Tri-generation System Analysis

In this section the micro-tubular SOFC is integrated alongside the SDCS to form the complete tri-generation system. As previously highlighted, the microtubular SOFC operation is fixed, it has no provision to modulate or alter output. Therefore, it is the liquid desiccant systems operation that is controlled in order to investigate tri-generation system performance.

In the tri-generation system the connection between the micro-tubular SOFC and liquid desiccant components is through the regenerator. As a result of this arrangement, the tri-generation system analysis evaluates the performance of the regenerator at three different desiccant solution flows using the micro-tubular SOFC thermal input. This is to determine the instantaneous performance of the novel system. Following this, a daily tri-generation performance analysis is presented which serves to demonstrate the novel system operating in a nonsynchronous fashion in a building application.

For efficient and effective tri-generation system operation, a moisture balance between the dehumidifier and regenerator is required. As a result, the achievable regenerator moisture addition rate presented in this section is equated, using the data presented in Sect. 6.3.1, to a suitable dehumidifier moisture removal rate.

From the dehumidifier moisture removal rate, the achievable cooling output can be obtained, and the tri-generation system efficiency calculated. In Chap. 6, the SDCS dehumidifier was tested in the environmental chamber to simulate real life operating conditions. Thus the tri-generation system results presented are representative of the novel system in a real working environment.

Testing of the tri-generation system is carried out by operating the microtubular SOFC CHP system until an outlet water temperature of 50 °C is achieved with a 2 L min^-1 water volumetric flow in the WHR circuit. This takes approximately 120 min from ambient. The by-pass loop is then closed and the hot water is directed to the regenerator desiccant solution plate heat exchanger (PX1). The regenerator desiccant solution and air flow is then turned on. The regenerator tests last for 90 min or until steady-state output data is achieved. The micro-tubular SOFC and regenerator are then turned off.

Three desiccant solution volumetric flows have been investigated: 1.2, 2.2,

3.2 L min^-1 at a potassium formate desiccant solution mass concentration of 0.65-0.7. The regenerator volumetric air flow used is 256 m³ h^-1. The aim of the investigation is to determine the conditions, at which the regenerator moisture addition rate is highest, and thus the instantaneous cooling output can be maximised. As previously highlighted, during tri-generation system testing the regenerator uses ambient laboratory air.

Figure 7.9 shows the performance of the regenerator operating on the thermal input from the micro-tubular SOFC. The plot shows the regenerator moisture addition rate and inlet solution temperature with respect to regenerator solution volumetric flow. As demonstrated in Figs. 6.12a and 6.13a, it is apparent that the regenerator moisture addition rate is related to both the desiccant solution volumetric flow and temperature. Operating with the micro-tubular SOFC thermal input, the highest regenerator moisture addition rate of 0.11 g s^-1 is achieved at a 2.2 L min^-1 desiccant solution flow. This desiccant solution volumetric flow

Fig. 7.9 Tri-generation system regenerator performance

Table 7.2 Operating values for experimental tri-generation system evaluation

achieves a balance between the volume and temperature of solution passing through the regenerator HMX. As a result, a 2.2 L min^-1 solution volumetric flow has been selected for tri-generation system evaluation.

For successful tri-generation system evaluation a dehumidifier operating condition which balances the regenerator operation is required. Figure 6.7a shows a balanced (with the regenerator) dehumidifier moisture removal rate of 0.11 g s^-1 is achieved when operating at a dehumidifier volumetric air flow of 102 m³ h^-1. This equates to a cooling output of 278.6 W. Table 7.2 provides the operating values selected for tri-generation system evaluation.

Figure 7.10a, b show the respective steady-state performance of the micro-tubular SOFC and SDCS regenerator during tri-generation system testing. Figure 7.10a shows that the micro-tubular SOFC is operating in CHP mode up until 125 min. During this period 150.4 W of DC electrical power is produced with a thermal output of 418 W. At 128 min the WHR flow temperature reaches 51.76 °C, and the regenerator is turned on for a 90 min test. During the regenerator testing period, the WHR flow temperature drops dramatically, to eventually stabilise at approximately 32 °C. During the regenerator operating period the power output remains unchanged, however the WHR thermal output increases to 572.8 W. This increase in thermal output is due to the regenerator desiccant solution acting as a thermal load and thus lowering the return water temperature to the recupertaor heat exchanger. At 218 min, the micro-tubular SOFC and regenerator are turned off, and the system takes 22 min to cool down (Fig. 7.10).

Figure 7.10b shows the moisture addition rate in the regenerator and the inlet water temperature to regenerator PX1 over the 90 min test period. For the first 15 min the results are unstable. Following this, the regenerator moisture addition rate reduces in proportion with the inlet water temperature. As the desiccant solution is circulated through regenerator PX1 it is heated using the hot water in the SOFC WHR circuit. The low thermal output from the SOFC cannot maintain the flow temperature of 50 °C in the WHR circuit and as a result the water flow temperature decreases over time. Over the regenerator test period, the inlet water temperature decreases from an initial value of 50 °C to approximately 32 °C after 50 min when it becomes steady. The moisture addition rate of 0.11 g s^-1, shown in Table 7.3, is an average value taken between 60 and 90 min as this is a period of steady-state operation. The average solution temperature in this period is 27.2 °C.

Table 7.3 presents the instantaneous performance of the novel micro-tubular SOFC liquid desiccant tri-generation system. The performance evaluation is

Table 7.3 Instantaneous performance of the novel tri-generation system

Fig. 7.10 Tri-generation system performance in the a micro-tubular SOFC, and b SDCS regenerator

provided at the 150.4 W electrical output. The 110 W parasitic energy consumption of the SDCS has been accounted for.

The marginal difference in the dehumidifier moisture removal and regenerator moisture addition rates is deemed insignificant enough for the purpose of tri-generation system evaluation. The novel system can generate 150.4 W of electrical power, 442.6 W of heat output or 278.6 W of cooling. Instantaneous trigeneration system efficiency is 24.77 %. At the original 250 W electrical output, the tri-generation system efficiency is 32.5 %. Without considering the parasitic energy consumption of the SDCS, the tri-generation system efficiency is 33.31 % at a 150.4 W electrical output and 41.04 % at the 250 W electrical output. When integrated with the micro-tubular SOFC, the SDCS demonstrates a COP_th of 0.62, an encouraging value for a waste heat driven cooling system of this capacity. Due to its low temperature regeneration requirement, potassium formate at a 0.65-0.7 mass concentration is an appropriate desiccant solution for a SOFC tri-generation system.

The tri-generation system efficiency is low. However, as highlighted in Sect. 7.4.1, the initial micro-tubular SOFC CHP system efficiency is below 50 %. Tri-generation system analysis shows that the low thermal output from the microtubular SOFC is insufficient to maintain a flow temperature of 45-50 °C and thus the regenerator moisture addition rate is low, resulting in a small instantaneous cooling output. Furthermore, almost all the micro-tubular SOFC electrical output is used for the parasitic energy consumption of the SDCS. However, the novel concept of integrating SOFC and liquid desiccant air conditioning technology into the first of its kind tri-generation system has been successfully demonstrated. The SOFC has been used to generate simultaneous electrical power, heating and dehu- midification/cooling. The inclusion of liquid desiccant air conditioning technology provides an efficiency increase of up to 13 % compared to SOFC electrical operation only, demonstrating the merit of the novel tri-generation system in applications that require electricity, heating and dehumidification/cooling. Improvements to the micro-tubular SOFC WHR provision will improve tri-generation system performance.

As highlighted in Sect. 6.3.4, an operational advantage of a SOFC liquid desiccant tri-generation system is the potential for nonsynchronous operation. Re-concentration of the desiccant solution over extended time periods is an effective and efficient form of storing the constant thermal energy output from the SOFC with minimal losses. Furthermore, to meet a specific cooling load the dehumidifier can be operated at a higher cooling capacity, and the regenerator can be operated for an extended period to make up the moisture addition shortfall, creating a solution mass balance. Based on the assumption that the SOFC operates for 24 h a day, with a 6 h cooling period, Table 7.4 presents the daily tri-generation system performance, and serves to demonstrate the novel tri-generation system operating in a building application. The SDCS dehumidifier performance figures are taken from the steady-state operating values shown in Fig. 6.5.

The proposed daily tri-generation system operating concept demonstrates that the novel system can produce a peak cooling output of 527 W over a 6 h period. The daily tri-generation system efficiency is 37.9 %. At the original 250 W electrical output the daily tri-generation efficiency is 45.6 %. As the cooling period is increased the daily tri-generation efficiency decreases. This is because the SDCS has a COP_th of less than one. In this scenario the system would require the

Table 7.4 Daily tri-generation system performance

provision of sufficient desiccant solution storage in order balance the dehumidifier and regenerator operation. Continuous micro-tubular SOFC operation is a reasonable assumption in a (domestic) building application as the small electrical output can be used for base load applications (lights, standby etc.).

The micro-tubular SOFC can operate on natural gas, in such a scenario the novel tri-generation system generates a cost and emission reduction of 56 and 42 % respectively compared to a base case scenario of grid electricity, gas fired boiler and electrical driven VCS. The constants used for the emission and economic analysis can be referred to in Table 4.6. The encouraging economic and environmental performance demonstrates the potential of the novel tri-generation system in applications that require simultaneous electrical power, heating and dehumidification/cooling.

Section 7.4.2 has presented tri-generation system integration and analysis. The novel concept has been proven, experimentally, in the first of its kind system; however the reported performance is low. This is primarily due to the low thermal output from the micro-tubular SOFC. Possible solutions to improve performance have been discussed. Next, Sect. 7.5 presents the chapter conclusions with particular respect to the achievement of the thesis aim.