Title:

High Efficiency and Large-Scale Subsurface Energy Storage with CO2

Authors:

M.R. FLEMING, B.M. ADAMS, J.B. RANDOLPH, J.D. OGLAND-HAND, T.H. KUEHN, T.A. BUSCHECK, J.M. BIELICKI, M.O. SAAR

Key Words:

energy storage, geothermal heat input, CO2, supercritical, high efficiency, large capacity, two reservoirs

Conference:

Stanford Geothermal Workshop

Year:

2018

Session:

Emerging Technology

Language:

English

Paper Number:

Fleming

File Size:

1351 KB

View File:

Abstract:

Storing large amounts intermittently produced solar or wind power for later, when there is a lack of sunlight or wind, is one of society’s biggest challenges when attempting to decarbonize energy systems. Traditional energy storage technologies tend to suffer from relatively low efficiencies, severe environmental concerns, and limited scale both in capacity and over time. Subsurface energy storage can solve the drawbacks of many other energy storage approaches, as it can be large scale in capacity and time, environmentally benign, and highly efficient. When CO2 is used as the (pressure) energy storage medium in reservoirs underneath caprocks at depths of at least ~1 km (to ensure the CO2 is in its supercritical state), the energy generated after the energy storage operation can be greater than the energy stored. This is possible if reservoir temperatures and CO2 storage durations combine to result in more geothermal energy input into the CO2 at depth than what the CO2 pumps at the surface (and other machinery) consume. Such subsurface energy storage is typically also large scale in capacity (due to typical reservoir sizes, potentially enabling storing excess power from a substantial portion of the power grid) and in time (enabling even seasonal energy storage). Here, we present subsurface electricity energy storage with supercritical carbon dioxide (CO2) called CO2-Plume Energy Storage (CPGES) and discuss the system’s performance, as well as its advantages and disadvantages, compared to other energy storage options. Our investigated system consists of a deep and a shallow reservoir, where excess electricity from the grid is stored by producing CO2 from the shallow reservoir and injecting it into the deep reservoir, storing the energy in the form of pressure and heat. When energy is needed, the geothermally heated CO2 is produced from the deep reservoir and injected into the shallow reservoir, passing through a power generation system along the way. Thus, the shallow reservoir takes the place of a storage tank at the surface. The shallow reservoir well system is a huff-and-puff system to store the CO2 with as few heat and pressure losses as possible, whereas the deep reservoir has an injection and a production well, so the CO2 can extract heat as it passes through. We find that both the diurnal (daily) and seasonal (6 months) CPGES systems generate more electricity to the power grid than they store from it. The diurnal system has a ratio of generated electricity to stored electricity (called the Energy Storage Ratio) between 2.93 and 1.95. Similarly, the seasonal system has an energy storage ratio between 1.55 and 1.05, depending on operational strategy. The energy storage ratio decreases with duration due to the pump power needed to overcome the increasing reservoir pressures as CO2 is stored.


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