Cogenerator Using a Power Electronic System: History
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Marian Kampik
,
Marcin Fice
,
Andrzej Jurkiewicz

Cogeneration sources play a very important role in the power industry with dispersed renewable sources with forced generation (e.g. photovoltaics and wind generators). They also fit into the circular economy by increasing the efficiency of fuel use, including biogas from agricultural or livestock waste. 

  • cogenerator
  • CHP
  • electronic power converter
  • induction generator

1. Introduction

Distributed combined heat and power (CHP) generation on a small scale (mCHP—micro CHP) is part of the promotion of renewable energy [1], energy efficiency [2], and the circular economy, promoted by legal acts of the European Union (EU) [3]. Cogeneration systems up to 50 kW are dedicated to agricultural biogas plants installed in small or medium-sized agricultural and breeding farms. The potential for biogas (biomethane) production in Poland is 7 billion Nm3 per year [4]. These types of farms are located in rural areas surrounded by other buildings [5]. However, more and more often new breeding farms are launched away from buildings, e.g., because of the odor nuisance. In this case, building of a local energy source in the farm may turn out to be less costly than building a new power line. Such solutions can also act as a guaranteed power supply system. In Europe, internal combustion engines (ICE) are most commonly used for CHP propulsion [6]. This is because, among other things, controlling the generated power is relatively easy [7]. In addition, the use of an ICE as a driving source for CHP still seems to be the cheapest solution despite the continuous development of other types of drives based on micro gas turbines, micro Rankine cycle, Stirling, and thermophotovoltaic technologies [8], especially in the case of systems with power up to several dozen kilowatts [9]. This applies in particular to mCHP technology. For farmers who operate and service agricultural machines on their own, a CHP operation built on a simple internal combustion engine is not difficult. For small CHPs, it is relatively easy to adjust the efficiency of the drive to match the system to the priority of electricity or heat [10], by controlling, for example, the rotational speed of the ICE [11][12].

2. Micro Electrical Systems with CHP

Micro power grids (micro electrical systems) have a great potential to become a solution to the electricity quality in power grids penetrated by micro renewable energy sources (micro-RES), especially photovoltaic sources with their own converter systems [13][14]. The influence of these sources is particularly visible in the values and shapes of voltages and currents in low-voltage (LV) grids. The solution may be an on-grid power system with its own control and balancing source. Such systems are connected to the power grid but are not visible to the active power (frequency) system control. This means that it balances the entire electrical energy demand in its own balancing shield (control shield, in particular an LV line). Renewable energy management in local LV grids usually uses the method of reducing the power generated from individual sources, to limit the voltage increase in the LV network and prevent the flow of energy to the medium-voltage (MV) grid [15]. There will be less and less sources with synchronous generators in the structures of microgrids powered by RES. This results in problems with ensuring sufficient system inertia for frequency control, especially in the case of micro systems covering a very small network area. Therefore, extensive frequency and voltage management systems in the LV network are necessary [16]. A good regulating and balancing source for local microgrids is a source driven by an ICE, i.e., a cogenerator. A good location for such a source is the LV grid on a farm with its own microbiogas power plant. The latter is not required if natural gas or other fuel is available.

3. Cogeneration Devices for Local Micro Electrical Systems

Typical solutions for regulation and balancing sources of electric energy are the use of synchronous generators connected to the power grid. Power converter systems are used more and more often to control power quality parameters in local microgrids. In local LV grids saturated with RES sources with their own inverters, there is a lack of kinetic energy stored in rotating machines. For such solutions, it is necessary to use connections that synchronize the operation of inverters in order to regulate the voltage and frequency [17][18]. This problem is particularly noticeable in a microgrid with one generator and a drive with a slight oversupply of torque (e.g., ICE). Frequency and voltage fluctuations may occur when the load power changes. The converter system, without kinetic energy resources, does not have sufficient control resources [19][20]. In such systems, it is necessary to use techniques to model hidden equivalent inertias for microgrids with RES using power converters [21][22][23].
Due to the low price, durability, and simplicity of the solution, induction machines driven by internal combustion engines are used for the construction of CHP generators and cogenerators up to 50 kW, fueled from microbiogas plants. For induction generators permanently connected to the grid, these solutions have been known for years [24]. However, the use of induction generators to supply the island network is associated with the problem of voltage regulation and excitation of the generator [25][26]. Cogeneration sources are currently treated as an excellent energy link in the circular economy. On the other hand, at the legislative level in the EU, they are a constantly underestimated flexible regulatory and balancing source [27][28][29]. Electrical power sources of up to 50 kW are formalized in Poland as prosumer energy sources [30]. Sources of up to 50 kW of electrical power can be connected to the grid on the basis of a notification submitted to the distribution system operator (DSO). The device must have a certificate confirming compliance with the technical requirements of the European NC RfG [31][32][33]. Induction generators are also used in small-scale water and wind energy, but permanently connected to the power grid [34][35][36].
The mCHP devices typically operate connected to the power grid (on-grid), and for proper operation induction generators only require a capacitor bank compensating inductive reactive power. They are also adapted to work with constant power with the lowest specific fuel consumption [37][38]. This is quite a limitation, such a solution means that mCHP devices most often work in intermittent mode, i.e., they are switched on periodically during the day when the demand for energy is greater. However, an important advantage of induction generators is the lack of the need to use a power supply for the excitation winding [39].
mCHP devices in the on-grid mode, although they have the ability to control the generated electric power by changing the fuel dose, are most often equipped with a manually controlled throttle and work with constant power. The manually controlled throttle reduces the cost of the device. However, such a system cannot properly generate energy in an off-grid installation without an excitation capacitance control system [40][41]. A major limitation in this case is also the low quality of electricity, especially large fluctuations in voltage and frequency [42]. Despite this, off-grid operation is increasingly required, e.g., in livestock farms requiring an emergency source of energy in the event of a power failure of the power grid.
Summing up the current state of the techniques used in practice to connect a source with an induction generator with a power of up to several dozen kilowatts:
  • devices operating in on-grid and off-grid modes are built based on synchronous generators. The time of switching between on-grid and off-grid modes is not shorter than several seconds, which cannot be considered as an uninterruptible power supply;
  • devices with induction generators operating in on-grid mode are permanently connected and usually operate with constant or variable electric power;
  • converter systems are used to control the excitation of synchronous generators or in systems with induction generators as inverters, allowing the rotational speed of the generator and the driving internal combustion engine to be changed.

This entry is adapted from the peer-reviewed paper 10.3390/app13105866

References

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