Power Plant Process at Nesjavellir Based on Experimental Tests

Abstract

This paper describes the result of experimental tests for geothermal heat exchangers and wellhead control valves at the Nesjavellir geothermal field. The power plant started in 1990 by heating of cold potable groundwater with steam to use in district heating in ReykjavÌk. In 1998 the plant became a combined heat and power (CHP) plant generating 60 MW of electricity. The heat exchange process was modified and geothermal brine was used to heat potable groundwater. Prior to exploitation a pilot plant was operated in the period 1974- 1990, supplied with geo-fluid from one of the wells. The purpose of the pilot plant was to explore and perform tests of the processes planned in the power plant. One of the tests was to use geothermal brine in the heat exchanger. These tests were mainly performed in the period 1985-1988 and later 1990-1996 on small-scale in the power plant. Test with geofluid heat exchangers was performed in the period 1997-1998. Long-run tests with utilization of geo-fluids and brine in conventional shell and tube heat exchangers indicate reduction of the heat transfer coefficient (k-value) in just a small degree. These results were important and valuable. Tests with wellhead control valves were performed in the period 1994-1998. The tests verified that both the geothermal brine heat exchanger and the wellhead control valves were able to operate sufficiently in the power plant process. At present, heat exchangers using separated geothermal brine are in full-scale operation and are one of the key elements in the utilization of high temperature geothermal fields for district heating. Control valves are in operation at the wellhead of every production well at Nesjavellir with considerable improvements of the plant efficiency. The geo-fluid heat exchangers are now used as reserve district heating for Nesjavellir area.