fluctuation of outlet temperature in case of using test flow rate and low temperature increasement in case of using lower flow rate in evacuated tube solar collectors of trnsys

edit

Hello I'm using an evacoaed tube solar collector (type 71). the colector efficiency curve, area and test flow rate is based on a test report. The collector pump is controlled by a differential controller to be turned on when th difference of collector inelt and outlet temperatures is higher than 5 and to be turned off when it is lower than 2. Based on the area multiplied by test flow rate, the flow rate of pump could be 375kgr/hour. But using this flow rate the pump is turned off and turned on repeatedly, using low flow rate about 100 kgr/hour, the pump works well, but the outlet temperature is really low. attached picture named 100 is for using 100kgr/hour wter flow rate and the other named 375 is for 100kgr/hour wter flow rate. As the longitudinal IAM data was not in the report, the default IAM data of trnsys is used. Please give me your comments to solve it Thank you system:

100kgr/hour water flow rate in collector : 375 kgr/hour water flow rate in collector: