Energy storage for wind integration: Hydropower and other contributions

Energy storage for wind integration: Hydropower and other contributions
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    Abstract   —   The amount of wind power and other time-variable non-dispatchable renewable energy sources (RES) is rapidly increasing in the world. A few power systems are already facing a very high penetration from variable renewables which can surpass the systems’ consump tion during no-load periods, requiring the energy excess to be curtailed, exported or stored. The limitations for electric energy storage naturally lead to the selection of the well-known form of storing potential energy in reservoirs of reversible hydropower stations, although other technologies such as heat storage are also being used successfully. This paper reviews the storage technologies that are available and may be used on a power system scale and compares their advantages and disadvantages for the integration of fast-growing renewables, such as wind power, with a special focus on the role of pumped hydro storage. Index Terms   —   Wind power, renewable integration, energy storage, balancing of wind power. I   I  NTRODUCTION  he fast growing capacity of wind and other time-variable RES is posing a set of new challenges to power systems operators (TSOs). One of the difficulties faced by TSOs is the non-dispatchability of these new renewable power sources, allied to daily and seasonal production profiles that, in most situations, do not follow the load consumption  profile. Several countries already have a very high wind contribution from these sources: in 2010 Denmark had 22% wind penetration; Spain covered 16% of their electric energy demand with wind power; Portugal had a wind participation of 17% and Ireland had 11% [1]. In order to achieve annual wind energy contributions between 10 and 25% these power systems faced several periods when the percentage of wind  power (and other non-dispatchable power stations) was very close to, and sometimes even above, total consumption. The solutions to handling excess wind power or other non-dispatchable generation are to store the energy, to export it or curtail it. Although curtailment of excess wind or solar  power seems a solution, its renewable time-dependent nature, the growing capacities being installed and the large investments in renewable power plants encourage co- ordination with the existing forms of energy storage, This paper has been written as a part of the IEA Wind Task 25 “Design and operation of power systems with large amounts of wind power”. Ana Estanqueiro is with Laboratório Nacional de Energia e Geologia, LNEG, (e-mail: as well as Luis Rodrigues, Juha. Kiviluoma and Hannele Holtinen are with VTT, Javier Revuelta is with REE, Emilio Gómez is with Univ. Castilla La Mancha, Ciara O’Dwyer and Damian Flynn are with UCD, Atle Rygg Årdal and D. Huertas-Hernando are with SINTEF , Erik Ela and Debra Lew are with NREL and Mikael Amelin is with KTH. especially when transmission capacity for exporting to neighboring countries is limited. For the future, and in view of the increasing role of variable RES, it is important to assess how systems with higher shares of these sources can  be operated and designed for efficient integration without violating system security, while maximizing the penetration and added value of these sources. Besides reducing the need for curtailment of RES, energy storage can also be used for smoothing net load variations allowing dispatchable base load units to be operated also when RES generation is high and reducing the need to dispatch peak power units when RES generation is low. Therefore, assessing the role and added value of energy storage, especially well-known technologies such as pumped hydro storage (PHS), is actually of utmost relevance for  power systems with existing and planned high RES  penetration. Section II of this paper presents typical wind and consumption profiles for different power systems that indicate the necessity of storage, exporting facilities or curtailment. Section III describes the energy storage technologies available at the power system scale that are  being used among IEA Wind Task 25 members, while Section IV provides a cost/benefit analysis of power system energy storage options with a particular focus on the advantages and disadvantages of PHS. Some conclusions are  presented in Section V. II   P ROFILES OF W IND G ENERATION AND L OAD C ONSUMPTION  In principle, large scale deployment of variable RES, namely wind, doesn’t necessarily imply the ins tallation and/or reinforcement of the energy storage capability. This is  justified by the fact that several European countries have already experienced a pronounced growth in wind capacity without changing (or even planning) their energy storing capability. Nevertheless, the benefits of coordinating wind generation (especially for high penetrations) with hydro generation in countries and markets has been recognized and extensively utilized by most system operators that possess that capability [2]. The fact that wind generation has little or no power regulation capability introduces the concern of excess wind generation during periods of reduced load. That concern is amplified when some countries such as Denmark, Portugal and Spain exceeded a wind energy penetration of 15% that, as a natural consequence, led to periods when wind power and other RES (added by the required reserves) was sufficient to match the entire system demand. Moreover, several countries have already felt the need to curtail wind generation during periods of excess generation, e.g. Spain [3], while Ireland has introduced an operational limit of 50% Energy Storage for Wind Integration: Hydropower and other contributions Ana Estanqueiro,  Member IEEE, Atle Rygg Årdal, Ciara O’Dwyer, Student Member, IEEE   Damian Flynn, Senior Member, IEEE  , Daniel Huertas-Hernando,  Member IEEE  , Debra Lew,  Member IEEE,  Emilio Gómez-Lázaro,  Senior Member IEEE  , Erik Ela,  Member IEEE,  Javier Revuelta, Juha Kiviluoma, Student  Member IEEE,  Luis Rodrigues, Mikael Amelin,  Member IEEE   and Hannele Holtinen,  Member IEEE    T    penetration from non-synchronous sources. Other countries, including Denmark and Portugal [4], have maintained wind  power plants in operation with zero (and even negative) market prices. From the above, and considering a purely energy balance approach, it becomes reasonable to conclude  –   apart from associated investment costs, ancillary service tariffs and market value issues  –   that using wind power plants in conjunction with energy storage units would be a desirable option for any power system with a relevant share of wind (and other) renewable variable generation. Some countries have a higher correlation between wind (and other variable renewable) generation and load consumption than others. Moreover, geographical, climatic and other socio-economic constraints strongly affect the shape of the consumption load pattern. Fig. 1 depicts two typical daily  profiles for consumption and wind generation in IEA Wind Task 25 countries. Figure 1 Typical normalized daily load and wind generation profiles (annual data).  While countries in northern Europe, such as Norway and Finland, show a “flat” load profile (curve A) others, namely Portugal, Ireland and Spain, present a greater variation from  peak to no-load hours (curve B). Although these load  profiles based on annual data are naturally smoothed by the intra-annual climatic variations, when compared with the two typical wind generation daily profiles depicted, the lack of correlation between wind and load becomes clear for most countries that follow a type “Wind 1” curve and only a few, such as Ireland and Norway show a positive correlation  between wind and load (Wind 2). In Ireland the correlation  between the daily wind profile and the demand clearly favors wind integration in a country that operates a near isolated  power system. The less marked daily wind power profiles obtained from wind power simulations in Nordic countries [5], allied to a smoother demand profile may indicate a reduced need for added energy storage. Although energy storage usually follows a daily pattern, its full range of operation, especially for hydro storage, also addresses weekly and seasonal patterns of consumption. Fig. 2 depicts typical high RES weekly profiles (run of river, ROR, wind and others) which periodically exceeds consumption during no load hours. Figure 2 Normalized weekly load and high wind generation typical profiles. An annual pattern of load with respect to wind generation is  presented in Fig. 3. It is clear that the country with load  pattern B (typical of northern Europe) has a variable seasonal consumption, while the country with load A (from southern Europe) shows an almost constant pattern. For the annual profile both Nordic and Southern wind profiles show a positive correlation with consumption. Figure 3 Example of two normalized annual consumption and wind generation profiles. Figures 1 to 3 highlight the daily, weekly and intra-annual load patterns that, together with the wind generation daily and seasonal variations, enable to visualize the technical difficulties in operating power systems with high wind  penetration  –  if energy storage or import/export capacities are not available. III   S TORAGE OF WIND POWER  :   P UMPING H YDRO A  ND O THER T ECHNOLOGIES .   In some high wind penetration countries (Portugal, Spain and Denmark) there is already some experience in handling excess wind energy and other RES during periods when generation exceeds load consumption. In others (e.g. US), the high load factor of transmission lines with periodic occurrences of local grid congestion makes storage almost mandatory with the systematic growth of wind capacity that is being installed. This section highlights different approaches of IEA Wind Task 25 countries towards using storage capacity as a means to optimize wind integration while adding value for wind energy.  A.    Portugal and Spain In 2007 Portugal conducted a review of the deployed hydro capacity versus the existing potential having in mind the growing installed capacity of wind power and other variable non-dispatchable renewable sources. This study, PNBEPH [6] characterized in detail the technical, environmental, social and economic conditions of 25 plants (including plants already operating and new projects) and identified 10 plants to be repowered and/or constructed that would enable to increase the hydro capacity to be installed by the end of 2011 from 5.85 GW to 6.95 GW in 2020. From the ten hydro  plants presented in Table 1 that will add approximately 1.1 GW of hydro capacity, eight new/repowered hydro power stations will be reversible with a total new pumping capacity close to 800 MW, that will add up to the already existing 1.2 GW in 2011. Pumped hydro stations (PHS) in Spain account for around 5 GW (2.75 GW of pure PHS, with 77 GWh capacity), being the European country with the highest PHS capacity installed. Effective total pump load rarely exceeds 3.5 GW  because of hydro constraints, high maintenance, or market driven strategies.    TABLE   I   C HARACTERISTICS OF THE NEW ( REVERSIBLE )  HYDRO PLANTS . Hydro Power plant Type Storage cap. (hm3) Turb. Rated Power (MW) Nominal Energy (GWh/y) Pump Rated Power  (1)  (MW) Actual Cost of Inv (  €/kW)   Foz Tua Rev. 310 234 340 255 822 Fridão - 195 163 299 886 Padroselos Rev. 147 113 102 77 943 Gouvães Rev. 13 112 153 115 951 Daivões Rev. 66 109 148 111 1404 Vidago Rev. 96 90 114 86 1256  Almourol - 20 78 209 1353 Pinhosão Rev. 68 77 106 80 1455 Girabolhos Rev. 143 72 99 74 1472  Alvito Rev. 209 48 62 47 1445 (1)Estimated pump rated power. Actual pumping capacity still under assessment.   The proposed PHS developments for the next years are 3 GW of new PHS in 2020, from potential projects amounting to nearly 6 GW between pure PHS and mixed inflow PHS (conventional hydro with reversible turbines). However, nowadays the operation of the PHS stations carries with regulatory barriers, since the TSO cannot operate directly energy pumped PHS stations, whose management is market driven, unless system security is at risk. It is a matter of discussion whether such a management will maximize the integration of RES, which suggests addressing this regulatory issue in the future, as RES integration difficulties increase.  B. Ireland Ireland has one large storage plant (pumped hydro), Turlough Hill, with a capacity of 292 MW, consisting of 4 x 73 MW units, and can generate at full power for 5 hours. The plant is used to fill the night time valley and can act as a fast source of reserve. A further 70 MW of pumped hydro has also been contracted, while a compressed air energy storage (CAES) plant has been proposed in Northern Ireland ( 100 MW). A time of use tariff called NightSaver, available to residential and business customers in Ireland, offers a reduced overnight tariff. Particularly during the winter months this promotes thermal storage by appliances such as storage heaters and night time water heaters. Demand increases in excess of 100 MW can be seen during the winter months after 11 p.m.  B.   USA In 2010, the U.S. had 79 GW of installed hydro capacity (about 8% of total capacity) and an additional 22 GW of  pumped hydro storage. In some regions, there are environmental and other operating restrictions that limit the ability of hydro plants to contribute to balancing of wind or net load. In 2010, the U.S. had 39 GW of wind capacity, representing about 4% of total capacity. There is interest in energy storage, to accommodate variable generation and for other reasons, and a number of PHS projects are actively  being developed. One of the Balancing Areas (BAs) with the highest  penetration of wind is Xcel’s Public Service of Colorado which sees a 12-13% annual average wind penetration which can reach 55-56% on an hourly basis. They are pursuing a move to faster interchanges with neighboring BAs to help them accommodate the wind variability and reduce wind curtailment. Incidentally their 10% wind integration study showed that using their 300 MW pumped hydro unit to  balance net load instead of load would reduce their wind integration costs by 26% [7]. Currently, in the U.S. areas with restructured electricity markets, pumped storage hydro generally must let the market know whether or not it would like to be operated in generation mode or pumped mode [8]. The operator of the storage plant therefore will usually give the scheduling of mode to the ISO based on the time of day (e.g., based on the load being high during the day and low at night). The ISO can then run the resource or keep it offline based on the market outcomes. The one exception is in PJM, where the full optimization of the pumped hydro storage plant is made  based on meeting the load at least cost considering the  pumped storage plants technical constraints [9]. However, even in this case, the decision is made in the day-ahead market, and therefore it is difficult to change the operating mode when large variable generation forecast errors occurs.  No market in the U.S. currently has the capability to change the operating mode of pumped storage during the real-time markets. Many of the areas are evaluating the complexities of being able to fully optimize the pumped storage plant from the system level, and to be able to update the decision as it gets closer to real-time.  D. Finland, Norway and Sweden The synchronous power system of Finland, Norway and Sweden is also integrated in an international electricity market. The total installed hydro power capacity in these countries is about 49 GW. Most of the larger hydro power  plants have reservoirs, where the storage capacity is ranging from a few hours of generation to seasonal storage. Pumped storage hydro is not common in the Nordic area. In Norway, there is 1.3 GW of pumped storage plants, primarily used for seasonal pumping, and the pumped hydro contribution in Sweden and Finland is negligible. However, balancing of wind power can instead be performed in the conventional hydro power units, by decreasing hydro generation during windy periods (storing more water in the reservoirs) and vice versa. The balancing capacity of the conventional hydro  power is considerable; the total storage capacity of the reservoirs is approximately 121 TWh, which can be compared to that in 2010 the total wind power generation in the three countries was 4.6 TWh. Another option for the Nordic system is to use heat storage in district heating systems for balancing fluctuations in the  power system. The stored heat will not be converted back to electricity  –   it will be used as heat in different end uses including space heating and cooling, industrial process heat, commercial heating and refrigeration, as well as household hot water usage. Heat use is responsible for a large share of  primary energy consumption and therefore presents a large  potential for power system flexibility. In order to get any  benefits for the power system, heat (or cool) has to be  produced with electric resistance heaters, heat pumps, or combined heat and power (CHP) plants. A combination of electric resistance heaters and heat storage can be a relatively inexpensive way to deal with the low residual demand situations. A heat storage with an electric heater can decrease boiler fuel use more than a plain electric heater could do by providing room for the excess heat during  periods of low power prices. Heat pumps are rather capital intensive and therefore require   high amount of full load hours to be profitable. At the same time, their operation during high power prices is expensive and in cold climates they will suffer from lower COP (co-efficient of performance) during high prices due to cold weather. Fuel based boiler can be used as a supplement, but the investment cost could be lowered with a heat storage. Moreover, during low heat consumption, heat pumps will be forced to produce at low capacity and hence low efficiency. In these situations, heat storage can be used to operate the heat pump intermittently at full capacity. CHP plants often have restrictions in operation due to the need to serve the heat demand. Heat storage can break this bond and by so doing, liberate the CHP plant to follow power price signals. This will be important during low residual demand situations, as CHP units can be shut down or operated at minimum load. Heat storage can also decrease the need for cycling of CHP units, which is likely to increase when the share of variable generation increases. IV   C OST -B ENEFIT A  NALYSIS OF W IND I  NTEGRATION U SING E  NERGY S TORAGE  For a particular system the challenges for integrating large amounts of variable RES depend mainly on: how often and how deep the demand valleys occur and their correlation with variable RES production; the flexibility of other power sources; management of grid congestion;  possibility to trade with neighboring systems. Major challenges to balance wind power and other RES occur in situations when the share of wind power is high (often in windy, low load situations), trading with neighbors is limited and the flexibility from other power stations in the system is constrained. An important factor is also whether it is necessary (required) to maintain large power reserves.  A. Portugal and Spain Portugal has currently (March 2011) 4.2 GW of wind power installed and had total installed capacity of 17.9 GW by the end of 2010. The minimum and maximum loads are 3.4 GW and 9.4 GW respectively and the only interconnection capacity is to Spain, with 1.3 GW. The Spanish total installed wind power capacity by 2010 was 20.1 GW. The minimum and maximum load were, in 2010, 18.2 GW and 44.1 G W −the maximum in January 11th, same   day in Portugal− respectively. The interconnection capacity to neighboring countries is 1.3 GW with Portugal, 1.4 GW (import) or 400 MW (export) with France and 900 MW with Morocco. The minimum and maximum wind production levels during 2010 were in Spain, 191 MW and 14.9 GW, respectively. During 2010, 17% and 16% of the energy consumption was covered by wind power in Portugal and Spain, respectively. Table II illustrates recent very high penetration situations in the Iberian countries of wind and other sources with no  power regulation capability (e.g. run of the river plants, industrial CHP with IPP contracts and PV power stations). As it is clear from Table II both the Portuguese and the Spanish systems  –   that share a common electricity market, MIBEL - have been operated several periods with more than half of its demand covered by wind generation, with record values in Portugal reaching 85% on the 13 th  of November 2011 and in Spain on the 6 th  November, 2011 with 59.6 % of the instantaneous demand fed by wind. In Spain wind power supplied 20.8 % of the demand during the month of March, 2011, making it the technology with the highest energy  produced during that month while in Portugal during the autumn/winter (from November to March) the monthly energy penetration has been systematically above 25% since 2009. T ABLE II I BERIAN EPISODES OF HIGH SHARES OF WIND AND OTHER RES   ( AVERAGE HOURLY POWER  ). Day Min. Load [MW] Min. Load & Pump (no exports) [MW] Max. Wind Power [MW] Max. Non-regul. Power [MW] Max. Wind Penetr. [%] Max. Penetr. Non-reg. P (incl. PHS. & export) [%]     P   o   r    t   u   g   a    l 15.Nov.09 3708 4365 2785 3958 70% 78% 31.Oct.10 3862 4137 3182 4093 75% 90% 15.May.11 3727 4206 3115 3811 81% 102% 13.Nov.11 3835 4401 3694 5011 85% 117%     S   p   a    i   n 24.Nov.08 20700 24200 9.300 17000 45% 35,5% 8.Nov.09 19000 21950 10250 13700 54% 42,8% 10.Nov.10 22900 25900 11300 15800 50% 41,8% 6.Nov.11 20800 23500 12000 13400 58% 47,3% Fig.s 4 and 5 depict the contribution of PHS in Portugal and Spain in handling the excess of non-dispatchable generation, and reducing (for Spain) the need to curtail part of the available primary wind resource (which was however needed). Such a situation occurred in Spain during approximately 200 hours in 2010, with an estimated curtailment of 0.6% of the annual wind resource. (a) (b) Figure 4: Portuguese load and generation profiles for a (a) high wind day (13/11/2011) and (b) average wind (28.10.2011) [10].   (a) (b) Figure 5: Spanish load and generation profiles for a) average wind (26/10/2010) and b) high wind (06/11/2011 [11]. The studies conducted by REE forecast in 2020 a frequency of occurrence of these situations ranging from 400 to 1400 hours, and the curtailment of 1 to 6% of the primary resource of RES, including the expected installation of new PHS. Similar studies show for Portugal in 2020 an excess of RES generation totaling 457 GWh that will represent a -5,00005,00010,00015,00020,00025,00030,00035,000     M    W Hourly demand coverage -06/11/2011 Estimation RES curtailmentsWindSolar CSPSolar PVMini-hydroCogen & WasteNet importsHydro Pump Stor.Hydro ConventionalCCGTCoalFuel&GasNuclearNet exportsPump loadDemand + Exports (Excl. Pump)Demand (Excl. Pump&Exports)   l l - . llll l l.   Estimation RES curtailmentsWindSolar CSPSolar PVMini-hydroCogen & WasteNet importsHydro Pump Stor.Hydro ConventionalCCGTCoalFuel&GasNuclearNet exportsPump loadDemand + Exports (Excl. Pump)Demand (Excl. Pump&Exports)   curtailment of 65 hours at full power if no storage is assumed (484 hours of partial power). Planned PHS for 2020 has the capacity to avoid RES curtailment in 39% of those periods. In Spain these high levels of wind penetration were possible thanks to several factors, such as a good monitoring from the CECRE (Control Centre for Renewable Energies), the  possibility to issue curtailment set points to all wind farms, the possibility to export part of the RES production surplus, and the contribution of PHS to increase the system load. In Portugal, where wind curtailment is still not applied, the  positive factors were the operation of the Distribution Centre for Wind Power that aggregates and controls a representative  part of the wind generation, the high contribution of PHS in articulation with wind (bilateral contracts between wind  power and hydro plants already exist) as well as exporting the RES surplus (almost at zero market price). The dynamic limits to the integration of RES in the Iberian system will be in the flexibility of the remaining generation, including thermal and hydro power plants. The contribution of PHS to increasing these limits on specific difficult situations is very beneficial both for the Portuguese and the Spanish sub-systems, and PHS is highly valued. Although Portugal already defined a plan to repower and install new PHS [6], the social and environmental implications of these  projects, as well as some risk in the return of investments in the present market, may delay or prevent a successful installation of some of the potential projects in both Iberian countries having, as a consequence, a higher curtailment of RES in Spain and a negative economic impact in Portugal, to allow for high yearly penetrations. The future market regulations and signals will play a key role in attracting the adequate investments, something a holistic cost/benefit analysis of the installation of PHS with respect to higher RES curtailments will indicate. PSH is considered as a highly valuable generation technology in the Portuguese system as well as the Spanish generation mix for the 2020 horizon and beyond, and is highly promoted in Spain by the regulator and the system operator. However, the regulatory  barriers not ensuring the return on investment in such volatile wholesale markets, and the social and environmental  barriers, do not ensure that all sustainable projects will be  built in the medium term.  B.Ireland The power system of Ireland and Northern Ireland has 1.73 GW of wind generation. There are 9.1 GW of conventional generation subject to central dispatch and a further 230 MW of embedded non-wind generation which is not subject to central dispatch. DC interconnection to Scotland also  provides an import capacity of 450 MW [12]. The maximum wind power generated on the island was 1.4 GW on 2nd November 2011 [13] which accounted for 39% of demand at that time. Wind power has reached 50% of demand on a number of occasions, limited by the non-synchronous penetration limit for the combined system. Wind provided 10.5% of electricity demand in 2010 and is targeted to rise to 37% by 2020. From Jan  –   Nov 2010 approximately 1% of total available wind energy was curtailed (26 GWh), the vast majority of which occurred in the second half of the year. This was due to the combination of Turlough Hill being offline since July 2010 (leaving no storage capability on the Irish system), an increase in the level of installed wind (200 MW) and an increase in capacity factor for the wind generation (20% up to 25%) [14]. An example of wind curtailment can be seen on 11 November 2011 (Fig. 6). The potential wind penetration exceeded 50%  before being curtailed (by over 1 GWh energy) to accommodate the demand night valley and adhere to the 50% system non-synchronous penetration limit. Recent studies on a 2020 scenario for Ireland have shown CO 2  emissions to grow with increasing storage capacity due to the increased participation of base-load plant, including coal plant in Great Britain, accessing the Irish system through DC interconnection [14]-[15]. The unprofitability of merchant storage units participating in energy arbitrage under current market conditions was also highlighted. Reduced curtailment in high wind scenarios using storage was demonstrated. However, due to high capital costs and low round-trip efficiencies, storage was not economically  justified until 50% of energy was supplied by wind [14], not considering alternative options such as demand side management and improved wind forecasts. Opportunities arising from additional benefits of storage, including fast start up and response times, have yet to be evaluated. For example, due to the significant reserve contribution which can be provided by storage (particularly at times of low load) the system can be operated with less synchronous generation online, although concerns arising from lower system inertia may result [12]. Fig. 6. Wind curtailment on Irish power system (11 November 2011).  Energy arbitrage alone does not justify the capital costs involved in large scale pumped hydro plants in Ireland. Considering additional value streams may improve the  business case for such projects. An upcoming review of ancillary services payments in Ireland [12], against a  background of increased flexibility requirements from high wind penetrations, may provide an opportunity for storage. C.   USA Pumped storage plants, like many other storage technologies, also provide a number of valuable services besides providing energy at high need times, which may often allow them further value streams than what is received from energy arbitrage. This may include voltage support, frequency control, energy balancing, transmission congestion, and capacity value. For example, they usually have fast ramp rates, and can be able to start and synchronize to the grid from offline mode in less than 30 minutes. This can allow them to provide balancing support and various forms of frequency control. Pumped storage can provide contingency reserve by allowing for head room in generating mode, as well as in pumping mode by shutting off, thereby cutting the  pumping demand (which is essentially the same equivalent as increasing power). Pumped storage plants also can provide very valuable secondary control reserve on automatic 1000   1500   2000   2500   3000   3500   4000   00:00   04:00   08:00   12:00   16:00   20:00      W   i   n    d   &   D   e   m   a   n    d   P   r   o    f   i    l   e    (   M   W   Wind Generation   System Demand  
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