The Malembo Micro Grid

The overall project has become clearer in our minds. Essentially we are going to set up a 12 V DC micro grid in Malembo that people can use for lights at night time. Perhaps we will expand this to general purpose items in the future. Phone charging can be done through the same power source at the station (power plant) itself and if you wanted to use 12 V DC for something else, like perhaps to run a radio, then it would require no infrastructural change.

For now the system will follow the diagram below but everyone is very keen for expansion as soon as possible.

The Malembo Micro Grid

The micro grid will use two 100 W solar panels to charge a 100 AH deep cycle battery and then power the loads that are connected. There are two ways in which the energy in this system is used. Firstly there is a load box just like the other system we set up in Lwemodde. This is used to hook up arbitrary loads directly to the 12 V DC lines. Most of the loads connected are likely to be phone chargers but there may be one or two other things such as a radio or even an inverter. When this branch is upgraded to an internet café the computers will connect directly to the load box.

The second way in which energy is distributed is via the distribution box which is essentially a bunch of electrical connectors and fuses. This is where electricity is taken from the battery and sent to individual houses via a fuse.

The uses of fuses comes about because of a very clever system that Lydia thought of. As mentioned in the first post about Malembo, the problem with the last attempt to supply people with lights at night was that they would attach other devices to the lines given to their house. These were 240 V AC lines that he connected to a generator he used at night. While they were only paying for one light a client would hook up other devices in parallel with their light and therefore take more watt-hours of energy than they should be.

To guard against this each client will have a fuse assigned to them which will only allow the current necessary for their number of lights to pass. If the lights are taking ~200 mA each and someone wants three lights we would give them a 700 mA fuse. This fuse will sit in the power station and be the critical link between their positive wire and the grid. If a client was to attach an unpaid load to the grid, such as a phone charger, then they would suddenly draw more current than was allotted to them and their fuse would break immediately.

For this to work we need racks of fuses with individual lines connected to each. Now the biggest issue here is getting the correct fuses. Specific fuses are extremely hard to get even in Kampala. The Ugandan way is to simply replace a blown fuse with an arbitrary/bigger fuse or simply hot wire the fuse. I found a shop in Masaka which had a few fuses but got them to order in 40 fuses of different types from Kamapla. What came in two days later was roughly half the order because the others were impossible to find.

What is even harder to find are fuse clips or holders. So we had to be innovative and make our own. After some prototyping we came up with this:

Our fuse holder design.

Mapping our the array of fuses. We staggered them in height and gave each  fuse enough horizontal space so that no shorts would occur.

The middle fuse in place on the distribution board.

The team from Lwemodde and Alex (our man in Malembo) all helping out to make these fuse holders.

Took a while but we got 12 connectors done. Will probably have to make a second rack before long...

The terms of the contract that each client will sign will state that to be reconnected upon blowing their fuse they will need to buy a replacement fuse from the Malembo Micro Grid. We get each fuse for 1,300 shillings and the grid will charge 2000 for a replacement. This system should keep clients in line and also, if they do attempt to take more electricity than they paid for then the grid will gain a small profit from selling the replacement fuse.

The fuse system is almost perfect however there is one major restriction. We must only use lights which have an extremely constant current usage. With LED lamps (which are the most expensive and efficient) this is not an issue however we cannot use fluorescent lights. Due to the surge of solar power in Uganda there are many fluorescence lights which run on 12 V DC and they are actually the cheapest 12 V lights you can get. The issues with a fluorescent light is that it requires a very high voltage to start the light which is usually achieved via a very large inductor giving a voltage spike. This requires a large amount of current in the first millisecond of operation and then the light settles to a current usage roughly half the initial demand.

This property means that if we wanted fuses which did not break after a light is simply turned on we would need to use fuses which are so large that they will accommodate clandestine loads attached to the client’s outlet. Our solution to this problem is to simply use LED lamps. They have an extremely constant current because usually they are a purely resistive load. They are also more efficient and last longer.

Other concerns with this micro grid are too many loads causing the battery terminal voltage drop to be too large and also the loss of power in long distance electricity transport at low voltages. Operating a 12 V means that for any significant power we need to draw hundreds of milliamps and considering that the power loss in wires follows a squared law proportional to the resistance of the wires we should be doing everything possible to lower the current consumption.

The answer to this very classical problem is elegantly demonstrated in Julian Ilett’s video. After we set up this first system we will have to access its performance and look into small step up and step down circuits.