Danish Development Center for District Energy CO2 based district ...

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Idea competition ‐ CO 2 based energy system d. 1/6 – 2011Danish Development Center for District Energynot the case. Instead the production from wind power will decrease the overall emissions of the system, sincethe thermal sources are mostly emissions free, see section 5.3, leaving only the emissions from powerproduction.That being said, more work should be done to consider the best way to ensure a certain degree of timecorrelation between renewable electricity production and consumer thermal production. Fortunately this isalready an area under intensive research in regard to the large number of individually installed heat pumps inDenmark, and their potential to stabilize the fluctuating production from wind turbines.5.2 Synergy effectOne of the most interesting things regarding the proposed system is the synergy effect that arises betweenheating and cooling consumption in the CO 2 network. When a customer produces cooling from liquid CO 2 thegas vaporizes and goes to the vapor supply line and thereby becomes available to other customers as a heatsource. The same synergy effect occurs when a customer produces heating from CO 2 where the gas condensesand goes to the liquid supply line and becomes available to other customers as a heat sink for production ofcooling or refrigeration.This synergy effect creates a balance in the network to some extent, and thereby limits the energy inputneeded to condense or evaporate CO 2 at the plant side in order to fulfill the demands of the consumers. Thiscould potentially contribute to high energy savings in periods where the demand for cooling and heating aresimilar in size. To illustrate this effect, the following Figure 2 has been created 1 , which illustrates an estimationof the average consumption of different energy services pr m 2 in a city environment during a year.Figure 2: Estimated average energy consumption pr m2 in an urban environment 11 Based on hourly consumption patterns from Residential buildings, supermarkets, Malls and office buildings and the areadistribution of Copenhagen. A range of simplifications has been made, for more information contact the authors.Christian Vang Madsen – s052309In cooperation withChristian Nørr Jacobsen – s052567 Rambøll A/S and DTU‐MEK p. 8/10
Idea competition ‐ CO 2 based energy system d. 1/6 – 2011Danish Development Center for District EnergyAs seen, the balance in the system is fluctuating throughout the year and day, but during the summer thedemand for cooling on a 24 hour average exceeds that of heating, meaning that in this period, all heatingenergy is produced by cooling equipment throughout the network, and some heat must actually be eitherstored or released to the surroundings.5.3 Inclusion of low temperature heat sourcesOne of the major advantages of the proposed system is the easy integration of various low energytemperature sources. Even temperatures below 0⁰C can be used to produce CO 2 vapor for the system,meaning that there will never be a shortage of thermal energy. It is however clear that energy sources abovethe pipeline temperature is far more desirable, since no energy would then be needed to compress the vaporback to system level. Even with this in mind, there are still a large number of different sources to choose from.Even in full condensing mode a power plant produces excess heat at approximately 20⁰C which needs to becooled to 10⁰C, cheap solar panels are far more efficient when only low temperatures are needed, inFrederikshavn sewage between 10⁰C and 20⁰C are already being used as a heat source for the district heatingsystem and geothermal energy is much more easily obtainable when only low temperatures are needed.The easy access to low temperature sources combined with the described synergy effects and increasedinsulation levels, means that it is highly likely that no additional thermal energy input would be needed for thesystem.5.4 Ability to supply all thermal energy services within one systemDue to the use of water as an energy carrier, the existing district energy system is only able to provide heatingand air condition through district cooling. District cooling however is only feasible when located near a coldsource, and have large customers in need of cold water for air condition. As water has a freezing point at 0°Crefrigeration cannot be produced centrally and circulated by the existing system. Another problem also arisesif the existing system is to provide district‐ heating and cooling and that is that the supply and return pipes willtake up quite some space in the underground infrastructure in a city.The proposed system is able to function as a heat source and a heat sink making it possible to utilized the CO 2to produce energy services between ‐70°C ‐ 150°C depending on the decentral equipment. A major benefit ofthe CO 2 system is that it needs only two pipelines, which do not need to have a large diameter due to lowermass flow rate, and thereby do not obtain the same space in the underground infrastructure as the existingsystem. Additionally every component providing a thermal energy service is connected into one districtnetwork, making it easier to balance the system centrally and exploit the synergy effect mentioned above.5.5 Risk assessmentThe proposed system does not come without potential drawbacks and issues, the main concern being to havepressurized CO 2 around 40‐45 bar in an urban environment. Though it rarely happens, hitting a pipeline andmaking a hole while digging in the city is a possibility, and it has to be investigated whether the pipe willexplode following decompression or just leak CO 2 to the surroundings. If CO 2 is leaked to the surroundingseither from a pipeline or inside buildings where decentral energy service equipment is placed, it is a potentialthreat to humans whom can suffocate if the concentration is too high. CO 2 is also without smell or colormaking it more difficult for humans to detect.Measures can be taken to minimize the extent of potential accidents. This could be that in the pipe lines, bothvapor‐ and liquid line, for every 100 meters shut off valves with pressure sensors are placed and if the pressureChristian Vang Madsen – s052309In cooperation withChristian Nørr Jacobsen – s052567 Rambøll A/S and DTU‐MEK p. 9/10
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Danish Development Center for District Energy CO2 based district ...

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