Specific tri-generation system case

The tri-generation system efficiency is defined as the ratio of the overall tri- generation system energy conversion (electricity + heating + cooling) over the total amount of energy input to the system. The net electrical output is defined as the total AC electrical output from the SOFC less the electrical requirement to operate the liquid desiccant systems pumps and fans, and has been assumed constant, from empirical data, at 110W. The net heat output is defined as the total thermal output from the SOFC CHP unit less the thermal energy input required to regenerate the liquid desiccant solution. The energy input to the tri-generation

system is the required fuel energy input ( ) to the SOFC CHP unit.

The results from the tri-generation system analysis are presented in three sections. First, section 4.3.1 presents a specific case based on the thesis technical objectives. The specific case will provide a theoretical benchmark value for the first of its kind SOFC liquid desiccant tri-generation system. Following this, section 4.3.2 provides a parametric analysis to investigate the effect changes in electrical and cooling capacities have on the overall tri-generation system performance. The aim of this is to understand the interaction between sub component operation and overall tri- generation system performance. A 24 hour tri-generation simulation is also

provided. Finally, section 4.3.3 presents a tri-generation system climatic

performance investigation. The aim of this is to evaluate the performance of the novel system under ‘real’ operating conditions to see how it performs in different geographical locations. Throughout the analysis, the SOFC tri-generation system performance is compared to a conventional separated system comprising grid electricity, gas fired boiler and vapour compression cooling system.

4.3.1 Specific tri-generation system case

This section presents a specific case based on defined selection criteria. As highlighted in section 4.2, the SOFC CHP system selection criteria are based on the technical objectives discussed in chapter 1 and realistic operational values. For tri- generation system optimisation it is essential that three criteria are met: (1) the

technical objectives, (2) realistic operating values, and (3) a thermal agreement between the SOFC CHP system and liquid desiccant regenerator. Table 4-4 provides the selected input values for the specific novel tri-generation system. The input values used for the SOFC CHP system are the same as those used in section 4.2.1.2 due to little or no variation in realistic operational values for the SOFC whilst maintaining the technical objectives of the thesis. As a result, it is the liquid desiccant system’s operation that has been optimised for successful tri-generation system integration. There are more variables in a liquid desiccant system that may be controlled in a realistic range. This conclusion is demonstrated during experimental tri-generation system integration presented in chapter 7. Unless listed in Table 4-4, the operating values used for the SOFC CHP and liquid desiccant system are those used previously and provided in Table 4-1 and Table 4-3 respectively.

Table 4-4 Selected input values for specific tri-generation system case

Parameter Value Parameter Value

SOFC current density (mA.cm-2_{) 600 Air volumetric flow (m}3_.h-1_{) 191}

SOFC temperature (°C) 800 Des vol. flow (L.min-1) 2.75

Number of cells in stack 53 Des mass concentration 0.65

Fuel utilisation factor (%) 80 WHR volumetric flow (L.min-1_{) 2.25}

The water volumetric flow in the SOFC CHP WHR circuit has been selected based on achieving the highest outlet desiccant solution temperature, as shown in Figure 4-10.

Figure 4-10 WHR circuit water volumetric flow optimisation

In the specific tri-generation system case a desiccant solution mass concentration

of 0.65 and solution volumetric flow of 2.75L.min-1 are used. These values have

been selected due to maximising the utilisation of thermal energy from the SOFC

1 1.5 2 2.5 3 3.5 4 4.5 5 40 45 50 55 60 65

WHR volumetric flow [L.min-1]

T e m p e ra tu re [° C ] Tw ,in Tsol,out

CHP system, and thus maximising the cooling output. As shown in Figure 4-10, at a

solution volumetric flow of 2.75L.min-1 and an assumed 45°C water return

temperature, the SOFC CHP WHR circuit heats the desiccant solution to a temperature of 44°C. As demonstrated in Figure 3-25 as desiccant solution mass concentration increases so does the required solution temperature to facilitate balanced regeneration. At a 44°C solution temperature, the maximum operating desiccant solution mass concentration is 0.65. The performance of the specific tri- generation system case is presented in Table 4-5.

Table 4-5 Performance data of specific tri-generation system case

Parameter Value Parameter Value

, ( ) 1526 , (° ) 44 ( ) 1116 ( ) 1108 ( ) 3345 (%) 45.63 ( ) 920 (%) 78.98 MRR (g. s ) 0.3168 (%) 70.07 Ta,out(°C) 28.2 0.83 RHa,out(%) 56.2 8.4

The results presented in Table 4-5 demonstrate that the novel system can reach 79% in co-generation heating (CHP) mode and 70% in tri-generation cooling mode.

Because the COPth of the desiccant system is less than one and the desiccant unit

has a small electrical load, CHP efficiency is higher than tri-generation. In tri- generation cooling mode, the selected (specific) system maximises the utilisation of thermal energy from the SOFC CHP system in the liquid desiccant system. A lower cooling capacity could have been selected, thus elevating tri-generation system efficiency; however the system would be more akin to a CHP operating scenario. The selected values fulfil two of the technical objectives of the thesis, namely a

1.5kWe system operating at an electrical efficiency of 45% or higher, and the

operating values for both the SOFC CHP and liquid desiccant air conditioning system are within a realistic range. Furthermore, the system produces a meaningful amount of cooling (almost 1kW). However, the overall system efficiency is lower

than the 85% target. This is primarily due to a liquid desiccant system COPth of less

than one (1.18 would be required). The inclusion of liquid desiccant air conditioning provides an efficiency increase of up to 24% compared to SOFC electrical operation only, demonstrating the potential of the novel tri-generation system in applications that require simultaneous electrical power, heating and dehumidification/cooling.

The energetic performance of the novel SOFC tri-generation system has been compared to a conventional separated system comprising: grid electricity, gas fired boiler and electrical driven vapour compression cooling, using an adapted version of the tri-generation primary energy savings energy methodology discussed in section 2.6 (Badami, Portoraro et al., 2012). The electrical efficiency of the conventional separated system has been assumed as 33%, a figure considering the efficiency of utility scale electrical generation plus transmission losses (Wu, Wang et al., 2014).

The thermal efficiency of the gas fired boiler has been assumed as 90%. The COPel

of the vapour compression cooling system is assumed constant at 2 (Welch, 2008). Thus, the overall efficiency of the conventional separated system can be calculated for any given electrical, heat and cooling output from the SOFC CHP / tri-generation system.

For the outputs listed in Table 4-5 the conventional separated system has an overall efficiency of 45.05% in CHP mode and 41.17% in tri-generation cooling mode. The relative performance, namely Primary Energy Demand (PED), cost and emission reductions generated by the novel tri-generation system compared to the equivalent conventional separated system are presented in Table 4-6.

Table 4-6 Percentage difference in PED, operating cost and emissions between the novel tri-generation system and an equivalent conventional separated system

Tri-generation system operating mode % PED % cost % emissions CHP -42.96 -55.25 -42.75 Tri-generation cooling -41.25 -56.41 -63.21

Electrical import cost and emission factor = 0.172£.kWh-1_{(Goot, 2013) and 0.555kgCO}

2.kWh-1

(AMEE, 2014) / Natural gas import cost and emission factor = 0.0421 £.kWh-1

and 0.184kg CO2.kWh-1(EST, 2014)

Compared to the conventional separated system, the novel tri-generation system demonstrates a percentage reduction in PED of 43–45%, an operating cost reduction of 55-59%, and an emission reduction of 43–65%. The encouraging PED, cost and emission reductions are primarily due to the high electrical efficiency of the SOFC and the replacement of electrically derived cooling with waste heat driven cooling. Because grid electricity has a high economic cost and emission factor, generating it at high electrical efficiencies from lower cost and emission factor natural gas produces great benefit compared to a conventional separated system. A comprehensive economic and environmental assessment is provided in chapter 8.

Wu, Wang et al. (2014) theoretically and experimentally evaluated a micro tri- generation system incorporating a 16kWe ICE with a vapour absorption chiller. Experimental results demonstrate a tri-generation system efficiency of 76.5% in CHP mode and 56% in cooling mode. Compared to a conventional separated system, a clear efficiency increase was gained in CHP mode, however only a marginal improvement was observed during cooling mode. This is because of a low

COPth of the vapour absorption chiller (~0.4). Compared to other theoretical

assessments of micro tri-generation systems employing either SOFC or liquid desiccant technology, the theoretical analysis of the novel SOFC liquid desiccant tri- generation system shows promising results. This is primarily due to the high

electrical efficiency of the SOFC (~45%) and reasonable COPth of the liquid

desiccant air conditioning system (~0.8).

Next, section 4.3.2 presents a tri-generation system parametric performance analysis.