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ANNUAL PERFORMANCE PREDICTION FOR OFF-AXIS ALIGNED LUG0 HELIOSTATS AT SOLAR TWO

Scott A. Jones SUN+LAB

Solar Thermal Technology Department Sandia National Laboratories

Albuquerque, New Mexico

ABSTRACT

The DELSOL computer code was used to model the annual performance for numerous off-axis alignments of the Lug0 heliostats located at the Solar Two site in Dagget, California Recommended canting times are presented for the Lug0 heliostats based upon their location in the field. Predicted annual performance of an off-axis alignment was actually higher than for on-axis alignment in some cases, and approximately equal if the recommended times are used. The annual performances of Solar One heliostats located nearby were also calculated, and illustrated the poorer performance expected of the Lug0 heliostats.

INTRODUCTION

One hundred and eight 95 m2 Lug0 heliostats were added to the original 1818, 40 m2 Solar One heliostats for the Solar Two project. The Lug0 heliostats were added to the south field to even out the flux distribution on the molten-salt receiver. These heliostats were fabricated with inexpensive mirror modules from Carrisa Plains and trackers from Hesperia, two now-defunct photovoltaic sites in California. The Hesperia facility is located at Southern California Edison’s Lug0 Substation, and this name has been assigned to the 95 m2 heliostats. Figure 1 shows a Lug0 heliostat installed at Solar Two.

The Lug0 heliostats were slated to be aligned or “canted” using an off-axis, sun-tracking method, rather than an on-axis approach, because it was believed to be faster and simpler to implement as well a better match for the imprecise Lug0 mirror modules (Jones et al., 1995). These advantages were being traded for a what was believed to be a lower theoretical annual

This work was supported by the US. Department of Energy under Contract

performance. This study was undertaken to determine the relationship between time of off-axis alignment and annual performance, and to make appropriate recommendations for canting times. Another benefit of the study was further insights into the modeling of Lug0 heliostats for plant control and performance predictions.

BACKGROUND

An on-axis alignment occurs when each mirror modules’ normal vector intersects at a point twice the focal distance. This alignment condition is termed on-axis because it is optimized for the geometry where the heliostat center, the target, and the sun all fall along a line. An on-axis alignment is normally implemented by mechanical means that do not involve tracking the sun such as using an inclinometer, or a lookback camera system (Jones et al., 1995). Because these methods rely on calculations of the desired mirror normal vector pointing, it is possible to implement any focal length in the canting process.

Conversely, an off-axis alignment is optimized for a geometry where the heliostat center, the target, and the sun do not fall along a line. The mirror module normal vectors do not all intersect at a single point for an off-axis alignment. This type of alignment is normally implemented by tracking the sun and adjusting the aiming of each mirror module to minimize the size of the reflected beam upon the target. The overall heliostat focal length, which may be different from the individual mirror module focal length, by default becomes the distance between the heliostat and the target. At Solar Two, the distance between the heliostats and the Beam Characterization System (BCS) target closely approximates the actual slant range, which is the distance between the heliostat and the receiver.

DE-ACO4-94AL85000.

DISCLAIMER

Portions of this document may be illegible in electronic image products. hiages are produced fbm the best available original document.

Since either alignment strategy optimizes the performance of the heliostat for only one particular geometry, and the sun’s position changes significantly with time, the relevant question becomes which alignment provides the best performance over the course of a year. The answer to this question can vary with heliostat and target geometry, location in the field, and aiming strategy.

Figure 1. Lug0 Heliostat at Solar Two.

The best figure of merit, or metric, to evaluate the performance of a canting approach is the Annual, Incident- Power-Weighted Intercept (AIPWI) because it is directly tied to the plant economics. This is the fraction of the beam power leaving the heliostats over a year that actually lands upon the receiver; a larger number is better.

The DELSOL computer code was used to calculate the AIF’WI values for a group of heliostats called a cell. In DELSOL, the AIPWI value is tabulated under the somewhat misleading title of “spillage”. Spillage is actually the fraction of power that misses the receiver and is simply equal to 1 minus the intercept. DELSOL computes the daily and annual average intercepts and performs the insolation weighting (Kistler, 1986).

APPROACH

The area surrounding the tower was divided into 12 rings and 12 slices, for a total of 144 cells. Only 21 of those cells contain Lug0 heliostats (10 east, 10 west, and 1 due south). Figure 2 shows the cell layout used by DELSOL. Heliostats may be located along the cell borders, leading to a fractional number of heliostats in each cell, as listed in Table 1. Cell location is denoted by its radius in tower heights from the center of the tower, and its azimuth location in clockwise degrees from north. For instance, the center of cell (5.27,90) is located 5.27 tower heights (282 m) from the tower’s center, and 90’ clockwise

from North (East). Blanks in the table indicate no heliostats are present in that cell.

The symmetry of the cells on the east and west halves of the field coupled with a symmetric insolation model1 permit the analysis of east cells to be applied to west cells with a simple change. Namely, the afternoon canting times for the east field cells discussed herein are equivalent to morning canting times for symmetric west field cells. For instance, the AIPWI for cell (3.97,150) canted on equinox at +3 hours from solar noon is equal to the AIPWI for cell (3.97,210) if canted at solar noon -3 hours on equinox.

1

180 deg

Figure 2. Matrix of Cells Used by DELSOL to Model the Heliostat Field.

In this study, the annual performance was calculated using one hour time steps on five days spaced evenly between winter and summer solstice (days 354.75, 35.38, 81, 126.63, and 172.25). The Lug0 heliostats use a simple belt-aim strategy, meaning they are aimed at a “belt” of points located around the circumference of the receiver at its midpoint.

Three east/west field cell combinations were analyzed and are shown in bold, italic typeface in Table 1. Those cell combinations were chosen for two reasons: (1) they had the largest concentrations of heliostats in them, and hence were representative, and (2) they covered the full range of cell azimuth values containing Lug0 heliostatsz. This permits an examination of the relationship between cell azimuth and off- axis canting time. For each cell combination, a number of off- axis canting times on summer solstice and equinox were evaluated, as was the on-axis performance. The Appendix describes the innovative DELSOL approach used in this study.

The sun is due south at solar noon, and provides equal amounts of

The 2 Lug0 heliostats at 1SO”are almost in the 150” and 210” cells insolation in the morning and afternoon.

Table 1. Number of Lug0 Heliostats in Each Cell of the DELSOL Model

I I Azimuth Anale I

ASSUMPTIONS

The only assumption required for this study is that the cell performance remains constant despite defining adjacent cells with heliostats as having none. Specifically, an incorrect blocking and shading factor calculated for the cell would introduce a second-order error in the weighting of the AIPWI. While this could be dismissed because of its small potential for introducing error, it was explored just to make certain of that.

The findings were that the cell approach used by DELSOL to model the field almost certainly eliminates this source of error. The cell approach to modeling assumes every heliostat in the cell has the performance of a representative heliostat located at the cell's center. The shading and blocking for that representative heliostat are calculated assuming it is surrounded by other heliostats at the pre-defined spacing interval, ignoring cell boundaries.

This was verified by comparing blocking and shading values for cases without heliostats in adjacent cells versus the same scenario except with heliostats located in adjacent cells. Both the shading and blocking values matched in all cases.

RESULTS

Figures 3 through 5 show the relationship between AIPWI values and the time of canting for the east field cells examined. AIPWI values are presented for canting times in one hour increments on both summer solstice (June 21) and equinox (March 21 and September 21). Since the canting is likely to occur between June 21 and September 21, these dates provide bounding cases on the expected AIPWI values.

DELSOL assumes the heliostats in each cell are canted instantaneously, whereas in reality it is predicted to take 2 hours. The average AIPWI value from two consecutive hours is therefore an acceptable figure-of-merit. On equinox, the data end at 5 hours after solar noon because of the earlier sunsets.

It is clear that differences are seen between the three cell azimuth values. For instance, the off-axis performance curves are fairly flat for go", curved for 120", and even more so for 150". Another interesting result seen in Figure 3 is the potential to achieve a higher AIF'WI value from an off-axis, sun cant done on equinox between solar noon and +4 hours than from an on- axis cant. However, sun canting can also lead to much poorer

performance if done at an inappropriate time. Figure 5 shows that annual performance would suffer severely if these heliostats were canted near solar noon, and by extrapolating to the left of the graph it is clear than canting in the morning would be disastrous. In general, the off-axis cant and on-axis cant are of equivalent performance if attention is paid to the time of canting.

The difference in performance between summer solstice and equinox off-axis canting is typically small. For the 150" cell in Figure 5, summer solstice gives better performance than equinox as well as providing sunshine later into the afternoon. For the 90" and 120" cells, equinox canting tends to give better performance than summer solstice, and the optimal time for canting falls closer to solar noon. Consequently for Solar Two, it would be beneficial to begin the canting operations at the south road and proceed simultaneously clockwise in the West half of the field, and counter-clockwise in the East half of the field.

Table 2 lists the optimal time period for canting heliostats by the cell they are located in, still assuming the canting occurs between March 21 and September 21. The two heliostats located in cell (3.10,180) should be grouped with their adjacent cells- add one to cell (3.10,150), and one to cell (3.10,210). The results suggest canting heliostats near the south road late in the afternoon for the east field, or early in the morning for the west field, would be beneficial.

Table 2. Optimal Hours from Solar Noon to Cant Lug0 Heliostats by Cell Azimuth3

Cell Azimuth Start Time Finish Time 90 +I +4 120 +I .5 +4.5

1 270 -4 I -1 I

Assuming the canting occurs between March 21 and September 21

0.850

0.800

0.750

0.700

AlPWl

0.650

0.600

0.550

0.500

- u - Equinox

-1 hr solar +I hr +2 hr +3 hr +4 hr +5 hr +6 hr +7 hr

Time of Canting noon

Figure 3. Lug0 Heliostat Annual Performance Cell (5.27, 90)

0.850

0.800

0.750

0.700

AlPWl

0.650

0.600

0.550

0.500

. a - Equinox

-1 hr solar +I hr +2hr +3 hr +4 hr +5 hr +6hr +7hr

Time of Canting noon

Figure 4. Lug0 Heliostat Annual Performance Cell (4.40, 120)

5 0.850

AlPWl

. Q - Summer Solstice

. - Equinox -On-Axis

+5 hr -1 hr solar noon

+I hr +2 hr +3 hr +4 hr

Time of Canting

Figure 5. Lug0 Heliostat Annual Performance

+6 hr +7 hr

Cell (3.97,150)

If the Lug0 heliostats are canted during the recommended times, then their Performance will approximately match that from an on-axis alignment. A simpler model based upon the assumption of on-axis alignment will therefore provide acceptable accuracy for most applications.

As expected, the AIPWI values calculated for the Lug0 heliostats are significantly lower than for the Solar One heliostats. The field average AIPWI is 0.964 for Solar One heliostats canted on-axis to slant range (Kolb, 1994). Table 3 lists the AIPWI values for south field Solar One heliostats located near the Lug0 heliostats. This is a more appropriate comparison, although the Lug0 heliostats are located further from the tower leading to lower AIPWI values due to beam divergence. However, the large size and the nominally flat glass of the Lug0 heliostats are really the primary contributing factors to the lower AIPWI values. Despite the expectedly high spillage, the Lug0 heliostats were added because they are a very cost-effective addition to the field.

Table 3. AlPWl Values for On-Axis Alignment of Lug0 and Solar One Heliostats Located Near Each Other

Solar One Heliostats Lug0 Heliostats Cell Azimuth Radius Radius

Heights) Heights) 90 4.66 ,896 5.27 .753 120 3.51 .951 4.4 .792 150 2.74 .966 3.97 ,801

(Degrees) (Tower AlPWl (Tower AlPWl

Please note that the DELSOL predicted AIPWI values assume perfectly executed on- and off-axis canting of the heliostats in addition to typical assumptions about the heliostat such as its focal length and optical error. In reality, a good quality canting job (on- or off-axis) on the rather imprecise Lug0 heliostats might provide slightly lower performance than predicted, and a careless job could be substantially lower. Also, the Carissa Plains modules were found to be slightly warped and have a focal length that varies with temperature (Jones et al., 1995).

CONCLUSIONS

The time of day at which Lug0 heliostats are off-axis aligned was found to substantially impact the predicted annual performance. The annual performance of an off-axis alignment can vary from out-performing an on-axis alignment to seriously under-performing one, depending on the time of alignment. A relationship between cell azimuth and optimal canting time of the year was also noted. By combining the two, a recommended strategy to be used at Solar Two was presented.

The cost-effective Lug0 heliostats were expectedly predicted to have lower AIPWI values than the original Solar One heliostats. A simplified, on-axis model of the Lug0 heliostats would provide acceptable accuracy for most power plant performance models assuming the recommended strategy is f o 11 ow e d .

REFERENCES

Jones, S.A., R.M. Edgar, R.M. Houser, “Recent Results on the Optical Performance of Solar Two Heliostats,” Proceedings of the American Society of Mechanical Engineers Solar Energy Division Meeting, March 19-24, 1995, Maui, Hawaii.

Kistler, B. L., 1986, A Users Manual for DELSOL3: A Computer Code for Calculating the Optical Performance and Optimal System Design for Solar Thermal Central Receiver Plants, Sandia National Laboratories, Albuquerque, New Mexico, SAND86-8018.

Kolb, G.J., 1994, “DELSOL Canting Studies for the Heliostats at Solar Two,” Sandia National Laboratories Memorandum to S.A. Jones, Albuquerque, New Mexico, October 31, 1994.

ACKNOWLEDGMENTS

I would like to thank Greg Kolb for his assistance in this study. This work was supported by the U.S. Department of Energy under Contract DE-AC04-94AL85000.

APPENDIX

This section describes the innovative approach needed to work around some inherent limitations of the DELSOL code and achieve the study’s objectives. It may be difficult to follow and of little interest for readers with no DELSOL experience, so it is included as an Appendix.

One of the useful outputs provided by DELSOL is the individual cell annual performances. Unfortunately, only east field cells are presented in this output section because the code assumes symmetric east and west cells are canted at the same time and also have the same annual performance. While this is appropriate for on-axis canting, it is not for off-axis canting. Consequently, this output was rendered meaningless. Instead, the field summary output section was utilized to determine the needed information. Two changes were needed to achieve this.

First, the field definition was limited to a single cell of interest, such as (4.40,120). The remaining cells were all set to a density of 10-5 (a density of zero causes program crashes). This effectively changed the field summary output to a cell summary. Second, the annual performance for each cell was determined by using two performance runs. This was required because DELSOL utilizes the symmetric insolation model and only calculates field performance for the afternoon. However, the total, annual performance for a cell is the sum of the annual- afternoon performance and the annual-morning performance. By symmetry, the annual-morning performance of an east field cell canted in the afternoon is equivalent to the annual-afternoon performance of the opposing west field cell canted in the morning. So the total, annual performance for an east field cell canted in the afternoon is the sum of its annual-afternoon performance and the annual-afternoon performance of the opposing west field cell canted in the morning. The two performance runs were then averaged with insolation weighting.

One drawback of this work around was that it required many more executions of the code. To limit the number of computer runs to a reasonable quantity, only 3 easvwest field cell combinations were analyzed. As mentioned previously, those cell combinations were chosen for two reasons: (1) they had the largest concentrations of heliostats in them, and hence were more representative, and (2) they covered the full range of cell azimuth values containing Lug0 heliostats (except for 1 80°, where only 2 Lug0 heliostats are located), illuminating any trends between canting time and cell azimuth. This led to a total of 99 individual DELSOL runs needed to complete this study.