WO2017148487A1

WO2017148487A1 - System and method for adjusting the temperature of cooling-liquid for a fuel cell

Info

Publication number: WO2017148487A1
Application number: PCT/DK2017/050058
Authority: WO
Inventors: Mads Bang; Anders Risum Korsgaard
Original assignee: Serenergy A/S
Priority date: 2016-03-02
Filing date: 2017-03-02
Publication date: 2017-09-08
Also published as: DK178852B1; DK201670121A1

Abstract

A fuel cell system, comprising a fuel cell, a liquid fuel supply for providing liquid fuel, an evaporator for evaporating the liquid fuel to fuel vapor, a reformer for catalytic conversion of the fuel vapor to syngas for the fuel cell, and a cooling circuit for cooling the fuel cell by cooling-liquid. The cooling circuit comprises a flow-channel serially connecting the fuel cell, a first heat exchanger and a valve in a manner that the cooling-liquid flows in circulation. Further, the cooling circuit comprising a bypass channel connecting an upstream and a downstream of the first heat exchanger by bypassing the first heat exchanger and the valve. The bypass channel passes through the evaporator for transferring heat from the cooling-liquid to the liquid fuel for evaporation of the liquid fuel in the evaporator. The first heat exchanger and the valve are relatively small and compact as the bypass flow is dominant and only a minor amount of coolant is flowing through the first heat exchanger and valve.

Description

System and method for adjusting the temperature of cooling-liquid for a fuel cell

FIELD OF THE INVENTION The present invention relates to a system and method for operating a fuel cell system. In particular, it relates to adjusting the temperature of cooling-liquid for the fuel cell.

BACKGROUND OF THE INVENTION

Fuel cell systems generate heat as a by-product when generating electricity. This heat is removed by cooling-liquid that circulating through channels in the fuel cell, where the flow of cooling-liquid through heat exchangers and radiators is adjusted to keep the fuel cell at a steady temperature for optimized operation

US patent application No. 2014/0147764 discloses a fuel cell system in which the cooling-liquid circulation system comprises a branch that divides the cooling-liquid into a first portion that passes through a radiator for cooling and a second portion that bypasses the radiator. The second portion is splits into two further sub-portions, where one sub-portion passes through a temperature regulating thermostatic three way valve and combines with the first portion inside the valve, and where a second sub-portion by-passes the valve in a by-pass channel before being combined with the cooling- liquid downstream of the valve. The by-pass channel is used to secure a flow of cooling-liquid through the fuel cell even if the valve is fully closed. In order to safeguard sufficient cooling-liquid passing through the radiator during normal operation, the bypass channel is relatively small, creating sufficient back pressure. Accordingly, the second sub-portion that flows through the by-pass channel is substantially less than the flow through the valve itself. In order to provide a compact technical solution, the bypass channel is provided as part of the valve housing, merely by-passing the flow- adjusting valve member. Thus, although, the objective of this disclosure is to provide a compact technical solution, useful for vehicles, the valve housing has to be of a substantial size in order to accommodate the entire flow of the cooling-liquid.

Also Japanese patent application JP2004- 178826 discloses a cooling circuit for a fuel cell in which part of the cooling-liquid is by-passing the radiator. However, also in this case, all of the cooling-liquid flows through a three way valve.

It would be desirable to provide an even more compact technical solution in order to reduce size, weight, and production costs.

DESCRIPTION / SUMMARY OF THE INVENTION

It is the objective of the invention to provide an improvement in the art. In particular, it is an objective to provide a compact fuel cell system. It is a further objective to provide a simple and compact technical solution for temperature adjustment of the cooling-liquid for the fuel cell. These objectives are obtained with systems and methods as explained in more detail in the following. The fuel cell system comprises a fuel cell, for example, a fuel cell stack. Typically, the fuel cells in the stack are interconnected to share a common cooling circuit. For example, the fuel cells are high temperature proton exchange membrane fuel cells, also called high temperature proton electrolyte membrane (HTPEM) fuel cells, which operate above 120 degrees centigrade, differentiating HTPEM fuel cell from low tem- perature PEM fuel cells, the latter operating at temperatures below 100 degrees, for example at 70 degrees. The operating temperature of HTPEM fuel cells is the range of 120 to 200 degrees centigrade, for example in the range of 160 to 170 degrees centigrade. The electrolyte membrane in the HTPEM fuel cell is mineral acid based, typically a polymer film, for example polybenzimidazole doped with phosphoric acid.

When using liquid fuel, hydrogen for the fuel cell is generated by conversion of the liquid fuel into a synthetic gas, also called syngas, containing high amounts of gaseous hydrogen. An example of liquid fuel is a mixture of methanol and water, but other liquid fuels can also be used, especially, other alcohols, including ethanol. For the conversion, the liquid fuel is evaporated in an evaporator and converted in a reformer for catalytic conversion of the fuel vapour to syngas prior to entering the fuel cell. HTPEM fuel cells are advantageous in being tolerant to relatively high CO concentration and are therefore not requiring PrOx reactors between the reformer and the fuel cell stack, why simple, lightweight and inexpensive reformers can be used, which minimizes the overall size and weight of the system in line with the purpose of providing compact fuel cell systems, for example for automobile industry.

For receiving the liquid fuel, the evaporator has an upstream liquid conduit to the liq- uid fuel supply and is configured for evaporating the liquid fuel to fuel vapour which is then fed into the reformer through a vapour conduit between the downstream side of the evaporator and the upstream side of the reformer. In addition, the reformer has a downstream syngas conduit to the fuel cell through which syngas is provided to the fuel cell.

In order to reach the temperature relevant for the conversion process in the reformer, for example around 280 degrees centigrade, a burner is optionally employed, although, also electrical heating can be used. For example, the exhaust gas of a burner is used for heating the walls of the reformer, typically by flow of the hot gas along the outer walls of the reformer.

A cooling-liquid circuit is provided for cooling the fuel cell by cooling-liquid. The cooling-liquid circuit comprises a first heat exchanger configured for cooling of the cooling-liquid prior to entering the fuel cell. The cooling-liquid circuit comprises a cooling-liquid circulation channel serially connecting the fuel cell, the first heat exchanger and a valve in a manner that the cooling-liquid flows in circulation. In addition, the cooling-liquid circuit comprising a bypass channel connecting an upstream and a downstream of the first heat exchanger by bypassing the first heat exchanger. This bypass channel is bypassing the valve and passing through an evaporator heat exchanger inside the evaporator and transferring heat from the cooling-liquid to the liquid fuel for evaporation of the liquid fuel in the evaporator.

A substantial amount of heat from the cooling-liquid is used for evaporation of liquid fuel. This way, a part of the necessary reduction in temperature for the cooling-liquid is caused by the evaporator. Another part of heat is removed by flow of cooling liquid through the first heat exchanger and the valve. However, as the heat loss in the first heat exchanger is efficient, the flow through the first heat exchanger and valve is substantially lower than the flow of cooling liquid through the bypass channel and the evaporator. For example, the flow through the valve is less than half of the flow through the evaporator. Therefore, also, the dimensioning of the first heat exchanger and valve can be kept small.

For example, the temperature of the cooling-liquid is 170 degrees centigrade at the exit of the fuel cell stack and needs to be cooled to 160 degrees centigrade. This drop of 10 degrees is mainly achieved with the evaporator. However, minor adjustments of a few degrees are made finally, for example downstream of the evaporator, by controlled addition in the bypass channel of cooling-liquid that has passed through the first heat exchanger. For this purpose, optionally, cooling-liquid is cooled to a temper- ature in the range of 60 to 120 degrees centigrade in the first heat exchanger. Only a small portion of cooling-liquid at 60 degrees centigrade has to be added to a cooling- liquid at about 160 degrees in order to cause a small temperature drop of a few degrees. This implies that the cooling-liquid circulation channel, the heat exchanger and the valve can be kept relatively compact and with low weight, which also reduces production costs and transport costs of the system. Especially, due to the serial configuration of the cooling-liquid circulation channel, the valve can be provided as a two-way valve. Such two way valve is in contrast to the aforementioned prior art, where a three way valve is necessary for the flow.

For example, the first heat exchanger comprises an air blown cooler configured for blowing air on the cooler for transfer of heat from the cooling-liquid in the first heat exchanger to the air.

It is for sake of clarity pointed out here that all temperatures herein are given in degrees centigrade. SHORT DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to the drawing, where FIG. 1 illustrates a fuel cell system with a cooling circuit

DETAILED DESCRIPTION / PREFERRED EMBODIMENT

FIG. 1 illustrates a fuel cell system with a cooling circuit that cools the fuel cell and heats liquid fuel for evaporation.

The fuel cell system comprises a fuel cell stack for which liquid fuel, for example a mixture of methanol and water is supplied from the fuel supply tank. In liquid conduit P-8, the liquid fuel is guided into the evaporator, in which the temperature of the liq- uid fuel is raised in the fuel heat exchange conduit P-10 until evaporation of the fuel. The vapour is fed into a reformer that converts the vapour catalytically into syngas, for example by using a catalyser, optionally comprising copper. Syngas mainly consist of hydrogen and carbon dioxide and a small content of water mist and carbon monoxide. The syngas is supplied through a syngas conduit (not shown) to the fuel cell stack anode side of the proton electrolyte membrane, while oxygen, typically from air, is supplied to the cathode side of the proton electrolyte membrane.

In order to reach the temperature relevant for the conversion process in the reformer, for example around 280 degrees centigrade, a burner is advantageously employed, typically, using anode waste gas for burning. For example, the exhaust gas of the burner has a temperature of 350-400 degrees centigrade and is used for heating the walls of the reformer, typically by guiding the exhaust gas along an outer wall of the reformer. For evaporation, cooling-liquid at a high temperature in the range of 120 to 200 degrees centigrade, for example at 170 degrees centigrade, is guided from the exit portion of the fuel cell stack through cooling-liquid channel P-2, a pump, and a further cooling-liquid channel P-3 into the cooling-liquid heat exchange conduit P-5 inside the evaporator, the cooling-liquid heat exchange conduit P-5 being in thermal connec- tion with the fuel heat exchange conduit P-10 for transfer of heat from the cooling- liquid to the liquid fuel for evaporation thereof, which causes a drop in the temperature of the cooling liquid. This drop brings the cooling-liquid close to the temperature needed at the entrance of the fuel cell stack and only minor adjustments of the temper- ature are necessary for precise control of the cooling-liquid temperature at the entrance of the fuel cell stack.

For example, the temperature of a high temperature PEM fuel cell stack is 170 degrees centigrade, and in the evaporator, the temperature drops close to 160 degrees, which is the temperature needed at the entrance of the fuel cell stack.

Accordingly, the flow of cooling-liquid through the evaporator is dominant and only a minor part of the cooling-liquid is led through a first heat exchanger by heat exchanger channel P-4 which branches off the further cooling-liquid channel P-3. The cooling- liquid through the first heat exchanger is cooled down to a lower temperature, typically in the range of 60-120 degrees centigrade. Only a relatively small volume of cooling-liquid through channels P6 and P-7 downstream of the first heat exchanger is necessary to add to the cooling-liquid in cooling-liquid heat exchange conduit P-5 inside the evaporator, the small amount being precisely controlled and adjusted through valve V-l, which advantageously is a two-way valve. For controlled and precise adjustment of the temperature of the cooling-liquid flowing into the fuel cell stack after combination from cooling-liquid heat exchange conduit P-5 and channels P-7 downstream of the first heat exchanger, a system controller (not shown) is provided which measures the temperature at the entrance of the fuel cell stack and regulates the flow of cooling-liquid through the valve V- 1.

As an example, the following parameters apply. For a HTPEM stack delivering 5 kW, typical dimensions are 0.5 m x 0.25 m x 0.14 m. The flow through the valve is less than half of the flow through the evaporator, which corresponds to 33% of the total flow through the first heat exchanger and the valve. Typically, the flow through the first heat exchanger and the valve is less than 25% of the total flow. For example, the total flow of cooling liquid is 12 1/min. A flow of 2 1/min through the first heat exchanger and the valve would suffice for the desired temperature reduction of the cooling liquid. This implies that the remaining 10 1/min would flow through the evapora- tor. Thus, the flow through the first heat exchanger and the first valve is less than 20% of the total flow of cooling liquid, which is only a minor fraction. This implies that the first heat exchanger as well as the valve can be made small. Examples of weight and dimensions of the first heat exchanger and the valve are 1 kg and within 100x100x300 mm. For example, the entire fuel cell stack with burner, evaporator and reformer have a weight of around 20 kg, and an entire fuel cell system including electronics, cooling- liquid pump, first heat exchanger and valve weighs in the order of 40-45 kg. Even a few kg saving by reducing the size and weight of the heat exchanger and valve implies a substantial increase of the efficacy in terms of W/kg. This efficacy increase relative- ly to the weight is important in cases where low weight and high power is essential, such as in automobiles and aircrafts.

For these reasons, the branched cooling-liquid system is an important contribution in increase of overall efficacy relatively to the weight.

Claims

1. A fuel cell system, comprising:

a fuel cell;

a liquid fuel supply for providing liquid fuel;

an evaporator configured for evaporating the liquid fuel to fuel vapor and having an upstream liquid-conduit to the liquid fuel supply for receiving the liquid fuel;

a reformer configured for catalytic conversion of the fuel vapor to syngas, the reformer having an upstream vapor-conduit to the evaporator for receiving the fuel vapor and a downstream syngas-conduit to the fuel cell for providing syngas to the fuel cell;

a cooling-liquid circuit for cooling the fuel cell by cooling-liquid, the cooling- liquid circuit comprising a first heat exchanger configured for cooling of the cooling- liquid prior to entering the fuel cell; the cooling-liquid circuit comprising a cooling- liquid circulation channel serially connecting the fuel cell, the first heat exchanger and a valve in a manner that the cooling-liquid flows in circulation through the fuel cell, the heat exchanger and the valve; the cooling-liquid circuit comprising a bypass channel connecting an upstream and a downstream of the first heat exchanger by bypassing the first heat exchanger;

characterised in that the bypass channel is bypassing the valve and passing through an evaporator heat exchanger inside the evaporator for transferring heat from the cooling-liquid to the liquid fuel and for causing evaporation of the liquid fuel in the evaporator.

2. A fuel cell system according to claim 1, wherein the bypass channel is configured for higher flow of cooling-liquid than the flow in the cooling-liquid circulation channel through the first heat exchanger and the valve.

3. A fuel cell system according to claim 2, wherein the bypass channel is configured for at least two times higher flow of cooling-liquid than the cooling-liquid circulation channel.

4. A fuel cell system according to any preceding claim, wherein the valve is a two- way valve.

5. A fuel cell system according to any preceding claim, wherein the fuel cell is a high temperature PEM fuel cell configured for operating at a temperature in the range of 120-200 degrees centigrade.

6. A fuel cell system according to any preceding claim, wherein the first heat exchanger comprises an air blown cooler configured for blowing air on the cooler for transfer of heat from the cooling-liquid in the first heat exchanger to the air.

7. A method of operating a fuel cell system according to any preceding claim, the fuel cell system comprising:

a fuel cell;

a liquid fuel supply for providing liquid fuel;

characterised in that the bypass channel is bypassing the valve and passing through an evaporator heat exchanger inside the evaporator, and wherein the method comprises causing a flow of cooling-liquid through the bypass channel inside the evaporator and causing a flow of liquid-fuel from the upstream liquid-conduit through the evaporator and transferring heat from the cooling-liquid to the liquid-fuel in the evaporator heat exchanger and causing evaporation of the liquid-fuel in the evaporator heat exchanger.

8. A method according to claim 7, wherein the method comprises providing a flow of cooling liquid through the valve of less than half of the flow of cooling-liquid through the evaporator.

9. A method according to claim 7 or 8, wherein the fuel cell is a high temperature PEM fuel cell, and the method comprises operating the fuel cell at a temperature in the range of 120-200 degrees centigrade.

10. A method according to claim 9, wherein the method comprises operating the fuel cell at a temperature in the range of 160-170 degrees centigrade, measuring the temperature of the cooling-liquid at the entrance of the fuel cell, and adjusting the temperature of the cooling-liquid at the entrance of the fuel cell to 160 degrees centigrade by controlled adding of a portion of cooling-liquid from the first heat exchanger to the cooling-liquid in the bypass channel.

11. A method according to claim 10, wherein the controlled adding of a portion of the cooling-liquid from the first heat exchanger to the cooling-liquid in the bypass channel is made at a downstream end of the evaporator.

12. A method according to claim 9 or 10 or 11, wherein the method comprises cooling the liquid in the first heat exchanger to a temperature in the range of 60 to 120 degrees centigrade.