Thermodynamic Performance

In order to investigate the thermodynamic performance of the SMBWR steam cycle, each component of the steam cycle is modelled in MATLAB and the loop model is designed following the steam cycle configuration displayed in Fig. 2.5. The assumptions used to provide consistent comparison of the steam cycle at various reactor operating pressures are constant turbine inlet temperature and constant turbine outlet pressure for both HP and LP turbines. By varying the steam dome pressure while keeping the turbine inlet temperature constant, the amount of external heat needed to reach the selected turbine inlet temperature will be varied. In addition, by having constant turbine outlet pressure, the turbine work will vary depending on the inlet condition, and, thus, the electric power produced and the cycle efficiency. The steam conditions assumed in this study are listed in Table 4.1. As mentioned in Chapter 2, these operating conditions are selected by considering two main criteria. The first consideration is to ensure saturated steam conditions at the LP turbine outlet so that no heat would be wasted. The second consideration is to ensure that both the HP turbine inlet stream and LP turbine inlet stream have the same enthalpy. Fig. 4.16 shows the T-s diagram (temperature versus entropy) of the selected 4 representative cases. The points displayed in Fig. 4.16 correspond to the state points specified in Fig. 2.5. Fig. 4.17 and Fig. 4.18 display the main effect of reactor operating pressure on the SMBWR steam cycle.

Table 4.1 Parameters Used for SMBWR Balance of Plant (BOP) Comparison

Parameter Value Unit

HP turbine inlet temperature 540 oC

LP turbine inlet temperature 500 oC

HP turbine outlet pressure 10 bar

70

Fig. 4.16 T-s diagram of the SMBWR steam cycle at selected system pressures.

71 Fig. 4.18 SMBWR steam cycle efficiency at various system pressures.

Fig. 4.17 shows that as the steam pressure in the reactor increases, the amount of external heat required to reach a given turbine inlet temperature is higher. This is expected as the enthalpy of steam is a function of both its pressure and temperature and the value is higher at higher pressure. The cycle efficiency, shown in Fig. 4.18, is defined as the ratio between the net of work (produced by both turbines less the work consumed by feedwater pumps) to the total amount of heat supplied to the cycle as stated in the following equation (Eq. (4-2)). As the steam dome pressure increases, the external heat required and the work produced by the turbines increase. The increased work from turbines is higher than the additional heat required by the system at the higher pressure resulting in higher cycle efficiency. It is shown in Fig. 4.18 that increasing steam dome pressure from 65 bar to 100 bar would result in an increase in thermal cycle efficiency by approximately Δη = 1.2%. It is also shown that regardless of the steam dome pressure, the thermal cycle efficiency of the SMBWR is reaching 40%, which is comparable to those of stand-alone gas turbines in general. It is obvious that the reason for the higher SMBWR cycle efficiency than the stand-alone LWR, which is normally 33 – 35%, is due to the introduction of an external superheater.

72 𝜂𝐶𝑦𝑐𝑙𝑒=

𝑊_{𝑇𝑢𝑟𝑏𝑖𝑛𝑒}−𝑊_{𝑃𝑢𝑚𝑝}

𝑄_{𝑁𝑢𝑐𝑙𝑒𝑎𝑟 𝑟𝑒𝑎𝑐𝑡𝑜𝑟}+𝑄_{𝑆𝑢𝑝𝑒𝑟ℎ𝑒𝑎𝑡𝑒𝑟} +𝑄_{𝑅𝑒ℎ𝑒𝑎𝑡𝑒𝑟}+𝑄_{𝐸𝑐𝑜𝑛𝑜𝑚𝑖𝑧𝑒𝑟} (4-2)

There are several other ways of defining efficiency that could be potentially interesting to examine. The first one is superheater efficiency, which is defined as ratio of the additional electric power produced by generator to external heat supplied to the superheaters, as shown in Eq. (4-3). By assuming that the external heat from the superheater and reheater comes directly from a gas burner, the superheater efficiency can be used to show how efficient the hybrid SMBWR is compared to the conventional gas fired power plant. The other efficiency value that is interesting to consider is the combined cycle efficiency, in which we assume that external heat for the superheaters is taken from the waste heat of a gas turbine system. The combined cycle efficiency is calculated using Eq. (4-4). In this method, the reference gas turbine used is the SIEMENS design SGT5-4000F, with specifications listed in Table 4.2 [51]. It is assumed that the gas turbine’s power is a function of the superheater’s coolant flow rate and will be proportional to the reference design. Fig. 4.19 displays the effect of reactor pressure on both superheater efficiency and combined cycle efficiency.

𝜂𝑆𝑢𝑝𝑒𝑟ℎ𝑒𝑎𝑡𝑒𝑟=

𝐴𝑑𝑑𝑖𝑡𝑖𝑜𝑛𝑎𝑙 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑃𝑜𝑤𝑒𝑟 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑏𝑦 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑜𝑟

𝑄𝑆𝑢𝑝𝑒𝑟ℎ𝑒𝑎𝑡𝑒𝑟 +𝑄𝑅𝑒ℎ𝑒𝑎𝑡𝑒𝑟+𝑄𝐸𝑐𝑜𝑛𝑜𝑚𝑖𝑧𝑒𝑟 (4-3)

𝜂_𝐶𝐶=𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑃𝑜𝑤𝑒𝑟𝐺𝑇 + 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑃𝑜𝑤𝑒𝑟𝑁𝑃

𝑄_𝐺𝑇 + 𝑄_{𝑁𝑢𝑐𝑙𝑒𝑎𝑟 𝑟𝑒𝑎𝑐𝑡𝑜𝑟} (4-4)

Table 4.2 The Specification of SGT5-4000F [51]

Parameter Value

Power output 329 MW

Gross efficiency 41.0%

Exhaust gas mass flow 724 kg/s

73 Fig. 4.19 SMBWR alternative efficiencies at various system pressures.

Fig. 4.19 shows that by using a conventional gas burner as the heat source for the superheater and reheater of the SMBWR steam cycle, the superheater efficiency is comparable to the stand-alone gas turbine, and it improves as the operating pressure increases. It also shows that, if waste heat from gas turbines is used instead of a direct gas fired boiler, the resulting combined cycle efficiency could improve by approximately Δη = 3% as the pressure increases from 65 bar to 100 bar. Therefore, it can be concluded that operating the SMBWR at higher reactor pressure would provide notable improvement to its thermodynamic performance.