EPE Delft 2008

Energy storage, a condition when feeding the electrical grid from natural sources

Werner LeonhardInstitut fur Regelungstechnik, Technical University Braunschweig, Germany

W.Leonhard@tu-bs.de

Keywords<<Wind power>>, <<Grid stability>>, <<Storage>>, <<Hydrogen>>, <<CO2 capture>>, <<Hydrogenation>>AbstractSustainability by using regenerative sources is a longterm political goal, the reasons are the finite fossil resourcesand the avoidance of toxic emissions and waste. Of the natural energy sources, wind energy is considered in Eu-rope to offer best promise for contributing in the coming decades a substantial portion of the electrical supply. Itis discussed, how the environment-oriented policy in Germany of feeding with priority large quantities of subsi-dized wind power into the supply affects the stability of the grid and whether it can substitute the dependance onfossil and nuclear fuels. Generation of storable secondary energy carrters such as hydrogen for decoupling thefluctuating supply and demand may be a preferable altemative. The possibility of processing the CO2 - emissionsof coal power stations with regenerative hydrogen, similar to natural photosynthesis, is briefly discussed.

Is the direct supply of wind power to the grid the best use of renewable energy?

When technical freaks began to feed a few kW of wind- and solar-power into the electrical grid, this was clearlythe simplest and least expensive method of using renewable energy, the small "negative consumer" load couldnot affect the grid in any way. Nobody would have imagined that some day wind power might assume magnitu-des to endanger the stability of a large interconnected power system, but this can no longer be excluded today.Through massive subsidies with an attractive fixed compensation/kWh, combined with the privilege to feed withpriority undispatched power into the grid, a scheme for rapidly increasing the use of renewable energy, at the sa-me time shielding it from competition, was created by Gernan law (EEG) in 2000. With new large converter de-signs in the MW-range, potent investors took over from small companies and today, more than 22 000 MW ofwind generation capacity are connected to the German grid; it has become an important partner in the electricitysupply, feeding about 37 TWh/a ofmainly undispatched power into the high voltage grid and becoming a seriousdisturbance for the grid control, Fig. 1.

.............................................The usual control plants such as hydro- (4%----4-.... .......... I ~~~~~~Dispatchedgenerationco tn)/ o -..........thsseertcontent) and gas-power stations (10%/con-

. based on Load model and wind prognosis

Gridad loadtent) are no longer adequate for balancing the1----------- ---------- -------------. .. ...............-..........................................-fuctutin win pow lrge ase-oad.her

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... ......................... Wind powerflctatngwid owr;lagebae-oaPter[0>. ----_|Windpower mal power stations are needed instead whichCt t} : :--.-::: \.:-.-::--------~~~~~~~~~~~~-------------

%rv: PWind ....../-.. has the undesired effect of increasing their--.--- _ _ , contraoarpower

I 9, r, /X,§ ; R31 gI l- 5 1 / gri 'n fuel consumption and emissions, besides

A1½~llR$]p costs and wear. Since the inacurately predic-

Fig 1 Wind power, a disturbing force when controlling the grid ted wind power is not coordinated with the

grid control, local overload of high voltage lines may occur, leading to protective switchings and even breakdownof the whole Europeanl UCTE grid, as in Nov. 2006 [1]. This raises the question whether the present approachcan be continued as it endangers the stability of the grid; large energy storage facilities are needed instead.

- 1-

What type of energy storage is available for protecting the grid.?Figs. 2 and 3 show the grid loads and wind powers in the four control areas of the interconnected Gen-nan highvoltage grid during the Kyrill-week in January 2007 as well as in June 2007 [1]. Fig. 2 indicates that the wind po-wer fed to the Vattenfall area amounted at times to 70 % of the load, while the total wind energy in the German

grid is only about 7 %; the consequence was a heavy pressure for exporting power to neighbouring control zones.

The projected increase of wind power by another 20 to 25 GW from Off-shore plants in the next 20 years willmake it very difficult to always maintain the (n-1) - stability criterion. Since the grid responds rapidly with emer-

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spectacular incident; for legal reasons the station has not yet been restored to normal operation.The financial value of the energy surge in terms of compensation to the wind industry is about 30 Mio 6, a minorsum when related to the total costs of the service interruption; also, the C02- saving achieved by collecting windenergy during the storm is irrelevant when compared to the emissions from fossil plants needed for replacing thenuclear station during many months. If for political reasons the German EEG-law cannot be modified by assigningmore effective control authority to the grid operators, it would clearly be preferable in future to autonomouslythrottle or shut-down wind farms during storms and compensate the wind industry for the lost revenue.In order to estimate which storage schemes could temporarily accept energy inputs of this magnitude, the new(2003) pumped storage hydro station Goldisthal in Thuringia may serve for comparison [1]. The dates are 1060MW, 8.5 GWh, employing a 12 Mio m3 reservoir with a 300 m head and a (round trip) efficiency between 0.7 and0.8. This indicates that a wind surge as in Fig. 3 can definitely not be handled with pumped storage hydro plants.The devastations while constructing such plants in the remote mountains and the needed additional high voltage li-nes from the Off-shore sites would prevent such undertakings which, after all, are intended to serve an "ecologicalpurpose".With compressed air energy storage (CAES), of which an early diabatic version is operating at Huntorf since the80's, the situation is more favourable because the energy is stored in underground salt domes existing in the Nor-thern part of the country. However, the dates of Huntorf with solution-mined salt caverns of 0.3 Mill m3 volume,pressures up to 70 bar and a power of 300 MW, storing 0.6 GWh at a round trip efficiency of about 0.42, showagain that the capacity is far too small to accommodate wind power surges as in Fig.3.

Underground Hydrogen storage

According to the results of a VDE study to be published later this year [5], energy storage on such a scale is onlyfeasible in chemical form by storing hydrogen at high pressure in large underground volumes. This was proven inthe former DDR, also in England, where city gas with a 50 % hydrogen content was stored in salt caverns; in USA

and England the chemical industry reports about hydrogen storage in salt caverns, observing minimal leakage los-ses [4, 5]. With an underground volume of 3 Mio m3, as they are employed for seasonal storage of natural gas andat a pressure of 140 bar, up to 1 000 GWh of energy could be stored, however at a lower efficiency of about 0.4,similar to the CAES plant at Huntorf.Fig. 4 depicts a scheme, where the fluctuating electrical power from renewable sources is separated - by suitablyreconnecting the grid or with under sea cables - from the dispatchable power and converted to hydrogen for under-ground storage at high pressure; it may be used in (combined cycle) gas power stations, producing dispatchablepower, thus protecting the grid from overload. Of course, hydrogen can also be employed for numerous other ap-plications, not discussed here. An important aspect ist that the compressors delivering hydrogen into the storagecavern have to supply only the mechanical energy for the compression, whereas the total potential and chemical

energy of the hydrogen becomes available when the medium is leaving the storage for final conversion with exi-sting or new gas power stations, parallel to their normal operation.The C02-emissions per kWh of power stations with natural gas amount to about 40%/ of the emissions of lignite-based plants, they are further reduced when hydrogen is mixed to the natural gas and are eliminated with pure hy-drogen. Since very large seasonal storage facilities for natural gas are already available, they could also serve forstorage of wind energy, mixing natural gas with hydrogen. In view of the low storage efficiency of about 0.4 thismay appear as a modest achievement, but the problems of directly balancing the electrical grid with thermal sta-

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tions and the danger of destabilisation by overload and possibly tiggering a breakdown would be avoided.

Separate electrical lines . ..Sun PPV, Solar- or uncritical H-thermal parts of the electrical grid storage

Wind 'Electro- 0 H2 Vehicles H20WEA Fluctuating power 1SIS (BC/ICE)

H 2 ' 2

WaterIWate Hydro-power st.

Oil Vehicles Emissions

Natural Gas (IE 002, NO1-*. Gas-

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power st.I !Electrical

~~~~~~~~~~~~~~~~~~~~~~c n Um 4ersUran IElectrical cosmrNuclear ~~~Dispatchable power GiPumnpedstorageCAES

Fig. 4 Scheme of an electrical energy supply with hydrogen storage

Clean coal, CO2- sequestration and underground deposition

The largest fossil resources worldwide are coal deposits, many countries including China and India are basingtheir plans for industrial development on coal and nuclear energy; in Germany, there are huge deposits of lignitethat could be mined for another 200 years, however at the cost of large C02-emissions. Since almost half of theelectricity in Germany is produced with coal power stations, there is a fundamental interest to continue their useuntil a sustainable solution based on renewable sources is available. Because nuclear power is to be phased outuntil 2023, "decarbonization" by sequestration and penranent underground deposition of CO2 is seen as the onlyacceptable solution; the necessary technology is proven on a pilot scale and will be available for use in power sta-tions. C02-sequestration and deposition in depleted oil or gas wells is already practized on a large scale [9], withthe planned emission charge serving as an important economic incentive.However, many are unaware of the dimensions of the problem, indicated in Fig. 5 with the example of a 1 000MW power station using either hard coal or nuclear fuel. In one case 60 m3/a of highly radioactive solid nuclearwaste is to be deposited in a salt dome, 1000 m below ground; in the other nearly 100 000 times this volume, 5.7

Mio m3/a of pressurized liquid CO2 must beHard coal power station Light waer nuclear power station

.1 m deposited underground, both for unlimitedHighly radioactve, solid nuclear waste:

. ... ER,,i.m /a time. Clearly, none of the methods is wi-* - --Efi. Ri.EitESEitiXE..,^'

thout risk, when recalling the volcanic erup--iiiii.

Pressursed (553bar) liqulfied C02tion in lake Nyos (West Africa) a few

months after Tschemnobyl, where the hca-Yearly vo/lume of waste tor be safet depositedvirta-rCOgsflwdnonehbuwhen operating a 1 000 MWpowerstat/onvirta-r Ogsflwdnonehbu

Fig. 5 Yearly volume of waste to be safely depositedrigvlesufoangvryiigbigwhens operating a 1 000 MW-power station including more than 1 500 people.

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Of the different methods of CO2 - sequestration in coal power stations only the "Integrated Gasification Combi-ned Cycle"-process (IGCC) is mentioned, whose large scale feasibility is established [7, 8]. It includes pressuri-

zed gasification of coal for producing synthetic gas which consists of CO and H2, similar to the Fischer-Tropsch-process for liquifying coal. With suitable catalysts, high temperature and high pressure this is converted to H2 undC02, to be separated, liquified and deposited, while the hydrogen is burned in combined cycle power stationswhose exhaust consists mainly of nitrogen from the air and water.

Carbon recycling by reprocessing C02 with regenerative hydrogen.

The enormous quantities of CO2 to be deposited, where even the slightest chance of release into the atmospheremust be avoided at all cost - assuming similar conditions as with radioactive waste - gave rise to the thought ofconsidering carbon as an energy resource, to be processed with regenerative hydrogen [2]. The hydrogen storagescheme (Fig. 4) and the IGCC process involving capture and deposition of C02, might be extended by hydrogena-tion, chemically reducing the captured CO2 with regenerative hydrogen for producing new synthetic fuel that canbe returned to the power stations in a closed carbon cycle, instead of releasing CO2 into the atmosphere or deposi-ting it.In Fig. 6 it is shown that this amounts to an attempt to imitate photosynthesis, a complicated process nature hascreated in millions of years by evolution, where sunlight splits water and the hydrogen captures CO2 containedwith < 0.04 % in the atmosphere for producing new biomass. A condition to simulate this fundamental naturalprocess is of course adequate availability of regenerative hydrogen for retuming to the CO2 all the energy thatwas extracted in the power plant by the combustion of coal. Assuming a power station with 40 % net efficiencythis would be more than 2.5 times the electrical energy produced.

|Suln < i \Separate electrical lines, Stage

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Fig 6 Imitating nature by chemically reducing CO2 with regenerative hydrogen,a way towards a sustainable electrical energy supply?

HIydrogenation of the chemically very stable CO2 with low free-energy calls for suitable catalysts, high pressureand temperatuDre as with the Fischer-Tropsch-process for liquifying coal. Hence electrolysis, C02- capture and hy-drogenation would have to be parts of an integrated technological process calling for extended research.

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To avoid energy losses, hydrogen storage and C02- reprocessing might be applied only to the part of the rene-wable power that otherwise could endanger the stability of the electrical grid; several options are indicated:Option 1, presently used, allows only a limited power fed to the grid to avoid instability, while Option 2 invol-ves hydrogen storage, without or with C02- capture and deposition. Only Option 3 includes C02- capture aswell as hydrogenation. The options may be used separately or in combination, allowing room for optimizationwith regard to grid stability, energy storage or the C02- volume processed.A major expansion of energy generation with off-shore wind- and solar converters is conditional to CO2 - pro-cessing, but this is believed to be necessary anyway for climatic reasons. However, just continuing our presentpolicies of feeding more and more fluctuating power into the electrical grid would miss both objectives, a sustai-nable energy supply as well as a stable electrical grid.

Conclusions

Hydrogenation of CO2 - emisssions from fossil power stations with regenerative hydrogen would affect our pre-sent environmental and energy policies:- Fluctuations of renewable power are separated from the eletrical grid and no longer endanger its stability.- Inversely, the massive increase of renewable power, believed to be necessary for climatic reasons, is notlimited by the grid, in fact it is made possible by the separation.

- With existing storage facilities for natural gas, hydrogen storage allows to bridge calm periods.- The planned North/South grid extensions for transporting large Off-shore wind power during a few daysof the year are unnecessary.

- CO2 - emissions from fossil power stations are converted to new synthetic fuel and recycled,instead of being released to the atmosphere or accumulated in underground deposits.

- Coal power stations would have a future perspective for some decades wihout a possibly dangerousaccumulation of CO2 - deposits.

- The chemical and process industry would have an important new goal for applying their technologicaladvances in the century after Haber-Bosch and Fischer-Tropsch.

With a long term research program of this kind there are no "losers" and consensus could return to the energydiscussion instead of short term politics and acrimony. Even the "nuclear issue" might be mitigated when post-poning the "nuclear exit" until the feasibility ofCO2 -hydrogenation is established.

References

[1] Leonhard, W., Wenzel, A.: Flauten, Orkane und eine verfehlte Energiepolitik - wie soll das elektrische Netz das richten?

ew, 2007, Heft 7, S. 52-57

[2] Leonhard, W.: Energiespeicher fir eine nachhaltige und zuverlhssige Energieversorgung, ew, 2007, Heft 19, S. 26-30

[3] Leonhard:, W.: (Unanswered) Open letters on energy issues, <www.ifr.ing.tu-bs.de>[4] Schulien, S.: Windenergie im deutschen Gasnetz, AGAFE-Mitteilungen, 2004, Hfte 2

[5] Stromspeicher zur Integrationl emeuerbarer Energie, VDE-Bericht, 2008

[6] Wasserstoff. Nachhaltige Energie - stationir, mobil. Zukunfisenergien. <www.energieland.nrw.de>[7] JGCC-Kraftwerk mit CO2 - Endlagerung im Grol3mal3stab realisierbar, Broschtire der RWEB-AG.[8] Vortmeyer, N.: Verbrennung mit Mehrwert, Puertollano. Siemens-Sonderverdffentlichung Klimawandel, FAZ 12/2007

[9] Statoil Sleipner Project, <www.statoil.com>

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