The electrical output efficiency of a photovoltaic thermal system can be restored from degradation due to light exposure by annealing a photovoltaic thermal cell at an elevated temperature. The elevated temperature at the photovoltaic thermal cell can be provided by redirecting the flow of a heat exchange fluid to bypass a heat exchanger unit. A boiler unit may be employed to provide additional heating of the heat exchange fluid during the anneal. Further, a variable configuration lid can be provided over a front surface of the photovoltaic thermal cell to control ventilation over the front surface. During the anneal, the position of the variable configuration lid can be set so as to trap heat above the front surface and to elevate the anneal temperature further.