Turbine Design

Part of Hydro Generator

Water turbines extract energy from falling water with far greater efficiency and at much higher speeds than traditional water wheels. While a water wheel turns at 3-10 RPM, a turbine can spin at 100-3000 RPM — fast enough to directly drive a generator for electricity production without complex gearing.

Why Turbines Over Water Wheels

Water wheels are excellent, but they have fundamental limitations:

Factor	Water Wheel	Turbine
Efficiency	60-90% (overshot)	70-95%
Speed	3-10 RPM	100-3000 RPM
Size relative to power	Very large	Compact
Generator coupling	Needs massive gear train	Direct or simple belt drive
Head range	0.5-10 meters	2-500+ meters
Construction difficulty	Moderate	High

For electrical generation, turbines are strongly preferred because generators need high rotational speed to produce useful voltage. A turbine at 500 RPM drives a generator directly; a water wheel at 5 RPM needs a 100:1 gear ratio — adding cost, complexity, and friction losses.

Turbine Types for Rebuilding

Pelton Wheel (Impulse Turbine)

Best for high-head, low-flow sites. Head range: 15-500+ meters.

How it works:

1. Water is directed through a nozzle, converting pressure to a high-velocity jet.
2. The jet strikes cup-shaped buckets arranged around a wheel.
3. The bucket shape reverses the water’s direction, extracting nearly all its kinetic energy.
4. The spent water falls away with almost zero velocity.

Construction:

1. The runner (wheel): A disc with cup-shaped buckets bolted or welded around its rim.

   • Buckets: Each bucket is split by a central ridge (splitter). The jet hits the splitter and divides into two streams that curve around and exit to the sides. This prevents the jet from hitting the back of the next bucket.
   • Number of buckets: 15-25, spaced evenly around the rim.
   • Bucket material: Cast bronze is ideal — strong, smooth, corrosion-resistant. Cast iron works but corrodes. Hardwood for temporary/testing purposes.
   • Bucket size: Width approximately 3-4 times the jet diameter.

2. The nozzle: A conical pipe that accelerates water into a focused jet.

   • Start with the penstock (supply pipe) diameter.
   • Taper smoothly to the nozzle opening (jet diameter).
   • Jet diameter: sized to deliver the available flow at the velocity corresponding to the head.
   • Jet velocity = sqrt(2 x g x Head) where g = 9.81 m/s2
   • Example: at 20 m head, jet velocity = sqrt(2 x 9.81 x 20) = 19.8 m/s

3. The housing: An open frame — Pelton wheels do not need a sealed casing since they operate in air, not submerged in water.

Parameter	Typical Value
Runner diameter	200 mm - 1 meter
Number of buckets	15-25
Operating speed	200-3000 RPM
Efficiency	80-92%
Minimum practical head	15 meters

Spear Valve for Flow Control

To regulate power output, use a spear valve (needle valve) in the nozzle. A tapered rod (spear) slides into the nozzle opening, varying the jet area. This allows precise flow and power control without changing the jet velocity — maintaining efficiency across load ranges.

Crossflow (Banki-Michell) Turbine

The most practical turbine for rebuilding scenarios — simple to build, efficient across a wide range of conditions.

How it works:

1. Water enters through a rectangular nozzle along the top of a cylindrical runner.
2. The water passes through the curved blades of the runner from outside to inside (first pass).
3. The water crosses the open center of the runner.
4. The water passes through blades on the opposite side from inside to outside (second pass).
5. Both passes extract energy, giving the crossflow turbine its name.

Why it excels for rebuilding:

• Simple blade geometry — blades are sections of pipe or curved plate, all identical
• Wide operating range — works from 2 to 200 meters of head
• Partial admission — can run on reduced flow without severe efficiency loss
• Self-cleaning — water passing through the runner flushes out debris

Construction:

1. The runner:

   • Two circular end discs, 150-500 mm diameter depending on power needed.
   • 20-30 curved blades mounted between the discs, arranged around the circumference.
   • Blade shape: Each blade is a section of a circle — approximately 60-70 degree arc. Cut from pipe of appropriate diameter or bend flat stock to shape.
   • Blade angle: The leading edge of each blade should be angled at approximately 30 degrees to the tangent of the runner circle.
   • Mount blades by welding, brazing, or fitting into slots cut in the end discs.

2. The nozzle:

   • A rectangular opening directing water across approximately 90-120 degrees of the runner’s circumference.
   • A curved back plate guides the water into the blades at the correct angle.
   • Width matches the runner width; height determines the flow area.

3. The housing:

   • A simple box-like casing enclosing the runner with the nozzle entry at the top and a drain at the bottom.
   • The runner shaft exits through bearings on each side.

Parameter	Typical Value
Runner diameter	150-500 mm
Runner width	50-300 mm (wider = more flow)
Number of blades	20-30
Blade material	Steel plate 2-3 mm thick, or pipe sections
Operating speed	100-1500 RPM
Efficiency	65-85%
Head range	2-200 meters

The Crossflow Is Your Best Bet

For a community rebuilding from scratch, the crossflow turbine offers the best combination of buildability, efficiency, and versatility. It works with medium and high head, tolerates dirty water, runs on partial flow without damage, and can be built entirely from flat steel plate and pipe sections. Prioritize this design unless your site has very high head (50+ meters), which would favor a Pelton wheel.

Penstock Design

The penstock is the pipe that delivers water from the intake to the turbine. Its design is critical to turbine performance.

Sizing

The penstock must be large enough that friction losses do not consume a significant fraction of the available head.

| Penstock Diameter (mm) | Maximum Flow (liters/second) for Less Than 10% Head Loss per 100m | |---|---|---| | 100 | 5 | | 150 | 15 | | 200 | 30 | | 300 | 80 | | 400 | 170 |

Rule of thumb: Water velocity in the penstock should be 1-3 m/s. Faster than 3 m/s creates excessive friction losses.

Material

Material	Pressure Rating	Notes
PVC pipe (salvaged)	Moderate	Lightweight, easy to join, degrades in UV
Steel pipe	High	Heavy, durable, welds or threads
Cast iron	Very high	Excellent but heavy and hard to transport
Wood stave	Low to moderate	Bored logs or barrel-stave construction
Concrete	Moderate	For large, permanent installations

Installation

1. Intake — submerge the penstock entrance below the water surface to prevent air entrainment. Install a trash rack (metal grate) to block debris.
2. Route — follow the terrain to minimize bends. Every bend creates turbulence and friction loss.
3. Anchor — secure the penstock against movement at bends and on slopes. Water pressure creates thrust at bends that can push the pipe off its supports.
4. Air valve — install at the highest point in the penstock run to release trapped air.
5. Shutoff valve — install near the turbine to control flow and allow maintenance.

Water Hammer

Closing a valve quickly on a long penstock creates a pressure spike (water hammer) that can burst the pipe. Always close valves slowly — take at least 30 seconds to fully close. For penstocks longer than 100 meters, install a surge tank (a vertical open pipe at a high point) to absorb pressure spikes.

Speed and Power Calculations

Optimal Turbine Speed

Each turbine type has an optimal ratio between water jet velocity and runner peripheral speed:

Turbine	Optimal Peripheral Speed / Jet Speed
Pelton	0.45-0.48 (bucket speed ≈ half jet speed)
Crossflow	0.45-0.50

Example — Pelton at 30m head:

• Jet velocity = sqrt(2 x 9.81 x 30) = 24.3 m/s
• Optimal bucket speed = 24.3 x 0.46 = 11.2 m/s
• For a runner of 300 mm diameter (circumference = 0.942 m):
• RPM = (11.2 / 0.942) x 60 = 713 RPM

Power Output

Power (watts) = Flow (kg/s) x Head (m) x 9.81 x Turbine Efficiency x Generator Efficiency

Example: 10 liters/second, 30 meters head, 80% turbine, 85% generator:

• Power = 10 x 30 x 9.81 x 0.80 x 0.85 = 2,001 watts (2 kW)

This is enough to power LED lighting, tool charging, radio communications, and a small workshop.

Generator Coupling

Belt Drive

The simplest coupling method:

1. Mount a large pulley on the turbine shaft.
2. Mount a smaller pulley on the generator shaft.
3. Connect with a V-belt or flat belt.
4. The pulley ratio adjusts the speed — a 3:1 ratio (large to small) triples the generator speed.

Direct Coupling

For turbines already spinning at generator speed (typically 1500 or 3000 RPM):

1. Align the turbine shaft precisely with the generator shaft.
2. Connect with a flexible coupling (rubber disc, universal joint, or Oldham coupling).
3. Perfect alignment is critical — even 1 mm of offset causes vibration and bearing damage.

Common Mistakes

1. Penstock too small — an undersized penstock wastes head in friction. The water arrives at the turbine with less pressure than the site provides. Always size generously.
2. Wrong turbine type for the site — a Pelton wheel on a 5-meter head site runs too slowly to be useful. Match turbine type to your head and flow.
3. Poor nozzle design — a rough, poorly shaped nozzle creates turbulence that reduces jet quality and efficiency. Machine the nozzle interior as smooth as possible.
4. Ignoring debris — sticks, leaves, and gravel damage blades and clog nozzles. Always install a trash rack at the intake and clean it regularly.
5. Forgetting about water hammer — slamming a valve shut on a long penstock can generate pressures 5-10 times normal operating pressure. Close valves slowly and install surge protection.

Summary

Turbine Design — At a Glance

• Pelton wheels excel at high-head (15-500m) sites — a jet strikes split-cup buckets at high velocity, achieving 80-92% efficiency
• Crossflow (Banki) turbines are the most buildable design for rebuilding — work from 2-200m head, tolerate debris, made from flat steel and pipe sections
• Size the penstock to keep water velocity below 3 m/s — undersized penstocks waste available head in friction
• Optimal runner speed is approximately half the jet velocity — calculate RPM from runner diameter and desired peripheral speed
• Power = Flow x Head x 9.81 x Turbine Efficiency x Generator Efficiency — even small streams can produce kilowatts
• Close penstock valves slowly to prevent water hammer pressure spikes
• The crossflow turbine is recommended as the default choice for most rebuilding communities