Conference Papers
Renewable Energy and Energy Storage Systems in Rural Electrical Power Systems: Issues, Challenges and Application Guidelines
Steven C. Smith P. K. Sen Benjamin Kroposki Keith Malmedal
Paper No. 10 B4
978-1-4244-5473-0/10/$26.00 ©2010 IEEE B4
Renewable Energy and Energy Storage Systems in Rural Electrical Power Systems: Issues, Challenges and Application Guidelines Steven C. Smith1, Member IEEE; P.K. Sen2, Sr. Member IEEE; Benjamin Kroposki3, Sr. Member IEEE; and Keith Malmedal4, Member IEEE
Abstract- There is an increasing demand both by legislation and the public for a more secured, reliable and efficient power system using dispatchable and non-dispatchable renewable resources. However, the existing design and operational practice of the electrical power grid does not lend itself easily to the incorporation of non-dispatchable renewable energy resources. Distributive Electrical Energy Storage (DESS) is a key to the development and future of all non-dispatchable renewable energy resources in the electrical power grid. This paper provides an overview, discusses the state-of-the-art status and will introduce how DESS can be used to incorporate non-dispatchable renewable resources into the power grid and also provide additional benefits to the power system. Index Terms—Distributive Electrical Energy Storage Systems, Battery Energy Storage System, Battery, Renewable Energy, Energy Storage, Distributed Resources.
I. INTRODUCTION Today renewable energy, global energy sustainability and climate change is a very common topic of discussions in the media, politics, society-at-large and academic institutions. Going Green through the use of renewable energy resources and make the planet sustainable and by so doing improve the air quality and environment is a common thread to the new paradigm shift in the thinking process, how electricity will be delivered in the twenty first century. Even in the monthly residential bill, the utility companies are starting to identify their portfolio of power generation and offer the ability to pay for newer renewable energy resources. To many this may seem as progress in making our power systems more efficient, flexible, secure and environmentally friendly. To this end the United States government through legislation has required that by the year 2030 that 20% of the electrical energy generated in the U. S. will be by renewable resources. The specifics and practicality of how to accomplish this, however, have not been determined and perhaps thought through. There are many different types of renewable energy resources such as hydroelectric, biomass, wind, solar, tidal and geothermal. The key to the usage of renewable resources is
that they are replenished by nature. In addition they also have the advantage of having low or no emissions of carbons which make them more environmentally friendly than their fossil fuel counter parts. This, however, is only part of the total story as seen by electric power systems engineers predominantly responsible to deliver the final product. Renewable resources can be categorized into two major categories: dispatchable and nondispatchable. Hydroelectric, biomass and geothermal are dispatchable resources, whereas, wind, solar and tidal waves would be classified as non-dispatchable resources. The key difference between the two categories is the controllability of electric power. The dispatchable resources, in general, have the energy stored, and could therefore be called upon at any given time to produce power. The non-dispatchable resources, on the other hand, inherently do not have any control of the input energy for later use when needed. One of the main concerns about using renewable resources such as wind, solar and tidal is their inability to dispatch power on demand. For example the wind can stop blowing or the sun can go behind a cloud which can significantly affect the power output. By their very nature these renewable resources are more variable than fossil fuel, nuclear, biomass, and hydroelectric power plants. This lack of control over the input is what causes the variability in power output. This basic principle of dispatchable versus non-dispatchable without any storage is one of the key concern. II. POWER SYSTEM BASICS The US electric power grid is the largest and one of the most complex electrical systems in the world today. Real and reactive power is generated by thousands of synchronous generators operating in parallel to create ideally a fixed frequency and voltage system. The basic principles that govern the real (P) and reactive power (Q) flow in the system can be described for a simplified two-bus power system without any resistance by the following equations. P1, 2 = (V1V2 / X) sin
The Power Systems Energy Research Center at Colorado School of Mines, Golden, CO 80401, USA (www.PSerc.org) partially supported this research and in the development of this paper. 1
Steven C. Smith is with Lockheed Martin, Littleton, CO 80122, USA (email:
[email protected]). 2 P.K. Sen is with Colorado School of Mines, Golden, CO 80401, USA, (e-mail:
[email protected]) 3 Benjamin Kroposki is with the National Renewable Energy Laboratory (NREL), Golden, CO 80401, USA (e-mail:
[email protected]) 4 Keith Malmedal is with NEI Electric Power Engineering, Inc., Arvada, CO 80001, USA (e-mail:
[email protected])
(1)
Q1 = (V12 - V1V2 cos ) / X
(2)
Q2 = ( V1V2 cos - V22) / X
(3)
Where X is the reactance and is the power angle [31][32]. In a physical system real power flows from a higher power angle to a lower power angle and the reactive power flow is controlled by the voltage in the system. Positive reactive power flows normally from bus (1) to bus (2) when V1 > V2, whereas, reactive power flows from bus (2) to bus (1) when
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V1 < V2. In most cases in a physical system all variables are fixed by the design except for the power angel and the bus voltage V [32]. The bus voltages with many parallel generators, however, is generally fixed but the induced voltage in the synchronous generator of the generator can be raised and lowered within certain tolerances to provide the reactive power needed to the system. The real power, hence the system frequency, is related to the speed and torque of the generator’s prime mover. Governors and automatic frequency controllers adjust the speed of the generator’s prime mover to maintain a constant system frequency and provide a proper division of power between the systems generators. The voltage and the reactive power output of the generator are controlled by the adjustment of the generator’s field current usually using an automatic voltage regulator. The total amount of real and reactive power that can be produced by a synchronous generator is limited by its physical capabilities. The limiting factors are the heating limits caused by the armature and field currents [31]. The adjustment of the governor (p-f controller) and the field current (Q-V Controller) of the generators provide the main production and control of the real and reactive power of the system. There are, however, other specialized equipment in the transmission and distribution system to help control the flow of power and reactive power in the grid such as synchronous condensers/generators, capacitors and FACTS (Flexible AC Transmission Systems). This discussion however is beyond the scope of this paper. In order to understand some of the complications that large penetration of renewable resources in the grid can cause, it is needed to understand how power (and reactive power) flows in the system. As discussed above the real and reactive power flow is controlled by the power angle (prime mover torque) and the voltage relationships. These are directly controlled by the governor and field current in the synchronous generators of the system. These basic principles and Laws of Physics are largely forgotten when discussing renewable resources. Non-dispatchable renewable resources usually do not have the capability to provide these controls needed to maintain the stability and quality control of the power system. The idea that they are not an equivalent replacement for a synchronous generator is usually not discussed. Non-dispatchable renewable resources are not designed to provide these functions to the power grid. They could be designed to provide these functions if the right amount of energy storage and controls were added. This, however, would significantly increase the cost of renewable resources. Modern power electronics can provide a current and voltage output in all four quadrants and energy storage can be provided depending on the amount and duration of energy needing to be stored. III. WIND Generation of electricity utilizing wind has been around in the US since the 1890’s [30]. The vast majorities of all the installations are small scale and isolated from the power grid. Currently larger scale wind farms are being planned and introduced in many different areas throughout the US wherever, it is economically justifiable. There are many factors that affect the power output of a wind generator such
as wind speed, air density, altitude, temperature, tower height, terrain characteristics, type of generator, number of blades, blade size, rotor efficiency, gear box efficiency, controls, wind speed distribution, to name a few [30]. The fundamental equation that shows the maximum power that can be extracted from the wind by the blades is given by the following equation. Pb = ½ A v3 Cp
(4)
Where, Pb is the power delivered by the rotor in watts, density of the air at 15ºC at 1 atm (=1.225/kg/m3), A is the cross sectional area the wind passes in m2, v is the wind speed in m/s, and Cp is rotor efficiency [30]. A simple observation from Equation (4), it can be seen that wind power increases by the cube of the wind speed [30]. There are, however, constraints to how much power can be extracted from the rotor. For example the maximum theoretical rotor efficiency of 59.3% is reached when the blade efficiency reduces the input wind speed to one third of its original speed. This maximum theoretical rotor efficiency is called the Betz Efficiency. Currently however rotor efficiencies are in the range of 40 to 50% and when the gear box efficiency is taken into account the overall machine efficiency is usually between 25 to 30% [30]. Therefore in order to generate more power from the wind higher wind speeds are needed. Higher and more consistent wind speeds are located at higher altitudes than those closer to the ground. Wind turbines are being located on high towers at the height of (typically) 50 meters in order to gain access to higher more consistent wind. Wind is classified into seven power classes ranging from 1 to 7 with seven being the highest wind speed classification. A map of the U. S. wind resources by NREL is shown in the appendix. According to the meteorological data, there are many areas of the US that are favorable to wind power generation. Although wind is available to produce power, the issue of its availability is of concern to the power system engineer. In order to determine the actual wind speeds throughout the year would require measuring the wind at the site. This would provide a statistical distribution of the wind speeds over the year. This data could be used to determine how much energy could be extracted throughout the year by the wind turbine or wind farm. This type of data may not always be available so a probability density function can be used to estimate the winds profile with relatively good accuracy [30]. As stated this will give the energy that can be produced throughout the year in kWh/yr. However the power system requires daily and even hourly planning of generators in order to meet the load demands. In order to accomplish this planning a daily profile of the wind velocities is needed. This profile is called the diurnal wind pattern. From Figure 1 it can be seen that the wind speed varies during the day. This one shows a favorable pattern to the daily power usage. Even this favorable pattern is still falling off during the period of peak power usage of 5 to 9 PM. Since the fuel or wind in this case cannot be controlled it needs to be predicted in order to determine when and how the power can be used in the system. Each location will have a
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different diurnal wind pattern and a massive control strategy is needed to optimize the wind source to the load profile.
Fig. 1. Diurnal Wind Pattern at Livingston, MT by NREL [34]
IV. PV AND CONCENTRATED SOLAR POWER (CSP) A detail comprehensive discussion of the other important renewable source of solar power is beyond the scope of this paper. Like wind, solar power availability depends on the location of sun, cloud, the angle at which the sunrays fall on the earth surface, etc. Needles to say, there is no solar power when there is no sun. This non-dispatchable and intermittent nature and the availability makes it difficult, if not impossible, to meet the electricity demands. This could, however, be integrated with the existing system to provide some smaller portion of total energy sources. There are a plethora of literatures and books are available discussing the available solar energy reaching the earth surface, the energy density and similar information. There are two types of solar energy systems currently available for commercial use: photovoltaics (PV) and concentrated solar power (CSP). CSP could be designed with or without the thermal storage. These are promising technology and have some many useful and cost effective applications. But in the grand scheme of total electricity usage in the US, they make insignificant contributions today. The electric energy usage growth has been around 2.0% for the past two decades (attributed to the growth in population and to the growth in residential sector electricity consumption and quality of life). It is foreseeable that the renewable energy sources like solar and wind could make substantial contribution to accommodate this growth rate. But the idea of replacing the existing power grid with renewable energy sources is impractical and completely unsubstantiated both from economical and operational point of view.
kept in standby and/or spinning reserve mode to augment the lack of power produced if needed. Combustion turbines such as diesel or natural gas turbines are smaller that thermal generation plants and can be brought on line quickly for this type of operation. However when it is mandated by law that 20% of the total generation of electricity has to come from renewable sources capacity by 2030, this is a vastly different problem. The current [2008] installed generating capacity of the US electric power system connected to the grid is approximately 1,000,000MW and the total electricity usage is approx. 4,000TWh. Twenty percent of today’s number would be approximately 800TWh and it doesn’t include the growth rate. One other implicit factor that must be taken into account, that the capacity factor (a measure of the installed capacity vs. the energy produced) for these non renewable energy sources is very low (20% for solar and 35% for wind) compared to the continuously running nuclear (90%) and coal-fired thermal power plant (80%). If a significant fraction of electricity has to be produced from these non-dispatchable resources such as wind and solar, a large amount of spinning reserve would be required to provide power when needed. Operating thermal plants at a partial load reduce their efficiency and increase their specific fuel consumption as well as their specific CO2 emissions [35]. There continues to be many advances in the design of wind generators to improve wind as a viable resource to the grid such as improved induction generators, doubly feed induction generators, gear boxes, controls, power electronics and even small scale DC energy storage in the turbine for short term ride through capabilities and stability. Even with these advancements the solution to the short to long term loss of power has still not been solved. Currently these renewable resources cannot be operated the same way as typical synchronous generators where the prime mover is controlled the produce the real and reactive power needed. This one simple difference in power generation capability is a cause for great concern. The inability to generate power on demand causes reliability and stability issues to the system. This spinning reserve margin is usually in the range of 10 to 15% of the total load. The existing power system does not have large electrical energy storage or system margins to compensate for the additional variability of non-dispatchable power generation. If 20% of the total electric power generation in the system is variable or intermittent, the margins of the system will not be able to handle the large loss or variability of power in global sense and therefore require some sort of additional spinning reserve or energy storage in the system. VI. ENERGY STORAGE
V. ELECTRIC POWER GRID AND FUTURE As mentioned earlier, the expectations of the politicians and the public-at-large that today the country could be powered by only renewable energy sources is completely wrong. Whether this could be accomplished in the next 50 years is questionable at best unless there is a major breakthrough in technology and a massive change in energy usage (energy conservation and improved energy efficiency) along with the philosophy of life due to very high cost. This is where many of the power system issues come into play. Other dispatchable resources must be
Energy storage can be defined as the stock piles of fuel such as coal, oil, natural gas, diesel and gas or the storage of energy in electrical, chemical or mechanical form. By using more renewable resources in the system it is assumed that less energy stock piles will be consumed by traditional power generating facilities. However the amount of saved energy stock piles produced by having renewable resources generating power such as wind and solar are not equivalent to a direct replacement of the fossil fuel generators. Their inabilities to supply real and reactive power on demand like
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traditional generators require that other generators be held as spinning reserve to compensate for this issue. However one solution to this issue is to store the energy generated by the renewable resource and save it for later use (available energy and load balancing act). This would require the addition of Electrical Energy Storage Systems such as Battery Energy Storage System (BESS), Flywheel Energy Storage System (FESS), Superconducting Magnetic Energy Storage (SMES), Pumped Hydro Storage, Compressed Air Energy Storage (CAES) and the Ultracapacitor. Currently the majority (more than 95%) of all large scale energy storage system in the US are pumped hydro facilities, which has been in existence since 1920’s. All the economically viable sites are essentially fully developed and there is very little room for additional pumped hydro in the US. There are, however, a potential for a few other energy storage facilities such as the CAES facility located in Alabama. The rest however are mostly BESS systems. These smaller facilities are at distribution levels and are not designed for large scale transmission level energy storage. In fact, other than pumped hydro there are no other energy storage facilities that can currently store the large amount of energy that could be created by a large wind or solar installation. The other electrical energy storage device that is currently available and can be installed almost anywhere in the power distribution system is the Battery Energy Storage System (BESS). BESS systems come in various sizes and storage capacities depending on the design and type of battery used. In Table 1 in the Appendix shows all the current BESS installation over 1 MW and 1 MWh in size. BESS systems of this size by themselves cannot provide the load leveling and spinning reserve needed for a large renewable resource like wind and/or solar farms. However if they are distributed throughout the distribution system they may be able to provide the necessary electrical energy storage to alleviate the need for additional spinning reserve generators and provide greater stability and reliability to the system at the distribution and local level. A number of such applications (microgrid and islanded system) has been designed and tested. However, these are all designed for a local solution rather than solving the “big picture.” The basic model of a Distributive Electrical Energy Storage System Model is shown in Figure 2. The details of sizing and performance on these Distributive Electrical Energy Storage Systems (DESS) in the Power Distribution System are the subject of current research by the authors and will be presented in future papers.
shaving, voltage regulation, VAR support, frequency control, spinning reserve and power quality mitigation that the power system so desperately needs.
Fig. 2. Distributive Electrical Energy Storage Model (DESS)
The addition to the BESS, storage system in the power distribution system offer many additional benefits. They not only can supply the benefits of load leveling, frequency control, voltage regulation, etc they can also provide a more controllable and dynamic energy supply or load to the system as needed. BESS are uniquely qualified to perform this function. Their ability to store or supply energy for long or short periods of time is ideal. In addition the BESS can be located very close to the load such as in neighborhoods, buildings, or substations. This adds another dispatchable asset to the “local” power grid that can be either a source or load as needed. The ability to provide distributed electrical energy storage throughout the power system provides a key function to the evolutionary development of the overall power system and in particular to the distribution systems. The distribution system is currently a radial feed system which means the power only flows in one direction from the source to the load. By installing BESS into the distribution system they can also serve other functions similar to Flexible AC Transmission System (FACTS) devices used in the transmission system to providing dynamic voltage control, oscillation damping, improve transient stability, active and reactive power control, and frequency control [21]. These functions will need to be provided in the distribution system if it is going to mimic the transmission system’s ability to dispatch power bidirectionally. This will also help provide the means to allow the dispatch of distributive generation and renewable resource in the distribution system.
VII. APPLICATIONS Currently there is a relatively few large scale Energy Storage Systems in the world other than pumped hydro facilities relevant to the transmission level. Overall Battery Energy Storage Systems have the most installations energy storage facilities other than pumped hydro. Their numbers however are not large when considering installations over 1 MW and 1 MWh in size as shown in Table 1. Their ability to provide energy storage in the distribution system however should not be underestimated due to the lack of a large number of installations. They have shown their ability to provide many of the auxiliary functions such as load leveling, peak
VIII. CONCLUSION The key to incorporating non-dispatchable renewable energy resources in large quantities in the power system is Distributive Electrical Energy Storage System (DESS). DESS can provide load leveling functions to the distribution system to help maintain reliability and stability to the consumer. In addition the DESS can help make renewable resource appear as a normal synchronous power generator to the load with its ability to change the real and reactive power output. However, a tremendous amount of research is needed, before
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the DESS is an integral part of the new way electricity will be delivered, as more and more non-dispatchable renewable energy sources penetrate the existing power grid. With the current state of technology, 20% renewable energy penetration as mandated in 35 states by 2020 (or by 2030) is not a viable solution. It is an impossible task and this ill conceived solution doesn’t have the sound foundation of any good electrical power practicing engineers. IX. REFERENCES [1] [2]
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[18] Benjamin L. Norris, Jeff Newmiller, and Georgianne Peek, NAS Battery Demonstration at American Electric Power – A Study for the DOE Energy Storage Program, Sandia National Laboratories, Sandia Report SAND2006-6740, March 2007. [19] Ali Nourai, Installation of the First Distributed Energy Storage System (DESS) at American Electric Power (AEP) – A Study for the DOE Energy Storage Systems Program, Sandia National Laboratories, Sandia Report SAND2007-3580, June 2007. [20] [Online]. May 2008, How Lead Acid Batteries Work, http://www.vonwentzel.net. [21] Anthony Green, “The Characteristics of the Nickel-Cadmium Battery for Energy Storage,” Power Engineering Journal, Vol. 13, Issue 3, June 1999, pp. 117-121. [22] K. Iba, R. Ideta, and K. Suzuki, “Analysis and Operational Records of NAS Battery,” 2006 IEEE Proceedings of the 41st International Universities Power Engineering Conference, Vol. 2, 6-8 September 2006, pp. 491-495. [23] L. Zhang, C. Shen, M. L. Crow and S. Atcitty, “A Comparison of FACTS Integrated with Battery Energy Storage Systems,” 2001 IEEE Transmission and Distribution Conference and Exposition, Vol. 2, 28 October – 2 November 2001, pp. 1151-1155. [24] W.R. Lachs and D. Sutanto, "Applications of Battery Energy Storage in Power Systems," IEEE Catalogue No. 95 TH8025, 1995, pp. 700-705. [25] Tongzhen Wei, Sibo Wang and Zhiping Qi, "A Supercapacitor Based Ride-Through System for Industrial Drive Applications," Proc. of the 2007 IEEE International Conference on Mechatronics and Automation, Aug. 2007, pp. 3833-3837. [26] Roger C. Dugan, Mark F. McGranaghan, Surya Santoso and H. Wayne Beaty, Electrical Power Systems Quality, 2nd Edition, McGraw-Hill, New York, 2003. [27] Robert B. Schainker, “Executive Overview: Energy Storage Options For a Sustainable Energy Future,” IEEE Power Engineering Society General Meeting, June 2004, pp. 2309-2314. [28] [Online]. May 2007, The Edison Papers, Rutgers University, http://edison.rutgers.edu/battery.htm. [29] R.F. Thelen, J.D. Herbst and M.T. Caprio,” A 2MW Flywheel for Hybrid Locomotive Power,” IEEE 58th Vehicular Technology Conference VTC 2003, Vol. 5, 6-9 Oct. 2003, pp. 3231-3235. [30] S.C. Smith and P.K. Sen, “Ultracapacitors and Energy Storage: Applications in Electrical Power System,” Proceedings of the North American Power Symposium (NAPS), Calgary, Canada, Oct. 2008 [31] S.C. Smith, P.K. Sen and B. Kroposki, “Advancement of Energy Storage Devices and Applications in Electrical Power Systems,” 2008 IEEE Power Engineering Society General Meeting, Pittsburg, PA, July 2008. [32] Gilbert M. Masters, Renewable and Efficient Electric Power Systems, John Wiley, 2004. [33] A. E. Fitzgerald, Charles Kinsley Jr. and Stephen D. Umans, Electric Machinery, 5th ed., McGraw-Hill, New York, 1990. [34] P. K. Sen, Power Distribution Systems Engineering, Course No. ECEN5767, University of Colorado, 1997. [35] [Online]. May 2009, D. Elliott, M. Schwartz and G. Scott, Wind Resource Assessment and Mapping, April 2003, http://NREL.gov/ NREL03172009.pdf. [36] Hans Roth, Philipp Kuhn and Ulrich Wagner, “Effects of Wind Energy on Thermal Power Plants,” IEEE 2007
X. BIOGRAPHIES Steven C. Smith (M’08) received his BS and MS degrees in Electrical Engineering from the University of Colorado at Boulder, Colorado in 1991 and 2000, respectively. He is currently working on his PhD in Engineering Systems (Electrical) at the Colorado School of Mines, Golden, CO. His research interests include energy storage, renewable energy, power generation and distribution systems, motors and power electronics. Mr. Smith is a Principle Systems Engineer with Lockheed Martin Corporation and is a Registered Professional Engineer in Oregon. P.K. Sen (SM’90) received his PhD in EE at the Technical University of Nova Scotia (Dalhousie University), Halifax, Canada. Dr. Sen is currently a professor of Engineering at the Colorado School of Mines, Golden, Colorado and the Site Director for Power Systems Engineering Research Center. Dr. Sen is a Registered
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Professional Engineer in Colorado. His research interests include application problems in electrical power systems, electrical safety and arc flash hazard, machines, renewable energy and distributed generation and power engineering education. Dr. Kroposki (SM’05) is the Principal Engineer and Group Manager for Distributed Energy Systems Integration at the National Renewable Energy Laboratory (NREL)’ Golden, CO. He has authored and co-authored more than 70 articles and reports and was selected to be the editor for IEEE Power & Energy Magazine special issue on solar energy integration. Dr. Kroposki participates in the development of distributed power standards and codes for IEEE, IEC, and NEC. He is instrumental and actively participates for IEEE Stds. P1547.4 and 1547.1. Dr. Kroposki is the US representative for International Energy Agency (IEA) Photovoltaic Power Systems Task 11 on Hybrid Power and Minigrid systems. Dr. Kroposki received his BS and MS in Electrical Engineering from Virginia
Tech, and his PhD in Engineering Systems (Electrical) at the Colorado School of Mines, Golden, CO. He is a registered PE in Colorado. Keith Malmedal (M’97) received his MSEE degree (Power) and a MSCE degree (Structures) from the Univ. of Colorado at Denver in 1998 and 2002, respectively. He received his PhD at Colorado School of Mines in Engg. Systems (Electrical) in 2008. He has over 17 yrs. of combined experience in design, system study, teaching and research. He is the President of NEI Electric Power, Colorado, specializing in power system design. Keith has published numerous technical papers, taught (co-taught) university courses, and short courses on the interrelated areas of power systems, machines, protection, renewable energy, and energy policy issues. Dr. Malmedal is a member of the ASCE and a registered PE in 15 states and Canada.
XI. APPENDIX Table 1. Battery Energy Storage Systems over 1 MW and 1 MWh in size
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g
p
Figure 3. United States Wind Resource Map by NREL [35]
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