You wouldn't store the CO2 gaseous at atmospheric pressure, you'd use the energy to compress it, presumably liquefying it. Liquid CO2 has a density of around 11

You wouldn't store the CO2 gaseous at atmospheric pressure, you'd use the energy to compress it, presumably liquefying it. Liquid CO2 has a density of around 1100kg/m3, according to a quick Google search, so your example power plant would need only about 200m^3 to store the CO2 produced in one hour, or a cube of about 6m wide. Or 120m wide per year. Still seems like a lot, and I don't know how much energy is needed to compress and cool all that CO2 but it's dubious the surplus renewable energy would suffice to compress all the CO2 produced by fossile-burning power plants.