Energy system components & system decomposition
Earlier this week, we discussed how systems thinking can be used in understanding energy or any other systems. Energy systems connect resources to end-users. We may, for example, consider our energy system to comprise of a collection of energy (supply) chains. This is an abstraction of reality. The importance of abstracting reality allows us to reduce the real complex interrelated components in our energy system (not only technological, but also social, environmental, etc.) to a more simple, yet effective representation that fits the topic you are interested in or the problem you want to analyse. In this activity, we will inform you about systems decomposition, an essential step in system analysis and design.
Analysing systems requires conceptual, qualitative, and information gathering activities. First, by gathering information you may determine the inputs and outputs of your system, and identify a suitable system boundary.
Figure 1. A coal-fired power plant, represented as a system with many in- and ouputs. Note that components are not displayed, which would make the representation much more complex.
For example, a coal-fired power plant requires a tonne of coal to produce 3 MWh of electricity1. The combustion requires oxygen in air. To remove sulfur dioxide, limestone is used in flue-gas cleaning, where it reacts to gypsum. Fly-ash is removed from the flue-gas, and the plant produces bottom ashes. Finally, cooling water is used to carry away the plant’s waste heat.
Inputs and outputs
Most conventional energy systems will use a non-renewable input, such as coal, gas, or oil products. Due to economies-of-scale, these have developed into systems that may run at a very large capacity – a 1,000 MW power plant is no exception; nor is a refinery that converts several 100,000s of barrels of crude oil a day2. From the analysis, we may observe that such systems translate to large point-sources of CO2 and other Greenhouse Gas (GHG) emissions.
The boundary of such an energy system is where the input enters the system (coal), and output leaves (electricity, heat, CO2, etc.). The system interacts with these in and outputs from and to its ‘environment’, and the components of the system itself also interact with each other. The system environment will depend on your boundary selection; part or all of the environment may also consist of a number of man-made systems. A coal-fired power-plant’s environment, for example may include the coal supply-chain (delivering coal), the electricity grid (absorbing electricity produced), and the atmosphere (providing air for combustion; accepting flue gas). Realise that the system ‘environment’ is not only what we can physically see, but also consists of socio-political interactions for example. As you can imagine, this systems thinking can become very complex. Yet, by clearly defining your scope, it can function as a helpful tool in giving structure to technological, but also socio-economic and ecologic systems.
Components and flows
Within the system, a number of processes take place which affect the transformation of inputs to outputs. Every energy system has distinct processes and characteristics. These ‘flows’ are internal interactions of the systems’ components. In a system diagram they are usually indicated with arrows. By connecting the elements with flows and in and outputs, the system starts to take shape.
Figure 2. A coal-fired power plant system representation, now including some key components and flows.
In the coal fired power plant, we can distinguish a number of major system components: the furnace, flue gas cleaning, steam cycle and generator, and cooling. Not shown in the diagram are grinders used to break down the coal to small particles suitable for combustion.
Wind and solar technologies enable us to harvest energy from the sun and convert them into a source of clean electricity and heat. The initial output of a wind mill is shaft power – long used by mankind in windmills to drive a great variety of machines. Today, power generators are integrated in the design of wind turbines. Solar energy can be captured as heat using collectors. Thermal collectors may be used in systems to produce hot tap water or solar heating. In concentrated solar power, solar energy is converted to heat at high temperature by using mirrors, which can then be converted to electricity. Photovoltaics capture solar radiation and convert it directly to electric power. For both solar and wind one could consider the surface area as input – the energy in air flow and solar radiation must be captured. Any required power rating of the system translates to a particular size of the “collectors” (the solar panels, the wind mill). This size is directly related to the density (of wind, solar radiation) per unit surface area and the efficiency of the conversion.
By successfully reducing technically complex systems into structured and simple representations through using decomposition, you can develop your ‘systems thinking’, something that you may already have been doing subconsciously. In the quiz for this topic, we encourage you to practise with systems thinking by analysing your own energy system. It doesn’t have to be a technological system, just look around and you will find systems everywhere.
- The Higher Heating Value of Coal ranges between 15 and 33 GigaJoule per tonne (GJ/tonne). Let’s say we use coal with a heating value of 25 GJ/tonne. One MegaWatt-Hour (MWh) of electricity equates to 3.6 GigaJoule. At 40% efficiency, our coal-fired power plant using then requires 3.6/25/0.4 = 0.36 tonne coal for one MWh of electricity; so a tonne of coal yields about 3 MWh of electricity.
- A list of large oil refineries, taken from wikipedia.
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