Electric Conversion of a Water-Pumping Windmill in Namibia

Multibladed windmills are a common sight in many parts of the US, Australia, and southern Africa. They are generally used to pump water from boreholes for agricultural purposes.

In recent years, more and more of these windpumps are being replaced by PV-powered electric pumps. Main drivers behind this development are the less consistent production and higher maintenance effort of windmills. Especially the laborious process of pulling and unscrewing the windpumps’ heavy and often corroded steel pipes and rods in order to service or repair the pump is a heavy drawback. Electric pumps, on the other hand, can be attached to flexible plastic pipes, making it much easier to pull the entire assembly out of the borehole.

This project explores the possibility of converting such disused windmills into producers of electric power. The idea came up during an internship on a rural farm in southern Namibia, when I (Louis Hillebrand) helped to pull old pipes out of a borehole in order to install an electric pump, while the windmill was just left standing over the well.

Motivation

There are two main motivational factors for the project. On one hand, windmills are often found in remote areas without a grid connection, as was the case on the project site, Farm Landsberg, in Namibia. In this setting, a repurposed windmill can provide renewable energy for an island system on a farm or village with little cost and minimal environmental impact. Working in combination with PV, compensatory effects between wind and solar can be utilized. This helps to stabilize power production in the system, putting less strain on the batteries and decreasing the time petrol or diesel generators need to be run.

On the other hand, windmills have become an iconic part of the landscape for many communities. In Namibia, for example, windmills can be found in the logos of numerous brands, and downscaled models are used as garden ornaments. Converting a disused water-pumping mill into an electricity-generating one can keep it from sitting idle and falling into disrepair, or being scrapped for materials.

Working Principle of Water Pumping Windmills

The rotation of the rotor is converted into a linear up-and-down motion via a gearbox, which powers a piston pump. Inside the gearbox, a set of small pinion gears on the wind wheel shaft engages with two crank (bull) gears in a ratio of 1:3 to 1:4 to increase the torque. One crank (pitman) arm is attached to each of the large crank gears and connected to the other arm via the guide wheel shaft. A guide wheel located on the guide wheel shaft between the crank arms travels inside a U-shaped pump rod guide, thereby converting the motion from rotational to linear.

Also connected to the guide wheel shaft is the pump rod, which is lifted and lowered with every turn of the crank wheel and moves the piston of the pump. The pump rod consists of a chain of steel rods connected by threaded couplings and runs down into the well inside the same pipes through which the water travels upwards. For illustration and a better understanding of the mechanism, please refer to the drawing.

Project Outcome

The result of this project is a working, power-producing windmill. However, the technical condition of the windmill (see Step 1) and compromises made during the construction make it unsuitable for unsupervised long-term operation. Planning from Germany, it was not possible to get exact dimensions or specifications of the windmill, and access to parts and machinery was very limited due to the remoteness of the site. As a result, the original plan to connect the generator to the windmill's gearbox had to be abandoned, as described in Step 5.

Consequently, this article is not meant as an assembly instruction for an electric windmill, but rather as a report on the engineering and construction behind this first prototype. The aim is to guide the reader through the different engineering tasks involved in such a conversion and point out possible ways to approach them. Many different manufacturers and sizes of windmills exist, and the technical details need to be tailored for the model at hand. Furthermore, the best route for the conversion depends on the engineering knowledge of the person undertaking the modification and their access to workshop machinery. A lathe, for example, can be helpful, as it provides the possibility to adapt new parts to the existing gears and shafts, or vice versa.

Lastly, if the main goal is to produce renewable electricity from the wind and keeping the original appearance is not a major concern, it is suggested to install a modern wind turbine on the existing tower. This will be more efficient and reliable, with comparable cost, as a modern three-bladed rotor has higher aerodynamic efficiency and does not require a gearbox to achieve sufficient rotational speed for the alternator.

Future Plans

With the pictures and measurements taken during the conversion, a CAD model of the windmill's head and gearbox is being implemented in Autodesk Fusion. The goal is to develop a gearbox that will fit into the existing housing with a simple drop-in installation and minimal modification, while providing the appropriate transmission ratio.

Furthermore, I plan to design an adaptor platform that will make it possible to mount a modern small wind turbine on the windmill's tower and mast pipe. The objective is to explore the possible benefits of solar and wind co-production for an island system in the area and compare the productivity to a converted windmill.

Disclaimer

The information provided in this tutorial is intended for educational and informational purposes only. The conversion should only be attempted by someone with skills in engineering and fabrication. The author does not assume any responsibility for harm, injury, or damage that may arise during the design, assembly, or operation of the described windmill conversion. All activities should be carried out with the utmost care and attention to safety protocols, especially when working with electricity and at heights.

A windmill in good working order is a prerequisite for a successful conversion. There are a couple of main points that need to be checked:

1.1: Tower and Working Platform

A stable and sufficiently sized platform is not only important for safety while working on top of a windmill, it also makes it much more comfortable. In my case, the old wooden platform was rotten and no longer safe to use. Therefore, it was completely replaced by a newly built larger one platform (pictures 1 and 3). Also verify that the bolted connections of the tower have not loosened.

1.2: Rotor

The rotor should not have any visible damage or runout. Check all bolted connections to ensure they are tight. In my case, the rotor had been chained to the tower in order to keep it from turning (picture 2). This resulted in a severe bend in the rotor and made it necessary to replace an entire section of three blades. During testing, it became clear that this repair did not straighten out the rotor enough, and there is still significant vibration during high RPMs.

1.3: Head (Gearbox)

Make sure the rotor shaft bearings are running freely. If the gears are used to power the alternator (step 5), also check the bearings of the crank gears and check all gears for excessive wear or cracks. Perform an oil change according to manufacturer specifications if necessary.

1.4: Tail Vane and Brake Assembly

The manual braking mechanism is activated by a hand-cranked winch at the bottom of the tower and has two effects. Firstly, the tail vane is pulled parallel to the rotor to turn (furl) it out of the wind. Secondly, a metal friction band is pulled tight around the rotor drum, keeping it from rotating (picture 4). A set of jam nuts is used to adjust the tension of the metal band. The tail vane swings around the head on a bolt (picture 5), and the upper connection on this windmill was worn out severely.

2.1: Rotor Power Estimation

The mechanical power of the rotor can be estimated by multiplying the power of the wind, flowing through the rotor swept area, with a specific Coefficient of Power. This CoP depends on a few factors, including the type of rotor and the so called Tip Speed Ratio (TSR), which is the ratio of the rotor blade tip velocity to the wind speed. A multi bladed windmill can achieve a maximum CoP of about 0.3 at a TSR of 1.0 (see figure 2). With these values, the maximum rotor power can be calculated as a function of rotor diameter and wind speed. The graphs in figure 1 show the power curves of three common rotor sizes in Namibia up to a wind speed of 15 m/s.

2.2: Choosing a Design Wind Speed

Multibladed rotors are designed for high torque and low RPMs, as can be seen by the low ideal TSR. It is difficult to find exact numbers, but the rated wind speed is around 10–12 m/s. At higher speeds, the rotor is turned (furled) out of the wind to prevent it from spinning too fast and getting damaged. The windmill has an oversized tail vane, and the rotor axis and tail bone are off-center on opposite sides compared to the mast pipe (figure 3). This causes the tail vane to never align exactly with the direction of the wind and to experience a sideways force. During regular operation, the tail vane is held in place by a large spring. In high winds exceeding the rated wind speed however, the sideways force overcomes the spring's tension and pushes the tail vane parallel to the rotor, furling it out of the wind. The tension on the spring and thus the wind velocity, at which the rotor furls, can be adjusted by selecting one of a series of holes in the tailbone. 

The rotor on the Climax brand windmill in this project has a diameter of 10ft and can produce 1.12 kW at the chosen design point of 10 m/s (figure 4). This does not include mechanical losses in the gearbox and bearings but is a good indication for the rated power of the generator. Depending on the local wind resource, the design point can be chosen lower or higher. It is important to adjust the tension in the spring at the tail vane so that it furls at winds faster than the design point.

2.3: Alternator Selection

For small wind turbines, three-phase permanent magnet alternators are the state of the art and are available in many different sizes. A series of alternators was tested on a test bench, and the cheapest models from China performed well below their specifications. Ultimately, the choice fell on the 1500i by IstaBreeze, which is a good compromise between price and performance. With a rated power of 1500 W, there is still a substantial safety margin to the design point, even if it performs worse than the manufacturer’s specifications.

3.1: Estimating the Rotor RPM and Torque

With the definition of the Tip Speed Ratio (TSR), the rotational velocity of the rotor can be calculated as a function of wind speed and TSR. Figure 1 shows the rotor RPM over wind speed for a 10 ft, 12 ft, and 14 ft rotor at a TSR of 1.0. At 10 m/s, the 10 ft rotor ideally turns at around 64 RPM (figure 2).

The rotor torque can easily be calculated from the power and rotational speed. Figure 3 shows the torque curve for a 10 ft wheel. With 1.12 kW at 64 RPM, the maximum torque at the design wind speed is 167 Nm.

3.2: Calculating the Transmission Ratio

For the next step, the spec sheet for the alternator is required (figure 3). The ratio between the ideal rotor RPM and the required alternator RPM determines the necessary transmission ratio. If the ratio is higher or lower, the TSR will change, and the rotor will lose aerodynamic efficiency. In my case, the alternator needs to be turning at least 1000 RPM to convert the 1.12 kW of mechanical power from the rotor into electric power. Therefore, a 64:1000 ratio gives a transmission ratio of approximately 1:16.

3.3: Gearbox Selection

I found a second-hand 1:10 planetary gearbox, intended for a linear drive with a stepper motor (figure 6). The idea was to further lower the ratio when connecting the gearbox to the windmill (step 5) or accept a faster-turning and less efficient rotor. This also ensures that the input torque on the gearbox is lower than the maximum wheel torque of 167 Nm, as the gearbox is only rated for 152 Nm. Another reason for this gearbox are the high permissible sideways force and bending moment on the low speed shaft, since it is connected to the rotor via a gear or pulley and experiences sideways load.

In the end, the overall ratio turned out to be 1:12. While the loss of power is not a concern, the higher RPM, together with the out-of-round rotor (step 1.2), generates some noise and vibration. It is therefore recommended to use a transmission ratio at or below the calculated value. After further research, a manual for Aeromotor windmills was found, which states the maximum wheel RPM for different sizes (figure 4). For the 10 ft wheel, a maximum of 85 RPM is recommended.

4.1: Using the Windmills Internal Gears

One option is to drive the gearbox from one of the large crank gears inside the windmill’s head (picture 1), which requires a suitable gear to be mounted on the new gearbox input shaft. The pinion gears for the wheel shaft can be purchased as spare parts, and they bring the rotational speed from the 3-4 times slower crank gear back to the level of the rotor. This approach is the preferred method, as it is reliable and does not require additional maintenance. It also has a small visual impact, and only the crank arms need to be removed from the windmill’s head for the installation. On the downside, the precision required to ensure that the new gear meshes properly with the crank gear is high. The frame holding the gearbox must place it accurately, and the adapter between the new gear and the gearbox input shaft requires access to a lathe.

4.2: Connecting directly to the Rotor

Alternatively, the gearbox can be driven directly from the rotor brake drum (picture 2) via a belt or chain. The alignment of a belt or chain is a lot less critical, especially at low speeds. V-belts and pulleys, in particular, are often used in agricultural applications and are readily available in Namibia. However, belts wear out, and chains need to be lubricated much more frequently than the windmill’s annual oil change interval, which increases maintenance effort and the risk of failure. Furthermore, the assembly is larger and more exposed, disrupting the windmill’s original appearance more than the first option.

I chose to go with the less favorable option of using V-belts and pulleys for two reasons: Firstly, the farm on which this project is being carried out has only basic tools in their workshop, and the remoteness of the location makes access to a machine shop impossible. However, an assortment of V-belts and pulleys can be found in the farm’s scrapyard.

Secondly, the windmill chosen for me by the farmer is in questionable technical condition and requires extensive repairs to make it suitable for long-term operation beyond testing. It is also located a few kilometers from the farmhouse, so it would need to be completely dismantled and reassembled to connect it to the electrical system. Consequently, it involves too much effort and cost to integrate the converted windmill into the farm’s power generation system.

5.1: Building a Frame for the the Gearbox and Alternator

To join the gearbox and alternator, a frame was fabricated from old pieces of angle iron (picture 2). Since this does not align the two shafts precisely, they are connected by a flexible coupling. The belt pulley is fitted to the gearbox's low-speed shaft, and the frame is mounted on a sliding rail to allow for adjustment of the belt tension (picture 3).

5.2: Mounting the Frame on the Windmill

First, the entire crank assembly is removed from the windmill’s head, leaving only the wheel shaft and pinion gears. Next, the sliding rail is mounted to the existing studs of the gearbox, and the rotor is placed on the rail (picture 4).

5.3: Attaching a Pulley and Belt to the Rotor

The pulley on the rotor is attached to the machined surface of the brake drum, as it provides a smooth surface and aids the alignment. Since no pulley with an exactly matching diameter could be found in the scrapyard, a similarly sized pulley was cut into sections with an angle grinder and mounted to the brake drum with machine screws (picture 5). To get the V-belts past the rotor, the rotor struts were loosened one by one, and the belts were slipped past them (picture 6). The original intention was to use two belts on the pulleys, as the transmitted torque is quite high, and this would also add redundancy. However, this would have required removing the original brake, which is useful for securing the rotor while working on the windmill, and after testing, when the windmill will be restored to its original state. As a result, the pulley segments were split in half to allow enough space for the friction brake, and only one belt was mounted (picture 1). Despite these concerns, one belt provides enough friction to transmit the torque, even when the alternator phases are shorted out to use it as an electrical brake. The diameter of the rotor pulley is slightly larger than the pulley on the gearbox, lowering the overall ratio from 1:10 to 1:12.5.

5.4: Building an Enclosure

For long-term operation, a cover should be designed to protect the components from dust and water ingress.

6.1: Power System Integration

The power generated by the windmill can be fed into the local power system either through the AC distribution side using a grid-tie inverter or via a charge controller on the DC battery side. Grid-tie inverters are usually cheaper than charge controllers but have higher requirements for the system. Unless the windmill is connected to a large grid with continuous demand, the inverter that powers the grid from the battery bank must be able to recognize when there is a surplus of power and be capable of feeding it back into the batteries. A DC charge controller, on the other hand, can simply be wired into the battery bank in parallel with the existing (PV) charge controller.

6.2: Selecting a Device

While there are wind-specific devices available, charge controllers and inverters for PV systems are much more common and less expensive. However, these devices lack several important functions, which must be retrofitted if they are to be used with a wind generator. During preparation in Germany, a few charge controllers and inverters were tested in combination with different alternators (picture 1). One observation was that the Maximum Power Point Tracking (MPPT) of the solar devices was much slower than the MPPT designed for wind. This is a disadvantage in situations with steep power gradients, which are more common in wind than in solar, such as during gusty conditions. In conclusion, the recommendation is to use a device intended for a wind generator, as it requires less effort to set up and will likely perform better. Ensure that the maximum voltage and current of your alternator are within the specifications of the chosen device.

I used a solar charge controller (1, picture 2) for the conversion, as it was already available on the farm. The following sections explain how it was adapted for this new purpose.

6.3: Adapting a PV Charge Controller

6.3.1: Phase Rectification

A typical alternator generates 3-phase alternating current, whereas the PV charge controller is designed to accept direct current. With a bridge rectifier (2, picture 2), the 3-phase AC is converted into DC. Ensure that the amperage rating of the rectifier is adequate and mount it to a heat sink (3, picture 2) to dissipate thermal losses. After rectification, the direct current still is not perfectly smooth. It has a ripple, which is smoothed out using a capacitor (4, picture 2). The required capacitance can be estimated using online tools (e.g. electronicbase). If an electrolytic capacitor is used, it is crucial to observe the correct polarity during installation.

6.3.2: Adding a Dump Load

During operation, a wind turbine must always be connected to a load that can accept the current from the alternator. Otherwise, the alternator does not provide enough resistance for the rotor, allowing it to spin too fast and potentially leading to catastrophic failure. For this reason, wind charge controllers typically come with a dump load in the form of a heating coil (picture 3), to which the current is diverted when the batteries are fully charged. A PV charge controller, however, simply disconnects the solar panels when the batteries are full. To retrofit this functionality, a voltage-controlled relay can be used to direct the alternator current to a dump load once the batteries reach their peak voltage.

A dump load is not necessary if constant demand can be guaranteed, for example, if the power is fed into a large grid. In my case, there was also no dump load installed because the windmill was only running during testing, and the batteries were discharged between tests.

6.4: Installing a 3-Phase Slip Ring

As the windmill's head rotates around the mast pipe to turn into and out of the wind, the cable transmitting power down to the charge controller twists. To prevent this, a slip ring (picture 4) can be installed. This solution is controversial, as the slip ring introduces additional losses and is a common cause of failure. It is often recommended to use a flexible cable instead, since the back-and-forth rotation would statistically even out over time. The best option depends on the wind conditions at each location.

The windmill is located on a saddle between a plain and a mountainous area (pictures 1 & 2), which results in an enhanced wind resource compared to the surrounding region. To measure the wind speed during testing, an anemometer with Bluetooth connectivity was mounted at rotor height, and the wind speed was recorded on a smartphone. Unfortunately, the wind was very gusty during the testing phase. Combined with the slow power point tracking of the solar charge controller, this made it difficult to record stationary working points of the windmill. The wind speed typically changed or died down while the MPPT was still ramping up. As a result, the following data contains a significant amount of uncertainty.

Figure 3 shows the recorded operational points of the converted windmill compared to the theoretical rotor power curve. The recorded power curve is more linear than the theoretical rotor power curve. Measurements at wind speeds of 7 and 8 m/s lie below the curve, which is expected due to losses in the transmission and alternator. However, at lower wind speeds, the windmill appears to have outperformed expectations. It is possible that the multi-bladed rotor has a higher coefficient of performance (CoP) at low wind speeds than assumed in Step 2, but it is also likely that this is due to measurement inaccuracies.