OUR TECHNOLOGY
POWER SOLUTIONS
As most households in rural Rwanda are in off-grid locations, our system must be capable of generating its own electricity. We have considered different generation methods that could provide sufficient energy to power our fan:
Solar Panels
Using the photoelectric effect to generate electricity is a well-known concept. As Rwanda is a country with a lot of solar irradiance, this option seems very promising. Furthermore, this option is self-sustainable and renewable. However, solar panels have a low efficiency of 15-19%, and are affected by many parameters making it an unreliable power source. More importantly, the price range of a 5W solar panel is of about £12, making it a quite costly option for locals.
Keeping solar panels as generation method in mind, we have considered developing a solution as extension to the project E.quinox, a student-led initiative from Imperial College that is already active in the area. E.quinox has got two solutions in place that could be useful to us:
E.quinox Battery Box
Making use of the battery boxes the E.quinox project have introduced in Rwanda would imply users would have to visit the solar powered energy kiosks to collect battery which would not only be consumed to charge their phones, but also to power the fans. This would imply visiting the kiosks more frequently.
E.quinox Standalone
Again, using E.quinox’s technology, locals can mount a small solar panel on their roof, all provided by E.quinox. One problem with this solution is that it would limit our product to E.quinox’s target area and prevent us from providing our product to other locations.
Click here to find out more about these solutions
Thermoelectic Technology
Through the use of thermoelectric generators (TEGs), which generate a voltage proportional to the temperature difference across its faces, we would power our fans with the heat provided by the fire. One of the TEGs sides would be connected to a heat probe reaching into the cooking fire, while the other side would be connected to a heat sink at ambient temperature. This is a cheap and very reliable method as it does not require high maintenance and is not affected by weather conditions. However, TEGs have a very low efficiency of just a few percent.
In order to find out more about the technology behind a TEG, click here.
OUR TESTS
Carefully considering all concepts, we decided to further develop our ventilation system based on thermoelectric generation. The factors that led to this decision were mostly the expected high reliability and independence of external weather conditions, making it adaptable to various different locations. Furthermore, the thermoelectric generator would only run when necessary, whenever the fire is on, and automatically switch off if no cooking is done.
Once we had decided to use thermoelectric technology, we conducted various experiments to test its performance. Our main criteria were reliability and stability of the system in order to achieve effective ventilation.
Therefore, we tested the fans and the generation TEGs separately in order to find the best way of matching their criteria.
Testing the Fans
When testing the fans available to us, our major concern was the achievable airflow. We have estimated that an airflow of about 20-30m3/h should suffice to provide effective ventilation. Using old computer fans, which are known to work efficiently, we found their air-flow to achieve airflows of up to 50m3/h, sufficient even for lower voltages that we expect to be generated by the TEG. Using this information, we ordered fans to match our desired criteria even better: A larger extractor fan that provides an even higher airflow and a small, low-voltage cooling fan.
Testing the TEGs
In order to test the characteristics of a single TEG and a combination of two modules, we used an experimental set-up based on a hot plate modelling the cooking fire, which we found to be the best approximation to the real heat source. On top of the hot plate, we put an aluminium block to model the heat probe, and added additional heat shielding in form of aluminium foil. The TEG’s hot side was put on top of the heat probe, while its cold side was connected to a heat sink.
Testing showed that it is necessary to constantly cool the heat sink in order to obtain a constant power output. Even if one TEG had a sufficiently large power output to drive one fan, its voltage was not high enough, which is why a second TEG was added electrically in series (whilst thermally in parallel). This had the additional advantage that the internal resistance increased and thus moved towards the parallel load impedance of the fans, providing a better match and therefore a higher total power output.
FINAL DESIGN
Finally, we got to the most exciting moment: Connect everything up and see if it works! We decided to mount two TEGs to power two fans, as this has given us best performance. This setup provided to be very stable without any significant changes in voltage or airflow for a measured time period of more than 30 minutes. On top of that, it was possible to connect even a third fan and keep the system stable and running in the expected way. This opens up the possibility of storing some excess charge to power external devices.
The ‘Starting Kick’
However, this design has one shortcoming: as the heat probe heats up very slowly, the temperature difference across the two sides of the TEG and therefore the voltage across it only increases gradually. This slow increase however does not suffice to overcome the inertia of the fan, which requires a quick change in voltage to start up.
In order to overcome this problem, we have included an extra component into our system, called a thyristor. Built out of two transistors with their respective bases and collectors connected, it provides a very fast switch once one of the transistors switches on. This quick change in current is used to switch on the fan. Furthermore, we might include capacitors to store extra charge and increase the ‘kick’.
FUTURE WORK
Future works for Project Guhungiza consist primarily of much more rigorous testing set-ups. With a real fire in a room of a similar size to the houses in Rwanda, we will be able to test the efficiency of our electrical and ventilation systems and will also be able to hone towards the best solution. As well as intense testing of our circuit, we will follow through with our design concepts for the casing and systems mechanical features; including cabling and mounting of the extractor fan to a wall/roof. A key consideration here will be the enclosure of the heat sink and its paired cooling fan; a closure is required for durability and safety reasons, but the restriction of surrounding air could potentially affect the heat sink's cooling due to the airflow being limited.
With a fully functioning final product in mind, we have thought up some enhancements that could improve our simple system:
- Control System - Temperature sensor to detect if the TEG is rising to a critical temperature, this would lead to a warning light flashing, signalling to the user that the probe must be removed from the fire.
- Battery charging/USB outlet - To utilise any excess electrical energy that is generated, despite the fact that the excess energy may not be huge in this case, we believe it still may be enough to put to use.
REPORT AND FURTHER READING
For more details on our background research on technologies and social impact, please see our Interim Report.
For details on our market research, concept selection and system development procedure, please see our Final Report.