| Abstract |
Since it was first visited by a United Nations scientific delegation in 1963, the Miravalles geothermal field in Costa Rica has developed into one of the most productive and reliable geothermal power complexes in the world. Currently five power units are in operation with a total installed capacity of 163 MW. This paper deals with power plant Unit 3, the only one that is not owned by the Instituto Costarricense de Electricidad (the Costa Rican Electricity Authority), known by its Spanish acronym ICE. This paper provides a brief summary and overview of the current Miravalles operation including a layout of the field showing the locations of power units, wells, and Satellite No.7 and its gathering and reinjection system pipelines. The type of power units and their installed capacities are given along with some performance data including electrical generation, as well as capacity, load and utilization factors. The history of the development of the single-flash 27.5 MW Unit 3 which began operating in 2000 is presented. The power plant occupies its own facility in the area of the field known as Las Mesas, approximately 2.5 km to the north-northeast of Units 1 and 2 at Miravalles. ICE is the owner and operator of the other four Miravalles units, as well as the field developer and steam supplier. However, Unit 3, the first and only Build-Operate-Transfer (BOT) power unit at Miravalles, presented specific problems for ICE which has the obligation to supply the steam to the unit as well as to purchase the energy generated by the unit from its owner and operator Geoenergia de Guanacaste (GdG). The conditions under which the steam must arrive at the fence of the unit are precisely defined in the BOT power purchase agreement (PPA). Since the steam conditions from the producing wells changed over the course of operation, ICE had to undertake certain actions to guarantee that they did not default on that obligation. The unique design aspects of the plant are presented and discussed. The performance of Unit 3 is given in terms of its thermodynamic design and its actual operating conditions. State diagrams are used to show the processes followed by the steam in the plant; state-point property tables are given for design and typical operating conditions; actual performance data are analyzed in terms of power generation, parasitic power requirements, utilization efficiency, specific steam consumption, and specific brine consumption. Also examined are how the plant performance and steam supply conditions have changed over the plant lifetime, and how ICE has managed the field to meet its contractual requirements. The paper includes a look into the environmental aspects of the unit. Although it is difficult to isolate the effects of one unit among a complex of five units, several possible impacts are examined including acid rain, air pollution and pollution of ground waters. |