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 Energy4.2.1.1 Open loop heat pumpWith an open loop heat pump vapor is drawn from the vapor supply pipe and thencompressed to the needed temperature, then a counter flow gas cooler transfersthe heat to a water loop. Hereafter the CO 2 gas is expanded through a turbineconnected to the compressor to regain some of the energy. Afterwards the CO 2 willbe a mix of vapor and liquid, which is separated in a separator tank from where gasand liquid can be led back to the supply lines, using valves to ensures the outputpressure is identical to the supply system pressure.This use of decentralized separator tanks has the added advantage that they willfunction as buffer capacity for the system, thereby ensuring that fluctuations indemand are smoothed out. It should however be noted that the tank should bethermally insulated and have a one way valve installed between the tank and theliquid line, since it otherwise will assume a steady state at approximately 10⁰C andcool the room where it is placed.4.2.1.2 Closed loop heat pumpIn the closed loop heat pump, the CO 2 is condensed at the system pressure around8⁰C, and then pumped back to the liquid line, thereby functioning as a heat sourcefor a secondary heat pump, which can then raise the temperature to the requiredlevel.The advantage of this setup is that the possibility of using another refrigerantmakes it easier to adapt the system to specific temperature needs, therebypotentially raising the COP of the system. Additionally a failure in the heat pumpwill not lead to release of CO 2 from the transmission system, thereby possiblydecreasing the need for safety valves.4.2.2 Cooling demandThe choice of cooling system depends on the temperature demand, the two most simple systems will bebriefly explained.4.2.2.1 Passive comfort coolingComfort cooling can be obtained passively simply by evaporating liquid directly in anevaporator. This is possible because the system has a slightly higher pressure in theliquid line to avoid unintended evaporation in the transmission pipes. Therefore thehigher pressure in the liquid line will force liquid into the evaporator without theneed for pumps or compressors, leading to a passive cooling effect at 8⁰C, which issuitable for comfort cooling.Christian Vang Madsen – s052309In cooperation withChristian Nørr Jacobsen – s052567 Rambøll A/S and DTU‐MEK p. 6/10
Idea competition ‐ CO 2 based energy system d. 1/6 – 2011Danish Development Center for District Energy4.2.2.2 RefrigerationIn case lower temperatures are needed, an expansion valve is used to lower thepressure from the liquid line to the desired temperature after which then anevaporator transfers the energy to the CO 2 , before compressing the CO 2 back to thecorrect pressure for the vapor line. After the compressor the hot CO 2 gas is cooledto the desired temperature by evaporating CO 2 directly from the liquid line to thevapor line, thereby producing additional vapor for the network.Using this method, temperatures down to approximately ‐70⁰C can be obtained,which should satisfy the needs of any normal consumer. If lower temperaturesshould be needed, the system can be used as a heat sink for another refrigerationcycle.4.2.2.3 Organic Rankine CycleThe increased electricity consumption of the proposed CO 2 system could besomewhat alleviated by using Organic Rankine Cycles in situations where relativelyhot temperature sources are available. In this case, instead of transferring theenergy directly to the network, an intermediate loop would extract part of theenergy as electricity before evaporating the CO 2 liquid to vapor.This method could turn sources such as industrial waste heat and solar thermalproduction into electricity production units, while still producing heat for thenetwork.5 Pros and cons of the CO2 district energy system.In the following section the most important advantages and disadvantages of the proposed system will bediscussed.5.1 Increased electricity productionOne of the major disadvantages of the system is the increased use of electricity for the compressors. Thishowever is alleviated somewhat by a range of different points which must be considered.Firstly, the decrease in needed output temperature from power plants will raise their electrical efficiency byapproximately 6%, while still supplying the surplus thermal energy to the system. Depending on which COP canbe assumed, more than 20% of the heat will still be supplied from the same amount of input fuel to the powerplants.Industrial waste heat can be used to supply both electricity and heat for the network using Organic RankineCycles, leading to a situation where many industrial thermal sources and solar production facilities will be ableto supply part of the need for electricity.In the future, an increasing amount of the Danish electricity consumption is expected to come from renewablesources such as wind. This is a problem for the current system since wind power does not produce heat,meaning that boilers are often employed during the winter to meet demand. With the proposed system this isChristian Vang Madsen – s052309In cooperation withChristian Nørr Jacobsen – s052567 Rambøll A/S and DTU‐MEK p. 7/10
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