## 4 Discussion  
It is clear that the hot gas helium temperature at the inlet to the precooler has a significant effect on the heat sink exit temperatures. The results show that the IC has the highest heat sink temperatures at the exit and this direct significance is evident in the average figures of hot gas at the inlet of each cycle. The IC has the highest average of 214.4 \({}^{\circ}\)C for the hot gas; the SCR is 195 \({}^{\circ}\)C and the ICR is 138.7 \({}^{\circ}\)C. With reference to Figure 6, the IC curve for the temperature ratio indicates a constant hot gas precooler inlet temperature due to the decreasing nature. The reason for this notable difference in comparison to the SCR and ICR is due to the recuperator effect. The temperatures at the inlet to the hot gas LP section of the recuperator have similar trends to the observations noted for the IC, suggesting that the heat exchange that takes place in the recuperator reduces with increasing precooler outlet/cycle  
Figure 11: Comparison of Heat Sink Outlet Temperature for the Co-Current Precooler. The heat sink outlet temperature is normalised by the respective cycle/compressor inlet temperatures. Figure 10: Comparison of Heat Sink Outlet Temperature for the Counter-Current Precooler. The heat sink outlet temperature is normalised by the respective cycle/compressor inlet temperatures. inlet temperature (\(T_{1}\)). Furthermore, the intercooler in the ICR ensures that the temperature of the compressed coolant is low, thereby utilising more of the hot gas temperature exchange. Thus the intercooler in the IC enables more of the generated power to be absorbed by the compressor and the turbine as more mass flow is compressed, but the SCR depends wholly on the recuperator when the compressor work is low. The benefit of this to the SCR (Figure 1) is the improved cycle efficiency of 4% in comparison to the IC (Figure 3), but the SCR has a more complex arrangement and plant size due to the recuperator, whereby the IC has a simpler arrangement. The ICR (Figure 2) has the most complex arrangement and is the largest of the plant cycles because of the recuperator and intercooler. The cycle efficiency benefit over the SCR is \(\sim\)3% at optimum turbine cooling conditions and 6.6% when compared to the IC (see Figure 12 for efficiency comparisons), but the increased efficiency is inversely proportional to the reduced hot gas temperature. The opposite of this statement holds true for the IC. For the SCR and ICR, the increases in hot gas at the precooler inlet due to the recuperator are primarily a result of the compressor/cycle inlet temperature (\(T_{1}\)). For every 1 \({}^{\circ}\)C rise in compressor/cycle inlet temperature, the resulting increases in the hot gas are 1.8 \({}^{\circ}\)C for the SCR and 0.92 \({}^{\circ}\)C for the ICR. When the focus turns to the effect of the hot gas on the heat sink outlet temperatures, the co-current precooler design shows that the average temperature increases are modest. For every degree rise in \(T_{1}\), the ICR has the highest degree rise of 0.82 \({}^{\circ}\)C for the heat sink outlet temperature; the SCR has an average rise of 0.29 \({}^{\circ}\)C per degree rise at \(T_{1}\) and the IC has no rise in the heat sink outlet temperature. With regards to the counter-current design, the rise in heat sink outlet temperatures are pronounced at \(T_{1}\) temperatures that are \(<\)35 \({}^{\circ}\)C and generally show a significant downward trend as \(T_{1}\) exceeds 35 \({}^{\circ}\)C. When temperatures between 25 and 35 \({}^{\circ}\)C are considered, a 1 \({}^{\circ}\)C increase in \(T_{1}\) results in a 7 \({}^{\circ}\)C increase for the SCR, a 5.5 \({}^{\circ}\)C increase for the IC, and a 4.5 \({}^{\circ}\)C increase for the ICR. When the full range of analysed temperatures are considered (25 to 55 \({}^{\circ}\)C), the averages are 5 \({}^{\circ}\)C for the SCR, 3.7 \({}^{\circ}\)C for the ICR, and 3.6 \({}^{\circ}\)C for the IC.