Different Experimental Techniques for Solar Flux

Measurement

Presented by:

Manojkumar Maurya

605008

Introduction

Different solar flux measurement techniques:1. direct method2. indirect method3. measurement supported simulation methods

In direct method, flux sensors are used directly to measure the flux

In indirect method, CCD camera are used measure solar radiation

Simulation should be supported direct or indirect method results

Classification

Following are some of the experimental technique to measure solar flux:

Indirect method

1. Flux density measurement by using a white diffuse moving bar target and CCD camera

2. Flux density measurement using a CCD camera on external surface

3. Flux density measurement by using a stationary stripe- shaped target and moving focus

Direct method

1. Flux density measurement by using a moving bar with mounted sensors

2. Flux density measurement by using stationary sensors

1. Flux density measurement by using a white diffuse moving bar target and CCD camera

• CCD cameras often used due to their dynamic range and signal quality

• The intensity of reflected solar light is calibrated using solar flux guage

• This principle is used since the end of 1970s in different R&D projects, eg:

Beam Characterization System (BCS) of Sandia (USA), Flux Analyzing

System of EIR (Switzerland), heliostat and receiver measurement system

(HERMES I+II, ProHERMES) of the German Aerospace Center (DLR),

Plataforma Solar de Almeria (PSA)

• Let us consider the “ProHERMES” system at the Plataforma Solar de

Almeria(PSA)

ProHERMES measurement system

• A moving bar is used to scan the whole focal area while a camera takes several photos

• After the image acquisition , the target region is cropped in each image and combined to a surface covering the whole region of interest

• The resulting image is corrected for camera non - linearities, dark current, shading and finally calibrated by a radiometer that is mounted directly behind the moving bar plane

• A periodic calibration of the radiometer is necessary

• The moving bars used in the ProHERMES have been found to working without any mechanical problems

• Different possible mechanisms of moving bars are shown

Possible mechanism of moving bars

Continued…

• Rotating moving bars are a good choice for small scale prototype receiver, as only one drive is needed to run them

• However, up - scaling of rotating bars is critical because of the high angular momentum and the influence of wind

• Up – scaling is possible by switching over the concept of linear movement of the bar

• For external receivers, vertical linear moving bar may be used which are moved along the circumference of receiver

• In case of cavity receivers, a feasible option is a horizontal linear moving bar with two motors operated in master/slave, master/slave operation helps to avoid tilting of moving bar

Continued…

• Ulmer et al. reported a low total measurement uncertainty of solar flux density, being in the range of -4.7% to +4.1%

• The highest contribution to uncertainty is caused by the uncertainties of calibration of the radiometer about 3%

• Strachan and Houser reports for the BCS a total error of 6%, being the calibration uncertainty of the flux gauge the principle source of uncertainty 5%

• Then there are uncertainties caused by distance between the measurement plane and aperture plane, spectral effects, positioning and sizing during image processing

2. FDM on external receiver surface using a CCD camera

• In case of external receivers, the receiver itself can be used as measurement target

• In this method, moving bars are not used and the calibration of reflected intensity from the receiver is avoided

• The PHLUX method by Ho and Khalsa uses a recorded image of sun, DNI reading and the reflectivity of the target to calibrate the brightness distribution of target surface

• This technique can be applied to cavity receivers, provided the camera can see the whole receiver cavity surface

• The PHLUX method would be explained later

3. Flux density measurement by using a stationary stripe-shaped target and moving focus

Continued…• This method mentioned by Pacheco et al. uses a stationary strip shaped target and

sweeping the focus over the fixed target

• The stripe shaped images are then merged to gain a composite flux image

• Pacheco et al. report a test with 30 heliostat moved over a horizontally oriented striped target

• A stripe target is mounted on the west side of the receiver

• Instead of moving the whole focus over a water cooled target at one instant, the authors suggested splitting up the focus in heliostat group

• 80% tracks on receiver and 20% is swept over the target in steps

• This avoids the stopping of receiver operation and the use of actively cooled target

Continued….

• A CCD image is taken, when the nearest heliostat has reached its new aim point, then the aim point is shifted to take the next image

• After the scan of these heliostat group, the images are assembled together

• A flux sensor calibrates the resulting gray value distribution

• After evaluation of all the heliostat group, the flux images are added up to get the final result

• The measurement uncertainties are related to the accuracy of heliostat tracking and the number of heliostat involved

• In a simulation study involving 120 heliostat, if a tracking error of 0.65mrad is considered, the uncertainty in the flux distribution is in the range of -6% to +5.5%

• If tracking error is increased to 1.6mrad, uncertainty would increase upto -9.9% to +8.8%

4. FDM using a moving bar with mounted sensors

• A moving bar can be directly equipped with sensors that scan the aperture surface during the movement

• Gardon flux gauge would scan that would take several minutes resulting in the necessity of active water cooling system

• For this reason, Ballestrin and Monterreal used in their MDF system thin film heat flux sensors having a fast time constant and allowing a rapid scan without water cooling

• They can operate upto 8500 C and measure heat flux and temperature by creating small temperature across the thermal resistance element of a thermopile

• For the application in solar heat flux measurement, they were coated with a black painting with a an absorption of 94%

• Due to the small size, numerous sensors can be mounted along the moving bar

• High frequency data acquisition data system allows a high density of measurement points without reducing bar speed and excessive heating of sensor

Continued….• Based on the work of Ballestrin et. al. , esolar designed, built and tested a

pneumatically actuated, 6.1m long rotary moving bar equipped with several thin film heat flux sensors in their plant, capable of measuring flux densities upto 5 MW/m2

• Being a direct measurement type, uncertainties caused by camera and target properties are avoided

• The accuracy of the heat flux microsensors is reported to ±3%

• They also integrated with the possibility of measuring heat flux by an indirect method using a remote camera and the moving bar

5. FDM by using stationary sensors

• Moving parts are avoided

• Heat flux maps are created by interpolation between measurement points

• Numerous sensors are required for accurate results

• Measurement accuracy at sensor location is high

• Spatial resolution may only be moderate when excessive number of sensors are not used

• Maintaining the reliable operation of high number of sensor in high temperature zone is a challenging task

• The lifetime of flux gauges in the receiver environment is about 6 months

• Stationary sensors were used at the PS 10 plant in Seville

The PHLUX Method• The method uses a CCD camera but does not need to have additional sensors,

calorimeters or the flux gauges

• The additional information required other than image recorded from CCD camera is the DNI and the reflectivity of the receiver

• The novel feature of PHLUX is using the recorded images of the sun to scale both the magnitude of each pixel value and the subtended angle of each pixel

• A test was performed to evaluate the PHLUX method using a heliostat beam on the receiver tower at National Solar Thermal Test facility in Albuquerque, New Mexico

• Flux map of the heliostat beam obtained with relative error in the peak flux of 2%

Pixel conversion using sun calibration

Continued….• Consider one square pixel, i, on a raw grayscale CCD image of a receiver

• This pixel captures an elemental portion of the receiver AR,i (sq. m), which receives an irradiance, ER,i [w/ m2], from the heliostat field

• The CCD response due to irradiance at this pixel is expressed in the arbitrary voltage units per pixel value and px denotes the unit length of a pixel, VCCD,i

[volts/px2 ]

• The recorded signal VCCD,i be converted the receiver irradiance ER,i

• The irradiance on the pixel, ECCD_w [w/px2], is given by:

Continued…

Continued….• The radiant intensity in the normal direction, IN [W/sr], in eq. 1 is calculated as

follows

(2)

• The solid angle, dΏ [sr], in eq. 1, can be determined assuming that the radius of the camera iris is small compared to r[m], which is the distance between the receiver and the camera:

• Substitute eq.(2) and eq.(3) in eq.(1) yields the following equation for pixel irradiance

Continued…• The pixel irradiance in eq.(4) can be expressed in terms of the CCD response, VCCD,I

by using a conversion factor between watts and volts

• The W/volt ratio is equal to the ratio of the power that entered the camera in the sun image to the sum of the pixel values within the sun image:

• Dividing equation 4 by equation 5, following equation for the CCD response, VCCD,i

• The receiver element area, AR,I [m2], in equation 6 can be expressed as follows:

Continued….

Continued…• ωR in equation 7, can be determined using the sun image as a reference for

images/angles projected between the nodal point and the CCD

• Combining equations (6 – 8) yields the following equation for the irradiance on a receiver element, ER,I , as a function each pixel value, VCCD,i ,

• From the above equation, irradiance at the receiver can be calculated

Error sources:• The accuracy of the method to determine the irradiance on a receiver from digital

images depends on the accuracy of the quantities measured in equation 9

• Ulmer et al. provide a error estimates for camera linearity, noise and spectral influences, which can be caused by a non – constant filter transmission as a function of radiation wavelength, each factor cause an error of approximately ±0.5%

• The attenuation factor is expected to contribute errors to the measured irradiance

• The PHLUX method assumes that the receiver is a Lambertian reflector

• Errors in Lambertian properties of the target were estimated by Ulmer et al. to cause errors in pixel values ranging from -2% to +5%

• The maximum total error was estimated to be approximately 10%

Summary of test condition and parameters

Testing and analyses

• The calculated peak flux is 10.4kW/ m2 with the accuracy of ±3%

• The relative error between the measured and predicted peak flux was 2%

• Additional tests are performed to determine the flux distribution for different heliostats and multiple heliostats to evaluate the PHLUX method

Summary• The direct and indirect method(1 and 4), are attractive due to low

measurement uncertainty, however the moving bars cause more costs and may have higher susceptibility to loads and environment

• In the second method explained, higher measurement uncertainty is observed, but technologically simple

• The third method may deliver reasonable receiver input power but is not recommended for aimpoint control or to measure the flux distribution

• The fifth method is good for measuring receiver input power, if sufficient number of sensors is used, but has reliability and cost issues

• During the last decades, technique 1,4 and 5 are widely used whereas technique 2 and 3 are in development phase

References [1] Marc Roger et. al., Techniques to measure solar flux density distribution on large scale receivers, Journal of

Solar Energy Engineering, august 2014

[2] Clifford K Ho, Siri Khalsa, A flux mapping method for central receiver system, Proceedings of the ASME 2011, 5th International conference on Energy Sustainablity

[3] J. Ballestrín, R. Monterreal, Hybrid heat flux measurement system for solar central receiver evaluation, Energy 29, 5-6 (2004) 915-924.