Germany’s Fraunhofer Institute for Solar Energy Systems ISE (Fraunhofer ISE) has investigated the thermal effect and net energy gains of a 3.2 kW vehicle integrated photovoltaic (VIPV) system in a refrigerated truck with a cargo storage box and has found that the generated solar power easily offset the additional energetic demand caused by the PV system itself and balance out the total annual demand of the vehicle's chiller.
“Although there is an expected rise in energy demand of the refrigeration unit, it was somewhat surprising to see how easily the solar power offsets this increase and remains extremely favorable in terms of the energy balance,” the research corresponding author, Luis Eduardo Alanis, told pv magazine.
In the study “Thermal effect of VIPV modules in refrigerated trucks,” published in Solar energy materials and solar cells, the research team conducted their analysis on the expectations that the VIPV system would increase the energy demand of the cooling unit and that headwinds would have a cooling effect on the PV array.
The team modeled several scenarios to make an informed assessment of the energy balance. “A one-dimensional thermal simulation model based on a Resistance-Capacitance methodology was created and validated experimentally,” said the academics.
The model took into consideration several parameters affecting the cooling load of the truck’s refrigeration unit, including geo-location, materials specification of PV modules, weather data, basic geometry and forced air convection caused by vehicle movements. It included bill of materials (BOM) data for two module designs, one with foam and plywood insulating layers for refrigerated applications, and one with foam layers and no plywood for non-refrigerated freight applications.
The VIPV module BOM used as a basis for the study was previously developed and validated in an earlier project led by Fraunhofer ISE, known as Lade-PV.
The model was used to predict the thermal behavior of the VIPV system in three cities in Europe: Stockholm in Sweden, Freiburg in Germany, and Seville in Spain. It predicted that, for a stationary case, the air in the refrigerated cargo area could be heated in Stockholm, Freiburg, and Seville by 0.36 C, 0.5 C, and 0.67 C, respectively, averaged over a year, and up to 3.12 C, 2.98 C, and 2.61 C as maximum value, respectively.
The team also modeled the PV cooling effect of forced convection at a wind speed of 50 km/h. It noted that the temperature of the “solar cell dropped significantly.” In the case of Freiburg, it meant that a maximum air temperature increase of 0.6 C was predicted, as opposed to the 2.98 C calculated for in a stationary scenario.
Noting that the convective heat loss due to the movement of a vehicle with integrated PV is “significant”, the team said it is a “highly relevant” aspect to consider when analyzing the thermodynamics and yield potential of such systems. This was shown in two scenarios at 2 C and -18 C.
The researchers concluded that although VIPV adds heat to the system affecting the cooling unit’s energy demand, the energy gained from the PV installation is “significantly higher than the increased cooling demand.” They added that “sufficient battery storage” could enable the PV to satisfy the energetic needs of the cooling unit throughout the year.
The team also stressed that the yearly results described in the study are “merely referential” and “numerous additional variables” could affect both the refrigerating unit’s energy demand and the total annual yield of the PV system.
“A simulation tool can only go so far in understanding a phenomenon with so many variables and which is so complex to model,” explained Alanis. “It would be interesting for us to find the right partners with whom we can take this further and potentially put this to the test in the field in real-life conditions. We may also consider extending the simulation model and considering more influences in the future. For example, cargo loading and unloading.”
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