The thermoelectric power factor for (Na1-y¿My¿)0.8CoO2 is dramatically increased when M = Sr, but it is essentally unchanged from the pure compound when M = Ca. Our previous SXD measurements on pure sodium cobaltate show how the superstructures control the thermoelectric properties. Our preliminary studies of strontium doped samples show that the dopant is incorporated in the lattice and orders long range. Furthermore, they show that the superstructures are stabilised over a wide temperature range, a feature that is important for power recovery applications. The aim of this proposal is to measure the superstructures in calcium doped samples on SXD, in order to understand why the thermoelectric properties are so different to those doped with strontium.