Making A Case for Flywheel Energy Storage
By Drew Devitt
Founder, Chairman, and Chief Technology Officer
American Offshore Energy
Aston, PA, USA
This Feature Article appeared on pages 68-70 of the January-February Issue of Renewable Energy World - North America Magazine
Electricity is the ultimate in a perishable commodity. If it is not used or transformed as it is generated it will be lost. So the systems to supply electricity have been designed with such flexibility that supply may be made to closely match whatever the demand happens to be.
We take this balance for granted but the electric company studies historical demand, accounting for the change in seasons, changes during the day, weather forecasts and even whether there is a baseball game at the stadium. When they turn to their supply side they have hundreds of generator sets all varying in kilowatt rating size, cost efficiency and on/off flexibility. They've taken their best bet as to which of the large inflexible turbines to have powered on. They would like to maximize the use of these turbines as they are generally the most efficient turbines to run and cleanest turbines for the environment. Due to the need to be able to follow demand though, they also need to have in the mix a spectrum of smaller turbines that maybe turned on and off easily. This is the most expensive electricity and these are also the dirtiest turbines but they are able to dispatch them on and off to follow demand. The electric company is more likely to own the larger capital intensive generation sets and issue supply contracts with independents for the smaller turbines (where there are deregulated markets). The contract prices are usually priced based on the kilowatts that can be provided and the speed at which they may be turned on or off. This spectrum of adjustability is referred to as load following on the broad scale and frequency regulation on the fine scale.
The need for frequency regulation is the main reason to match the supply to the demand. It would be much easier sometimes just to create more electricity than is being demanded, but this is more dangerous than not supplying enough electricity. When there is more supply of electricity than is demanded the frequency of the alternating current goes above 60 Hz and when the supply is exceeded by demand the frequency drops below 60 Hz. In Europe and in some other parts of the world this standard is 50 Hz. Electric companies are mandated by federal laws to maintain 60 Hz on the grid. The bigger the disparity above or below 60 Hz the bigger the fines that may be imposed on them by Regulators.
This balancing act is based on attempting to forecast demand and dispatching smaller turbines with known kilowatt delivery capability. With this understanding of the electric companies need to closely follow demand it is possible to conceptualize the smart grid in a more realistic context. Let's first consider the variability of renewable energy sources like wind and solar.
The power companies are used to having a deterministic supply-side. If they tell a supplier that they've contracted with to fire up a turbine that is rated for 30 MW they can count on having 30 MW within the contracted time with something like a 98% certainty With wind and solar energy we are now asking the power company to deal with variability on their supply-side too, not just on their demand side. Today, renewable energies (not counting hydro) account for less than 2% of the total energy generated in the United States. The popular press and the Legislative Powers are talking about and making laws that suggest we will have 20% of our electricity generated by renewables within ten years. It is difficult to argue that load following and frequency regulation will not become more difficult and expensive with this increase in variability on the supply-side.
On the counter side though, it can be seen that there is a great degree of flexibility built into electricity supply already. Having looked to the classic demand curve in figure 1 it can be seen that within a particular regulation area that has an average of 10,000 MW of electricity being produced all the time, the demand at any one time varies from 20,000 MW to 5000 MW, which is a very significant spread. It is sometimes insinuated that these renewable energy sources will require a whole new fleet of turbines standing on the ready when the wind dies or when the clouds obscure the sun, this is simply not the case as they will have 1000s of megawatts in reserve most of the time. And even at peak demand times they will be glad for the extra capacity.
Larry Barth- NJ Clean Energy Program
The electric company though, has a serious obligation to always be able to supply the electricity that is being demanded. As the percentage of possible variability increases in their supply, they will have to increase the value of contracts with deterministic generation sources. If the wind does stop blowing, the turbines they will have to turn on will not be as efficient or as clean as the turbines they would select to use in the absence of wind variability. These contracts require payments just for the turbine to be ready to turn on, even when the wind is blowing. This will have a negative impact on the value of wind energy but new turbines and huge storage faculties will not be necessary.
The great hope for renewables is aggregation. The idea is that as more renewables come on line their variability will average out, at least to some significant degree. The wind blowing harder at night will average with the sun shining bright in the day on a macro level and as a gust of wind blows through a wind farm it won't much change the average output of the farm on a micro level. There is no doubt that there will be an averaging of renewable generated power but on the macro side this averaging is limited by transmission constraints.
Energy storage technologies are often referred to as a way to time shift and smooth the delivery of renewable energy such as wind and solar. But the cost of energy storage infrastructure is not insignificant. Today the cost for advanced lithium batteries (one of the leading energy storage candidates) that could store a megawatt hour of electricity is about $2 million. This is about the same capital cost per megawatt hour as the wind turbine. If a 1 MW rated turbine has good wind and is able to produce its megawatt hour rating for 10 hours in an evening it will produce 10 MWh of energy. To store this energy for use the next day you would need $20 million worth of batteries. This is obviously not an economic model.
Pumped Hydro and compressed air storage have much more realistic capacity costs but their round-trip efficiency is relatively low. As you can imagine there are a lot of losses in the pumps and pipes that are used to push the water up to some elevated reservoir and there are losses on the way back down, and the inefficiency of the impeller and the generator set adding up multiple times. Compressed air storage faces these same issues. Aside from their low round-trip efficiency pumped hydro and compressed air storage also require natural features; an elevated reservoir or an underground cavern that can store the water or the compressed air. These features are not normally found near demand centers.
What can we learn from the Navy's electric ship program?
One way to look into the future or see examples of how energy storage is used in smart grid applications is to have a look at what the Navy has been doing. Developments from military and space programs have a habit of trickling down to more pedestrian applications and the Navy has been working on an electric ship program for many years.
Navy ships have a lot going on and have historically had mechanical, hydraulic and even steam operated equipment on board. The Navy would like to reduce the complexity and increase the efficiency of these systems by switching over to all electric systems. This is not classified information and is consistent with trends in the general industry.
For instance, large earth moving equipment, like a huge dump truck, used to employ a transmission with a drive shaft going to a differential and then large axles to actually turn the wheels. This mechanical driveline was replaced by hydraulic hoses and hydraulic motors were placed directly on the wheel hubs. This reduced the cost, weight and the maintenance requirements on equipment while increasing the flexibility with which the power could be independently transferred to the wheels. Today we are seeing large frameless electric motors being placed on the wheel hubs, as the same sort of thing is done on Navy ships the hydraulic fluid is removed as a potential fire hazard, electrical wires are more efficient at transmitting the energy and significant weight and space is saved, all high priorities for the Navy. This is also a general trend in Mecatronics, putting motors directly where work is to be done. An example is that where we used to see a screw and nut to achieve linear motion from rotary motion, linear motors are often being used to provide axial force directly.
A Navy ship at sea has its own independent smart grid with multiple generation sources and a very high requirement on the reliability and capability of supply. The ship needs to be able to go from economical cruising to full on Battle ready within seconds. If there is an application where energy storage would be valuable, this is it.
One of the energy storage projects which the Navy is working on is the electromagnetic aircraft launch system (EMALS). Everyone has probably seen the Naval War film footage with planes being launched off the aircraft carrier decks with huge steam pistons located just below decks. This is still the way it's done today on modern aircraft carriers, but the Navy is planning to switch to a large linear motor that would be lighter, would have less maintenance and would have more capability than their current steam catapults.
The energy requirements of such electronic catapults are impressive. A 20 ton air plane needs to be accelerated to 200 miles an hour in about two seconds. This is equal to about 500,000 kWh or 0.5 MWh of energy. Remember though this energy is consumed in two seconds, and in order to maintain a constant acceleration much of that energy will be consumed in the last half a second. Even if we spread the energy evenly across the two seconds, the power required approaches 1000 MW or 1 GW of power. This is equivalent to the power from larger utility steam turbines which are obviously not practical to put onboard a Navy ship. So some type of energy storage is required.
There are multiple ways of storing energy and energy storage is becoming a hot topic in our national smart grid effort. Energy can be stored chemically, electrically, potentially or kinetically. A battery stores energy chemically, a capacitor would store energy electrically, pumped Hydro would store energy potentially and a flywheel would store energy kinetically. Pumped Hydro would have one of the largest capacities but is relatively slow and certainly very heavy. It would be possible to supply 0.5 MWh of energy storage via batteries but batteries cannot discharge that much power that quickly. Capacitors have the capability to respond quickly enough but do not yet have the capability to hold that much energy. So the Navy has selected a flywheel system for kinetic energy storage for the EMALS project. The fly wheels do not require periodic replacement of hazardous chemicals as with batteries which was a factor.
Flywheels are not a new technology, they have been employed in trains and amusement park rides for over a hundred years. The principal behind the flywheel is you can have a relatively small generator spinning up or charging a flywheel over a period of say a minute and then take the power off the flywheel over a period of several seconds to initiate acceleration of the train or amusement park ride at a much higher power than is available from the generator. This is the same way the flywheel would be used by the Navy, it takes about a minute between planes to get them staged and ready to be launched and the flywheel is charged during this time. What is impressive is that utility scale power has been demonstrated even if for only short periods of time.
Although energy storage may not be practical as a method for load following, there appears to be an application for energy storage on the finer side, helping with frequency regulation. Earlier we noted that this is the most expensive electricity to the electric company, with them contracting for it based on the general principle that the faster capacity can be supplied the more they pay for it.
Brendan Kirby - Oak Ridge National Laboratory
Have a look to the frequency regulation chart in Fig.2. The Green line trending upward represents the electricity demanded, the blue line represents the supply and the utilities effort at load following. It can be seen the electric company increased the supply of electricity to meet increasing demand by about 400 MW between 7 a.m. and 9 a.m. Notice also that electricity demand is not a perfectly smooth line, there is some randomness which cannot be predicted, at least with today's technology. The red line represents the difference between what is being instantaneously demanded and instantaneously supplied. So when the red line is above zero there is more electricity on the grid then is being demanded and the frequency will be above 60 Hz, this is wasted energy. When the line is below zero there is not enough electricity on the grid and the frequency will be below 60 Hz. In this example the supply line crosses the demand line about 10 times in each in hour.
This presents a huge opportunity for energy storage technologies as today this variability is dealt with by the electric company by telling the contracted suppliers to turn turbines on or turn turbines off on a per minute or per second basis.
In the effort to reduce carbon emissions of utility scale electrical generation we should look to dirtiest and most expensive generators first. These are typically the generators that are used for frequency regulation. It would be much more efficient to capture the excess energy when it is available, store it, and then put it back on the grid when demand exceeds supply.
It can be seen in the example that a 1 MW hour capacity energy storage device could have been completely charged and discharged 5 times in each hour, meaning that 5 MWh of electricity could have been sold in a single hour. In contrast a 1 MW rated wind turbine would require one hour to generate 1 MWh of electricity in the best wind conditions. The price for electricity in the regulation market is about 10 times that of what can be negotiated in a power purchase agreement for wind energy. This is not to be disparaging of wind generated electricity, the object here is to point out the possibility of very healthy returns on investments in the energy storage sector and the possibility to eliminate carbon output from the very dirtiest generators.
Returns on an energy storage investment targeted at frequency regulation are more predictable than other renewable energy efforts as frequency regulation is a problem that needs to be addressed 24 hours a day 365 days a year. It is also a safer and easier to implement investment, in the case of flywheels they are very sustainable, having no limitation on their cycle capability, no gearbox to wear out and no visible presence.
When you consider that almost 4 TW hours of electricity were generated in the United States in 2008 a 1% regulation market would represent 40 GW hours for profit opportunity. And there is yet another opportunity when we consider carbon capture. Energy storage for frequency regulation would be one of the most cost-effective alternatives to carbon capture, or for earning carbon credits. Remember by eliminating the dirtiest 1% of turbines by definition you eliminate significantly more than 1% of all the carbon generated.
How much carbon is made creating 40 GWh? My best estimate from looking at NERL information is that saving 40 GWh of electricity generation would be 30 million metric tons of carbon. But remember that this is based on overall carbon generated, where energy storage in this frequency regulation application will address the dirtiest 1% of generators. The larger utility scale generators create two or three times less carbon per megawatt hour generated. So the carbon savings would likely two or three times more than the 30 million metric tons per year mentioned above.
There are other very significant advantages for grid reliability and safety. The ability to distribute electric potential away from actual generators and close to demand centers or substations increases the effectiveness of energy storage systems. This is especially true with other ancillary services like reactive power and voltage support, which are much more effective when implemented locally rather than trying to affect them through transmission lines. And last but not least, energy storage systems with the capacity to supply large power ratings for short periods of time, like our 1 MW hour capacity flywheel that could supply 30 MWh of power for two minutes, are a perfect way to make up for instantaneous outages and so giving time to get other generators started.
So why don't we already have more energy storage built into our grid distribution system? There are multiple answers to this question. One is that energy storage technologies with the capacity to deal with utility scale demand are only just being developed. The Navy's recent accomplishments are an example of this. Another is that the cost of natural gas or even kerosene used in frequency regulation turbines has been relatively low and there is no additional cost penalty to the turbine being dirty (no carbon tax). Another is that frequency regulation has been perceived as a marginal issue and not as sexy as wind turbines or solar power to talk about. Probably the most significant reason is that the electric companies are not inclined to pay what these services are actually worth. Contracts and rules are formatted toward what can be done with small turbines. There are rather antiquated rules which govern the contracting of purchase agreements for providing the marginal power for frequency regulation.
An example of this is co-generation, which became popular in the energy crisis of the late 70s. Typically these generators use natural gas to run an internal combustion engine to generate electricity. The heat that is created as a byproduct of cooling the engine is then used in some sort of an industrial process, for instance industrial washing and drying machines at hotels or nursing homes. The electric company has required these generators to provide ancillary services like reactive power and voltage control as a condition to be able to connect to the grid. In those days it was quite a privilege to be able to connect to the grid. In a free market it would be a compensated service.
Many recent stimulous awards have gone toward smart metering efforts. This is essentially an effort to try to bring market pricing to the use of electricity. When electricity costs more to the user of electricity, as during periods of high demand, it is likely that less will be demanded. There are also efforts for utilities to be able to contract with large users of electricity, for them not to use electricity at certain times. This begins to give the utility some control over demand, not just their supply and is referred to as Demand Response. Demand Response will be competitive with energy storage in load following and possibly in frequency regulation too.
Electric vehicles could also be a mechanism to affect energy storage. The object would be to charge them during non-peak times and then use the cars that are plugged in during the day as banks of distributed batteries.
All these efforts are to be applauded. But we need to also look to the utility side of the meter. A lot of opportunity exists there for utility scale energy storage and just as the Department of Energy is making an effort to bring market forces to the use of electricity they should apply the same emphasis in bringing market forces to the way electric utilities procure electricity. This would actually be faster to deploy then Demand Response through smart meters or batteries though electric vehicles and could be stimulated by changing rules and laws meaning; no stimulus needed.