Analytical Study of Tri-Generation System Integrated with Thermal Management Using HT-PEMFC Stack

: Recently, extensive studies on power generation using clean energy have been conducted to reduce air pollution and global warming. In particular, as existing internal combustion engines lose favor to power generation through hydrogen fuel cells, the development of tri-generation technology using e ﬃ cient and reliable fuel cells is gaining importance. This study proposes a tri-generation thermal management model that enables thermal control and waste heat utilization control of a high-temperature PEMFC stack that simultaneously satisﬁes combined cooling, heating, and power (CCHP) load. As the high-temperature PEMFC stack operates at 150 ◦ C or more, a tri-generative system using such a stack requires a thermal management system that can maintain the operating temperature of the stack and utilize the stack waste heat. Thus, to apply the waste heat produced through the stack to heating (hot water) and absorption cooling, proper distribution control of the thermal management ﬂuid (cooling ﬂuid) of the stack is essential. For the thermal management ﬂuid control design, system analysis modeling was performed to selectively design the heat exchange amount of each part utilizing the stack waste heat. In addition, a thermal management system based on thermal storage was constructed for complementary waste heat utilization and active stack cooling control. Through a coupled analysis of the stack thermal management model and the absorption cooling system model, this study compared changes in system performance by cooling cycle operation conditions. This study investigated into the appropriate operating conditions for cooling operation in a tri-generative system using a high-temperature PEMFC stack.


Introduction
With rising air pollution due to fossil fuel-based power generation, improving energy efficiency through environmentally friendly power generation technology has increasingly gained widespread attention. Currently, cooling, heating, and power use in buildings comprises approximately 40% of the world's main energy consumption [1]. However, most of the energy used is generated from city-based centralized power stations with efficiency of less than 40%, resulting in high emissions of environmental pollutants and high operating costs [2]. Researchers have investigated various methods to address these issues, among which micro-generation technology has garnered attention for achieving fuel efficiency above 70% and system efficiency of up to 90% [3]. Micro-generation technology enables direct conversion to the required electric energy using the energy source from the application target. It is being introduced in hotels, airports, hospitals, and commercial buildings [4][5][6].
In addition to micro-generation technology, micro combined heat and power (CHP) and micro combined cooling, heating, and power (CCHP) systems have also emerged as environmentally friendly energy-saving systems [7]. Both systems provide power and thermal energy, which are mainly management and system performance changes according to the main operating conditions of the cooling system. Figure 1 shows a schematic diagram of the constructed tri-generative system including a high-temperature PEMFC thermal management system. TEG (the thermal management fluid of the overall tri-generative system) and the LiBr aqueous solution (the hydraulic fluid of the cooling system) are represented by the black and blue lines, respectively. For the CHP cycle, TEG was selected as the operating fluid in which phase change did not occur above 160 • C, which is the operating temperature of the high-temperature PEMFC. TEG is typically used as the cooling fluid from 100 to 280 • C or less. For the absorption cooling cycle, LiBr-Water was used as the refrigerant aqueous solution with the concentration maintained from 50 to 65%.

Tri-Generative System Description Based on HT-PEMFC System
The operating components of the high-temperature PEMFC thermal management and tri-generation cycle consist of the high-temperature PEMFC stack, thermal storage, partial TEG control valves, TEG circulation pump, heat exchanger, and absorption cooling system. This system controls the flow of TEG from the thermal storage to realize tri-generation with high-temperature PEMFC thermal management. TEG from the thermal storage is transferred through a high-temperature TEG circulation pump; to regulate the entire CHP cycle flow, the TEG passes through position (1) through the internal circulation (IC) valve and returns to the thermal storage. Through the TM valve, the total TEG flow determined by the IC valve replenishes heat through high-temperature PEMFC cooling (positions (2), (3), and (4)). The necessary hot water is provided through the CHP valve (consideration of heating and stack cooling amount is possible) (positions (5), (6), and (4)), and the amount of heat required for absorption cooling (positions (7), (8) and (4)) via the CCP valve is provided to the variable heat exchanger and then returned to the thermal storage. By single plate heat exchange, the heat quantity transferred to the generator reduces the concentration of LiBr-Water to generate vapor, the refrigerant in the absorption cooling cycle. The steam thus generated enters the condenser through (17), is condensed by the cooling water, and then passes through the liquid phase (18). The states of position (18) and (19) are divided by the expansion valve and the solution pump into relatively high and low pressure states, and evaporation is then performed with the latent heat in the evaporator through throttling. The vaporization efficiency Qe can be obtained from the evaporated vapor. The vapor evaporated at low pressures is absorbed by the high-concentration LiBr aqueous solution in the lower portion of the absorber through position (20) and (21). The LiBr aqueous solution, which absorbs the refrigerant vapor, is transferred to the generator by the solution pump (positions (11), (12) and (13)), and the high-concentration LiBr aqueous solution remaining in the generator is transferred to the absorber by the height difference (positions (14), (15) and (16)).

Stack Thermal Management and CHP System Cycle
We made the following assumptions to model the generation and exhaust heat changes of the high-temperature PEMFC stack according to the operating temperature. The operating state of the cell is steady and one-dimensional. The temperature is evenly distributed over the electrodes in the stack, and the reactive gas mixture is an ideal gas fluid. In addition, the outlet temperature of the cathode is equal to the operating temperature of the stack. The heat loss due to ambient temperature is relatively low and therefore ignored. All the heating value of the stack is transferred to the cooling fluid. The parameters applied to the model are shown in Table 1.
The anode (1), cathode (2), and overall electrochemical reaction (3) of the high-temperature PEMFC are as follows: V cell , the voltage of a single cell unit, is calculated through (4) using V act (activation voltage loss), V ocv (open circuit voltage), and V ohmic (ohmic overvoltage).
To calculate V act and V ohmic , R conc and R ohmic are obtained by using the temperature-only function through the linear regression method ((5) and (6)) [23].
V act , and V ohmic were calculated as shown in (7) and (8) through the calculated R conc and R ohmic [24].
V ohmic = I·R ohmic (8) where R is the ideal gas constant, R conc is the concentration resistance, R ohmic is the ohmic resistance, T cell is the operating temperature, α a is the anode charge transfer coefficient, F is the faraday constant, I is the current density, k eh is the hydrogen elecro-oxidation rate constant, θ h2 is the hydrogen surface coverage, α c is the cathode charge transfer coefficient, I 0 is the exchange current density, and λ air is the air stoichiometry. The P stack (total electric power) and P thermal (thermal power) of the stack can be calculated through (9) and (10) under the assumption that the individual stack performances are all the same.
where A cell is the Single cell active area, HHV is the higher heating value of the ideal gas constant, T cell is the operating temperature, and N is the number of cells in the stack [25]. We assume that the heat transferred to TEG, the coolant of the CHP cycle, is equal to the heat generated by the stack. All cycles were analyzed using the commercial program FloMaster based on the Energy (11), Mass (12), and Species conservation (13). In Equation (13)

LiBr-Water Absorption Refrigerator Cycle
The input values in Table 2 were used to observe basic performance. All flow control devices, such as control valves and flow pumps, are assumed to be adiabatic from the external. In addition, we assumed the same pressure for the evaporator and absorber, and the same pressure for the generator and condenser. The cooling performance of the evaporator is defined by the energy balance equation: . m 20 is the liquid mass flow rate, and  (15) The pressure at the outlet of the evaporator P 20 and the enthalpy of the vapor state h 20 are calculated solely by the function of the outlet temperature of the evaporator. The enthalpy of a certain liquid phase h 21 is set as the enthalpy of the saturated liquid state at the same pressure through the pressure and enthalpy diagram of water [26]: (16) h v = −1.2539 × 10 −3 T 2 + 1.88060937T + 2500.559 (17) The enthalpy at the inlet end of the evaporator h 19 and the enthalpy at the outlet end of the condenser h 18 do not change through the throttling process; this is used to obtain h 19 . h 19 is the enthalpy of the saturated liquid state in high-side pressure; high-side pressure is calculated using generator solution outlet temperature T 14 and LiBr strong solution mass fraction X 14 as follows [27]: The evaporator side mass flow rate  (14) and (15). The heating value of the absorber is defined by the energy balance equation: The heating value of the generator is defined by the energy balance equation: Through the P-T graph of water, the superheated steam T sh (the difference between T sat and T 17 , the temperature of the dry saturated steam at the same pressure) was calculated using Equation (33). The enthalpy at point 17 is defined by Equation (36).
The heating value of the condenser side is defined by the energy balance equation: COP is defined as: Table 3 shows the results of the single-effect LiBr-water absorption refrigerator cycle using the Engineering Equation Solver (EES). We use a script and controller signal to apply this to FloMaster as shown in Figure 2.

Selection of Appropriate Cooling Capacity
First, we use the integrated tri-generative system model to confirm the operating temperature change of the high-temperature PEMFC stack according to the load variation of the cooling system. We set operating conditions to ensure appropriate thermal management of the 5 kW rated stack model system as shown in Table 4. To set the flow distribution conditions for TEG (the stack thermal management and waste heat exchange fluid), we applied the opening of each valve and the operating conditions for each position of the absorption cooling system as shown in Table 5. TEG is configured to circulate through a single drive pump operating at a constant power, while the changes in stack temperature and cooling and heating load were checked through each valve control. For each operating condition, we compared whether it is possible to obtain an appropriate operating point between the stack thermal management and waste heat recovery in the tri-generative system according to the above-configured absorption cooling load.

Parameters Values
Heat balance of total system (kW) ±0.5 TEG temperature, T TEG ( • C) -7 < T TEG Stack outlet temperature, T S-out ( • C) 150 < T S-Out < 200 Hot water temperature, T hw ( • C) 40 < T hw As shown in Figure 3, we confirmed the optimal cooling capacity with the thermal management fluid distribution case satisfying the operating temperature of the stack. The cooling capacities including the required operating temperature range of the stack are 2.0 and 2.5 kW, respectively. As shown in Figure 4, the cooling capacities are 2.0 and 2.5 kW, in which the deviation of the stack load fluctuation is within 3.96% of 5 kW. Table 5 shows the valve open ratio and flow rate of each part of each case satisfying the stack operating conditions at the selected cooling capacity. Based on these selected conditions, we compared the optimum operating temperature range of the absorption cooling system, accounting for the cooling performance and stack thermal management according to the generator temperature and the concentration of the LiBr aqueous solution in the absorption cooling system.   As a result of this system analysis, the distribution of the stack outlet temperature according to the mass flow rate and corresponding to the open ratio of each valve is compared for each absorption type cooling capacity in Figures 5 and 6, respectively. According to the analysis condition setting, analysis cases satisfying the required operating condition in a total of 625 analysis cases showed 68 analysis cases and 96 analysis cases when the absorption cooling capacity was 2.0 kW and 2.5 kW respectively. A relatively wide range of valve open ratios was allowed when the cooling capacity was 2.5 kW.

LiBr Aqueous Solution Temperature in Generator
In the absorption cooling system, we compared the variation of cooling performance, stack thermal management, and waste heat distribution for each system according to the concentration of the LiBr aqueous solution circulating in the generator and the absorber serving as the heat pump. Figure 7 shows the TEG and hot water (heating) temperature changes of the PEMFC stack according to the configuration of the LiBr aqueous solution temperature at the generator outlet in each cooling capacity system. Accounting for the crystallization of LiBr, the temperature range of the LiBr aqueous solution at the outlet of the generator was set to approximately 65 to 120 • C. We conservatively set the optimal range of the stack outlet TEG temperature to approximately 150-160 • C. At cooling capacities of 2.0 kW and 2.5 kW, respectively, the temperature of LiBr aqueous solution at the outlet of the generator ranged from approximately 106-111 • C and 94-97 • C, respectively. This indicates that the optimal cooling operating temperature range for stack thermal management changes according to the applied cooling capacity; thus, we were able to compare the performance and efficiency of the implemented cooling systems. Figure 8 shows the stack waste heat amount and the actual cooling and hot water (heating) load according to the temperature of the LiBr aqueous solution at the outlet of the generator. In the temperature range of LiBr aqueous solution at the outlet of the generator set in Figure 7, the cooling load was approximately 3.7-4.0 kW. Comparing the COP of the absorption cooling system according to the temperature of LiBr solution at the outlet of the generator (Figure 9), at cooling capacities of 2.0 and 2.5 kW, the COPs of the appropriate temperature range were approximately 0.47-0.52 and 0.6-0.65. Thus, we concluded that the 2.5 kW cooling system was more effective for the 5.0 kW high-temperature PEMFC stack capacity.

Concentration of LiBr Aqueous Solution
We compared the cooling performance and the load of each part of the system including the stack thermal management according to the concentration of the generator LiBr aqueous solution constituting the absorption cooling system. The temperature of the LiBr aqueous solution at the outlet of the generator was set to 90 • C; Table 5 shows the selected operating conditions excluding the high concentration side of the LiBr aqueous solution. As shown in Figure 10, at cooling capacities of 2.0 and 2.5 kW, the concentration of the LiBr aqueous solution is approximately 54-56% and 54-57%, respectively, when the stack outlet TEG temperature ranged from approximately 150-160 • (review of operating conditions and generator temperature needed). Figure 11 compares the stack heating value with the load of each system according to the concentration of the LiBr aqueous solution in the generator. When the high concentration side of the LiBr aqueous solution is approximately 58% or more, the stack waste heat and the load of each system converge to an acceptable level which is common among the cooling capacity conditions.

Conclusions
To design a partial heat transfer (stack waste heat) fluid distribution method for a tri-generative system considering the thermal management of a high-temperature PEMFC stack, this study modeled and compared the heat transfer fluid (TEG) valve control systems according to the applied cooling capacity. Based on the selected optimal stack waste heat distribution conditions, we compared stack thermal management and the performance of the absorption cooling system. The results enabled us to determine the optimal cooling capacity and operating conditions for the high-temperature PEMFC stack applied to the tri-generative system through the cooling system.
The conclusions of this study can be summarized as follows.
(1) We modeled the integrated thermal management tri-generative system, including a high-temperature PEMFC stack heating model and absorption cooling model, to compare the suitable operating conditions according to the stack waste heat and partial load. (2) When the appropriate operating range of the 5 kW high-temperature PEMFC stack model is set to about 150-160 • C, the applied cooling capacity ranged from approximately 2.0 to 2.5 kW, allowing us to set a proper flow control distribution for the partial thermal management fluid in the model.
(3) When a 2.5 kW absorption cooling capacity is applied, considering the stack thermal management, if the operating temperature of LiBr aqueous solution at the outlet of the generator is judged to be in the appropriate range from 94 to 97 • C, then COP is expected to range from approximately 0.6-0.65.
This tri-generative system model can be improved based on the capacity and thermal management conditions of the applied high-temperature PEMFC stack, thereby contributing to the selection of initial system operating conditions and post-design operation optimization. Through experimental verification based on the operating conditions of the tri-generative system applying the improved high-temperature PEMFC stack, we plan to conduct correlation analyses with the improved model reflecting partial loss and detailed structural information.