Optimal flow control of a forced circulation solar water heating system with energy storage units and connecting pipes

摘要

Solar water heating systems (SWHS) have fast become a suitable alternative to conventionalwater heating systems due to growing energy demands. A SWHS generally consists of a solarcollector (which is used to convert solar radiation to heat), a water storage tank, and a flowcontrol device such as a pump in the case of forced circulation SWHS. Extensive researchand analysis on the operation and performance of these systems has been conducted, andresults show that optimal flow control is an important factor that can be used to improve theperformance and efficiency of SWHS.This study focuses on pump flow rate optimization for forced circulation SWHS with pipes.The system analyzed consists of an array of flat plate solar collectors, two storage tanks (onefor the circulation fluid and one for the water), a heat exchanger, two pumps, and connectingpipes which are considered as one of the components of the SWHS so as to account for theirthermal effects. The proposed model is developed using mainly the first and second laws ofthermodynamics. The model is used to maximize the difference between the energy extractedfrom the solar collector and the combined sum of the energy extracted by the heat exchangerand corresponding energies used by the pumps in the primary and secondary loops. Theobjective function maximizes the overall system energy gain whilst minimizing the sum of the energy extracted by the heat exchanger and energy used by the corresponding pumpsin the secondary loop to conserve the stored energy and meet the user requirement of watertank temperatures.The model is solved using the fmincon solver in MATLAB’s optimization toolbox. Whencompared to other flow control techniques, in particular the most suitable energy efficientcontrol strategy, the results of this study show a significant increase in the system’s overallenergy gain. The results also illustrate the effects of system pipe thermal losses for thedifferent control strategies, hence highlighting the importance of developing a model thattakes such losses into account so as to improve the overall accuracy of the model.