Nesjavellir Combined Heat and Power Plant

The Nesjavellir geothermal field is a high-enthalpy geothermal system within the Hengill area of SW-Iceland.  The plant is a combined geothermal heat and power plant (CHP) wherein it generates electricity and hot water for district heating. Construction of the geothermal power plant began in 1987 and the first stage of the thermal plant was commissioned in 1990, following an intensive drilling and testing phase in the 1980s. The last 30 MWe turbine generator unit was commissioned in 2005.

Mannvit Services:  

Project management	Assistance during contract negotiation
Pre-feasibility study	Site supervision
Feasibility study	Site engineering
EIA and report	Supervision for turbine and cold end installation
Design management	Steam gathering system
Overall power plant design	Technical supervision
Detailed mechanical design of the plant	Electrical work supervision
Control system supervision	Civil work supervision
Geotechnical design	Mechanical work supervision
Electrical design	Commissioning work and start-up
Design of control systems	Acceptance testing
Process- and system design	Training of operators
Design of steam gathering systems	Groundwater modelling
Well design	Drilling bid documents and procurement
Reservoir engineering	Drilling work supervision
Design of HVAC systems	Project planning report
Review of manufacturing design	Supervision for drilling operations
Procurement management	Inspections
Bid documents preparation and evaluation	Supervision of construction work
Document control

Process

The plant itself is a combined cycle plant, wherein a mixture of steam and geothermal brine is transported from the wells to a central separation station at 200° C and 14 bars. From there the fluid (steam and liquid) goes into a steam separator and the two phases are separated. Moisture is removed from the steam, which is then sent through the turbine after which it is condensed in a condenser.  Within the condenser fresh water is preheated.  The preheated fresh water is then run through a system of heat exchangers, which utilize the heat from the liquid part of the brine after the steam separator.  The fresh water is heated to the required temperature and sent through de-areators, which remove the bulk of the oxygen. Then finally a small amount of geothermal steam containing acidic gases (hydrogen sulfide) is injected into the water to remove any remaining oxygen, thereby preventing corrosion and scaling.  

This hot water is then pumped to a large storage tank at an elevation of 406 meters.  From there, the hot water flows by gravity to two smaller storage tanks on the outskirts of Reykjavik to be used for heating and hot tap water.

Plant details

Combined Heat and Power cycle geothermal plant

• Electricity generation: 120 MWe, developed in three phases
  
• District heating supply: 300 MWth
  
• 25 boreholes were drilled, from 1,000-2,200 meters