# Design Point Performance and Optimization of Humid Air Turbine Power Plants
## 3 Results and Discussion
### Heat Exchanger Size and Cost Analysis  
The breakdown of the total weight and cost for the heat exchanger configurations are highlighted in Figure 9. Cycles \(A\), \(B\), and \(C\) (see Table 3) show the variation when the effectiveness of all heat exchangers vary together. On the other hand, cycles \(D\), \(E\), \(F\), and \(G\) show the effect of reducing the technology level of one heat exchanger at a time. The reference cycle (\(A_{5}^{40}\)) has been used to normalize the weight and cost of the components. The total reference weight of all heat exchangers is estimated up to 47.76 tones, whereas the reference cost estimate is 4.2 m USD. These estimates are broadly in accordance with the cost estimates previously presented by [PERSON] and [PERSON] [1] for a power plant of 55.1 MW (3.8 m USD updated to 2015). It is important to highlight that the thermal efficiency achieved by the reference cycle (61.4%) is 11.9 pp higher than the one reported by [PERSON] and [PERSON] in [1], which partially justifies the discrepancy in the heat exchanger acquisition cost estimate. For cycle \(A_{5}^{40}\), the recuperator is the heaviest component--33.12% of the total weight--followed by the humidifier, the economizer, the intercooler, and finally the aftercooler--with relative contributions to the overall power plant weight of 33.01%, 20.96%, 9.98%, 2.94%, respectively. Although the economizer and the humidifier are among the heaviest components, their contribution to the overall cost of the system was found to be approximately 3% (Figure 9). In terms of heat exchanger cost, the recuperator is the most expensive heat exchanger--48.11% of the total cost, followed by the intercooler, the aftercooler, the economizer, and finally the saturator--with relative contributions to the overall cycle cost of 34.95%, 13.43%, 1.99%, and 1.52%, respectively. Reducing the \(OPR\) from 40 to 30 increases the total weight across a range between 5% and 10% depending upon the cycle technology level (Figure 9a), and the total cost by 8% in all cases (Figure 9b). The observed increment in heat exchanger weight is primarily driven by the recuperator and the economizer due to the excess of available heat downstream the power turbine that needs to be managed by these two components whose size is further increased by the higher flow rate requirement of this low \(OPR\) cycle. As a result, the size increase of the recuperator primarily drives the increase in the estimated acquisition cost of the system. It is worth noting that when the \(OPR\) becomes 30 from 40, the humidifier is 7% less expensive, driven by the need for thinner shell walls. However, an increase in weight is observed as the required material for packing increases by 12% compared to the baseline case to facilitate the larger enthalpy flux. Finally, although the reduction of the \(OPR\) to 30 increases the weight and the cost of the components, it is associated with a reduction in the overall thermal efficiency of the system by 0.7 pp. When the \(\Delta T_{ps}\) becomes 15 K from 5 K, the size, and, consequently, the weight and cost of all the heat exchangers increases due to the demand of a higher mass flow to achieve the required power output. The saturator is the only component whose weight and cost reduce as it becomes shorter. Except for \(A_{15}^{40}\), which increases its total weight compared with \(A_{5}^{40}\), the reduction in the weight of the humidifier outbalances the increment in the weight of the heat exchangers, but not the increment in the cost. When the pinch point temperature is increased to 15 K, the total acquisition cost of the system marginally increases by around 3% while the thermal efficiency falls by approximately 0.45 pp. The comparison of cycles \(A_{5}^{40}\), \(B_{5}^{40}\), and \(C_{5}^{40}\) (Figure 9), where the effectiveness of all the heat exchangers is reduced at the same time, shows the exponential relationship between the technology level of the components, and their weight and cost.