Electrical Generating Capacities of Geothermal Slim Holes

Abstract

A growing market exists for off-grid electrification projects in the developing countries of the world. Small wellhead geothermal generating plants (from 100 to 1000 kW electrical capacity) could supply these needs in nations with geothermal potential. Theoretical calculations are presented to estimate the electrical generating capacity of the hot fluids discharged from individual geothermal wells, particularly slim holes, over a wide range of reservoir and operating conditions. Slim holes (75 to 150 mm in diameter) can provide enough hot geofluid for such projects, and can also substantially reduce project costs and adverse environmental effects. The study examines both self-discharging wells and wells equipped with downhole pumps, as well as a variety of design concepts for the surface generating equipment (including flash-steam and binary systems). Resource temperature is the single most important parameter influencing electrical capacity. A downhole pump will usually be required if the reservoir temperature is below 150?C or so. Downhole submersible pumps are preferable to conventional lineshaft pumps for slim-hole applications. Downhole pumps cease to be advantageous (compared to selfdischarge) above ~200?C for standard production wells and above ~160?C for slim holes. Wells as small as 75 mm diameter can supply enough fluid to generate 100 kW of electricity so long as reservoir temperatures exceed 170?C, and can attain 600 kW if temperatures reach 240?C. Larger (150 mm diameter) slim holes can provide over 1000 kW for resource temperatures above 180?C and can reach 3000 kW for a 240?C fluid supply.