Simulation model of a flat plate air solar collector
Keywords:air solar collector, simulation model, computer system, control, nonlinear, dynamics.
Analysis of recent research and publications. According to the design of solar collectors, generalizing dependencies are known, the values of the main parameters of the collector are determined, which makes it possible to determine the possible ef-ficiency of the solar heating system quite simply at the stage of the preliminary de-sign stage. Systems where thermal solar collectors are used are usually equipped with fairly simple control systems. These systems are characterized by the fact that they use static mathematical models and the management of work processes is per-formed by periodic switching on and off of the executing devices. Thus using more effective, continuous, local control systems, the solar collector is a rather complex object. Firstly, the control is possible only by adjusting the efficiency of the fan, which provides the circulation of the coolant. Secondly, the temperature of the coolant at the entrance to the heated room is also adjusted by the regulation of the fan. Consid-ering that in real conditions, the arriving of the solar energy to the collector is a proc-ess determined by many random factors, the operation of the control system must provide an appropriate response to such changes. Problem formulation. The operation of two heating systems is necessarily equipped with a computer information-control system of automatic control, which al-lows the maximum usage of solar energy. As a result of that the energy savings can reach quite significant values. The main functions of the used control systems in-clude: the algorithms for maintaining the necessary temperature parameters in the heated room, the energy consumption control, the regulation of the thermal power of the main heating system depending on the thermal power that the solar collector can provide. Regulation of the solar collector work must necessarily takes into account the indicators of solar radiation power, the temperature conditions of the external en-vironment, the features of the heated room, the inertia of the objects used in the heating system. Main material. To build a simulation model of a solar collector for heating a room where air is used as a heat carrier, known dependencies which describe ther-mal processes were considered. It is shown that the effective thermal power of the solar collector is determined by the difference between the thermal power of solar ra-diation and the thermal power of losses. Taking into account that the operation of the solar collector is possible only during the day and when the sun is clear, there regula-tion is necessary to provide the highest rate of the room heating, stabilization of the room temperature, ventilation mode (mixing of the heated air from the room with the part of the outside air) and other possible work options. As a control object, we will consider the following elements: a circulation fan, the dependence of the air temperature at the outlet of the collector (on the power of the solar radiation, the heat losses, the flow rate of the heat carrier), the inertial com-ponent of the heat transfer process to the heat carrier. We can consider a fan with a power regulator as a non-inertial element. This assumption is based on a preliminary comparison of the collectors inertia in common and the inertia of fans of relatively low power. The transmission coefficient of the so-lar collector is a non-constant and non-linear value. Therefore, in the structure of the collector as a control object, non-linearity is highlighted by a separate block. The heat transfer from the absorber to the air in its channels will be as faster as the mass of the air is smaller than the mass of the absorber. Considering the mass of metal and air, design features of air collectors, the weight of heated air is approximately 102 times less than the weight of the absorber. Therefore, in the first approximation, we can assume that the inertia model of the collector is a transfer function of the first or-der. Further use of the mathematical model of the control object for the local auto-matic control system synthesis and selection of the regulator type and its parameters determination requires of the object characterizing. The main feature of the control object is that it is a non-linear object. The general proportionality coefficient can take values in the range from 0.2 to 30. Conclusions and further research. The mathematical model of an air solar col-lector should display not only proportional relationships but also the dynamics of air heating. It is shown that the model that describes the dependence of the air tempera-ture at the outlet of the air collector on the speed of its flow is non-linear. The dynam-ics of air heating can be described by a dynamic element of the first order. For the computer control system operation, it is necessary to use air temperature sensors at the inlet and outlet of the solar collector. It is also necessary to use data from the sensor of the current power of solar radiation.
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