Replacing Na ions in NaxCoO2 with divalent Ca ions results in the formation of ordered arrays of divacancy clusters, rather than the trivacancy clusters found for the pure compound at high x. Our previous experiment on SXD showed that Na0.7Ca0.1CoO2 adopts one of the superstructures with a low Coulombic ground state energy. We now wish to explore the ground states of the system doped with divalent Ca ions as a function of concentration in order to compare directly with theory. Doping with divalent ions results in a much lower hole concentration in the cobalt layers, and this is of fundamental interest since it will lead to new electronic and magnetic properties, and technological importance because it is expected to improve thermoelectric performance. We shall determine the ground states of NaxCayCoO2 with x+y=7/9, 11/13 and 13/15, where commensurate superstructures are predicted.