Seasonal Storage of Air Cooled Water for Arid Zone Geothermal Power Plants

Abstract

This paper presents a new option for heat rejection from geothermal plants. Our 20 years experience with small geothermal plants at Mulka Station and Birdsville has convinced us that heat rejection is a major issue in Australian conditions. In arid zones air cooling has been the option of choice due to lack of water and the availability of industrial air condensers. This option has proved expensive, with costs in the U.S. of 20 - 30% of the total capital costs, including wells for 5 major geothermal plants As noted previously by several authors this option also comes with severe performance penalties due to the extreme summer daytime temperatures. When the demand for electricity is greatest the geothermal electric output will drop by up to 40%. The geothermal industry needs to begin development and testing of new heat rejection options to solve this problem. Just as the desert presents problems with high daytime temperatures, it offers opportunities with very low night time temperatures, large areas of land available and in most regions, shallow, saline aquifers suitable for water storage. A simple heat rejection system taking advantage of these characteristics is illustrated in Figure 1. A water cooled condenser using saline water from a shallow aquifer provides the heat rejection sink for the geothermal plant. The area of the aquifer used is sized to provide approximately 6 months of cooling water. The water that is returned to the aquifer is intermittently cooled by air coolers when the air temperature is below a set value. As long as the heat rejected to the air during cooler periods matches the heat going into the water form the condenser over the year, a constant water temperature will be maintained.