The technical feasibility of superconductive magnetic energy storage (SMES) is being studied by an interdisciplinary group at the University of Wisconsin, including fundamental research aimed at designing safe and economic undergound rock caverns to house the magnets. Evaluation of cavern stability for alternative magnet designs and proposed rock sites in Wisconsin for a prototype magnet present unique engineering problems requiring comprehensive field, laboratory and numerical studies. Analysis of three promising magnet designs and five possible rock units (granite, quartzite, rhyolite, two dolomites) indicate possible advantages and potential problems. Several conclusions and recommendations regarding tunnel geometry, load transfer, and the interaction between geology and cavern stability and deformation are made.
The continued development of superconductive magnetic energy storage (SMES) units (Fig. 1) indicates the promise and feasibility of building these large capacity, highly efficient (>90%), quick responding and virtually topographically unrestricted electrical energy storage devices. The virial theorem states that a charged SMES magnet will require a confinement structure proportional to the electromagnetic energy stored, or else, as it charges and discharges to level out the variable daily power demand, it will break apart. For a 10,000 MWh magnet, a prohibitive 777,000 tons of steel would be needed. Thus SMES magnets will be built at depth in rock, a similarly stiff and strong material available for the cost of excavation. The rock chamber for a 1,000 MWh to 10,000 MWh magnet consists of annular shaped tunnels, 5–8 m wide, 8–20 m high, with annular diameters of from 100–1,000 m. The charged magnet develops radial and axial forces which are transferred to the rock by regularly spaced polyester struts. In addition to magnet stresses, there are gravitational and anisotropic tectonic stresses whose effect varies from point to point around each tunnel's circumference.
The annular geometry also insures that each tunnel will intersect joint sets at anywhere from very favorable to very unfavorable orientations around the circumference. Moreover, the stacked annular tunnels create two different stress regimes in the rock mass: one on the outside of the tunnels in which the high horizontal stresses are active, and the other on the inside of the tunnels which shield the rock core bounded by the annular caverns. Excessive radial movement (>15 cm) of the rock walls under the complex applied loads will damage the conductors. Groundwater contact with the conductor will result in corrosion and icing which interfere with the magnet's operation. Rock mechanics research seeks to determine the suitability of different rock types and sites for housing an SMES magnet. Such rocks must possess sufficient strength for long-term stability, be stiff enough to prevent excessive conductor deformation, and have groundwater inflows which are neither difficult nor prohibitively expensive to control. The following sections describe the geological and mechanical properties of rocks with potential for SMES sites and their engineering classification.
An energy storage magnet site requires a suitable rock mass with sufficient horizontal and vertical extent to contain the cavern system.