Magma-Sourced Geothermal Energy and Plans for Krafla Magma Testbed, Iceland

Super-hot geothermal systems (SHGS) would be much more efficient in generating electric power than conventional systems. The heat source is expected to be magma accumulated just below the producing reservoir. These comprise a system of coupled, stacked liquid reservoirs, one of magma and one of hydrothermal fluid. Between them is hot rock, so hot that it will be ductile. The liquids in both reservoirs are expected to convect. In the hydrothermal reservoir, convection is by porous flow, where the fluid constitutes < 10 vol.% of the reservoir. For the magma reservoir, the circulating liquid+crystal suspension constitutes 100 vol.% of its container. Heat is advected upward through the magma, conducted through its ductile rock lid, and then advected upward by hydrothermal fluid where it can be extracted for power production. The rate-controlling step in transporting energy from deep crust to near surface is conduction through the magma’s lid, for which thickness is the critical factor. Heat flux from magma to hydrothermal fluid is inversely proportional to the thickness of the lid. The response time between a perturbation in one reservoir and its effect on the other is proportional to the square of the lid thickness. Most of the thermal energy in the system is contained within the magma, because magma’s energy is released not just be cooling but by latent heat of crystallization. Direct evidence for such a model is provided by accidental encounters with silicic magma by geothermal drilling at Kilauea Volcano, Hawaii; Krafla Caldera, Iceland; and Menengai Caldera, Kenya. The most complete data comes from the Iceland Deep Drilling Project’s IDDP-1 within Landsvirkjun’s (National Power Company of Iceland) Krafla Geothermal Project. IDDP-1 produced a sustained power output estimated at >100 MWt. The magma’s lid is < 20 m with a thermal gradient of > 20C/m, yielding a heat flow of > 40 W/m and a characteristic response time of about one year or less, well within the lifetime of a power plant. Thus, extracting superheated fluid from adjacent the magma body would in effect be using magma energy. There are, however, major challenges to putting magma energy into practice, including finding alloys and cements that will make the boreholes sustainable, treating the fluids so they can be introduced to turbines, and successfully prospecting for other magmatic sources. Besides its potential for power production, understanding where magma is and how it behaves is critical for mitigating risks to communities under threat of explosive eruptions. Thus was born the concept of the Krafla Magma Testbed (KMT). KMT will provide long-term infrastructure where science and engineering teams can conduct sampling, observations, and experiments in magma and its superhot rock envelope. Example analogues from other science fields are particle accelerators and telescope arrays. Critical experiments in Phase One of KMT include: 1) core through the rock-magma transition; 2) emplace a thermocouple string to measure heat flux through magma’s conductive lid; 3) provide (under)ground truth for testing geophysical techniques for locating magma. As the project progresses, further tests of drilling materials, borehole design, extreme sensors, and energy extraction will be conducted and a time series of magma samples obtained. KMT will be the first deep laboratory in the last frontier of Earth’s crust, with the potential to revolutionize both geothermal energy and volcanology.