Heat transfer coefficient for convection in hot water tank.

Hey gang, I can't believe that finding this elusive value is eating my lunch, so I'm looking for some help.

The situation: I'm trying to calculate the amount of temperature rise in a coil of 3/4" copper tube immersed in a solar hot water tank. In order to arrive at the optimum lengths required for the three different coils (incoming cold water make-up to the domestic water heater, the temporarily cold return line from the instant hot water loop, and the hydronic heated floor loop) I must evaluate the heat transfer using the various temperature conditions of tanks & loops. My Major Overkill idiom compelled me to buy a double walled, ss, dairy tank of 600 gal cap and I plan on keeping the water inside it as close to 180-200°F as I can get with my 6, 4 x 10 panels. In the winter here, I expect to see the overnight water temp drop to 120°F (possibly lower, but won't know till we flip that switch) after a couple of cloudy days with normal daily heat extraction.

My problem is that I can not find a good equation for calculating the h-sub-m for the laminar boundary condition undergoing natural convection on the outside of the 3 pumped loops. Aaaarrg. Without this I can't get to "U" to solve my cases. It's largely academic after I get this value. I'm betting that one of you bright folks will have seen a good value, or will know of a way to calculate a reasonable value given that the tank will likely NOT be stratified given the delta T of Min: 30°F and Max 150°F between wall of copper tube and bulk water storage temp in the tank.

Please remember I'm only looking for the external convective film coefficient.

Thanks,

Major Overkill