Electric Energy Storage Systems

Transcription

1 Electric Energy Storage Systems Energy storage systems include various means of storing and recovering energy for later use (Table 1). Electric energy storage systems accept and return the stored energy as electric power, although they may store the energy in another form. Non-electric energy storage units store the energy in some other form. Despite this, they may be of interest to electric utilities and power engineers since they can materially affect the shape of daily demand curves. Among other benefits and features, energy storage of either type permits peak shaving in electric usage: power can be consumed at one time (off peak or slowly over a long period to time) and the energy actually used at another time and perhaps at a much higher rate than when put into storage. The most widely used energy storage system is one most people to not think of as energy storage: the storage water heater, which retains 30 to 100 gallons of hot water that it heats gradually over an hour or more but in a way that it can deliver it for us in only a few minutes if asked. This and other non-electric energy storage devices are discussed elsewhere (see References at end of this page). The rest of this discussion will focus on electric energy storage. Contents 1. Parts of an Energy Storage System 2. Basics of Electric Energy Storage Systems 3. Capabilities Provided by Electric Energy Storage 4. Benefits and Applications of Energy Storage 5. Making Renewable Energy Dispatchable 6. Complicated Studies 7. Fixing the Apples-to-oranges Conundrum 8. Energy Storage at Quanta Technology References and Resources Energy Storage Technologies A number of very different methods exist to store electric energy, some of which are listed in Table 1. Only two of those shown actually store the energy in electric form: super-capacitors and SMES (Superconducting Magnetic Energy Storage which keep the energy as electric charge or magnetic fields respectively. Batteries (first four lines in table) actually store the energy in a chemical form, but the natural operation of the battery converts the power to direct current electric power upon being provided with a pathway for the power to flow. Page 1

2 Mechanical storage includes several types of flywheels, compressed air, and pumped hydro systems. The last two are practical and widely used on a system (100 MW peak capacity or larger) scale. Thermal storage systems use electricity to heat a liquid to very high temperatures and then use that, via a heat exchanger, to heat steam to drive a steam turbine generator or a sterling cycle generator. The table below is taken from Chapter 10 of Distribution Power Generation Planning and Evaluation by H. L. Willis and W. G. Scott and is reproduced by permission of the publisher s power systems series editor. More detailed discussion of each of the technologies listed can be found in that book. (See References at end of this document for links to that book). Table 1 Qualitative Comparison of Various Energy Storage Methods Electric First Energy Power Electrical Smallest Lifetime Levelized storage viable density density efficiency size in annual system type in year kwhr/ft 3 kw/ft 3 % - 24 hr. kwhr years $/kwhr Lead-acid $25 Nickel-metal hydride $80 Lithium polymer $120 Sodium-sulfur $85 SMES $200 Super-carbon capacitor $85 Low-speed flywheels $40 High-speed flywheels $80 Compressed Air 1975 n.a. n.a $20 Pumped Water 1950 n.a. n.a $20 Thermal (STES) $15 From Distribution Power Generation Planning and Evaluation by H. L. Willis and W. G. Scott. Parts of an Energy Storage System An energy storage system consists of four main components as shown in Figure 2. The first and the element that sets the basic system storage capability limits is the energy storage medium itself: the battery, or flywheel, compressed air reservoir or the lake or reservoir that can be filled with water in pumped hydro systems. Page 2

3 Monitoring And Control Grid Charging system Discharging system Storage Mechanism (battery, flywheel, etc) Figure 2: Basic components of an energy storage system Second is the charging mechanism, which takes power from the utility system and converts it into the form that can be put into and stored by the storage medium. For example in flywheel systems this is basically an electric motor that can rev up the flywheel to very high speeds. In a thermal storage system, it is a set of heating elements. Third is the equipment that takes energy from the storage and converts it back to electricity and inputs it out onto the grid for an energy consumer s demand. In a flywheel, this is just the aforesaid motor (its field is reversed to turn it into a generator that produces electric power while gradually slowing the momentum of the rapidly spinning flywheel). For thermal storage it is a heat exchanger and steam generator. Regardless, it is this component that shapes what the unit looks like to the system and electric demands as far as the quality and quantity of power provided, voltage regulation and other aspects it provides. Finally there is the control system, which consists of two sub-systems. First, there is the equipment needed to monitor and control the unit itself: for example with a flywheel, signals need to be sent and equipment activated to turn the motor generator into a motor, to permit power flow to the motor so it accelerates the flywheel, storing energy. This must include sensors that can determine when the unit is full (the flywheel is spinning at its maximum allowed rate) and shut off this process. Similarly it must control the discharge cycle, too. And typically the equipment acts as a diagnostic and protection system, monitoring the unit, setting alarms if there is any anomaly in condition or operation, and activating protective equipment such as fuses, breakers, brakes, etc., in the event of a contingency. The second part of the control system is the electrical energy storage system control the system that determines how, when, and why the unit performs. This may be a simple UPS control system (when you sense power from the grid has stopped, you provide power from the battery backup system). Or it could be a very complex computer algorithm that takes a number of factors, including weather (the forecast is for record temperatures tomorrow demand will likely be high), utility load (its peak season, demand is already high), generation on line (the big nuke unit south of here is down for emergency inspection and thus electric prices are 12% higher than normal) and perhaps many other factors (status of renewable Page 3

4 energy and other storage owned) into account to determine when it should charge, how much it should charge, and when it should release energy. Basics of Electric Energy Storage Systems Electric energy storage systems do not generate power themselves. They have to be charged have power put into them before they can store it for later use. Major characteristics of an electric energy storage system are: Energy the amount of energy it can store. Electric energy units vary from those that store only a tiny amount of energy (flashlight battery) to systems that store enough energy to feed an entire city for several hours. Power the rate at which the electric energy storage unit can accept and release power, is a function of both the type of energy storage technology used and the design of the unit itself. Certain technologies (pumped hydro) can release the energy they store only relatively slowly going from full to empty even at their highest release rate over a period of an hour or even several hours. Other energy storage technologies (super-capacitors) can release all their stored energy in the blink of an eye, or very quickly (some types of batteries). Most technologies and devices fall somewhere in between these two extremes. Rate of power release is a major factor in selecting the type of energy storage technology. In practice, due to design and other factors, an energy storage system may have different ratings for the power input (the rate energy can be put into it) and its power rating (the rate at which it can output power the maximum power it can provide). Density is the rating of the unit compared to its weight or its size. Energy density measures how much energy the device can store per pound of weight or cubic foot of required space. Power density measures how much power the device can provide per pound of weight or cubic foot of required space. Both are critical elements in non-stationary energy storage applications such as in electric vehicles and portable backup units. They are design factors but far less important in stationary applications such as on the utility grid. Service lifetime. Some types of energy storage systems wear out over time and therefore have only a finite life, no matter how well they are cared for. This varies greatly among available technologies, and depends very much on how the energy units are used. Batteries are particularly notorious for having short lifetimes in applications where they are repeatedly charged and discharged completely. Many often last only a few hundred cycles. Fault current is often a major consideration and concern. An energy storage device will feed a fault (short circuit in the power system) just as readily as it will feed power to a legitimate demand. Particularly for technologies that have very high power ratings (e.g. super-capacitors, batteries) the short-circuit capability is a concern and considerable attention may have been devoted in the design of the system to assure that the unit is controlled so that it will not feed prodigious amounts of current into a fault. In some energy storage systems the maximum rated power is heavily constrained by design constraints required to keep fault feeding capability within certain required limits for the application. Page 4

5 Regulation depending on the design of the device, and to some extent, the type of energy storage technology, an electric energy storage system may be able to vary its output power and voltage rapidly and very precisely. Some units are designed with this as the foremost feature of their control systems and are used to provide very stable power or as regulation units in power systems, to assure adequate voltage and stable power flow. Dispatching control. How the energy storage unit and the energy it stores is managed is a function of how it is used. Typically the operator has a particular purpose (see Purposes for and Benefits of Energy Storage below) and operates the unit, usually via computerized control, to maximize that particular benefit. Dispatching and other energy storage use algorithms can be quite involved and their design and programming is both a science and an art. Often their owners consider this part of the system to be highly proprietary intellectual knowledge. Where in the system? Energy storage can be designed to work with and connected to the transmission system, to the buses at a substation, to the primary distribution feeder, to the secondary (utilization voltage) distribution systems, or to the wiring within a building or facility like a factory. Depending on the location in a power system, energy storage will be able to provide some types of benefits more than others. For example, to provide bulk power for system stability and balance needs electric energy is best to attached to the transmission system. By contrast, if reliability of supply at a factory is the main goal, the energy storage is best installed on the custmer s side of the meter at the factory. Losses. A portion of the energy put into an electric storage system will never be recovered. Losses occur in the electric equipment that converts electric power form the grid into that stored in the system, and vice versa, and in the storage itself (a battery, flywheel, or super-capacitor might be able to return only 95% of the input power due to natural, physical or chemical phenomena and their limits). The control equipment required to operate the device requires power and that, too, is characterized as energy losses (power that goes into the unit but never comes out). Losses can be as high as 20% in some types of systems, or as low as 5% in others, and are usually a design element that is optimized: they cannot be avoided entirely but most systems have been designed to limit them to the minimum lifetime cost, spending just enough up front to reduce them but only to the extent the long-term savings justifies that initial cost. Capabilities Provided by Electric Energy Storage Electric energy storage units can be used to provide a number of benefits to a power system. These are discussed below: Peak reduction and load flattening. Energy storage is often used to store power during times of low usage and to output that power to meet power needs when demand is high. Figure 1 shows the daily usage of a small community in the western US with HR, 7.5 MW storage units to flatten daily load cycle. Peak demand that must be supplied by generation and T&D delivery to the town is reduced by 10%. Making renewable energy sources such as wind and solar generation into dispatchable sources of power. Both wind and solar generation are somewhat erratic in their expected output depending on the weather. Solar generation definitely will not provide any power at night. Without electric energy storage, the power they generate must be Page 5

6 MW of Power Needed consumed at the instant generated. Otherwise it is lost: they are non-dispatchable sources of power. A properly sized and configured electric energy storage system can accept power from the solar and wind generation when they produce it, save it for later use, and then release it upon command (dispatch it as requested) when needed, making the energy produced by the renewable energy generation dispatchable power. Reliability backup. Electric energy storage can be used to provide power when the grid is unable to supply it due to equipment outages, storms, or other reasons. Millions of homes and businesses throughout the US have Un-interruptible Power Supplies (UPS) that provide backup power for Noon 6 12 Hour of the Day Figure 1. The daily demand curve of the substation feeding a small town (blue line). Peak demand is 75 MW. A 15 MW-hour electrical energy storage system charges between midnight and 8 AM (yellow) and release power between noon a 6 PM, reducing system peak by 10%. computers, telephone switching equipment, and similar high-value electric demands in the event of a utility or grid outage. These provide the power required to operate the equipment their owners wish to operate for at least as long as it takes to shut down smoothly or while backup generation is started. Millions of these units are on the size of a very large book and provide backup to only one personal computer. Others provide backup power for large digital equipment installations and entire buildings in some cases. Spinning reserve. Regional power systems are required to keep significant amounts of generators running, but not producing power. This is so that they can take over, quite literally in the blink of an eye, in the event of a sudden emergency such the failure of a major power plant. This is quite expensive. One or more large power plants have to be operated, which uses fuel and personnel and creates wear and a need for maintenance and service on the equipment. Electric energy storage can be used to defer the need to keep that machinery running. The utility still needs the extra generation capability but can now operate it as cold standby. It is not started or running but is available to start in a short amount of time a period less than that over which the energy storage unit can provide the required power. Improved efficiency of generation. Almost all generators are most efficient at a certain design point, a certain amount of outputted power. Some generators are also just much more efficient than others. A power system is normally operated with only the most efficient generators available operating at any one time. That means that in daily cycles like that shown in Figure 1, the least efficient generators are left until last (peak time) to be run. A curve like that shown also means that they have to be run to follow load they are throttled up or back and so do not always run at their most efficient level. For these reasons, the mix of generators and the way they are operated is much more efficient when there is energy storage as compared to when there is not. Page 6

7 Regulation and Stabilization. Energy storage can be used to inject, or draw out, power from the grid to even out fluctuations, and/or to keep voltage and frequency at a highly regulated level. Generally electric energy storage units designed for these purposes are all about the control and power electronics : the focus of design is mostly on the characteristics of the control system and the ultimate performance and value of the device is a function of those characteristics. Reduced emissions. In many cases, not only efficiency but emissions of generators vary one from the other. Again, the best units are run most often. By flattening the load curve, energy storage permits the utility to run its least emitting units more and its most emitting units less. Benefits and Applications of Energy Storage Arbitrage involves buying power cheap, storing it, and then selling it at a higher price sometime later. The power bought to charge a storage unit (the yellow shaded area in Figure 1) might be purchased at night, when power can be purchased on the wholesale grid for 4 /kwhr, and sold during the peak demand period of mid-day when power is selling for 12 /kwhr. Even allowing for the inevitable electrical losses in the system and the need to pay for the storage unit and its operation and maintenance, this may provide a steady daily profit to the owner/operator. Avoidance of congestion charges. Electric energy storage that is connected to the right places in a transmission grid can mitigate or eliminate congestion and congestion charges, which occur when demand for power transmission exceeds capability. A power transmission line might be limited to 300 MW, but there could be a peak demand for an hour of 325 MW on the other end of it. If the line is congested and additional fees would be charged for the line s use in order to reduce demand back to 300 MW. A 25 MW storage unit could avoid the need for any reduction or cost increases. Deferral of capital additions to utility systems. In many utility systems there is a slow but steady growth of local peak demand as communities and neighborhoods gradually expand and fill out. At times, new or expanded facilities to deliver power into these areas must be added, requiring the utility to invest considerable money to pay for them. Suppose that a community has a 75 MW peak day demand like that diagrammed by the base (black) line in Figure 1, and is growing at about 2%, or 1.5 MW per year, and that the substation is limited in capability to serve no more than 75 MW. The only recourse available to the utility is to upgrade the substation, and typically such upgrades are available only in relatively large steps perhaps here the next lowest option for this substation is 90 MW capability. Installing 15 MW of additional capacity to handle the growth the first year it becomes an issue, when peak demand is only 76.5 MW, means the new capacity is only utilized by 10%. Even in the second year, it is used only to a 20% utilization. Temporarily adding 7.5 MW energy storage means that the utility can defer this expensive addition of this 15 MW upgrade for 5 years, at which point when it does the new upgrade it will be utilized by 60% in the following year. The utility saves the carrying charges on the upgrade for five years. The storage can then be moved to another location at the end of the five year period or kept at the substation to be used again (if the growth is expected to continue, this same problem will reoccur in year 11, even with the substation capability upgrade). Page 7

8 Whether there is a business case for the use of the energy storage unit in this application is something that needs to be determined through detailed study. Often there is not. But there are cases where the deferral is cost justifiable. Rate Reduction. With electric energy storage, the power system serving the community shown in Figure 1 will be more efficient in regards to equipment utilization and electric losses, and the power generation required will be more efficient over the course of a day, a month, or a year (see discussions above). Even allowing for the cost of the storage system, the total cost may be lower than in the case with no energy storage. In cases where the cost savings is not taken as profit through arbitrage, it may result in a net savings for the utility and/or its customers. Reliability Augmentation. Electrical energy storage can provide improved reliability (see above). Whether there is a business case for a utility or an energy consumer to use it for that purpose depends on a host of factors that need to be analyzed for the specific situation and need. Among them are, how good (or bad) is reliability now? Is there even a need for improvement? Numerous demandside (utility customer) issues and factors including peak demand, energy needs and load curve shape need to be included, along with the value of uninterrupted, or less interrupted, service to the particularly energy consumer. A host of utility system issues have to be considered including the cost of fitting the equipment to the site and controlling and monitoring it so it will perform as needed. Making Renewable Energy Dispatchable This benefit was listed under the basic capabilities of energy storage but it is perhaps the most valuable benefit energy storage can provide and almost certainly the application that will lead to very widespread use of electric energy storage in the 21 st century. Renewable energy from solar, wind, and other technologies makes good sense from so many perspectives. But except for a few niche technologies that are difficult to site and fit to systems (solar tower generation), renewable generation systems are not dispatchable sources of power. Energy storage makes them so. Renewable energy systems also contribute to existing reliability and regulation problems for widespread power grids. Energy storage provides the means to mitigate those, too. Many of the fundamental concepts and mainstream ways electric energy storage is used in power systems have not yet been determined or set in the power industry. There may never be a typical way electric energy storage is used for this type of application. One reason is that the storage does not have to be located at or operated in conjunction with the renewable generation in order to provide this benefit. For example, the owner of a 50 MW wind plant could install energy storage at the plant or at a site electrically convenient to it. A study of past wind and weather cycles, and the plant s design and expected reliability, and the region s grid load and operation might determine that 110 MW hours of storage with a 65 MW peak output capability would give the owner a 99.98% probability of meeting peak commitments if contracted better than for a coal plant. Installed and operated in conjunction with the wind farm, this would make the farm s output dispatchable power. Page 8

9 But a farmer 80 miles away could also install storage, and what is actually a fairly simple buying control system to operate it, sufficient to allow her to buy non-dispatchable power from the grid when it is a bargain (when wind farms are producing lots of power) and store it for use when she needs it to run her business or power her home. Again, a study would be needed to determine the characteristics of the storage how much energy it would store, what peak load it could serve, how fast it could recharge, etc. That would need to include a comprehensive look at a number of factors, but the unit that would do the job for the farmer could be determined and once installed, she would get power when she needed it but buy cheap non-dispatchable power from the grid when she could. The interesting point here is that both of these alternatives are very realistic: current technologies can do either well, and also permit a range of choices between these two extremes. The storage required to make all the wind farms output into dispatchable power could be added in one large system at the wind farm site, or dispersed as dozens, perhaps hundreds of smaller installations at customer sites. Characteristics would vary: probably more net storage capability would need for the dispersed scenario, but that would provide more benefits, too (in addition to having dispatchable power all the time, the farmer would have power, for a while, if the utility system was experiencing an outage of service). Furthermore, a range of options between these two extremes are both technically feasible and sometimes economically feasible. Storage could be installed at key waypoints in the regional transmission grid. There is no need to store wind energy at the wind farm. Decision makers can transport it to convenient central locations and keep it there until needed. It could be stored in numerous smaller but still utility size locations at substations, etc. Each option would have different characteristics, different initial and operating costs, and different benefits to different stakeholders. In some sense, all these potential electric energy storage options compete against one another: someone is going to see economic benefits from the electric energy storage, either the wind farm owners who can sell the output of their plant for much more when it is dispatchable, or the farmer who will buy cheaper power to operate her business. And only so much is needed. Potentially, a power system could have more storage installed on it than needed, causing a glut of storage capability and reducing the margin between the cost of dispatchable and non-dispatchable power. Currently, however, there is no reason to be concerned about this. Complicated Studies In every scenario and option discussed above, a study of the electric energy storage system would be needed in order to determine how a particular system would perform, and particularly what type of system would be best for any specific site and application. Such studies are among the most intricate and complicated planning studies required in the power industry, for several reasons. The studies must be probabilistic, because they deal with weather and other uncertainties and only a risk-based method using a probabilistic approach ultimately determines the likelihood of success and the expectations of payoff. Technology SME expertise is required in a number of fields. One needs experts in the various storage methods: what works best at the wind farm might not be even an Page 9

10 acceptable option at for the farmer herself. And the key to operational success in many cases is not the energy storage as much as the control and communication and inverter technologies employed. Finally, the economic viability as well as the operational constraints on the unit will be a function of regional grid and utility rules. Knowledge of those and expertise in interpreting them in the particular case is needed. Intricate and complicated technical analysis and evaluation is required. To have any detail in determining requirements, costs, benefits, and reliability, etc., these studies need to model demand, output, and operation of the units on an 8760 hour a year basis, for a variety of scenarios (contingencies, etc) and include a very wide range of considerations and costs, many of which are not required in most T&D or DR studies. A very wide range of factors, issues, and interactions must be considered, for equipment capabilities to control capabilities to peak and energy needs and peak and energy losses and reliability of energy and power, and cost and value of benefits. Finally, the economic viability of any electric energy storage system is the key to its success. While requiring a comprehensive consideration and attention to detail, cost are actually relatively easy to estimate and tally. It is the determination of the value of the benefits that is usually challenging, requiring detailed study, again on a probabilistic basis at 8760 hours per year and over many years. Fixing the apples-to-oranges conundrum. The complexities listed above soon become obvious to anyone who gets into such studies. What may not be obvious early, but is the real key to success, is that the study must have a much wider consideration of benefits and options to the energy storage capabilities than just for the primary purpose. Energy storage, by its nature, changes the basic operating cost framework at/near its location. Therefore planning alternatives are not necessarily apple to apple comparisons. For example, in the case of the farmer looking to install electric energy storage on her property, for her use, the benefits of the system will include improved reliability when the utility has an outage, she can still have power. The value of that additional reliability, if any, can be determined fairly easily. But in addition, given that she is buying an energy storage system, it might be worth it to her while to buy additional storage capability (e.g., more batteries), or a higher peak capability in the systems inverters and controllers, in order to provide her with more reliability or operation flexibility. She might not seek those benefits if she were not already buying the base system for its primary purpose, but given that she is, it might be more than worthwhile to change the specifications and design to add these additional benefits. Furthermore, other changes to the system might allow her to arbitrage power, and perhaps she is in a class of consumer/actor permitted to do so and it would make sense to her. Something very similar occurs at the other end of the scale. The wind farm operator/owners do not have to and perhaps should not limit their considerations to only storage systems that fit their plant s output. They can oversize the system and buy other wind power from the grid when it is cheap and sell it when in demand. Perhaps they can add advanced control and other features and the use the storage to provide regulation and other services and benefits to the regional grid operator. These strategies might permit them to make more money, or be more successful in meeting their business goals. Conversely, perhaps they should undersize the plant slightly and depend on others doing some of that for them. Page 10

11 Energy Storage at Quanta Technology At Quanta Technology, Energy Storage studies are performed by a cross-functional team of experts led by Senior Vice President Lee Willis. A team appropriate to each project is assembled from the experts and experienced engineers, planners, and operators in the various Quanta Technology divisions. Quanta Technology has done and continues to offer studies in all aspects of and all applications for energy storage, electric, thermal, cold, and mechanical, for utilities, government, industrial, and private applications. Quanta Technology Experts in Energy Storage Dr. Julio Romero Aguero Dr. Richard E. Brown PE Dr. Ati Edris Dr. Gerald Sheble H. Lee Willis Carl Wilkins References and Resources Articles Overview of Energy Storage Storage-Overview.pdf Example of valuing the benefits of renewable energy; Looking at high penetration needs for renewable energy technology.com/sites/default/files/doc-files/grid-impacts-and-solutions-of-renewables-executive- Summary.pdf Books by Quanta Technology Authors Power Distribution Planning Reference Book 2 nd Edition discusses basic planning conscepts needed as a foundation for many energy storage studies Reference-Second- Engineering/dp/ /ref=sr_1_1?s=books&ie=UTF8&qid= &sr=1-1 Business Essentials for Utility Engineers cove r the basic methods for valueing and compared costs and benefits in a valid manner Richard/dp/ /ref=sr_1_1?s=books&ie=UTF8&qid= &sr=1-1 Distributed Power Generation and Distribution, chapter 10, provides an accessible overview of energy storage technologies Engineering/dp/ /ref=sr_1_1?s=books&ie=UTF8&qid= &sr=1-1 Page 11

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