| The district heating (DH) system, especially with combined heat and power (CHP)plants, has been used widely in China these years. But it is a little hysteretic of theresearch in this field when the technologies of cogeneration plants have changed. Thereis no accepted method for deciding reasonable thermalization coefficient of districtheating systems in theory. Based on energy savings and economic evaluation, theoptimal thermalization coefficient and heat sources combination of district heatingsystem are studied in this research.At first, this dissertation introduce and analyze the energy performance evaluatingindicators of combined heat and power system in detail, it suggest that the relativeprimary energy savings (RPES) and relative avoided irreversibility (RAI) are moresuitable for representing the energy performance of combined heat and power system,and should be involved into the evaluating system. In this study, the mathematic modelsfor calculating RAI and RPES are built.In order to find out the relationship between heat load, power load and energyconsumption of dual-purpose and back pressure plant on different conditions, thethermodynamic circle system of them are simulated. For dual-purpose cogenerationplants, five main influence factors on the energetic and exegetic efficiency are analyzedby Taguchi method. The results shows that feed steam flow and extraction steam floware the most important factors of energetic efficiency, and then extraction steampressure has the most significant influence on exergetic efficiency. A novel conceptwhich called minimum extraction ratio is proposed in this dissertation. The values ofminimum extraction ratio based on relative primary energy savings and relative avoidedirreversibility are almost equal to0.2for plant NC145, NC200and NC300.Based on the relative primary energy savings, the mathematic model forthermalization coefficient optimization is built (RPES model). For district heatingsystem with dual-purpose plants, the meteorological parameters and the average energyefficiency of heating boiler are the most important factor, so different area has differentoptimal thermalization coefficient. The range of optimal thermalization coefficient forcold region A, cold region B, severe cold region A, severe cold region B and C are0.57~0.7,0.66~0.73,0.53~0.62,0.55~0.65and0.57~0.65, respectively. The resultscalculated with boiler’s energy efficiency70%can be accepted as an approximationvalue of other cases. For space heating system with back pressure cogeneration plant,the range of optimal thermalization coefficient varies from0.74~0.89to0.84~0.94when the number of plants change from2to4. If year-round heat load involved intodistrict heating system, the optimal thermalization coefficient of cold zone varies from0.6to0.7, and it corresponds to outdoor temperature for heating calculation. The value of severe cold zone changes between0.5and0.65.This study proposes a mathematic model for economic optimal thermalizationcoefficient calculation (economic model). The key of economic model is to decide theallocation ratio for heating of combined heat and power plants. An improved combinedmethod is given and used in this research. To compare different heating plants, dual-purpose combined heat and power plants have the highest fixed cost, and back pressurecogeneration plants have the lowest variable cost. From the perspective of economy, thedual-purpose plants with lager design capacity are more useful, but it is contrary forback pressure cogeneration plants. After all, the back pressure plants are moreeconomical than dual-purpose plants. For plant NC300and B80, the range of economicoptimal thermalization coefficient are0.65~0.86and0.57~0.8, respectively.Finally, a practical district heating project located in Harbin is involved into thisstudy. To take into account whether includes the existing heating plants, some differentoptimal heat plants combination are given for recent planning and long-term planningon the perspective of energy savings. |
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