M. Anderson, A. Cauchon
Icarus RT, Inc.,
Keywords: graphene, hybrid photovoltaic/thermal
Summary:A graphene layer between the polymer backsheet of a photovoltaic (PV) panel and an aluminum heat exchanger designed to extract heat from a PV panel is explored. The thermal properties of graphene are well-documented, and even large (m^2 scale) graphene films demonstrate great promise as heat spreaders (1). Thermal stability of graphene is known to be sufficient at the operating temperatures of PV panels. Graphene exhibits a very high fatigue life under load cycles, and mechanical failure of the graphene layer is not expected, depending on normal operating conditions. However, when thin (monolayer) graphene does fail due to fatigue, the failure is usually global (2). Graphene’s thermal conductivity also decreases quickly with exposure to strain in-plane (3). This thermal conductivity decrease may be an objection to the theoretical inclusion of a graphene layer, but the strained graphene still possesses a much higher thermal conductivity (~3000 W/m*K at .1 strain) than polyethylene terephthalate (PET) (~.3 W/m*K), a common material for PV panel backsheets. Current standards for backsheet materials include a required Water Vapor Transmission Rate (WVTR) and UV Exposure Failure Rate. If a graphene layer is present on the polymer backsheet, it should also meet these standards. The WVTR of graphene is much lower than common materials for backsheets, at ~.5 g/m^2*day (4). When layered on PET, graphene drastically reduces the WVTR compared to bare PET (~18 g/m^2*day) (5). Graphene oxide is vulnerable to photolysis from sunlight, but a graphene layer upon a polymer surface has been shown to reduce UV photodegradation in the polymer (6). Since UV aging is a notable failure cause in PV panels, the inclusion of a graphene layer may increase average panel lifespan. In addition, the graphene layer will be evaluated as a medium between the different thermal expansion characteristics of the backsheet and the aluminum heat extractor. Graphene has also been explored as a candidate for an additive in the EVA encapsulant, in the form of nanoplatelets (7). Nanoplatelets increased the tensile strength of the EVA by a modest amount, but even .001 wt% of graphene nanoplatelets decreased WVTR by ~70% (8).