Successful Utilization of Low-Temperature Geothermal Resources in Iceland for District Heating for 80 Years

Abstract

Geothermal energy provides at present about 2/3 of the primary energy supply in Iceland. Its principal use is for space heating, but other direct uses and electricity generation are also highly significant. About 90% of the space heating is currently by geothermal energy. The majority of the country’s district heating services use energy from many of the numerous low-temperature geothermal systems, which are all located outside the volcanic zone. Many of the geothermal district heating services (hitaveitur) have been in operation for several decades, the oldest ones for more than 80 years and several others for 30 – 60 years. Much can be learned from their operation, in particular regarding long-term management of low-temperature geothermal resources. In most cases the utilization is through the operation of down-hole pumps, but there are examples of large-scale free-flow being maintained for up to 50 years. The production- and response (pressure, chemistry and temperature) histories of seven low-temperature geothermal systems are presented. Three of the systems are very productive and reach pressure equilibrium at constant produc¬tion. Two are much less productive and don’t attain pressure equilibrium. One of the systems is in-between these two. Only one of the systems is plagued by consider¬able cold ground-water inflow that has resulted in temperature decline and chemical changes. Several problems have faced the Icelandic low-temperature operations, such as overexploitation manifest¬ing itself in excessive pressure draw-down as well as problems related to colder water inflow and sea water incursion. None of the district heating systems have ceased operation and solutions have been found to these problems. The solutions include improving the energy efficiency of the associated heating systems, deeper and more focussed drilling (e.g. directional drilling), finding new drilling targets (even new drilling areas), reinjection as well as technical solutions on surface. The long utilization case histories provide important information pertaining to sustainable management of geothermal resources. The future of all of these operations appears bright.