Gridflex Energy, LLC utilizes the two commercially-proven technologies available for bulk energy storage today: pumped hydroelectric energy storage (PHES) and compressed air energy storage (CAES). As new technologies emerge, we will evaluate them for inclusion in our portfolio.
Pumped Hydroelectric Energy Storage (PHES), more commonly referred to simply as “pumped storage,” creates potential energy by pumping water from a lower reservoir to an upper reservoir during off-peak periods. During the generation period, stored water is released to flow downhill through a turbine to produce power. Because pumped storage plants can operate flexibly and respond quickly, they can offer a variety of grid-support (ancillary) services to a utility in addition to power generation.
Pumped storage has been utilized for the past hundred years. There are currently more than 120,000 MW of pumped storage generating capacity installed around the world.
Most Gridflex projects involving pumped storage utilize smaller-scale, efficient, closed-loop projects that do not involve natural bodies of water. These projects have a smaller environmental footprint and can be sited more easily and permitted more quickly.
Compressed Air Energy Storage (CAES) utilizes pressurized air as its storage medium. During the charging period, air is compressed and stored in an underground cavern at pressures exceeding 1,000 psi. During the generation period, this air is released through a turbine to generate power. A relatively small amount of natural gas is also combusted in the generation cycle. Caverns can be built in salt beds, in aquifers, or in hard rock. As with pumped storage, CAES plants can provide ancillary services to the grid due to their operational flexibility and speed of response.
CAES has been utilized for bulk energy storage since 1979, when the 290 MW Huntorf plant in Germany began operation. This was followed in 1991 by the 110 MW McIntosh plant in Alabama. Both plants have operated at high levels of availability and reliability. Both plants utilize salt domes for storage caverns. Currently under development is a 260 MW aquifer-based project in Iowa, with additional projects in various earlier stages of development.
In addition to being proven and affordable solutions, PHES and CAES are geographically complementary because each may be viable where the other is not. PHES requires a high head, steep slope, and suitable landforms. Where this kind of topography is not available, the below-ground geology may be suitable for CAES.
The Pumped Storage Advantage
A variety of other energy storage technologies—batteries, flywheels, thermal energy storage, and others—are in various stages of development. None are yet commercially viable for multi-hour storage at the scale of hundreds of megawatts. Pumped storage hydro is sometimes compared with emerging storage technologies like batteries. However, pumped storage facilities are in a class by themselves when it comes to value and versatility:
1. Good pumped storage sites have a far lower cost per kWh of storage capacity than batteries and other short-duration storage technologies.
2. With full-power storage capacities well beyond eight hours, pumped storage can easily qualify as firm peaking capacity, competing directly against simple-cycle gas turbines or even combined-cycle gas turbines. Other storage technologies cannot do this, so they are far more dependent on ancillary services and arbitrage revenues.
3. Pumped storage facilities can provide the same level of fast-response frequency regulation service that batteries, flywheels, and other emerging storage technologies are designed for.
4. Pumped storage facilities have lifetimes exceeding 75 years, whereas batteries are still working toward a 15 year lifetime.
In sum, pumped storage facilities should be viewed as firm generating assets with the capability to provide the full range of ancillary services more quickly than gas peakers and more cost-effectively than other storage approaches.