The next critical issue in enabling diverse energy solutions is energy storage. Let's start by asking ourselves, why is energy storage such a critical issue now, when we haven't talked much about it in the past? As we discussed earlier in the course, more than 80 percent of today's global energy is supplied by coal, oil, and natural gas. Nuclear and hydro-electricity are also important while intermittent renewables, wind, and solar supply less than 10 percent of the energy mix. In the future, regardless of which scenario we follow, a much higher percentage of our energy supply will come as electricity generated by low emission sources, nuclear, hydro, and a large proportion of wind and solar. The energy we obtain from oil, gas, coal, and nuclear, primarily uranium, is stored in the fields themselves and not released until we need it. This happens when we burn oil products and our cars and natural gas to heat our homes or coal at the electrical generation station or industrial plant. We can even think of the water stored behind a hydro dam as a fuel that generates energy upon demand when we let it flow over the dam and through the powerhouse but we can generate electricity from the wind or the sun only when those power sources are available. When the wind is blowing or the sun is shining. In the future, if we are going to obtain a lot of our energy in the form of electricity from the sun and the wind, we need to be able to store that electricity when more than enough as being generated and pull it from storage when the sun isn't shining and the wind isn't blowing. That's the basic concept but of course, it's never that simple. To run an electrical grid with a large component of intermittent renewable power supply, we need a variety of storage technologies in different places within the system to ensure reliable electricity supply at all times. Let's dig into that. Storage can take many forms and be used in a number of applications. Electrical generation and transmission companies already used short-term storage and other services to help ensure that their systems can manage the minute by minute demand changes that occur with normal operation of the grid. We're all familiar with the lights dimming momentarily at home when a power hungry appliance like a printer comes online suddenly. Imagine the short-term resources your electricity provider must draw on when huge factory equipment or massive air conditioners suddenly switch on while everyone else on the grid expects their power supply to continue uninterrupted. There are a number of storage strategies to address that challenge and any others like it. Behind the meter. Commercial and industrial customers install their own storage so that they can buy and store electricity when it's cheaper during low-demand periods, generally overnight, and use it to reduce electricity purchases during expensive high-demand periods. Bulk energy arbitrage, a business model where the operator purchases and stores electricity during cheap, low-demand periods and resells it on the open market during expensive, high demand periods. Renewable energy firming, using storage to maintain a steady output of electricity from intermittent wind and solar sources at different timescales, ranging from clouds passing in front of the sun or gusty winds, to more predictable changes like the day, night fluctuation of solar energy. As we see more and more wind and solar come on stream, long-term seasonal storage becomes more critical. In most places, the sun provides far more energy during the summer and much less during the winter. Wind energy also varies seasonally and may partially offset solar variation if it's strongest during the winter. There are other strategies to offset intermittency as well but almost everywhere where wind and solar are to be the main generators of electricity will require a lot of long-term storage that needs to be charged up in the summer and gradually consumed over the winter. We've seen that there are many applications and business models for energy storage but what about the technology that actually stores the energy? When we think of storage, we often think of batteries, such as the lithium batteries that power electric vehicles and many of our tech applications today. However, there's a huge range of energy storage technologies, from small one kilowatt to huge one gigawatt electrical output, producing electricity for mere seconds, hours, or days. We'll review the major storage technologies in the next lesson but a quick comparison here is useful to set the stage. There are a variety of battery technologies. They are all basically electro-chemical, storing energy using a chemical reaction and delivering electricity by reversing that reaction. Batteries range in output up to ten megawatts, which is a good size for an industrial operation or a small community. But they provide that energy for only minutes to a few hours, making them useful for short-term applications, but not long-term storage. A good example would be in backing up intermittent wind and solar when the wind is gusty or on a partly cloudy day. Batteries can also provide extra power in the early evening when customer demand is high and solar power is going upstream. Electricity is stored directly in supercapacitors, but they generally have lower power ratings than most other storage technologies and are suitable only for very short-term applications with many rapid charge and discharge cycles. Covering momentary demand fluctuations in electrical grid and regenerative braking in electric or hybrid vehicles are good examples. Heat energy can be stored directly for later use where space heating or water heating is required. Heat storage is also a feature in some small nuclear reactors, where power output can be modified by cycling heat in and out of a storage medium, such as molten salt. Mechanical storage technologies, where energy is stored by imparting kinetic or potential energy to large masses of material offer very large power ratings and long-term storage potential. In fact, one such method, pumped hydro, accounts for more than 90 percent of the storage capacity in existence today. Compressed air energy storage and other mechanical technologies are being developed and can be useful in particular situations. Because they can generate and deliver electricity for longer periods of time, these systems will have tremendous importance as our electricity grids evolve. They're also very short-term mechanical storage technologies. Fly wheels stores kinetic energy by spinning rapidly, and that energy can be recalled very quickly to deliver additional electrical or mechanical power when needed, such as to cover very rapid fluctuations in electrical supply. We'll talk about some of the important mechanical technologies in the next lesson. Hydrogen can be viewed as an energy storage medium. We expend energy to manufacture it, and then it can be stored and later used as a fuel in hydrogen fuel cells or for heat. We'll talk about hydrogen in more detail in a separate lesson. Let's look at an example of how energy storage can be used to supplement conventional power sources. The West Kauai energy project on Hawaii's western most populated island is scheduled to come online in 2024. A solar array on the sunny South Western beaches is planned to deliver 35 megawatts of electricity to the grid and also power a battery that can store up to 240 megawatt hours. So it could deliver 24 megawatts of electricity for 10 hours. The hydro project features three reservoirs that can deliver an average of 24 megawatts daily, sourced in the northeast from an area advertised as the rainiest place on Earth. More than ten meters of rain annually. It includes pumped hydro storage that can deliver power overnight. The project overall will meet about 25 percent of the Island's electricity needs and will assist in reaching a target of 70 percent renewables generation by 2030 from firm hydro-power output, intermittent solar and strategic battery and pumped hydro storage. The storage is essential to deliver electricity overnight and on those rare days when West Hawaii isn't sunny. Besides the fundamental characteristics of different storage sources, there are a number of factors to consider in designing energy storage systems. Capital costs and operating expenses of the storage system under the anticipated conditions of use. We have to be able to afford to build and run the systems in order to use them effectively. The round trip efficiency of conversion, the amount of energy that comes out of storage divided by the amount that went in. The more energy lost in the conversion process, the less favorable the storage economics will be and the more storage capacity will need to backup our systems. Availability of the selected technologies. Pumped hydro storage requires a large reservoir at a significant elevation above the base level. Large-scale battery use requires reliable supply chains to provide the various raw materials to build literally millions of batteries. Hydrogen storage requires cheap production and efficient transportation of hydrogen to where it's needed. Now that we have some background on energy storage, our next lesson, we'll look more closely at some important technologies available for storage of various forms of energy.