General Comparisons of Turbines

Introduction

It is important to understand turbines because using a more efficient turbine will create more electricity. The choice of turbine depends on circumstances such as water flow and head.

Note this article is written primarily for turbines related to hydropower, including micro-hydro systems. However, some of the information here will apply equally well to turbines for other types of power. In those cases, the term “water” can be replaced with other types of molecules such as “gas molecules” or “steam”.

 

Turbines are Individual to the Situation

Selecting the turbine is one of the most important parts of the entire process. Always remember this concept: every micro-hydro turbine is custom built for a specific location. No two are exactly alike. If you are actually going to install a micro-hydro system then you must work closely with a turbine company. Together you will come up with the best turbine for your location.

The exact choice of turbine, the size, the blade design, the number of jets, and choice of other features depends entirely on the flow and the head at your location. Experts have compiled empirical data over the years based on actual turbines in use. This extensive data tells you which turbines work best in which circumstances. I have summarized and compared the main points of these turbines in the sections below.

(Note that full descriptions and drawings of each turbine can be found in my book Hydropower Explained Simply).

Classic Water Wheel

The classic water wheel has low efficiency, around 65%. The classic water wheel needs a head of at least 10 feet. The flow rate can be much lower than all of the other turbines. The upper limits of the water wheel have not been fully tested.

The classic water wheel is generally used in areas where the flow rate is very low, which means that the wheel has a low rotational speed. Higher rotational speeds can be obtained using smaller diameter wheels. However, a series of gears are absolutely required to boost the speed for power generation.

The classic water wheel does have some advantages for small power use. It is quiet, peaceful, and simple to maintain. For small power needs the water wheel can be sufficient.

Pelton

Pelton turbines can work on quite a range of heads, but is best suited for heads over 150 feet. Pelton turbines generally work better with slow flows than with fast flows. Pelton turbines have a good efficiency of 80% to 90%.

 Turgo

The Turgo turbine can work with quite a range of heads and range of flow rates. The usefulness of the Turgo turbine is similar to the Pelton in most respects, with a few distinctions. The Turgo turbine rotates faster than the Pelton, usually requiring no gears. The Turgo has a better efficiency with changing flows than the Pelton. Specifically, if the flow is slower than the design flow value, the Turgo is more efficient.

Propeller

Propeller turbines work best where there is a low head (less than 30 feet) and a constant flow. Constant flow is the real key to using a propeller turbine. If you can control the flow then a propeller turbine works well.

Kaplan

The Kaplan turbine works best with heads less than 250 feet. The Kaplan can work with a wide range of flow speeds, second only to the range of speeds offered by the Francis turbine. One primary advantage of the Kaplan turbine over other turbines is that the Kaplan can have a reasonable efficiency regardless of the changing flow.

Francis

In general, Francis turbines are used on high heads and fast flows. Francis turbines work on quite a range of heads, however the expense of Francis turbines makes them more cost-effective for higher heads than for lower heads. Francis turbines are the best choice (and have the best efficiency) for fast flows. Francis turbines are capable of taking the fastest flows anywhere. The Francis Turbine is the ideal choice for megawatt hydropower plants.

Cross-Flow

The Cross-Flow turbine can work on a wide range of heads, but is most effective with heads of 250 feet or lower. The Cross-Flow turbine generally works best at lower flow rates than the other turbines. However, the Cross-Flow has an advantage over the other turbines in that it has a higher efficiency with partial flow than the other turbines.

This article is an excerpt from my book “Hydropower Explained Simply”. You can find the full book here.