Gravity Power

The challenges in implementing PHS severely limit the future potential for new PHS systems. However these challenges have also been the impetus behind a new water based energy storage technology proposal; the Gravity Power Module developed by Gravity Power LLC. The Gravity Power Module is a modular, closed underground system consisting of a large piston that moves up and down in a water filled storage shaft that is connected to a pump-turbine through a return pipe (Figure 1). During discharge, gravity pulls the piston down in the storage shaft forcing water into the return pipe. This water drives the pump-turbine to generate electricity. During charge, electricity is used to drive the pump-turbine to force water down the return pipe and under the piston, thereby forcing the piston back up the storage shaft (Mason, 2011).

Figure 1 showing the Gravity Power Module (Gravity Power LLC., 2010).

Compared to a traditional PHS system, the footprint of the gravity power system is rather small; for a 40MW unit the storage shaft is 30 meters in diameter and 500 meters deep, while a 250 MW unit is 80 meters in diameter and 500 meters deep. The balance of the plant (the pump-turbine, the return pipe, the housing for the motor/generator, etc…) is also comparatively small, fitting an entire module in less than an acre of land. Proposals to combine eight 150 MW modules into a 1.2 GW storage facility estimate land usage at less than 3 acres (Mason, 2011). This efficient, underground land usage greatly mitigates the siting concerns of traditional PHS.

As a fully closed, underground system, there would be negligible water loss compared to other PHS facilities. The fact that it is a closed system also means that the pressure within the system is constant. This allows the pump-turbine to be optimally designed to a single pressure resulting in estimated efficiencies on the order of 75% to 80%. Because the pump-turbine is always engaged in constant pressure water, the response times, compared to PHS, are also predicted to be lower than traditional PHS (Mason, 2011).

The power rating is a function of the shaft diameter and the piston size/weight which determine the hydraulic head and the water’s outflow rate. The energy rating is a function of the shaft length (excluding the piston length) which also determines the hydraulic head and the size of the reservoir. There is great flexibility in the possible sizes of the systems, however, a system with a 30m diameter storage shaft that is 2000m long, would have a power rating of 150 MW, an energy rating of 600 MWh and a cost of $1000/kW (Mason, 2011). It is predicted that Gravity Power Modules would serve the same applications as a traditional PHS system while being able to correct power quality events on an even shorter time scale (Mason, 2011). Though the Gravity Power energy storage system has not yet been deployed commercially it is another example of the innovation and investment driving the current energy storage industry.

Recognizing of course my strong bias in favor of mechanical energy storage, I see a lot of potential in the Gavity Power module as a large scale energy storage device. With power and energy ratings that rival large scale PHS at a fraction of the land use (and in some instances the potential to repurpose abandoned mine shafts) I predict that gravity power will get more and more consideration in the years to come. Recently, the Neckar-Alb Regional Government Association of the state of Baden-Württemberg in southern Germany has stated that the gravity power module will be evaluated, along with two other technologies to determine its ability to deliver large scale energy storage (Gravity Power LLC, 2013). This could be a significant win for the company.

Works Cited

Gravity Power LLC. (2013, June 12). Gravity Power Energy Storage System Approved By German Regional Planning Authority. Retrieved October 7, 2013, from Gravity Power LLC: http://www.gravitypower.net/newsitem.aspx?id=34

Gravity Power LLC. (2010). Gravity Power Modules. Retrieved January 12, 2013, from Gravity Power LLC.: http://82.80.210.34/gravitypower/index.aspx

Mason, T. (2011, October 11). Gravity Power: Utility-Scale Electricity Storage Systems. Retrieved January 12, 2013, from Definitive Solar Libary: http://www.youtube.com/watch?v=CujxJFXwOns