24 Advanced fuelcell control systems

This section considers the development of a fuel-cell controller and power converter for a vehicle weighing 2 tons, for operation in an urban environment10. The techniques employed can be used with either PEM membrane fuel cells or alkaline units. The main challenge is to re-engineer a high cost system into a volume-manufactured product but this is unlikely to be achieved'overnight'. What is required is a new generation of components which are plastic as opposed to metal based.

The power electronics are practical, but need integrated packaging to reduce costs. Equally important is improvement in the fuel-cell stack specifications. This section considers the requirements and performance of a low pressure scheme at the current state of the art and predicts the measures needed to achieve significant cost reduction.

Modern hybrid cars are demonstrating major improvements in fuel consumption (3 litres/100 km) and emissions (ULEV limits) compared to conventional thermal engines. These designs use small peaking batteries which weigh less than 100 kg, for a family sedan, and store perhaps 2 kWh.

A new aluminium battery chemistry has been identified whereby it should be possible to store 50 kWh in a weight of 150 kg in perhaps 3/5 years from now. Nickel-metal hydride needs 500 kg with current technology to achieve 50 kWh. This makes a new type of hybrid an interesting long-term contender - the electric hybrid with a small fuel cell. In this vehicle a 2-5 kW fuel cell would charge the battery continuously. The only time the battery would

Mounting rail

Hull-cooled power supply

Self-managed cells 76 each stack

Mounting rail

Hull-cooled power supply

Self-managed cells 76 each stack

Hull-cooled water circulating heat exchangers

Voltage/temp. electronics

Interface and vehicle through connectors

Hydrogen and oxygen sensors

Oxygen sphere

FCPS-IBM processor (mounted with AUV electronics)

Hull-cooled water circulating heat exchangers

Hydrogen and oxygen sensors

Oxygen sphere

FCPS-IBM processor (mounted with AUV electronics)

Voltage/temp. electronics

Interface and vehicle through connectors

Performance:

Power

Capacity

Voltage

Endurance

Fuel

Oxidant

Buoyancy

Time to refuel

100 kWh

120 V nominal

40 h at full power

25 kg aluminium anodes

22 kg oxygen at 4000 lb/in2

Neutral, including aluminium hull section

Dimensions: Mass

Battery diameter Hull diameter System length

360 kg 470 mm 533 mm 2235 mm

Non-dimensional performance: Volumetric energy density 265 Wh/l Gravimetric energy density 265 Wh/kg

Fig. 2.9 Aluminium/oxygen power system and its characteristics (courtesy Alupower).

become discharged would be if one travelled more than 400 km in one day. In this case the battery would be rapidly charged at a service station. Since the battery is light the cost is moderate and because it is not normally deep cycled a long life can be expected. Aluminium test cells have already demonstrated over 3000 deep discharge cycles and operation down to -80°C, as seen in the previous section.

At the present time we need to use larger fuel cells and smaller batteries similar to the hybrids with thermal engines. The vehicle which is going to be the development testbed is the new TX1 London taxi chassis made by LTI International, a division of Manganese Bronze in Coventry, shown in Fig. 2.10. This vehicle has been chosen because of growing air quality problems in London. The City of Westminster is now an Air Quality Improvement Area. This is mainly due to a large increase in diesel use which has resulted in unacceptable levels of PM10 emissions. Public Transport is a major contributor, with the concentration of large numbers of vehicles in the central zone.

Two types of fuel cell are attractive for use in vehicles - the PEM membrane and the alkaline types, as described in the following chapter. Both types have undergone a revolution in stack design in the last few years with the result that the stack (Fig. 2.11) is no longer the major cost item in small systems, it is the fuel-cell controller and the power converter. In this section we shall review the problems to be solved and offer some suggestions as to the likely course of development. As always the fundamental issue is to convert a high cost technology for mass production civilian use. Current (1998) fuel cells cost $1000 per kW and most of that cost lies in the control system and power conversion. Stacks will cost less than $100 per kW in mass production. The challenge is to reduce the control system cost. It is for this reason that most vehicle fuel-cell manufacturers are opting to supply the stacks, and leave the car industry to manufacture the controller, Fig. 2.12. This is an opportunity that Fuel Cell Control Ltd intends to take up by offering control systems commercially.

Fig. 2.10 TXI London taxi.
Fig. 2.11 Developed PEM fuel cell: (a) plate; (b) stack; (c) anode; (d) cathode.
2.4.1 WHAT IS IN A FUEL-CELL SYSTEM? Here is a typical specification:

Power:

7.2 kW max

Output voltage:

96 V DC no-load

64 V DC full-load

Output current:

110 amps

Operating temperature:

70°C

Fuel:

Air 45 cubic metres per

Pure hydrogen 5 cubic metres per h

Hydrogen storage:

Cryogenic - 180°C

High pressure 200 bar

DC/DC converter 1

Input:

60-100 V DC

Output:

0-396 V at 2.45 V per cell, lead-acid - 18 A

Current ripple less than 1 part in 10 000

Fuel-cell controller

Intelligence:

80 I/O programmable logic controller

at 24 V DC

Control:

Close loop: hydrogen 0-1/10 bar; air 0-45

cubic metres/hour proportional to demand

Purge:

Dry nitrogen loop

Pumps:

Valves: Preheat:

DC/DC converter 2

Input: Output:

Fig. 2.12 Fuel-cell system specification.

(1) Hydrogen 80 watts

(2) Air 320 watts

(4) Water 10 watts

10 off electropneumatic control 2 kW - 312 V DC

200/400 V DC

27.6 V DC 600 W for control system

Figure 2.13 shows the cell layout of the two fuel-cell types, alkaline and proton exchange membrane (PEM). In the alkaline type, the electrolyte is a liquid - potassium hydroxide or KOH. This is the same material used in alkaline batteries. The anode membrane is porous and has eight very small amounts of platinum catalyst (1 g would cover three football fields of plate area). The cathode side has a silver catalyst made by Hoechst. It is possible to use platinum but the cost is much greater.

In the PEM type the electrolyte is a solid, Nafion 115 sheet - a proprietary Du Pont product; there are competitors such as Dow Chemical and Ashai in Japan. The anode is similar to the alkaline type. The cathode membrane has a platinum catalyst and much research has been aimed at reducing the cathode loading which is why many PEM cells use the high pressure approach, since it helps to reduce the amount of catalyst for a given current density. Catalysts are the main cost in stack construction and optimizing their use is a major research area. Other differences between the two types are cooling and source gas purity.

In both systems about 40% of the fuel expended is given out as heat. In the PEM type, water cooling plates are used to remove this heat. In the alkaline type the electrolyte does this job and has the added advantage that it does not freeze at 0°C. Consequently both systems need a liquid cooling system.

In Europe, Esso is already committed to making hydrogen available at vehicle service stations. Hydrogen may also be used to power aircraft in the future. In America, petrol is effectively subsidized (see Chapter 4) which makes it very hard for other fuels to compete. One of the main interests there has been reforming petrol and methanol to produce hydrogen. If done in the vehicle this produces a hydrogen supply which contains a high concentration of carbon monoxide. PEM systems can be made to tolerate this impurity. To date alkaline stacks need pure hydrogen.

However, the whole business of on-board reforming is undesirable in terms of cost and complexity and is inefficient in terms of fuel consumption compared to using pure hydrogen made at a central

HYDROGEN GAS

ANODE (NICKEL MESH)

HYDROGEN GAS

ANODE (NICKEL MESH)

HYDROGEN GAS

ANODE (NICKEL MESH)

HYDROGEN GAS

ANODE (NICKEL MESH)

^ CATHODE (NICKEL MESH) OXYGEN GAS

Fig. 2.13 Alkali (left) and PEM cell layouts compared.

OXYGEN GAS

CATHODE (NICKEL MESH)

^ CATHODE (NICKEL MESH) OXYGEN GAS

Fig. 2.13 Alkali (left) and PEM cell layouts compared.

OXYGEN GAS

CATHODE (NICKEL MESH)

facility. There are two main ways hydrogen can be stored: gas or liquid. As a gas it is usually compressed to 200 bar and stored in steel tanks with man-made fibre reinforcement and carbon additives to assist in the absorption. This technique works for large vehicles where bottles can be roof mounted - buses, for example. As a liquid the energy density is three times that of petrol, being 57 000 BTUs per lb compared with 19 000 BTUs per lb for gasoline. Two gallons would be needed to travel 500 miles in a 3 litre/100 km (80/100 mpg) PNGV specification vehicle. The gas liquefies at -180°C and 20 bar. In modern super-insulated vehicle tanks, hydrogen can be kept liquid for 2 weeks without refrigeration. A 20 watt Sterling cycle refrigerator can keep it liquid indefinitely. This system is suitable for application where space is limited, such as aeroplanes and cars. Many people believe the compression process uses too much energy. In fact it is the LIND refrigeration cycle, which is used to take hydrogen down to -269°C from -180°C where hydrogen is liquid at atmospheric pressure, that is the heavy consumer of compressor energy.

Polaron believe the technique permits the early use of hydrogen because tank exchange is possible until the investment in on-board refuelling is possible. The tank for a car would only be the size of an outboard-engined boat fuel tank. Cryogenic storage is already well established in the natural gas industry where liquid natural gas (methane) at -160°C is used to fuel 1000 bhp heavy duty trucks in Europe and Japan.

Considering again fuel-cell stacks, in either system an anode or cathode plate is 2.5 mm thick, so a pair of plates give a 5 mm build-up. Each pair of plates gives 1 V at no-load and typically 0.66 V at full load. This means that the stack length is around 500 mm, plus manifolds, for a 64 V, 7.2 kW continuous rating stack. It should also be pointed out that stack power doubles, at least, if pure oxygen is used instead of air. This is unlikely, however, as on-board enrichment to 40% oxygen is promised in the near future, as are some significant improvements in stack chemistry, especially in the catalyst area.

The fuel-cell stack is controlled by regulating the hydrogen pressure in the range 0-30 millibars. A recirculation loop permits water vapour to be added as PEM fuel cells work best with wet hydrogen. The air pressure is regulated by changing the blower speed in conjunction with the fuel-cell current demand. This takes 10 seconds to rise in a low pressure system, but may fall rapidly. The DC/DC converter determines the load applied to the fuel cell.

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