Energy

You need to be able to put a load on your fan. A known load.

Sorry i have no idea on load. Could you explain. My theroy is that water flowing through a pipe would spin the blade which in turn would produce enery through a genrator of some type

Yes, i could imagine that. In one way it's as simple as the old mill or the drop pool for a hydro-electric plant. That's easy enough to look into. Another way would also be simple but different: Couple your center shaft to your generator and load it down. When it stops that's too much. Electric current pumped at a given voltage equals wattage. Wattage can be directly converted to Horse Power (around 750w = 1hp).

In order to calculate the amount of energy you need to have some type of force. The flow of seawater down the pipe is not enough. The head pressure of the seawater is the force needed. Even if the energy output were calculated this way it would be an ideal calcualtion in practice there are losses. The fan construction, tolerance of the blades to the walls in the pipe all factor in.

Is the pressure of the sea at any one point not uniform if it is moving at a consitent speed.

Are we talking about tidal flow? If so it not constant.

My Theroy is to create energy by having water flowing through the pipe hence turning the blade which in turn is attached to a generator which i would hope then to store in battery packs.

The average speed of water in the sea is 2.5m/s with up to 4.5 in rivers.

How do i go about calcualting head pressure

CHeck out this link Section 4 micro turbines http://www.districtenergy.org/06CampConfProceeding/Track_A/6A3_Valencia%20.pdf

Several companies such as Carrier are doing this already in food plant, and large building facilities.

Steam is usually the best agent fro propulsion. The reason for this is in practical applications the water colum needs to be very high, unless ussing a large diameter pipe, in both cases ther is the law of diminishing return. Uses more power to pump the water up to the hight in the first place.

How much wattage are you trying to generate?