EGEE 101
Energy and the Environment

Electricity Storage

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Challenges of Electricity Storage

We have already examined how the electricity demand cycle changes according to the time of day, day of the week, season, and needs to accommodate temperature changes (due to electricity use in heating and cooling). Here are examples of the weekly electricity demand cycle for various months. Note that day and time have an impact on demand. Also, these are averages so the variance due to weather (especially cold/hot days and holidays are muted). When you use electricity it has a very significant impact on the cost to generate that supply (the peak will be more expensive than the electricity generated for the trough).

Electricity demand cycle changes for various months
Average hourly U.S. electricity load during a typical week (selected months)
What else influences the non-averaged electricity demand? Would California differ from Vermont? Does season or weather impact usage? What about the economy? Our electricity management in PA is controlled by the PJM which is a regional transmission organization for the movement of wholesale electricity. It covers PA and parts or all of 11 other states.
Credit: Energy Information Agency

Thus, to meet this demand we need to have either extra generation capacity or electricity storage. The traditional approach was to have additional generation capacity, often smaller natural gas-fired "peaking" units that were activated when the demand was high. As these were not used for extended periods (and they were small) they were/are expensive to operate. The owners also need to be paid (a financial incentive) to keep this electricity generation in reserve for when it is needed (the hotter afternoons in the summer, colder winter mornings/afternoons). A few locations used pumped-storage: using coal-fired and nuclear-derived electricity at night (when the demand was lower) to effectively store the electricity as potential energy in water that could be returned to electricity when demand was again high via the hydroelectricity approach. In some cases, this was a cheaper option than the addition of new capacity that was needed for the then growing electricity demand (currently electricity demand growth is very low). Our current situation however is very different. There is now an ongoing transition towards increasing the renewable energy contribution from solar and wind. Both suffer from an intermittency challenge: so we need either many more wind turbines and solar farms (well-dispersed) to accommodate the expectation for always available electricity or we need electricity storage options. The closure of coal-fired plants and the impending closure of nuclear plants will make this challenge much greater (electricity was available with a high probability unless maintenance or unscheduled off-grid issues). 

Figure shows two pie charts, the first showing pumped storage being ~95% of the U.S. capacity. The other 5% is mostly thermal storage and battery storage with limited contribution from compressed air and flywheel technologies.
Pumped storage is the leading contributor to our electricity storage but large-scale battery farms are now being added.
Credit: Department of Energy

Pumped Storage

This was covered earlier in the lesson. It is one of the few electricity "storage" options that have been available for decades and at scale. Here the electricity is converted to the potential energy in the water that is pumped to a higher elevation. This potential energy is converted back into electricity when needed (peak demand or to fill in the gap if renewable electricity is not meeting the need). This supplies 95% of the electricity "storage" in the U.S. There are some other emerging options, however.

An illustration showing the pumped-storage concept of an upper and lower reservoir with a pump house / generator
Pumped storage illustration (not to scale).
Credit: energy.gov

Large-Scale Battery Storage

A large-scale battery storage
location in Germany
Credit: BMW

We have used rechargeable batteries for decades, such as the lead-acid battery in gasoline and diesel vehicles (used to start the engine). However, the cost of alternative batteries has significantly declined and large-scale electricity storage plants are now possible using advanced Lithium-ion batteries. These plants make money by buying electricity when it is cheaper (when the demand is low) and selling it when the demand is higher (and the electricity cost is higher). Alternatively, they are now being paired with wind and solar farms to permit a more consistent electricity supply. The cheaper batteries are also "driving" the transportation transitions to electric vehicles (the pun was intentional — sorry) such as Tesla vehicles. If we electrify the transportation market, we will need much more electricity production. Some experts also predict we may have electricity storage at home in the future using some of these batteries (not just for rooftop solar cells). Again the system would store cheaper electricity and would provide electricity to the home when electricity costs are higher — as well as providing backup power for grid disruptions (blackouts).

Thermal Storage

Thermal storage was discussed with concentrated solar plants. By using a molten metal salt, there is a heat source (heat storage) that can be used when the sun goes down or on cloudy days. This could be heated by other power-plants.

Concentration solar thermal plant
A concentrated solar power plant. Notice the addition of the thermal energy storage tanks to increase the mass of the molten (very hot) metal salt from the receiving tower. These units extend the solar electricity generation period into the evening hours.
Credit: Clean Leap