Tag Archives: power stations

Energy efficiency

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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

Are electric cars back?

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roadchaosDid you know that before Henry Ford developed the Model T Ford motorcar, the nearly 40% of automobiles on US roads were electric vehicles? I think we will be heading back in this direction if we are to have any hope of achieving reductions in carbon dioxide emissions. The implications for the national electricity grid of a major shift to plug-in cars would be very serious and has been the subject of several recent studies including a third year undergraduate dissertation that I have been supervising and from which came the opening factoid.

It is relatively easy, through not without obstacles, to envision a shift to all-electric cars; after all there are several models on the market now. However, an all-electric aircraft seems further in the future, if only because of the weight of the batteries required. Engineers would talk about the energy density, i.e. the amount of energy that can be extracted from a kilogram of kerosene compared to a kilogram battery. However, perhaps the future is not far away because the New Scientist reported earlier in the month [3rd May, 2014] that Airbus had completed the test flight of an electric plane, the E-fan. It is a two-seater plane with a pair of 65 kilogram lithium battery packs driving a pair of 30 kilowatt motors attached to the fans. The E-fan will cruise at 185 kilometres per hour and flies for an hour. Relative to a modern computer jet, this performance is similar to the early plug-in cars relative to their internal-combustion-engined rivals. But, it is an indication of bigger things to come. In the meantime, if you want an E-fan then a new division of Airbus called Voltair will be producing them by 2017.

I mentioned undergraduate dissertations because they have filled a sizeable chunk of my waking hours for a few weeks. This is an annual ritual in the UK during May when final-year undergraduate students are busy submitting and defending their dissertations. I had a pile of twelve dissertations to read and assess. Eight of them belonged to students that I have supervising in weekly one-to-one meetings since last October and the remainder were dissertations for which I was the assessor. All of the students that I supervised were studying either Mechanical or Aerospace Engineering and so the topics of their projects were associated mainly with energy and, or transportation. Some of these projects are provided by engineering companies (those with an asterisk in the list below), which guarantees their topicality and relevance, while others spin-out from my interests and research activities. So many of the topics in the list below will come as no surprise to regular readers of this blog.

Dissertation projects supervised during 2013-14:

Investigation into a redesign of graphite re-entrant seals for a nuclear power station*

Conceptual design for a carbon sequestration system for automobiles

Recommendations for achieving a low carbon airline industry

Strain-based defect analysis of industrial pipe-work*

Investigation of random frequency excitation of an aerospace body panel

Assessment of preload control of threaded fasteners in motorcycle production*

Recommendations for technology-based approaches to reduced ecological footprints

Investigation of low carbon power for plug-in electric vehicles

Impossible perfection

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Carnot's equation for ideal efficiency of a cyclic device converting heat to work and operating between two temperatures specified on the Kelvin scale

Carnot’s equation for ideal efficiency of a cyclic device converting heat to work and operating between two temperatures specified on the Kelvin scale

In my last post [National efficiency on 29th May, 2013] I calculated the efficiency of the nationwide process of electricity generation in the UK [35.8%] and made no comment on the relatively low value.  It will be similarly in all industrialised countries as a consequence of the second law of thermodynamics and the requirement for all real processes to increase entropy.  A French engineer / scientist, Sadi Carnot [1796-1832] demonstrated from the second law, that the maximum efficiency achievable in ideal conditions by a process operating in a cycle to convert heat into work is a ratio of the temperatures of the heat source and cold sink to which excess heat is dumped.  In a power station the heat source might be a fossil-fuelled furnace, a nuclear reactor or a solar concentrator.  The cold sink is usually the environment, perhaps in the form of river or sea water.  So both source and sink temperatures are limited.  The sink by the local climate and the source by the temperatures that modern materials can withstand.

The very best efficiency based on Carnot’s expression for a maximum material temperature of 350 degrees Centigrade [=623 Kelvin] and environment temperature of 5 degrees Centigrade [278 Kelvin] is 55%.  Of course a real power station will never operate at this level because ideal conditions are not achievable – perfection is impossible.

The ideal efficiency improves as the operating temperatures of the heat source and sink are moved further apart and this quest to raise this temperature difference drives a substantial proportion of materials research.  However, even operating with a heat source at 800 degrees Centigrade, using expensive, high temperature alloys, such as Hastelloy N  [a nickel-chromium alloy], on a winter day in the Canadian capital, Ottawa where the average January daytime temperature is -7 degrees Centigrade, the Carnot efficiency of a power station would be only 75%  [=1-(266/1073)].

National efficiency

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Thermodynamics, especially the first and second laws, are usually perceived as boring and perhaps mysterious by most people, including many engineers, as well as irrelevant by many non-engineers.  However, thermodynamics is fundamental to how engineers deliver products and services to society.  The name ‘thermodynamics’ does not help much, perhaps it would be better to call it ‘energy science’, since it is about energy transfers, conversions and flows.

The national energy flow charts mentioned in my post about ‘Energy Blending’ on 22 May 2013 illustrate nicely the first and second laws of thermodynamics (or energy science).  The underlying basis of the flowcharts is to treat the nation as a system and to account for the energy flows in and out across the system boundaries.  The first law, which is about conservation of energy, demands that the inflow and outflow balance one another, so for the UK and USA the annual inflows were 12.5 and 92 quintrillion joules respectively.  A quadtillion is a million million million or 1 with 18 zeros.

The second law demands that any real process involves an increase in entropy, which is a measure of energy dispersion, essentially lost or wasted energy, and this is also present in the flow charts.  In the centre of the UK chart is electricity generation or conversion with an input totally 82.4 Mtoe [millions tons oil equivalent], an output of 29.5 Mtoe and losses of 48.2 Mtoe, which are demanded by the second law of thermodynamics.  So the overall efficiency of electricity generation in the UK is 35.8% [=desired output/required input].

Footnote: the raw data for the UK and USA energy inflows were 299.2 Mtoe [millions tons oil equivalent] and 97 quadrillion Btu [British Thermal units] respectively which I converted into the SI unit for energy, the joule.  The links for the energy flow charts are:

UK Energy flow chart: http://www.gov.uk/government/uploads/system/uploads/attachment_data/file/65897/5939-energy-flow-chart-2011.pdf

USA Energy flow chart: http://www.eia.gov/totalenergy/data/annual/pdf/aer.pdf