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.

Solar Collector Assembly (SCA)

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

Current SCA inputs

The name of the collector in the SCA library

SCA Length (m)

Length of a single SCA. Used in SCA end loss calculation.

SCA Aperture (m)

Mirror aperture or width of a single SCA. Used in the row-to-row shadowing loss factor and HCE thermal loss calculations.

SCA Aperture Area (m2)

The net aperture area of a single SCA. Used in the solar field size calculations.

Average Focal Length (m)

Average trough focal length. Used in end gain and end loss factor calculations.

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 Reflectance

The mirror reflectance input is the solar weighted specular reflectance.  The solar-weighted  specular reflectance is the fraction of incident solar radiation  reflected into a given solid angle about the specular reflection direction. The appropriate choice for the solid angle is that subtended by the receiver as viewed from the point on the mirror surface from which the ray is being reflected. For parabolic troughs, typical values for solar mirrors are 0.923 (4-mm glass), 0.945 (1-mm or laminated glass), 0..906 (silvered polymer),  0.836 (enhanced anodized aluminum), and 0.957 (silvered front surface).

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.

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 equation for calculated values 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 calculate the HCE heat loss. The default values are based on NREL modeling and test results. (See Forristall R, 2003. Heat Transfer Analysis and Modeling of a Parabolic Trough Solar Receiver Implemented in Engineering Equation Solver. National Renewable Energy Laboratory NREL/TP-550-34169. http://www.nrel.gov/csp/troughnet/pdfs/34169.pdf.)

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.

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.

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 16. Default collector types.

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

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 17. Default HCE types.

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 18. HCE conditions.

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,

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:

Where,

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:

Where,

Thermal Losses (Weighted W/m)

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

Where,

Thermal Losses (Weighted W/m2)

And the SCA aperture area:

Where,