Direct Use of Boiling Saline Water in the Oxafjordur Heating System in NE Iceland--Production Problems and Solutions

Abstract

In 1994 a geothermal heating system was built in Öxarfjörður (NE Iceland), serving the village of Kópasker and surrounding rural area. This heating system was built following and based on 15 years geothermal exploration of the area. The geothermal system Skógalón utilized for the Öxarfjörður heating system, is a boiling geothermal field with a reservoir temperature, of about 150°C. The wellhead temperature has increased over time from 96°C to 116°C. The water is saline, about 3‰, devoid of oxygen and with a low concentration of hydrogen sulfide. Such as all Icelandic low-temperature geothermal waters, the Skógalon water is in equilibrium with calcite at the reservoir temperature. Geothermal water of similar chemical composition as the Skógalon water has been used in two other heating systems in Iceland without significant problems. In those heating systems both original design and later improvements have prevented major corrosion and scaling problems. In the Öxarfjördur heating system, however, there have been extensive production problems, both due to steel corrosion and calcite scaling. The rate of steel corrosion is higher than in any other heating system in the country. The troubles in production are partly due to an increase in temperature in the geothermal field over time, but mainly due to improper design and material choice. At present considerable energy is wasted to pump the water through a cooling unit in order to cool it below 100°C, thus avoiding boiling and cavitation in the system pumps. This prevents supersaturation and successive precipitation of calcite in the distribution system, however, it is very costly and a waste of energy. A good solution to improve the efficiency and profitability of the heating system as well as minimizing scaling and corrosion problems, would be to install a binary unit at the well head to produce electricity by cooling the water from 116°C to about 80°C. By doing that, the water cooling produces energy instead of wasting it. Due to the high capital cost this has, so far, not been a financially feasible option in the sparsely populated rural area.