Rough Calculations

This week in Design & Technology we had a talk from the PDE departments most loved man, Ben Craven.

Ben is famous amongst all PDEers for his participation in every engineer's favourite childhood show, Scrapheap Challenge!

Ben addressed the importance of doing rough calculations before developing concepts. There is no point progressing with an idea if the science isn't sound.

Thought I'd apply his teachings by performing some rough calculations. This is one of the concept ideas I developed for our current project.

This concept aims to collect rainwater in a tray on the roof of public transport (bus, train etc...). The rain water would then be filtered and distributed to the passengers on the bus via a water dispenser. The idea is to make travel more comfortable by keeping passengers hydrated throughout their journey.

When I suggested this as a concept, I by no means thought it was a feasible product but it's time to put the science to the test!

So, how long would it take to collect enough rainwater to supply 1 person with a bottle of water on the bus?

Assuming the bus is outside 365 days a year with an average rainfall over the 12 months the tray could collect 79 litres a day. Meaning that a 500ml bottle of water could be filled every 10 minutes! This system sounds way too good to be true.

Ok, so let's not get too carried away. This is assuming the collection tray catches every drop of water within each square meter, every second of every day. This would not be the case in real life. Glasgow has plenty of tall buildings and bridges to prevent direct rainwater reaching the roof of the bus, never mind when the bus is under shelter at stations and service areas.

It's fair to assume the bus is only outside for 300 days a year. Of these 300 days, only 60% of the rainfall reaches the collection tray.

From these assumptions, I calculated the collection of 38 litres/day with a bottle filled every 18 minutes. This still sounds unlikely but it does rain a lot in Glasgow!

To put this into context, a full bus travelling from Buchannan Street to Queen Elizabeth hospital (30min) only supplies 1 person with a bottle of water from the dispenser on board. This is clearly not a very beneficial system.

Using rough calculations could easily prevent development of pointless concepts like this one.