Research On Cascaded Energy Utilization And Energy Saving In District Heating

With the coming of a new round of industrial revolution,science technology is developing rapidly and energy requirement is rising dramatically.Therefore,contradictions between resources and environment become more and more sharpen.Hence sustainable development has drawn worldwide attentions.And its critical part is energy saving.There are many phenomena of severe waste during the existing utilization of energy,especially thermal energy.Electricity can be totally transformed to thermal energy through an electric heater.However,40%of thermal energy can be converted into electricity.It's of paramount value for energy saving that improving the methods of using energy based on the second law of thermodynamics,also known as the principal of cascade utilization of energy.Energy saving from heating system of combined heat and power generation units,of which the low pressure cylinder is split during the period of peak heating,was studied in this paper.Screw expander was adopted to recover the exergy of steam in the light of the great exergy lost in heating system.In consideration of optimizing system operation a scheme of multi-parameter expender was proposed to warm up return-water with cascade heating,by which exergy loss could be further reduced thus improving exergy efficiency.And thermal economy of the unit before and after transformation was analyzed.The main contents are as follows:Above all,energy composition of high parameter gas was analyzed theoretically.The cascade utilization method of steam energy was perfected.And research direction of energy saving in heating system was pointed out.Furthermore,upgrade was performed aiming at a 'one on one' gas-steam combined cycle cogeneration unit of JingXi power plant,which directly takes intermediate pressure cylinder exit steam into heating system.Aiming at the shortcoming of high exergy loss in heating system,the scheme of recovering exergy by screw expander was put forward and mathematical model analysis on the complete process of steam energy utilization was carried out.Operation parameters were optimized according to thermal economy.Finally,the influence of general thermal load fluctuation and heat load fluctuation caused by the increase of ambient temperature on the system,which is the upgraded heating system,was analyzed.Then a more flexible heating method was presented:Steam exergy was recovered by differential screw expanders thus improving system flexibility with respect to the heat load fluctuation by equipment switching.In the same time,return-water of heating system was warmed up by cascade heating using exhaust steam with different pressure.Mathmetical model of the improved scheme was conducted and optimal design aiming at parameters of screw expander and operation stragegies of cascaded heating system were performed with the lowest exergy loss as the optimization target.And thermal economy of the unit before and after transformation was analyzed.Results suggested that multi-parameter screw expanders not only enhanced the flexibility of heating system,but also further increasd the exergy efficiency and made more profits for the plant with adding electricity in the guarantee of thermal energy produced.