The Solar Field page displays variables and options that describe the size and properties of the solar field, properties of the heat transfer fluid. It also displays reference design specifications of the solar field. See Input Variable Reference for a description of the solar field input variables.

Solar Advisor provides two options for specifying the size of the solar field: Option 1 specifies the field area as a multiple of the area required to drive the power cycle at its rated capacity under design conditions, and Option 2 specifies the field area as an explicit value in square meters. See Input Variable Reference for details.

You can specify the heat transfer fluid by choosing from a list of pre-defined fluids, or by creating your own fluid. See Specifying a Custom Heat Transfer Fluid for details.

Solar Advisor assumes that all collectors in the field use single-axis tracking with the collector tilt and azimuth defined by the collector orientation input variables. See the variable descriptions in Input Variable Reference for details.

The mirror washing variables determine the quantity of water required for mirror washing. See the variable descriptions in Input Variable Reference for details.

Solar Field Parameters

Option 1 and Option 2

For Option 1 (solar multiple mode), Solar Advisor calculates the total required aperture and number of loops based on the value you enter for the solar multiple. For option 2 (field aperture mode), Solar Advisor calculates the solar multiple based on the field aperture value you enter. Note that Solar Advisor does not use the value that appears dimmed for the inactive option.

Solar multiple

The field aperture expressed as a multiple of the aperture required to operate the power cycle at its design capacity.

Field aperture

The total solar energy collection area of the solar field in square meters. Note that this is less than the total mirror surface area.

Row spacing (m)

The centerline-to-centerline distance in meters between rows of collectors, assuming that rows are laid out uniformly throughout the solar field.

Stow angle (degrees)

The collector 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.

Deploy angle (degrees)

The collector angle during the hour of deployment. A deploy angle of zero for a northern latitude is vertical facing due east. Default is 10 degrees.

Solar Field

Describes how collectors and header pipes are laid out in the solar field. See Solar Field "H" and "I" Layout Options for details.

Header pipe roughness (m)

The header pipe roughness is a measure of the internal surface roughness of the header and runner piping. Solar Advisor uses this value in calculation of the shear force and piping pressure drop in the headers.

HTF pump efficiency

The electrical-to-mechanical energy conversion efficiency of the field heat transfer fluid pump. This value accounts for all mechanical, thermodynamic, and electrical efficiency losses.

Freeze protection temp (ºC)

The temperature at which freeze protection equipment is activated. The fluid temperature is maintained at this value during hours that freeze protection is operating.

Irradiation at design (W/m2)

The design point direct normal radiation value, used in solar multiple mode to calculate the aperture area required to drive the power cycle at its design capacity. Also used to calculate the design mass flow rate of the heat transfer fluid for header pipe sizing. See Choosing a Design Irradiation Value for details.

Allow partial defocusing

Partial defocusing assumes that the tracking control system can adjust the collector angle in response to the capacity of the power cycle (and thermal storage system, if applicable). See Defining Collector Defocusing for details.

Heat Transfer Fluid

Field HTF fluid

The heat transfer fluid (HTF) used in the heat collection elements and headers of the solar field. Solar Advisor includes the following options in the HTF library: Solar salt, Caloria, Hitec XL, Therminol VP-1, Hitec salt, Dowtherm Q, Dowtherm RP. You can also define your own HTF using the user-defined HTF fluid option

User-defined HTF fluid

To define your own HTF, choose User-defined for the Field HTF fluid and specify a table of material properties for use in the solar field. You must specify at least two data points for each property: temperature, specific heat, density, viscosity, and conductivity. See Specifying a Custom Heat Transfer Fluid for details.

Design loop inlet temp (ºC)

The temperature of HTF at the loop inlet under design conditions. The actual temperature during operation may differ from this value. Solar Advisor sets the power cycle's design outlet temperature equal to this value.

Design loop outlet temp (ºC)

The temperature of the HTF at the outlet of the loop under design conditions. During operation, the actual value may differ from this set point. This value represents the target temperature for control of the HTF flow through the solar field and will be maintained when possible.

Min single loop flow rate (kg/s)

The minimum allowable flow rate through a single loop in the field.

Max single loop flow rate (kg/s)

The maximum allowable flow rate through a single loop in the field.

Header design min flow velocity (m/s)

A calculated value that indicates the minimum flow velocity in the field  corresponding to the specified minimum single loop flow rate. This value is calculated using the density of the HTF at the design inlet temperature and the maximum specified receiver diameter.

Header design max flow velocity (m/s)

A calculated value that indicates the maximum flow velocity in the field corresponding to the specified maximum single loop flow rate. This value is calculated using the density of the HTF at the design outlet temperature and the minimum specified receiver diameter.

Initial field temperature (ºC)

Temperature of the HTF in the solar field in the first time step of the simulation (Hour one, typically the hour beginning at midnight on January 1). The value affects the system's performance in the first hours of the simulation, but typically has little impact on subsequent hours and total annual plant performance.

Design Point

Single loop aperture (m2)

The aperture area of a single loop of collectors, equal to the product of aperture width, reflective area, times the structure length times the number of collector assemblies per loop according to the distribution of the up to four collector types in the field. This area does not include non-reflective surface on the collector or non-reflective space between collectors.

Loop optical efficiency

The optical efficiency when incident radiation is normal to the aperture plane, not including end losses or cosine losses. This value does not include thermal losses from piping and the receivers.

Total loop conversion efficiency

The total conversion efficiency of the loop, including optical losses and estimated thermal losses. Used to calculate the required aperture area of the solar field.

Total required aperture, SM=1 (m2)

The exact mirror aperture area required to meet the design thermal output for a solar multiple of 1.0. Solar Advisor uses the required aperture to calculate the actual aperture. The actual aperture may be slightly more or less than the required aperture, depending on the collector dimensions you specify on the Collectors page.

Required number of loops, SM=1

The exact number of loops required to produce the total required aperture at a solar multiple of 1.0. This number may be a non-integer value, Solar Advisor rounds this value to the nearest integer to caculate the value of the actual number of loops variable.

Actual number of loops

The actual number of loops in the field, equal to the solar multiple times the required number of loops at a solar multiple of 1.0. The  required number of loops is rounded to the nearest integer to represent a realistic field layout.

Actual aperture (m2)

The actual aperture area based on the actual number of loops in the field, equal to the single loop aperture times the actual number of loops.

Actual solar multiple

For Option 1 (solar multiple mode), the calculated solar multiple based on the actual (rounded) number of loops in the field. For Option 2 (field aperture mode), the solar multiple value corresponding to the thermal output of the field based at design point: The actual aperture divided by the field thermal output.

Field thermal output (MWt)

The thermal energy delivered by the solar field under design conditions at the actual solar multiple.

Collector Orientation

Collector tilt (degrees)

The angle of all collectors in the field in degrees 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.  Solar Advisor assumes that the collectors are fixed at the tilt angle.

Collector azimuth  (degrees)

The azimuth angle of all collectors in the field, where zero degrees is pointing toward the equator, equivalent to a north-south axis. Solar Advisor assumes that the collectors are oriented 90 degrees east of the azimuth angle in the morning and track the daily movement of the sun from east to west.

Mirror Washing

Solar Advisor reports the water usage of the system in the results based on the mirror washing variables. The annual water usage is the product of the water usage per wash and 365 (days per year) divided by the washing frequency.

Water usage per wash

The volume of water in liters per square meter of solar field aperture area required for periodic mirror washing.

Washing frequency

The number of days between washing.

Single Loop Configuration

Number of SCA/HCE assemblies per loop

The number of individual solar collector assemblies (SCAs) in a single loop of the field. Computationally, this corresponds to the number of simulation nodes in the loop. See Specifying the Loop Configuration for details.

Edit SCAs

Click Edit SCAs to assign an SCA type number (1-4) to each of the collectors in the loop. Use your mouse to select collectors, and type a number on your keyboard to assign a type number to the selected collectors. Solar Advisor indicates the SCA type by coloring the rectangle representing the collector in the diagram, and displaying the type number after the word "SCA." See Specifying the Loop Configuration for details.

Edit HCEs

Click Edit HCEs to assign a receiver type number (1-4) to each of the collectors in the loop. Use your mouse to select collectors, and type a number on your keyboard to assign a type number. Solar Advisor indicates the HCE type by coloring the line representing the receiver, and displaying the type number after the word "HCE." See Specifying the Loop Configuration for details.

Edit Defocus Order

Click Edit Defocus Order to manually define the defocus order of the collectors in the field. Click an assembly to assign the defocus order. You should assign each collector a unique defocus order number. See Defining Collector Defocusing for details.

Reset Defocus

Click to reset the defocus order to the default values, starting at the hot end of the loop and proceeding sequentially toward the cold end of the loop. See Defining Collector Defocusing for details.

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

For Option 1, Solar Advisor calculates the field aperture area in square meters based on the solar multiple value you specify, along with the power cycle design gross output, irradiation at design, and the design heat and optical loss parameters. A solar multiple of one represents the field aperture area that, under design conditions, would deliver sufficient thermal energy to drive the power cycle at its design point, accounting for thermal and optical losses in the solar field. Option 1 allows you to specify the field aperture area as a function of the power cycle's capacity -- Solar Advisor calculates the field aperture area for you.

For Option 2, you specify the field aperture explicitly, and Solar Advisor calculates the equivalent solar multiple.

Option 1 (solar multiple) 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.

Option 1 (solar multiple) is best for analyses involving a known or fixed power cycle capacity. Option 2 (field aperture) is best for analyses involving a known or fixed field aperture, but requires that you adjust the power cycle capacity to match the solar field output.

The two solar field layout options describe the location and shape of header piping that delivers heat transfer fluid to the power block.

The "I" layout represents a field where two headers emerge from the power block in opposite directions, each directly feeding one half of the loops in the field:

The "H" layout represents a system where two runner pipes emerge from the power block in opposite directions and feed into two headers each, where each header feeds one quarter of the total loops in the field:

Solar Advisor uses the irradiation at design value to calculate the the relationship between the solar multiple and field aperture. 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.

If you are modeling a system outside of those two regions, or if you want to choose a more precise value for the irradiation at design value, you can use the maximum annual incident direct normal radiation in watts per square meter for the system at your location. Solar Advisor reports that value in the hourly results as Collector_DNI-x-CosTh. See the procedure below for instructions.

The irradiation at design value has a significant impact on the field aperture that Solar Advisor calculates based on the solar multiple. For example, if one system with an irradiation at design value of 950 W/m2 and a solar multiple of 2 requires a field aperture of 862,000 m2 to drive a 110 MWe power cycle, the same system with an irradiation at design value of 800 W/m2 would require a field aperture of 1,030,000 m2 to drive the same power cycle.

Two 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 an oversized solar field, which in turn will cause  the collectors to defocus excessively. On the other hand, using too high of a reference direct normal radiation value results in an undersized solar field that can rarely drive the power cycle at its design point.

To find the maximum annual incident direct normal radiation value:

You can use default values for the remaining inputs.

If the heat transfer fluid you want to use in the solar field is not included in the Field HTF Fluid list, you can define a custom heat transfer fluid using the User-defined option in the list. To define a custom fluid, you need to know the specific heat, density, viscosity, kinematic viscosity, conductivity, and enthalpy of the fluid for at least two temperatures.

Table 24. Heat transfer fluids on the Field HTF Fluid list.

To define a custom heat transfer fluid:

You can also import data from a text file of comma-separated values. Each row in the file should contain properties separated by commas, in the same the order that they appear in the Edit Material Properties window. Do not include a header row in the file.

Keep the following in mind when you define a custom heat transfer fluid:

The solar field consists of loops of collector-receiver assemblies. On the Solar Field page, you specify the characteristics of a single loop in the field.

When you configure a loop, you specify the following characteristics using the single loop configuration diagram:

Each rectangle in the diagram represents a collector-reciever assembly. Solar Advisor allows you to specify a single loop of up to 35 collector-receiver assemblies, and up to four different receiver and collector types.

Note. In the current version of Solar Advisor, it is not possible to specify more than one loop. If your field consists of different types of collectors and receivers, you must represent the proportion of different types in a single loop.

Assembly #1, at the cold end of the loop, appears at the top left corner of the diagram. Depending on the collector defocusing option you use, you may need to know each assembly's number to assign a collector defocusing order. See Defining Collector Defocusing for details.

The color of the rectangle and SCA number indicates the collector type of each assembly. Similarly, the color of the line representing the receiver and the HCE number indicates the receiver type. The "DF" number indicates the collector defocusing order:

The characteristics of each collector type are defined on the Collectors page, and of each receiver type on the Receivers page.

To specify the loop configuration:

During hours when the solar field delivers more thermal energy than the power cycle (and thermal storage system, if available) can accept, or when the mass flow rate is higher than the maximum single loop flow rate defined on the Solar Field page, Solar Advisor defocuses collectors in the solar field to reduce the solar field thermal output. Mathematically, the model  multiplies the radiation incident on the collector by a defocusing factor. In a physical system, the collector tracker would adjust the collector angle to reduce the amount of absorbed energy.

Solar Advisor provides three defocusing options:

To define collector defocusing option:

Option 1: Clear Allow partial defocusing.

Option 2: Check Allow partial defocusing, and choose Sequenced.

Option 3: Check Allow partial defocusing, and choose Simultaneous.

If you choose Option 1 or Option 2, you should define the defocus order as described in the next procedure. If you choose Option 3, Solar Advisor ignores the defocusing order displayed in the single loop diagram.

To define the defocus order:

Click Reset Defocus if you want the defocus order to start at the hot end of the loop and proceed sequentially to the cold end of the loop.

This section will describe equations for the calculated values on the Solar Field page. It is currently under development. For general descriptions of the variables, see Input Variable Reference.

Min field flow velocity

Max field flow velocity

Single loop aperture

Loop optical efficiency

Total loop conversion efficiency

Total required aperture, SM=1

Required number of loops, SM=1

Actual number of loops

Actual aperture

Actual solar multiple

Field thermal output