SCA / HCE

To view the SCA / HCE page, click Solar Field on the main window's navigation menu. Note that for the SCA / HCE page to be available, the technology option in the Technology and Market window must be Concentrating Solar Power - Parabolic Trough System.

The SCA / HCE page displays the characteristics of the solar collector assembly (SCA) and heat collection elements (HCE) in the solar field. Note that the SCA is often referred to as the collector. The HCE is often referred to as the receiver.

A solar collector assembly (SCA) is an individually tracking component of the solar field that includes mirrors, a supporting structure, and heat collection elements or receivers.

A heat collection element (HCE) is a metal pipe contained in a vacuum within glass tube that runs through the focal line of the trough-shaped parabolic collector. Seals and bellows ensure that a vacuum is maintained in each tube. Anti-reflective coatings on the glass tube maximize the amount of solar radiation that enters the tube. Solar-selective radiation absorbing coatings on the metal tube maximize the transfer of energy from the solar radiation to the pipe.

Note. See http://www.nrel.gov/csp/troughnet/solar_field.html for more information on solar collector assemblies and heat collection elements. Also see relevant articles in the list of publications on the Troughnet website.

For a more detailed description of the model, please download the CSP trough reference manual from the Solar Advisor website's support page: https://www.nrel.gov/analysis/sam/support.html.

Contents

►	Input Variable Reference describes the input variables and options on the SCA / HCE page.

►	About the SCA Parameters describes the physical charateristics of the four SCAs included in the default library.

►	About the HCE Parameters describes the four HCE (receiver) types and five HCE conditions included in the default library.

►	About the Mirror Reflectivity Value describes guidelines for choosing a mirror reflectivity value.

►	Equations for Calculated Values describes the equations used to calculated the calculated values on the SCA / HCE page.

Input Variable Reference

Solar Collector Assembly (SCA)

The solar collector assembly (SCA) input variables describe the dimensions and optical characteristics of the SCA or collector.

Name	Description	Units
Current SCA inputs	The name of the collector in the SCA library
SCA Length	Length of a single SCA. Used in SCA end loss calculation.	m
SCA Aperture	Mirror aperture of a single SCA. Used in the row-to-row shadowing loss factor and HCE thermal loss calculations.	m
SCA Aperture Area	Area of aperture of single SCA. Used in the solar field size calculations.	m2
Average Focal Length	Average trough focal length. Used in end gain and end loss factor calculations.	m
Incident Angle Mod Coeff (1-3)	Incident angle modifier coefficients. Used to calculate the incident angle modifier factor, which is used to calculate the HCE absorbed energy and the solar field optical efficiency.	--
Tracking Error and Twist	Accounts for errors in the SCA's ability to track the sun. Sources of error may include poor alignment of sun sensor, tracking algorithm error, errors caused by the tracker drive update rate, and twisting of the SCA end at the sun sensor mounting location relative to the tracking unit end. A typical value is 0.985. Used to calculate SCA field error factor.	--
Geometric Accuracy	Accounts for SCA optical errors caused by misaligned mirrors, mirror contour distortion caused by the support structure, mirror shape errors compared to an ideal parabola, and misaligned or distorted HCE. A typical range of values is between 0.97 and 0.98. Used to calculate SCA field error factor.	--
Mirror Reflectivity	The solar-weighted hemispherical reflectance of the mirrors. For 4-mm low iron, pristine, second surface tempered glass mirrors, a reasonable value would be 0.95. Used to calculate SCA field error factor.	--
Mirror Cleanliness Factor (avg)	Accounts for dirt and dust on the mirrors that reduce their effective reflectivity. Typically, mirrors are continuously cleaned, but a single mirror may be cleaned once each one or two weeks. The expected overall effect on the total solar field would be an average loss of between one and two percent. A typical value would be 0.985. Used to calculate SCA field error factor.	--
Dust on Envelope (avg)	Accounts for dust on the HCE envelope that affects light transmission. A typical value would be 0.99. Used to calculate HCE heat loss.	--
Concentrator Factor	A additional error factor to make it possible to adjust the SCE performance without modifying the other error factors. Useful for modeling an improved or degraded SCE. The default value is 1. Used to calculate SCA field error factor.	--
Solar Field Availability	Accounts for solar field down time for maintenance and repairs. Used to calculate absorbed energy.	--

Heat Collection Element (HCE)

The HCE variables describe the properties of up to four HCE types that can make up the solar field. This makes it possible to model a solar field with HCEs in different states. Each set of properties applies to one of the HCE types. The Fraction of Field variable determines what portion of the solar field is made up of a given HCE type.

Name	Description	Units
Current HCE inputs	The name of the receiver and its condition. Vacuum refers to an HCE in good condition, lost vacuum, broken glass, and hydrogen refer to different problem conditions. You can define up to four HCE (receiver) conditions.	--
Fraction of Field	Fraction of solar field using this HCE type and condition. Used to calculate HCE field error factor and HCE heat loss.	--
Bellows Shadowing	The portion of the HCE tube that does not absorb solar thermal radiation. Used to calculate HCE field error factor.	--
Envelope Transmissivity	Used to calculate HCE field error factor.	--
Absorber Absorption	Accounts for inefficiencies in the HCE black coating. Used to calculate HCE field error factor.	--
Unaccounted	Allows for adjustment of the HCE performance to explore effect of changes in performance of the HCE without changing the values of other correction factors. A typical value is 1. Used to calculate HCE field error factor.	--
Optical Efficiency (HCE)	The design optical efficiency of each of the four receiver type and condition options. SAM uses the values to calculate the design weighted optical efficiency.	--
Optical Efficiency (Weighted)	The design weighted optical efficiency, representing the average optical efficiency of all receivers in the field (see equations below). SAM uses the value to calculate the solar field area. Note that SAM also calculates a separate HCE optical efficiency value for each hour during simulation that counts for the loss factors on the SCA / HCE page that also accounts for the incident angle modifier factor, which depends on the time of day and collector orientation.	--
Heat Loss Coefficient A0...A6	Used to calculated the HCE heat loss. The default values are based on NREL modeling and test results. (See Forristall 2003 in References.)	--
Heat Loss Factor	The design heat loss factor that applies to the active HCE type and condition. Used to calculate design HCE heat loss that is part of the solar field area equation. The heat loss factor scales the heat loss equation and can be used to fine tune the results when measured heat loss data are available. The default value of 1.0 is valid for the current version of SAM using the default heat loss coefficients.	--
Min windspeed (m/s)	Used to calculated the HCE heat loss for hours when the wind speed from the weather file is lower than the minimum wind speed.	m/s
HCE Heat Losses (W/m) Thermal Losses (Weighted W/m) Thermal Losses (Weighted W/m2)	These values are provided for reference. SAM calculates the HCE heat loss for each hour during simulation based on the loss factor coefficients on the SCA / HCE page and other values from the weather data.	W/m, W/m2

About the SCA Parameters

The default SCA library includes a set of parameters for four types of SCAs described in the table below. These SCA types are either installed in currently operating systems, or were used in past system designs. See Working with Libraries for information about managing libraries.

Table 18. Default collector types.

Name	Description	Location
Euro Trough ET150	Torque box, galvanized steel	SEGS V, Kramer Junction, California
Luz LS-2	Torque-tube, galvanized steel	SEGS I - VII, Kramer Junction, California
Luz LS-3	Bridge truss, galvanized steel	SEGS VII - IX, Kramer Junction, California
Solargenix SGX-1	Organic hubbing structure, extruded aluminum	Nevada Solar One, Boulder City, Nevada

The values of input variables on the SCA / HCE page are stored in libraries. See Working with Libraries for information about managing libraries.

About the HCE Parameters

The HCE library includes four HCE types, and for each HCE type, five HCE conditions. See Working with Libraries for information about managing libraries.

For each HCE type and condition, you can assign a Percent of Field value. For example, in the figure below, the receiver type is Schott PTR70, and 98.5% percent of the HCEs are in normal condition, 1.0% have lost vacuum, 0.5% have glass damage, and 0% have allowed hydrogen to enter the tube.

When you select a name from the Receiver Type and Condition list, Solar Advisor populates the optical and heat loss parameters using values stored in the library. When you change one or more of these values, Solar Advisor creates a copy of the parameter set and adds it to the library under the name "CUSTOM CUSTOM."

The four HCE types are described in the table below.

Table 19. Default HCE types.

HCE Type	Description
Luz Cermet	Original HCE design. Low reliability of seals.
Schott PTR70 Vacuum	Newer design with improved reliability.
Solel UVAC2	Newer design with improved reliability.
Solel UVAC3	The newest HCE available as of May 2008.

The performance of the HCE is highly dependent on the quality of the vacuum in the glass tube. Solar Advisor models the HCE under the five conditions described in the following table.

Table 20. HCE conditions.

HCE Condition	Description
Broken glass	Glass tube is damaged, increasing heat transfer between tube and atmosphere.
Fluorescent	Selective coating on metal tube is compromised, reducing absorption of solar radiation
Hydrogen	Hydrogen from hydrocarbon-based heat transfer fluid (e.g., mineral oil) has permeated through metal tube into the vacuum, increasing heat transfer between metal tube and glass.
Lost vacuum	Glass-to-metal seal is compromised
Vacuum	HCE is not damaged and is operating as designed.

About the Mirror Reflectivity Value

The following information is intended to help choose a value for the mirror reflectivity factor. The solar weighted hemispherical reflectance (SWV) of mirror glass depends on the iron content, thickness, and tempering of the glass, and the thickness of the reflective coating of the mirror:

•	Glass transmittance and mirror reflectivity both depend on the iron (Fe2O3) content of the glass. The higher the iron content, the lower the transmittance and the higher the reflectivity of the mirror. Iron contents of more than 0.02% typically result in unacceptably low mirror reflectivity values.

•

Mirror reflectivity increases as glass thickness decreases. The thinner glass requires faster pulling during manufacturing and is easier to break during shipping and handling than thicker glass. A glass thickness of one millimeter mounted with a substrate is a reasonable compromise to maximize mirror reflectivity and minimize the risk of mirror breakage. Five millimeter thick, non-tempered, low-iron, self-supporting glass mirrors are typically recommended for mirrors at the periphery of the parabolic trough field that are exposed to wind. Normally, five to ten percent of a solar field is equipped with 5 mm glass.

•	Glass tempering generally raises mirror reflectivity.

•	Mirror coating typically uses a silver thickness between 800 - 1200 Å or 0.8 -1.2 g/m2. Silver thicknesses less than 0.8 g/m2 result in unacceptably low mirror reflectivity values. Silver thicknesses greater than 1.2 g/m2 do not improve reflectivity, and have a tendency to delaminate.

Table 21. Suggested mirror reflectivity values for different types of commercially available glass solar mirrors using pristine second surface glass.

Glass Thickness (mm)	Iron Content	Mirror Reflectivity
4	low	0.93 ±0.002
1	low	0.96 ±0.002
4	low	0.948 ±0.003
4	very low	0.946 ±0.001
3	very low	0.956 ±0.001

Equations for Calculated Values

Optical Efficiency (HCE)

The design optical efficiency of each receiver type and condition option is a function of the efficiency and loss factors for each option.

Where,

FOptEffD,n	Optical Efficiency (HCE) for each of the four receivers types.
FSCAFieldError,n	The SCA field error factor, which is the product of Tracking Error and Twist, Geometric Accuracy, Mirror Reflectivity, Mirror Cleanliness Factor and Concentrator Factor. (Note that the Dust on Envelope factor is used for the HCE field error calculation above, not here.)
FDustEnvelope,n	Dust on Envelope (avg) specified in the SCA parameters above. The same value applies to each of teh four receiver types.
FBellows,n	Bellows Shadowing for the receiver type n.
FTransmissivity,n	Envelope Transmissivity for the receiver type n.
FAbsorption,n	Absorber Absorption for the receiver type n.
FUnaccounted,n	Unaccounted for the receiver type n.
n	The receiver type number (1 through 4)

Optical Efficiency (Weighted)

The design weighted optical efficiency is a design value that Solar Advisor uses to calculate the solar field area. Note that the design optical efficiency equations differ from the optical efficiency factor equations used in the hourly simulation. It is a function of the four design optical efficiency factors and fraction of field values for each receiver type option:

EQ_F-OptEffD

Where,

FOptEffD	Optical Efficiency (Weighted)
FOptEffD,n	Optical Efficiency (HCE) for each of the four receivers
FPercentOfField,n	Percent of Solar Field for each of the four receivers
n	Receiver number (1 through 4)

HCE Heat Losses (W/m)

The heat loss for each HCE type depends on the value of the six heat loss coefficients and heat loss factor for each HCE type, and on the solar field heat transfer fluid temperature design parameters:

EQ_SCAHCE-heatlossterms

Where,

QHCELosses (W/m)	Receiver Heat Losses
FHeatLossFactor	Heat Loss Factor
TSFout (°C)	Solar Field Outlet Temperature from the Solar Field page
TSFin (°C)	Solar Field Inlet Temperature from the Solar Field page
FA0 ... FA6	A0 Heat Loss Coefficient through A6 Heat Loss Coefficient
nWind (m/s)	Reference wind velocity from the Solar Field page
TAmb (°C)	Reference ambient temperature from the Solar Field page

Thermal Losses (Weighted W/m)

The total, or weighted HCE losses are expressed both in terms of the SCA aperture length:

EQ_Q-HCELossesWm

Where,

QHCELossesWeightedW/m (W/m)	Thermal Losses per SCA aperture length.
QHCELosses,n (W/m)	Receiver Heat Losses for receiver number n
FPercentOfField,n	Percent of Solar Field for each of the four receivers

Thermal Losses (Weighted W/m2)

And the SCA aperture area:

Where,

QHCELossesWeightedW/m2 (W/m2)	Thermal Losses per SCA aperture area.
QHCELossesWeightedW/m (W/m)	Thermal Losses per SCA aperture length
ASCAAperture (m2)	SCA Aperture Area