scope:

INTRODUCTION

In building-integrated photovoltaic (BIPV) systems, photovoltaic modules are installed as functional components of the building envelope (typically, replacing cladding on façades or shingles on roofs). Since high temperatures are detrimental to the performance of photovoltaic arrays, the circulation of a cooling fluid can be used to remove thermal energy from BIPV systems. The fluid can be used for space heating or domestic hot water heating, and in the case of open loop air systems the heated air can also be used as fresh air for ventilation or for drying clothes. The integrated system is called "building integrated photovoltaic/thermal" (BIPV/T). These systems have several advantages. First, the multiplicity of functions significantly reduces costs. Second, the electrical efficiency of the photovoltaic modules is considerably improved. Finally, the proximity to the loads reduces electrical and thermal transmission losses. Properly designed BIPV/T systems may even play an aesthetic role, since they can be used to cover entire roofs, thus allowing seamless integration.

In open loop air systems, outdoor air passes through a channel under the outermost layer of the BIPV/T system which is typically the PV module or metal-roof with directly attached PV laminates (see Figure 1 for example). Although water or glycol systems have the advantage of a much higher specific heat, air-based systems have reduced risks such as no possibility of freezing or damages to the roof due to leaks. Also, less maintenance is required and will last as long as the PV system operates (20 to 50 years).

Air-based BIPV/T systems are usually installed in an open-loop configuration (see Figure 2), in which outdoor air is used to cool the PV modules by convection (commonly forced convection). The heated air is used to provide thermal energy to one or more functions in the building before being exhausted to the exterior. Open-loop air systems are normally preferred over closed loop air systems as the latter would likely lead to overheating of the PV (reducing its durability and possibly causing delamination) unless fins are built into the PV design. Also, open-loop systems allow for the potential use for fresh air preheating. Since the inlet temperatures are lower than in the case of closed-loop systems, the BIPV/T system normally operates with higher thermal efficiencies, although its air exit temperatures are lower.

BIPV/T systems contain several features that complicate their study, such as heating asymmetry and a relatively complex geometry. Mathematical models of different levels of complexity, emphasizing different phenomena, have been developed over the years (a brief literature review is presented below). This paper presents a model bringing together some of the ideas presented in previous works by the authors, and the most relevant findings obtained from measurements at the experimental facilities and demonstration projects of the Canadian Solar Buildings Research Network (Athienitis 2008). This model could readily be adapted as a design tool for air-based open-loop BIPV/T systems in cold climates. By incorporating meteorological data, this model can be used as a decision-making tool in pre-feasibility studies.