JP2001251869A - Dc/ac conversion circuit, power converter and power generating system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a DC/AC conversion circuit of high conversion efficiency which can be used at a high-voltage range, while ensuring a practical transistor size is ensured. SOLUTION: In this DC/AC conversion circuit, two or more series circuits are connected in parallel across DC input terminals, which circuit is constituted by connecting two switching elements turning on and off, independently in series, and on/off operations of the respective switching elements are repeated alternately with respective prescribed timing, so that AC power is outputted between intermediate nodes of the series circuits. The switching element is a MOSFET using GaN based material, or a switching circuit formed by connecting two or more FETs in series. The product of the stationary on- resistance of the switching element and the effective current value of AC output is 0.0001-0.01 (units of A.Ω).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quadrature conversion circuit provided with a switching element and having high power conversion efficiency, a power conversion device provided with the orthogonal conversion circuit, and a power conversion device and a power generation device such as a fuel cell. The present invention relates to a power generation system provided.

[0002]

2. Description of the Related Art As a power generation system of this kind, a reformer for producing hydrogen from a raw fuel such as natural gas and a reverse reaction of electrolysis of water from hydrogen produced by the reformer and oxygen in the air. A fuel cell that exhausts high temperature and generates DC power, an inverter that orthogonally converts the DC power generated by the fuel cell into an AC, and recovers heat energy of high-temperature exhaust heat from the fuel cell There is a fuel cell power generation system provided with an exhaust heat recovery device. Such a fuel cell power generation system does not emit sulfur oxides SO _x , emits very little nitrogen oxides NO _x , and emits carbon dioxide CO ₂ ,
It is a power generation system that is low in noise and vibration and is friendly to the global environment. In addition, the power generation efficiency is as high as about 40%, and the total energy efficiency including the power generation efficiency and the recovery heat efficiency is as high as about 85%, making effective use of energy. It has a wonderful feature that it can be made, and is attracting attention as a new energy. An inverter is used to orthogonally convert DC power generated in the above-described fuel cell into AC power. As a switching element in the inverter, for example, S
An IGBT (insulated gate bipolar transistor) using i is used. Further, for the purpose of reducing the on-resistance of the transistor, transistors with low on-resistance are connected in parallel.

[0003]

At present, there is a demand for improving the power conversion efficiency of the power converter inside the power generation system in order to increase the efficiency of the total energy of the power generation system. That is, it is required to improve the orthogonal conversion efficiency in the orthogonal conversion circuit inside the power conversion device. Here, the loss that occurs in the orthogonal transform circuit includes conduction loss, switching loss, drive loss, and the like. However, there is a physical limitation in improving the conversion efficiency of the power conversion device while using the IGBT manufactured using Si as the switching element used in the orthogonal conversion circuit unit of the power conversion device. There are other structural issues as well. The problem is that the presence of a pn junction as a barrier in the transistor's current path increases its on-resistance, resulting in a large conduction loss. Therefore, when the IGBT is used as the switching element in the orthogonal transform circuit, the power conversion efficiency cannot be improved. Furthermore, if transistors are connected in parallel, the on-resistance can be reduced.
If the number of connected transistors increases, the input capacitance increases, and the drive loss increases, so that the conversion efficiency cannot be improved. From the above, when an FET is used as a switching element, there is no pn junction serving as a charge barrier in the current path, so that conduction loss can be reduced and the conversion efficiency of the power conversion device can be improved. Conceivable. However, the inventor of the present application has found that, when an FET manufactured using Si is adopted as a switching element, the size of the transistor becomes too large if the power conversion efficiency is optimized. On the other hand, it has been found that when the size of the transistor is made to be a practical size, the power conversion efficiency is reduced.

[0004] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to improve the conversion efficiency of an orthogonal conversion circuit having a switching element. Another object of the present invention is to improve the overall energy efficiency of a power generation system including the power conversion device and a power generation device such as a fuel cell.

[0005]

In order to achieve this object, the orthogonal transform circuit according to the present invention is characterized in that it has an independent on / off operation as described in claim 1 of the claims. By connecting two or more sets of series circuits in which two switching elements are connected in series and connecting them between DC input terminals, and repeating the on / off operation of each of the switching elements at a predetermined timing, respectively, An orthogonal transformation circuit for outputting AC power between intermediate nodes of a series circuit, wherein the switching element uses a GaN-based material.
One FET, or a switching circuit formed by connecting two or more FETs in series, wherein the product of the steady-state on-resistance of the switching element and the effective current value of the AC output is:
The point is 0.0001 to 0.01 (unit: A · Ω).

To achieve this object, the orthogonal transform circuit according to the present invention is characterized in that two switching elements that are independently turned on and off operate as described in claim 2 of the claims. By connecting two or more sets of series circuits formed in series in parallel and connecting them between DC input terminals, and alternately repeating the ON / OFF operation of each of the switching elements at a predetermined timing, an intermediate node between the series circuits is Wherein the switching element is a single FET using a GaN-based material or a switching circuit in which two or more FETs are formed in series. The point is that the normal on-resistance is 0.00001 to 0.001 (unit: Ω).

In order to achieve this object, the power converter according to the present invention is characterized in that the DC power is converted into the DC power and the DC power is converted into the AC power, as described in claim 3 of the claims. Or, a power converter for converting AC power to AC power, comprising at least one orthogonal conversion circuit unit for converting DC power to AC power, at least one of the orthogonal conversion circuit unit, An orthogonal transformation circuit according to item 1 or 2.

[0008] A first characteristic configuration of the power generation system according to the present invention for achieving this object is as described in claim 4 of the claims, wherein the power generation device outputs DC power, and A conversion device comprising at least one of an inverter for orthogonally converting the DC power output by the device and converting the DC power into an AC, or a converter for converting the voltage level of the DC power, the conversion device comprising: Comprises at least one orthogonal conversion circuit unit for converting DC power into AC power, and at least one of the orthogonal conversion circuit units is the orthogonal conversion circuit according to claim 1 or 2.

A second feature of the present invention is that, in addition to the first feature, the power generation device is a fuel cell, as described in claim 5 of the claims.

The operation and effect will be described below. According to the characteristic configuration of the orthogonal transform circuit according to the present invention, the conventional IGBT and Si are used while securing a practical transistor size by employing an FET manufactured using a GaN-based material as a switching element. FE made
The orthogonal transform circuit can be used in a voltage range higher than T. Further, the product of the steady-state ON resistance of the FET and the effective current value of the AC output is 0.0001 to 0.01.
By setting the range (unit: A · Ω), a loss in the orthogonal transform circuit can be reduced while a high operating voltage is secured. Further, by setting the on-state resistance of the FET in a steady state to 0.00001 to 0.001 (unit: Ω), it is possible to reduce the loss in the orthogonal transform circuit while securing a high operating voltage. The steady-state ON resistance referred to here is a resistance value between a drain and a source in a triode characteristic region of the FET, and includes a parasitic resistance component of each terminal of the drain and the source. The GaN-based material is Ga
N, AlGaN, InGaN, InAlGaN, or a material combining these.

According to the characteristic configuration of the power conversion device according to the present invention, since the power conversion is performed using the above-described orthogonal conversion circuit, it is possible to provide a power conversion device with good conversion efficiency.

According to the characteristic configuration of the power generation system according to the present invention, the power generation system is configured by using the power conversion device having the above-described orthogonal conversion circuit, so that a power generation system with good overall energy efficiency is provided. You can do it.

According to the second characteristic configuration, the overall energy efficiency of the fuel cell power generation system can be improved.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of an orthogonal transform circuit according to the present invention will be described with reference to the drawings. As shown in FIG.
The orthogonal transform circuit 100 according to the present invention includes four switching elements SW ₁ , SW ₂ , SW ₃ , and SW ₄ , a driving transistor circuit 12 that individually drives each switching element, and a free wheel diode. ing. In the operation cycle, SW ₁ and SW ₄ are turned on and SW ₂
A half cycle and SW ₃ is positive in the off and, SW ₁
And a negative half cycle in which SW ₄ is off and SW ₂ and SW ₃ are on, whereby an AC current I _AC flows in the load L. FIG. 2 is a cross-sectional view of an FET 200 that constitutes each switching element and is manufactured using GaN. Next, a loss affecting the conversion efficiency of the orthogonal transform circuit will be considered. The losses in the switching elements of the orthogonal transform circuit include the conduction loss L ₁ and the drive loss L
_2, and a switching loss L _3. Hereinafter, calculating each loss of SW ₁ in FIG. As described above, since the orthogonal transform circuit has a positive cycle and a negative cycle, the conduction loss L1 is expressed by Equation ₁ as follows.

[0015]

L ₁ = L _1P + L _1N

Here, L _1P is the conduction loss in the positive cycle, and L _1N is the conduction loss in the negative cycle, given by Equations 2 and 3, respectively.

[0017]

[Number 2] ^{_{L 1P = R ON × I 2}} /2

[0018]

Equation 3] ^{_{L 1N = R ON × I 2}} /2

[0019] Here, R _ON is the on resistance of the SW _1, I is the effective value of the current flowing through the SW _1. Incidentally, since the leakage current is very small through the SW ₁ in the negative cycle, L _1N can be regarded as zero. Therefore, the conduction loss L1 is given by Equation 4.

[0020]

Equation 4] _{L 1 = L 1P = R ON} × I 2/2

Next, an explanation will be made for a driving loss L _2. Drive loss L ₂ is given by equation (5).

[0022]

L ₂ = C × V _GS ² × f / 2

Here, C is the input capacitance of the switching element, V _GS is the gate-source voltage of the switching element, and f is the switching frequency.

Next, the switching loss will be described with reference to FIG. The loss here is obtained by integrating the product of the voltage change and the current change during the switching period, assuming that the time change of the voltage and the current is linear. Take the product with
The required switching loss can be obtained. In one switching operation, the voltage change and the current change occur in each of the ON operation and the OFF operation, so the number of the voltage change and the current change is represented by 2 × f. Therefore, Equation 6 holds.

[0025]

(Equation 6)

Here, V 'is a voltage change in the switching element, I' is a current change in the switching element, and τ is a switching period. As is clear from the right side of Equation 6, the switching loss depends only on the switching period. Here, the switching period refers to a rise time and a fall time of a current flowing through the switching element or a potential difference between both ends of the switching element in the ON / OFF operation of the switching element. The switching period is adjusted by adjusting the driving capability of the driving transistor circuit 12.

Based on the above equation, using GaN when the source-drain voltage is changed to 60, 300, and 1000 (V) and the drain current is changed to 10, 20, and 50 (A). Tables 1 to 9 show the relationship among the on-resistance, the input capacitance, the conduction loss, the drive loss, the total loss obtained from the sum of the conduction loss and the drive loss, and the transistor area of the switching element employing the manufactured FET. In the evaluation of the loss here, the difference in the input capacitance of the switching element is evaluated in terms of the driving loss, and the driving capability of the driving transistor is adjusted so that the switching period is constant. Therefore, the switching loss is constant and is not included in the total loss here.

Here, the on-resistance and the input capacitance indicate the on-resistance or the gate capacitance when the switching element is composed of one GaN-FET.
When the FETs are connected in parallel, the combined resistance or combined capacitance is shown. However, the combined capacitance is a combined capacitance corresponding to the number of GaN-FETs involved in switching. Therefore, an inversely proportional relationship is established in which increasing the on-resistance decreases the gate capacitance. The transistor area is mainly represented by the channel cross-sectional area when a transistor having the above parameters is manufactured. The transistor area is represented by the relationship between the blocking voltage (V) and the on-resistance (mΩ · cm ² ) in FIG. The theoretical area is determined from the ON resistance (Ω) used. However, when the GaN-FET is manufactured in a horizontal structure, the area of the source, drain and gate electrodes must be taken into consideration. Is shown. The gate-source voltage is 20 (V), and the switching frequency is 20 (kHz).

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

[Table 5]

[0034]

[Table 6]

[0035]

[Table 7]

[0036]

[Table 8]

[0037]

[Table 9]

When the on-resistance and the output current are changed, the relationship between the on-resistance of the switching element and the total loss in the switching element is as shown in FIGS. Value and the optimal on-resistance value are determined. FIG. 13 to FIG.
12 summarizes the relationship between the on-resistance and the loss shown in FIG. 12 with respect to voltage and current. As is clear from FIGS. 13 to 15, the on-resistance in the steady state is 0.000.
When the value is in the range of 01 to 0.001 (unit: Ω), the effect of reducing the total loss of the switching element is particularly exhibited. Further, the product of the optimum value of the on-resistance and the effective current value of the AC output is obtained, and the total loss for the product is shown in FIG. 16 when the source-drain voltage is 300 (V).
Referring to FIG. 16, the product of the on-resistance and the current value is equal to 0.
When it is between 0001 and 0.01 (AΩ), the effect of reducing the total loss is particularly exhibited. Referring to FIG. 8 and Table 5, when the transistor area when the efficiency takes the optimum value in the case of 300 (V) and 20 (A) is obtained, about 3 (c)
m ² ), which is within the usable transistor area including the source, drain and gate electrode areas.

Next, as a comparative example, Si was used instead of GaN.
1 shows an example in which an FET manufactured by using the above is used as a switching element. The structure of the orthogonal transformation circuit and the FET is
This is the same as the configuration shown in FIGS. Hereinafter, GaN
As in the case of the above, the source-drain voltage is set to 60, 30
0, and 1000 (V), and the drain current is 10, 2
From the on-resistance of the switching element adopting the FET manufactured using Si and the input capacitance, the conduction loss, the drive loss, and the sum of the conduction loss and the drive loss when changing to 0 and 50 (A). Tables 10 to 18 show the relationship between the calculated total loss and the transistor area. The switching loss is not included in the total loss here, as in the case where the switching element is an FET using GaN. Although the transistor area was obtained in the same manner as in the case of the GaN-FET,
Since the Si-FET is manufactured in a vertical structure, the theoretical area is shown as the transistor area without considering the electrode area. However, in the case of the horizontal structure, the same consideration as in the case of the GaN-FET is necessary. The gate-source voltage is 20 (V), and the switching frequency is 20 (kHz).

[0040]

[Table 10]

[0041]

[Table 11]

[0042]

[Table 12]

[0043]

[Table 13]

[0044]

[Table 14]

[0045]

[Table 15]

[0046]

[Table 16]

[0047]

[Table 17]

[0048]

[Table 18]

The relationship between the on-resistance of the switching element and the total loss of the switching element when the on-resistance and the output current are changed is as shown in FIGS. A minimum value and an optimum on-resistance value are obtained. 26 to 28
9 summarizes the relationship between the on-resistance and the loss shown in FIGS. 17 to 25 with respect to voltage and current. FIG.
As is clear from FIG. 28, the on-state resistance at steady state is:
When it is 0.00001 to 0.001 (unit: Ω), the effect of reducing the total loss of the switching element appears. The product of the optimum value of the on-resistance and the effective current value of the AC output is obtained, and the total loss for the product is shown in FIG. 29 only when the source-drain voltage is 300 (V).

Referring to FIG. 29, when the product of the on-resistance and the current value is between 0.0001 and 0.01 (AΩ), the effect of reducing the total loss appears. However, since the relationship shown in FIG. 30 is established between the blocking voltage and the on-resistance, it is not appropriate to employ an FET manufactured using Si as a switching element. The reasons are as follows.

When an AC output of about 100 (V) is obtained as an effective value, the orthogonal transform circuit of the present invention uses about 300 (V).
Is required for the switching element. Here, the voltage is 30 in the Si-FET.
The on-resistance value in the case of 0 (V) is approximately 13 (mΩ) from FIG.
Cm ² ). On the other hand, for example, FIG.
(V) and 20 (A)) are about 0.0023 (Ω), so that
When a transistor capable of withstanding the voltage of (V) is manufactured, the transistor area becomes as large as about 5.7 (cm ² ). In practice, it is inappropriate to employ a transistor of such a size.
In ET, the transistor area becomes too large to optimize the power loss. This means that the power loss cannot be reduced while maintaining a practical transistor area. On the other hand, GaN-FET
In FIG. 3, the on-resistance value when the voltage is 300 (V) is shown in FIG.
From 0 to about 0.025 (mΩ · cm ² ). Further, for example, the optimum on-resistance value obtained in FIG. 8 (300 (V), 20 (A)) is about 8 × 10 ⁻⁵ (Ω). Therefore, a GaN-F that can withstand a voltage of 300 (V)
When an ET is manufactured, its transistor area is about 3 (c
m ² ), which is within the usable transistor area including the source, drain and gate electrode areas.

Next, FIG. 31 shows an example of a power conversion device provided with an orthogonal conversion circuit. The power conversion circuit 300 includes a converter unit 1, a smoothing circuit 2, and an orthogonal conversion circuit 3, and converts a three-phase AC input to a commercial frequency (50 Hz or 60H).
and z) three-phase AC output. Orthogonal transformation circuit 3
Consists of a switching element 5 and a freewheel diode 6. The three-phase AC input is obtained by orthogonally converting DC power from a fuel cell or the like, and then boosting the DC power using a transformer. Unlike the orthogonal transformation circuit 200 shown in FIG. 2, the three-phase AC output can be obtained by using the six switching circuit units 4 in the orthogonal transformation circuit 3 of FIG.

Next, FIG. 32 shows an example of a power generation system provided with a power converter. The power generation system 400 includes a fuel cell body 7 that receives a supply of hydrogen and oxygen to generate a DC current, a reformer 8 that generates hydrogen from city gas or the like to supply hydrogen to the fuel cell body 7, a fuel cell body A converter 9 for orthogonally converting the DC power output by the generator 7 to generate AC power, an exhaust heat recovery device 10 for recovering exhaust heat of the reformer 8 and the fuel cell body 7 as steam or hot water, and a fuel cell body 7 And a control device 5 for controlling operations of the reformer 8 and the conversion device 9. The conversion device 9 includes the orthogonal conversion circuit having good conversion efficiency according to the present invention.
The overall energy efficiency of the power generation system 400 can be improved.

The present invention has been described above by way of example. However, the present invention is not limited to the above description, and other embodiments are possible. For example, although the present invention has been described using GaN, a similar effect can be obtained with a GaN-based material.
GaN, InGaN, InAlGaN, or a material combining these can also be adopted.

Further, in this embodiment, the horizontal structure is used as the structure of the FET, but a vertical structure may be used.

Further, although the fuel cell is used as the power generation device in the present embodiment, other power generation devices such as a gas engine generator and a solar cell can be used.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an orthogonal transform circuit according to the present invention.

FIG. 2 shows an FE constituting a switching element according to the present invention.
It is sectional drawing of T.

FIG. 3 is a graph exemplarily showing a time change of a voltage and a current.

FIG. 4 is a graph showing a relationship between ON resistance and total loss in a GaN-FET.

FIG. 5 is a graph showing a relationship between on-resistance and total loss in a GaN-FET.

FIG. 6 is a graph showing the relationship between on-resistance and total loss in a GaN-FET.

FIG. 7 is a graph showing the relationship between on-resistance and total loss in a GaN-FET.

FIG. 8 is a graph showing a relationship between on-resistance and total loss in a GaN-FET.

FIG. 9 is a graph showing a relationship between on-resistance and total loss in a GaN-FET.

FIG. 10 is a graph showing the relationship between on-resistance and total loss in a GaN-FET.

FIG. 11 is a graph showing a relationship between on-resistance and total loss in a GaN-FET.

FIG. 12 is a graph showing the relationship between on-resistance and total loss in a GaN-FET.

FIG. 13 is a graph showing the relationship between on-resistance and total loss in a GaN-FET.

FIG. 14 is a graph showing a relationship between on-resistance and total loss in a GaN-FET.

FIG. 15 is a graph showing a relationship between on-resistance and total loss in a GaN-FET.

FIG. 16 is a graph showing the relationship between the product of the on-resistance and the output current in the GaN-FET and the total loss.

FIG. 17 is a graph showing the relationship between on-resistance and total loss in a Si-FET.

FIG. 18 is a graph showing the relationship between on-resistance and total loss in a Si-FET.

FIG. 19 is a graph showing a relationship between on-resistance and total loss in a Si-FET.

FIG. 20 is a graph showing the relationship between on-resistance and total loss in a Si-FET.

FIG. 21 is a graph showing a relationship between on-resistance and total loss in a Si-FET.

FIG. 22 is a graph showing a relationship between on-resistance and total loss in a Si-FET.

FIG. 23 is a graph showing a relationship between on-resistance and total loss in a Si-FET.

FIG. 24 is a graph showing the relationship between on-resistance and total loss in a Si-FET.

FIG. 25 is a graph showing a relationship between on-resistance and total loss in a Si-FET.

FIG. 26 is a graph showing the relationship between on-resistance and total loss in a Si-FET.

FIG. 27 is a graph showing the relationship between on-resistance and total loss in a Si-FET.

FIG. 28 is a graph showing a relationship between on-resistance and total loss in a Si-FET.

FIG. 29 is a graph showing the relationship between the product of the on-resistance and the output current in the Si-FET and the total loss.

FIG. 30 is a graph showing the relationship between blocking voltage and on-resistance in an FET.

FIG. 31 is a circuit diagram of a power conversion device according to the present invention.

FIG. 32 is a configuration diagram of a power generation system according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Converter part 2 Smoothing circuit 3 Orthogonal conversion circuit 4 Switching circuit unit 5 Switching element 6 Reflux diode 7 Fuel cell main body 8 Reformer 9 Conversion device 10 Exhaust heat recovery device 11 Controller 12 Drive transistor circuit 100 Quadrature conversion circuit 200 FET 300 Power conversion device 400 power generation system

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Hirokazu Sasaki, Inventor, Osaka Gas Co., Ltd. 4-1-2, Hirano-cho, Chuo-ku, Osaka-shi (72) Inventor Ken Tabata Hirano-cho, Chuo-ku, Osaka, Osaka 1-2, Osaka Gas Co., Ltd. (72) Mitsuaki Echigo, Inventor 4-1-2, Hirano-cho, Chuo-ku, Osaka City, Osaka Prefecture Inside, Osaka Gas Co., Ltd. (72) Osamu Yamazaki, Chuo-ku, Osaka City, Osaka 4-1-2 Hiranocho Osaka Gas Co., Ltd. F-term (reference) 5F102 GA01 GB01 GB04 GC01 GD01 GJ04 5H007 BB07 CA02 CB05 CC23 5H027 AA02 BA01 5H420 CC03 DD03 DD04 EA12 EA45 EB04 EB39

Claims (5)

[Claims] 1. A series circuit comprising two switching elements that are independently turned on and off in series and connected in parallel between two or more DC input terminals, and each of the switching elements is turned on and off. A quadrature conversion circuit that outputs alternating-current power between intermediate nodes of the series circuits by repeating alternately at a predetermined timing, wherein the switching element is a single FET using GaN-based material, or two or more switching elements. FET is a switching circuit formed in series, the product of the steady-state ON resistance of the switching element and the effective current value of the AC output is 0.0
001 to 0.01 (unit: A · Ω). 2. A series circuit comprising two switching elements which are independently turned on and off in series, and two or more sets of series circuits are connected in parallel and connected between DC input terminals, and each of the switching elements is turned on / off. A quadrature conversion circuit that outputs alternating-current power between intermediate nodes of the series circuits by repeating alternately at a predetermined timing, wherein the switching element is a single FET using GaN-based material, or two or more switching elements. A switching circuit in which FETs are formed in series, and a steady-state ON resistance of the switching element is 0.00001 to 0.001 (unit:
Ω). 3. A power converter for converting DC power to DC power, DC power to AC power, or AC power to AC power, wherein the quadrature conversion circuit converts DC power to AC power. And at least one of the orthogonal transformation circuit units has
A power conversion device, which is the orthogonal conversion circuit according to claim 1. 4. A power generator for outputting DC power, and at least one of an inverter for orthogonally converting the DC power output from the power generator into AC and a converter for converting a voltage level of the DC power. A power generation system comprising: a conversion device; and the conversion device includes one or more orthogonal conversion circuit units that convert DC power into AC power, and at least one of the orthogonal conversion circuit units is configured to be a power conversion system. 3. A power generation system, which is the orthogonal transformation circuit according to 2. 5. The power generator according to claim 4, wherein the power generator is a fuel cell.
The described power generation system.