combined purposes of power transmission to shore and energy storage.
Buoyant solutions
The conference also showcased several new energy storage propositions, including three schemes storing energy in buoyancy, and several variants on compressed-air storage.
The symposium also featured a session dedicated to power-to-gas, and featured presentations on an island-based hydrogen production project (Neil Kermode, EMEC) and plasmonic dissociation of CO2 to produce hydrocarbon fuels (Adelbert Goede, DIFFER). Battery technologies were discussed in detail, with particular focus on innovative variations of copper-zinc and nickel-iron batteries and how to pull their costs down towards £100/kWh of capacity. System modelling papers shed light on where offshore energy storage could add most value to the system and the extent of that value. Several detailed aspects of thermal energy storage were also explored, with a view to achieving high-performance, low-cost and compact stores suited to the offshore environment. Professor Markus Mueller, head of the Institute for Energy Systems at Edinburgh, was one of several to note the importance of public engagement and community interest for successful projects.
The conference sessions on the final day focused on policy and how to achieve progress in the sector. A panel session featuring Andrew Scott of ETI, David Wallace of the Offshore Renewable Energy Catapult, Andrew Lever of the Carbon Trust, Nick Kitchin of Cumulus, and Professor Rupp Carriveau, who chaired OSES2014 at the University of Windsor, addressed how to deliver affordable and dispatchable offshore energy and produced a recommendation for the Department of Energy and Climate Change, National Grid and the Crown Estate on how to ensure that the sterling work being done on cost-reduction of wave, tidal and offshore wind generation is not choked by lack of flexibility. There was also a consensus that we should not be pursuing technologies that have very low chances of being part of a long-term solution to the problem of secure, low-carbon energy supply.
Earlier presentations – by Professor Ian Arbon of Glasgow University, Dr Rick Jefferys of Edinburgh and Professor Richard Green of Imperial College London – had established that there is a strong case for developing offshore energy storage, but highlighted severe shortcomings in the way that this is presently considered. Four significant issues they mentioned explicitly were:
• Inconsistency in the carbon price
• Penalisation of storage assets with unsuitable transmission charges
• A lack of recognition that energy storage fulfils two functions (consuming low-value electricity to hold the price up as well as supplying high-value electricity to prevent the price from sky-rocketing) but under the Electricity Market Reform is only rewarded for one, and
• The significant penalty imposed on any integrated renewable generation plus energy storage unit that prevents it from claiming rewards for being both flexible and low-carbon.
However, the academics said that these four issues are all dwarfed by three more ponderous problems:
• The intrinsic short-termism associated with reliance on private finance for national energy assets
• The lack of funded demonstrators enabling the community to learn from experience at scale what can never be learned from laboratory experiments and, most importantly,
• The investment impotence caused by uncertainty about the structure of the future energy system.
Community spirit OSES2015 resonated with the realisation that there is a community interested in offshore energy storage and genuine business opportunities – probably amounting to more than £1 billion a year for the UK from 2020 on. The timing and location of OSES2016 will be announced soon.
To access the proceedings of OSES2014 and OSES2015 and to find out more about OSES2016, contact enquiries@oses2015.com