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<< Click to Display Table of Contents >> Array Thermal losses |
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Thermal Model
The objective is to evaluate the array (or cell) temperature during the simulation. The cell temperature is a basic input parameter of the one-diode model.
This is evaluated by an energy balance, accounting for all incoming and outcoming energy fluxes in the array.
At the thermal equilibrium, the energy fluxes should be compensated by the array cooling thermal loss, which is mainly convective:
Ginc · Alpha · (1 - Effic) = U · (Tcell - Tamb)
where:
oGinc is the incoming irradiance on the module or PV array. PVsyst uses GlobInc for simplicity, although the relevant value should be the effective irradiance GlobEff.
oAlpha is the absorption coefficient of solar irradiance, i.e. (1 - reflection). The usual value chosen for the Absorption coefficient Alpha is 0.9. It is eventually modifiable in the PV module definition dialog, but this is not recommended. The measured U-value is closely related to the choice of Alpha.
oEffic is the PV efficiency (related to the module area), i.e. the electrical energy removed from the module. When possible, the PV efficiency is calculated according to the operating conditions of the module. Otherwise it is taken as 15%.
oTamb is the ambient temperature, according to the meteo data,
oU is the "heat loss factor", expressed in [W/m²·k]. This is a heat transfer coefficient, determining the heat flux as proportional to the temperature difference between two media. This U-value is quite equivalent to the Heat transfer factor [W/m²·k], used in building physics for the characterization of walls or windows.
The left side of this equation describes the different energy fluxes specific to a PV array, the right side defines the necessary heat transfer for ensuring the thermal equilibrium.
From this equation, we can easily extract the cell temperature:
Tcell = Tamb + 1 / U · ( Alpha · Ginc · (1 - Effic) )
U-value
The heat loss factor is the main input parameter used during the simulation for the evaluation of the PV array behavior due to the temperature with respect to running at 25°C (may be a loss when TArray greater than 25°C, or a gain beloiw).
However this parameter may be varying with the wind speed: as the heat loss is mainly convective, it is sensitive to the air circulation on the array. Therefore this parameter can be split into a constant component Uc and a factor proportional to the wind velocity Uv :
U = Uc + Uv · WindVel (Uc in [W/m²·k], Uv in [W/m²·k / m/s], WindVel = wind velocity in [m/s]).
Using the wind dependency Uv is really difficult. Reliable knowing of the wind velocity is seldom in the meteorological datasets. When the monthly values are specified, some programs construct synthetic hourly values from these monthly data, but on which basis and with which models ?
NB: According to the meteorological standards, the wind velocity should be measured on a mast at a10 meter height, in a free environment. This is rarely the case when we measure a wind velocity for the monitoring of a PV system. The value at the collectors level may be lower by a factor of -35 to -50%. Therefore the Uv-value should be adapted to the way of recording the wind velocity.
U-value determination
The U-factor depends on the mounting mode of the modules (sheds, roofing, facade, floating system, etc...).
For free circulation all around the tables (i.e sheds arrangement), this coefficient refers to both faces, i.e. twice the area of the module. If the back of the modules is more or less thermally insulated, this should be lowered, theoretically up to half the value when the back side is fully insulated (i.e.doesn't participate anymore to thermal convection and radiation transfer).
The determination of the parameters Uc and Uv is indeed a difficult question. We have some measured data for usual free mounted arrays, but there is a severe lack of information when the modules are semi-integrated.
We don't have any simple way for the evaluation of the U-values in a general case. The the only reliable way of determining this parameter is to measure it on-site.
Default and proposed values
In the absence of reliable measured data, PVsyst proposes default values without wind dependency (i.e. assuming an average wind velocity):
| - | For free-standing (open-rack) systems, i.e. with air circulation all around the collectors), according to our measurements on several installations: |
Uc = 29 W/m²·k, Uv = 0 W/m²·k / m/s
| - | Therefore for fully insulated backside (no heat exchange at the backside, only one side contribution to the convecting heat exchange), the U value should be divided by 2: |
Uc = 15 W/m²·k, Uv = 0 W/m²·k / m/s
| - | For intermediary cases (semi-integration, air duct below the collectors), the value should be taken between these 2 limits, but preferably lower than 22 W/m²·k as the air heat removing is often not very efficient. The default value proposed by PVsyst for any new project is |
Uc = 20 W/m²·k, Uv = 0 W/m²·k / m/s
| We have chosen this value as general default, because we consider that it is more representative of usual rooftop systems, managed by "less professional" people who will not necessarily modify the PVsyst default. For big systems, we suppose that trained engineers will indeed adjust this parameter (for example at 29 W/m² for row-like big power plants). |
| - | For domes, a manufacturer has measured the U-value on several installations (height about 40 to 70 cm above the ground): |
Uc = 27 W/m²·k, Uv = 0 W/m²·k / m/s:
Now if reliable hourly wind velocity data are present in the data, we don't have any reliable measured data with wind.
PVUSA proposes the following thermal correlation, widely used for the free-standing (open-rack) situations when wind speed data are available.
Uc = 25 W/m²·k, Uv = 1.2 W/m²·k / m/s
This corresponds to our default 29 W/m2, when the average wind velocity is 3.3 m/sec (rather usual in continental - not coast situations).
Thermal behaviour of an array on a tilted roof
When installing a PV array on a tilted roof, with an air-duct between the roof and the modules, the air circulation is driven by the temperature difference between the incoming air (ambient) and the outcoming air; i.e. a rather weak motor (low air speed).
As the thermal capacity of the air is low, the air will be heaten up when passing under the very first modules, so that there will not be a significant heat exchange in the upper modules, they will be in the "fully insulated" situation. In this situation, the array temperature is very difficult to evaluate, and may be strongly inhomogeneous. Developing an accurate model would require a detailed description of the air duct thickness, its length, etc. This is out of the scope of PVsyst.
PVsyst doesn't treat this inhomogeneity in the present time, it considers an average temperature.
NOCT Values
Some practicians - and most of PV module's catalogues - usually specify the NOCT coefficient ("Nominal Operating Cell Temperature"), which you are advised to completely forget... Please have a look at this topic !