Energy can be simple and complicated at the same time. We should therefore spend a few minutes on explaining everything from the start.
Energy is the basis of all life: it provides us with light, warmth and food. Everything in the entire universe has been made from energy. Energy can be neither created nor destroyed: It can only be converted, from one form to another.
The law of conservation of energy is one of the cornerstones of the universe. Regardless of what process that is considered, the total of the energy in the system will be unchanged at the end. All the energy in today’s universe must therefore have originated from an unimaginably small volume at the time of the Big Bang, about 13 000 million years ago.
The sun is the ultimate basis for a gigantic energy system which supplies the Earth with about 15 000 times as much power as we use today. Put another way, the sun supplies enough energy in 15 minutes to meet human energy demands for one year. The sun is the engine for all photosynthesis processes, all growth, coal, oil, gas, hydro power, wind power and solar energy.
Power and energy
When comparing different forms of energy it is vital to use the same concepts. Unfortunately, confusion often creeps into the debate, with one of the reasons being a misunderstanding of the basic concepts of power and energy.
Power and energy are not the same
Energy comes from the Greek ‘ι’ and ‘ergon’, with the approximate meaning of ‘inherent force’. Energy is movement, or the potential for movement. Its fundamental unit of measurement and expression is the internationally standardised joule (J). However, the joule is a very small quantity of energy, so that we need, for example, a good 4 joules to heat one gram of water through one degree. For practical applications, we therefore use the larger unit of the kilowatt hour (kWh), which is the same as 3 600 000 J.
Power, which we measure in watt (W), can be described as the capacity of a machine to do work, or the capacity of a lamp to provide light. If 1 joule (J) of energy is converted in a time of 1 second (s), then the rate of doing so (the power) is 1 watt (W).
This means that, for example, a 40 W light bulb uses about 1 kWh in a day, which is the same as 1000 Wh. Strictly, the energy in the lamp has not been used, but merely converted into light and heat.
Does this relationship between power and energy seem difficult to you? If so, you’re not alone. A survey among the country’s HVAC consultants found that about 25 % of them could not correctly describe the difference. We can simplify the concepts of power and energy by comparing them with water from a tap. The rate of water running out of the tap represents the power: the amount of water supplied represents the energy. With the tap only partly turned on, it might take a minute to fill a one litre bowl. By turning the tap on fully, it can fill the same bowl in a few seconds: the volume, in other words, is given by the rate of flow multiplied by time. This illustrates the link between power and energy: energy is equivalent to the water volume, while power is equivalent to the flow.
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Energy has different qualities (exergy)
Energy can have different qualities, depending on what type of energy is concerned. Everything depends on how convertible the energy is. The measure of quality of energy is referred to as exergy , and varies for different types of energy. Electrical and mechanical work, for example, can be completely converted, while heat is considerably less easily converted. Electrical energy has a high exergy, and can be relatively easily converted to (for example) thermal energy. However, attempting to convert thermal energy into electricity is more difficult, as thermal energy has a low exergy. This means that 1 kWh of electricity is of a higher quality than 1 kWh of heat, and can be used to perform more work. To raise an exergy value from a lower level to a higher level requires the input of higher level energy, e.g. as in a heat pump.
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What is the practical use of exergy?
As opposed to energy, which cannot be destroyed, exergy is something that we have to make the best use of, and utilise in the most efficient manner. It is, for example, wasteful to use electricity for providing heat, when there are other forms of energy that would be more suitable.
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