| Due to the increasing global demand for energy,as well as the strict emissions targets,the application of renewable energy technologies has attracted much attention.Distributed polygeneration systems driven by solar energy offer enormous and promising potential,with the advantages of multi-products,compact structure,less land occupation and pollution-free.As the core of such systems,among the other prime movers like inner engine combustion and Stirling engine,organic Rankine cycle(ORC)features applicability and effectiveness for the low-grade conversion of solar thermal heat into useful power by using low boiling point organic working fluids.However,since the unexploited overall energy efficiency as well as the intermittent of solar energy,the practical operation characteristics turn out to be more complex and urgent tasks,inhibiting the engineering application.This study focuses on a solar driven ORC-based CCHP system with a single-effect absorption chiller(ACH)and heat exchanger for the heating system.The innovation lies in the following three fields.(1)Determination of the operation boundary and solution domain with considering the multidimensional variables in the practical working condition.An algorithm was proposed to explore the performance distribution and its change rule,where four-dimensional constraints were determined with a specific condition of heat and cooling source.With this method,the limitation of operation performance and the goal of control strategy were revealed.(2)Identification,quantification and comparison of sequential and parallel configurations of the solar driven ORC-based CCHP system.As a result,the bifurcation point between the design of solar-ORC and solar driven ORC-based CCHP system was clarified.A balance between the thermal efficiency of ORC and PTC efficiency was achieved.The enhancement potential of optimal system configuration was quantified by a tradeoff among three objective functions.(3)Multi-time scale mechanism for the system simulation and verification of the dynamic model.The mechanism is consisted of long-,mid-,and short-term simulation with yearly,hourly and second time step,respectively.The corresponding targets include:designing an optimal system;analyzing supply-and-demand matching performance;and revealing the dynamic characteristic under the variable working condition,which was compared with dynamic experimental results.The specific conclusion is as follows:First,the configuration effects on the solar driven ORC-based CCHP system were explored,of which two configurations are concisely clarified into sequential and parallel connections.The corresponding performance was compared and evaluated by establishing the thermodynamic performance,system size,and economic models.Under reasonable thermodynamic boundary conditions,the optimal operational parameters were obtained via a Pareto frontier solution for the sequential system with an ORC of 200 k W using MM as a working fluid.The sequential system achieved an annual overall economic energy efficiency of 45.82%,which improved up to 26.21%than the parallel systems.The annual cooling,heating and power output is 1940,1650,and 369 MWh/yr.,respectively.And it shows better economic profitability with less land occupation,with about seven year's payback period.Second,the preliminary test of a pilot-scale solar driven ORC-based CCHP system with 200k W power capacity was analyzed,which could sustain 10%ORC thermal efficiency with the working fluid of R123 and a self-made expansion valve.Targeting a whole optimized system in practical application,a multi-time scale simulation was proposed and consisted of long-,mid-,and short-term simulation with yearly,hourly and second time step,respectively.It guided an optimal sequential system with MM,which achieved a high performance ratio of 67.61%,an efficient cost of$0.12 million,and less land occupation of 3774.20 m~2.The system shows acceptable supply-and-demand matching performance.During a typical sunny day,it showed a steady thermal efficiency of 9.6%within 15,000s and captured a smaller period of safety state only within 6,000s.Whereas,the system performance degraded dramatically under a typical cloudy day condition due to the sharp insolation fluctuation,even though the peak insolation was higher than that in the sunny day.Third,a test rig of ORC with a closed-loop heat source and a self-made expansion valve was built for the dynamic test and model verification.The test was conducted by manipulating three common freedoms,namely the mass flow rate of the working fluid,heat source and cooling source.Results showed that the internal disturbance of the mass flow rate of working fluid significantly affected the cycle state parameters.The external disturbance of the mass flow rate of the cooling source had a less effect on the state parameters,while the state parameters were insensitive to the external disturbance of the mass flow rate of the heat source.By imposing the same real-time experimental conditions,the system-level dynamic simulation was performed on the Modelica/Dymola software.The simulation results were in agreement with the experimental results,which predicted that the maximum desired power output of this ORC test rig was about 1.9 k W with the thermal efficiency of 6.94%.Finally,the baseline model corresponding to the test rig was improved by integrating the solar loop and incorporating an off-design expander model.It revealed that neglecting the variation of expender efficiency may lead to an underestimation of power output up to 54.7%.And the required PTC aperture area is 70.3 m~2 for the baseline model,with considering a designed solar irradiance of 800W/m~2.In addition to summarizing the optimal operation principle of ORC with closed-type heat source,an algorithm was proposed to determine the operation boundary and solution domain.It considered a specific condition of heat and cooling source,and a multidimensional constraint consisting of the variables of evaporation temperature,condensation temperature,pinch point temperature in evaporator and mass flow rate of working fluid.The results indicated that the feasible operation boundary and maximum ORC thermal efficiency showed the opposite variation trend to the variation trend of mass flow rate of working fluid.However,the maximum ORC power output showed the same variation trend to the trend of mass flow rate of working fluid.As a result,within the above four-dimension constraint,the maximum thermal efficiency is 9.42%,corresponding to a relatively low power output of 697.06W,while the maximum power output is 2251.54W,corresponding to a relatively low thermal efficiency of 8.04%.The experimental and simulation results were in agreement with the solution domain.On this basis,a model-guided threshold range of step change of solar irradiance was obtained,which ranges from 774W/m~2 to 874W/m~2.A PI control strategy on the solar ORC was developed and simulated in response to the real-time variation of solar irradiance.Compared to the system operation without this control strategy,the instant power output and thermal efficiency were improved up to 7.0%and 35.5%,respectively. |
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