Walk the Li-ion: Students develop energy harvesting backpack for hikers

A team of students at Bath University was tasked with designing an energy harvesting system that could charge a battery through the power generated by walking. The Student Engineer caught up with team leader Alex Kyriakides to find out more.

energy harvesting

The team at the Bath University Design Exhibition

Hi Alex. Firstly, can you tell us what you’re studying at Bath?
I am currently studying Electrical and Electronic Engineering at the University of Bath. I have just completed my third academic year or my fourth year with a placement. In between my second and third year I undertook a year industrial placement at a building engineering company in London.

What inspired the idea for eWalk?
In the third year of study at the Bath, each student is required to complete a business and design module in the second semester. Each student is given a survey to complete, with a list of different engineering topics to rank in order of preference. Students are then put into groups based on the choices they make. I was put into a group with five other engineers and we were tasked with designing a product that could harvest energy through the motion of walking. Ideally, it would be aimed at hikers, campers and walkers; people who walked for extended periods of time. The group thought it was essential for the consumer to be comfortable when using the product. Out of all the energy harvesting products on the market, none looked particularly well fitted. This was the inspiration for the idea, to offer a hybrid of comfort and practicality. When hiking, a person will always take a bag with them and so it was decided that the product should be attached or built into one.

energy harvesting

How does eWalk work?
eWalk works via electromagnetic induction. Essentially, the whole product will be built into a hiking bag (approximately 60 Litre capacity). Inside the bag, multiple tubes with magnets and coils will be connected in series. As the consumer walks, the magnets inside the tubes oscillate, leading to an induced EMF (electromagnetic field) in the coils. This is used to charge a lithium-ion battery inside the bag. After walking for a couple of hours, the consumer will then be able to charge an electronic device.

What sort of materials does the system use?
eWalk uses magnets, springs, plastic tubes, copper wire, lithium-ion batteries and various electronic components such as resistors and capacitors.

Did you explore other energy harvesting technologies?
Other methods of generating electricity were researched such as piezoelectric. It was found that the frequency of walking was too low to work effectively. Photovoltaic cells were also considered but proved to be inefficient due to the fact that they required sunlight. If the bag were to be taken out on a hike in an area with little sunlight or dense forests with branches blocking the sunlight, then it would not work.

Equipment shows rates of energy harvesting

Will it be embedded in a hiking bag or just designed to sit in one?
The device will be designed to be embedded into a hiking bag, as opposed to just sitting in one. This was deemed to be the better option because it could allow for the whole system to be tweaked together to produce the most efficient product. Giving customers the option to put the product into a bag of their choosing would offer a level of flexibility, albeit at the expense of producing a product that may not work as advertised.

Why was this option chosen over others, such as a worn device?
Some earlier concepts of the design included having wearable energy harvesters. One option was to design a product that could be worn as a leg brace. After conducting some research, it was discovered that a large amount of energy could be produced. The issue with wearing a leg brace is the level of comfort. If a consumer is uncomfortable whilst on a long hike, they are not going to use the product, regardless of how much energy can be produced. Another concept was to design an energy harvester that was built in/ could be attached to a shoe. This would not work because of battery size limitations and the effectiveness in rough terrain such as mud.

What do the charging cycle and power output look like?
We aim to charge the battery inside the product after walking for 8 hours, which can charge a typical mobile phone twice. Based on preliminary calculations, once the lithium-ion battery has adequate charge, the time taken to charge a 3300mAh battery is just under 3 hours and 20 minutes.

Where are you now in relation to development and what are the next steps?
At this current moment in time, the team have completed a technical feasibility report for the product and prototyped different aspects of the design. If interest was shown by a potential backer, then the team would continue to design and make a working prototype.

Thanks Alex!

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