The Heliostat Field page displays the variables that specify the position of the heliostats in the solar field along with the heliostat geometry and optical properties. Unlike parabolic trough and dish system designs, which can be based on modular designs of individual components, power tower system designs typically require optimization of the tower height, receiver geometry, and distribution of heliostats around the receiver as a complete system.

Page numbers relevant to this section from the Wagner (2008) and Kistler B (1986) references are:

You can define the heliostat field layout in two ways: If you have a field layout in mind, you can enter values by hand. Or, you can use Solar Advisor's optimization wizard to determine the optimal layout for you.

Heliostat Properties

The heliostat properties define the area of a single heliostat mirrored surface, shape of the heliostat, and the boundaries of the solar field area. Note that Solar Advisor assumes that each heliostat employs a two-axis drive system with a pivot at the center of the mirrored surface.

Heliostat Width (m)

The width of the heliostat surface in meters, including the mirrored surface, edge supports and any cutouts or slots.Sandia 44

Heliostat Height (m)

The height of the heliostat surface in meters, including the mirrored surface, edge supports and any cutouts or slots.

Ratio of Reflective Area to Profile

The fraction of the area defined by the heliostat width and height that actually reflects sunlight. This value determines the  ratio of reflective area on each heliostat to the total projected area of the heliostat on a plane normal to the heliostat surface. The ratio accounts for non-reflective area on the heliostat that may cause shading of neighboring heliostats.

Use Round Heliostats (D=W)

Check the box to use round heliostats in place of the standard rectangular shape. For round heliostats, the heliostat diameter is equal to the value of the Heliostat Width variable.

Heliostat Area (m2)

The area of the heliostat mirrored area. For rectangular heliostats, the area is the product of the heliostat width and height (or the product of the square of half the width and pi for round heliostats) and the ratio of reflective area to heliostat profile.

Mirror Reflectance and Soiling

The mirror reflectance input is the solar weighted specular reflectance.  The solar-weighted  specular reflectance is the fraction of incident solar radiation  reflected into a given solid angle about the specular reflection direction. The appropriate choice for the solid angle is that subtended by the receiver as viewed from the point on the mirror surface from which the ray is being reflected. For parabolic troughs, typical values for solar mirrors are 0.923 (4-mm glass), 0.945 (1-mm or laminated glass), 0..906 (silvered polymer),  0.836 (enhanced anodized aluminum), and 0.957 (silvered front surface).

Heliostat Availability

An adjustment factor that accounts for reduction in energy output due to downtime of some heliostats in the field for maintenance and repair. A value of 1 means that each heliostat in the field operates whenever sufficient solar energy is available. Solar Advisor multiplies the solar field output for each hour by the availability factor.

Image Error (radians)

A measure of the deviation of the actual heliostat image on the receiver from the expected or ideal image that helps determine the overall shape and distribution of the reflected solar flux on the receiver. This value specifies the total conical error distribution for each heliostat at one standard deviation in radians. Solar Advisor applies the value to each heliostat in the field regardless of its distance from the tower. The image error accounts for all error sources, including tracking imprecision, foundation motion, mirror waviness, panel alignment problems, atmospheric refraction and tower sway.

Heliostat Stow Deploy Angle (degrees)

Solar elevation angle below which the heliostat field will not operate.

Wind Stow Speed (m/s)

Wind velocity from the weather file at which the heliostats defocus and go into stowed position to protect them from possible wind damage.Wagner, 10 and 68. Mentions "ground level."

Circular Field Optimization Wizard

When the you are specifying the heliostat field using radial sections, Solar Advisor can find the optimal number of heliostats for each section automatically. See Optimization Wizard for more information.

Note. The optimization wizard will not work if you are specifying the solar field using x-y coordinates.

Field Parameters

Total Reflective Area (m2)

Total mirrored area of the heliostat field, equal to the heliostat reflective area multiplied by the number of heliostats. Solar Advisor uses the total field area to calculate the site improvements and heliostat costs on the Tower System Costs page.

Number of Heliostats

The total number of individual heliostats in the field. Solar Advisor displays the number of heliostats based either on the results of the optimization wizard, or based on the data in the heliostat layout file when the heliostat locations are loaded from a text file.

Radial Step Size for Layout (m)

The radial distance between centers of heliostat field zones. The zone centers are indicated by the symbol + in the zone layout sample diagram shown on the Heliostat Field page.

In the x-y coordinate mode, Solar Advisor disables the radial step size variable.

When you define the number of heliostats per zone by entering values in the field layout table by hand or by loading a file, the radial step size is the difference between the initial maximum distance from the tower and initial minimum distance from the tower divided by the number of radial zones.

When you use the optimization wizard to specify the field, Solar Advisor calculates the radial step size as a function of the initial minimum and maximum distances from the tower, which it in turn calculates as a function of the ratio of the optimized tower height to the minimum and maximum tower height specified on the Receiver/Tower Sizing tab of the optimization wizard.

Solar Field Layout Constraints

Max Heliostat Distance to Tower Height Ratio and Min Heliostat Distance to Tower Height Ratio

The maximum and minimum ratio of the distance from the heliostat furthest and closest from the tower to the tower height.

Max Distance to Tower and Min Distance from Tower (m)

The maximum and minimum allowable radial distances in meters between the center of the tower base and heliostats furthest from the tower. Under certain conditions, Solar Advisor uses this value to calculate the radial step size. (See radial step size variable description below.)

Tower Height (m)

The height of the tower in meters. Specify this value on the Tower and Receiver page.

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.

Land Area

Non-Solar Field Land Area

Land area required for components other than solar field components, such as the power cycle, storage, buildings, etc.

Solar Field Land Area Multiplier

The ratio of the total solar field land area to land occupied by heliostats.

Calculated Total Land Area

The total land area required for the system.

Solar Advisor allows the heliostat locations in the field to be specified either by a set of rectangular coordinates (x-y) or as a number of heliostats per radial section of the field (number of radial and azimuthal zones).

Radial Zones

To specify the field as a number of heliostats per radial zone enter the number of radial zones and azimuthal zones to divide the heliostat field into radial zones shown in the field diagram. You can then specify the field manually or automatically. To specify the field manually, either type values in the Number of Heliostats Per Zone table or import the data as a text file. To specify the field automatically, use the optimization wizard to specify a set of optimization parameters and allow Solar Advisor to optimize the heliostat field design and calculate the optimal number of heliostats per zone, receiver tower height, receiver height and diameter, and other variables.

The solar field is divided into evenly distributed sections of a circle called zones, as shown in the sample diagram on the Heliostat Field page. The rows of the table specify the radial position if each zone relative to the tower located at the center of the field. The zone closest to the tower is assigned the number one, with each successively farther zone incrementing by one. The columns specify the position of the zone's center in degrees east of due north, where zero is north, 90 degrees is east, 180 degrees is south, and 270 degrees is west. The number of heliostats per zone can be  a non-integer value because Solar Advisor converts the value to a mirror surface area for each zone that is equivalent to the total mirrored surface of all heliostats in the zone.

Rectangular (x-y) Coordinates

To specify the field as a set of rectangular coordinates, change the value of Azimuthal Zones to 2, and enter the number of heliostats for # of Heliostats. You can then either type the x-y coordinates of each heliostat in the field, or import a text file of x-y coordinates. Solar Advisor displays the location of each heliostat on the field diagram. It models the system based on the heliostat locations specified by the set of x-y locations, and based on the values you specify for the tower height, receiver height, receiver  diameter, and other input values. This approach is appropriate for predicting the output of a system with a known design. The optimization wizard does not work in the x-y coordinate mode.

Each row specifies the position of an individual heliostat relative to the tower. The first column in the table specifies the x-coordinate along the east-west axis of the field, with negative values indicating positions west of the tower, and positive values indicating positions east of the tower. The second column specifies the y-coordinate along the north-south axis, with positive values indicating positions north of the tower, and negative values indicating positions south of the tower. The tower is assumed to be at 0,0. Note that this convention also applies to systems in the southern hemisphere. In the x-y coordinate mode, Solar Advisor requires that the field be symmetric about the north-south axis.

Solar Advisor allows you to use text files to save and load field layout data when you specify the field layout by hand instead of relying on the optimization wizard to calculate the optimal layout.

For radial zone data, each row in the file represents a radial step (distance away from the center of the circle), and each column represents an azimuthal division (distance clockwise around the circle from the zero degree line pointing north), as shown on the sample layout diagram. The first row must contain data for the radial step closest to the center of the field, and subsequent rows should be in consecutive order away from the center. The first column of each row must contain data for the azimuthal  division containing the north line at zero degrees, and the second column the next division moving counterclockwise from the first column, and so on. Zones with no heliostats should be indicated by a zero. Each column in the file should be separated by a space, and each row by a new line. For example, a text file with the following contents would describe a field with three radial steps and four azimuthal divisions:

9.0 10.0 9.0 10.0

15.5 15.5 15.5 15.5

22.5 18.0 18.5 22.5

For rectangular coordinate data, each row represents an individual heliostat position in the field, with the x coordinate in the first column and the y coordinate in the second column. A positive x value is east, and a positive y value is north of the tower. Use negative values for positions west and south of the tower. The heliostat coordinates do not have to be in a particular order in the file. Each column in the file should be separated by a space, and each row by a new line. A file with the following contents would describe a solar field with three heliostats at (x = 0.0, y = 75.0), (x = 7.5, y = 70.0), and (x = 15.0, y = 65.0):

0.0 75.0

7.5 70.0

15.0 65.0