| In recent years, the state encourages the development of energy–savingtechnology and clean energy replying to the energy shortage, which has made a highperformance and environment–friendly air conditioning technology—geothermal heatpump (GHP) to grow quickly in China. At the same time, many researchers havebegun to conduct in–depth study on GHP. Most of current studies of GHP focus onthe heat transfer model and mechanism of the borehole heat exchanger (BHE),simulation of the process parameters, combined application under complementaryenergy sources and development of alternative refrigerants. However, the research ofcontrol strategy and optimization operation on GHP is rarely seen. Therefore, we havestudied a GHP system integrated with advanced control technologies based on moderncontrol theory methods and nonlinear optimization methods. Also, a comprehensiveanalysis and study on the running process and energy saving effect of this system hasbeen done.A feedforward GHP energy–regulation control strategy is proposed on the basisof modern control theory. A detailed analysis of implementation method on thiscontrol strategy is also carried out. Then the experiment system of GHP and thematched intelligent control system are established. A dynamic simulation model ofGHP has been developed by using mechanism modeling and experiment modelingmethods. Simulation and experimental studies were conducted on water supplytemperature of GHP under the conditions of setpoint changes, flow rate disturbancesand abrupt changes of frequency multiplication coefficient. The results show that, theovershoot of gearshift integral PID controller is higher, the regulation time ofgearshift integral PID controller is less than that of fuzzy controller as the setpointmakes a change. And there is a the steady-state deviation of the water supplytemperature under the fuzzy control. The control performance of the two controlmodes has been improved significantly with the help of feedforward strategy. Thefuzzy controller will be in unstable state gradually while gearshift integral PIDcontroller has good robustness with the abrupt changes of frequency multiplicationcoefficient.It has also shown that the feedforward-feedback control strategy makes the GHP control performance improve significantly. The water supply temperature of GHP cankeep within a variation of0~1.0℃in a big terminal cooling/heating load fluctuation,in comparison to0~3.4℃under conventional control strategy. The mean indoortemperature under the feedforward-feedback control strategy is kept in24.4~25.9℃,in comparison to23.1~26.1℃under conventional control strategy. Therefore, thefeedforward-feedback control strategy provides a good noise immunity and dynamicresponse characteristics. Moreover, the stability and response speed of control outputare superior to the GHP system with conventional control strategy.An optimized operation method is developed for the GHP system in combinationwith energy storage device on the basis of load forecasting. And the implementationprocess of this method is described in detail. Experimental validation has been carriedout from a small load system established at Tianjin University. It is found that a lot ofrunning costs can be reduced and the saving ratio to daily running cost can be morethan20%. Also the COP of the system rises to4.5from3.0, which will make theenergy consumption decrease significantly.Based on the experimental system, three typical operation modes areinvestigated, They are: geothermal and solar coupling operation mode, energy storageoperation mode and independent geothermal heat pump operation mode. Through theanalysis on performance, environmental influence, energy structure and exergyefficiency of the system, the conclusions are as follows:(1) The short–term effect of solar on improving the COP of the heat pumpsystem is not obvious when it is geothermal and solar coupling operation mode. Thesolar plays an important role in maintaining the high efficiency and stability of thesystem during long-term operation.(2) The internal structure of the water thermalstorage device is an important factor to affect the overall efficiency of the energystorage process.(3) The highest energy efficiency link of the system is the heatexchange process in BHE (Energy efficiency of which is96.1%). However, thisprocess has the lowest exergy efficiency (Exergy efficiency of which is7.2%).Therefore, the effects of energy and exergy efficiency to the whole system need to betaken into consideration when the system is designed. |
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