Milling machine

A Solar powered Electric Milling Machine

Many of the communities we work with are agricultural, with maize flour contributing to an integral part of their diet. In the rural Maasai Tanzanian communities we work with, villagers often have to travel miles to neighbouring towns to access a diesel milling machine. Once there, they can face long waiting periods for sufficient customers to arrive so it becomes worth the operator switching on the machine, due to the inefficiencies of diesel engines at start-up and shut-down.

Electrical maize milling is therefore a priority productive-use application in energy access projects, and potentially an important contributor to overall commercial sustainability in a system, since a milling machine will be one of the most significant loads connected to a system. 

Little information is available online regarding the performance of low-power electric milling machines, and there were concerns that the quality of flour might not meet locals’ expectations. Very small DC and single phase mills have sometimes had a bad reputation in this regard. 

We wanted to test this, so we recently acquired a 2.2kW electric 3-phase hammer mill, from Agsol in Kenya, to test at our existing rural off-grid productive use mini-hub in Ormoti. The tests assessed (1) speed of milling, and (2) quality of output.

For each test, a measurement was made on the time taken for a 2kg bucket of maize kernels to completely empty from the feed tray. The maize feed rate was adjusted by changing the size of the entrance hole into the hammer-mill, and screen size was adjusted from 0.8mm (finest) to 3mm (coarsest).

Nine tests were done in total, using eight different setting combinations, and one repeat test of the ‘best’ setting to check repeatability. Samples of flour from each test were placed into separate cups, with a cup of locally milled flour included as a reference sample. The samples were presented to a group of 3 local cooks who were asked to rank the flour in order of best to worst, separating out the flour samples that were of too low a quality for cooking (In practice these would be used to make animal feed). A local sample of flour was added into the blind test for comparison.

Testing was a success, with results indicating that screen size is the single most important factor influencing flour quality. Only the finest screen (0.8mm) produced flour of a quality ‘high-enough’ for cooking. Interestingly, the locals unanimously ranked their own flour as of too poor quality for cooking’ in the blind test, showing that flour quality from the small electric milling machine could surpass what was currently achievable, and ‘acceptability’ can depend largely on convenience and what is available at the time.

This 3-phase electric milling machine is significantly smaller than most milling machines we’ve seen in rural markets, and consequently the milling speed cannot compete. Milling speeds ranged from 2 times to 12 times slower than the existing diesel-fed milling machines found in the market (310kg/hr). However, the increased energy efficiency, cost effectiveness and convenience of the electric milling machine could outweigh the speed being a barrier to acceptability by local people. In addition, the increased energy efficiency at start-up would reduce the necessity to wait for several customers before turning on the machine.

Given flour quality standards were met, the milling machine was connected to the VFD solar pump inverter at the Ormoti borehole site, in parallel with the borehole pump. The AgSol milling machine comes with a bundled soft starter, which enables it to be started and stopped whilst the borehole pump is running. As expected, the VFD was able to power the milling machine without problem. 

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