Experimental Tri-generation System Results and Analysis
The performance of the SDCS is documented in detail in Chap. 6, in which the regenerator thermal input values (water volumetric flow and water flow temperature) were selected based on a range that matched the 1.5 kWe (BlueGEN) SOFC CHP system. However, due to the BlueGEN SOFC not being available for tri-generation system integration, a 250 We (Adelan) micro-tubular SOFC unit had to be acquired. Actual thermal output values for this unit were not known before testing because the micro-tubular SOFC WHR system was developed specifically for this project by The University of Nottingham. Prior to this the micro-tubular SOFC had only been used for electrical production. Section 7.4.1 presents micro-tubular SOFC CHP system component analysis, which aims to quantify the thermal performance of the SOFC before tri-generation system integration and analysis in Sect. 7.4.2.
Micro-tubular SOFC CHP System Component Analysis
In this section the micro-tubular SOFC CHP system component test results are presented and analysed. The results are provided to show (a) the performance of the micro-tubular SOFC unit (electrical and thermal) over time and (b) to characterise the thermal performance to facilitate effective tri-generation system integration in Sect. 7.4.2. The micro-tubular SOFC CHP unit operates at a constant output. The input fuel flow rate is fixed and thus so power and thermal output is approximately constant. The water volumetric flow in the WHR circuit is set to 2 L min-1 to replicate the SDCS testing conditions presented in Chap. 6.
Before being supplied to The University of Nottingham, long term stability testing of the micro-tubular SOFC was carried out at The University of Birmingham.
This testing data is presented in Appendix 4. The micro-tubular SOFC was run for 130 h, with 19 thermal cycles. A steady-state electrical output of 250 W has been demonstrated. However, after 75 h of operation severe sulphur poisoning of the micro-tubular SOFC stack occurred. The sulphur trap had been previously used and went over the 250 h operating limit. In the hours following sulphur trap replacement very little power output could be gained from the micro-tubular SOFC. However, after approximately 95 h of operation, the micro-tubular SOFC had recovered to a final maximum power output of 140 W. The micro-tubular SOFC has now recovered a little more to a 150.4 W electrical output, which is approximately 60 % of the manufacture’s quoted 250 W capacity. Sulphur poisoning and the regeneration of Ni-based anodes in SOFCs has been investigated by Zha et al. (2007). The degradation in cell performance is attributed to rapid adsorption of sulphur onto the Ni surface to form nickel sulphide, which blocks the active sites for hydrogen adsorption and oxidation. Following removal of H2S (hydrogen sulphide) from the fuel stream, the anode performance can, depending on operating conditions and duration of H2S exposure, recover fully or partially. The rate of the recovery process increases with operating temperature and cell current density.
Figure 7.8 shows the electrical and thermal performance of the micro-tubular SOFC CHP unit over a 350 min test period. The first 28 min shows the microtubular SOFC heat-up. During this period the micro-tubular SOFC has a 35 W parasitic load on the batteries, however there is a thermal output. Following heat-up, the micro-tubular SOFC goes into power production. During this period there is an electrical and thermal output. At 322 min, the micro-tubular SOFC is turned off, and it takes 22 min to cool down.
Figure 7.8a shows that during the power production period (minute 28-322) the micro-tubular SOFC produces an average of 150.4 W of DC electrical power (12.2 V at a current flow of 12.33 A). This equates to an electrical efficiency of 11.68 %, in comparison to 19.4 % at the original 250 W electrical output.
Figure 7.8b shows the inlet and outlet water temperatures in the WHR circuit. At a 2 L min-1 water volumetric flow, a maximum WHR outlet water temperature of up to 65 °C is possible, demonstrating the potential for desiccant solution regeneration in a tri-generation system context. Over the power production period, the average water temperature difference across the recuperator heat exchanger is 3.2 °C, this equates to an average thermal output of 446.9 W. The pinch temperature shown in Fig. 7.8b is the difference between the outlet flue gas temperature and outlet WHR water temperature. The pinch temperature reaches a peak of
20.01 °C at 70 min then gradually decreases to 11.53 °C at 322 min. The decline in pinch temperature is because the rate of increase in flue gas outlet temperature over time is greater than the rate of increase in the WHR outlet water temperature. This indicates a reduction in thermal energy extraction.
Based on the averaged values over the power production period, the microtubular SOFC achieved a CHP efficiency of 46.39 %. This compares with
54.1 % based on the micro-tubular SOFC unit before sulphur poisoning, which had an average power output of 250 We. The stable operational nature of the
Fig. 7.8 a Micro-tubular SOFC CHP electrical and thermal output, and b WHR inlet/outlet, flue gas outlet and pinch temperatures
micro-tubular SOFC CHP unit demonstrates the potential for tri-generation system integration. Using Eq. 5.3, the maximum calculated relative uncertainties in the SOFC QWHR and пснр are ±9.1 and ±6.8 % respectively.
In comparison to other combustion based micro-CHP technologies of this electrical capacity, the electrical efficiency of the micro-tubular SOFC is reasonable. Compared to planar type SOFC systems, the micro-tubular SOFC has a low electrical efficiency. However, the significant advantage of micro-tubular SOFC technology has been confirmed. Quick start-up and shut-down times of 20 min have been demonstrated, meaning the unit can respond to rapid supply and demand requirements. The company (Adelan Ltd.) supplying the micro-tubular
SOFC have not previously attempted to provide WHR, and thus the WHR system was developed for this project by the University of Nottingham. The low thermal output from the micro-tubular SOFC highlights the need for future work on optimising and refining the WHR provision in order to maximise the thermal output and thus elevate the system efficiency. Future work should aim to improve the connection between the afterburner outlet and recupertaor heat exchanger flue inlet. As demonstrated in Chap. 6, a regenerator thermal input of less than 500 W will result in limited regeneration capacity. In order to maintain balanced liquid desiccant system operation, a restricted cooling output will have to be assigned to the dehumidifier.
Section 7.4.1 has presented component testing of a micro-tubular SOFC CHP unit. The micro-tubular SOFC was acquired at short notice to replace the building-installed 1.5 kWe BlueGEN SOFC. Due to sulphur poisoning the micro-tubular SOFC unit has suffered a 40 % drop in electrical output from 250 to 150 W. Water flow temperatures in the WHR circuit of up to 65 °C at a 2 L min-1 water volumetric flow have been demonstrated. The micro-tubular SOFC has a low thermal output of approximately 450 W, thus it is not the ideal match for the developed SDCS and it is anticipated that regeneration capacity in the tri-generation system will be limited. However, the novel concept of integrating SOFC and liquid desiccant technology is still successfully demonstrated in Sect. 7.4.2
Next, Sect. 7.4.2 presents the results and analysis from tri-generation system integration and testing.