Buildings.Fluid.SolarCollectors.UsersGuide
User's Guide
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 Buildings.Fluid.SolarCollectors.BaseClasses. Accordingly, data records for both test methods are available in Buildings.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:
Input data | SRCC | Solar Keymark | SPF |
---|---|---|---|
A | Gross area | Gross area | Gross, aperture, or absorber area |
C | (mDry*385, V) | C | (CDry or mDry*385, V) |
mperA_flow_nominal | mperA_flow_nominal | mperA_flow_nominal | m_flow_nominal/A |
dp_nominal | / | / | dp_nominal |
incAngDat, incAngModDat | IAM({0°,10°,...,90°}) | IAM({10°,20°,...,90°}) | IAM(angle)-plot |
y_intercept, slope (ASHRAE93) | y_intercept, slope | / | / |
IAMDiff, eta0, a1, a2 (EN12975) | IAM(50°), eta0, a1, a2 | Kd or IAM(50°), eta0, a1, a2 | IAM(50°), eta0, a1, a2 |
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 Buildings.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 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 areNumber
, allowing the user to enter a number of panels, orTotalArea
, allowing the user to enter a system area.-
Number
: IfNumber
is selected fornColType
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
: IfTotalArea
is selected fornColType
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 areSeries
andParallel
. The handling ofdp_nominal
is changed depending on the selection.-
Series
: IfSeries
is selected it is assumed that all panels in the system are connected in series. As a result there is a pressure drop corresponding todp_nominal
for each panel and the effectivedp_nominal
for the system isdp_nominal
*nPanels
. -
Parallel
: IfParallel
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 thedp_nominal
for the system isdp_nominal
specified in the collector data package if the collector field has a mass flow rate equal tom_flow_nominal
. -
Array
: IfArray
is selected it is assumed that the panels are mounted as a rectangular array withnPanelsPar
rows in parallel. As a result, the pressure drop is calculated usingdp_nominal = nSer * per.dp_nominal
, wherenSer
is the number of panels in series, which is internally computed, andper.dp_nominal
is the pressure drop of a panel as obtained from the performance recordper
at the respective nominal mass flow rate.
-
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.
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