12th International Symposium on District Heating and Cooling

The <strong>12th</strong> <strong>International</strong> <strong>Symposium</strong> on District Heating and Cooling,September 5 th to September 7 th , 2010, Tallinn, EstoniaTable 3 – Results for the primary energy factor and CO 2emission factor for heat from a geothermal CHP plantSource ofheatHeat,HellisheidiCHP plantHeat,HellisheidiCHP plant,reinjectionDISCUSSIONPrimary energy factors f p[MWh primary energy /MWh produced energy]Non-RenewableTotal>0.01 1.78 0.98>0.01 0.69 0.98CO 2productioncoeff. K[Kg/MWh]The following discussion highlights the most significantresults from this study:1) By comparing the energy performance indicatorscalculated in this study and shown in Table 3 to theindicators given in Table 3 it can be seen that electricityfrom geothermal power plants has the highest total f pfactor while the share of non-renewable energysources is the lowest. The main reason for the high f pfactor is the low conversion efficiency of geothermalpower plants due to low working temperatures andpressures. The CO 2 production coefficient is relativelylow compared to the other energy conversiontechnologies and could be lowered even further ifmeasures are taken to control the emissions from thepower plant. The results for the Hellisheidi geothermalCHP plant cannot be used to represent all geothermalpower plants producing either electricity alone or with acombined production of electricity and heat. Furtherstudies are needed on different types of geothermalpower plants, such as single flash and organic Rankinecycles, to be able to produce specific or averagefactors representing geothermal utilization.2) The results for the heat production at theHellisheidi geothermal CHP plant, given in Table 3,show that the energy performance indicators arerelatively low and, in the case of reinjection, belowunity. This is because the primary energy needed topreheat the DH water is not accounted for in the heatproduction but rather assigned to the electricityproduction. This is due to the fact that the preheating ofthe DH water from 5 °C to 41 °C is done in thecondenser for the high pressure steam turbine as seenin Figure 1 and is a necessary step in the electricityproduction, but a beneficial step in the heat productionfor the DH system.3) Values for the indicators for both electricity andheat are calculated with and without reinjection of thecooled geothermal brine from the energy conversionprocess. Reinjection of geothermal brine is recognizedto improve heat mining and stabilize the productioncapacity of geothermal fields, if successfully carriedout. It can also counteract pressure draw-down in thereservoir by providing an artificial water recharge [18].In this study, reinjection of the waste stream ismodelled, which decreases the use of primary energyin the energy conversion process, since a part of theprimary energy from the geothermal fluid it is returnedback to the reservoir. Reinjection is present at theHellisheidi geothermal CHP plant so the values of theenergy performance indicators with reinjection are validfor the power plant.4) Life cycle assessment is especially useful toevaluate the total impact of geothermal power plantswith respect to their emission of greenhouse gasses.Figure 3 and Figure 4 show how the different phases inthe life cycle of the power plant significantly contributeto the overall emission in CO 2 equivalents. If LCA hadnot been carried out for the process, 12% of the CO 2emissions resulting from the electricity generationwould not have been accounted for and no emissionswould have been found for the heat production, sincethe emissions from drilling, construction of buildings,and manufacture of components had not beenaccounted for.ACKNOWLEDGMENTSSpecial thanks are given to the following partners:Nordic Energy Research (NER) for funding the studyand the Energy Research Fund of Landsvirkjun for theirsupport. To Orkuveita Reykjavíkur for providing data forHellisheidi Power plant, to Mannvit engineering fordiscussion and data provision and to Ragnar Gylfasonfor his contribution in the data gathering phase.REFERENCES[1] EU. (2003, January 4). Directive 2002/91/EC ofthe European Parliament and of the Council of 16December 2002 on the energy performance ofbuildings. Official Journal of the EuropeanCommunities .[2] Bertani, R. (2010). Geothermal Power Generationin the World 2005 – 2010 Update Report.Proceedings World Geothermal Congress 2010,(April), 25-29.[3] Lund, J. W., Freeston, D. H., & Boyd, T. L. (2010).Direct Utilization of Geothermal Energy 2010Worldwide Review. Proceedings World GeothermalCongress 2010, (April), 25-29.[4] EN 15603:2008. Energy performance of buildings.Overall energy use and definition of energy ratings.191