JPH08163881A - Power generation system utilizing coil of lng - Google Patents

Abstract

PURPOSE: To obtain a highly efficient power generation system employing the cold of LNG by arranging two types, i.e., P type and IN type, of thermoelectric elements on the inner wall part of LNG channel in an LNG evaporator where LNG flow through the evaporator and sea water flows on the outside thereof. CONSTITUTION: The power generation system utilizing the cold of LNG comprises an LNG evaporator having an LNG channel arranged with a pair of P type and N type thermoelectric elements. A module is constituted by arranging the thermoelectric elements in series electrically but in parallel thermally so that the electromotive force per a pair of thermoelectric elements is lowered to pass thermal flux as high as possible. Preferably, the evaporator has a parallel-plate structure, the thermoelectric element is disposed between two evaporator panels, i.e., on the LNG channel side, and a ceramic board is employed for the evaporator fixed to the sea water side. Furthermore, a low temperature end electrode and an evaporator panel are provided with curved parts which function as a fin thus enhancing the heat transmission rate. This structure realizes a highly efficient power generation system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power generation system using LNG cold heat. More specifically, the present invention relates to an LNG cold heat utilization power generation system having excellent power generation efficiency and a simple structure.

[0002]

2. Description of the Related Art Conventionally, electric energy has been an important foundation for industrial society, and fossil fuels such as coal, oil, and natural gas are widely used to obtain the electric energy. . However,
Such fossil fuels do not exist inexhaustibly,
If it continues to be used in this state, it may be exhausted several hundred years later.

A number of measures have been made to date for the depletion of fossil fuels, one of which is as follows.
A highly efficient power generation system capable of obtaining the maximum output energy with respect to the minimum input energy is being studied. Liquefied natural gas (LN), which is one of fossil fuels
The power generation system using G) is LNG low NO _x low SO.
It's getting more attention than _x- sex.

The power generation system using LNG is roughly classified into a direct expansion system, another fluid Rankine cycle system,
The direct expansion method and the separate fluid Rankine cycle method are known. The direct expansion method is LNG that has flowed out of the LNG tank.
Is boosted using an LNG pump, and this boosted LNG is
Is heated using a heat source such as seawater to vaporize LNG, and the vaporized high pressure natural gas (NG) is sent to a turbine generator to directly drive a turbine to generate electricity. In this direct expansion method, LNG after driving the turbine is further heated with seawater,
It can then be used as fuel for thermal power plants. This direct method is generally used when the LNG discharge pressure is not so high, and its efficiency is generally about 12 to 22%.

On the other hand, in the separate fluid Rankine cycle, a condenser is used to cool and condense a fluid such as propane and chlorofluorocarbon, and then the cooled and condensed fluid is boosted with a pump, and the boosted fluid is seawater or the like. The above heat source is used to heat and vaporize, and the vaporized fluid is used to drive the turbine to generate electricity. This separate fluid Rankine cycle system can be used even when the LNG discharge pressure is high, but the efficiency is poor at around 10 because a secondary medium such as propane or CFC is used.

Further, the direct expansion system + CFC rankine cycle system is a combination of the direct expansion system and CFC rankine cycle system as shown in FIG. 1, for example, and the efficiency is very good at about 17 to 27. . However, this direct expansion method + CFC rankine cycle method uses a heat cycle, and thus has poor maintainability, and when CFCs are used as a secondary medium, there is a problem of ozone layer depletion. As the secondary medium, for example, propane or the like can be used in addition to CFC, but in that case, the efficiency is extremely lowered.

Therefore, in recent years, LNG direct expansion + thermoelectric generators have been studied in comparison with the conventional direct expansion + flon rankine cycle system. In this LNG direct expansion + thermoelectric generator system, for example, as illustrated in FIG. 4, LNG flowing out from the LNG tank is boosted by using an LNG pump, and this boosted LNG is passed through the LNG evaporator, Then, power is generated by a thermoelectric generator installed in this LNG evaporator, and the temperature of LNG is also raised at the same time.

In the LNG direct expansion + thermoelectric generator system, not only the thermoelectric generator can generate electric power, but also the temperature-raised LNG can be guided to the LNG turbine to drive the turbine to generate electric power. Becomes Furthermore, in this LNG direct expansion + thermoelectric generator system, NG after driving the turbine in the LNG turbine
Is used as the fuel for ordinary thermal power generators.

In this LNG direct expansion + thermoelectric generator system, the LNG evaporator plays a very important role. Up to now, there are mainly two general LNG evaporators, an open rack system and a submerged combustion system. In the open rack system, for example, as shown in FIG. 2, aluminum pipes are joined together to form an evaporator panel, and LNG is passed through this pipe. By applying seawater or the like to the outside of this evaporator panel, LNG is vaporized (NG). The open rack system does not use fuel as a heat source and can be operated only by the seawater supply power, so that the structure is very simple. For this reason, this open rack system is used as an evaporator for base load in normal operation.

On the other hand, in the submerged combustion system, as shown in FIG. 3, for example, a submerged combustion burner capable of burning even in liquid is used to burn the heating fuel in the water tank to be heated. This is a method of evaporating LNG with warm water. This submerged combustion method requires fuel as a heat source, but the device is smaller than the open rack method,
Since it can be quickly started, it is often used for operation during peak load of generators.

However, even in the LNG cold heat utilization power generation system using these current LNG evaporators, in reality, it is still not sufficiently satisfactory in terms of efficiency. Therefore, the present invention has been made in view of the circumstances as described above.
While taking advantage of the features of the power generation system using the NG evaporator,
It is an object of the present invention to overcome the limitations of the conventional technology and provide a more efficient LNG cold heat utilization power generation system.

[0012]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a P-type and an N-type in the LNG flow passage inner wall portion of an LNG evaporator in which LNG flows in the evaporator and seawater flows outside the evaporator. Provided is an LNG cold heat utilization power generation system characterized in that two types of thermoelectric elements of a mold are arranged.

The present invention, as one embodiment,
Provided is an LNG cold heat utilization power generation system characterized in that two or more parallel flat plate-shaped evaporator panels are provided with two types of thermoelectric elements of P type and N type.

[0014]

That is, in the system of the present invention, the LNG evaporator utilizes seawater as an external heat source and flows it to the outer surface of the evaporator, as compared with the conventional open rack type LNG evaporator. It is similar in that it vaporizes, but is largely different in that a thermoelectric element is arranged in the evaporator, and when seawater is flowed outside the evaporator to vaporize LNG, the temperature difference between LNG and seawater is used to There is a great feature that the thermoelectric elements arranged also generate electricity.

In the LNG cold heat utilization power generation system of the present invention, two types of thermoelectric elements, P type and N type, are arranged in a pair in the flow path of the LNG, the electromotive force per pair of thermoelectric elements is made small, and the passing heat flux is as high as possible. Since it is desirable to increase the size, a module in which thermoelectric element pairs are arranged so as to be electrically in series and thermally in parallel is configured. In the present invention, the panel forming the evaporator is preferably a parallel plate shape, and the thermoelectric element is arranged on the evaporator panel at a place between the two evaporator panels as shown in FIG. 5, for example. The roadside.

If this thermoelectric element pair is arranged outside the evaporator panel, it is necessary to prevent erosion against seawater. Furthermore, in such arrangement of the thermoelectric element pair, the thermoelectric element is located on the high temperature side. In addition, it is not possible to use a Bi-Sb thermoelectric element that has high performance at low temperatures. As the thermoelectric element used in the present invention, for example, the one illustrated in FIG. 6 can be used, but of course, it is not limited to such an arrangement.

When the flow path of the LNG is changed from a conventional circular tube to a plate shape, it is not necessary to provide anticorrosion insulation on the low temperature side of the thermoelectric element, and on the high temperature side of the thermoelectric element, an evaporator panel as a structural material protects it from seawater. In addition, the electrical connection is free, which is advantageous in terms of optimizing the current density. Further, since the whole is a flat plate, processing such as integral vacuum brazing is possible.

In the present invention, the evaporator panel to which the thermoelectric element is attached may be, for example, one shown in FIG. 7 as one mode. Generally LNG
Is a non-conductor, the thermoelectric element attached to the LNG side of the evaporator panel does not need to be insulated, and may be exposed and arranged on the LNG side in order to lower the temperature on the low temperature side of the thermoelectric element.

On the other hand, for the evaporator panel mounted on the seawater side of the evaporator panel, it is desirable to use a ceramic base for the insulation of the high temperature side electrode. Further, in the present invention, it is desirable to provide a curved portion on the low temperature end electrode and the evaporator panel in order to relax thermal stress due to the difference in thermal expansion coefficient between ceramics and metal.
Furthermore, since this curved portion projects into the fluid on the high-temperature side and the fluid on the low-temperature side, it functions as a fin and the heat transfer rate can be improved.

For the thermoelectric element of the present invention, a Bi-Te type thermoelectric element can be generally used for low temperatures, but in the low temperature region, a Bi-Sb type thermoelectric element is used as a thermoelectric element having a high figure of merit. You may use the thermoelectric element of. In general, it is known that the performance of the Bi-Sb-based thermoelectric element is higher than that of the Bi-Te-based thermoelectric element in the low temperature range from around 0 ° C.
The outer metal temperature on the seawater side of the NG evaporator is mostly in the region of 10 ° C or lower, and the average temperature of the thermoelectric element is considered to be 0 ° C or lower in most regions. Therefore, in the present invention, Bi-Sb is used. It is desirable to use a thermoelectric element of the system.

Hereinafter, the present invention will be described in more detail with reference to examples.

[0022]

Example 1 When the evaporator in the cold heat power generation system was cylindrical and had a diameter of 27 mm × 150, the optimum thermoelectric element shape of the cold heat power generation system and its output were examined.

The amount of heat passing through the power generation system needs to consider not only the amount of heat due to heat conduction but also the amount of heat absorption and heat dissipation due to the Peltier effect. The amount of heat absorbed and the amount of heat released by this Peltier effect depend on the current density. Therefore, in addition to the thermoelectric element characteristics, the heat transfer coefficient and the fluid temperature on the high temperature side and the low temperature side are important parameters for the conditions of the power generation system. Need to be promoted. Further, since the temperature at the end of the thermoelectric element changes depending on the heat flow rate, a convergence calculation for optimizing the thickness of the thermoelectric element and the current density is performed.

On the other hand, the temperature of the fluid changes as it flows on the heat transfer surface, and the optimum thickness and the optimum current density change on the heat transfer surface. In this example,
The heat transfer surface was divided in the flow direction of the fluid, the liquid temperature was calculated for each section, and the convergence calculation of the high temperature end temperature and the low temperature end temperature was performed to optimize the LNG cold heat power generation system and set the optimum operation.

The conditions are shown below. To be exact, LN
Although the heat conductivity and the specific heat of LNG change along the flow direction due to the phase change of G and the freezing of seawater, both the specific heat and the heat transfer coefficient are assumed to be constant regardless of the flow direction for simplicity. The conditions are as shown in Table 1.

[0026]

[Table 1]

Here, the fluid flow rate is a flow rate per cross-sectional length of the heat transfer surface. The change in the optimum element thickness depending on the position of the heat transfer surface of the optimized LNG cold heat generation system of the present invention is
This is as shown in FIG. 8, and changes in the fluid temperature and the temperature at both ends of the element are as shown in FIG. Furthermore, changes in the optimum current density and output density are as shown in FIG. 10, and changes in the heat absorption amount and heat dissipation amount at both ends of the element are shown in FIG.
As shown in.

From these results, the optimum thickness of the element is 4.
It can be seen that it is 2 mm. Furthermore, from these results, the temperature distributions of the fluid and the element have a large temperature difference near the LNG inlet and a small temperature change near the outlet (pinch). Thus the distribution of the optimum current density is greatly changed, in the vicinity of LNG inlet at the outlet portion whereas exceeds the 200,000 / m ² is 14,300A / m ^2. The power density also changes, 1.5m from the LNG entrance
You're getting most of the output up to the position.

The actual LNG cold heat power generation system is φ27.
Assuming that it is composed of 150 mm × 150 evaporators, the calculation result of the output per evaporator is as shown in Table 3 below. In addition, the numerical value in () is a design value of the conventional general LNG evaporator.

[0030]

[Table 2]

The schematic shape of the thermoelectric generator determined from the above results can be shown, for example, as shown in FIG. 12 as one mode. The LNG cold power generation system having the basic design here has a specific output per LNG: 1T / H of 11.9 kW / (T / H-LNG) at the power generation end, and a power generation efficiency of 5.1%. Power generation end ratio output of CFC Rankine cycle combined with direct expansion: 10.9kW
It is possible to obtain a specific output exceeding / (T / H-LNG). Furthermore, when this LNG cold heat utilization power generation system is combined with a direct expansion type LNG power generation plant, the specific output becomes 63.7 kW / (T / H-LNG). Example 2 Next, with reference to the case of the tubular evaporator in the cold thermal power generation system of Example 1, the optimum shape of the plate-shaped evaporator was examined.

That is, first, assuming that the heat transfer area per evaporator panel is 39.4 m ² from Example 1, the required thermoelectric element volume is 0.158 m ³ , 110 T per evaporator panel. The thermoelectric element volume required for evaporation of LNG / h is 8.83 m ³ . The optimum thermoelectric element thickness is 4 mm, the shape of the thermoelectric element used is 10 × 10 × 4 mm, and the gap between the thermoelectric elements is 1 mm, the shape of the evaporator panel is 6,990 × 3.
410mm, the required number of thermoelectric elements is 1.97 x 1
0 ⁵ × 2, about 400,000.

Of course, it is possible to reduce the shape of the thermoelectric element by enlarging the shape of the thermoelectric element, but the representative length of the thermoelectric element / electrode coupling portion becomes large, and it is necessary to consider thermal stress. The schematic shape of the power generation system 1 block according to the present invention is, for example, as illustrated in FIG. Example 3 Next, based on Example 2, the heat balance of the LNG cold heat utilization power generation system of the present invention was examined.

The system and heat balance of the LNG cold heat utilization power generation system of the present invention are as shown in FIG. 14 when the LNG turbine generator end output is 4,900 kW and the thermoelectric power generation system output is 1.313 kW (DC). Becomes The power generation system designed by this trial can obtain a high power density exceeding 1500 W / m ² in the low temperature region, but the power density in the high temperature region near the pinch is low and hardly contributes to the power generation system. this is,
For example, as illustrated in FIG. 15, low temperature seawater and LN
The G temperature difference is large and the temperature difference between the thermoelectric elements is large, while the temperature between the fluids is close to each other in the vicinity of the pinch, so that the temperature difference between both ends of the thermoelectric element is small.

In such a case, for example, as shown in FIG. 16, the thermoelectric elements in the high temperature region may be removed to reduce the number of thermoelectric elements to reduce the heat transfer area. Embodiment 4 Furthermore, in the present invention, the mounting length of the thermoelectric element is changed with respect to the heat transfer surface length of 3.1 m required when mounting the power generation system on the entire heat transfer surface to improve the performance of the power generation system. It was investigated. The result is as illustrated in FIG. 17, and it is possible to obtain an output of about 1,000 kW even when the length of the mounting portion of the thermoelectric element is shortened to 1 m. Example 5 Next, exergy evaluation was performed on the LNG cold heat utilization power generation system of the present invention. The exergy evaluation indicates which of the exergy received by the exhaust heat recovery system has been converted to electricity. The definition formula is

[0036]

[Equation 1]

It is as follows. FIG. 18 shows a calculation example of evaluation by Exergy. Exergy transfer between LNG cold heat and seawater is shown in Fig. 18 from <1> to <2> and <3>.
To <4>. The exchange rate of exergy per unit flow rate of LNG is 443.41 kJ
/ Kg, LNG of LNG evaporator in this study
Since the flow rate is 110 T / H, the total amount of exchange of exergy is 13,548.6 kW. When the power generation system output of 946 kW is added to the turbine generator output of 4950 kW, the electric output is 5896 kW and the exergy efficiency is 43.5%.

The exergy flow in a general thermal power plant is as shown in FIG. 19, and the amount of entropy produced in each part is as shown in FIG. Example 6 Next, the energy balance of the LNG cold heat utilization power generation system of the present invention was examined.

The energy balance represents the ratio of the energy input to the system and the amount of energy produced, and is expressed by the following equation.

[0040]

[Equation 2]

Here, the density of Bi is about 9820 kg /
m ³ and Sb are about 6620 kg / m ³ . Bi-Sb
Assuming that the density of the thermoelectric elements of the system is 9564 kg / m ³ , the LNG cold heat utilization power generation system of the present invention has a thermoelectric element volume of 8.83 m ³ , so that the amount of energy required to manufacture the thermoelectric elements is 8.83 × 9564. × 200 = 16,8
It is 90,024 kWh. Here, assuming that the amount of energy required to operate the power generation system is equal to the vaporization power of 2 kW in the direct expansion cycle and the service life of the power generation system is 20 years, the energy balance is approximately 467.

[0042]

As described in detail above, according to the present invention, the output can be improved. Specifically, the specific output is 11.8 kW / (t / h-LNG) at the power generation end, and the power generation efficiency is 5.1%. This value exceeds the specific output of the Freon Rankine cycle of 10.9 kw (t / h-LNG).

Further, when the present invention is combined with LNG direct expansion, the specific output is 63.7 kW (t / h-L).
NG), direct expansion + Freon Rankine cycle 6
Over 2.7 kW (t / h-LNG). Further, the present invention does not require a heat exchanger between LNG / Freon or a Freon evaporator, and has a simple system configuration and easy maintenance.

Furthermore, in the present invention, LNG
By changing the flow path of the conventional circular tube to a plate shape, there is no need for anticorrosion insulation on the low temperature side of the thermoelectric element, and on the high temperature side of the thermoelectric element, it is protected from seawater by the evaporator panel which is a structural material, and Since the electrical connection is free, it is advantageous in terms of optimizing the current density, and since the whole is a flat plate, processing such as integral vacuum brazing is possible.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a conventional LNG cold heat power generation system.

FIG. 2 LNG in a conventional LNG cold heat generation system
It is the schematic which showed the evaporator.

FIG. 3 LNG in a conventional LNG cold heat generation system
It is the schematic which showed the evaporator.

FIG. 4 is a schematic diagram showing an LNG cold heat generation system of the present invention.

FIG. 5: L in the LNG cold heat power generation system of the present invention
It is the schematic which showed the NG evaporator.

FIG. 6 is a schematic diagram showing a heating element in the LNG cold heat generation system of the present invention.

FIG. 7 is a sectional view showing an evaporator panel in the LNG cold heat generation system of the present invention.

FIG. 8 is a relationship diagram showing the position on the heat transfer surface and the optimum thickness of the element in the LNG cold heat generation system of the present invention.

FIG. 9 is a relationship diagram showing the position and temperature on the heat transfer surface in the LNG cold heat generation system of the present invention.

FIG. 10 is a relationship diagram showing a position on a heat transfer surface and a current density in the LNG cold heat generation system of the present invention.

FIG. 11 is a relationship diagram showing a position on a heat transfer surface and a heat flux in the LNG cold heat generation system of the present invention.

FIG. 12 is a schematic view showing an embodiment of the heat transfer panel of the present invention.

FIG. 13 is a schematic view showing an embodiment of the heat transfer panel of the present invention.

FIG. 14 is a schematic view showing an embodiment of the present invention.

FIG. 15 is a relationship diagram showing characteristics of the present invention on a heat transfer surface in the embodiment of the present invention.

FIG. 16 is a schematic view showing an embodiment of the present invention.

FIG. 17 is a relational diagram showing a decrease in output due to a decrease in element attachment in the embodiment of the present invention.

FIG. 18 is a schematic diagram and a relational diagram showing an exergy calculation in the embodiment of the present invention.

FIG. 19 is a relationship diagram showing general exergy loss in an LNG-only burning thermal power plant.

FIG. 20 is a relationship diagram showing the amount of entropy generation in each general part in an LNG-only burning thermal power plant.

Claims (2)

[Claims] 1. An LNG flow passage inner wall portion of an LNG evaporator, in which LNG flows inside an evaporator and seawater flows outside the evaporator,
An LNG cold heat utilization power generation system characterized in that two types of thermoelectric elements of P type and N type are arranged. 2. The LNG cold heat utilization power generation system according to claim 1, wherein the evaporator has a parallel plate opposing structure.