How much does a 4 kW microhydro power plant cost, complete with all of the components?
John Sabor • Memphis, Tennessee
There are so many variables behind your simple question that a simple answer would not be helpful. We need to look at the site and what it has to offer. Equally important is the load you will be running and how best to meet your needs.
My first question is: What does “4 kW” mean? If it means producing 4 kW all the time, this would be a fairly large amount of energy produced for a residential application (96 kWh per day), unless you are planning to heat with electricity. If 4 kW is only the peak load, then you might not need a 4 kW turbine; it could be more sensible to choose a smaller turbine that produces the daily energy that you need. You would meet the peak power demand from batteries (via an inverter), or from the grid.
If this is an off-grid situation, adding a battery and an inverter will maximize the usefulness of the energy from your hydro turbine. Producing 4 kW all the time calls for a very good hydro resource—and a big investment. If you cannot store energy, then much of the energy from your 4 kW turbine will arrive when you do not need it. The control system will have to discard this excess as heat, which may have no value in summer. It is unusual to find a site where significant amounts of water can be stored in a dam, so battery/inverter systems (like the ones used for off-grid solar and wind) are a popular option.
Similarly, an on-grid turbine that is sized to meet your peak demand will export a lot of energy. Will this pay off? If you have a very good hydro resource, then you may not care about wasting energy because it is cheap, but most of us don’t have that kind of resource. In most cases, you should size your hydro in line with your daily kWh needs, rather than thinking peak power. A smaller turbine that uses less water will also tend to have a better capacity factor or, in other words, it will run for more hours per year.
A small stream that runs steadily and falls steeply can be a very good electricity source. The potential power in watts would be roughly the flow in gallons per minute times the vertical fall in feet, divided by 10. To get 4 kW, you might, for example, use 400 gallons per minute falling 100 feet in height. The water will need to pass through a pipe between the source and the turbine 100 feet below, and the friction of the pipe walls and any angles in the pipe will use up some of the pressure, diminishing the energy available at the bottom. On a steep slope, you might use 500 feet of 5-inch pipe. But if the slope is gentler, you might need 2,500 feet of 8-inch pipe. By the time it is laid, the pipe often costs much more than the turbine itself.
There are many options to consider. Will the turbine adjust its flow automatically to make the best of the water in the stream? Will it be better to buy several off-the-shelf turbines or one big custom-built unit that might not work so well at times of low flow? Should the generator produce AC directly? Should you connect this system directly to the grid, or even with the output of an inverter? Or should it produce DC that charges batteries or feeds a grid-tied inverter like a PV array? Should you invest in a foolproof intake system or will you be happy to hike up there from time to time and clear blockages? Hydro system costs can vary widely depending on the energy needed, the head, the flow, and the distances between intake, turbine, and wherever the energy is being used. It’s easy to look at the cost of the turbine—home- to ranch-scale models range from perhaps $1,500 to $10,000. But that is only a fraction of the cost of the whole system, which includes intake, pipeline, turbine, tailrace, controls, perhaps batteries, inverter, etc.
A fully installed hydro system for the average use of a modern household might cost $20,000 to $100,000 (or more). But it’s not wise to look at these numbers as anything more than a very general range of possible costs. The cost for your system will depend on the resource available (head and flow), the demand (watts and kWh), the distances involved, and the specific components you choose.
Hydro can often be the most economical renewable electricity resource over the long haul. Investing well up-front will make your system last a long time, which decreases the cost per kWh. Let’s hope you have a good hydro site, because it’s a wonderful thing to develop. I hope that helps you start the planning process.
Hugh Piggott • Scoraig, Scotland