This paper presents models/strategies for optimum performance of solar collector in closed loop systems. These models aim to maximize the obtained energy by thermal conversion of solar energy. The mass flow rate of the fluid from the primary circuit of the system is the control parameter. The semi empirical models and optimal control methods are in brief presented. The volume of the storage tank is important and the ratio Vs/Ac between this volume and area of the collectors is a key factor in appropriate sizing of the DHW system. Therefore, the paper establishes a relationship between this ratio and the mass flow rate of the fluid in the collector This paper also analyses the variation of the energetic performance (useful heat flux transferred to the storage tank, heat flux transferred to the water, water temperature in the storage tank) with the volume of the storage tank. Analysis was performed on an extensive set of meteorological data from Timisoara, Romania, with instantaneous data (measured at 15 seconds) for summer days, from July 2009, with different relative sunshine values, σ. Important differences have been observed between days with different stability levels - days more or less stable.
 S. Furbo, I. J. Shah. Optimum solar collector fluid flow rate, Proceeding of Eurosun, Department of Building and Energy (1996) 996
 L. M. Nhut, Y. C. Park. A study on automatic optimal operation of a pump for solar domestic hot water system Solar Energy 98 (2013) 448-457
 T. Persson, M. Ronnelid. Increasing solar gains by using hot waterto heat dishwashers and washing machines Applied Thermal Engineering 27 (2007) 646-657
 V. Badescu. Optimal control of flow in solar collector systems with fully mixed water storage tanks Energy Conversion and Management 49 (2008) 169–184
 V. Badescu, S. Budea, M. Paulescu. Empirical versus optimal control of flow in solar domestic hot water systems. J Energ Eng. 2015, 142 (3) (2015), 15-38
 S. Budea, V. Bădescu. Improving the Performance of Systems with Solar Water Collectors Used in Domestic Hot Water Production, Energy Procedia, 112 (2017), 398-403
 J. A. Duffie, W.A. Beckman Solar energy thermal processes (New York: Wiley) (1991)
 I. G. Carabogdan et al. Industrial thermal installations (Bucharest Editura Tehnica) (1978)
 D. E. Prapas et al. Large DHW solar systems with distributed storage tanks. Solar Energy; 55(3) (1995) 175–183
 S. Knudsen. Consumers’ influence on the thermal performance of small SDHW systems Solar Energy73(1) (2002) 33–42
 Retscreen International 2004 Solar water heating. Project analysis chapter, Retscreen Engineering and Cases Textbook, ISBN: 0-622-35674-8, Catalogue no.: m39-101/2003e-pdf Minister of Natural Resources Canada. (See also: www.retscreen.net)
 TableCurve 2D v5.01 for Windows 2002 San Jose CA 95110 USA: SYSTAT Software Inc. 1735 Technology Drive Suite 430.