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The investigation of heat exchanger dynamics has been carried out. The three kinds of exchangers havebeen taken into consideration: the low power plate heat exchanger, the high power industrial heatexchanger and the model of heat exchanger. The dynamics of the examined exchangers has been ana-lyzed using the parametric identification methods and their implementation in MATLAB. The characterof observed phenomenon is not unique. In some of the cases it shows inertial character, in the othersoscillatory character. It might indicate an appearing of the kind of a thermal oscillation during thenon-stationary states of the heat exchanger. 42320
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1. IntroductionThe problems corresponding to obtain and, in particularly, toconverse and storage of an energy become more and more impor-tant. The costs of an energy generation and the environmentalrequirements have caused some efforts to use the renewable re-sources of an energy being an alternative for the classical exhaust-ible energy resources.1.1. Genesis of the workOur team for many years has investigated the different systemsusing renewable resources and also the hybrid systems. One ofthem has been created in Budy Grabskie at about 100 km fromWarsaw, being the complex energy system for small hotel locatedin the forest. The experimental stand uses the different forms of anenergy: solar, geothermal, electrical and chemical (of heating oil).This system consists of the solar installation with the solar liquidand vacuum collectors, the plate heat exchangers, the geothermalinstallation with the heat pump and the soil exchanger, the electricboilers and the boiler-room for the heating oil [1]. The system hasbeen analyzed in different aspects for many years (about 15) andmainly for the purpose of the preparation of an optimal algorithmof the automatic control of the system in order to maximize itseffectiveness during the whole year span. For this purpose the ele-ments of system have been analyzed, mainly in the respect of theirdynamics for the best fit to the physical reality.
The problems of hybrid systems are more often analyzed in theliterature in different aspects. There are systems using solar energy[2–4] but also fuel cell systems [5,6] and polygeneration systems[7]. The computer simulations of the system working under differ-ent work conditions are necessary in order to prepare a control sys-tem. If the elements of the system transform different kinds of anenergy then one is searching some unified diagram for the cooper-ating elements. Then the energy fluxes are easier to analyze andthe optimum control system may be found. The first step is to findthe system working in the stationary state and the next is to find itin the transient state. Methods using in order to solve this problemare also different and based usually in numerical solutions of dif-ferential equations and theirs implementations. The MATLAB/Sim-ulink platform is often used for solve this problem [4–6] anddifferent Toolboxes or other programs cooperated with MATLAB[7,8] but also the Artificial Neural Networks [9,10] and the others.While searching for the dynamics identification (the testing of thereaction to the standard step function) of the elements of the sys-tem the step response to different input function is investigated[10,11]. In the presented paper the dynamics identification of allelements of the system should be done. Then correct algorithmof the control system can be easily found.In the first step of an examination the battery of the plate col-lector dynamics was analyzed. Then the models of solar collectorswere prepared [12,13]. The heat exchanger plays an important rolein hybrid systems, separating the primary and the secondary sidesof the cycle. Due to this reason the dynamics of the plate heatexchangers cooperating with collectors has been analyzed. The soilheat exchanger cooperating with the heat pump has been analyzedseparately for its specific properties. The dynamics of the exchang-ers is widely analyzed [14–17] and also using the step function analysis [17,18]. The dynamics of the elements of the system inBudy Grabskie has been analyzed using MATLAB and their Tool-boxes, mainly the System Identification Toolbox.It is assumed that dynamic properties of the object (element)are known if the transfer function form corresponding to the in-put/output signals is known or the element transient responsefor the step function (STEP) is known. The transfer function formas well as STEP characteristic can be calculated on the base ofthe results of the measurements of the input/output signals (usingthe numerical tools offered by the MATLAB packet).The thermal objects, in the dynamic sense, have usually theinertial character of superior order. It was observed that the heatexchanger demonstrated the oscillatory properties in some workcases, it means that after the step function is used, the oscillationsof short duration in the output temperature course are present.At the beginning the measurements were performed for lowpower heat exchanger. Next the research for higher power heat ex-changer was carried out because the stronger oscillations with thelonger duration were expected. The laboratory investigation of theelements modeling heat exchanger were performed after that, be-cause the observed phenomenon have not unique character. Theresults of all these analysis are presented in this work.The long standing investigation of complex hybrid system inorder to create the coherent system describing their operating re-vealed phenomenon which are not described in the accessible lit-erature. It is possible to carry out the more precise analysis ofthe physical phenomenon thanks to that more advanced numericaltools used at present. The results of the model elements resistanceheating were previously analyzed using the EXCEL packet. The re-sults of this analysis were presented in [19] and they were not sat-isfactory, because they did not show precisely dynamic characterof the heating process course. Only when the MATLAB packetwas used and in particularly the System Identification