In order to control the response of a capsule-type thermal energy storage system, a new method using a bypass is introduced. The bypass is set so as to connect the inlet to the outlet directly, to obtain the required temperature by mixing low and high temperature fluids. The bypass works simply using a proportionally controlled 3-way valve. Then the performance of a storage tank is defined by the amount of energy released to obtain the required temperature. Finally the performance map is shown as a function of the required temperature and flow rate.