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Pumped hydro energy storage

A pumped hydro storage system (PHES) relies on gravitational energy using the difference in height between two water reservoirs to store energy. During periods when electricity demand is low, electricity is used to pump water from the lower reservoir to the higher one. During periods of high demand this water is released through the pumps now acting as turbines to generate electricity. Systems can operate using reversible pump-turbines or separate turbines and pumps.The size of the reservoirs determines the charge/discharge duration.

Pumped hydro is the most widely deployed stationary storage technology with 158 GW installed in 2020. This is ~90% of the installed stationary storage capacity globally. The first PHES plant was built in Switzerland in 1907. Significant deployment started in the 1960s in line with nuclear power plant deployment to charge with low-cost nuclear electricity overnight and discharge during peak demand mid-day. With deployments of ~5 GW each year, pumped hydro is still one of the fastest growing stationary storage technologies.


Figure 1 - Schematic of a pumped hydro energy storage plant.

Heindl Gravity Storage

Heindl Energy's Gravity Storage concept is based on the hydraulic lifting of a very large rock mass using water pumps. The rock mass acquires potential energy and can release this energy when the water that is under pressure is discharged back through a turbine. The figure below shows the concept as well as its lifetime cost compared to the the four most common stationary electricity storage technologies for bulk electricity storage.


Figure 2 - Levelized cost of storage in US$/MWh discharged electricity for investigated bulk storage technologies of 5 GWh system size, 8 hours discharge duration, 330 full equivalent charge cycles per year, electricity price of 20 US$/MWh and 8% discount rate. Values are compared to results from studies by Lazard.

Based on the given data, Gravity Storage is the most cost-effective bulk electricity storage technology for systems larger than  1 GWh, followed by compressed air and pumped hydro. Low specific energy investment costs represent the key advantage for these technologies at the required discharge duration of 8 hours. Gravity Storage further benefits from moderate specific power investment costs and more significant scale effects with increasing system size.

Schmidt, O. Levelised Cost of Storage - Gravity Storage. Imperial College Consultants and Storage Lab. 2018. 

Please contact us for the consulting report. 

Gravitricity Gravity Storage

LCOS can be expressed in power terms as $/kW-year, also called annuitised capacity cost (ACC). This metric represents the minimum payment required for each “kW” that is made available for power provision for an entire year to achieve a net present value of zero. It is used for applications that require electric power like frequency response services. The figure below shows the ACC for four technologies providing frequency response compared to the gravity-based concept by Gravitricity.


Figure 3 - Gravitricity concept and comparison of its ACC in a frequency response application (US$/kW-year) to other suitable electricity storage technologies.

Low specific power cost and high cyclability represent the key advantages of Gravitricity over the comparison technologies. These characteristics match best with the power-to-energy ratio and cycle requirements in the given application.

Schmidt, O. Levelised Cost of Storage for Gravitricity storage systems. Imperial College Consultants and Storage Lab. 2018. 

Please contact us for the consulting report. 

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