Category Archives: energy science

Closed systems in nature?

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Engineers use a number of types of boundaries when defining systems.  For instance, a boundary can be completely porous to material and energy so that an open system is defined in which energy and mass can flow in and out.  Alternatively, a boundary can be impervious to material but permit energy transfers in which case the system is term a closed system; and when the boundary does not allow energy or mass transfers then an isolated (& closed) system is defined.

In a recent thermodynamics lecture I explained how systems are defined by the type of boundaries used to describe them and gave an engineering example, an everyday example and a biological example for each type of system. So for an open system: a jet engine, a cup of coffee and the human body; for a closed system: a vehicle braking system, a water bottle and an egg; and for an insulated system: a refrigerator with the door closed, a sealed flask of coffee and I could not think of an insulated naturally-occurring biological system.

I challenged my students to identify an insulated system found in biology.  So far none of them have suggested any; however, one of them has challenged my example of an egg as a closed system.  He cited a web article from Scientific American [http://www.scientificamerican.com/article.cfm?id=bring-science-home-chick-breathe-inside-shell] to point out that the developing chick inside an egg has to breathe and hence gas molecules must cross the boundary thus making an egg an open system.  Perhaps, one could claim the soft-boiled egg that you have with toasted soldiers of bread is a closed system?  Are there any closed systems in biology?

Drawing boundaries

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147-4792_IMGMy family and I have been settling into a new home during the last few months, which is why there have been no posts for some time.  You could say that we have new boundaries which define our space in the city.  Indeed, as part of the process of buying our house we received copies of the entry in the UK Land Registry which defined the extent of the property we were purchasing. On a larger scale, ‘boundaries are lines drawn on a map and fought over by man’; this is a quote that I came across sometime ago but unfortunately I have lost the source.  It implies that the judgments made in drawing national or regional boundaries are fraught with difficulty.

Engineers have to draw boundaries in order to define a system for analysis.  In thermodynamics, which is the study of energy, a system is defined as the part of the universe that is the centre of attention and everything outside of the system is described as the ‘surroundings’.  This approach provides enormous freedom in defining the system for analysis and as a consequence there is some significant skill involved in drawing the boundaries so that an analysis is both viable and useful.  Students learning thermodynamics might say it was ‘fraught with difficulty’.

Drawing appropriate boundaries to define a system allows us to evaluate energy and mass transfers in and out of the system and thus assess the capabilities and efficiency of the system.  The system could be a jet engine, a refrigerator or a biological cell.  Of course, the freedom available in drawing system boundaries is open to abuse because organisations can draw the boundaries to optimise the claimed efficiency of their product, so we need to be careful about accepting such claims.  For instance, fuel efficiency values for electric cars look impressive alongside a conventional petrol or diesel vehicle and thus imply less use of the world’s resources; however, such values rarely take account of the generation of electricity at the power station, which might be oil-fired depending on where you live.  Thus a ‘well-to-wheel’ efficiency would be more appropriate if you are interested in global sustainability, or Euros/kilometre if you are more interested in financial efficiency.

Waste is unavoidable

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Image from http://www.nucleartourist.com/systems/ct.htm
Courtesy KKN Liebstadt NPP

If you read my previous post on perfect engines, then you might have thought a heat engine that did not discharge any heat would be more efficient.  However, this would contravene the second law of thermodynamics, which requires that every real process must generate an increase in disorder, in this case by the discharge of waste heat.  Thermodynamicists like to call this increase in disorder, an increase in ‘entropy’.

A consequence of the second law of thermodynamics is that the entropy, or disorder, of the universe is always increasing; but now I have strayed from engineering to physics.  Together with Bob Handscombe, I wrote a book on this topic called the ‘Entropy Vector: Connecting science and business’.  It was not a best-seller but it got some good reviews, see http://www.worldscientific.com/worldscibooks/10.1142/5365#t=reviews.

Perfect engines

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We can’t build perfect engines and even if we could they would not be 100% efficient. A heat engine generates power [or does work] by absorbing heat from a source into a working fluid, often water,Image using the hot fluid to create motion, e.g. via a turbine, then discharging waste heat to a heat sink before pumping the fluid back to the heat source.  This is the operating cycle of most power stations.  The heat source might be a fossil fuel furnace, a nuclear reactor or a solar concentrator; and the heat sink is often the environment.

A Frenchman, Nicolas Leonard Sadi Carnot [1796-1832], deduced that the best efficiency achievable by a heat engine was given by one minus the ratio of the temperatures [in Kelvin] of its heat sink to heat source.

A perfect heat engine operating with a heat source at about 350°C [623K] and a heat sink at 20°C [293K] would have a Carnot efficiency of about 45%.  We can only hope to increase this efficiency by finding a naturally occurring very cold heat sink or by increasing the temperature of the heat source, which is why we are interested in strain measurement in very hot components (see post on ‘hot stuff’) –  we don’t want our super-efficient engines to break!