Joule is a basic unit
Calculating with energy and power
With what you have learned here, you will no longer confuse kilowatts and kilowatt hours and you will also avoid other mistakes. The above-mentioned wind turbine may produce a peak output of 350 kW in full wind. Depending on the wind conditions at the location, it would then be z. B. 80 kW on an annual average, which would lead to an annual production of 80 kW · 24 (hours per day) · 365 (days per year) = 700 800 kWh = 700.8 MWh. However, the information in the introduction could also be meant to mean that 350 kW is the average output, corresponding to an annual production of a good 3 million kWh. It can be seen that clearer information is very desirable.
A load of one ton (1000 kg) has a weight of 9810 N on earth. If a crane is to lift this load by 1 m, it needs an energy of 9810 N · 1 m = 9810 J - plus energy losses e.g. B. in the electric motor. If the load is to be lifted at a constant vertical speed of 2 m / s, a power of 2 m / s · 9810 N = 19 620 J / s = 19.62 kW (without losses) is required.
An older, fully automatic coffee machine needs 24 seconds with 1500 watts to heat up after being switched on. This results in an amount of energy of 1500 W · 24 s = 36,000 W s = 10 Wh = 0.01 kWh. When coffee is then drawn off, an additional 13 Wh per cup (150 ml) is added. In principle, one would have expected that 150 g of water would have to be heated from 20 ° C to 95 ° C (i.e. by 75 Kelvin), which equates to an amount of energy of 4.19 J / g K · 150 g · 75 K = 47.1 kJ = 13 Wh. (We can neglect the power requirement for the mechanical drives for the coffee grinder, the pump, etc.). So that fits exactly, and it couldn't get any better; Significant energy is only lost through heating, not through heat losses during the short time it takes to prepare coffee. In addition, there is a standby consumption of 3.3 W (measured); in 24 hours this results in 3.3 W · 24 h = 79 Wh. Otherwise, that would have been enough for another 6 cups of coffee per day, or for heating up 8 times. A new device should no longer have such a standby consumption.
A private person usually handles the greatest benefits in connection with the car. The engine of a small car delivers z. B. a maximum of 50 kW (= 68 PS) drive power from 180 kW through the combustion of gasoline, so it emits 130 kW as heat. The efficiency is then 28%. When idling, it does nothing (efficiency = 0) and still consumes 10 kW in the form of gasoline (a good 1 liter per hour), which it gives off as heat to the environment. This 10 kW would be enough to keep a somewhat thermally insulated house warm in winter. The idling of an SVU can easily be enough for a two-family house. This explains why an informed energy saver becomes restless when he finds a car idling unnecessarily.
Gas power plant
A large gas power plant supplies z. B. 300 MW (1 MW = 1000 kW) electrical and emits a similarly large power in the form of waste heat to the environment. A large nuclear reactor delivers 3 GW = 3000 MW thermally, which results in approx. 1 GW electrical and 2 GW in the form of waste heat. So much heat in one place is difficult to utilize and is therefore usually released into the atmosphere and / or a river via a cooling tower.
Sun exposure on a small town
A small German town may have a cadastral area of z. B. 20 km2 to have. With full sun exposure in summer, this leads roughly to a solar heating output of 20 km2 1 kW / m2 = 20 million kW = 20 GW on the urban area. This corresponds to ten times the waste heat output of the above-mentioned gas power plant.
Even if it is significantly less in winter: Only a small part of the area would have to be occupied in order to cover a large part of the heat demand with solar collectors. The main problem is the energy storage required. However, this can be achieved cost-effectively with a communal approach (central heat storage + local heating network).
Water has a heat capacity of 4.19 kJ / (kg K) - so you need 4.19 kJ to heat one kg of water by one degree. If 15 liters (15 kg) per minute pass through the sink, which have to be heated up by 50 degrees in the heating system, this corresponds to an amount of energy of 4.19 kJ · 50 · (15/60) = 52 kJ per second, i.e. one Thermal output of 52 kW. If you compare this z. B. with the 60 W of the ceiling lighting, one understands why the hair is standing on end of the knowledgeable person at the sight of a useless running hot water jet, while the ten minutes uselessly burning light leaves him comparatively cool.
One liter of heating oil has a calorific value of just under 10 kWh. In the ideal case (negligible energy losses in burners, storage tanks, pipes etc.) this is sufficient to provide approx. 170 liters of hot water.
A cooling device, for example a cooling machine in an air conditioning system, often supplies the cold generated in the form of cooled water. Similar to hot water (see above), the transported cooling capacity can be calculated from the product of volume flow, specific heat capacity and, in this case, the temperature spread (the temperature difference between the outward and return lines). Example: A water volume flow of 10 l / min (mass flow 10 kg / min) and a temperature spread of 20 K results in a transported cooling capacity of 10 kg / (60 s) · 4.19 kJ / (kg K) · 20 K = 14 kJ / s = 14 kW.
Energy expenditure of the human body
Compare such figures with the average turnover of an adult human body. In this context, for historical reasons, the unit kcal (kilocalories) is common. A daily intake of 2000 kilocalories through food corresponds to 4.19 · 2000 kJ = 8.38 MJ.
The operation of a car already when idling (with 10 kW = 100 · 100 W by burning gasoline or diesel fuel) corresponds energetically to the use of 100 energy slaves, and 5 to 10 times more with moderately fast motorway travel.
Many believe that they have a natural right to use such resources as intensively as they want, regardless of climate risk - just as in the past many claimed a natural right to be a slave. According to the motto, how should it work without that!
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See also: energy, power, joules
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