For almost three years, I lived and worked in Southern Africa (South Africa and Mozambique). As most of the occidentals who discover the continent, one of the first thing you notice is that most people walk very slowly. For the first months of my stay I was surprised by this fact, that unfortunately led to a lot of prejudice and judgements. In Northern American or European cities, people walk fast. Because they are busy, because they want to make to most of their day, etc. So when you go to Africa for the first time, you think “Well, people here are not in a hurry of doing anything”. Which obviously is not true. People in Africa do not walk slowly because they have nothing to do or because they are lazy. They walk slowly because they are smart. They know that under a 42 Celsius sun, walking fast is a bad idea. Because short after you started, you are sweating, your clothes are wet and therefor do not look clean, and most importantly, you are thirsty. Walking fast in Africa means that you lose energy faster. If you slower your pace, you do not sweat as much and you can walk longer. You go through your day and have done tons of things without feeling exhausted as much.
This article was really interesting as it uses and explains the concept of COH, cost of harvesting:
COH = difference between harvesting mode metabolic power – non-harvesting mode metabolic power/difference between harvesting mode electrical power – non harvesting mode electrical power
COH = 1/device efficiency*muscle efficiency
When designing a wearable device, or even any mechanical device that would harvest energy, it is very important to measure the COH. We do have to measure the efficiency of the device in various modes and then compare them. This comparison allows us to understand the efficiency of the whole structure. A device that is producing a lot of energy but demands a lot of efforts is not worth it. As the energy needed to activate it cancels the energy produced.
The researchers of this study had the great idea to deconstruct a motion into various parts and measured the COH for every one of them. The results showed that it is more efficient to activate the power harvest only in one of the walking phase. Using the whole walking cycle would not be efficient as it would demands to much power to the walker to produce energy. Having identified this, the researches will rework the design of the wearable.
This study is really important in the creation process because it will allow them to create a wearable that is lighter and can be used for longer by the user.