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Energy Systems and Technologies for the Coming Century ...

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2/3 Sun1 Coal1/3 ElPower plantHP1 Heat(a) Heat pump output is 100% fossil2/3 Sun1/3 WindWind turbine1/3 ElHP1 Heat(b) Heat pump output is 100% renewableFigure 2: Extremes of fuel consumption of a heat pump. Power coefficient of wind turbine isneglected.only 4% because <strong>the</strong> heat is to be used <strong>for</strong> space heating at 20°C. There is thus a significant exergyloss of about 85% in distribution of district heating. The loss is related to both <strong>the</strong> heat loss (30%)<strong>and</strong> <strong>the</strong> temperature difference between district heating network <strong>and</strong> indoor space (55%).3.2 Heat Supply <strong>Technologies</strong> Integrated with ElectricityProductionFor <strong>the</strong> domestic boilers it is straight<strong>for</strong>ward to calculate primary fuel consumption <strong>and</strong> thus CO 2emissions based on <strong>the</strong> in<strong>for</strong>mation in 1. However, <strong>for</strong> <strong>the</strong> technologies that are integrated wi<strong>the</strong>lectricity production is it to a much larger extent a matter of definition to calculate <strong>the</strong> key figures.3.2.1 Heat PumpsHeat pumps are often suggested as a renewable energy source as well as a means <strong>for</strong> integratingfluctuating energy sources in <strong>the</strong> energy system. It must, however, be kept in mind that heat pumpsare operated by electricity that is today produced mainly based on a fossil fuel. This means that <strong>the</strong>energy supplied by a heat pump actually does emit greenhouse gases. The CO 2 emission from heatpumps is connected to <strong>the</strong> electricity production, as well. In figure 2 two extremes of <strong>the</strong> heat supplyfrom a heat pump are illustrated. In one situation (figure 2(a)) a coal-fired power plant produces all<strong>the</strong> electricity <strong>for</strong> <strong>the</strong> heat pump at an electrical efficiency of 33%. With a COP of 3 <strong>the</strong> heat from<strong>the</strong> heat pump equals <strong>the</strong> fuel consumed <strong>and</strong> <strong>the</strong> CO 2 emission will be <strong>the</strong> same as <strong>for</strong> coal. In <strong>the</strong>o<strong>the</strong>r situation (figure 2(b)) <strong>the</strong> heat pump is supplied with wind power <strong>and</strong> thus <strong>the</strong> heat from it willbe Carbon neutral. In <strong>the</strong> actual system each of <strong>the</strong>se situations may occur at different times of <strong>the</strong>year. However, <strong>the</strong> average annual energy supply to <strong>the</strong> electricity production is significantly morecomplex, including both fossil fuels <strong>and</strong> biomass <strong>for</strong> <strong>the</strong> <strong>the</strong>rmal power plants <strong>and</strong> wind power.The overall characteristics of <strong>the</strong> energy system are presented in table 1:Two distinct types of heat pumps are suggested.Small-scale heat pumps <strong>for</strong> domestic use , mainly intended <strong>for</strong> floor-heating systems. Measurements[8] show an annual COP of 2.8-4.3. We assume <strong>the</strong> COP to be 3.3. The energy flows<strong>for</strong> production of heat <strong>for</strong> this situation is illustrated in figure 3. We find that <strong>the</strong> share of fossilfuels required <strong>for</strong> production of heat is 48%. Out of <strong>the</strong> produced heat 15% does not comefrom <strong>the</strong> heat pump but from <strong>the</strong> CHP production.Large-scale heat pumps <strong>for</strong> use in district heating system As <strong>the</strong> district heating systems oper-Risø International <strong>Energy</strong> Conference 2011 Proceedings Page 142

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