… and why it matters a lot
Have you ever wondered what the difference is between power and energy? With the climate crisis, the political and the economic dislocations in the recent months, many discussions around our energy systems arose. In these discussions, terms like “power”, “energy”, or “energy storage” are used frequently together with units like “Megawatts (MW)” or “Terawatt hours (TWh)”. Very often, all these terms are used interchangeably and often not in a quite correct context. There is far more than units and formula behind them, so let us shed light on these terms and the challenges behind them.
What is Energy?
Energy is the capacity to do work. Work is the process of moving or changing something. For example, when you lift a box, you are doing work on the box by applying a force to move it against gravity. The amount of work you do depends on how heavy the box is and how high you lift it. Energy can be measured in units of for example joules, calories, or kilowatt-hours (kWh).
What is Power?
Power is the rate at which work is done or energy is transferred, or even at which energy is transported. Power is measured in units of watts (W), which are equal to joules per second. We often see also other units such as kW (kilowatt = 1’000 watts), but also horsepower (hp), which is equal to 746 watts. Power tells you how fast something can do work or use energy. For example, if you lift a box with a high power (fast) or a low power (slow), you will still do the same amount of work and use the same amount of energy. The only difference is how long it takes you to do it.
Energy vs Power – an Analogy
Are you confused by all these technical terms and mindboggling units? So let us try to build up an analogy: Let us imagine a beautiful landscape with gentle streams and picturesque lakes, where water gently flows down from the mountains to the valleys and the lakes. In this case, you can compare the water to the energy: It is stored in the lakes and flows down from the mountains. The rate, at which water flows in this example would correspond to the power. Hence, to transport water from one lake to the other, it does so with a certain rate or power.
So far for the analogy… What now?
Let us stick for a while with the analogy. After a hot summer day, thunderstorms may come up in the evening in our beautiful landscape. All of a sudden, rain starts pouring and turns the gentle streams into roaring rivers. Vast amounts of water run down the mountains for a few hours to fill up the lakes. At some point, the streambeds cannot cope with the water anymore and floods occur. After some hours, the thunderstorms are over, and the roaring rivers turn back into gentle streams again. The water level of the lakes may have increased a bit, but maybe not too much. So, what happened? A considerable amount of water came within a very short time or – in our analogy – with a high power. The capacity of the streambeds was not sufficient, and floods occurred. The levels of the lakes only changed marginally and so – again in our analogy – not huge amounts of energy were accumulated.
Thunderstorms in the Energy System
In our energy systems, we are gradually increasing the capacity of solar and wind power generation. When the wind blows gently or the sun shines weakly during an average autumn or spring day, the stream of generated electricity resembles the gentle streams described above. The produced electrical power is not very high, and the grid can easily cope with it. However, if a sudden strong wind comes up or during a hot and clear summer day at lunchtime, the gentle streams of electricity may turn into roaring rivers: The electrical power output increases dramatically and may exceed the capacity of the electrical grid.
One possibility to mitigate the risk for an “overflooding” of the electrical grid is simply to increase the capacity of the lines, as we would do with the streambeds: We would reinforce them and make sure that there is enough capacity to absorb all the water from the thunderstorms.
Increasing the capacity of the grid to transport the high amounts of energy e.g. to storage systems such as pumped hydro stations in the mountains is very challenging because of the cost and often political and bureaucratic issues. A better option is to build a couple of local reservoirs. We may use energy storage devices such as batteries to absorb the roaring electricity stream at midday to gently release the electricity during the night. Hence, we can limit the need of power capacity by including energy reservoirs in the system.
This is a principle, which is widely applied in any energy system: Temporary high energy supplies or demands are equalized using energy storage devices in order to limit the power of the energy transport or conversion. A battery smoothens the power peak demands and supplies in a hybrid powertrain. A pumped hydro powerplant smoothens the power demand peaks of electrified trains. A hydrogen storage system helps to absorb the power supply peaks from a wind farm.
The Challenge in Today’s and Tomorrow’s Energy Systems
In any energy system, the demand of any consumer or the supply of any producer must be satisfied at any time. As shown above, this may lead to enormous temporary power peaks. Over the year, the total amount of produced and consumed energy may still be the same. However, the distribution and thus the power demand changes completely.
Up to now, many of these energy streams or powers could be easily absorbed by the enormous energy capacity of carriers such as gasoline or diesel fuel. A stream of such fuels in a pipeline or fuel supply system exceeds the power capacity of electrical lines by orders of magnitudes. It often appears economically more viable to transport high amounts of energy (i.e. at a high power) with a pipeline of renewable hydrogen or methane rather than via the electrical grid, especially over large distances.
When discussing renewable energy systems, it is not enough to simply calculate the number of wind turbines or area of photovoltaic panels to cover the annual energy supply for a region or a country. It is equally important to evaluate, where and at what time electrical energy is produced or consumed and how it is distributed. In other words, power matters equally much as the energy itself. The grid has to withstand the power demand at any time in any location, down to the end-consumer, be it a home, an industrial facility, or a charging station for an electric truck. This is the real challenge in the energy systems on any level.
Challenge accepted!
Transportation, transformation, and storage of energy will be an increasingly difficult challenge for the design of our future energy systems. To place and size energy conversion, transmission, and storage devices is a highly complex task, which incorporates many considerations such as functionality, efficiency, safety, feasibility, and cost. At the end of the day, everything boils down to a big optimization and controls problem with many constraints and boundary conditions. This can only be solved using appropriate simulation, optimization, and controls tools and the right level of expertise. We at AIMotroniX are ready for this challenge. Dealing with dynamic systems including energy conversion, transmission, and storage is our daily business. Contact us for more information and challenge us with your problem!
This text has been written by real people from AIMotroniX.