Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyMulti-Physics Analysis of a Photovoltaic Panel witha Heat Recovery SystemP. Maffezzoni, L. Codecasa, and D. D’AmoreDipartimento di Elettronica e InformazionePolitecnico di Milanop.zza Leonardo Da Vinci, n o 32I-20133 Milano, Italy.Abstract— This paper presents a multi-physics analysis of ahybrid solar panel equipped with a solar concentrator and acooling interface with heat recovery capability. It is shown thatthe analysis allows one to predict the temperature profile alongthe panel as well as the I-V electrical characteristic as a functionof the cooling strategy.I. INTRODUCTIONPhotovoltaic (PV) generation represents today one of themost promising source of renewable green energy. Unfortunately,the widespread diffusion of PV systems is still limitedby the large surface which is needed for a significant energygeneration and by the high cost of the silicon raw material [1].For this reason, novel solutions are emerging in which solarpanels are equipped with some optics, such as mirrors orlenses, that concentrate light from a broad collection areaonto a much smaller active surface. Light concentration leadsto a significant saving of the active material but unavoidablyimplies also a much higher power density over the cell surfaceand thus a great increase of the local temperature.This imposes the adoption of a proper cooling system soas to remove a portion of the generated heat and to limitthe temperature increase. The cooling effect is commonlyaccomplished through a cold fluid that flows into one or morepipes connected to the back of the panel and unavoidablyproduces some kind of temperature gradient over the panel.A secondary advantage of such a solution is that a significantportion of the solar energy which would be wasted inthe photovoltaic process is actually rescued in term of fluidheating thus increasing the overall conversion efficiency.A correct and reliable design of such multiphysics systemsoperating in nonuniform thermal conditions, and in whichmany different factors and design choices interact together,relays on the capability to quantify the temperature increasedistribution over the panel as a function of the adopted coolingstrategy. Moreover, accurate electrical and electro-thermalsimulations of the cells forming the panel are needed in orderto predict the actual electrical and energetic performances ofthe system [2].This paper presents a multi-physics analysis of an innovativehybrid solar system composed of a solar concentrator, a stringof series connected PV cells and a water cooling system.First the power contributions that are involved in the multiphysicsprocess are analyzed. Then, the temperature profilethat establishes along the string is deduced as a functionof the speed of the incoming fluid. From the electrical I-Vcharacteristic of the single cell, the I-V characteristic of thewhole panel is determined.This analysis allows the designer to predict the current andvoltage levels that can be sustained by the solar panel as wellas the maximum suppliable power. All of these aspects are akey to the proper design of the electronic control interface thattracks the maximum power point [3], [4], [5].II. THE HYBRID PV SYSTEMFig. 1 shows the section of the photovoltaic system andthe longitudinal cells displacement along the single stringthat form the panel. A paraboloid collector concentrates theincidence radiation, with a concentration factor of 50 onto theactive solar cells active area. The cells are turned upside-downand are connected on the backside to a tube in which flowsa cooling fluid. The interconnection between cell and tube isestablished through a thin-film thermal material which assuresa low thermal resistance. The panel is composed of a singlestring of 142 series connected solar cells.A) Analysis of the power contributions.We refer to as P sol = 1000 × 50 W/m 2 the specific irradiatedsolar power that hits the PV string. P el =0.18 · P sol is thefraction of specific incident power which is transformed intoelectrical power by means of the photovoltaic effect. P dis =0.80 · P sol is the portion of the specific incident power whichis neither reflected nor transformed into electrical power: thiscontribution is the specific dissipated power and constitutesthe primary heat source of thermal analysis.Since each cell is rectangular with dimensions a =0.010 m,b =0.014m, we have that the supplied electrical power andthe dissipated power of a single cell are P el−cell = A cell P eland P dis−cell = A cell P dis where A cell = a × b respectively.B) Determination of the temperature profile of the coolingfluid along the tube.The temperature profile that establishes (at steady-state condition)into the cooling fluid into the tube depends on the initialtemperature of the inlet fluid and on the average fluid speed w(measured in m/s). In our example we suppose to employ waterfor which physical parameters are: k th =0.6 W/mK thermal©EDA Publishing/THERMINIC 2008 93ISBN: 978-2-35500-008-9