Now that we've been introduced to the concept of energy storage, let's look at some of the leading energy storage technologies in more detail. Recall from the last lesson that there are many different energy storage technologies, each with its own characteristics, advantages, and challenges. Let's look first at batteries, which most of us think of when we think of storage. We're familiar with several types of batteries in everyday use. Alkaline batteries for a wide variety of applications, the traditional lead acid batteries used in cars, trucks and boats, and lithium-ion batteries that power the latest electric vehicles. But there are many others, each with their own advantages and disadvantages. For energy storage purposes, we'll focus on batteries that can be charged and discharged many times. There are several types of electrochemical batteries, all operating on the principle of storing electrical energy by converting electricity into chemical energy in the form of charged ions that flow between the cathode, a negative terminal, and the anode, a positive terminal, then converting the chemical energy back to electricity when discharging. Electrochemical batteries can be built to produce up to 10 megawatts of electricity, but generally operate for only up to a few hours. Electrochemical battery types include lead acid batteries are the most mature and widely used battery technology, commonly in vehicles and in various stationary applications where they are not subject to rough treatment and where weight is not an important consideration. Metallic lead and sulfuric acid are the primary components. Lithium-ion batteries are also a mature technology, although advances are still being made. Ions of the metal lithium carry the chemical energy during charging and discharging cycles. Other electrochemical batteries have specialized applications and/or operate at elevated temperatures. Research and development to improve their performance characteristics are ongoing. Flow batteries also convert electricity into chemical energy and then back again to electricity. But instead of relying on charged ions flowing between cathode and anode, the chemical energy is provided by two chemical components dissolved in liquids pumped through the system on separate sides of a membrane. Flow batteries also use various specific chemical reactions involving different materials and chemical processes. They offer flexible layouts, long cycle time, and quick response times, but have relatively low-energy density and slow-charge discharge rates, meaning they need to be large. Let's move on now to look at other energy storage technologies. We're joined by Dr. Maurice Dusseault, who teaches geological engineering at the University of Waterloo. He works on subsurface energy technologies including hydrocarbon development, hydraulic fracturing, energy storage, geothermal energy, carbon sequestration, and deep injection disposal of granular solids and liquid wastes. Maurice is going to address a wide variety of energy storage technologies, including many that most people don't think about. Energy can be stored at the surface of the Earth or in the subsurface. There are many different surface storage methods. Perhaps the most common that we see are oil tanks and natural gas pressure vessels. There is great merit to consider subsurface energy storing methods for a number of reasons. For example, the surface footprint can be extremely small. Consider salt cavern at depth just as a wellhead, for example. We can store fluids in salt caverns, in porous media. We can store compressed air as an energy storage system in porous media, in salt caverns, or even in steel case cemented wellbores. We can store heat in the rock mass by heating up the rock mass, say, during the summer, and then using that rock mass heat beneficially during the winter. The emerging hydrogen storage technologies that are going to help us towards a partial hydrogen economy needs storage. We need to store the hydrogen as molecular hydrogen, which is very, very incompressible or very difficult to compress, or we can store it as a hydrogen-rich molecule such as ammonia or methanol. Of course, underground pumped hydro energy storage is an option. The energy density issue arises whenever we're looking at subsurface energy storage or even surface energy storage. It has to do with the density of energy per volume per liter versus the density of energy per kilogram. Let's just see two examples here. On the far right, you'll see that hydrogen has a huge amount of energy per kilogram. But even at a condition of liquid hydrogen at 27 degrees Kelvin, the energy density per volume is only about a third or a quarter of the energy density of diesel fuel or gasoline. By contrast, methane at 25 megapascals has a energy density of about 53 megajoules per kilogram, but only about nine megajoules per liter. So energy density is an important concept in storage. Focusing more on the specific storage technologies, we see that all of the common storage methods, heat storage, distributed batteries, compressed air storage, pumped hydro, all of them tend to have an extremely low density per kilogram of construct and a low density per volume as well. This means that these technologies require very large volumes in order to be economically interesting to grids and to large industries. Subsurface methane storage, large volume storage has been around since the 1950s when salt caverns in Saskatchewan were among the first ever used for the storage of natural gas. We can also use saline aquifers and depleted reservoirs. Some of the advantage: less costly than surface tanks, the footprint is very small, and we can refill these storage facilities seasonally to smooth out pipelining demands based on climatic conditions, especially in cold climates where we have a huge demand in the winter compared to the summer. Subsurface hydrocarbon storage is big business whenever you're in an area that has large refining capabilities. For example, along the Texas Gulf Coast, there are many refineries that ship oil and products all the way to Maine and New York, and even export products such as liquefied natural gas to Europe. Well, these facilities need storage. In the Gulf Coast, salt domes are very common and these are often replete with caverns to store natural gas. In China, the national strategic oil storage project currently stores twice the American volumes of crude oil. In India, they're generating storage facilities by excavating in hard rock. In the compressed air energy storage cycle, we take renewable power, such as sun and wind, which are irregular sources and they're variable even from second to second in many circumstances, and we take excess power from the grid. For example, if we have nuclear power plants, there's excess power being generated at certain times when the power demand in society is weak. We compress that air and we store it in a salt gallery, an open cavity that is excavated in salt by solution mining. When we need this energy, we pass the compressed air through an air turbine and we also use some of the heat of compression which we stored to heat the air up going into the turbine so that we're recovering more of the energy of the compressed air. If you don't store the heat, you'll have to use some natural gas as fuel to heat the air to avoid icing. A highly efficient storage system of this type can achieve just over 70 percent round-trip efficiency. If you don't store the heat in some way, if you just waste the heat of compression, then your efficiency will drop down to about 48-52 percent. Salt is an ideal material for the storage of compressed air and also for the storage of methane or hydrogen and other petrochemical liquids and crude oils, any kind of fluid that doesn't dissolve the salt. The advantages are that salt is really very, very low permeability. It's essentially impermeable and it's not soluble in oil-type hydrocarbons, but it is soluble in water. So it's easy to mine. We've been doing this for 70 years in our societies. We know how to build stable storage structures of large volume and how to control them within pressure limits that are set by geomechanics analysis and thermodynamic considerations. We can provide grid scale storage, even long-term power provisions on the range of hundreds of megawatts and, of course, storage of gigawatts, a gigawatt hours of energy over a period of, let's say, a day, or four hours, or maybe several days. However, good, thick salt beds are present only in a few locations in the world. For example, Northeast of Edmonton, we have many salt caverns in the Fort Saskatchewan area, but Southwest of Edmonton there's no salt. So we can't store energy as compressed air in a salt cavern, for example, in the Hinton region. There is no salt. An alternative is to use oil field technology based upon steel case dwells of large volume, say, 70 cubic meters per kilometer depth. This will provide you with approximately eight hours of energy at one megawatt output. Now, clearly, this is almost off-the-shelf technology from the oil industry and we see here at the bottom a compressor and an expander, these are standard items used in a system of this type. The nice thing about this is that it's essentially irrelevant to the nature of the geology locally. So we can put these in Toronto. We can put these in Montreal. We can put these in Winnipeg, where we don't have the depth of sediments and we don't have salt. Another approach to energy storage, not really in the subsurface this time, for this example, we can see the upper pond just at the top of the picture here. That is a pond that is pumped up by spare energy when it's not needed and then that water is passed through the powerhouse when it's needed. Pumped hydro approaches efficiencies of about 85 percent on full cycle. But of course, it has a huge surface imprint and you need to build dams. You need to build a retaining structure. Socioeconomically, that might no longer be very acceptable to society in general. Here is a configuration using a mine that is no longer being exploited to store power from an upper reservoir, passing through a pump house to generate electricity. The water level in the mine is much lower than the water level in the upper reservoir, which is probably a surface reservoir. That's your difference in height and that's where your energy comes from. The literal equation shows you the absolute maximum at 100 percent efficiency. So here's a way to put part of the system underground. Is this actually happening in the world? Yes. Pumped hydro exists at a very small scale in many parts of the world. Now here's a project. TC Energy is the proponent of this project and it takes advantage of the height difference in the Niagara Escarpment on the Bruce Peninsula and Lake Huron. They propose to put all of the powerhouses and the pump turbines underground. So the surface footprint from that point of view will be small. The storage pond, however, is very large area up high on the escarpment. Because there is environmental perturbation, many people oppose this and the project is under discussion and under consideration and is currently applying for permits. In a country like Canada, heat is very important. Heat is currently provided largely by natural gas or hydropower across Canada. Here are two projects, one in Germany on the left, where heat is stored for a period of a day or two or three in water. There's many other approaches. There are thermal oils. There are phase change salts. There's other media that you can use to store heat and use that heat beneficially later on when you need it. Well, we're doing this to a considerable degree in Canada. Here is an image of the Nordic Condos in Toronto and a company that is started by actually an ex-professor, Geosource Energy. He provides services to drill boreholes for heat exchange down to 850 feet and create a geological heat repository underneath this condo development. That means that in the summer, when there's excess heat and you need air conditioning, the excess heat is sucked out of the rooms and pumped down into this geo reservoir. In the winter, the opposite is true. You are pumping the heat into the rooms and cooling down the reservoir. So this is seasonal heat storage using a geological medium as your storage facility. Heat storage is actually already happening and it's based upon ground source heat pump technology, as I mentioned in the previous slide. Now can we store the heat in a geothermal system at higher temperature? If we have waste heat that is higher temperature, let's say we could heat up water to 100 degrees Celsius, or maybe more, from diesel power generators or from industrial processes that normally waste the heat, well, that water could be pumped into a geothermal reservoir and be used when we need it. Alternatively, you could perhaps collect low-grade heat or even higher-grade heat from some other source and pump it into the rock as liquid. The nice thing about geological storage of heat is that we can do this on an annual cycle. That means there are definite advantages to a cold country like Canada where, in January, everyone needs heat. Here I'm showing the concept of parabolic trough reflectors that focus the sun's energy onto a focal point which has a black painted tube that absorbs the heat to heat the fluid inside the tube. In hot climates like, say, Southern Saudi Arabia, you can actually heat the liquid in those tubes to in excess of 250-260 degrees Celsius. Maybe not so high in Canada, but maybe 200 degrees Celsius. If we can provide that fluid at high temperature, we can actually push that fluid down into the ground to store it in the rock mass. So we have to heat up some volume of granite, and that volume turns out to be quite large because granite does not have as high a specific heat capacity as water, for example. Here I show the concept of a solar heat farm with parabolic collectors and the injection of the hot fluid into the crystalline bedrock thermal repository. Now on the left-hand side, I'm suggesting this could be as much as five kilometers deep. But in practical terms, we would probably be limiting it to several kilometers deep. But it depends. There's many parameters to assess. The heat required for a home can be stored in a cube of granite 12 meters on its side with a Delta T of 20 degrees Celsius. That means that, on average, we can extract that amount of heat from the granite. Remember that a small cube, like a small cup of coffee, has a rapid heat loss. But a very large cube, say 100 by 100, that cube can store heat seasonally with very low losses and enough to heat an entire community. Not to provide electrical power, but enough to heat the community, which, of course, is valuable. The subsurface can be used in many ways to store energy. We can store hydrogen, natural gas, compressed air, heat. The subsurface is a good way to do it. Very low risk, very small environmental impact, small surface footprint, and we can store valuable things like low-grade heat to keep you warm in the winter. That's something that we need in many of our communities, especially in the North where the only fuel that they use, the only energy source that they have is diesel fuel. We can do better. We've seen that there is a large variety of energy storage technologies that can provide electricity at all scales. From your personal appliances and vehicles all the way up to gigawatt scale electrical grid support. Batteries and electrical devices or supercapacitors can react quickly to provide power locally, such as to your electrical vehicle. Larger ones can help ensure the smooth, continuous operations of our electrical grids over short periods of time, ranging up to several hours. But for longer-term storage, to maintain electrical supply for longer periods when intermittent energy sources like wind and solar cannot deliver, we need to look at innovative energy storage mechanisms like subsurface storage and pumped hydro. In our next lesson, we'll look at hydrogen, which many think of as an energy source, but it's actually an energy storage technology as well.