ASME FEATURE The efficiency factor as a function of tip velocity ratio (to design tip velocity) is obtained from:[17]

Equation 3

The turbine efficiency was then calculated as product of the design point efficiency and the two efficiency coefficients and is used in the iterative cycle calculations:

rate and high pressure are therefore considered free variables and are optimized during simulation.

Results and discussion The produced power is shown as a function of produced heat for the different cases in Figure 2. As expected, the power output is in general higher for the Northern case compared to the Southern. This is due to the lower cooling water temperature and the higher exhaust mass flow. For the dual WHRU system, the power output is not affected by the heat production up to 5 MW, since this heat

Equation 4

Calculation procedure The design of the bottoming cycle model was performed in Aspen HYSYS [18] to enable simple modifications and tuning of the processes. The design case is based on 10 MW heat production. The design point parameters shown in Table 2 were used to define the HYSYS model. Then advanced geometry-based models were designed for individual components using the in-house code to match the HYSYS model. The advanced model will allow for realistic off-design calculations. Stationary solutions for the whole bottoming cycle are solved using a sequential quadratic programming (SQP) method (NLPQL).[19] More details about the calculation procedure can be found in reference [4]. For off-design simulation, a control strategy must be chosen for the bottoming cycle. The condensation pressure could, to some degree, be controlled by the flow of cooling water, but in these simulations it was decided to keep the cooling water flow constant for the DWHRU system. For the northern conditions, where the system operates in trans-critical mode, the condensation pressure will be controlled by the heat rejected in the condenser. For the IHRU system the cooling water flow rate is variable in order to obtain necessary control flexibility. For the southern conditions, when the system is operating fully in supercritical mode, pressures are controlled by mass repartition between the low- and high-pressure sides. The mass flow and pump outlet pressure is controlled by the turbine and pump operation. The VFD of the CO2 pump/compressor enables a high efficiency in a wide range of flow rates and pressure ratios. The VIGV allows the turbine to operate with constant pressure ratios across a broad flow range.[20] The mass flow APRIL 2015 | ASME Power Division Special Section

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# April 2015

Heat Exchangers – Retrofit/Rebuild/Equipment Upgrade – Bearings – Turbine Tech: Steam – ASME: Combined-Cycle Plants

# April 2015

Heat Exchangers – Retrofit/Rebuild/Equipment Upgrade – Bearings – Turbine Tech: Steam – ASME: Combined-Cycle Plants

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