Massive energy storage systems enable secure electricity supply from renewables
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Climate change is mainly attributed to the burning of fossil fuels. To solve the problem, current inhabitants have to dispense with fossil fuels as a source of power. It has been demonstrated that this can be secured before 2050 by transitioning to renewable sources of energy. Massive energy storage (MES) incorporated into long distance high voltage direct current (HVDC) transmission systems is the key technology for the transition. This paper describes the current state-of-the-art in electricity grid configurations. It demonstrates how MES, and other back-up local electricity storage schemes represent a natural way of buffering electricity consumers from the intermittency of inherent in the primary renewable systems. Consequently, it can solve the base-load issues and facilitate a global transition to 100% renewable energy sources over the next fifteen to twenty years.
KeywordsElectricity grid Energy storage Pumped-hydro Lagoon storage Hydraulic piston Climate change
By five years from now, in 2020, a rational assessment of the climate science suggests that, to avoid dangerous levels of global warming, the burning of fossil fuels needs to be heavily proscribed, and mankind should be directing maximum effort towards creating renewable energy power systems. The early transition to renewable energy sources is necessary to improve our ‘green’ credentials and enable the abandonment of fossil fuels, without sacrificing our advanced technological societies.
Developing evidence suggests that planning for these renewable energy delivery systems will have to be predicated on operation at the continental level. For example, in Europe, a viable direct current (DC) super-grid, mooted in several recent reports, would connect geothermal power stations in central Europe, solar power stations in southern Europe, wind farms in Western Europe, wave/tidal systems off Scotland, Norway and Portugal, hydroelectric stations in Northern Europe, and nuclear power stations in France. This system would be backed up by massive storage facilities based on compressed gas and hot water thermal storage using cathedral sized underground caverns, on massive battery farms, and on pumped storage employing conventional high altitude reservoirs, and artificial lagoons constructed in shallow sheltered bays, as planned for the coastal waters off Denmark. Continuity of supply from diverse intermittent sources dictates that a super-grid, which is geographically extensive and employs a multiplicity of sources, will assist in this goal.
In a future world weaned off fossil fuels, the power to drive all sectors of our modern economies will have to be delivered through the agency of electricity – except perhaps for a limited number of licensed users of fossil fuels and some users of biofuels. Consequently, nations will have to unite to expand their electricity grids to continental coverage  and to enhance efficiency by large scale adoption of ‘smart’ technology . The European Commission is edging in this direction. Unfortunately doubt continues to be expressed on the practices of the current electrical power industry, that without flexible ‘base load’, provided today by fossil fuel power stations, a geographically extensive grid based electricity supply system capable of delivering dependable power is incompatible with renewables, particularly wind, wave and solar power. While this attitude prevails, progress towards the realisation of a renewables based electricity grid will be slow.
The source of the above assertion lies in the need, in our traditional AC grid system, to actively intervene to balance supply and demand, otherwise frequency stability is degraded. In fact, the grid frequency is a system-wide measure of overall power imbalance. For example, if demand becomes too high, thereby drawing excess current through any given electrical power generator, an unavoidable Lorentz braking force is initiated resulting in a decrease in its rotational speed. If the generator is operated in frequency-response mode, whereby it normally runs at reduced output, a buffer of spare capacity is available. In this case the generator can continually alter the power it delivers to the grid on a second-to-second basis using what is termed droop speed control - a feedback signal to the turbine directing it to boost its power. If demand exceeds this level of control it becomes necessary to power up ‘idle’ additional turbine/generator sets. This mode of operation is effective for the power utilities particularly when accurate predictions of the demand profile are available. Needless to say this ‘balancing act’ is deemed to be severely compromised by the introduction of ‘inflexible’ and ‘intermittent’ renewable sources.
Over shorter distances, the grid voltage is generally transformed down to about 100 kV as indicated by the transformer symbol between the blue and orange transmission line representations. At this voltage level, power is fed onto the grid, typically from gas power plants (150 MW – central symbol in 100 kV box), and industrial plants (50 MW), while simultaneously power is distributed to factories and other high voltage consumers. Primary distribution is normally carried out at about 50 kV. In this segment, the grid predominantly provides electrical power to consumers, such as factories, administrative offices, hospitals, schools and housing estates. Historically, on conventionally operated systems, generators have seldom formed part of the distribution grid.
The favoured electrical power source on the conventional grid is the synchronous generator. This is tricky when, for any given machine on the grid, both parameters are influenced by any imbalance between the machine load and the power supplied to the input shaft. Hence the need to predictively balance supply and demand. In simple terms smart monitoring provides signals to a local governor that regulates the turbine driving torque by controlling the steam supply to it, and hence the speed of each generator is maintained. Generation and consumption must be balanced across the entire grid, because energy is consumed almost at the instant it is produced. Energy storage is largely absent.
In general electrical engineering practice outside of the power supply industry, this deliberate need for predictive balancing of supply and demand is actually not an issue. For example, any standalone complex electronic device, such a mobile phone, laptop computer or electric car, is a small but self-contained power system. It exhibits varying power demand from a multiplicity of electrical circuits and components which are influenced by changing operational modes, while the electrical supply comes from an energy storage device (usually a battery). Obviously, but significantly, this energy source delivers power only when it is needed. Thus supply and demand are automatically balanced. If the battery is rechargeable, which it usually is, then balanced demand/supply is maintained as long as the battery contains sufficient charge. Importantly, it does not matter how erratic or intermittent the charging process is as long as on average over a defined time period, the charging power supplied to the battery is equal to or greater than the power demanded over the same interval, by the electrical circuits of the device in question.
The inference of the above routine electronic circuit behaviour is that an electrical power supply system, at the national grid level, should function adequately with intermittent renewables acting as the primary power sources, provided the supply side is buffered from the demand side by massive energy storage (MES) plants, such as pumped hydroelectric schemes. The grid has to carry AC power, for transmission and distribution reasons, the buffering storage plants will be required to drive turbine/generator sets little different from those currently employed in today’s power stations. Demand management will very likely parallel current practice. A few, but far too few, of these storage facilities are finding a place in global grid systems. There are many other massive storage technologies which could be implemented for example the Nordhavn ‘green island’ storage scheme near Copenhagen, but unfortunately effective development is almost non-existent at the present time.
2 Literature review - massive energy storage
A power transmission grid is generally a routine electrical circuit except that it entails carrying power generated at geographically separated locations over long distances to customers or users (loads) at other geographical locations. In addition to this geographical element, a further difference from a conventional DC or very low frequency AC circuit is a multiplicity of generators and loads (Fig. 1). Conventional grid systems operate satisfactorily provided the network exhibits spare capacity, both in terms of redundant transmission line routes, to facilitate the bypassing of damaged lines, and in terms of power generators to ensure a balance between supply and demand can be maintained in all circumstances. The latter, arguably, has been made easy over the past century by the abundance of fossil fuels driving the development of affordable and reliable thermal power stations. These until very recently have represented the core of the electricity grid system. Unfortunately, the effluent from these fossil fuel power stations is harmful to the planetary eco-system, because of the warming effect of carbon dioxide in the atmosphere [3, 4, 5]. Carbon capture  could solve the problem, but technological implementation remains much too far into the future to meaningfully address the climate change crisis.
2.1 Distribution level storage
The currently favoured solution [1, 7, 8] envisages all of our power being supplied from renewable sources, with some backup from already existing ‘clean’ nuclear power stations in the form of integral fast reactors (IFR’s), plus any which can be brought ‘on stream’ by 2020. However, in order to achieve abandonment of fossil fuels by 2030, as dictated by the climate science, the major drive by mankind has to be towards securing clean energy from renewables. The major ‘stumbling block’ for this strategy is intermittency, particularly from wind, solar and wave sources . It is usually claimed that security and quality of supply on a grid which accommodates renewable sources of electricity requires ‘base load’ generating stations fueled by coal, oil or natural gas to smooth out power fluctuations.
2.2 Transmission level storage
Figure 3 shows that the MES methods require the assistance of nature with conducive geological and geographical sites. Unfortunately, pumped-hydro energy storage based on high altitude reservoirs, and compressed air storage based on geologically formed air tight underground caverns, are a rather limited resource, with all of the best sites already largely commandeered. Yet in any future electricity generation system, based on renewable sources, reliability of supply will require many more MES plants than are currently in existence.
2.2.1 Lagoon hydraulic storage
For the sake of simplicity it has been assumed that a range of hydraulic, electrical and dynamic loss mechanisms, which are generally relatively small, have been ignored . Interestingly while (7) is independent of the lagoon area (A) it is strongly dependent on the head (h) and the outflow cross-section (a).
For storage lagoons in the typically 5~10 m depth range, Fig. 6b provides a good indication of the cross-sectional areas (a) of the discharge/recharge valves which will be required to achieve grid level power delivery. For example, one or more apertures exhibiting a combined area of 9 m2 will be needed to extract 2 MW from a 10 m deep lagoon storage facility.
2.2.2 Hydraulic piston storage
The operational principle of each can be expounded by reference to the deep shaft option in Fig. 7a. This shaft will generally be of circular cross-section and invariant with depth. However, unlike the lagoon alternative (Fig. 5) piston storage achieves the bulk of its gravitational storage, not by shifting water vertically, but by raising a heavier than water piston. This is done by pumping water into the lower cavity of the shaft which is located below the piston (Fig. 7a). This lifting fluid is extracted from the upper chamber via a pump and penstock to the lower one. Fig. 7a depicts the discharging phase of the storage system with the turbine/generator powered by the descending piston at a time of high electricity demand. At low demand the opposite process occurs with electrical power from the grid driving the pump to lift the piston back against gravity to the top of the shaft, whereupon maximum potential energy is stored. For efficient operation of this system, the quality of the pressure seal between the curved surface of the piston and the lining of the shaft is absolutely key. Arguably, this component is likely to be materially, and mechanically, the main hurdle to successful implementation of the technology.
The well storage concept in Fig. 7b and the basin configuration in Fig. 7c operate in a largely similar manner, except that since they are less deep for practical constructional reasons the recharging and discharging of the piston chamber can be implemented less expensively by employing a natural or artificial reservoir.
The engineering guidance offered by Fig. 8 to potential developers of hydraulic piston energy storage systems, is that perhaps not unexpectedly, in a single unit, storage levels fall short of ‘massive’ proportions (GWh’s). Nevertheless, significant storage levels in the MWh range are available in shaft type systems exhibiting depths approaching 1 km, or in basin configurations of sizeable area (six or more Olympic scale swimming pools). In civil engineering terms the former is practicable only if the shaft diameter is no more than about 5 m, or in the latter case only if the basin depth is less than 20 m. These restriction are primarily related to the fact that any developer seeking, simultaneously, to maximize both depth and width will soon discover that this course rapidly leads to very expensive levels of landscape reshaping. Furthermore, the requirement to seal the gap between the piston and its well by means of compression gaskets of some form, promises to be much easier in narrow lined shafts or in shallow smooth sided basins, than in other possible configurations.
It is perhaps pertinent to note that while a single hydraulic piston unit is unlikely to exceed the 100 MWh capacity, several units, particularly of the shaft geometry, could readily be assembled on a single urban site. Visually it would be no more intrusive than a gas storage complex housing multiple gasometers. Such a site could certainly achieve electrical energy storage levels in excess of 1 GWh.
3 Storage enhanced renewables grid
From a technological perspective this article has demonstrated that gravitational energy storage in the ‘massive’ category is feasible by means of sea-level hydro storage (SLHS) techniques. This assertion is supported by simple mathematical models (see section 2) and is confirmed by recent reports, of planned developments and of some progress on prototype systems, in the engineering literature. The accumulating statistical data on sea-level pumped hydro storage configurations suggest that it compares to alternative storage technologies by providing a bridge between rapid charge/discharge low energy systems and the ‘massive’ capacity systems associated with high altitude pumped hydro. This is summarized in Fig. 3 where SLHS capability, in power versus energy storage terms, is represented by the blue ribbed ellipse extending from 1 MWh to 104 MWh on the energy axis, and from 1 MW to 104 MW on the power axis. This technology arguably fulfils a similar role to flow-battery energy storage (FBES) but without the restriction in instantaneous electrical power delivery which is inherent to batteries.
The middle third of the Fig. 9 diagram represents middle distance grid operation at typically 100 kV transformed down from the very high voltage sector as represented by the transformer/control icon. At this voltage, power is again generated by the full range of renewables (wind turbine + solar array icons), again replacing the fossil fuel generators in Fig. 1, while most of the storage facilities listed in Fig. 3 (green battery icons in Fig. 9), could contribute to voltage quality maintenance. Some power distribution to factories and high voltage users is suggested by the factory symbols (see Fig. 1). The lower third of Fig. 9 represents the power distribution network. While some power generation can occur at this level (solar panel symbol), this network mainly provides electrical power to users (as suggested by the administrative tower, factory and housing estate). Voltage smoothing and fault mitigation is secured by employing storage facilities which are matched to the needs of the community being served. These are again represented by the battery icons.
At the distribution level, it is clear that users, from households to hospitals, have very different requirements with respect to the quality, stability and reliability of their electricity supply. This probably means that in addition to the pumped-hydro or compressed air plants capable of delivering significant power levels over long enough time periods to ensure continuity of supply, the distribution grid will also have to incorporate low energy, high discharge rate systems, such as BES, FBES, FES and SMES (see Fig. 3) to guarantee quality. Unfortunately, all of these technologies are intrinsically generators of DC power on discharge. While this may have represented a problem in the past for the AC grid system, recent technology advances in the power supply industries, based on modern power electronics converters, can provide an answer to this mismatch. Developments in flexible AC transmission systems (FACTS) aim to provide robust grid control at times of complex operational demand, usually associated with random external disturbances or fluctuations . FACTS are multi-megawatt proven power electronic devices which are being introduced at the present time into the electricity supply networks. Rather than following conventional practice and switching to stand-by, or base load, fossil fuel powered generators in this power interruption scenario, FACTS enable the maintenance of stability on the grid by permitting storage techniques based on DC devices, such as SMES, FES, BES and CES to be incorporated into grid control systems. Thus by employing storage in conjunction with voltage source converters, it is possible to effectively negate or damp oscillations caused by sudden changes in load conditions .
From a fundamental electrical engineering perspective there is little doubt that an electricity generating and transmission system powered by renewable sources, and backed up by MES facilities, is viable. Recent contributions to the engineering literature attest to this assertion, yet claims that renewables are incompatible with high quality voltage supplies continue, somewhat erroneously, to persist. It is suggested that this is largely attributable to the fact that developments in energy storage lag hugely the growth in renewables.
Generally, in a sustainable electricity supply system, if the renewable power levels available to the grid as a whole are, on average, well in excess of the demand level, then with sufficient storage capacity electricity supplies will be reliable, and capable of meeting quality standards to which users have become accustomed. In this article, it is demonstrated that the range and versatility of energy storage methods steadily becoming available to the electrical supply industries, is by no means inconsiderable. Unfortunately, much of it remains at the prototype stage of development. Additionally, a continuing major technology gap exists in the ‘massive’ energy storage sector, which the paper highlights. It is emphasised that this storage category is essential to the stabilization of any future high voltage transmission grid relying on renewables.
While MES already exists in the form of high altitude pumped storage hydro-electric complexes, these cannot easily be increased in numbers to meet future grid needs, because they are strictly limited by geology and geography, with the best sites largely commandeered. CES is also similarly restricted by geology. This paper argues that a solution potentially lies with lagoon hydro-storage and hydraulic piston storage systems. It is demonstrated that these novel alternatives display power versus energy storage characteristics which very usefully bridge the gap in performance between compact rapid discharge systems including batteries, and storage in high altitude reservoirs. Furthermore, since they are much less dependent on geology and are intrinsically safe, these systems can readily be sited near towns and cities where electricity demand is inevitably at its most acute.
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