Power Cycle |

| To view the Power Cycle page, click Power Cycle on the main window's navigation menu. Note that for the physical trough input pages to be available, the technology option in the Technology and Market window must be Concentrating Solar Power - Physical Trough System.
| Overview
The power cycle model represents a power block that converts thermal energy delivered by the solar field and optional thermal energy system to electric energy using a conventional steam Rankine cycle power plant. The power cycle can use either an evaporative cooling system for wet cooling, or an air-cooled system for dry cooling. The power cycle may include a fossil-fired backup boiler that heats the heat transfer fluid before it enters the power cycle during times when there is insufficient solar energy to drive the power cycle at its design load. The power cycle model for the Solar Advisor physical trough model is the same as that used for the power tower model. For a detailed description of the power cycle model, see Chapter 4 of Wagner M, 2008. Simulation and Predictive Performance Modeling of Utility-Scale Central Receiver System Power Plants. Master of Science Thesis. University of Wisconsin-Madison. http://sel.me.wisc.edu/theses/wagner08.zip.
| Input Variable Reference
Plant Capacity Design gross output (MWe) The power cycle's design output, not accounting for parasitic losses. Solar Advisor uses this value to size system components, such as the solar field area when you use the solar multiple to specify the solar field size. Estimated gross to net conversion factor An estimate of the ratio of the electric energy delivered to the grid to the power cycle's gross output. Solar Advisor uses the factor to calculate the power cycle's nameplate capacity for capacity-related calculations, including the estimated total cost per net capacity value on the System Costs page, capacity-based incentives on the Payment Incentives page, and the capacity factor reported in the results. Estimated net output design (nameplate) (MWe) The power cycle's nameplate capacity, calculated as the product of the design gross output and estimated gross to net conversion factor. Power Block Design Point Rated cycle conversion efficiency The thermal to electric conversion efficiency of the power cycle under design conditions. Design inlet temperature (ºC) The heat transfer fluid temperature at the power cycle inlet under design conditions. Design outlet temperature (ºC) The heat transfer fluid temperature at the power cycle outlet under design conditions. Boiler operating pressure (bar) The steam pressure in the main Rankine cycle boiler at design, used to calculate the steam saturation temperature in the boiler, and thus the driving heat transfer temperature difference between the inlet heat transfer fluid and the steam in the boiler. Boiler LHV efficiency The back-up boiler's lower heating value efficiency, used to calculate the quantity of gas required by the back-up boiler. See Storage and Fossil Backup Dispatch Controls for details. Heat capacity of balance of plant (kWht/ºC-MWhe) A term to introduce additional thermal capacity into the solar field to account for thermal inertia effects not directly linked to the mass of heat transfer fluid in the solar field. The units for this value are thermal kilowatt-hours per megawatt of gross electric output capacity needed to raise the balance of plant temperature one degree Celsius. Steam cycle blowdown fraction The fraction of the steam mass flow rate in the power cycle that is extracted and replaced by fresh water. This fraction is multiplied by the steam mass flow rate in the power cycle for each hour of plant operation to determine the total required quantity of power cycle makeup water. The blowdown fraction accounts for water use related directly to replacement of the steam working fluid. The default value of 0.013 for the wet-cooled case represents makeup due to blowdown quench and steam cycle makeup during operation and startup. A value of 0.016 is appropriate for dry-cooled systems to account for additional wet-surface air cooling for critical Rankine cycle components. Plant Control Fraction of thermal power needed for standby The fraction of the power cycle's design thermal input required from storage to keep the power cycle in standby mode. This thermal energy is not converted into electric power. Solar Advisor does not calculate standby energy for systems with no storage. Power block startup time (hr) The time in hours that the system consumes energy at the startup fraction before it begins producing electricity. If the startup fraction is zero, the system will operate at the design capacity during the startup time. Fraction of thermal power needed for startup The fraction of the turbine's design thermal input energy required during startup. This thermal energy is not converted to electric power. Minimum required startup temp (ºC) The temperature at which heat transfer fluid circulation through the power cycle heat exchangers begins, typically near the power block design heat transfer fluid outlet temperature. Max turbine over design operation The maximum allowable power cycle output as a fraction of the electric nameplate capacity. Whenever storage is not available and the solar resource exceeds the irradiation at design value from the Solar Field page, some collectors in the solar field are defocused to limit the power block output to the maximum load. Min turbine operation The fraction of the nameplate electric capacity below which the power cycle does not generate electricity. Whenever the power block output is below the minimum load and thermal energy is available from the solar field, the field is defocused. For systems with storage, solar field energy is delivered to storage until storage is full before the field is defocused. Cooling System Condenser type Choose either an air-cooled condenser system (dry cooling) or evaporative cooling system (wet cooling). Ambient temp at design (ºC) The ambient temperature at which the power cycle operates at its design-point-rated cycle conversion efficiency. For the air-cooled condenser option, use a dry bulb ambient temperature value. For the evaporative condenser, use the wet bulb temperature. Ref. Condenser Water dT (ºC) For the evaporative type only. The temperature rise of the cooling water across the condenser under design conditions, used to calculate the cooling water mass flow rate at design, and the steam condensing temperature. Approach temperature (ºC) For the evaporative type only. The temperature difference between the circulating water at the condenser inlet and the wet bulb ambient temperature, used with the ref. condenser water dT value to determine the condenser saturation temperature and thus the turbine back pressure. ITD at design point (ºC) For the air-cooled type only. Initial temperature difference (ITD), difference between the temperature of steam at the turbine outlet (condenser inlet) and the ambient dry-bulb temperature. Condenser pressure ratio For the air-cooled type only. The pressure-drop ratio across the air-cooled condenser heat exchanger, used to calculate the pressure drop across the condenser and the corresponding parasitic power required to maintain the air flow rate.
| Modeling a Fossil-fired Backup Boiler
To model a system with a fossil-fired backup boiler, set the boiler LHV efficiency input variable value to an appropriate value, and define the boiler's operating schedule on the Thermal Storage page. See Storage and Fossil Backup Dispatch Controls for details.
| Equations for Calculated Values
This section will describe equations for the calculated values on the Power Cycle page. It is currently under development. For general descriptions of the variables, see Input Variable Reference. Estimated net output at design (nameplate) Design inlet temperature Design outlet temperature