The Solar Field page displays variables and options that describe the size and properties of the solar field, properties of the heat transfer fluid, reference design specifications of the solar field, and collector orientation.

For a more detailed description of the model, please download the CSP trough reference manual from the Solar Advisor website's support page: https://www.nrel.gov/analysis/sam/support.html.

Field Layout

Option 1: Solar Multiple and Option 2: Solar Field Area

For option 1, (solar multiple mode), SAM calculates the solar field area and displays it in Solar Field Area (calc). For option 2 (solar field area mode), SAM calculates the solar multiple and displays it in Solar Multiple (calc). Note that SAM does not use the value that appears dimmed for the inactive option.

Distance Between SCAs in Row (m)

The end-to-end distance in meters between SCAs (solar collection elements, or collectors) in a single row, assuming that SCAs are laid out uniformly in all rows of the solar field. SAM uses this value to calculate the end loss. This value is not part of the SCA library on the SCA / HCE page, and should be verified manually to ensure that it is appropriate for the SCA type that appears on the SCA / HCE page.

Row spacing, center-to-center (m)

The centerline-to-centerline distance in meters between rows of SCAs, assuming that rows are laid out uniformly throughout the solar field. SAM uses this value to calculate the row-to-row shadowing loss factor. This value is not part of the SCA library, and should be verified manually to ensure that it is appropriate for the SCA type that appears on the SCA / HCE page.

Number of SCAs per Row

The number of SCAs in each row, assuming that each row in the solar field has the same number of SCAs. SAM uses this value in the SCA end loss calculation.

Deploy Angle (degrees)

The SCA angle during the hour of deployment. A deploy angle of zero for a northern latitude is vertical facing due east. SAM uses this value along with sun angle values to determine whether the current hour of simulation is the hour of deployment, which is the hour before the first hour of operation in the morning. SAM assumes that this angle applies to all SCAs in the solar field.

Stow Angle (degrees)

The SCA angle during the hour of stow. A stow angle of zero for a northern latitude is vertical facing east, and 180 degrees is vertical facing west. SAM uses this value along with the sun angle values to determine whether the current hour of simulation is the hour of stow, which is the hour after the final hour of operation in the evening.

Heat Transfer Fluid

Solar Field HTF Type

Name of the heat transfer fluid type. The Minimum HTF Temp value depends on the HTF type. The available fluid types are limited to those described in the HTF Properties section.

Property table for user-defined HTF

When the Solar Field HTF type is "User-defined," click Edit to enter properties of a custom HTF.

Solar Field Inlet Temp (ºC)

Design temperature of the solar field inlet in degrees Celsius used to calculate design solar field average temperature, and design HTF enthalpy at the solar field inlet. SAM also limits the solar field inlet temperature to this value during operation and solar field warm up, and uses this value to calculate the actual inlet temperature when the solar field energy is insufficient for warm-up.

Solar Field Outlet Temp (ºC)

Design temperature of the solar field outlet in degrees Celsius, used to calculate design solar field average temperature. It is also used to calculate the design HTF enthalpy at the solar field  outlet, which SAM uses to determine whether solar field is operating or warming up. SAM also uses this value to calculate the actual inlet temperature when the solar field energy is insufficient for warm-up.

Solar Field Initial Temp (ºC)

Initial solar field inlet temperature. The solar field inlet temperature is set to this value for hour one of the simulation.

Piping Heat Losses @ Design Temp (W/m2)

Solar field piping heat loss in Watts per square meter of solar field area calculated based on design variables. Used in solar field heat loss calculation.

Piping Heat Loss Coeff (1-3)

These three values are used with the solar field piping heat loss at design temperature to calculate solar field piping heat loss.

Solar Field Piping Heat Losses (W/m2)

Design solar field piping heat losses. This value is used only in the solar field size equations. This design value different from the hourly solar field pipe heat losses calculated during simulation.

Minimum HTF Temp (ºC)

Minimum heat transfer fluid temperature in degrees Celsius. SAM  automatically populates the value based on the properties of the solar field HTF type, i.e., changing the HTF type changes the minimum HTF temperature. The value determines when freeze protection energy is required, is used to calculate HTF enthalpies for the freeze protection energy calculation, and is the lower limit of the average solar field temperature.

HTF Gallons Per Area (gal/m2)

Volume at 25°C of HTF per square meter of solar field area, used to calculate the total mass of HTF in the solar field, which is used to calculate solar field temperatures and energies during hourly simulations. The volume includes fluid in the entire system including the power block and storage system if applicable. Example values are: SEGS VI: 115,000 gal VP-1 for a 188,000 m2 solar field is 0.612 gal/m2, SEGS VIII 340,500 gal VP-1 and 464,340 m2 solar field is 0.733 ga/m2.

Solar Multiple (Design Point)

Note. The ambient temperature, direct normal radiation, and wind velocity reference variables differ from the hourly weather data that Solar Advisor uses for system output calculations. Solar Advisor uses the reference ambient condition variables to size the solar field. Hourly data from the weather file shown on the Climate page determine the solar resource at the site.

Solar Multiple (calc)

The solar field area expressed as a multiple of the exact area (see "Exact Area" below). SAM uses the calculated solar multiple value to calculate the design solar field thermal energy and the maximum thermal energy storage charge rate.

Solar Field Area (calc) (m2)

The solar field area expressed in square meters. SAM uses this value in the delivered thermal energy calculations. The solar field area is the total collection aperture area, which is less than the mirror area. The solar field area does not include space between collectors or the land required by the power block.

Ambient Temp (ºC)

Reference ambient temperature in degrees Celsius. Used to calculate the design solar field pipe heat losses.

Direct Normal Radiation (W/m2)

Reference direct normal radiation in Watts per square meter. Used to calculate the solar field area that would be required at this insolation level to generate enough thermal energy to drive the power block at the design turbine thermal input level. SAM also uses this value to calculate the design HCE heat losses displayed on the SCA / HCE page. The appropriate value depends on the system location. For example, 950 W/m2 is an appropriate value for the Mohave Desert and typical locations under consideration for development in the U.S., and 800 W/m2 is appropriate for southern Spain. See below for more information.

Wind Velocity (m/s)

Reference wind velocity in meters per second. SAM uses this value to calculate the design HCE heat losses displayed on the SCA / HCE page.

Exact Area (m2)

The solar field area required to deliver sufficient solar energy to drive the power block at the design turbine gross output level under reference weather conditions. It is equivalent to a solar multiple of one, and used to calculate the solar field area when the Layout mode is Solar Multiple.

Exact Num. SCAs

The exact area divided by the SCA aperture area. SAM uses the nearest integer greater than or equal to this value in the solar field size equations to calculate value of the Solar Field Area (calc) variable described above. The exact number of SCAs represents the number of SCAs in a solar field for a solar multiple of one.

Aperture Area per SCA (m2)

SCA aperture area variable from the SCA / HCE page. SAM uses this value in the solar field size equations to calculate the value of the Solar Field Area (calc) variable described above.

HCE Thermal Losses (W/m2)

Design HCE thermal losses based on the heat loss parameters from the SCA / HCE page. SAM uses this value only in the solar field size equations. This design value is different from the hourly HCE thermal losses calculated during simulation.

Optical Efficiency

Weighted optical efficiency variable from the SCA / HCE page. SAM uses this design value only in the solar field size equations. This design value is different from SCA efficiency factor calculated during simulations.

Design Turbine Thermal Input (MWt)

Design turbine thermal input variable from the Power Block page. Used to calculate the exact area described above.

Orientation

Collector Tilt (degrees)

The collector angle from horizontal, where zero degrees is horizontal. A positive value tilts up the end of the array closest to the equator (the array's south end in the northern hemisphere), a negative value tilts down the southern end. Used to calculate the solar incidence angle and SCA tracking angle. SAM assumes that the SCAs are fixed at the tilt angle.

Collector Azimuth (degrees)

The azimuth angle of the collector, where zero degrees is pointing toward the equator, equivalent to a north-south axis. Used to calculate the solar incidence angle and the SCA tracking angle. SAM calculates the SCAs' tracking angle for each hour, assuming that the SCAs are oriented 90 degrees east of the azimuth angle in the morning and track the daily movement of the sun from east to west.

Solar Advisor provides two options for defining the size of the solar field: Solar Multiple (Option 1) and Solar Field Area (Option 2).

In Solar Multiple mode, Solar Advisor calculates the solar field area based on the solar multiple, the power block's rated thermal input capacity, reference weather conditions, and design heat loss parameters. For a solar multiple of one, Solar Advisor calculates the solar field area that, under reference weather conditions and accounting for heat losses from the field, generates a thermal energy amount equal to the design turbine thermal input value from the Power Block page.

In Solar Field Area mode, SAM uses the user-defined solar field area, and calculates the equivalent solar multiple.

The solar multiple mode is useful for determining the optimal solar field area for a given location. By varying the solar multiple, you can find the value that minimizes the levelized cost of energy for a given power block capacity. The levelized cost of energy metric captures the tradeoff between the benefit of higher annual electricity output and the cost of increased capital expenditures associated with increasing the solar field area.

Using the Solar Multiple mode is best for analyses involving a known or fixed power block capacity because Solar Advisor automatically calculates the solar field area based on the power block capacity. The Solar Field Area mode is best for analyses involving a known or fixed solar field area, but requires that the power block capacity be manually adjusted to match the solar field output.

The third case in the Sample Parabolic Trough System template "Phys Trough - Parameterized Storage," illustrates this approach using the physical trough model. The case compares the levelized cost of energy for systems with different solar multiple values with and without storage. You can use the same approach with the empirical model. For a description of the case, see Solar Multiple Optimization.

The three reference condition variables, ambient temperature, direct normal radiation, and wind velocity, are the ambient conditions at which the solar field thermal output is equal to the power block's design thermal input multiplied by the solar multiple. In other words, under reference conditions, the system operates at the system's design capacity. Note that these reference condition variables are system design parameters, and do not describe the weather conditions at the project site. Weather conditions are determined by the data in the weather file shown on the Climate page.

The reference ambient temperature and reference wind velocity variables are used to calculate the design heat losses, and do not have a significant effect on the solar field sizing calculations. Reasonable values for those two variables are the average annual measured ambient temperature and wind velocity at the project location.

The reference direct normal radiation value, on the other hand, does have a significant impact on the solar field size calculations. For example, a system with reference conditions of 25°C, 950 W/m2, and 5 m/s (ambient temperature, direct normal radiation, and wind speed, respectively), a solar multiple of 2,  and a 100 MWe power block, requires a solar field area of 871,940 m2. The same system with reference direct normal radiation of 800 W/m2 requires a solar field area of 1,055,350 m2. Note that with a solar multiple of 2, both systems would produce two times the thermal energy required to drive the power block at its rated capacity during hours in which the direct normal radiation, temperature, and wind speed from the weather file are equal to the reference conditions.

For systems in the Mohave Desert of the United States, a value of 950 W/m2 is reasonable, and for southern Spain, a value of 800 W/m2 is reasonable.

Four factors affect the choice of a reference direct normal radiation value for a given system:

Using too low of a reference direct normal radiation value results in excessive dumped energy: The actual direct normal radiation from the weather data is frequently greater than the reference value so that the solar field sized for the low reference radiation value often produces more energy than required by the power block, and excess thermal energy is either dumped or put into storage. On the other hand, using too high of a reference direct normal radiation value results in an undersized solar field that produces sufficient thermal energy to drive the power block at its design point only during the few hours when the actual direct normal radiation is at or greater than the reference value.

Method 1 for Choosing the Reference Direct Normal Radiation Value

The first approach to choosing a value for the reference direct normal radiation value is to set the value to the maximum value of the incident direct normal radiation (Q_nipCosTh) reported in the hourly results.

To display the cumulative distribution function for the direct normal radiation data:

You can either read the maximum value off of the graph as the right-most value on the x-axis, or right-click the graph to export values to a text file and read the maximum value there.

You can also find the maximum Q_nipCosTh in the simulation output file (.out) as described in Viewing Hourly Output Data. Use this option if you are running Solar Advisor on a Mac.

Method 2  for Choosing the Reference Direct Normal Radiation Value

Another approach to determine the reference direct normal radiation value for a given location is to find the value that minimizes the amount of thermal energy that the system dumps.

To minimize dumped thermal energy:

To determine the reference solar radiation value based on dumped thermal energy:

The solar field heat transfer fluid (HTF) absorbs heat as it circulates through the heat collection elements in the solar field and transports the heat to the power block where it is used to run a turbine. Several types of heat transfer fluid are used for trough systems, including hydrocarbon (mineral) oils, synthetic oils, silicone oils and nitrate salts.

When you choose a heat transfer fluid, Solar Advisor populates the minimum HTF temperature variable with that oil's minimum operating temperature value. Solar Advisor will not allow the system to operate at a temperature below the minimum HTF temperature. Electric heaters in the system maintain the fluid temperature. Solar Advisor accounts for the electric power requirement for heating on the Parasitics page.

The remaining heat transfer fluid parameters describe characteristics of the solar field that affect the performance of the heat transfer fluid. The two area-related parameters refer to square meters of solar field area. If you are unsure of what values to use for these parameters, refer to the Solar Field page for the case in Sample Parabolic Trough Systems.zsam.

Note. Solar field outlet temperature and solar field area data for U.S. parabolic trough power plants are available on the Troughnet website at http://www.nrel.gov/csp/troughnet/power_plant_data.html.

Table 15. Heat transfer fluids.

Calcualted values appear on the Solar Field page in blue type with blue backgrounds.

Solar Multiple and Solar Field Area

When the Layout option is Solar Multiple (Option 1), Solar Advisor calculates the solar field area based on the value you enter for the solar multiple:

When the Layout option is Solar Field Area (Option 2), Solar Advisor calculates the solar multiple based on the value you enter for the solar field area:

Where,

Exact Area and Exact Number of SCAs

The exact area is the solar field area for a solar multiple of one calculated as follows:

The values used for these equations are displayed under Solar Multiple Reference Conditions and Values From Other Pages, except for the five FET factors, which are on the Power Block page.

Where,

Note. Direct Normal Radiation does not represent weather conditions at the site, but is the reference radiation value used to calculate the solar field area when the solar multiple is one.

Where,

Solar Field Piping Heat Losses

The solar field piping heat losses are calculated using parameters of the heat transfer fluid and the reference ambient temperature:

Where,