When fracking is not allowed, or considered to be insufficient to lead to economic circulation rates in a conventional EGS system, drilling for geothermal energy exploitation in hot hard rock becomes futile, unless alternative concepts are investigated. One such alternative is offered by closed-loop geothermal systems, which do not require fracking to harness the geothermal energy. The idea of this proposed closed-loop concept is based on the Geyser principle, which has several advantages compared to conventional deep borehole exchangers. In particular, no downhole lifting is needed to run the system, which eliminates the costs of operating and maintaining pumps, and also the risk of well downtime when the pumps present issues. A Geyser produces steam, which can then be used to drive directly the energy producing turbines and generators, without an additional heat exchange cycle. The challenge of such a system is the technical feasibility, as certain conditions and requirements must be fulfilled before it can efficiently work. This paper introduces this novel concept based on the Geyser principle to produce high-enthalpy overheated steam downhole. The boiling point of each liquid depends significantly on both temperature and pressure, which in turn depend on well location and depth. In general, the higher the temperature, the better it is for the system to produce sufficient amount of hot steam. Thus, temperature and temperature are key design parameters. The novelty of the system consists of a downhole boiler with a special valve. Due to the hydrostatic column of water in the annulus, the pressure is relatively high at the boiler’s inlet, i.e. 700 bar. A special valve controls the inlet of the boiler according to a predetermined pressure value. This mechanism leads to a low pressure within the boiler and, consequently, to a lower temperature boiling point. The hot rock heats up the fluid within the boiler, so that the liquid fluid transitions to steam. The overheated steam has a significantly lower density than liquid and will therefore flow from the boiler through the tubing to the surface without the need for artificial lift, and at greater rates.

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