Fluctuating wind power must be smoothed before supplied to a power grid in order to prevent potential damages of sensitive electrical devices connected to the grid. This paper considers a hybrid wind-solar thermal plant, where the output from the wind turbine is smoothed by bypassing through the thermal plant. We propose a chance-constrained MPC-based control method that optimally decides the ratio of the wind output that is bypassed through the thermal plant so that the rapid fluctuation of output is suppressed while the direct power output is maximized. The controller's parameters are designed in a way that result in a Pareto-optimal performance. We demonstrate the performance of the proposed method through simulations using real data.