Modeling of a Magma Energy Geothermal Open Cycle Power Plant

Abstract

We are currently investigating the engineering feasibility of drilling into an active magma body at a depth of roughly 5 km from the earth’s surface, establishing a downhole heat exchange region, and extracting thermal energy from the magma body by circulating fluid through this heat exchange region. In the present paper, we evaluate the overall thermodynamic performance of a conceptual magma energy system in which energy is added as heat to the fluid within the magma region and is converted to useful work in a power conversion cycle at the surface. Unusually high return temperatures and pressures may be available at the wellhead of such a circulating well. Investigated here is an open Rankine power system in which heated water from the magma well is circulated directly through a power conversion cycle. The downhole heat exchange region is established during the drilling process. As drilling proceeds into the magma, a solidified layer forms about the drilling tube due to heat exchange to the fluid. This solidified layer thermally fractures because of large temperature gradients between the cooled inner region and the heater outer region, thereby opening secondary flow paths. Two models of the downhole behavior have been used. In the simplest approach, denoted as the “infinite area model,” the water entering the pipe to return to the surface is assumed to be always at the temperature of the magma, independent of mass flow rate and other parameters. The other model is more detailed and the fractured heat exchange region is modeled as a cylindrical porous layer through which fluid flows vertically. The net power and other performance aspects for the systems are investigated in terms of various parameters, including the characteristics of the downhole heat transfer. It is concluded that the open Rankine cycle probably will not be appropriate for this application; however, the analysis provides the first insights into possible characteristics of this energy resource.