• nuclear? (with a question-mark, because it's not clear whether nuclear power counts as "sustainable")

As we estimate our consumption of energy for heating, transportation, manufacturing, and so forth, the aim is not only to compute a number for the left-hand stack of our balance sheet, but also to understand what each number depends on, and how susceptible to modification it is.

In the right-hand, green stack, we'll add up the sustainable production estimates for the United Kingdom. This will allow us to answer the question "can the UK conceivably live on its own renewables?"

Whether the sustainable energy sources that we put in the right-hand stack are economically feasible is an important question, but let's leave that question to one side, and just add up the two stacks first. Sometimes people focus too much on economic feasibility and they miss the big picture. For example, people discuss "is wind cheaper than nuclear?" and forget to ask "how much wind is available?" or "how much uranium is left?"

The outcome when we add everything up might look like this:

Total consumption

Total conceivable sustainable production

If we find consumption is much less than conceivable sustainable production, then we can say "good, maybe we can live sustainably; let's look into the economic, social, and environmental costs of the sustainable alternatives, and figure out which of them deserve the most research and development; if we do a good job, there might not be an energy crisis."

On the other hand, the outcome of our sums might look like this:

Total consumption

Total conceivable sustainable production

- a much bleaker picture. This picture says "it doesn't matter what the economics of sustainable power are: there's simply not enough sustainable power to support our current lifestyle; massive change is coming."

Energy and power

Most discussions of energy consumption and production are confusing because of the proliferation of units in which energy and power are measured, from "tons of oil equivalent" to "terawatt-hours" (TWh) and "exa-joules" (EJ). Nobody but a specialist has a feeling for what "a barrel of oil" or "a million BTUs" means in human terms. In this book, we'll express everything in a single set of personal units that everyone can relate to.

The unit of energy I have chosen is the kilowatt-hour (kWh). This quantity is called "one unit" on electricity bills, and it costs a domestic user about 10p in the UK in 2008. As we'll see, most individual daily choices involve amounts of energy equal to small numbers of kilowatt-hours.

When we discuss powers (rates at which we use or produce energy), the main unit will be the kilowatt-hour per day (kWh/d). We'll also occasionally use the watt (40 W ~ 1 kWh/d) and the kilowatt (1 kW = 1000 W = 24 kWh/d), as I'll explain below. The kilowatt-hour per day is a nice human-sized unit: most personal energy-guzzling activities guzzle at a rate of a small number of kilowatt-hours per day. For example, one 40 W lightbulb, kept switched on all the time, uses one kilowatt-hour per day. Some electricity companies include graphs in their electricity bills, showing energy consumption in kilowatt-hours per day. I'll use the same unit for all forms of power, not just electricity. Petrol consumption, gas consumption, coal consumption: I'll measure all these powers in kilowatt-hours per day. Let me make this clear: for some people, the word "power" means only electrical energy consumption. But this book concerns all forms of energy consumption and production, and I will use the word "power" for all of them.

One kilowatt-hour per day is roughly the power you could get from one human servant. The number of kilowatt-hours per day you use is thus the effective number of servants you have working for you.

People use the two terms energy and power interchangeably in ordinary speech, but in this book we must stick rigorously to their scientific definitions. Power is the rate at which something uses energy.

Maybe a good way to explain energy and power is by an analogy with water and water-flow from taps. If you want a drink of water, you want a volume of water - one litre, perhaps (if you're thirsty). When you turn on a tap, you create a flow of water - one litre per minute, say, if the tap yields only a trickle; or 10 litres per minute, from a more generous tap. You can get the same volume (one litre) either by running the trickling tap for one minute, or by running the generous tap for one tenth of a minute. The volume delivered in a particular time is equal to the flow multiplied by the

Figure 2.1. Distinguishing energy and power. Each of these 60 W light bulbs has a power of 60 W when switched on; it doesn't have an "energy" of 60 W. The bulb uses 60 W of electrical power when it's on; it emits 60 W of power in the form of light and heat (mainly the latter).

volume is measured in litres energy is measured in kWh flow is measured in litres per minute power is measured in kWh per day time:

volume = flow x time.

We say that a flow is a rate at which volume is delivered. If you know the volume delivered in a particular time, you get the flow by dividing the volume by the time:

volume flow time

Here's the connection to energy and power. Energy is like water volume: power is like water flow. For example, whenever a toaster is switched on, it starts to consume power at a rate of one kilowatt. It continues to consume one kilowatt until it is switched off. To put it another way, the toaster (if it's left on permanently) consumes one kilowatt-hour (kWh) of energy per hour; it also consumes 24 kilowatt-hours per day.

The longer the toaster is on, the more energy it uses. You can work out the energy used by a particular activity by multiplying the power by the duration:

energy = power x time.

The joule is the standard international unit of energy, but sadly it's far too small to work with. The kilowatt-hour is equal to 3.6 million joules (3.6 megajoules).

Powers are so useful and important, they have something that water flows don't have: they have their own special units. When we talk of a flow, we might measure it in "litres per minute," "gallons per hour," or "cubic-metres per second;" these units' names make clear that the flow is "a volume per unit time." A power of one joule per second is called one watt. 1000 joules per second is called one kilowatt. Let's get the terminology straight: the toaster uses one kilowatt. It doesn't use "one kilowatt per second." The "per second" is already built in to the definition of the kilowatt: one kilowatt means "one kilojoule per second." Similarly we say "a nuclear power station generates one gigawatt." One gigawatt, by the way, is one billion watts, one million kilowatts, or 1000 megawatts. So one gigawatt is a million toasters. And the "g"s in gigawatt are pronounced hard, the same as in "giggle." And, while I'm tapping the blackboard, we capitalize the "g" and "w" in "gigawatt" only when we write the abbreviation "GW."

Please, never, ever say "one kilowatt per second," "one kilowatt per hour," or "one kilowatt per day;" none of these is a valid measure of power. The urge that people have to say "per something" when talking about their toasters is one of the reasons I decided to use the "kilowatt-hour per day" as my unit of power. I'm sorry that it's a bit cumbersome to say and to write.

Here's one last thing to make clear: if I say "someone used a gigawatt-hour of energy," I am simply telling you how much energy they used, not how fast they used it. Talking about a gigawatt-hour doesn't imply the energy is measured in kWh or MJ

power is measured in kWh per day or kW or

MW (megawatts) or

GW (gigawatts) or

TW (terawatts)

energy was used in one hour. You could use a gigawatt-hour of energy by switching on one million toasters for one hour, or by switching on 1000 toasters for 1000 hours.

As I said, I'll usually quote powers in kWh/d per person. One reason for liking these personal units is that it makes it much easier to move from talking about the UK to talking about other countries or regions. For example, imagine we are discussing waste incineration and we learn that

UK waste incineration delivers a power of 7 TWh per year and that Den- 1 TWh (one terawatt-hour) is equal to mark's waste incineration delivers 10TWh per year. Does this help us say one billion kWh. whether Denmark incinerates "more" waste than the UK? While the total power produced from waste in each country may be interesting, I think that what we usually want to know is the waste incineration per person. (For the record, that is: Denmark, 5kWh/d per person; UK, 0.3kWh/d per person. So Danes incinerate about 13 times as much waste as Brits.) To save ink, I'll sometimes abbreviate "per person" to "/p". By discussing everything per-person from the outset, we end up with a more transportable book, one that will hopefully be useful for sustainable energy discussions worldwide.

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Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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