We often get asked about using a small turbine to directly produce heat. This is a bit more complicated than it seems, and in general, we don’t recommend it, but in Ireland, the feed in tariff is only 9c and in other countries, DIY turbines are not eligible for the feed in tariff, and in such cases DIY enthusiasts might want to take this course.
The dynamic of turbine blades is usually that the available energy is proportional to the cube of the RPM (or voltage) whereas the power consumed by a heater is proportional to voltage squared. So as the turbine speeds up, a heater becomes less capable of controlling turbine speed, and at low speed it tends to stall the turbine.
You usually also cannot run a turbine safely with no load, so you need to be careful that there is always a load available. So if your water heater is up to max temperature, you need to be sure that there is a reliable pump sending the heat elsewhere, and you should still have a backup that shorts out or brakes the turbine in the event of a failure.
Ideally you would switch the turbine to a DC load using pulse width modulation (PWM), and the duty cycle of the PWM would be proportional to the voltage, so that the power harvested is the cube of the voltage. We can do this using an IGBT and a microprocessor, but the cost doesn’t justify it for small systems. However, a DIY enthusiast could go down this route.
An low-cost DIY alternative might be to use a small PLC to step in two or three loads at different voltages. You can do this using a low cost PLC such as the Crouzet Millenium 3. I would suggest having two or three loads – say 500W, 1kw and 2kw so that you can step up the total load to 3.5kw in 500W steps. Bear in mind that the power will be much lower at lower voltages, so for example if the heaters are rated for 230V, at 100V, the 500W heater will only pull 105W. Connect the turbine to a rectifier, capacitors and use a resistor-divider to provide an analogue voltage sensor to the Crouzet controller.
We could design this for you, but it’d cost yer! Much better if you can DIY.