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Theoretical Tri-generation System Integration Study

The original SOFC that was intended for tri-generation system integration and field trial testing in a building application was the BlueGEN CHP unit manufactured by Ceramic Fuel Cells Ltd. (CFCL). However, unit failure has meant that the SOFC is no longer available for tri-generation system integration. Although the BlueGEN SOFC is no longer operational, a theoretical integration analysis, based on collected empirical SOFC and SDCS data is presented. Published journal articles, by the author, on SOFC operation in a real building context and tri-generation system integration can be referred to here; Elmer et al. (2015a, 2016a).

BlueGEN is a commercially available SOFC CHP system designed for small to medium scale building applications. Operating on natural gas (NG), the unit can be power modulated from 500 We (25 %) to 2 kWe (100 %), however it achieves its highest net electrical efficiency of 60 % at a 1.5 kWe output. As a result, CFCL have optimised the default operation of the unit at 1.5 kWe to provide the highest electrical efficiency and thus greatest economic benefit to the user. The BlueGEN unit consists of 51 planar type YSZ electrolyte layer sets (each layer consist of 4 cells), and operates at 750 °C. Hydrogen is produced from natural gas by internal steam reforming (endothermic) on the fuel cell anode, utilising the heat of the electrochemical reaction (exothermic) to create a chemical combined cycle. The BlueGEN unit was selected for field trial testing because (1) it is commercially available, (2) it fulfils the technical objectives of the thesis, and (3) is certified for domestic building installations and qualifies for the UK FiT (feed-in-tariff); a tariff paid to the consumer per kWh of generated electricity. The unit is installed at The University of Nottingham’s Creative Energy Homes as shown in Fig. 7.1.

BlueGEN SOFC CHP system installed at The University of Nottingham

Fig. 7.1 BlueGEN SOFC CHP system installed at The University of Nottingham

The SOFC is connected electrically, in parallel, to the national grid in order to export or import power as required. The SOFC is connected to the natural gas grid. A heating water circuit delivers the generated heat from the SOFC unit directly to the homes 300 L hot water cylinder, which is supplemented by an auxiliary gas boiler. For tri-generation system integration, the intention was to install the SDCS outside of the home in-line between the SOFC and hot water cylinder, as shown in Figs. 7.1 and 7.2. Three way diverter valves direct thermal energy to the regenerator during tri-generation system operation.

Figure 7.3 shows field trial electrical performance data collected from the SOFC, using the online CFCL interface, from 24 March 2014 (point 1) to 12 December 2014 (point 8). This is equivalent to 4865 h of operation (8 months

BlueGEN and SDCS connection

Fig. 7.2 BlueGEN and SDCS connection

BlueGEN field trial electrical performance data

Fig. 7.3 BlueGEN field trial electrical performance data

  • 18 days). During this period the SOFC unit shows stable operation, i.e. electrical efficiency of 55-60 %, with availability for power generation of 91.7 %, therefore demonstrating the potential for the development of an efficient and effective tri-generation system. Due to the time taken to heat the stack to 750 °C and to avoid thermal cycling, the unit operated continuously, always aiming to maintain a
  • 1.5 kWe output. As seen in Fig. 7.3, as the stack efficiency degrades over time the fuel input is increased to compensate for this. At an electrical efficiency of 60 % the fuel input is 2.5 kW. After 4000 h of operation (point 2-5), the stack displayed an electrical efficiency degradation of approximately 6 %.

Three key events in the lifetime of the unit have meant that it is not available for tri-generation system integration. These events were as follows: (1) an unforeseen gas shut-off causing stack cool down and thermal contraction, leading to an electrical efficiency drop, and eventual stack failure (point 5) and replacement (point 6), (2) A 415 V voltage surge at The Creative Energy Homes causing irrevocable damage to the power electronics and thus stack cool-down, again leading to the requirement of power electronic and stack replacement (point 8). (3) CFCL going into administration, and thus not being able to carry-out the required repair works post voltage surge. At the time of writing the BlueGEN SOFC unit is not operational. During the operational period, the WHR circuit was only connected for a short period, thus there is limited thermal output data. However, Sommer and Messenholler (2013) and Foger (2013) have carried out extensive electrical and thermal performance characterisation of an identical BlueGEN SOFC CHP system in a building application. The results from a BlueGEN power modulation study are shown in Fig. 7.4. During the performance evaluation, Foger (2013) used a 2 L min-1 water volumetric flow in the WHR circuit. This is equal to the value used in the SDCS performance analysis in Chap. 6, thus a theoretical integration study is a rational idea.

From Fig. 7.4a it is evident that the net electrical efficiency increases as the electrical capacity increases, from 14 % at 200 We up to a maximum of 60 % at 1500 We, it then decreases to approximately 56 % at a 2000 We capacity. The thermal output from BlueGEN increases fairly linearly from 320 Wth at 200 We up to 540 Wth at 1500 kWe. The thermal output increase is then much steeper, up to a maximum of 1000 Wth at 2000 We. At the optimised 1500 We output a CHP efficiency of 81.6 % is achieved.

The field trial data shown in Fig. 7.3 demonstrates a net electrical efficiency of 60 % is achievable and maintainable at a 1.5 kWe output in a real life building application and thus the data presented in Fig. 7.4a can be used with confidence.

Figure 7.4b shows the flow water temperature in the SOFC WHR loop as a function of electrical power output. The flow water temperature is calculated based on the thermal output data presented in Fig. 7.4a, a 2 L min-1 water volumetric flow and a 45 °C return water temperature. The flow water temperature ranges between 47 °C at 100 We electrical power output up to a maximum of and 52 °C at a 2000 We electrical power output. As highlighted in Chaps. 4 and 6, it is primarily the desiccant systems operation that needs to be optimised to facilitate effective tri-generation system integration. Using the WHR flow water temperature and SDCS empirical data shown in Fig. 6.16c, the COPth of the SDCS can be

a SOFC electrical efficiency and thermal output, and b WHR flow temperature as a function of electrical output (Foger 2013)

Fig. 7.4 a SOFC electrical efficiency and thermal output, and b WHR flow temperature as a function of electrical output (Foger 2013)

identified, and the cooling output calculated. Table 7.1 presents the results from the theoretical integration of the BlueGEN SOFC and SDCS into a complete trigeneration system at a net 1.5 and 2 kWe output, operating with a 30 °C and 70 % RH inlet air condition. The parasitic energy consumption (110 W) of the SDCS has been included in the evaluation. The constants used for the PED, cost and emission analysis can be referred to in Table 4.6.

The theoretical integration study based on empirical data, demonstrates that high tri-generation efficiency in the range of 68-71 % is attainable when combining SOFC and liquid desiccant air conditioning technology; values in good agreement with the simulations presented in Chap. 4. The SOFC unit has a low heat to power ratio, particularly at the 1.5 kWe condition, this is because it is an electrically optimised device (fuel utilisation of ~85 %). As a result, there is limited thermal output available for desiccant solution regeneration. However, the SDCS

Table 7.1 BlueGEN SDCS tri-generation system performance evaluation

Variable

1.5 kWe

2 kWe

nelec(%)

60

56

Q CH4 (W)

2500

3571

Q whr(W)

540

1000

TwHR,flow(° C)

48.87

52.16

^CHP(%)

81.6

84

Desiccant volume (L min-1)

1.74

3.16

COPth

0.614

0.649

Q cooling (W)

332

649

MRR (g s-1)

0.2515

0.2941

ntri (%)

68.9

71.1

Д% PED (CHP/TRI)

51.41/46.98

50.21/46.79

Д% Cost (CHP/TRI)

62.84/60.67

61.53/60.53

Д% Emissions (CHP/TRI)

51.21/68.96

50.01/68.26

operating with a potassium formate solution at a 0.65-0.7 solution mass concentration has a low regeneration temperature requirement, and thus makes good use of the low-grade SOFC WHR output to generate a meaningful quantity of dehu- midification/cooling. At the 2 kWe condition, electrical efficiency is lower, but the thermal efficiency is higher. As a result, almost 650 W of cooling is produced. The inclusion of liquid desiccant air conditioning technology provides an efficiency increase of 9-15 % compared to SOFC electrical operation only. The performance of the novel tri-generation system is competitive with other systems of this capacity reported in the literature (Kong et al. 2005; Jradi and Riffat, 2014c; Wu et al. 2014; Buker et al. 2015), and the tri-generation system simulations presented in Chap. 4.

Table 7.1 shows that CHP and tri-generation efficiency is highest for the 2 kWe case. However the PED, cost and emission savings are highest for the 1.5 kWe case. Electricity has a higher associated cost and emissions compared to natural gas, therefore greater savings are made for the 1.5 kWe case due to the higher electrical efficiency. In tri-generation cooling mode, relative cost and emission reductions compared to a conventional separated system for the 1.5 and 2 kWe cases are around 60 and 70 % respectively, demonstrating the potential of the first of its kind SOFC liquid desiccant tri-generation system for building applications. A more comprehensive economic and environmental assessment is provided in Chap. 8. The operational issues encountered with the BlueGEN SOFC illustrate the real challenge of fuel cell deployment in the built environment. Reliability, durability and cost currently pose a great barrier to their wider use. Not until these issues are addressed will the operational advantages of fuel cells operating in the built environment be realised. Although experimental integration was not possible due to unforeseen circumstances, the stable nature prior to stack failure of the BlueGEN SOFC unit highlights the potential for the development of an efficient and effective tri-generation system.

Section 7.2 has presented a theoretical tri-generation system integration study based on empirical data. The work has established that SOFC and liquid desiccant are a viable technological pairing in the development of an efficient and effective tri-generation system. It has been demonstrated that high tri-generation system efficiency is attainable at low system capacities. The encouraging performance is primarily due to the high electrical efficiency of the SOFC and the reasonable COPth of the liquid desiccant system. The proof-of-concept study has achieved two of the technical objectives of the thesis, namely a 1.5 kWe system operating at an electrical efficiency of 45 % or higher. Meeting the overall system efficiency objective would require a higher desiccant system COPth.

Next, Sect. 7.3 presents the experimental tri-generation system, system configuration, experimental set-up, instrumentation and experimental method.

 
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