Geothermal District Heating in Reykjavik Today

 District heating - DH - is a system which distributes heat from a centralised generation plant to end (residential, tertiary, commercial, recreational facilities...) users, connected via a heating grid and substations. DH has replaced, in most instances, traditional central heating systems where each building is heated by an individual boiler. Clearly, DH achieves higher energy, economic and environmental performance. Heat supply is best adjusted to users demand. Individual building boilers are replaced by a heat exchanger three way valve piping outfit, fuel supplies and operation/maintenance are optimised, all factors resulting in significant cost savings. Last but not least, it reduces greenhouse gas emissions and excess heat losses, thus securing upgraded environmental control. As of early 2000’s European DH market penetration stands as follows (percentage of district heated houses) : Iceland: 96%; Baltic States / Poland / Sweden / Denmark / Finland: 50-60%; Austria / Germany: 12-15%; UK/Netherlands: 1-4%.

This record reflects (i) the fact that Iceland enjoys abundant geothermal resources added to a consistent energy policy of the state in favour of energy savings and renewable energy sources (RES), the latter adopted by Scandinavian, Baltic and Polish states, and (ii) an almost negligible DH share in the UK and Netherlands, most likely attributed to an adverse natural gas lobby competition and, at a lesser extent, to milder climatic conditions. Despite its “modernity” DH is nothing new. As a matter of fact, it dates back to Roman ages as witnessed by remnants evidencing city homes and baths heated via natural hot water catchments and piping. At Chaudes Aigues, in Central France, a city DH system, pioneered in year 1330, fed by the Par hot spring at 82°C, is still operating to date. Heated homes were charged, in those times, a tax by the local landlord in exchange of maintenance duties, as reported in the city annals. Noteworthy is that these early DH systems could be completed thanks to local hot springs and shallow wells, i.e. (sub)surface evidence of geothermal heat conveyed by water.

So, everything considered, engineering of geothermal district heating - GDH - ambitions nothing more than revisiting DH sources. However, no way does this “revival” imply a geothermal archaeological itinerary, but a thorough technological accomplishment instead. DH represents 35% of the European installed power dedicated to direct uses, i.e. an online capacity nearing 5,000 Mwt. Major GDH sites (over 35 exceeding 5 MWt capacity) highlight the dominant role played by Iceland and Turkey, two countries enjoying favourable, volcanically and tectonically active, geodynamic settings on the Mid Atlantic Ridge and the Aegean façade/Anatolian plateau respectively, demonstrating also relevant entrepreneurial skills. The two largest schemes address the heating of the city of Reykjavik and of the Paris suburban area.

GDH provides almost the whole of the Reykjavik demand with an installed capacity of 830 MWt serving 180,000 people, 60 million m3/yr of water at an average 75°C (user inlet) temperature. The city grid elsewhere exhibits several distinctive features compared to most of its European replica. An important part of the hot water supply is piped from distant wells and there is no injection whatsoever of the heat depleted water (ca 35°C) underground.

The Paris Basin GDH system is based on a dependable sedimentary resource environment and on the doublet concept of heat extraction. Here, hot waters at an average 70°C temperature are hosted in permeable carbonate rocks (the Dogger limestone reservoir) at depths of 1500 to 1800 m. The geothermal fluid, a hot saline brine including a solution gas phase, is pumped to surface from a production well and the heat depleted brine pumped back into the source reservoir via an injection well; the doublet well spacing is designed in order to avoid premature cooling of the production well.