Estimate the energy extracted from 1 km³ of granite if cooled from 300 ^◦C to 200 ^◦C. Assume that a power plant runs between the (time-varying) temperature T_g of the granite and an ambient temperature of 20 ^◦C and reaches 1/2 of the Carnot efficiency at each T_g. For how long could a 100 MW power plant run off this energy?