Well, I guess I'm going to have to get a bit technical as to why this is inefficient.
Heat transfer is based on the thermal conductivity of the materials involved. see
http://en.wikipedia.org/wiki/List_of_thermal_conductivities for a list of their properties of various materials.
This is using a steel tank with a glass lining, the glass lining does two things help holds corrosion down within the tank, and also acts as the first layer of insulation. Also, the coldest part should be used for transfer, heat rises it's as simple as that so the coldest water is in the bottom.
Also this is not enough coil space given the size of the tank, common usage of heat exchangers declares that the area with large storage should be at least the size of the collector to get maximum efficiency. This doesn't apply to flowing heat exchangers just stationary, like tanks.
Dual wall heat exchangers are quite common, the easiest to build is just wrap soft copper pipe around a larger one. of course a pump needs to be used with that design, low flow pumps don't use much energy.
If you wish to call me a nay-sayer, feel free. However I am just offering the facts,trying to save someone headaches down the road before they get too far along.