Readers who are new to the subject of energy may find it useful to look over our brief documents on important energy terms and electricity grid terms.

This week we are taking a look at electric energy storage. There are a number of important reasons why energy storage systems are desirable for power grids. Here are some of the major uses for energy storage on an electric power grid:

Peak shaving

Different amount of power are demanded from second to second. Power grids controllers must respond to changes in power usage on rapid time scales. However, even the most rapidly controllable forms of generation, such as dispatchable hydro or natural gas turbines, can take many seconds or even minutes to ramp up. The amount of time that is needed to ramp up a given form of generation is called its lead-in time. Conversely, some energy storage methods allow for much faster lead-in times. Capacitors and flywheels for instance can have lead-in times numbered in milliseconds.

There is always some gap between what power is being produced and what is being demanded. Depending on the specifics of the power grid, this gap may vary in size. This depends primarily on two factors: 1) how quickly the peak-matching generating equipment can be ramped up, and 2) how quickly demand can be expected to rise. All else being equal, power grids that have slower-responding generating equipment will need a larger gap. Since their system would be unable to respond quickly to a fast change in demand, they would need to have extra power being produced in case more is suddenly needed. Similarly, if there have historically been sharp demand spikes (where demand increased very rapidly), the power grid will need to maintain a larger gap.

Peak shaving is the concept of using fast-responding power equipment to match relatively small increases in demand. This gives grid operators more flexibility when managing their power grid. Thanks to fast-responding power systems, they can make the gap smaller between production and demand. This means less fuel is wasted since we are maintaining a smaller gap between production and demand. Additionally, peak shaving means that natural gas units that are used for peak-matching can be run at specific power outputs. It can be relatively costly, inefficient, and polluting to run a natural gas power system at less than ideal levels. Most thermal power plants have specific levels at which they are most efficient and non-polluting.

Energy storage for power quality improvement

There is also another distinct use for energy storage. When a power grid increases or decreases the amount of power they produce, it often introduces power disturbances. These can be very short lived, generally not longer than tens of seconds. Most power generation systems cannot adjust their power production in this time. This may lead to ‘dirty power’ being delivered to customers, or an additional cost in cleaning up the power before delivery. The term ‘dirty power’ refers to power that differs significantly from the local standard. This is essentially instability of the power being delivered, and can have detrimental effects on devices that use this power.

Current solutions involve allowing frequency to change, and then injecting more real power into the grid using a fossil-fuel fired system or dispatchable hydro. A lot of investigation has been done on flywheels for this sort of work. With the advent of magnetically-levitated flywheels, the number of discharges these systems can undergo has massively increased. Additionally, there are none of the associated problems that occur with some forms of batteries such as the memory effect or disposal concerns of toxic battery materials.

It seems that a fast-responding energy storage system can provide the same functional frequency regulation of a fossil-fuel facility that has twice the capacity. This is very interesting, since it illustrates the tremendous value to the power grid of a fast-responding energy source. Fast-responding energy storage is estimated to have a 67-89% smaller carbon footprint than an equivalently capable fossil-fuel system.

Large-scale energy storage systems

Our electrical utilities are largely structured around the largely baseload power sources we currently derive most of our power from. The majority of emerging renewable energy is coming in intermittent forms, such as wind and solar photovoltaics. To leverage these immense renewable energy sources, we must develop ways to produce reliable power while including sources that are unreliable on their own. One way of doing this is to utilize our steadily improving systems of energy storage.

Energy storage for intermittent power source balancing

In order to produce reliable power, a grid must generally use dispatchable power sources or energy storage to cope with the unreliability of intermittent power sources. Intermittent power sources sometimes synergize to some extent, such as how wind tends to be stronger at night, and solar produces power during the day. In general however, we cannot rely on these natural synergies to deliver reliable power. We need dispatchable electricity sources to call upon for balancing the grid, or we require a method to storage energy produced in excess of demand to be called upon when demand outstrips supply. Alternatively, we can request that customers reduce their demand when supply is low, this is known as managing the demand response.

Here we will discuss two interesting examples of energy storage systems that we believe show tremendous promise today.

Compressed air energy storage (CAES)

The Basics

Energy is required to compress air into a storage system, and energy can be extracted when air is uncompressed. Using a variety of storage methods and compression techniques, as well as playing on the thermodynamic realities of compression and decompression of gases, energy can be stored for later release back into the grid.

Different types of reservoirs

A CAES storage reservoir requires a large confined volume in which to store compressed gases. The maximum pressure attainable depends on the reservoir characteristics. The greater the compression of the air, the greater the energy storage density. Reservoir types include salt caverns, old mines, underground aquifers, and sealed underwater vessels. The solid reservoirs perform similarly, while the water ones have distinct advantages.

In any reservoir, air can be compressed to within a safe margin of the reservoirs’s breaking point, which varies from reservoir to reservoir. As this is done, the pressure in the cavern will vary from the ambient (atmospheric) pressure upwards. In an aquifer or underwater vessel, the compressed air will displace a volume of water until the water pressure is in balance with the air pressure; providing bonuses to turbine operation due to the narrowed range of operating pressures, and a variable reservoir.

The difficulties

During compression some of the energy will go into heating the air, all other important variables remaining the same. During decompression, each unit of work extracted reduces the temperature of the remaining air, reducing its pressure and therefore the work that can be extracted from the same mass of air. Increased temperature due to compression will make the air more difficult to compress, and much of this heat energy will be lost as the air reservoir cools to the ambient surrounding temperature.

The thermodynamic efficiency of the compression and decompression stages is determined by the ratio of compression/decompression undergone per stage. For acceptable efficiencies to be achieved, the change in pressure per stage should be minimized, but each additional stage adds time and equipment cost, and there is a declining rate of return as we add more stages. When a proper balance is struck, roughly 75% of the energy stored can be retrieved, including loses due to equipment.

When compressing gas into the reservoir, this heat energy will be lost unless it is channelled to alternative uses, such as space heating or process heat. To make efficient use of the compressed air, it must be heated during the decompression stage. This heating can be achieved by allowing time and utilizing heat exchangers to assist the air in rising to the ambient temperature, but this is costly. In practice, natural gas turbines are often used to provide additional heat. This practice strikes the most effective balance between cost, speed, and efficiency. In a purely renewable compressed air energy storage system, sustainable sources of biogas may be used as the heat source and a different operational balance struck.

There are a few large scale commercial air storage systems in use, such as the Iowa Stored Energy Park with a peak output capacity of 270 MW at a cost of 200 to 225 million USD ($740 to $830 per kilowatt peak output) to and a few others we are aware of. Costs have also been estimated at around $600 to $700 per kilowatt of generation capacity for a wind-integrated system .

Cambridge gravel-argon battery: Great potential

Researchers at Cambridge have experimented on a gravel-argon battery that can store energy in the temperature difference between two large gravel masses. Essentially the system acts like a refrigerator when it is storing the energy, so it causes one silo to heat up to about 500ºC, and the other to cool down to about -160ºC. This temperature difference leads to a theoretical thermal efficiency (initially) of about 85%. The researchers claim a practical conversion of up to 80% efficiency.

The cost of the system is quoted at about 10-55 dollars/kWh of storage. Also, the energy leaks away only very slowly. The designer is quoted as saying that a large insulated system (50m diameter, 50m tall) would lose half its energy to leakage in about three years. Additionally, this storage mechanism would occupy a very small area compared to pumped-hydro storage, our best energy storage mechanism today.

Dispatchable renewables

Update: A later issue of the renewable energy review covers the issue of how we can produce dispatchable power using renewable sources.

Call for submissions

This concludes the second installment of the Renewable Energy Review Blog Carnival. For a complete list of all publications in this series, see our post regarding the launch of the carnival. If you are interested in submitting an blog post or article to this carnival, see our submission page on the Blog Carnival website. This carnival is published regularly, and we are always interested in seeing new material.

The intent of this publication is an ongoing investigation of the progress and potential of renewable energy in our world. Our goal is to collect the best writing and news on the subject of renewable energy projects and policies. We have observed that humanity is innovating rapidly as the energy security of the future becomes a global priority.