Modification of the solar heating system diagnostic method under operating conditions

Abstract

One of the problems with the operation of solar heating systems is the variable capacity of the system dependent on the prevailing operating conditions. One of the methods for increasing the capacity of a solar system is to control the volume flow of the working medium. Volume flow of working medium in solar heating installation should be follow operating conditions. Designing a continuous control system based eg. on the PID controller requires knowledge of the dynamic properties of a solar system under operating conditions: gain coefficient, equivalent time constant and delay time of the solar collector response to a change in solar radiation intensity and the inlet temperature of a working medium. To date, the delay time has been omitted in the analyses. It is necessary to know the value of the delay time for proper design of the solar system controls, because the object response cannot be simultaneous with the moment of the input function. The paper presents the method for determining the dynamic properties of a solar system under operating conditions. The method is based on the comparison of two models: an analog model based on the energy balance of a collector and a digital model developed with the use of parametric identification based on a short-term measurement of solar radiation intensity and working medium temperatures. In the collector analog model, on the basis of the design and operating parameters of the collector, the transport delay time has been defined, which is an innovative aspect of this study. The modified analog collector model was verified on the basis of the step response determined experimentally in accordance with the applicable ISO 9806: 2017 standard. A very good convergence of the model with actual data was achieved for the time constant – 87%, effective thermal capacity – 85.9%, and the delay time – 96%. Practical application of the modified diagnostic method is presented on the example of a solar heating system composed of five flat-plate collectors with a total absorber area of 10 m2.