Solar Field Optical Characterization at Stillwater Geothermal/Solar Hybrid Plant

Concentrating solar power (CSP) can provide additional thermal energy to boost geothermal plant power generation. For a newly constructed solar field at a geothermal Guangdong Zhu power plant site, it is critical to properly characterize its performance so that the predic- National Renewable Energy Laboratory, tion of thermal power generation can be derived to develop an optimum operating strat-

15013 Denver West Parkway, egy for a hybrid system. In the past, laboratory characterization of a solar collector has Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/139/3/031002/6409184/sol_139_03_031002.pdf by guest on 01 October 2021 Golden, CO 80401 often extended into the solar field performance model and has been used to predict the e-mail: [email protected] actual solar field performance, disregarding realistic impacting factors. In this work, an extensive measurement on mirror slope error and receiver position error has been per- Craig Turchi formed in the field by using the optical characterization tool called distant observer National Renewable Energy Laboratory, (DO). Combining a solar reflectance sampling procedure, a newly developed solar char- 15013 Denver West Parkway, acterization program called FIRSTOPTIC and public software for annual performance mod- Golden, CO 80401 eling called system advisor model (SAM), a comprehensive solar field optical characterization has been conducted, thus allowing for an informed prediction of solar field annual performance. The paper illustrates this detailed solar field optical character- ization procedure and demonstrates how the results help to quantify an appropriate tracking-correction strategy to improve solar field performance. In particular, it is found that an appropriate tracking-offset algorithm can improve the solar field performance by about 15%. The work here provides a valuable reference for the growing CSP industry. [DOI: 10.1115/1.4035518]

Keywords: concentrating solar power, geothermal energy, parabolic trough, optical characterization, optical error, hybrid system

1 Introduction A key task is to properly characterize the performance of the new Stillwater solar field. Optical characterization of a utility- Concentrating solar power (CSP) uses mirrors to concentrate scale solar field is a broad topic. SolarPACES has developed mul- sunlight and convert solar power into heat, which can then be tiple international guidelines on solar mirror reflectance [12,13] directed to a thermodynamic cycle to generate electricity [1–4]. and solar system annual modeling [14–16] that provide guidance As a mature CSP technology, parabolic troughs can produce heat to the growing CSP industry. Solar reflectance correlates mirror at a temperature of 400 C or higher [2,5] and generate electricity reflectance with the solar spectrum [13,17–22] and becomes a through a standalone power plant. In addition, the produced heat fairly complex topic in the CSP applications. Solar reflectance can be hybridized naturally with other types of heat sources to used to be measured at a single small acceptance angle (often with produce electricity, such as the integrated solar combined-cycle a narrow band of visible light wavelength) and now is more [6,7] and geothermal/solar hybrid systems [8]. For the latter con- appropriately modeled in a more comprehensive way as more cept, adding solar heat from CSP to a geothermal power plant can modern instrumentation becomes available. No guidelines exist boost power generation during the daytime, when the increasing regarding optical characterization of the solar field. One major ambient temperature and/or degrading geothermal resource result reason is that a variety of optical tools exist and each has its limi- in lower power production. tations regarding measurement results and conditions [23–28]. The Stillwater geothermal plant located in Fallon, NV is an Most tools are designed to measure one specific type of optical example of a geothermal operation where Enel Green Power has error, such as mirror specular reflectance [12,19], mirror slope recently adopted solar hybridization [9,10]. The solar field is error [23,24,27–32], or receiver temperature [33–35]. Further- about 17 MWth at the design point and uses SkyTrough parabolic more, very few tools are suitable for use in the field. Past work trough collectors []. The heat-transfer fluid (HTF) in the trough [12,19,23–25,27–31,33–35] has predicted solar field performance receiver is pressurized water, and the design HTF inlet and based on laboratory measurement; such prediction inappropriately outlet temperatures are about 149 C and 199 C, respectively. disregards the deviation of a solar collector from its design-point The heated HTF is used to heat the brine flow with a degraded performance measured in the laboratory. temperature. Solar field specifications at the Stillwater plant site To fully characterize the optical performance of a solar field, are summarized in Table 1. one needs to obtain precise optical and geometric measurements of solar collectors in the field, including mirror specular reflec- tance, receiver absorptance and receiver glass-envelope transmit- Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING:INCLUDING WIND ENERGY AND BUILDING ENERGY tance, mirror slope error, receiver position error, and collector CONSERVATION. Manuscript received July 26, 2016; final manuscript received December tracking error. Among these, mirror slope error has been consid- 2, 2016; published online January 27, 2017. Assoc. Editor: Wojciech Lipinski. ered the dominant error source [36,37]. Other error sources, such The United States Government retains, and by accepting the article for publication, as mirror reflectance, receiver absorptance, and receiver position the publisher acknowledges that the United States Government retains, a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of error, could also significantly impact the collector performance, this work, or allow others to do so, for United States government purposes. depending on specific cases [38]. In this work, mirror specular

Journal of Solar Energy Engineering Copyright VC 2017 by ASME JUNE 2017, Vol. 139 / 031002-1 Table 1 Solar field specifications at Stillwater geothermal/ solar plant includes 11 SkyTrough collector loops, each of which solar hybrid plant site consists of four solar collector assemblies (SCAs). Each SCA includes eight solar collector elements/modules (SCEs), about Solar field aspects Value 110 m long and 6 m in aperture width. The solar field layout is a standard row-to-row formation. See Table 1 for further properties Location Fallon, NV of the Stillwater plant. Solar field area (acres) 20 Collector aperture area (m2) 25,000 The objective of the optical test is to obtain the average optical Solar collector model SkyTrough [11] performance of the solar field. In reality, it is not possible to mea- Solar collector aperture (m) 6 sure every single mirror facet or receiver tube. A sampling process Solar collector focal length (m) 1.71 needs to be developed to manage measurement time and ensure Solar absorber size (m) 0.08 sufficient accuracy at the same time. For this solar field, loop 5 is Solar collector module length (m) 13.9 equipped with sophisticated instrumentation to measure real-time Reflector material ReflecTech PLUS [11] loop temperature, mass flow rate, and pressure, and it is intended Nominal power output (MWth), 17

2 to be used as a test loop for the plant acceptance test to represent Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/139/3/031002/6409184/sol_139_03_031002.pdf by guest on 01 October 2021 assuming a DNI of 900 W/m , the entire solar field. For this reason, a detailed optical test plan is and an ambient temperature of 21 C Solar field configuration U shape specifically designed for the whole field and loop 5, as summar- Number of collector loops 11 ized in Table 3. Number of solar collector modules per loop 28 It is straightforward and efficient (about 30 readings per hour) Inlet temperature (C) 149 to use reflectometers (D&S 15R and 410-Solar) to measure mirror Outlet temperature (C) 199 reflectance in the field [13,42,43]. The DO implementation Heat-transfer fluid (HTF) Demineralized water requires certain preparation steps and may take a half hour to 1 h to measure each module [39,40]. The measurement number for D&S was selected based on previous work [13]; because this is reflectance, mirror slope error, and receiver position error were the first time to implement a slope error/receiver position error measured in the field to best predict the actual solar field perform- measurement in the field, a maximum number of sampled meas- ance. Mirror reflectance measurements use a newly developed urements is selected during the planned two-day test period. specular reflectance model, and mirror slope error and receiver position error measurements use a newly developed optical tool called distant observer (DO) [39,40]. DO is one of the very few 3 Solar Reflectance Measurement tools suitable for outdoor field environments, along with the For the SkyTrough collector as specified in Table 1, the airborne method proposed by Prahl and others [32,41]. receiver will only accept the reflected sun rays within about The focus of this paper is to: (1) exhibit a comprehensive and 1.35 deg around the specular direction, outside of which the light accurate field optical characterization by using state-of-the-art will not be intercepted/absorbed. It is the specular reflectance of a models and tools, which will provide valuable guidance to future mirror that determines optical performance of a trough collector field optical characterization work; and (2) employ an optical (and other types of CSP collectors). Many specular reflectance model and thermal performance model to predict annual thermal models have been proposed and often suffer from certain theoreti- energy production as a function of time. The modeling of a geo- cal/realistic limitations [12,13], such as only being suitable to spe- thermal/solar thermal hybrid system is a separate work and can be cific instrumentation or a specific type of reflector. Here, a newly found elsewhere [9]. proposed solar reflectance model is adopted [13] that is generic In this paper, Sec. 2 gives the optical test plan. Sections 3 and 4 and flexible in accommodating various levels of measurements. present and discuss in detail the solar reflectance measurements and collector optical error measurements, respectively. Sections 5 and 6 provide calculations of the overall optical performance and 3.1 Solar Reflectance Model. The mathematical model of annual energy generation of the solar field, based on the optical solar-weighted specular reflection is defined as follows: measurement results. Finally, discussion and conclusions are ð XðuÞ given in Secs. 7 and 8. SW SW SW qspecðuÞ¼qspec;tot f ðuÞdX (1) 0

2 Optical Test Plan SW Here, qspec is the solar-weighted specular reflectance at the accep- SW The optical tools used in this field optical test are summarized tance angle u,andqspec;tot is the total solar-weighted specular reflec- in Table 2. By using these tools, measurements of mirror specular tance. A general description of mirrorreflectionisprovidedinFig.1. reflectance, mirror slope error, and receiver position error can be A simple and effective form for mirror specularity profile is a conducted in the field. The solar field at the Stillwater geothermal/ Gaussian distribution

Table 2 Technical specifications of optical test tools used in this study

Instrument Manufacturer Measured parameters Specifications

410-Solar Surface Optics Solar-weighted hemispherical reflectance Incidence 20 deg wavelengths: Corporationa solar-weighted total specular reflectance (6 deg seven bands half aperture) D&S 15 R Devices and Services Specular reflectance at 7, 15, 25, and 46 mrad. Incidence 15 deg wavelength: Companyb A D&S model can be customized with either 660 nm repeatability: 0.002 three aperture angles. The adopted D&S model reflectance unit uses 7, 25, and 46 mrad Distant observer (DO) NREL under DOE funds Mirror slope error: transversal direction receiver Major components: high-precision position error: two directions in transversal camera photogrammetry-based plane program www.surfaceoptics.com www.devicesandservices.com

031002-2 / Vol. 139, JUNE 2017 Transactions of the ASME Table 3 Optical test plan summary

Measurement metrics of interest Instrument Procedure

Specular reflectance at one acceptance angle D&S 15R Select 44 modules across the whole solar field and select 14 modules at loop 5 Clean the mirror patch of interest for measurements for each module Use D&S 15R to measure specular reflectance at 25 mrad aperture Specularity and solar-weighted specular D&S 15R410-solar Select four modules at loop 5 Clean the mirror patch of interest for measurements for each module Use D&S reflectometer to measure specular reflectance at 7, 25, and 46 mrad Use 410-solar reflectometer to measure solar-weighted specular reflectance Mirror slope error and receiver position error DO Select ten modules across the whole solar field and select four modules at loop 5 Implement DO technique for measurements Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/139/3/031002/6409184/sol_139_03_031002.pdf by guest on 01 October 2021

Fig. 3 Specular reflectance (at a wavelength of 660 nm) as a function of acceptance aperture size for four mirror panel samples

Fig. 1 Schematic of mirror reflectance [13] full acceptance angle of 25 mrad and at a single wavelength of 660 nm. As seen in the figure, loops 1, 5, 7, 9, and 10 have a slightly higher reflectance than the average. Some reflectance variance across loops in the figure comes from measurement uncertainties (including instrumentation repeatability of 60.2%), whereas other variance comes from damage to reflector panels in certain loops (e.g., loop 8) due to rainwater trapped within some mirror panel storage containers during their long-time outdoor storage prior to solar field construction. Next, for selected mirror modules, mirror reflectance at various acceptance angles is also measured using the D&S reflectometer. The results are plotted in Fig. 3, and the measurements by the D&S reflectometer are denoted by

660;m qspc ðuiÞ; i ¼ 1; 2; 3 (3)

Here, m is the sample number, m ¼ 1, 2,…, Mspc, and

u1 ¼ 3:5 mrad; u2 ¼ 12:5 mrad; u3 ¼ 23 mrad (4) Fig. 2 Distribution of solar specular reflectance (at 25 mrad and at a wavelength of 660 nm) among loops. Variability comes In the formula, the half acceptance angle is used; the full accep- from measurement uncertainties and damage to some panels tance angle is twice the half acceptance angle, which is more often prior to construction. referred to in practice. Specular reflectance increases with acceptance angle. How ÀÁ u 2 fast the specular reflectance increases with the increasing accep- 1 pffiffi 2rs f ðÞu ¼ 2 e (2) tance angle defines the mirror specularity. In addition, another 2p rs reflectometer, the SOC 410-Solar [43], is also used to measure

2 solar-weighted hemispherical and specular reflectance. The two The coefficient 1=2p rs ensures an overall integral equal to 1 reflectometers complement one another—the D&S measures when assuming rs 1. Here, rs is the root mean square (RMS) reflectance at various acceptance angles, but at a single wave- of the Gaussian function, expressed in radians. length, whereas the SOC measures solar-weighted reflectance with multiple bands of wavelength representing the solar spec- trum, but at a single large acceptance angle. 3.2 Solar Reflectance Measurements. First, the D&S R15 reflectometer [13,42] is used to measure the mirror reflectance across the entire solar field. The measurements are plotted for 3.3 Solar Field Average Reflectance Characterization. different loops in Fig. 2. The mirror reflectance is measured at a Based on the data over varying acceptance angles, a single-

Journal of Solar Energy Engineering JUNE 2017, Vol. 139 / 031002-3 rSW ¼ r660nm (8)

SW Here, qspec;tot is the average solar-weighted specular reflectance SW 660nm derived from the D&S measurements. qspec;SOC and qspec;SOC are the solar-weighted value and the 660-nm-wavelength value measured by the SOC reflectometer, respectively; and SW 660nm ðqspec;SOC=qspec;SOCÞ is the average of the ratio of two reflectance measurements by the SOC reflectometer. rSW is the solar- weighted specularity RMS value. The solar reflectance measure- ments using the SOC reflectometer comply with solar reflectance test standard ASTM E903 and solar spectrum standard ASTM G173. Equations (7) and (8) are used to approximate solar- Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/139/3/031002/6409184/sol_139_03_031002.pdf by guest on 01 October 2021 Fig. 4 Fitted mirror specularity profile using a single-Gaussian weighted specular reflectance due to the availability of in-field approximation instrumentation according to the SolarPACES solar mirror reflec- tance measurement guideline [12]. The measurement uncertainty at a 95% confidence level is also Gaussian specularity profile can be derived. For three measure- given for the average values and is calculated by using a simple u ; ; ; ments at i i ¼ 1 2 3 of each mirror panel m, Eqs. (1)–(3) student t method based on the number of measurements. expand to ÀÁ u 2 pffiffi i 660;m 2r660nm 660;m qspec;tot 1 e ¼ qspec ðÞui ; i ¼ 1; 2; 3 (5) 4 Measurement of Mirror Slope Error and Receiver Position Error 660;m 660nm The unknown parameters qspec;tot and r can be solved from DO is an optical tool developed by NREL to characterize solar the equations above by using least-squares fitting, based on the collector optical errors, including mirror slope error and receiver measurements at three acceptance angles. The single Gaussian fit- alignment/position error [39,40]. It uses photogrammetry to ting curves are then plotted against the raw data points in Fig. 4. capture the collector error through the reflection images of the Combined with the average solar field reflectance and the solar- receiver on the reflector. An example reflection image is given in weighted reflectance, the average solar-weighted reflectance is Fig. 5. The distortion of the receiver reflection indicates the calculated for the whole solar field and loop 5 in Table 4. The inaccurate mirror shape and/or misplaced receiver locations. Four average solar-weighted reflectance is derived using the solar targets (black dot centered on a white background) are attached reflectance model in Sec. 3.1. More specifically on the collector module corners and provide the position reference ÀÁ for the photogrammetry analysis, as shown in Fig. 6. The original u 2 pffiffi SW SW SW image can then be rescaled to a regular square image (right image qspec;tot 1 e 2r ¼ q ðÞu (6) spec in Fig. 5). By combining with the camera optical specifications, the mirror slope error can be derived for one horizontal segment Here, the calculation of solar-weighted values is simplified with of the mirror module by using a sophisticated software package. available measurement capabilities by assuming The characterization of the full collector module requires a series ! of photos to be taken, while the receiver reflection sweeps from qSW one module edge to the other with the collector’s rotation. During SW 660 spec;SOC qspec;tot ¼ qspec;tot (7) this measurement, the collector tracking angles stay between q660 spec;SOC 10 deg and 20 deg above the horizon. It should be noted that the

Table 4 Average solar reflectance measurement and the uncertainties at a 95% confidence level for the solar field and loop 5. The average solar field value is affected by measurement uncertainties and damage to some panels prior to construction. Loop 5 will be used in the future performance acceptance test at the plant.

Solar field Loop 5

SW Average solar-weighted specular reflectance qspec;tot 0.9043 6 0.0033 0.9124 6 0.0052 Average specularity RMS value rSW 1.92 6 0.26 1.92 6 0.26

Fig. 5 A snapshot of distant observer (DO) optical characterization: raw photo (left) and scaled photo (right)

031002-4 / Vol. 139, JUNE 2017 Transactions of the ASME that the receiver position error indicates the receivers’ distortion due to gravity. The average position error is about 26.4 mm along x and 11.0 mm along z. The SkyTrough collector is designed with some receivers mounted below focus and some above, so that when they expand at operating temperature, all of the receivers move closer into focus. The receiver offset along z will be reduced under normal operational conditions. The mirror slope error on the entire sample mirror module is also calculated and plotted in Fig. 8. The average and RMS values of the mirror slope error are 3.8 mrad and 3.1 mrad, respec- tively. Note that the receiver misalignment is included as part of the mirror slope error. A number of collector modules were measured using DO. A

total of 12 data sets were identified to provide valid measure- Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/139/3/031002/6409184/sol_139_03_031002.pdf by guest on 01 October 2021 ments. The mean value and RMS of the mirror slope error for the valid data sets are plotted in Fig. 9. The average mean value is about deg 4.0 mrad. This nonzero mean value of slope error is due Fig. 6 Rescaling of the raw image based on the target loca- to gravity-induced displacement of the receiver and frame, which tions [40] is greatest at low tracking angles. The average RMS value is about 3.0 mrad. typical operating range is 10–170 deg. Thus, one underlying assumption is that the slope error measurement does not vary at a 5 Optical Characterization of Solar Collector significant degree with varying collector orientation. The receiver position error along x and z directions are plotted 5.1 Intercept Factor. The optical performance of a solar for one 13.9-m-long module at the Stillwater plant in Fig. 7. Note collector largely depends on two aspects: optical properties and

Fig. 7 Receiver position error along x and z directions. Because these measurements were taken near the horizon, they represent a worst-case scenario that occurs during operation. Receiver position error during normal operating angles may be substantially less.

Fig. 8 Slope error distribution over one collector module of a SkyTrough collector module (indexed by L5R2M10)

Journal of Solar Energy Engineering JUNE 2017, Vol. 139 / 031002-5 Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/139/3/031002/6409184/sol_139_03_031002.pdf by guest on 01 October 2021

Fig. 10 Intercept factor calculation for all sampled collector modules

Fig. 9 Slope error distribution attributes (mean value and appropriate tracking offset for each measured collector module is RMS) for all sampled collector modules. Note that the mean recalculated against the one without the tracking correction in value is dominated by the receiver’s gravity-induced displace- ment, which is greatest at the low tracking angle while the DO Fig. 11. Here, it is assumed that the tracking-offset algorithm is measurements were conducted. Its impact on the collector per- able to reduce the mean value of slope error for any collector to formance is compensated by including a tracking-offset algo- be 1 mrad. As seen in the figure, the collector performance is rithm, to be discussed later. improved substantially. With the tracking offset, the intercept fac- tor becomes close to the unity for all cases. The average intercept factor increases from 0.827 without tracking offset to 0.991 with geometric accuracy. Optical properties include mirror reflectance, tracking offset. According to the vendor, field calibration of the absorber-tube absorptivity, and receiver-envelope transmissivity; tracking-offset algorithm is an anticipated step of the field com- geometric accuracy refers to mirror slope error, receiver position missioning, and it clearly improves solar field performance to a error, and tracking error. The solar collector optical specifications significant degree. and optical error measurements are summarized for the Stillwater The large slope error is induced by the effect of gravity on the solar field in Table 5. collector. The gravity-induced deflection is greatest at low Once the collector optical characterization parameters are tracking angles and is less when the collector is oriented straight obtained, an optical performance calculation program called up. When the impact of gravity-induced deflection varies with FIRSTOPTIC can be used to derive the collector optical performance. tracking angle, the tracking-offset value is a function of collector FIRSTOPTIC employs the most accurate optical treatment of collector position. In reality, a tracking-offset algorithm can be directly optical error measurements, and it derives analytical mathematical incorporated into the tracking control program of the drive so that formulae to calculate the intercept factor of a trough collector the collector performance can be improved. [37,38,44]. It can provide fast, accurate calculation of intercept factor and incidence-angle modifier (IAM) [44]. The intercept fac- tor for each collector module is plotted in Fig. 10. The intercept 5.3 Incidence-Angle Modifier (IAM). An IAM is typically factor varies from 0.65 to 0.95, closely correlated to the slope used to describe the solar collector performance under nonzero error measurements in Fig. 9 because the collector slope error incidence angles, that is, for different sun positions throughout the typically dominates the collector performance. year. A common form for the IAM is

h h2 5.2 Tracking Offset. The predicted intercept factor values IAMðÞh ¼ kIAM;0 þ kIAM;1 þ kIAM;2 cosðÞh cosðÞh above are substantially lower than the anticipated target perform- 3 ance. It can be seen that the slope error magnitude strongly h þ kIAM;3 (9) impacts the collector performance. From Fig. 9, the mean value of cosðÞh each data set is negative between 2.9 mrad and 5.0 mrad. The- oretically, the slope error with a mean of 1 mrad can be compen- The fitting function is typically of second or third order. In the sated by þ2 mrad tracking error, which results in an effective work described here, it is found that a better fit is provided slope error with a mean of 0 mrad. Thus, this tracking-offset strat- with the third-order function because of its higher accuracy. In egy can be applied to each collector sample with exactly twice the either form, the optical efficiency at a nonzero incidence angle is mean value of the measured slope error. The intercept factor with given by

Table 5 Solar collector optical specifications and optical error measurements

Parameters Value Source

Receiver Absorptance 0.96 Manufacturer Glass-envelope transmittance 0.97 Manufacturer Mirror Solar-weighted specular reflectance 0.904 Measured Specularity—RMS 1.92 mrad Measured Slope error—mean value 4.0 mrad Measured, including the receiver position error Slope error—mean value with tracking offset 1.0 mrad Slope error—RMS 3.0 mrad Collector Tracking error 1 mrad Design parameter

031002-6 / Vol. 139, JUNE 2017 Transactions of the ASME Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/139/3/031002/6409184/sol_139_03_031002.pdf by guest on 01 October 2021

Fig. 12 Incidence-angle modifier (IAM) curve: circles mark the predicted data points, and the line indicates the fitting function

Fig. 11 Intercept factor calculation for all sampled collector modules with and without tracking-offset algorithm characterization results from Tables 5 and 6, the SAM solar field module can calculate the thermal power generation as a function of time. The design outlet temperature of the solar field is about Table 6 Stillwater solar field optical performance 200 C, but varies during real-world operation because the HTF mass flow cannot be readily controlled at the actual plant. No tracking offset With tracking offset Thermal energy generation results from SAM can be input to the INL geothermal power-cycle model. The INL model combines Nominal optical efficiency 0.696 0.840 these results with the solar heat input and geothermal heat to then Nominal intercept factor 0.827 0.991 calculate real-time power generation. The detailed modeling pro- kIAM;0 11cess and results can be found in Ref. [9]. kIAM;1 0.0138 0.0140 Three SAM cases are summarized in Table 7, and the hourly kIAM;2 0.2234 0.0622 k 0.1225 0.0909 output for a typical year in Stillwater, NV, is given in Fig. 13. IAM;3 Case 1 provides a reference annual performance for the Stillwater solar field by using the default performance metrics provided by the vendors; case 2 and case 3 are the performance modeling gðhÞ¼g IAMðhÞ¼qsac IAMðhÞ (10) based on the field test results given earlier in this paper. Case 2 o does not apply the tracking-offset algorithm, whereas case 3 does Here, h is the solar incidence angle in radians (always positive in include the algorithm. The comparison shows that the actual solar Eq. (9)), q is the parabolic mirror reflectivity, s is the receiver field performance will be about 9% lower than that expected from glass-envelope transmissivity, a is the average receiver coating the reference case (case 1). The vendor indicated that it was a rea- absorptivity, and c is the collector intercept factor. The nominal sonable value considering the storage history and fabrication optical performance and the IAM coefficients are provided in details of the Stillwater equipment. The greatest deviations are in Table 6. The IAM is also plotted as a function of incidence angle the mirror reflectance (3%) and geometric accuracy of the col- in Fig. 12. lector (7%). It is also shown that a significant boost in performance can be obtained if an appropriate tracking-offset algorithm is 6 Annual Thermal Generation From Solar Field implemented. In the perfect case, this tracking offset could boost After detailed solar field optical characterization, system the as-measured performance by almost 15%, as shown by case 3 advisor model (SAM) software can be used to calculate annual in Table 7. energy generation performance of a solar field [45]. SAM was developed by NREL to evaluate technical and economic perform- ance of renewable energy systems and is available to the public. 7 Discussion SAM allows users to configure a renewable energy system with Optical characterization is always a great challenge in the field, specific technology and evaluate its performance at certain loca- especially at a utility scale. Practically speaking, it is impossible tions under specific financial conditions. With a temporal weather to measure every solar collector module in the field; therefore, a resource file at a location, SAM can calculate annual performance statistical sampling procedure is implemented to ensure a certain of a solar field, such as hourly based thermal energy, electricity, level of measurement uncertainties. In reality, numerous factors and cash flow. Combined with system cost models, SAM can cal- affect the prediction of solar field performance. So the overall culate financial matrices of a solar energy system, such as level- uncertainty of the nominal optical/thermal efficiency may be at a ized cost of energy (LCOE) and internal rate of return (IRR). level of 3–5% [15]. By taking this as a reference, an appropriate Models for different renewable energy systems are regularly number of sampling measurements can be determined. In the real added and updated by NREL researchers. Currently, however, a world, the measurement accuracy is often constrained by certain solar/geothermal performance model is not available in SAM. The time demands, and the maximum number of measurements is typ- strategy to estimate the hybrid plant performance is to combine ically taken within the allowable test period. If the measurement SMA’s solar field performance module and a separate geothermal uncertainty is not acceptable, additional measurements will be power-cycle model developed by Idaho National Laboratory needed. (INL). The SAM solar field performance module uses the It is more complex to calculate the uncertainty of the average dimensions of the Stillwater solar field and the collector/receiver solar field optical efficiency. As shown in Fig. 11, with tracking parameters from the solar collector manufacturers (Huiyin [46]). offset, the collector intercept factor values are all above 0.99, and Using a local weather file at the hybrid plant location and optical their variation is not obviously correlated with the slope error

Journal of Solar Energy Engineering JUNE 2017, Vol. 139 / 031002-7 Table 7 Revised input parameters and annual energy output based on measured values at Stillwater

Case 2: revised value Case 3: revised value based on measurement based on measurement at Stillwater at Stillwater

Case 1: original value Without tracking-offset algorithm, from SAM default assuming maximum observed With tracking-offset SAM input parameter or vendor literature error at low tracking angles algorithm

Absorber absorptance (receiver) 0.963 0.96 0.96 Envelope transmittance (receiver) 0.964 0.97 0.97 Mirror reflectance 0.93 0.904a 0.904a Geometry effects 0.952 0.883 0.991 Tracking error 0.988 1b 1b IAM coefficient F0 1 1 1 Downloaded from http://asmedigitalcollection.asme.org/solarenergyengineering/article-pdf/139/3/031002/6409184/sol_139_03_031002.pdf by guest on 01 October 2021 IAM coefficient F1 0.0327 0.0138 0.0140 IAM coefficient F2 0.1351 0.2234 0.0622 IAM coefficient F3 0 0.1225 0.0909 SAM annual performance prediction for 11-loop, 24,778-m2 solar field Field thermal power produced (MWh/yr) 38,900 35,500 40,700 aThe measurement value accounts for damage to some panels prior to construction. bIncluded in geometry effects.

Fig. 13 Hourly SAM-predicted solar field thermal energy output for Stillwater based on measured parameters listed in Table 7 variation. This is because the relatively large receiver (80-mm field average solar-weighted reflectance and specularity is allowed diameter) can tolerate a larger optical error. Thus, the uncertainty by measurements of mirror specular reflectance at various accep- of solar field average intercept factor is very small compared with tance sizes and solar-weighted reflectance. The mirror slope error the uncertainty of the solar field average slope error. The domi- and receiver position error are measured by DO, one of very few nant source of solar field uncertainty is therefore the uncertainty tools suitable for both types of collector errors in the field condi- of the solar-weighted total specular reflectance. According to tion. During the optical test, sufficient measurement data were Eq. (10), the uncertainty of solar-weighted total reflectance results taken to obtain the solar field average values. To the authors’ in the same amount of uncertainty on the performance prediction, knowledge, this represents the most comprehensive optical char- assuming that the uncertainty from other parameters (all manufac- acterization in a utility-scale field. turer’s values) can be reasonably neglected. By employing the FIRSTOPTIC software, the measurement results It is also noted that not every optical error source is measured are readily interpreted to derive the solar field average optical in the field. Measuring certain optical metrics, such as receiver performance under normal incidence and non-normal incidence absorptance and receiver-glass transmittance, are not feasible in angle. Then, all measured and derived optical performance the field due to unavailability of required instruments. The sun metrics are input to SAM for the thermal energy annual perform- shape is always varying under instantaneous weather conditions. ance at an hourly basis by using local satellite weather data. This In addition, the work in this paper measured mirror reflectance, analysis indicates that annual delivered energy from the solar field mirror specularity, mirror slope error, and receiver position error, would be about 9% lower than the collector manufacturer’s and it provides the most comprehensive optical characterization prediction if an appropriate tracking-offset algorithm is not imple- of a large-scale solar field. The metrics missed by the analysis are mented. The analysis also shows that further calibration incorpo- mirror slope error along the longitudinal direction and collector rating an SCA tracking-offset algorithm can potentially improve error as a function of tracking position. The former was shown to performance by as much as 15%. have a minimal impact on collector optical performance compared The detailed performance predictions from SAM will serve as with the measured mirror slope error along transversal direction an input to INL’s plant model of the Stillwater power block, thus [44,47]. The impact of the latter is still unknown. allowing engineers to model plant performance and explore opti- mal integration strategies of solar thermal power into a geother- mal binary power-cycle. 8 Conclusions The work in this paper illustrates a detailed process on how To conclude, a comprehensive optical test has been performed to collect and interpret various types of measurement data by at the Stillwater plant solar field. First, the calculation of solar using state-of-the-art optical tools and models, and it will serve

031002-8 / Vol. 139, JUNE 2017 Transactions of the ASME as a valuable guideline in the area of solar field optical Power Plants—A Study on the Performance and Economy of Integrated Solar characterization. Combined Cycle Systems,” Energy, 29(5–6), pp. 947–959. [8] Turchi, C., Zhu, G., Wagner, M., Williams, T., and Wendt, D., 2014, “Geothermal/Solar Hybrid Designs: Use of Geothermal Energy for CSP Feed- Acknowledgment water Heating,” Geothermal Resources Council 38th Annual Meeting, Portland, OR, Sept. 28-Oct. 1, OSTI No. 1170300. The work at NREL was supported by the U.S. Department of [9] Wendt, D., Mines, G., Turchi, C., Zhu, G., Cohan, S., Angelini, L., Bizzarri, F., Energy under Contract No. DE-AC36-08-GO28308. Funding was Consoli, D., and A. De Marzo, 2015, “Stillwater Hybrid Geo-Solar Power Plant Optimization Analyses,” Geothermal Resources Council Annual Meeting, supplied by the DOE’s Geothermal Technologies Office (GTO). Reno, NV, Sept. 20-23, OSTI No. 1253710. 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