Output & Distribution of heat and electrtricity

Hitaveita Reykjavikur (the Reykjavik Municipal District Heating Service) supplies geothermal water to the greater Reykjavik area (except Seltjarnarnes), where over 53% of the population of Iceland lives.
Today the Hitaveita utilizes three low-temperature geothermal fields. The present maximum capacity of the low-temperature fields is about 460 MEW. Hitaveita Reykjavikur needs a further 140 MWt in order to meet peak power demands. It is not possible to increase production from these geothermal fields. They are fully exploited and their production capacity has declined by about 4% annually over the past few years due to extensive pressure drop in the reservoirs. Temperature decrease and scaling due to changes in fluid chemistry have been observed in a few production wells so their utilization was stopped.
Hitaveita Reykjavikur has carried out detailed geothermal exploration in the vicinity of Reykjavik in search of new exploitable low-temperature reservoirs. No usable reservoirs have been discovered so far. Exploration was also carried out at Nesjavellir and Kolvidarholl, which belong to the Hengill high-temperature area situated some 20-30 km east of Reykjavik. Nesjavellir has been found to be the most economical alternative to increase the geothermal production capacity of Hitaveita Reykjavikur.
New connections to the distribution system increase by 3-4% a year on average. During the past few years this has been met by increasing storage capacity from 18,000 m3 to 72,000 m3 and by the installation of additional oil-fired boilers.
The City Council of Reykjavik decided on 20 November 1986 to begin construction of a geothermal power plant at Nesjavellir. The first stage in this development is a thermal power plant with a capacity of 100 MWt. The plant went into operation in September 1990.


Well Capacity at Nesjavellir

Since 1972, all exploratory wells have been designed so as to function as production wells later. Results have been good. On average, each well has a thermal power of 60 MWt, which would yield a net output of 30 MWt from a thermal power plant. This would be sufficient to supply space heating for a community with 7,500 inhabitants.
Of 18 wells drilled so far at Nesjavellir, 13 are production wells.
Five of them have so far been activated to allow the Nesjavellir plant to produce a total thermal power of 150 MWt. The plant is designed for a maximum capacity of 400 MWt.
A numerical simulation model of the geothermal reservoir indicates that it can support an operation of the power plant over 30 years.
Since energy demand can be expected to grow beyond the productivity of the present area, Hitaveita Reykjavikur is already securing land with geothermal sources for future development in the vicinity of Nesjavellir and the Hengill fissure system. There is intense geothermal activity in a wide area around the Hengill volcanic system, both adjacent to Nesjavellir and as far west as Kolvidarholl. Studies of their potential for development are already under way.
Alongside thermal production for space heating, the Nesjavellir plant offers the possibility of economical co-generation of electricity, using steam turbines. Some of the wells produce virtually pure steam. The maximum electrical generation capacity is 80 MWe, comprising 40 MWe from a back-pressure turbine and 40 MWe from a condensing steam turbine. The plant's own energy requirement for driving the wells at a production capacity of 400 MWt is 14 MWe.

Transmission Pipe To Reykjavik

The Nesjavellir power station is situated at an elevation of 177 meters above sea level. The water is pumped through a main pipeline of 90 cm in diameter to a storage tank in the Hengill area at an elevation of 406 meters.
From there the water flows by gravity , through a pipeline which is 90 cm. In diameter, to storage tanks on Reynisvatnsheidi and Grafarholt on the eastern outskirts of Reykjavik. Those tanks are at an elevation of 140 meters above sea level, and have control valves to regulate the flow of water through the pipeline and maintain a constant water level in the tank in the Hengill area.
From the storage tank, near Reykjavik, the water is fed through pipelines to the communities which are served by Hitaveita Reykjavikur.
From Nesjavellir to Grafarholt, the transmission pipe measures about 27 kilometers in length. It is designed to carry water at up to 96 °C, with a transmission rate of 1870 liters/s.
During phase I of the project, its flow rate was around 560 liters per second, whereby the water took 7 hours to run the length of the pipe and cooled by 2 C on the way. Good insulation and a high volume of water are the most crucial factors contributing to this low heat loss. At later construction stages at Nesjavellir, the flow rate will be tripled, reducing the heat loss to less than 1 C.
The steel pipe is insulated with rockwool and covered with aluminum sheets where it lies above the ground, and insulated with polyethylene and covered with PEH plastic where it lies underground. Its high insulative properties are shown by the fact that snow does not melt on the part that lies above the surface. For environmental and traffic reasons, a 5 km section of the pipe is underground. The surface section also runs under automobile crossings at several points which have been well marked.