JPH02260371A - Ion exchanging fuel cell aggregate wherein water and heat are improved - Google Patents

Abstract

PURPOSE: To provide a battery assembly with improved management of water and heat by employing an ion-exchange membrane fuel cell which produces required hydrogen fuel through the emission of hydrogen from hydrides with the use of heat generated therein. CONSTITUTION: A fuel cell 22 comprises a plurality of layers including a catalytic fuel electrode 28 supplied with fuel, a catalytic oxygen electrode 30 supplied with oxygen-including gas and an ion-exchange membrane 32 interposed between the electrodes 28 and 30 so as to contact with them. A device for activating the fuel cell assembly with the use of hydrogen gas as the fuel comprises a transformed fuel cell assembly 20C cooled with a reflux coolant and a hydride container 70 receiving a hydride floor 72. Heat generated from a fuel-cell coolant flow 74 serves to generate hydrogen fuel inside the hydride floor 72. Excess heat that has been generated upon the operation of the fuel cell but not used for the emission of hydrogen is dissipated inside an air-cooled heat exchanger.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a fuel cell having an ion exchange membrane cell as taught in U.S. Pat. Regarding a battery assembly. More specifically,
The present invention provides a battery assembly with excellent water and heat management.

BACKGROUND OF THE INVENTION It is known that fuel cell assemblies having heat exchange membrane cells are suitable as a power source. This type of battery assembly starts at or below room temperature and does not require corrosive electrolytes for operation. What is used for power generation is a reaction between hydrogen and oxygen, and the reaction product is water, so water is the only liquid that needs to be treated.

Hydrogen is a potential fuel of choice for batteries with ion exchange membrane electrolytes. Hydrogen is obtained from known fuel sources such as methanol.

However, hydrogen stored in the form of metal hydrides with reversible reactivity is particularly attractive. Representative examples of compounds that form reversibly reactive metal hydrides include LaNi3.
FeTi and M m N i <, xs + F ea,
ss can be mentioned. The abundant storage concentration of hydrogen in hydrides makes it an attractive alternative to other hydrogen supply methods from a safety standpoint. Because the hydrogen release reaction is endothermic (typically on the order of 7,5 Kcal/H, 1 mole), the release of hydrogen from the hydride storage vessel is limited by the available heat supply.

The development of suitable assemblies using ion exchange membrane cells presents various problems, primarily those related to the need for water and heat control management.

<Conventional technology> Highly conductive ion exchange membranes are manufactured by E.I. DuPont Nemours & Company (E,L DuP).
ont Nea+ours and Company)
A conductive sulfonic acid membrane is commercially available under the trade name NAFI○N. Dow Chemical Company
ls Co., Ltd.) has also developed ion exchange membranes with high performance properties. Dow U.S. Pat. No. 4,417,969 discloses substantially fluorinated polymers with pendants containing sulfonic acid ion exchange groups. When the fuel cell is operated fully hydrogenated and substantially saturated with water, excellent operating properties are obtained with the membranes described above. However, it is difficult to maintain such a saturated state in an accumulated aggregate. Cell flooding due to build-up of product water in the stack, membrane dehydration caused by thermal gradients, or excessive water evaporation by the reactant gas stream can individually or jointly reduce the performance of the membrane and degrade the performance of the membrane. results in deterioration.

The reactive gases must be distributed over the large electrode surface without causing local drying of the ion exchange membrane, and the distribution of the reactive gases must not be disturbed by accumulation of product water.

Under such circumstances, the temperature of the battery is a very important factor. If the temperature gradient in the electrode is not minimized by cooling, the current density will not remain uniform.

Appropriate cell and system designs that solve the above problems are essential to the development of fuel cell power plants utilizing ion exchange membrane cells.

The problem of water control management was intended to be solved by our lead inventors in U.S. Pat. This promotes the outflow of produced water. However, hydrophilic coating solutions are effective only for small-scale battery structures and are not suitable as protection against drying out of the battery.

General Electric Company
U.S. Pat. No. 4,125,183, assigned to the US Pat. The solution of US Pat. No. 4,125,183 also does not solve the problem of water removal from the cell assembly or membrane dewatering.

Problems to be Solved by the Invention Accordingly, there is a need for an ion exchange membrane fuel cell and an assembly thereof having an improved configuration in terms of heat and water control management. Also, such a fuel cell assembly is desirable, in which hydrogen stored in a hydride is used for battery operation, and when the battery is operated, heat is generated for releasing the hydrogen. .

The above-mentioned drawbacks of the prior art are solved in a useful, novel and inventive manner. Each fuel cell in the stack is provided with a water transport layer that allows flexible design choices for water control management to optimize each fuel cell and the stack.

Therefore, one of the main objects of the present invention is to provide an ion exchange membrane battery that allows controlled management of water and heat. Stated another way, it is an object of the present invention to provide a fuel cell which overcomes membrane flooding and dehydration and further provides effective means for heat dissipation.

Another object of the present invention is to provide a fuel cell that utilizes evaporation of water stored in the cell or supplied to the cell to remove heat from an integrated cell structure.

Yet another object of the invention is to provide a fuel cell compatible with liquid cooling and air cooling.

Yet another object of the invention is to provide a fuel cell that allows the integrated structure to be scaled up.

Yet another object of the present invention is to provide an integrated fuel cell system configured to utilize heat generated within the fuel cell to release hydrogen from the hydride to obtain the required hydrogen fuel.

<Means for Solving the Problems> The fuel cell assembly according to the present invention consists of a plurality of integrated fuel cells, each fuel cell having a fuel electrode, an oxygen electrode,
an ion exchange membrane located between the two electrodes and engaged with the two electrodes; a fuel distribution plate for supplying fuel to the fuel electrode; and an oxygen distribution plate for supplying oxygen to the oxygen electrode;
It consists of a fluid permeable, electrically conductive member separated from both electrodes, and an impermeable, electrically conductive outer member. The position of the fluid-permeable and conductive member (produced water storage/transport member) can be selected as appropriate in the design, but it must be located between the outer members on both sides and away from both electrodes. be.

Typically, an assembly according to the invention will form part of an integrated system that removes heat from the fuel cell assembly during operation. Alternatively, heat can be removed by evaporative cooling, direct air cooling, or liquid cooling.

The integrated system preferably has a hydride bed that generates gaseous hydrogen fuel for the fuel cell.

Gaseous hydrogen is the fuel consumed within the fuel cell.

The integrated system routes waste heat from the fuel cell to the hydride bed, where it is used to release hydrogen from the hydride bed for fuel. The heat transfer method is determined by the battery cooling method. The method of heat transfer depends on the condensation of water from the hot reaction air leaving the assembly. Similarly, heat transfer by direct contact between the cell assembly and the hydride container can be utilized. Additionally, the liquid coolant used to remove heat from the cell can also provide the heat necessary for hydrogen release.

BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned and other features and advantages of the invention will become apparent from the following description of preferred embodiments, with reference to the accompanying drawings.

In FIG. 1, a preferred fuel cell assembly consisting of a plurality of stacked fuel cells 22 is designated by the reference numeral 2o.
are stacked in a bipolar mode, and the U.S. patent tube 4,175,
They are held together by end plates 24 and 26 in the known manner disclosed in the '165 patent.

Each fuel cell 22 consists of the following members. The gas-impermeable, electrically conductive sheet 44 is connected to the hydrophilic, fluid-permeable member 42.
The porous, hydrophobic distribution plate 34 is in mechanical contact with the opposite surface of the member 42. The hydrophilic, fluid-permeable member 42 and the hydrophobic distribution plate 34 are made of silicone rubber or Vitron, which forms an airtight seal on the sides of the fuel cell.
It is sealed inside a gasket 33 consisting of, etc. The distribution plate 34 is in mechanical contact with the catalytic fuel electrode 28 over its entire surface and has a plurality of grooves 4o facing the fuel electrode 28. Fuel electrode 28 is adhered to an ion exchange membrane 32, which has a catalytic oxygen electrode 30 adhered to an opposite side. A second hydrophobic porous distribution plate 38 is in electrical and mechanical contact with the oxygen electrode 30.

A porous distribution plate 38 has a plurality of grooves 36 facing the oxygen electrode 3o and is enclosed within an insulating cathode frame 37 of plastic or hard rubber or the like having reactant air slots 39. The cathode frame 37 and distribution plate 38 are in electrical and mechanical contact with a gas-impermeable and electrically conductive separator sheet 44, which is the latest component to complete one fuel cell 22. A plurality of fuel cells are stacked and compressed between end plates 24 and 26 to complete a fuel cell assembly.

A more detailed structure of the fuel cell 22 is shown in FIG. 2. The cell 22 includes a catalytic fuel electrode 28 to which fuel is supplied, a catalytic oxygen electrode 30 to which oxygen-containing gas is supplied, and an intermediate electrode 28 and 30. It has a plurality of layers including an ion exchange membrane 32 located at and in contact with both electrodes. The ion exchange membrane 32 functions as an electrolyte member. As the layer of the fuel cell 22,
Furthermore, the conductive reagent distribution plate 34 is in contact with substantially the entire surface of the fuel electrode 28. The distribution plate 34 may be made of a porous material and may include a plurality of grooves 40 facing adjacent fuel electrodes 28. can. However, the groove 40 can also be omitted, as shown in FIG.

The fuel cell 22 further includes an electrically conductive reactant distribution plate 38 in contact with substantially the entire surface of the oxygen electrode 30, the oxygen distribution plate 38 being comprised of a porous material, as shown.
It has a plurality of grooves 36 facing adjacent oxygen electrodes. However, these grooves 36 can also be omitted.

Fuel cell 22 further includes a fluid permeable, electrically conductive member 42 located outside of reactant distribution plate 34 in the illustrated embodiment and in contact with the surface of the reactant distribution plate remote from electrode 28. . In alternative constructions, the member 42 may be located outside the reactant distribution plate 38 or outside both plates 34 and 38. The outer surface of fuel cell 22 is comprised of a gas-impermeable, electrically conductive sheet 44 to isolate reactant gases from adjacent stacked fuel cells.

The reactant distribution plates 34 and 38 preferably consist of metal screens, metal plated plastic screens, porous plastics or ceramics, or perforated or corrugated metal sheets. Alternatively, the distribution plate 34 may consist of a carbon foam, such as reticulated vitreous carbon. A further suitable material for plates 34 and 38 is porous graphite with or without grooves as shown in FIG. It is. Distribution plates 34 and 38 may be overlaid with gold or nickel to reduce contact resistance.

Furthermore, distribution plates 34 and 38 are hydrophobic and can also be impregnated with tetrafluoroethylene (Teflon) to make them hydrophobic.

A fluid-permeable, electrically conductive member 42 provides a means for storing water, transporting water to and from the fuel cell, and consists of mineral or organic fibers embedded in a metal screen to carry electrical current. is conducted through a fluid permeable member 42. For example, member 42 is comprised of reticulated graphite foam. Alternatively, the fluid permeable member 42 may include:
It consists of porous graphite or carbon or metal-plated ceramics made hydrophobic by impregnation with colloidal silica.

Electricity generation substantially occurs within the catalyst-activated membrane 32. Membrane 32 becomes ionically conductive as a result of the migration of hydrogen ions through the polymer chains. Electric power is generated by the reaction of hydrogen, which releases electrons and enters the polymer as protons, at the catalytic electrode 28 . At the oxygen electrode 30, protons from the catalytically activated membrane 32 react with oxygen and electrons returned from the electrical load to produce water, which must be removed from the fuel cell 22. be.

In the fuel cell assembly 20, a reactant stream consisting of an oxygen-containing gas and a reactant stream consisting of a gaseous hydrogen fuel enter the assembly 20, and the fuel is consumed within the assembly 20, generating heat. , the heat needs to be removed. Figures 3, 4, and 58 illustrate methods for removing heat by evaporative cooling, direct air cooling, and liquid cooling, respectively.

FIG. 3 shows a fuel cell assembly 20 and a finned plate 50 having a condensing surface 51 spaced apart from the assembly 20. In FIG. The reactant stream 53 leaving the assembly 2o is saturated with water.The reaction side stream 53 hits the plate 50 and releases heat, condensing liquid water, as indicated schematically at 52. It can also be refluxed to the fuel cell assembly 20.

The evaporative cooling heat removal device shown in Figure 3 can also be referred to as an open heat pipe type. One of the advantages of this device over other cell cooling methods is that it allows the amount of reactant air flow to be much higher than the stoichiometric requirement, yet provides pre-humidification to prevent cell drying. is not necessary.

FIGS. 4 and 5 show modified examples of the fuel cell assembly 20. In an air-breathing system, air is supplied in the form of a diffused or forced flow directly into the cell assembly 2 through a plurality of openings (not shown) along the sides of the fuel cell assembly 22.
Enters 0. Hydrogen enters through a common manifold (not shown).

In the fuel cell assembly 20A shown in FIG. 4, a cooling member 60 is disposed between the fuel cells 22 or a group of fuel cells 22, and cooling air is passed through the assembly 20A as indicated by the reference numeral 62. flows. In the fuel cell assembly 20B shown in FIG. 5, a cooling member 64 is arranged between the fuel cells 22 or groups of fuel cells 22. The liquid coolant enters the fuel cell assembly 20B as separated at 66, traverses the assembly 20B through member 64, and exits the cooling member 64 as separated at 68 at an elevated temperature.
The zero members 60 and 64, which may also direct the hot liquid coolant exiting the cooling member 64 to the heat exchanger, must be electrically conductive members. Tubes 66 and 68 must be electrically isolated from the fuel cell assembly.

The fuel cell assembly of the present invention can be operated using hydrogen gas as fuel. FIG. 6 schematically shows this type of device, and the illustrated device is a modified fuel cell assembly 20G cooled by the recirculating coolant as described above.
and a hydride container 70 containing a hydride bed 72. Heat extracted from fuel cell coolant stream 74 generates hydrogen fuel within hydride bed 72 . The excess heat generated by fuel cell operation and not required for hydrogen release is dissipated in an air-cooled heat exchanger.

A water recovery system for use in zero gravity conditions is shown in FIG. 7, where water is transported through a plurality of fluid permeable membranes 42, which act as water transport and storage.

The function of the membrane 42 is to distribute externally introduced water entering the fuel cell 22 and to draw produced water out of the fuel cell 22.

A wick pad 82 is connected to a water absorbing blotting member 84 and water is forced into a collection port 88 by an inflatable bladder.

For short-term use when ground gravity or gravity load is zero, the device shown in Figure 7 may be modified to accommodate storage tank 8.
4. The need to place a wick pad 82 and sac 86 can be eliminated.

In this case, the produced water is stored inside the fluid-permeable member 42 inside each cell and is removed at the end of each period of use by applying a vacuum through the same passageway provided for the wick pad 82. All you have to do is remove the generated water. Drainage can be carried out using a suction pump, and most of the water present inside one member 42 can be recovered in the form of liquid water.

For this purpose, depending on the length of the period of use, the member 42
can be expanded.

An assembly of five batteries was operated using hydrogen and air. The cell assembly was cooled by evaporation of water into the reactant air stream. The active area of each battery is 100cnf (5
csX (20 countries) and Nafion
11 A ion exchange membrane was used. Both sides of the membrane were activated with platinum catalyst layers laminated to the membrane.

A 0.2 cm thick reactant distribution structure was placed in contact with the catalyst surface on the anode. For the same purpose, on the cathode there is a thickness O,
An IQI+ sheet was used. The bodies of both distribution structures were made from porous graphite (0.36 g/aJ) and impregnated with a Teflon emulsion to make them hydrophobic. In addition to having a water-repellent structure, a hydrophilic water transport member was placed in the cathode compartment. A hydrophilic member was placed between the reactant distribution plate and a Grafoil sheet with a thickness of 0.06 (!I) serving as a gas-impermeable separator plate, which sucked water into the cell.Hydrophilic member is a porous graphite plate with a thickness of 0.18 mm (0.36 mm)
g/d) and grooves were provided to allow gas to enter the cell. The porous graphite was coated with colloidal silica to make it hydrophobic.

Hydrogen pressure 0, 105 kg/al (1.5 psig)
The battery was operated in dead end mode. Atmosphere air was supplied at substantially atmospheric pressure.

The air flow rate was 4-6 liters/min (approximately 5 times the stoichiometric requirement). According to the results of electrical measurements of five battery assemblies, the assemblage voltage at an output current of 12 amperes is 3.5
It was a bolt. At this voltage, 28,2 Kcal/hour of heat was generated, calculated based on the downward heating value of hydrogen. A portion of this heat is dissipated as natural convection from the assembly surface and as detectable heat in the reactant gas flow.

However, under certain conditions, the removal of most of the heat from the battery assembly depends on water evaporation. For example, if the temperature of the stack is 55° C. and the ambient temperature is 22° C., 26 g of water per hour will evaporate from the stack in addition to the water produced by the electrochemical reaction (20 g/hour). This is 13
．． 7 kg/hour or 48.6% of the heat generated. Under this operating condition, the energy required to evaporate the water stabilizes the temperature of the assembly at 55°C. This water is supplied from an external source and drawn up into the aggregate from the bottom.
With hydrogen derived from a six-generation shear hydride storage vessel distributed to each cell via a hydrophilic plate.

An air breathing assembly consisting of 17 batteries was operated. In this example, heat from a fuel cell was used to release hydrogen from a hydride storage vessel. The method of heat transfer is evaporation of water from the fuel cell to the reactant air stream and subsequent condensation on the surface of the hydride storage vessel.

Loss of seeds ■y The assembly of five batteries used in Example 1 was operated using hydrogen and oxygen. In this embodiment, the produced water is stored in a member 42 inside the cell5. 0 member 42 removed by applying vacuum to manifold 82 after battery operation is complete.
The thickness is 0.18a to accommodate the added water!
The number of countries increased from 1 to 0.5 countries. The battery assembly was operated in "dead end" mode at 10 amps. Hydrogen and oxygen.

Gauge pressure 0, 105 kg/aJ (1,5 psig
) was supplied. The voltage across the assembly was 4.0 volts.

After 5 hours of incubation, battery operation was discontinued and the produced water was removed. Vacuum was applied to manifold 82 to collect 93 g of water and battery operation was resumed. The assembly was cooled by heat dissipation through the end plates.

The structure of the assembly is the same as described in Example 1. The hydride storage vessel consists of two cylindrical stainless steel vessels. These containers have a diameter of 3.20 m and a length of 1
7.5C! I, Ergenics (Erlcan)
H is a mischmetal-nickel-iron alloy manufactured by ics Inc.
The total hydrogen storage capacity when Y-8TOR209 is filled with 950g is 100 liters (STP), and the hydrogen pressure at room temperature is 10.5 to 14 in gauge pressure.
．． 1 kg/d (150-200 psig).

Using a pressure regulator, adjust the above hydrogen pressure to 0.105 in gauge pressure.
kg/aJ (1.5 psig). A small DC blower was used to supply air from ambient air to the fuel cell. Airflow was restricted by an orifice on the suction side of the blower. Air flows through the assembly, providing oxygen for the electrochemical reactions and evaporating water to cool the cell, so that the temperature of the assembly at a given load varies depending on the air flow. . For example, a load of 7 amperes (12° 4 volts, 87
Watt), the incoming air reaches an equilibrium value at a flow rate of 12 l/min (5.5 times stoichiometric) at 55-57°C. The airflow leaving the battery is directed to a hydride storage vessel.

When the air comes into contact with the hydride storage vessel, the temperature of the air is 5
Dropping from 0 to 53°C, moisture condenses on the stainless steel cylinder, releasing the hydrogen consumed to supply the load. After 110 minutes of continuous operation of the battery under the above load conditions, the hydrogen storage capacity in the hydride storage vessel is completely consumed. Continuous operation was performed at a load of 140 watts at 12 amps.

The recovered water can be returned to the cell for evaporation again. The reflux water is only a portion of the air that is evaporated, and is tasteless in sufficient quantity to cool the battery; water is added before or after the air contacts the hydride container, and is condensed by a heat exchanger for self-retention. Sufficient water must be recovered to operate.

The invention achieves the objects and advantages mentioned above. The details of the embodiments described above are provided for purposes of illustration and are not to be construed as limitations on the invention except as contained in the claims.

[Brief explanation of drawings]

FIG. 1 is a schematic exploded perspective view of a preferred fuel cell assembly according to the present invention. FIG. 2 is an end view of one of the fuel cells of the assembly shown in FIG. FIG. 3 is a schematic perspective view of a vertically organized assembly, illustrating how heat is removed from the assembly by evaporative cooling. FIG. 4 is a schematic perspective view of a vertically organized assembly, illustrating how heat is removed from the assembly by direct air cooling. FIG. 5 is a schematic perspective view of a vertically organized assembly, illustrating how heat is removed from the assembly by liquid cooling. FIG. 6 is a schematic perspective view of a vertically organized assembly illustrating the use of hydrogen gas from a hydride bed in a hydride vessel as fuel to be delivered into a fuel cell; FIG. . FIG. 7 is a side sectional view showing an arrangement for removing water from a fuel cell assembly under zero gravity. 2o...Fuel cell assembly, 22...Fuel cell, 24.2
6... End plate, 28... Fuel electrode, 30... Oxygen electrode, 32... Ion exchange membrane, 34... Distribution plate,
38...Distribution plate, 42...Fluid-permeable and electrically conductive member. FIG. 2. Engraving of drawings (no changes in content) Procedural amendment (method) June 6, 1999

Claims (1)

[Scope of Claims] 1) A first electrode and a second electrode in contact with an ion exchange electrolyte member located in an intermediate portion, and a first reactant in fluid communication with the first electrode and supplying a first reactant to the first electrode. a second reactant distribution member in fluid communication with the second electrode to supply a second reactant to the second electrode; The agent distribution member and the second reactant distribution member are electrically conductive and are housed between gas impermeable and electrically conductive members, and the fluid permeable and electrically conductive member is interposed between said gas impermeable members. An ion exchange membrane battery, characterized in that the ion exchange membrane battery is located between and outside of the electrodes and is spaced from the electrodes for storing and transporting liquid. 2) a fuel electrode and a reactant electrode, an ion exchange electrolyte member located between the fuel electrode and the reactant electrode and in contact with both electrodes, and in fluid communication with the fuel electrode to supply fuel to the fuel electrode; a reactant distribution member in fluid communication with the reactant electrode to supply a reactant to the reactant electrode, the fuel distribution member and the reactant distribution member being electrically conductive; , a fluid-permeable, electrically conductive member located between the gas-impermeable members and external to the electrodes for storing a liquid; An ion exchange fuel cell characterized in that it is separated from both electrodes for transport. 3) A planar fuel electrode, a planar oxygen electrode, a planar ion exchange electrolyte member located between the fuel electrode and the oxygen electrode and in contact with both electrodes, and in contact with the surface of the fuel electrode. a permeable and planar conductive fuel distribution member that covers the entire surface of the fuel electrode and supplies hydrogen gas fuel to the fuel electrode; a permeable, planar, electrically conductive oxygen distribution member for supplying an oxygen-containing gas, the fuel distribution member and the oxygen distribution member being housed between a pair of gas impermeable, electrically conductive separators; An ion exchange fuel characterized in that a fluid-permeable and electrically conductive planar member is located between the separators and is located outside the electrodes and is spaced apart from the electrodes for storing and transporting water. battery. 4) A catalytic fuel electrode to which fuel is supplied, a catalytic oxygen electrode to which oxygen-containing gas is supplied, and an ion exchange membrane located between the fuel electrode and the oxygen electrode and in contact with both electrodes to function as an electrolyte member. an electrically conductive reactant distribution member that contacts the fuel electrode and serves as a means for supplying fuel to the fuel electrode; and an electrically conductive reactant distribution member that contacts the oxygen electrode and serves as a means for supplying oxygen-containing gas to the oxygen electrode. a reactant distribution member having a chemical composition; and a means for storing water, supplying water to the fuel cell, and transporting water from the fuel cell, the member being in contact with at least one of the reactant distribution members and being isolated from the electrodes. A fuel cell comprising: a water-permeable, electrically conductive member; and a gas-impermeable, electrically conductive outer member that separates a reactant gas. 5) Direct heat conduction using a fuel cell, a hydride container, a metal hydride housed in the container that generates gaseous hydrogen, which is the fuel for the fuel cell, and a separator, to remove waste heat from the fuel cell. means for transmitting heat to the cell edge and to the hydride bed for conduction to the hydride bed. 6) An ion exchange membrane fuel cell system and water recovery system for use in zero gravity conditions, the water recovery system comprising: a fluid permeable distribution plate; a wicking pad;
A system comprising a fluid-absorbing member and an inflatable bladder. 7) An ion exchange membrane fuel cell system and water storage and recovery system, characterized in that said water storage and recovery system consists of a large fluid permeable distribution plate connected by a manifold to a vacuum line.