DYNAMIC CONTROL OF HEAT EXCHANGE PROCESSES FOR TRIGENERATION POWER SYSTEMS WITH FUEL CELLS

. The article considers the issues related to the development of energy technologies based on trigeneration with the use of fuel cells, which is expedient to use for additional generation of electrical energy. The possibilities of fuel cells to integrate them into traditional energy systems are shown; technological schemes using PEMFC, PAFC, and MCFC fuel cells are given. Maintaining a stable operating mode depends on temperature stability, so models of heat exchange processes are given and calculations of dynamic parameters for process stabilization under various external influences and dynamic control correction schemes are presented. The tools MatlabSimulink are used as a research tool.

Energy technologies belonging to the new generation should be able, in addition to providing their own high energy efficiency, should also successfully "embed", integrate into the existing energy system to improve the efficiency of energy conversion and ensure a more complete controllability and operational THEORY AND PRACTICE OF SCIENCE: KEY ASPECTS 218 reliability. One of such new generation energy technologies are energy technologies based on fuel cells, topless cells, creating technological opportunities for successful and full productive use of energy, which previously could not be used by the technological capabilities and equipment of the "old" generation. The development of fuel cell technology creates great opportunities for research in the field of power generation. The following are various subsystems that integrate a fuel cell (FC) system into a conventional power, metallurgical or other technological system. In such a system, the residual heat not used in the technological process makes it possible to additionally obtain hot water with a temperature in the range of 80C as a useful product. This temperature is usually sufficient to start absorption cooling cycles. Figures 1, 2 and 3 show the system structures proposed for fuel cell (FC) trigeneration to obtain a higher level of overall energy efficiency.
Trigeneration with PEMFC-type fuel cells. About 25% of the primary energy consumed in some countries is for heating and hot water. Since the temperature level required to meet these demands is relatively low, this is where PEMFC fuel cell technology becomes most appropriate. Figure 2 shows the PEMFC technology operating in trigeneration mode. of about 30%. This creates real possibilities for the use of this technology in polygeneration processes by showing that the generator system operates in a very suitable mode with a generator temperature in the range (60...65)C. This temperature is compatible with the temperature of the heat released by the PEMFC, which is usually in the 80C range.
Trigeneration with PAFC type fuel cells. Consideration is given to using PAFC technology to operate an air conditioning system at a site with a hot summer climate.
Under these conditions, air conditioning equipment can consume more than 75% of the electricity generated, typically during peak hours. Figure 2 shows the configuration adopted in an absorption refrigeration system using PAFC technology, located in Kuwait in particular. This system is rated at 200 kW and produces 105 kW of thermal energy at 120C and 100 kW at 60C. The system uses lithium bromide water as the working vapor; the electrical efficiency (EE) at full load is 45% and the thermal efficiency (TE) is 35%. Trigeneration with MCFC technology. Figure 3 shows the MCFC technology integrated into the process cooling system by absorption. This MCFC fuel cell cooling system showed electrical efficiency (EE) results of 42.27% and thermal efficiency (TE) of 44.21%. Thus, the total, total energy conversion efficiency, expressed in terms of fuel efficiency, is 86.48%. Considered SOFC technology, used to obtain air conditioning or hot water for use in residential and non-residential buildings. The system uses water/lithium bromide in an absorption cooling cycle as the operating couple. The results show that this combination of technologies presents major technical and environmental advantages. The SOFC proposed in this example is a pre-commercial 110 kW tube model developed by Siemens-Westinghouse. According to the developer, the system has the following efficiency figures: electrical efficiency is 43.3%; thermal efficiency in heating is 43.7%; thermal efficiency in cooling is 52.6%; thermal efficiency in hot water production is 46.7%. This figure yields global efficiency results in three operating modes of up to 87.95%, 95.9% and 90%, respectively.
Of particular interest are systems capable of using heat sources with low temperature (below 100C), i.e. capable of using residual heat of industrial origin or produced in cogeneration plants. In relation to industrial processes that use heat energy, they are classified according to the temperature level of the heat required: processes with low temperature, below 100C; processes with high temperature, from 300C to 700C: some chemical industries. To study the operation of the heat exchanger, as one of the main types of heat power and heat engineering equipment, let's consider a generalized, in a sense, model of the heat exchanger. Technological scheme of the model is given in figure   4 and it fully reflects all processes, which with sufficient degree of reliability describe similar equipment of different sizes and thermal capacities. Technological process is as follows. Hot steam flow passing through the heating pipeline inside the working tank of the exchanger gives its energy and heats the cold liquid inside the working tank to a certain temperature. The heating process is monitored by a temperature sensor. By regulating the flow of steam with a valve, as well as the volume of flowing liquid, we provide the specified parameters in terms of temperature, as shown in Figure 5. As already mentioned, the main task under consideration is to stabilize the temperature parameters.   It should be noted that without participation of regulating devices, for example, the specified steam valve, the processes will have uncontrolled character and may have quite arbitrary dependences of the output coordinate -temperature. There can be an oscillatory character with a very significant value of overregulation. Figure 5 shows curves of heat-exchange processes of the heat exchanger without regard to THEORY AND PRACTICE OF SCIENCE: KEY ASPECTS 224 the regulator and optimized in terms of speed to "technical optimum" (TO) with speed not more than 5.0 values of the control time constants and the value of overshoot not more than 5%. The process of setting the automatic control system to "symmetrical optimum" (SO) is also possible. In this case, a better response time of about 3.0 values of time constants will be achieved, but the overshoot will reach a value comparable to 50% of the steady-state value. Figure 5 below shows curves roughly comparable to the optimal process criteria listed above.
Mathematical description of the controlled process of exchange of thermal energy. Figure 6    Digital calculation models of heat exchange process and results of computer simulation. For the analysis of dynamic processes, which would provide the desired characteristics of heat exchange processes to the best extent, we will use a digital model, which is fully adequate to the ongoing physical processes in the heat exchange apparatus and implement the structural diagram in Figure 8, left.
Schematic diagram of the digital model is given in the MatlabSimulink modeling system and is presented in Figure 8, right. The above studies and calculations show that in the open state the dynamic system describing heat exchange processes according to the model shown in Fig. 9 does not satisfy the above requirements with respect to the desired stability of heat exchange processes. Therefore, in order to investigate the possibilities of improving the regulating properties of the heat exchange system, we perform further additional research.