Motor as Generator

In general, electric motors do function decently as generators. You can try this out by measuring the voltage on the motor leads when you spin the shaft by hand. I know it works well for AC, but I haven't tried it for DC. On some motors, you may have to provide some charge to excite the field. Contacting a motor vendor would probably yield a lot of information on this. Contacting a junk yard would probably yield a low cost dc motor.

While it wouldn't be quite as good as a real turbine, you may be able to get a rough idea of your project by piping a centrifugal pump backwards (swap the outlet and inlet), and measuring the power when the water flows through it.

I'm not sure just what you intend to do with the power you generate but there are a few caveats you might be interested in.

Firstly, if you are going to hook this up to the grid, you must synchronize your output with the grid, both in frequency and in phase. As you are going DC, you'll probably not worry about that and simply use the vast amounts of energy off grid.

The second thing to worry about is just how much power you are getting from the water. It is only going to produce power when you are taking water from the grid, and only in proportion to the amount of water you are using.

The third item is that you will be dropping the water pressure to your building with this device. In fact, the more efficient your blade is in gathering energy from the water, the more efficient it will be in reducing water pressure. That won't matter too much for some, but it could get folks unhappy on upper floors of an apartment building. Using a pump to repressurize the water after you remove the pressure will not be efficient.

Lest you are falling into the pseudogreen trap of thinking that utility energy is free and clean, someone had to put energy into the water to increase its pressure. Taking off the excess kinetic energy of the water that is going to your house would be close to getting it for free, with only the minor inconvenience of low water pressure. I suppose you could argue that you are taking advantage of wasted excessive kinetic energy in that case.

There are probably a lot of opportunities to be had in trimming power from those who don't need it in a single source network. These networks are designed to meet the needs of the furthest customer. If you can put in a little turbine instead of a balancing valve, you might do well for air handling too.

Good luck on your project.