The role of pumped storage power plants in the power system operation

Transcription

1 The role of pumped storage power plants in the power system operation Miroslav Vítek, Tomáš Králík, Ji í T ma Faulty of Eletrial Engineering Czeh Tehnial University in Prague Prague, Czeh Republi vitekm@ fel.vut.z, tomas.kralik@fel.vut.z, tuma@fel.vut.z Abstrat The paper disusses the funtion of pumped storage power plants in the power system as an aumulator of alternating-urrent power, a soure and also an eletri appliane whih does not produe eletriity by onversion from other forms of primary energy, e.g., onversion from hemial energy of fuel, but only shifts this eletriity in time from periods of surplus to periods of defiieny during the re-pumping yle. After evaluation of the usefulness of servies provided by pumped storage power plants to maintain the balane of output of the Czeh power system in real time, we propose an effiient way to inrease the installed pumped storage power plants apaity by utilizing a flooded strip mine after mining termination as part of the landsape relamation in North Bohemia. Keywords pumped storage power plant; flooded quarry; relamation after maining; wind turbines; aumulator of energy I. INTRODUCTION In the last deade, we have been witnessing in Central Europe a rapid inrease in the installed apaity of intermittent power soures using renewable energy soures (hereinafter, RES) suh as the sun and, more importantly, wind, whih has been inreasing the requirements on the transmission apabilities of power lines and the quantity and quality of support servies purhased by power transmission systems. Pumped storage power plants (PSP) have been a traditional and effiient way of aumulating eletriity. The problem with them is that suitable loations for their onstrution are in mountains and usually inside proteted landsape areas. That omes up against opposition by environmentalists, onservationists and loal population interest groups [3,5]. This opposition an be minimised by utilising the site of a depleted oal mine for the onstrution and operation of a PSP, whih is also the main advantage of the proposed design in addition to the high apaity and power output of this PSP. II. CLASSIFICATION OF POWER PLANTS BASED ON THEIR FUNCTION IN THE POWER SYSTEM In terms of their ability to adjust the grid frequeny, i.e., ability to adjust the power output supplied to the system upon the ontroller s instrution, power soures within the power system (PS) an be divided into two large ategories: 1. manageable by the ontrol entre, referred to as system s; 2. other (small), whih are dependent on the network frequeny, run synhronously with it and are not managed by the ontrol entre. Besides, they an be distinguished aording to the type of load for whih they are intended: 1. system s (manageable by the ontrol entre for frequeny adjustment): a) primary (nulear power plants NPP, run-of-the river hydropower plants ROR) b) semi-peak (ondensing oal plants COP, ombined-yle plants CCP) ) peak (gas turbines GT, storage hydropower STH) d) aumulators (PSP) 2. dependent: a) manageable at least partly (fatory power plants, heating plants, ombined heat and power plants with piston engines or miroturbines, small hydropower plants SHP, et.) b) intermittent unadjustable, an only be swithed off (wind WPP, photovoltai PVP). Generally speaking, systemi power plants intended to over the base load are more apital-intensive; thus, they tend to have higher fixed osts but lower variable osts. The opposite applies to power soures intended to over peak loads. PSP an also be viewed as peak power soures, as they are used to over peak loads and their fuel is eletriity onsumed in periods of low loads for the re-pumping. However, their funtion as an eletriity aumulator has ome to the fore in reent times muh more than previously, beause as the share of intermittent power plants using RES to supply eletriity inreases, there is more demand (and appreiation) for their ability to quikly pump surplus power out of the system at any given time during low demand rather than just supply it in periods of peak loads. PSP therefore deserve to be regarded as a speial type of power plant in terms of their funtion in the PS beause, unlike onventional peak power soures, whih flexibly supply eletriity at times of power shortage by means of onversion from other primary /16/$ IEEE

2 forms of energy, the typial feature of PSP is that, like aumulators, they are not apable of supplying eletriity by onverting a primary form of energy other than eletriity again (negleting their natural inflow). Eletriity is only shifted in time by means of them. PSP shift eletriity from periods of surplus to periods of defiieny within their repumping period. The length of that period is derived from the ratio of the nominal power output of their turbines and generators (or pumps and motors, respetively) to their apaity, i.e., the maximum quantity of energy aumulated. Typially, they are operated in a daily yle, although in the ase of the PSP in the depleted strip mine the storage apaity would be so enormous that it would even allow a weekly repumping yle. III. PSP IN THE CZECH PS Aording to the statistial data drawn from [1], the utilisation of the installed PSP apaity in the Czeh Republi (CR) has been steadily inreasing sine 2008 (see Table 1). That said, the installed apaity was at a steady rate of MW from the ommissioning of the Dlouhé Strán PSP in 1996 until 2014, when the renovation of the mahinery set at the Dalešie PSP resulted in a apaity inrease to the urrent total of MW. TABLE I. TREND OF UTILISATION OF INST. PSP CAPACITY IN CR [1] Year Generation Consumption for pumping Time of utilisation of inst. apaity (hrs/year) The quantity of eletriity re-pumped using PSP has been inreasing along with the growing generation from intermittent power soures in Central Europe, primarily in neighbouring Germany, as their operation is apable of levelling out even short-term osillations within several hours. Often it is the ase that a PSP runs in the pump mode for an hour during the day and swithes to the turbine mode the next hour, to again shift to the pump mode an hour later. The mode swithing therefore ours several times a day, not like in the past without intermittent soures, when pumping was done at night and then the turbine mode was swithed on for the morning peak and then again only for the afternoon peak in the PS onsumption following a possible midday pumping period. It follows from the above that it would be very wise to inrease the installed PSP apaity in the ountry s system and, together with a higher transmission apability of the rossborder power lines to Germany, offer the servies of the Czeh Republi s PSP to German eletriity traders and/or transmission system operators for a payment. The payment would have to be at a rate suh that would make the new PSP profitable. However, it should ertainly not exeed the opportunity osts of fored swithing off of wind turbine power plants, i.e., not taking the opportunity to store eletriity not needed at the moment for later meaningful use. The determining of these opportunity osts ould be the subjet of further researh. IV. VALUE OF SERVICES PROVIDED BY PSP Servies provided by PSP to the network an essentially be divided into two ategories: 1. stati, attrative (valuable) for eletriity traders; and 2. dynami, attrative for entralised network dispathing in real time. Both the ategories of servies are aimed at minimising the differene between planned and atual rates of supply and demand at any time during PS operation. The value of the stati servie depends on the business margin, i.e., differenes between eletriity pries during peak and low loads, i.e., hours of high demand, when the prie per MWh tends to be higher than in hours of low demand, when eletriity is traded at lower pries. Power eletriity pries on the market have been dereasing in reent years preisely thanks to the inflow of eletriity from subsidised RES, whih generate at short-term marginal osts near zero, thus pushing onventional power plants out of the market; the latter are fored to redue their bidding prie for eletriity generated to stay on the market. PSP may admittedly benefit from moments when eletriity is sold at a negative prie, whih will inrease in future, but it is still obvious that PSP will not pay bak for themselves by stati servies thanks to dereasing power eletriity pries, thus dereasing the average business margin, and that the prie of dynami servies that they provide, i.e., support servies sold to the transmission system, is muh more important for their eonomy. Solving unplanned surplus of transmitted power output is urrently more frequent and serious an issue than solving its defiieny, beause the ountry s transmission system has long been overloaded by overflows from wind turbine plants from North Germany. A onvenient plaement of a new PSP with a high power output and aumulation apaity in North Bohemia with a onnetion to the Hrade substation, where the line from Röhsdorf, Germany, also terminates, would enable elimination of sudden inflows of power from German wind by pumping water into the upper reservoir for later use. However, this would require implementing in the international trade a payment for suh a servie provided by the Czeh Transmit System Operator (TOS) EPS to German s TOS 50Hz or a diret purhase order for servies from the German side to the Czeh operator of the assumed PSP. V. PROPOSED DESIGN OF PSP Imagine the dimensions of the roughly retangular pit of the depleted strip mine [9]: 1. 4 km long, 2. 2 km wide, m deep, being the dam height too

3 Fig. 1. Proposed PSP design in depleted lignite strip mine. In ontrast with proposal in [7] where is upper reservoir proposed out of mineshaft and this would be used like down reservoir, we propose diretly damming mining pit. It would be onvenient that the upper reservoir ould be formed already during the oal mining proess using removed overburden in the oldest part of the mine pit (in terms of mining progress), where the overburden is being tipped anyway (internal depot). This part ould be approximately one third of the total planned length of the depleted pit. It would be advisable to reinfore and seal the bottom and the walls using a stabiliser and power gypsum, the latter being a onstrution material onvenient for land relamation purposes and produed by oal power plants in abundant quantities. An important rule would be to ahieve a maximum gradient of the banks at 30 from the horizontal plane so that instability of the modelled slopes ould not be a risk in any plae. In fat, if surplus material exists, it would not be out of the question to raise the banks of the upper reservoir in relation to the surrounding terrain so that its apaity (volume and head) is as large as possible. The bottom of the upper reservoir ould be raised as needed in relation to the lower reservoir with a bakfill to prevent the need for as muh water for filling it, but that would depend on the amount of the removed overburden. If overburden is laking, the bottom of the upper reservoir ould be lower down, but more water would be needed to fill the reservoir. This water would remain onstantly at the bottom of the upper reservoir and would not be engaged in the re-pumping yle. Nevertheless, it would inrease the store of water in the landsape for inidenes of prolonged drought. The water supply for the first filling would be arried from the river Oh e and Bílina, respetively, from Podkrušnohorský anal (drain) [4] that onnets the both rivers and ollet water from mountain Krušné hory. To omplement evaporation would be enough water from the springs flowing into the lower reservoir, whih now in mining time has to pump up into the drain. It is important to ahieve a ertain thikness of the stabiliser-reinfored bottom at the entre of the strip mine for the foundations of the onrete portion of the dam, in whih the mahinery room would be installed. This reinfored onrete portion would be inserted in an adequately sized earth dam, whih would also support it. On the surfae, it would be reinfored with a thik surfae layer of stabiliser, whih would be of a water-resistant omposition, and resistant to pressure similar to lean onrete. Figure 1 outlines the floor plan and setion view of the formed pit to be used for the PSP operation purposes. The original retangular shape of the mine pit would be altered to a figure-eight shape. The pit is narrowed down with an earth dam

4 at approximately one third of its length to minimise the length of the onrete dam ore resting on the embankments. The advantage of this layout is the very short supply lines to the reversible turbines linking the upper and lower reservoirs, whih leads to a signifiant redution in the energy loss due to frition and turbulene of water in the pipelines during both pumping and turbine operation ompared to the onventional layout in mountains, where the supply lines are relatively long. Therefore, the overall effiieny of the repumping yle an be expeted to be no lower than 75%. The theoretial aumulation apaity of the PSP is derived from the usable apaity of the upper reservoir, whih in this ase is more than 110 million m 3 with a median hydrauli head of approx. 90 metres. With an 89% effiieny in the turbine mode, this results in the maximum amount of energy that the PSP ould supply in the turbine mode per re-pumping yle: 6 12 W = h ς g Vη = GWh (1) t s where W t energy supplied by PSP in turbine mode per yle h s median hydrauli head (m) ς water density (kg/m 3 ) g aeleration due to gravity (m/s 2 ) V usable apaity of upper reservoir (m 3 ) η t effiieny in turbine mode This is eight times more than the ountry s largest existing PSP in at Dlouhé Strán [2,3]! Conversely, the filling of the upper reservoir from the minimum to the full ondition would onsume about the following amount with a pumping effiieny of about 85 %: W = h s ς g V η 116 TJ 32 GWh This result in the overall effiieny of the re-pumping yle is approximately Sine the Czeh Republi generates about 220 GWh of eletriity a day (exlusive of own onsumption), it an be onluded that this new PSP would be able to aumulate almost one eighth of the daily prodution of the CR s PS [1]. VI. CONCLUSION ECONOMIC EFFICIENCY OF THE PLAN The advantage of this proposal is the possibility to distribute the onstrution works over a longer time period and, more importantly, that the works onneted to the formation of the upper reservoir will merge with standard land relamation, whih will have to be arried out anyway. Thus, the expenditures on this part of the onstrution of the strutural part of the future PSP an be regarded not as apital investment osts of PSP onstrution, but as expenditures on (2) land relamation of the strip mine after the oal seam is depleted. The estimated total investment osts of the onstrution part is thus only somewhere in the region of CZK 10 billion. The expenditures on proess equipment aquisition depend on the installed apaity of the mahinery set. Assuming the nominal investment osts of equipment at CZK 10 thousand per kw and 6 units with 300 MW eah in the turbine mode allowing a slight speed inrease (by about 10% ompared to synhronous) in the pumping mode [6,8], thus ahieving a power input of about 390 MW for the pumping to aelerate the depletion of surplus power in the PS, the total apital investment osts of proess equipment would be CZK 18 billion. The total diret apital investment osts an thus be estimated at CZK 28 billion, whih is very little ompared to seleted mountain sites in the CR as a onsequene of the reservoir apaity that is an order of magnitude greater. For instane, the Šumný D l site would ost CZK 27 billion aording to [3], with an installed apaity of 880 MW and a power apaity of 5.3 GWh per re-pumping yle, whih is relatively more ostly. The operating osts an be estimated at approx. 0.5% of the apital investment osts, i.e., less than CZK 150 million. To ahieve a Internal Rate of Return at IRR = 6 % with a 35 years of servie life, it is neessary that the annual revenue from servies provided be around CZK 2 billion. The effiieny of the stati PSP servies depends on the differene in power eletriity pries in the different time periods. The ratio of the selling to the purhasing prie of eletriity has to be at least equal to the inverse value of the re-pumping yle effiieny, i.e., the so-alled re-pumping oeffiient: k 1 = = η = 1.3 The ahievement of the required rate of return on the invested apital requires a higher ratio, whih need not neessarily be guaranteed. Therefore, revenues from the dynami servies are more important, i.e., revenues from support servies to the power transmission system. The required daily revenue from sales of the MZ±5 servie is approx. CZK 5 million, translating to at least CZK 96 per MW per hour of stand-by. The standard pries urrently attained for this servie are usually several times higher, whih is why selling this servie by Czeh Transmit System Operator EPS ould attain relatively high revenue and the simple paybak time on the PSP investment ould be even shorter than 5 years. Naturally, the supply of support servies from this PSP would ertainly push down their prie over time, but its very low operating osts would ertainly guarantee a high ash flow for its owner and, alongside the other PSP, it would represent a ertainty for entralised ontrol of the Czeh power system in an unertain future full of intermittent power soures in Europe. To guarantee annual revenues at the neessary rate, it would be enough to sell support servies, whih would be of advantage (3)

5 to the operator beause it would mean revenue without standard wear of the PSP mahinery due to performing everyday stati servies. Besides the importane of this projet for the energy setor, it would be no less important for water management. The PSP would have a vaant (retention) volume of 220 million m 3 in its lower reservoir, whih is a volume that ould handle even large-sale floods assuming a suffiient feed apaity. REFERENCES [1] Annual and monthly reports on the CR s PS operation for the years ERO, available from [2] J. Budín, Parametry a použití p e erpávaíh vodníh elektráren, Bakalá ská práe, FEL VUT, Praha [3] R.Portužák, P e erpávaí vodní elektrárny, Prezentae, VŠB Ostrava, [4] M. íha, I. Dejmal, Územní ekologiké limity t žby v SHP, Spole nost pro krajinu, Praha [5] Synergion, a.s., P e erpávaí vodní elektrárna Šumný d l, Základní informae o projektu, Praha [6] Lorenz T.: Regulae p íkonu PVE v erpadlovém provozu. Dipl.práe, VÚT Brno, FEKT, [7] T.D Thanh, D. Shulz, Potential of new pumped-storage power plants in open ast mining strutures and omparison of the storage apability with eletro-mobility, Poster, Helmudt-Shmidt Universität, Hamburk [8] V. Bartoš, J. Bendl, F. Žížek, Dvojit napájený asynhronní stroj pro p e erpávaí vodní elektrárn, In: Elektroenergetika, Praha, [9] M. Vítek, PVE v zatopeném hn douhelném lomu po jeho vyt žení, In Energetika 1/2016.