If you ever read an energy statistic report you will see value stats, such as primary, final, or useful energy. What are we talking about? Why these values amounts differ significantly? To explain and define these values stats. I will use a simple example, the heating of one individual house. Let's consider an individual house of one hundred square meter. The amount of energy needed to heat this house during one year could vary significantly from one house to another, let's say from 5,000 to 15,000 kilowatt hour. This specific amount of energy depends on the effective need of the end user and its goal, the useful energy. This term is conceptual and the corresponding amount of energy is quite difficult to estimate. Let's say, for this example, that the house is currently insulated and the family who lives there is comfortable with a mean temperature of 20 degree Celsius and needs 10 kilowatt hour to warm the house every year. Value systems could be used for heating. If we consider a boiler working with fuel oil, we have a mean efficiency of 85 percent. The family living in this house needs to buy a given quantity of fuel oil. The same amount of energy, 11.8 hundreds of kilowatt power, in this example is called the final energy. The final energy is the energy which the consumer buys or receives. It is the energy billed at the delivery point and therefore it could be accurately quantified. Now, if we take into account the transportation of fuel with 10 percent losses and roughly 7 percent of losses induced by the conversion from crude oil to fuel oil, the initial amount of crude oil needed to heat the individual house will be around 30,000 kilowatt hour. This amount of energy is called the primary energy. It corresponds to the energy resources under their primary states as we can find them in the nature without any conversion or transformation process. We see in this example that the amount of primary energy is 30 percent more than the useful energy needed by the final consumer. However, this ratio could change drastically if another system of energy conversion is used. If, instead of a standard boiler, the house is warm with electric heaters, the efficiency of an electric heater is 100 percent. All the incoming electric energy is converted to heat. Hence, the amount of energy billed, in other words the final energy, is exactly the amount of the useful energy. However, the electricity could be produced from various ways. If the electricity comes from coal, the energy conversion is made by a thermal power plant with a typical efficiency of 35 percent. Hence, the primary energy used to warm the house will be almost 30,000 kilowatt hour, almost three times the useful energy. If now the electricity comes from hydro power instead of coal, the primary energy takes into account on this electricity transportation through the network and the primary energy use to warm the house will be 10,700 of kilowatt hour. This conversion process from hydro power to electricity seems optimal. The difference between the primary energy and the useful energy is very weak. However, we can do even better if instead of electric heater we are using a heat pump to warm the house. The heat pump will capture the heat from a cold space outside the house and release it to a warmer one inside the house. The electricity needed to accomplish this energy transfer could be three times less than the amount of heat. Hence, the electrical efficiency could reach 300 percent to bring 10,000 kilowatt hour of heat inside the house only 3,300 kilowatt hour of electricity will be used. Hence, the amount of energy paid by the final user, in other words the final electric energy, will be strongly reduced. However, to be relevant the energy analysts take into account the extra heat transfer from the outside to the inside. In the global energy budget they consider the difference between the amount of heat released by the heat pump minus the electric energy used. Therefore, we should had 6,700 of kilowatt hour of heat in the energy budget. By doing so, the final energy, the sum of the renewable heat and the electricity, will always be as close as possible to the useful energy. I hope that you better understand the complexity of the energy production and conversions with these few examples. Thank you.
