.AixLib.Fluid.SolarCollectors.UsersGuide

Information

This package contains models for solar thermal systems. Top-level models are available for solar thermal collectors based on the ASHRAE93 (American) and EN12975 (European) test protocols. The two models use different models for solar gain, heat loss, and use different data packages. The model applied to (un)glazed flat-plate solar thermal collectors, as well as evacuated tube collectors.

Model description

The solar thermal collector model is developed based on the flat-plate solar thermal collector model of EnergyPlus. The model determines the solar heat gain and heat loss of the collector seperately, and the difference of both is transferred to the collector. The ASHRAE93 and EN12975 collector model calculate the heat gain and heat loss differently. The details of these calculations can be found in AixLib.Fluid.SolarCollectors.BaseClasses. Accordingly, data records for both test methods are available in AixLib.Fluid.SolarCollectors.Data.

Performance data

Different sources exist to find ratings data of individual collectors. However, not all data might be available in one single data sheet. The table below specifies which input data of the model can be found in several well-known data sources:

Some extra important remarks regarding the performance data:

• Different areas can be defined for a solar collector: the gross, absorber, and aperture area. The performance parameters of the solar collector vary depending on the area for which they are defined. Therefore, the performance parameters used in the data record should match the area that is used.
• When the thermal capacity of the solar collector without fluid is not known, the thermal capacity is calculated based on the dry mass of the collector and the specific heat capacity of copper (385 J/kg/K).
• All data sources report a nominal mass flow rate (per unit area of collector), but only SPF reports a corresponding nominal pressure drop. If a specific collector is used that is not included in the SPF database, one can likely find this via the manufacturer (website or on request). Some examples of (mperA_flow_nominal, dp_nominal can be found in AixLib.Fluid.SolarCollectors.BaseClasses.
• Pressure drops depend on the medium that is used in the collectors. If the modelled solar thermal collector uses a different medium than the medium that was used to determine the nominal pressure drop in a data sheet, one should therefore correctly take this into account (e.g. using an empirical correction factor).

• The relation between the incidence angle modifier (IAM) and incidence angle θ is calculated using cubic splines and measurement data provided in the data sheets.

• Evacuated tube collectors have bi-axial IAMs due to its axisymetric geometry. Therefore, data sheets report both a longitudinal and transversal IAM. The model however only allows the definition of one (symmetrical) IAM. Two possible approaches to deal with this are:
  • multiplying the longitudinal and transversal IAM;
  • using either the longitudinal or transversal IAM.
  The model should therefore be used with care when dealing with evacuated tube collectors.
• The Solar Keymark database sometimes reports a value for Kd which is the incident angle modifier for diffuse irradiance. This value differs from the IAM at an incidence angle of 50 degrees because the former is determined by integrating the values of the IAM for all incidence angles over the hemisphere.

Other model parameters

Apart from the performance parameters, several other parameters must be defined. Most of the parameters are self-explanatory. The complex parameters are used as follows:

• nSeg: This parameter refers to the number of segments between the inlet and outlet of the system, not the number of segments in each solar thermal collector.
• nColType: This parameter allows the user to specify how the number of collectors in the system is defined. Options are Number, allowing the user to enter a number of panels, or TotalArea, allowing the user to enter a system area.
  • Number: If Number is selected for nColType the user enters a number of panels. The simulation then identifies the area of the system and uses that in solar gain and heat loss computations.
  • TotalArea: If TotalArea is selected for nColType the user enters a desired surface area of panels. The model then uses this specified area in solar gain and heat loss computations. The number of panels in the system is identified by dividing the specified area by the area of each panel.
• SysConfig: This parameter allows the user to specify the installation configuration of the system. Options are Series and Parallel. The handling of dp_nominal is changed depending on the selection.
  • Series: If Series is selected it is assumed that all panels in the system are connected in series. As a result there is a pressure drop corresponding to dp_nominal for each panel and the effective dp_nominal for the system is dp_nominal * nPanels.
  • Parallel: If Parallel is selected it is assumed that all panels in the system are connected in parallel. As a result the fluid flows through only a single panel and the dp_nominal for the system is dp_nominal specified in the collector data package if the collector field has a mass flow rate equal to m_flow_nominal.
  • Array: If Array is selected it is assumed that the panels are mounted as a rectangular array with nPanelsPar rows, each having nPanelsSer panels in series. As a result there is a pressure drop corresponding to dp_nominal for each panel per row and the effective dp_nominal for the system is dp_nominal * nPanelsSer.

References

ASHRAE 93-2010 -- Methods of Testing to Determine the Thermal Performance of Solar Collectors (ANSI approved).

CEN 2022, European Standard 12975:2022, European Committee for Standardization.

EnergyPlus 23.2.0 Engineering Reference.