NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Final Technical Progress 

Report: Development of 

Low-Cost Suspension Heliostat

December 7, 2011 — December 6, 2012

William Bender

Solaflect Energy

Norwich, Vermont

NREL Technical Monitors: Harin Ullal, Kaitlyn VanSant, and 

Allison Gray

Subcontract Report

NREL/SR-5200-57611

January 2013
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Final Technical Progress 

Report: Development of 

Low-Cost Suspension Heliostat

December 7, 2011 — December 6, 2012

William Bender

Solaflect Energy

Norwich, Vermont

NREL Technical Monitors: Harin Ullal, Kaitlyn VanSant, 

and Allison Gray

Prepared under Subcontract No. NEU-2-11979-02

Subcontract Report

NREL/SR-5200-57611

January 2013
This publication was reproduced from the best available copy

submitted by the subcontractor and received no editorial review at NREL.

NOTICE

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1

Background

SunShot Incubator Program

The SunShot Incubator project is a significant effort of the U.S. Department of Energy (DOE) to 

partner with U.S. industry to accelerate the commercialization of solar energy systems that can 

meet aggressive cost and capacity goals by the end of the decade. Specifically, the SunShot 

Incubator projects fund technologies with a disruptive potential to reach the DOE SunShot 

Initiative goal of an unsubsidized cost-competitive levelized cost of energy by the end of the 

decade. This is estimated to be approximately 6 cents/kW-hr at utility scale. 

Concentrated Solar Power

Concentrated solar power (CSP) uses a large field of mirrors (heliostats) that continuously adjust 

their position so that they reflect the sun to a receiver mounted on a central tower. This 

concentrated energy can be used to make steam or heat molten salt, which can then be stored or 

used to generate electricity. Heliostats constitute between one-fourth and one-half of the total 

cost of a CSP system. Many of the remaining components of a CSP system comprise mature 

components, such as steam turbines and electric generators. These are unlikely to be reduced in 

cost, so reducing the cost of the 

heliostat field is one of the most 

viable paths to reducing the 

overall cost of a CSP plant. 

The SunShot goals for CSP solar 

field costs are on the order of $100 

per square meter or less1, and ideally 

would cost as little as $70 per square 

meter2. However, the base case 

reference heliostat used by Sandia 

National Laboratories uses 59 

pounds of fabricated steel per square 

meter, estimated to cost $1.00 per 

pound or more in large volumes3. It 

seems unlikely that the SunShot 

goals can be met using the base case 

style of heliostat, as it is optimistic to 

think that all other components of 

the heliostat, including mirrors, 

motors, electronics, and foundation, 

would cost only $11 to $41 per square meter ($70 - $59 = $11; $100 - $59 = $41).

 1 National Renewable Energy Laboratory (2011). Solicitation for Letters of Interest (LOI) No. REU-1-11979, pg. 35. 2 Tex Wilkins (2011), at DOE/CSP Industry Meeting. Washington, DC: March 8-9, 2011. Personal communication. 3 Kolb, Gregory J., Scott A. Jones, Matthew W. Donnelly, David Gorman, Robert Thomas, Roger Davenport, and 

Ron Lumia, (2007). Albuquerque, N.M.: Sandia National Laboratories, SAND2007-3293. Pages 47-49. MarmonKeystone (2011): personal communication. (www.marmonkeystone.com).

All photos and figures in this document are property of Solaflect Energy.

Figure 1

Two Solaflect Energy Baseline Heliostats in Stow Mode

60% to 65% Less Steel per Square Meter at Baseline
2

Solaflect Energy’s Suspension Heliostat

Solaflect Energy has developed a fundamentally new and innovative approach to heliostat 

design. It provides a quantum leap in material efficiency with comparable performance. The 

traditional heliostat is built like a truss bridge, with a large steel truss structure built behind the 

mirrors to hold them in place (Figure 2). In 

contrast, the Solaflect heliostat is built like a 

suspension bridge, stabilizing the mirror 

structure with steel cables from the front 

and rear (Figure 1). As with a suspension 

bridge, this design is very material efficient 

yet structurally very effective. This design, 

which is called a Suspension Heliostat, is 

differentiated from the traditional design by 

utilizing steel primarily in tension and 

compression, where it is strongest, in 

comparison to a traditional design, which 

uses a steel support structure that must resist

bending. This disruptive technology has been 

demonstrated in a 16 m2 heliostat to use 60%

to over 80% less steel per square meter than 

the traditional design, allowing the SunShot 

cost goals to be within reach for heliostats.

Project Goals

There were several principal goals for this 

project, including:

1. Reduce heliostat costs by a total of at least $25 per square meter, when manufactured in 

volume;

2. Rigorously test and validate the performance of the heliostat; and

3. Create additional high-quality information that can be used to further reduce the cost and 

improve the performance of the heliostat in the future.

These overarching goals were achieved through five principal research and development (R&D) 

objectives in the project. These five objectives included:

1. Redesign the heliostat elevation assembly to reduce component cost and improve 

manufacturability;

2. Optically test the heliostat in different wind environments to determine the contribution 

of different heliostat components to overall optical errors;

3. Redesign the heliostat controller to significantly reduce costs and improve 

manufacturability;

4. Use a variety of accelerated testing methodologies to evaluate reliability and identify 

components to be redesigned; and

5. Test the cable drive system and create new design options.

Figure 2

Traditional “Truss” Heliostats

Too Much Steel to be Grid Competitive
3

Baseline Heliostat

Solaflect Energy is a small R&D-based start-up developing the Suspension Heliostat design. 

Two patents have been issued on this technology, and more are pending. Numerous hurdles in 

making this design operational have been overcome, and the concept is successfully proven. The 

first generation design used 28.5% less steel per square meter than the base case heliostat, and 

the second generation saves more than 65% of the steel per square meter.

The initial size of the Solaflect Energy heliostat is a 16 m2 heliostat. This is composed of a fourby-four matrix of facets, each of which is 1 meter by 1 meter in size. The innovation of this 

heliostat is that there is a compression rod that is perpendicular to the mirrors and runs through 

the center of the mirror array. A side view of the heliostat in stow in Figure 1 shows that this 

compression element is very slender in front of the mirrors, and is structural behind the mirrors. 

Sets of steel cables run from the front and rear of this compression rod to the corners of each 

facet.

The bi-directional shading from the components in front of the mirror is very small, on the order 

of 1%–2%. In addition, the slight curvature of the heliostat can be seen in Figure 1. By adjusting 

the position of each set of cables, the individual facets are symmetrically canted to the exact slant 

range for each heliostat. This process is much faster than the manual canting method on each 

facet traditionally done upon installation. It can be seen that there is no steel superstructure 

behind the mirrors, in comparison to the traditional heliostat designs shown in Figure 2.

Redesign Heliostat Elevation Assembly

The elevation assembly (Figure 3) is a 

structural component perpendicular to and 

behind the mirrors. It has several functions, 

including forming one of the points to which 

the mirror connectors are attached, resisting 

rotational forces on the mirror structure, and 

controlling the elevation position of the 

heliostat mirrors. Table 1 indicates the 

success that Solaflect Energy had in 

reducing the material content of the 

elevation assembly on the heliostat. There 

was a nearly 50% reduction in material 

usage, along with a significant reduction in 

part count. The final prototype was 

manufactured using high-volume processes 

wherever possible. For example, cuts were 

made on a CNC bandsaw (see Figure 4); 

holes were drilled using a CNC mill; and welding was accomplished using a robotic welder (see 

Figure 5).

Figure 3

Robotically Welded Elevation Assembly
4 

Table 1

Summary of Steel Used in Elevation Assembly

(In Comparison to Baseline Heliostat at Start of Project)

Initial Prototype Final Prototype

Carbon Steel in Elevation Assembly (pounds) -39.0% -37.1%

Stainless Steel (pounds) -46.9% -46.9%

“Rabbit Ears” (pounds) 0.0% -54.1%

Part Count -5 -8

Optically Test the Heliostat

The goal at the beginning of this project was for the Solaflect Suspension Heliostat to have a

canting accuracy of better than 3 mrad RMS, and to have vibration of less than 5 mrad RMS in a 

20 mph average wind speed. Solaflect surpassed both of these goals.

Canting accuracy was examined with three methodologies, but the primary method utilized was 

photogrammetry (see Figure 6). Nine photogrammetry targets were placed on each of the 16 

facets on the heliostat. Photogrammetry software was used to accurately determine the location 

of each target. The nine targets on each facet determined the plane of the facet, and this tilt was 

compared to the desired canting. On multiple heliostats, the desired canting accuracy could be 

replicated upon installation of the mirrors. The photogrammetry calculations were in agreement 

with optical observations of the heliostats reflection of the sun onto a target at the focal distance 

of the heliostat.

Figure 5 

Robotic Welder on Elevation Assembly

Figure 4 

CNC Bandsaw
5 

Finite element analysis (FEA) was used to improve the canting accuracy. The basic assumption 

here was that the stiffness of the cables was very high relative to the transverse stiffness of the 

mirrors. The validity of this assumption was later investigated using FEA (Figure 7) and was 

found to be incorrect. Canting procedures were significantly improved by accounting for the 

interaction of the mirrors and the suspension structure.

Another factor found to substantially affect canting accuracy is the stiffness of the gaskets used 

to hold the mirrors in place. An Instron materials testing machine was used to determine the 

elastic modulus of the gasket material in compression for a range of strain states. A sample of 

these results is shown in Figure 8. It is clear that the material is highly nonlinear, but over small 

ranges of strain it was assumed to be nearly linear to allow for linear FEA. 

Figure 6 

Photogrammetry Targets Used for Measuring 

Canting Accuracy

Figure 7 

Finite Element Analysis of Mirror Structure

(Displacement)

Figure 8

Elastic Modulus of Gasket 

Material in Compression
6

Finally, an important aspect of 

canting accuracy is 

consistency in the length of 

the suspension cables. A 

custom swaging machine 

(Figure 9) was initially 

designed by a student team at 

the Thayer School of 

Engineering, and this is 

capable of producing wire 

lengths 10 times more 

consistent in length than the 

industry standard. 

The vibration characteristics 

were measured at the highwind test site in Cheyenne, 

Wyoming. At this location, winds consistently exceed 10–20 mph in the summer, and regularly 

exceed 50 mph in the winter. This also frequently happens in conjunction with sunny days, so it 

is ideal for optically measuring the movement of the reflected image of the heliostat on a 

receiver. The target, at a focal distance of 25 meters, is viewed by a machine vision camera at 

approximately 16 frames per second. Each of these images is then processed to calculate the 

centroid of the fully saturated white focal spot in the image. The vibration characteristics of the 

heliostat at differing wind speeds and relative azimuth orientations of the wind angle of attack on 

the heliostat were calculated from this centroid data.

Redesign the Heliostat Controller

Solaflect Energy examined a number of commercial controller options from three continents, but 

did not find a controller that met the cost and performance criteria for this project. As a result, 

Solaflect designed a new controller circuit board integrating the desired features. This is now 

successfully controlling Solaflect heliostats with the desired tracking accuracy. The new 

Heliostat Controller includes a number of desirable features relating to operation, 

communication, and power.

Motor control on the new controller utilizes pulse width modulation (PWM) to ensure optimal 

performance. Normal operation involves moves of 0.5 to 1.0 milliradians every 10 seconds or as 

needed. A separate circuit is utilized for a high-wind emergency stow, which requires 

significantly more current than normal heliostat operation. This ensures that the heliostat can 

successfully stow at wind speeds of up to 70 mph. The controller can receive feedback from 

encoders on the heliostat and/or from the heliostat area controller (HAC, or field controller) to 

ensure it is at the desired position. Heliostat positioning can be measured to considerably less 

than one-tenth of a milliradian.

The controller has been designed so that either wired or wireless communication can be used, 

and either wired or wireless power can be used. The preferred alternative is to use both wireless 

communication and wireless power to eliminate the need for heliostat field wiring. Continued 

Figure 9

Custom Swaging Machine
7

development and testing is ongoing to prove reliability and longevity of the current wireless 

solutions in larger fields.

The controller is housed in an IP65 (weatherproof) box on the heliostat. It is designed as a field 

replaceable unit (FRU) to enable easy field maintenance or upgrades in minutes.

Conduct Accelerated Testing

There were two strategies for testing the performance of the heliostat in high winds. First, 

Solaflect Energy has a high-wind test facility in Cheyenne, Wyoming. Solaflect defines a “wind 

event” as a wind speed that would cause a heliostat to stow, typically around 30 mph. With this 

definition, it can be seen in Figure 10 that Cheyenne sees about 40 times as many wind events as 

Phoenix, or a full heliostat lifetime of high wind events per year of testing. This ratio is even 

higher for more extreme winds: Cheyenne has about 100 times as many recorded wind speeds 

averaging 40–45 mph when compared to Phoenix, and about 100 times as many gusts of wind in 

the 50–60 mph range. It is a great location to study a heliostat’s performance in high winds.

Several components and design features have been modified as a result of this testing to improve 

performance, reliability, and longevity.

Figure 10

Cheyenne Winds as Number of Years of Phoenix Winds

(Ratio calculated from number of minutes of each wind speed in the two locations)

Second, Solaflect used a portable ground-mounted jet engine to create extreme wind events on 

demand (see Figures 11 and 12). Using a heliostat of the most recent design, Solaflect conducted 

a series of tests at varying wind speeds and heliostat orientations relative to the wind, including 

heliostat motions between orientations while under wind load. 

0

20

40

60

80

100

120

140

160

180

200

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64

Years of Phoenix Wind Events

Wind Speed in Knots

Wind Speed

Stow Speed Gust Speed
8

The testing indicated that the Solaflect heliostat can survive the following conditions:

• Can survive 50 mph winds in any orientation;

• Can stow above 50 mph in multiple orientations;

• Can survive 70 mph winds with vertical mirrors in multiple orientations; and

• Can survive 90 mph winds in stow (see Figure 13).

Figure 11

Testing Heliostat with Jet Engine
9

Figure 12

Suspension Heliostat Test Using Jet Engine

3 meters

10 – 15 meters

Mirror direction

0 degrees

(away from wind)

90 degrees

(cross wind)

180 degrees

(into wind)

Center of cone

Edge of cone

wind

Heliostat Anemometers Jet Engine

Mirror direction

0 degrees

(vertical)

90 degrees

(stow)

wind

Top View

Side View
10

In addition to the accelerated wind testing, Solaflect also created a bank of heliostat motors that

are tested under heavily loaded conditions at an acceleration rate of 48 times actual heliostat 

operation (see Figure 14). The accelerated testing identified a failure point in the control circuitry 

that has been eliminated through a redesign of the controller. These tests have continued, and 

most of the motors have exceeded 15 years of equivalent life by mid-December. There have been 

no motor failures or excessive wear other than the redesigned control circuitry. Finally, full-scale 

heliostats are being tested at an acceleration rate of 20 times actual operation.

Figure 14

Accelerated Winch Testing (as of October 3, 2012)

Equivalent operation time (years)

1 2 3 4 5 6 7 8 9 10

6.14 5.13 5.25 5.16 4.71 5.01 4.06 3.49 5.23 3.28

Figure 13

Sample Wind Speeds during Jet Testing

(Red Line is Standardized to Meteorological 

Anemometer Height of 10 Meters)
11

Test the Cable Drive System and Consider Redesign 

Options

As a result of the wind testing in Cheyenne and the accelerated motor and heliostat testing, it was 

decided to redesign the cable drive system. The revision solves a potential problem of the cable 

drive system becoming detensioned when the highest operational wind was coming from a 

particular azimuth orientation. 

The major design change in the updated heliostat inverted the azimuth cable drive system and 

incorporated a new plastic bushing. The Pedestal Tube Welded Assembly is now much shorter 

and constructed of thinner-wall, less-expensive steel tubing. An additional section, the Base 

Welded Assembly, was added, but this configuration offers more flexibility in foundation 

options, including the possibility of having steel piles that extend above grade. The inversion of 

the azimuth cable drive also created a more straight forward manufacturing process. The 

previous version had rectilinear drive components attached to round structural steel tubing, and 

curvilinear drive components attached to square structural steel tubing. The inversion of the 

azimuth cable drive swapped these components so that the attachment mechanism is much 

simpler and less expensive to manufacture. With the cable drive more compactly secured to the 

stiffer vertical tube, the detensioning issue in the old design in certain wind orientations is 

eliminated.

The drive system inversion also results in improved performance and easier manufacturing. The 

drive cable tensions have been tested and optimized to reduce drive system backlash. A 

significant side benefit of this redesign is that the primary heliostat shipping unit is reduced in 

size by 32%. This increases the heliostat packing density that can be achieved for shipping by 

truck, container or rail. This configuration also offers more flexibility in foundation options,

enabling different foundation designs in different soil conditions.

Cost Reduction

An important goal of this project is to redesign the heliostat to enable high-volume 

manufacturing and to reduce the overall cost of the heliostat in volume. Costs were reduced by 

more than $25 per square meter through a reduction in material and improvements in the design 

for manufacture. The redesigned elevation assembly, discussed above, contributed more than $10 

per square meter to the reduction in cost of the heliostat in large volumes. Nearly two-thirds of 

this cost reduction was from a reduction in steel, with the remainder composed of a reduction in 

fabrication costs from fewer holes and savings in labor costs. The lessons learned redesigning for 

robotic manufacture can be applied to additional components moving forward, resulting in 

further cost reductions.

An additional reduction of about $17 per square meter was obtained from the redesigned 

heliostat controller. This cost savings resulted from replacing individual components and wiring 

with a printed circuit board, with obvious savings in materials and assembly labor costs. In 

addition, expensive relays were replaced with solid state motor controllers, and wiring and 

enclosure costs were reduced.
12

Figure 15

In comparison to the Sandia base case heliostat, the Suspension Heliostat saves $50 per square 

meter in steel alone when produced in high volumes. In Figure 15, the red segments of the pie 

charts represent the steel cost as a proportion of the $75 per square meter SunShot goal, when 

heliostats are produced in high volume. The blue segments of the pie charts represent the portion 

of the total cost of the heliostat that can be allocated to non-steel components such as mirrors, 

heliostat controller, power and communication, foundation, and installation. The Suspension 

Heliostat design makes the SunShot goal feasible due to the extreme steel efficiency of the 

design.

Conclusion

The project succeeded in demonstrating that the Suspension Heliostat design is viable for largescale CSP installations. Canting accuracy is acceptable and is continually improving as Solaflect 

improves its understanding of this design. Cost reduction initiatives were successful, and there 

are still many opportunities for further development and further cost reduction. The suspension 

design is inherently very material efficient, and further experience with this heliostat will 

continue to lead to further design and performance improvements.