Energy Systems: Definition, Types, and Properties
A system is a collection of elements or components that are organized for a common purpose. The main difference between a system and a set of elements is that a system is defined by the relationships between elements. A set, instead, is simply a collection of elements.
An energy system is a sequence of energy processes, isolated from the surroundings and interacting with them, including plants and devices realizing those processes of determined properties and parameters. A surrounding of an energy system is a set of all elements which do not belong to the system but have an impact on the system or are affected by the system due to their features or operation. The interactions between the energy system and surroundings consist in the system giving or taking energy from the surroundings.
Energy systems are being created to cover a demand for useful energy in the surroundings at a defined place and time, with a required power and in a required amount, and of demanded parameters. Examples of energy systems: boiler, pipeline, PV panel.
Positive impact: surrounding development, due to the energy supply (quality of life, increase of goods and services value, culture and information development). Negative impact: Depleting of non-renewable primary energy sources (fossil fuels). / Change of properties and parameters of fossil fuels deposits during exploitation, and changes at the surroundings (pressure reduction at the deposits of natural gas, removal of salted waters, mining wastes disposal). / Emission of atmospheric pollutants, emission of greenhouse gasses…
Autonomous Systems
A system is autonomous if during its operation it doesn’t receive from the surroundings and does not transmit to the surroundings any streams of mass, any streams of energy or any information which could influence its action. There are no autonomous systems in energetics because they all transmit streams of energy. A closed chemical reactor, thermally isolated from the surroundings, can be considered as a temporary autonomous system.
Coherent Systems
A system is coherent if all elements of the system are mutually related in such a way that any change at any element results in changes at all other elements of the system. All elements are active, all elements are interactive.
High coherence is a disadvantage. Examples of coherent systems: Power plants based on the simple gas-cycle, power turbojet, and chiller.
Independent Systems
A system is independent (not-coherent) if its active elements are not related to each other. A change at any element results in no change at any other element of the system.
Examples: Systems consisting of similar elements in parallel connection (thermal collector-kind systems, power plants consisting of power units); systems with storage elements (accumulators); systems which can be supplied with different energy carriers (biomass-coal duo-units).
Centralization in Energy Systems
A system has the property of centralization if there is an element or sub-system in the system, which has a steering role in relation to other elements. Examples of energy systems with centralization: Small systems with a station for monitoring and control of the system (control room of the power unit, control room at a chemical plant) / Bigger systems (national power dispatch center, regional dispatch centers, dispatch center at the national gas transmission system).
Importance of Centralization
- The central element (sub-system) receives, converts, and transmits information to other elements.
- Relations between the central (nerve-center) element and other elements or subsystems are enforced.
System Availability
Availability of a system is a ratio of time of operation or readiness to operate in a considered period of time. Time availability is the amount of time in a given period in which a computing system can be used by its normal users. Production availability is the ratio of production to a reference level (e.g. the design or contracted rate), over a specified period of time.
Available Power and Nominal Capacity
- Available power: a maximum power of a generation unit or a set of generation units with the present actual conditions.
- Maximal Reachable Power: maximum available power of a generation unit or a set of units that can be obtained with favorable conditions.
- The nominal capacity of a device is a capacity of a generation unit for which the generation effectiveness is the highest.
System Stability and Adaptability
A system is stable if values of its characteristic parameters do not exceed accepted limits. If a system is able to react to external impacts (stimuluses) in such a way that it chooses the best possible way of the operation, it is called the adaptive system.
A fully adapting system recognizes its dynamic characteristics (in a process of a permanent identification) and applies them for control of the operation at fluctuating conditions. – human being – thermostasy, a chiller with feedback elements.
Physical and Mathematical Models
A physical model is a representation of this energy or technological system in the form of a physical object or model, they provide a tangible, visual representation of the system in order to study it and test it in various ways. Ex: Bridges, cell models, human organs, crash test dummies, landform models, models of buildings, and models of chemical compounds
A mathematical model is a description of a system using mathematical concepts and language. Mathematical modeling can determine our system’s structure: simulation of the system operation with different loads, with modified structure and with additional constraints; to estimate e.g.:
- Electricity generation, fuel consumption, and pollutants emission.
- Transmission capacities, and bottle-necks in the network, etc.
- Optimization of a system being constructed or modernized
- Structure fixing and devices selection
System Structure
A system structure is a network of relations existing between the system elements, distinguished (identified) due to their kind and role. We need to know a system structure if we are creating a mathematical model because mathematical modeling can determine our system’s structure:
- Simulation of the system operation with different loads, modified structure and additional constraints.
- Optimization of a system being constructed or modernized