Engineers need to be experts in resolving conflicts…
Every man-made device that moves required energy to make it and uses energy when it moves. Heavier devices have greater inertia than small ones and hence more energy is needed to set them in motion – think about peddling an old-fashioned steel-framed bike compared to a modern alloy one. So, designing for sustainability requires engineers to minimise the quantity of raw materials and energy used to manufacture a device AND to minimize its weight if the device moves as part of its function.
Now, here comes the conflict.
Sustainability also implies that devices should have a long, maintenance-free service life so that resources used in maintenance and replacement are minimized. Service life is usually limited by fatigue and, or wear and the probability of these failure mechanisms occurring can be reduced by lowering stress levels. However, stress is inversely proportional to cross-section area and so can be reduced by adding material, i.e. increasing the mass of the device which will also increase its inertia, or resistance to motion. The probability of failure can be reduced by using stronger, more sophisticated materials that are lightweight and almost always more expensive, e.g. composites. Customers also want performance and additional expense might be acceptable if it is accompanied by additional performance – some people will pay for a carbon-fibre frame for their bicycle. Elegant engineering design requires resolution of the conflict between cost, safety and reliability, performance and sustainability.
This is why engineers are trained in conflict resolution or as it is more commonly known: problem-solving.
We were sent a summary of our annual gas and electricity consumption recently by our local utility company. The utility quantified our consumption of both gas and electricity in units of kilowatt-hours (kWh). It is usual to be sold electricity in kilowatt-hours but most people are confused by this unit. Perhaps because they learnt at school that the units of energy are Joules in the SI system and the power rating of appliances is usually given in Watts. They might know that a Watt is a Joule of energy per second, so what is a kilowatt-hour? Well it is about 3.6 x 106 Joules or 3.6 MJ, because it is 1000 Joules per second (= Watt) for one hour. So, I think the utility company should be telling me how many MegaJoules (MJ) we have consumed. After all we are used to seeing the energy content of our food quoted in kiloJoules (kJ), as well as calories.
The situation with our gas consumption is rather different because the utility does not supply energy but gas. The amount of energy that I get from it depends on what I do with it. If I burn it under conditions of constant volume, e.g. in a closed rigid container with exactly the correct concentration of oxygen then it will generate more energy in terms of heat than when it is burnt in constant pressure conditions, such as at atmospheric pressure in air. This is because in constant pressure conditions some of the energy released by combustion is used to expand the exhaust gases against the constant pressure, i.e. to do work, and only what is left is released as heat. So the utility should sell the gas by weight. If they sold it by volume then I would be paying more for the same amount of gas (i.e. number of hydrocarbon molecules) when the supply pressure was reduced.
Oil companies don’t sell gasoline or diesel in Joules for the same reason but they can sell by volume because it is always supplied to our cars at atmospheric pressure and the volume of a liquid is essentially constant.
We like to compare the efficiency of cars in terms of miles per gallon, or kilometres per litres. Efficiency can be loosely defined as what you want divided what you have to put in [See my post entitled ‘National Efficiency‘ on May 29th, 2013]. So for a car, what you want is kilometres travelled and what you put in is litres of fuel. However, when we are all driving plug-in electric cars then we will probably talk about how many kilometres per megajoule our car achieves [see my post entitled ‘Are electric car back?‘ on May 28th, 2014] . Unfortunately, while we are in transition with plug-in hybrids, car manufacturers like to quote very attractive kilometres per litre and ignore the electricity supplied via the plug – as if it were free!
Image courtesy KKN Liebstadt NPP from http://www.nucleartourist.com/systems/ct.htm