ECONOMIC MODELLING OF GEOTHERMAL ENERGY PRODUCTION USING AN INNOVATIVE HIGH-EFFECIENCY SUPERCRITICAL CO2 TURBOEXPANSION CYCLE IMPLEMENTED DURING CARBON CAPTURE AND STORAGE

Large costs associated with carbon capture and storage (CCS) act as deterrents for its widespread market acceptance and implementation. In order to help mitigate these costs, a power generation scheme based on a closed natural circulation loop (NCL) is proposed. Previous studies by Brown, (2000) Pruess (2006) and Friefield, et al. (2012) have examined the efficiency of supercritical carbon dioxide as a heat mining fluid and Friefield, et al. (2012) has investigated its use integral to a carbon sequestration project. This study examines the economics of implementing a supercritical carbon dioxide-geothermal generation scheme simultaneously with geological sequestration of carbon dioxide.

Supercritical CO₂ is injected into a sedimentary reservoir at depth and then warmed by the geothermal heat flux. A production well within the zone affected by the injection well creates a natural convection cycle in which the warm CO₂ at depth is allowed to ascend up the production well and drive an electrical turbine. This fluid then descends down the injection well again creating a NCL requiring no pumping to sustain flow.

In order to evaluate costs and sequence for the best build out scenario for a minimized Levelized Cost of Electricity (LCOE), a model was created specific to the proposed power-generation scenario. Prior economic models, including the Department Of Energy’s Geothermal Electricity Technology Evaluation Model (GETEM) (Mines, 2008), were developed for flash or binary power generation turbines and conventional build out scenarios. The present model is developed for simultaneous power production and ongoing plant construction where the rate of plant construction, mass flow rate, well cost, plant construction cost, discount rate, loan payoff period, and other various costs can be manipulated to determine an ideal build out scenario.

Results suggest the LCOE for this scheme is as low as 3-4 ¢/kWhr for a 25 kg/s flow rate with a reservoir temperature of 100 °C (212°F) and 4.5-7 ¢/kWhr for a 50 kg/s flow rate with a reservoir temperature of 100 °C (212°F).