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SITEMAP   School Physics Notes: Energy 7.3 Saving energy, costs and pay-back time

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Conservation of energy: 7.3 Costs, efficiency and pay-back time for a variety of energy strategies AND reducing unwanted energy transfers including friction and air resistance

(2 sections 7.3A and 7.3B)

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7.3A. Costs, efficiency and pay-back time for a variety of energy strategies

  • You can compare the efficiency and cost effectiveness of methods used to reduce ‘energy consumption’,

    • You should know what the term ‘pay-back time’ means - the time it takes to recover your energy investment from the savings you make from eg installing insulation, low energy light bulbs, any new-replacement appliance etc.

    • A general formula to calculate payback time

    •   cost of installation/device etc. (£)
      payback time (years)   = ---------------------------------------------------------
        savings per year in energy/fuel costs (£)
    • cost of installation (£) ÷ savings per year in fuel costs (£)

    • You should be able to make judgements about the cost effectiveness of different methods of reducing energy consumption over a set period of time.

      • This is not restricted to a consideration of building insulation but may include:

        • low energy light bulbs and LED lighting, these are 4-10 times more efficient than the old filament bulbs in terms of useful output of light energy. They are more expensive but are designed to last a lot longer and these lighting devices are cost effective with a payback time of months. If an LED bulb cost £5 and saved £15 a year on the electricity bill, the payback time is 4 months.

          • payback time = £5/£15 = 1/3 year (4 months)

          • LED bulbs are more costly than low energy bulbs but can provide even greater savings.

        • replacing old appliances with energy efficient ones

        • ways in which ‘waste’ energy can be useful, eg heat exchangers. Heat exchanges are a means of using potentially waste heat. To extract the heat from a device or industrial process, a cooler fluid (gas/liquid) is brought into contact with the heat source and so heats up via a heat exchanger. The now hotter fluid can now be passed through another heat exchange system to re-release the heat to some useful purpose.

      • Example of heater exchangers

        • Some of the heat from a car engine is passed into a heat exchanger and released through the car's interior heating system.

        • In some industrial processes which involve an exothermic (heat releasing) reaction eg manufacture of ammonia from hydrogen and nitrogen, some of the heat from the reaction is used to heat the incoming reactant gases to the correct high temperature for the reaction.

    • Be able to describe the energy transfers and the main energy wastages that occur with a range of appliances.

      • You should be familiar with common electrical appliances found in the home as these will be examined on.

      • Examples will not be limited to electrical appliances; however, in this case all the information would be given in the question.

      • Modern appliances are much better designed these days to be 'greener' and waste much less energy, but this does come at a price when you come to buy your more expensive replacement.

        • You payback time will depend very much on the cost effectiveness of your purchase!

      • See Methods of reducing heat transfer eg in a house and investigating insulating properties of materials

7.3B. Examples of reducing unwanted energy transfers including friction and air resistance

How we can reduce wasted energy

A system consists of an object or objects and the total energy in a system is constant - one expression of the law of conservation of energy - energy cannot be created or destroyed.

e.g. no mechanical device cannot work perfectly, there are always energy losses.

When anything moves, in most cases there is a friction force operating which causes energy to be lost.

When things rub together, work is done against the resistive force of friction raising the temperature of the system.

This also includes air resistance - so in a moving car you get resistance as it moves through air as well as all the friction associated with the moving parts of the car (engine and wheels) and friction between the tyre and road surfaces.

This work generates thermal energy which is lost and spread out into the surrounding thermal energy store - dissipated, and is therefore not useful energy - waste energy can't be used during the overall energy transfer .

This raises the temperature of the surrounding thermal energy store e.g. air, water or road surface etc. and you cannot extract or reclaim this lost thermal energy

There are several ways you can reduce wasted energy i.e. energy lost from a useful energy store to a useless energy store e.g.

In the case of moving machinery an application of oil and grease considerably reduces the friction and therefore the waste heat energy generated by surfaces rubbing together.

The lubricant smoothes the surfaces so they rub against each other with less friction.

The lubricant must be liquid (e.g. oil) or semi-liquid (eg grease) so that it spreads easily over the contact surfaces so that the two surfaces move smoothly over each other e.g. wheel bearings on a car axle or the pistons in the a engine's cylinder.

lubrication helps anything that moves on wheels to move more slowly - less friction - less energy wasted and lost to the surroundings - heat energy is dissipated to the thermal energy store of the surrounding air.

Apart from lubricating machinery in road vehicles to reduce friction, a more stream-lined aerodynamic design of a car body reduces air resistance - less friction - less wasted energy.

For examples of reducing energy losses in the home see ...

More on methods of reducing heat transfer eg in a house and investigating insulating properties of materials

For examples of reducing water friction and air resistance see ...

Acceleration, friction, drag effects and terminal velocity experiments

and in these examples you are reducing the loss from a kinetic energy store to the surrounding air/water thermal energy store


INDEX of notes on conservation of energy, costs and wasted energy

Keywords, phrases and learning objectives on energy conservation

Be able to discuss costs, efficiency and pay-back time for a variety of energy strategies to reduce wasted energy for transfers between energy stores e.g. lubricating machinery to reduce friction and aerodynamic design of a car to reduce air resistance.


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