Office of the CTO
January 2013
ENBALA Power Networks®
Malcolm Metcalfe Eric Hoevenaars mmetcalfe@enbala.com
ehoevenaars@enbala.com
ENBALA Power Networks® |#211 – 930 West 1 st St | North Vancouver | V7P 3N4 | www.enbala.com

ENBALA Power Networks®
The concept of grid-scale storage has become an increasingly sought after key technology to support our current electric grid. In fact, storage, in one form or another, has played a key role since almost the beginning of the power system, but its perceived value has increased dramatically in recent times. The history and application of storage has some curious twists.
Thomas Edison started the first commercial electric utility in 1882. The grid was a DC (Direct
Current) system that delivered power to a handful of local customers. Unfortunately, it was difficult to switch or to step voltage up or down by a significant amount. To transmit power over any significant distance, the voltage needed to be high to reduce losses, but for safety, a low voltage at the point of application was better. Edison’s system delivered generator voltage directly to the lights used at customer locations. The voltage used was a compromise between transmission loss and user safety.
Westinghouse and Tesla, two bright young engineers, recognized the need for a relatively high delivery voltage and claimed that DC was therefore unsafe for users. They recommended the use of AC (Alternating Current) for power delivery. AC allowed power to be generated at one voltage, delivered to the user site at a high voltage, and used at a third voltage. Unfortunately, the alternating voltage made storage much more difficult. Almost all forms of direct storage of electricity now involve conversions to and from DC. These conversions, as we will see, are a large source of loss. Ironically, new electronics has made voltage changes for DC feasible. With today’s technology, DC transmission can be delivered in the same way as AC, retaining the capability for efficient storage that is not available with AC.
Electricity can in fact be directly and efficiently stored as either an electric or magnetic field.
Electric field storage uses devices called capacitors, and these store energy that increases with the voltage applied. Magnetic field storage is done with inductors, large coils of wire, and the energy stored increases with the current in the device. But because we use AC electricity, all of these devices essentially charge, and discharge with each electric cycle – 60 times each second.
The net storage over each cycle is zero. As a direct result, the AC electricity grid has no inherent storage, and the amount of generation must be adjusted almost continuously to match the load that is connected to the grid.
Other fuels such as natural gas or gasoline have storage throughout their delivery systems. This was demonstrated when a pipeline rupture occurred on the TransCanada natural gas line in
North West Ontario during cold winter weather. The line was out of service for a few days while repairs were completed.
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The customers in southern Ontario did not even have to curtail use; the stored gas in the line across Ontario was sufficient to meet customer demands for the entire period of the outage.
This is definitely not the case in the electric grid. Remember the outages of 2003 in the
Northeastern US and Canada. Failure in one system plunged an area including many utilities into blackouts that lasted several days.
In general, storage can be applied in three areas of the grid, with varying results. The three areas are:
• Fuel storage : storing fuel before it is used to create electricity. This may be in the form of water behind a dam, a pile of coal, or a supply of uranium or other fuel.
• Grid storage: taking electricity off the grid, converting it to a form that can be stored, and then converting that stored energy back to electricity when needed. This is what we conventionally think about when we discuss storage – energy stored in forms like batteries, flywheels, compressed air or even pumped hydro storage.
• Demand-side or process storage : storing energy in the form in which it is to be used.
These three forms of storage have very different applications and equally different results. Fuel storage was the first method used, and it has seen extensive use by utilities. The other two methods are currently being exploited in a variety of different ways.
Canadian utilities initially relied heavily on hydro resources for generation, while their counterparts in the US used a mixture of hydro and coal for their sources of energy.
Hydroelectric plants were initially built to generate power based on a “firm” supply – the maximum water power likely to be continuously available. This resulted in a need to spill water for most of the year. The utilities soon realized that by capturing some of the spring runoff behind storage dams, and releasing the water later, during low water periods, that they could generate much more energy. The Columbia River treaty, signed in the 1960’s between Canada and the US is an excellent example. By building four storage dams, three with their own generation, the “firm” capacity of the existing hydro plants on the US portion of the Columbia
River could be almost doubled. “Spilling” is now a rare event, benefitting both the energy supply and the marine life in the river.
So just what is storage in this case? This form of storage is very simply the shifting in time of the water used for generation. There is no additional water in the rivers, just a smoothing of the flow.
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Prior to constructing the storage dams, water was low though much of the year, and the runoff that lasted a few weeks, far exceeded the generation capacity of the hydro plants that were on the river. By capturing some of the freshet for later release, time shifting allows the electricity to be produced when it is needed.
The principle is important because the method is essentially 100% efficient. Shifting the timing of the generation has very little impact on efficiency.
A similar process was used at coal fired generating stations. A plant that had a fixed and regular supply of coal, could store energy simply by reducing generation. The pile of coal at the plant would grow, essentially providing storage. There would be no additional energy generated – just a change in timing.
The second form of storage, which is getting the most attention these days, is grid storage - batteries, flywheels, pumped storage and other means that take electricity off the grid, convert it to another form for storage, and then convert it back to electricity when needed. This process is very different in that it requires at least two added conversion processes that would not exist without the storage system. Conversion efficiencies generally result in 85-90% of the stored energy being returned under ideal conditions. While this may seem relatively efficient, it must be recognized that in 7-10 storage cycles, the entire capacity of the storage system will be lost.
The highest value storage applications, such as PJM’s fast regulation service can require more than 5 full storage cycles each hour.
But grid storage efficiency also has other issues. First, it is extremely expensive both to install and to operate. These high costs, coupled with the current low cost supply of natural gas have made gas turbine generation the best option to provide effective grid storage. The high operating costs are caused by overall low return efficiency. The advertised efficiency is generally achieved by storing and returning energy at the maximum power rating as well as storing the energy for a short time. In addition, where the energy is returned to the AC grid with an inverter, the voltage provided by the inverter is assumed to match the grid voltage.
Neither of these conditions is typical, so return efficiencies are often less than 70-80% and where storage/return cycles occur frequently, the cost of loss is large.
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The third form of storage is demand-side or process-based storage. Just as fuel storage is achieved by modulating the generator power output based on changing fuel inputs, loads can deliver storage by varying the rate at which they consume power.
Many loads have capability to store energy without impacting the intended use by simply converting more (or less) energy into the form used than is required immediately by the load device.
This assumes that there is some capacity to store some energy in the form that it is to be used.
A simple example is a heated tank of hot water. Power consumption heats the water in the tank and it is then kept until needed. If the tank has a range of temperature that is acceptable for use, then the tank can be heated a little sooner or a little more than required and this stores the energy for use when needed. Like the generator, the overall energy over time is not altered, but the pattern of use is changed. Again, this form of storage is 100% efficient. This capability allows the grid to harvest unused short term storage to provide badly needed flexibility.
Many loads have this capability. Some of these include water pumping to a reservoir, water heating, building HVAC systems and frozen food freezer storage to name a few. A recent study, sponsored by the DOE suggests that the capacity of demand-side loads capable of storage exceeds 26 GW in the US alone.
This opportunity is exciting because not only is this storage available today but it is remarkably cost effective too! Because the underlying “storage” is based on capturing storage that already exists in the power system, it is available at less than 1/10 th the cost of more traditional forms of grid-scale storage. And because it is near 100% efficient, it is also less expensive to operate than most conventional forms of storage. With intelligent load management, the harvesting of this storage is generally not visible to the end users and so this application has the potential to play a very large part in the operation and optimization of the electric grid with little cost or impact.
Grid-scale energy storage technologies have an important role in the future of the power system. They have the ability to improve grid efficiency and reliability through many applications such as integrating renewables and optimizing power flows. But it is important to identify the technologies that can accomplish this in the most economic and efficient way. The storage that must be taken advantage of is ALREADY IN THE POWER SYSTEM on the load side – it simply needs to be connected to and managed.
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