Energy Conservation, Efficiency and Generation

Is it worth it?

Energy is never created or destroyed. When a home is heated, the energy bought does not vanish: it is transferred through the walls, roof, windows and gaps to the air outside, and it keeps going until it has been replaced. So saving energy really means wasting less of it, and almost every energy decision in this topic comes down to one question: is it worth it? A measure such as insulation costs money once, at the start, and then saves money every year after that, by cutting how much energy has to be bought to replace what escapes. Energy bought from a supplier is measured in kilowatt-hours, and its cost is the number of kilowatt-hours multiplied by the price of each one. This module works through four judgements: how long a measure takes to pay for itself, how to compare two measures, how to read where the energy goes on a Sankey diagram, and why electricity has to be stored and what storing it costs. The energy stores and the idea of efficiency are the starting point here rather than the subject. Every price, saving and lifetime in this module belongs to its own worked scenario. None of them is a real price, and real prices change.

Five words for energy decisions

Five terms this module relies on. The first two are easy to swap for each other, so read those definitions closely.

Match each energy situation to what it means

Two ways to spend the money

In this scenario a family can afford one of two measures. Measure A costs 240 pounds and saves 60 pounds a year. Measure B costs 900 pounds and saves 150 pounds a year. Judged by payback time, which is more cost-effective?

• Measure A, because it pays for itself in four years against six years for Measure B
• Measure B, because it saves more money every year
• Measure B, because a dearer measure must be better quality
• Neither, because both of them pay for themselves in the end

Two ways to keep electricity for later

Electricity cannot be kept waiting in a wire. To use it later, the energy has to be moved into a store and then transferred back when it is wanted. Storage is needed because the output of wind turbines and solar panels depends on the weather and the time of day, and not on when people want to boil a kettle. Two methods are named in this topic, and they do the same job in different ways.

Work out the payback time

In this scenario, fitting cavity wall insulation costs 1155 pounds. It cuts the energy the home has to buy for heating by 840 kWh every year, and each kWh costs 25 pence. Work out the payback time, in years.

Order the pumped storage cycle

Five stages in one day at a pumped storage station. Put them in order, starting at a quiet time when there is spare electricity.

• Spare electricity from the grid runs the pumps
• Water is lifted to the upper reservoir, filling its gravitational store
• The water is held there until demand for electricity rises
• The water is released and flows downhill, gaining kinetic energy
• Turbines turn generators, and electricity goes back to the grid

Picture it as a mill stream

Think of a mill that takes water from a river. At the mill the water splits into channels: one drives the wheel, and the others run off into the fields and a marsh. Draw each channel as a band whose width matches how much water it carries, and the picture does the arithmetic for you. The bands leaving always add up to the band arriving, because no water is made or lost on the way. The water in the marsh still exists, but it is spread so thinly across the ground that nothing can ever be turned by it again. A miller wanting a better mill does not ask how much water arrived. The useful question is what share of it ended up turning the wheel. And keeping the channel walls in good repair works like lining a roof: the leak is not stopped completely, it is only made slower.

Complete the energy flow facts

On a _____ diagram, the width of each arrow shows how much energy goes that way. Energy that has spread into the surroundings, where it can no longer be usefully used, is _____. The useful energy out divided by the total energy in gives the _____ of a device. Insulation does not stop heating losses completely: it reduces the rate at which energy is _____ to the outside.

Read the lamp Sankey in words

A Sankey diagram for a lamp shows 200 joules supplied each second. One arrow shows 30 joules each second leaving as light. The other, much wider arrow shows 170 joules each second heating the surroundings. Which THREE of these follow?

• The efficiency of the lamp is 0.15
• The wider arrow is the wasted energy, so most of the energy supplied is wasted
• The two arrows leaving add up to the arrow arriving, as conservation of energy requires
• The 170 joules of heating are destroyed once they leave the lamp
• The efficiency of the lamp is 0.85

One home, two measures, worked

A family has a budget for one improvement and two quotes. Draught strips around the doors and windows cost 45 pounds and would save 30 pounds a year. Loft insulation costs 390 pounds and would save 130 pounds a year. Start with payback time, which is the cost divided by the yearly saving. The draught strips pay for themselves in one and a half years and the loft insulation in three years, so on payback alone the strips win. Now ask the question payback leaves out: how long does each measure last? In this scenario the strips perish and need replacing every two years, while the insulation carries on working for decades. Over ten years the strips save 300 pounds but have to be bought five times, costing 225 pounds, which leaves the family 75 pounds ahead. Over the same ten years the insulation saves 1300 pounds against its single cost of 390 pounds, which leaves 910 pounds ahead. Both answers are correct, and they answer different questions. Payback time tells you how quickly the money comes back. The total over the life of the measure tells you how much comes back altogether. A strong answer uses payback to compare, then checks whether the lifetime changes the verdict.

Tap the claims that break the energy rules

Five claims overheard about saving and storing energy. Tap the TWO that break the law of conservation of energy.

• Insulation slows down the transfer of energy from a warm room to the cold air outside.
• Once heat escapes through the roof, that energy has been destroyed.
• A pumped storage station gives back less electricity than it used to lift the water.
• A good battery gives back more energy than was put into it when it was charged.
• The energy heating the air around a lamp is still there, just spread too thinly to use.

Build the verdict on a measure

A strong verdict uses payback to compare and then checks the lifetime. Choose the option for each gap.

The energy decisions run

Five quick questions on payback, Sankey diagrams and storage. Three lives.

Three decisions for a village hall

A village hall committee has a small budget, an old building, and a new wind turbine on the hill behind it. Work through three decisions. Every figure belongs to this scenario.

• The committee is offered two measures. One costs 200 pounds, saves 50 pounds a year and lasts three years. The other costs 600 pounds, saves 100 pounds a year and lasts thirty years. Choose the advice to give.
• On windy nights the turbine produces far more electricity than the empty hall uses. On still winter evenings, when the hall is busiest, it produces almost none. Decide what would help most.
• At a public meeting about a larger wind farm on the hill, one resident argues against it because it would change the view, and another argues for it because it would cut the carbon dioxide released by burning fuels. Decide how the committee should report the debate.

Advise the hall committee

The committee asks for written advice on one energy-saving measure for the hall and one way of storing energy from its turbine. Using figures you choose for your own scenario, explain your advice.

• Work out a payback time and show the working
• Say what payback time tells the committee and what it leaves out
• Explain why saving energy means wasting less, not creating any
• Explain why the turbine needs storage and name one way to store it
• Give one argument on each side of the wind farm debate, then a reasoned view