**3. Discussion**

TriGenSCA is used to determine the minimised power, heating and cooling targets and optimise the sizing of utilities in the trigeneration system. The final iteration of TriGenSCA shows PWR required energy of 3,000 MW (72 GWh/d) to overcome a deficit of demand load. The VHPS from the steam generator needs to be transferred to the lower temperature of utilities. Reference [46] has stated that 0.45 t of Uranium-235 can create 3,000 MWh/day of thermal energy. This means that PWR as a trigeneration system with integration requires 10.8 t of Uranium-235 as a fuel. Figures 8–10 show that the final network of three different systems in the same demand load. Analysis has been made by comparing between conventional PWR producing power, heating and cooling in a separate system, PWR as a trigeneration system without integration and PWR as a trigeneration system with integration.

The highest value of the power demand is 245 MW. The power that needs to be generated in a conventional PWR and PWR as a trigeneration system without integration is the same as the highest value of the power demand because any HPS and LPS surpluses cannot be cascaded to produce additional power in a conventional PWR and PWR as a trigeneration system without integration.

As a result, PWR as a trigeneration system without integration and conventional PWR producing power, heating and cooling in a separate system dump an excess power of 1,656 MWh/day. On the other hand, PWR as a trigeneration system with integration can only produce excess HPS, LPS and HW of 0.12 MWh, 3.13 MWh and 1.35 MWh. The power deficit of 1.07 MWh can be obtained by converting 3.13 MWh of HPS and 0.12 MWh of LPS using a condensing turbine with an efficiency of 33% [40]. The remaining power deficit of 1.86 MWh can be bought from the local grid. The excess 1.35 MWh of HW, on the other hand, is sent back to the steam generator through the deaerator. Production of power, heating and cooling in a separate PWR system needed more Uranium-235 as a fuel for energy source as compared with PWR as trigeneration with and without integration.

**Figure 7.** Graphical TriGenSCA (**a**) before, and (**b**) after iterations.

The amount of Uranium-235 required as a fuel in the conventional PWR system is 12.9 t whereas PWR as a trigeneration system without integration requires 10.95 t of Uranium-234 as a fuel. The total energy required for a conventional PWR system to produce the same energy as the PWR with trigeneration system is 3,598 MW or 86,352 MWh/day. PWR as a trigeneration system without integration requires a total energy of 3,040 MW that translates into approximately 73,000 MWh/day. Energy losses on the whole system are shown in Equation (12). Based on Equation (12), the energy losses per day for a conventional PWR producing power, heating and cooling is the highest value which is 64,000 MWh, followed by PWR as a trigeneration system without integration (53,000 MWh) and PWR as a trigeneration system with integration (50,000 MWh):

$$E\_{loss} = (TE - E\_{useful}) \times 24 \text{ hours} + E\_{\text{excess}} \tag{12}$$

where *Eloss* = Energy losses MWh/day; *TE* = Total energy required in MW; *Euseful* = Useful energy produced by trigeneration system in MW; *Eexcess* = Excess energy MWh/day.

**Figure 9.** Final network for a PWR as a trigeneration system without integration.

**Figure 10.** Final network for a PWR as a trigeneration system.

In terms of cost, the equivalent annual cost is calculated as the annual cost of owning, operating and maintaining an asset over its entire life. The equivalent annual cost can evaluate the cost of each system as well as optimise the systems for the lowest life-cycle cost. Equation (13) shows the equivalent annual cost. Operational and maintenance costs for fuel and non-fuel of PWR are 0.49 USD/kWh and 1.37 USD/kWh [47]. The initial investment for PWR is assumed to be 770 USD/kW, and life-cycle of PWR is 30 y [48]. The rate of return is assumed to be 10%. For PWR as a trigeneration system with integration, the initial investment for power and HW storages are taken into consideration where storage for HW is assumed to be 70 USD/kWh by using thermo-chemical storage and power is assumed to be 100 USD/kWh by using lead-acid battery [49]. Based on Table A18, maximum storage systems needed for power, HPS, LPS and HW are 180.59 MWh, 0.12 MWh, 3.13 MWh and 1.41 MWh. Cost for buying power from the local grid due to insufficient power in the trigeneration system with integration is also considered in an operational cost where the price of power is assumed to be 0.1 USD/kWh [50]. Based on the calculation using Equation (13), conventional PWR has the highest value of equivalent annual cost which is MUSD 364/year followed by PWR as a trigeneration system without integration (MUSD 307/year) and PWR as a trigeneration system with integration (MUSD 262/year):

$$EAC = IC\_{initial} \times \frac{i(1+i)^n}{(1+i)^{n-1}} + OM \times 365 \text{ days} \tag{13}$$

where *EAC* = Equivalent annual cost in USD/year; *i* = the rate of return; *n* = life-cycle of PWR in year; *ICinitial* = Initial investment cost in USD; *OM* = Operation and maintenance costs in USD.

Table 10 summarises the three different systems at the same demand load. Based on the table, PWR as a trigeneration system with integration is the best choice in terms of cost and also energy as compared with PWR as a trigeneration system without integration and conventional PWR producing separate power, heating and cooling. PWR as a trigeneration system with integration can create a savings of 28% for the equivalent annual cost and 17% of energy production as compared with conventional PWR producing power, heating and cooling. For energy loss, trigeneration PWR with integration can save up to 22% whereas trigeneration PWR without integration can save up to 17%. PWR as a trigeneration system without integration can only create a saving of 16% for equivalent annual cost and 16% for energy production when a comparison is made with a conventional PWR system. Moreover, trigeneration PWR with integration only required external power of 1.86 MWh as compared with trigeneration PWR without integration and conventional PWR which produce an excess power of 1,656 MWh. Excess power in trigeneration PWR without integration and conventional must be dumped as it cannot be used to serve a load [51]. This will create a waste if the power is in excess. As compared with trigeneration PWR with integration, deficit power can be bought from the power grid. Trigeneration PWR with integration is the best choice in term of cost and energy saving as compared with trigeneration PWR without integration and conventional PWR.

**Table 10.** Comparison between conventional PWR, PWR as a trigeneration system with and without integration.

