Hellisheidi Geothermal Power Plant 

Energy Solutions

Hellisheidi Geothermal Power Plant is a flash steam combined heat and power plant (CHP) located in the south-western part of Iceland. The plant aims to meet the increasing demand for electricity and hot water for space heating. From 2009 to 2018, an incremental expansion was implemented, increasing output to 303 MWe and 200 MWth, making it the largest geothermal CHP plant in Iceland and the fourth largest geothermal power plant in the world.  

The installed heat capacity is planned to be 400 MWt as demand for heating increases in the Reykjavik capital area. Overall, 66 wells were drilled, at depths from 1,000 to 2,200 metres, for the production and reinjection of fluid. The plant owner and operator is ON Power. 

COWI in Iceland played a leading role in the preparation, drilling, design, project management and construction of the entire project in an integrated team with the plant owner. Mannvit, Verkís, Tark Architects and Landslag Architects provided supervision, detailed piping, as well as electrical and control design for all phases. 


Geothermal fluid from production wells is collected in central separation stations. The separated water is flashed, and the steam is used in the low-pressure turbine, whereas the water is used in heat exchangers to heat the preheated water up to the required temperature. Electricity is generated by condensing steam turbines, six high-pressure turbines and one low-pressure turbine. The condensers from four turbines are used to preheat fresh water. The heated water is treated in deaerators to suit the requirements of the distribution system. The separated water is diluted with condensate and then pumped into re‐injection wells. Co-generation of electric and thermal power is thus utilised in the most economical way possible. 

Transmission of energy 

Electric energy is transmitted to the substation, which is directly connected to the national grid. The line voltage from each unit is 220 kV. The hot water is pumped from the plant to a tank at the highest point of the transmission pipe. From there, the water gravitates to the storage tanks of the district heating system from where it is distributed to the consumers in Reykjavik.  

Circular economy 

Next to the power plant, the plant owner and operator, ON Power, owns Jarðhitagarður (Geothermal Park). The Geothermal Park encompasses a wide range of operations by third-party operators, which aim to utilise the resources of the Hellisheidi geothermal power plant in the best way possible, to benefit the environment and create value. The resources available are electricity, cold water, hot water, steam and carbon dioxide from the adjacent geothermal plant. 

The park is a prime example of a circular economy where CarbFix and Climeworks capture CO₂ and store it underground. VAXA Impact Nutrition’s cultivation production facility converts clean energy into food, producing the most sustainable crop in the world; microalgae rich in Omega-3 and protein. GeoSilica produces a 100% natural silica mineral supplement from geothermal fluids. 



303 MW of installed power 
200 MW installed heating capacity 
66 production and injection wells 




ON Power  



  • Project management
  • Well design
  •  Overall plant design
  • Environmental impact assessment
  • Mechanical design 
  • HVAC systems 
  • Design management
  • Site supervision 
  • Technical supervision
  • Supervision of turbine and cold end installation 
  • Supervision of steam collection system 
  • Drilling bid preparation and tender evaluation
  • Electrical work supervision 
  • Inspection of rigs, equipment and materials
  • Supervision of control system 
  • Supervision of drilling operations
  • Supervision of civil works 
  • Process and system design
  • Supervision of mechanical work 
  • Detailed plant design
  • Commissioning work and start-up
  • Preparation of bid documents and bid evaluation 
  • Acceptance testing 
  • Training of operators.