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Advanced fuel-cell control systems

Viable energy storage systems

2.4 Advanced fuel-cell 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.

Mounting Endurance 40 h at full power Fuel 25 kg aluminium anodes Oxidant 22 kg oxygen at 4000 lb/in2 Buoyancy Neutral, including aluminium

hull section Time to refuel 3 h

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

Dimensions:

Volumetric energy density 265 Wh/l Gravimetric energy density 265 Wh/kg 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

Fig. 2.10 TXI London taxi.

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. 2.11 Developed PEM fuel cell: (a) plate; (b) stack; (c) anode; (d) cathode.

(a) (b)

(c) (d)

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 hour

Pure hydrogen 5 cubic metres per hour 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: (l) Hydrogen 80 watts (2) Air 320 watts (3) KOH 85 watts (4) Water 10 watts

Valves: 10 off electropneumatic control

Preheat: 2 kW – 312 V DC

DC/DC converter 2

Input: 200/400 V DC

Output: 27.6 V DC 600 W for control system

Fig. 2.12 Fuel-cell system specification.

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

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

HYDROGEN GAS HYDROGEN GAS

ANODE (NICKEL MESH) ANODE (NICKEL MESH)

POROUS MEMBRANE (PLATINUM CATALYST) POROUS MEMBRANE (PLATINUM CATALYST) LIQUID KOH (POTASSIUM HYDROXIDE) SOLID NAFION POLYMER ELECTROLYTE POROUS MEMBRANE (SILVER CATALYST) POROUS MEMBRANE (PLATINUM CATALYST)

CATHODE (NICKEL MESH) CATHODE (NICKEL MESH)

OXYGEN GAS OXYGEN GAS

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.

2.4.1 FUEL-CELL OPERATING STRATEGIES

As we can now see in Fig. 2.14, a fuel cell is a complex system and the key problems are that the feedstock must be kept pure and power consumption minimized in the auxiliaries. There are two main fuel-cell operating strategies: high pressure, 1–3.5 bar, and low pressure, at 1/20 bar The benefit of high pressure systems is fast hydrogen diffusion in the membrane which results in fast response – less than 1 second. Consequently it is possible for the fuel cell to follow the vehicle load profile and operate without a battery. This strategy is spoilt by warm-up issues.

The stack must be at 70°C to deliver rated power. Warm-up can take 15 minutes. Another problem is that the power to supply the compressed services is significant – perhaps 25% of output at peak power.

Low pressure systems have modest auxiliary power needs, perhaps 10% of rated output at full power and proportionally less at low power. The main consumers are the air compressor and the KOH pump. The price is slower response. It typically takes 10 seconds for the fuel cell to ramp up to full power, consequently a peaking battery is needed to provide power during acceleration. This means that generally a smaller fuel cell may be used.

Fuel cells are the opposite of most electrical devices in that peak efficiency occurs at minimum load. In a high pressure system this profile is ideal for a motorway express coach where most time

Fig. 2.14 Fuel-cell system schematic.

is spent cruising at 20/30% of maximum power. Hence efficiency is good in cruise and less so in continuous urban cycle duty. However, efficiency and emissions are always better than the equivalent thermal engine. Efficiency is 60% at no-load and 40% at full-load, at the current state of development – and the theoretical maximum is 83%.

2.4.2 COST REDUCTION STRATEGIES FOR FUEL-CELL CONTROLS

The cost of the fuel-cell controller (Fig. 2.15) is split up, at present, as follows: 20% each on pumps and compressors; valves and actuators; programmable logic controller; DC/DC converters;

sensors and transducers. The challenge is to achieve a 5:1 cost reduction for mass-market viability.

CO2

FUEL CELL STACK DC/DC CONVERTER

7.2kW

Fig. 2.15 Fuel-cell control system.

Fig. 2.16 Hydrogen/air blowers shown to left of drive electronics.

The hydrogen pump (shown left in Fig. 2.16) is a side channel blower and has to operate at 1/30 bar at 5 cubic metres/hour with wet hydrogen at 70°C, plus slight KOH contamination. The pressure criterion usually results in a choice of blower made by Gast and Rietschle. The standard unit is a 150 mm cube and weighs 5 kg. The operating point is 2800 rpm and power consumption by the pump is around 85 W, with an additional 36 W of copper loss in the motor. Fuel Cell Control Ltd rewound the 2 pole D56 induction motor as a 4 pole 20 V unit. Our second attempt will be an 8 pole design which should reduce the copper loss to about 8 watts. The low voltage is chosen for safety and the unit will be driven with a linear sine wave inverter (shown centre in Fig. 2.16). The dv/dt is kept low to avoid spontaneous ignition, in case hydrogen enters the motor chamber. The windings are potted, to avoid direct electrical contact and reduce the free volume in the motor chamber. All parts in the hydrogen contact area are nickel plated (zinc–copper–nickel). The blower is made of aluminium.

The air pump (shown right in Fig. 2.16) is about a 300 mm cube and weighs about 15 kg. The motor is a 1/2 hp D63 induction machine and has been rewound as 8 pole 20 V, 15 A, 325 W at 192 Hz (2900 rpm). The inverter is a switching unit to minimize losses, as the ignition risk is lower than with hydrogen. A 30 amp inverter provides good efficiency and the speed of this blower is adjusted with variations in power demand.

For the future, the company are working on a high speed channel blower, to operate at 10 000 rpm, using a brushless DC motor. In star-winding form, at 4000 rpm, it will satisfy 5 cubic metres per hour and at 10 000 rpm 45 cubic metres per hour. Thus a single design could do both jobs and it only weighs about l kg. However, silencing must be carefully considered.

The water and KOH pumps are standard 10 and 85 watt capacity permanent-magnet brushed DC driven pumps running at 3000 rpm on 28 V DC. The pumps are magnetically coupled with Talcum parts to resist aggressive fluids (such as potassium hydroxide).

2.4.3 FUEL-CELL CONTROL VALVES AND ACTUATION

Selecting suitable valves with such a diverse array of media and operating conditions has not been easy, Fig. 2.17. In fact the valves themselves are neither expensive nor heavy. The problem area is the actuators and it is intended to redesign these for the next version. Currently there is no safety legislation in place for hydrogen powered vehicles. The onus is on the supplier to demonstrate

Fig. 2.17 Kit of valves.

fitness for purpose and that all reasonable precautions have been taken. It is felt this will change once meaningful experience has been achieved. Clearly, declaring a vehicle to be a class 1 safety area would destroy all economic viability. Consequently, as with petrol and propane, safe techniques need to be established and demonstrated before regulations are enforced. Some are obvious, such as no fuel or fuel processes to be contained in the passenger compartment. Others require experience such as fuel storage and distribution. Storage in a closed building needs careful consideration.

The valves can be neatly split into two groups, high and low pressure. The high pressure units are standard metal valves with electric solenoid actuators and spring return; they operate with 28 V coils. The larger units were chosen as 2 and 3 way plastic ball valves, using polypropylene bodies and EPDM seals for KOH compatibility plus high temperature operation (70–80°C).

Polaron had a major problem with the actuators. Fail-safe operation with low power consumption was needed. Solenoid valves in larger sizes use the controlled medium as a pilot fluid and consequently do not operate reliably with pressures as low as 1/30 bar. The solenoids are direct acting, with no economy measures or permanent magnet biasing, and thus consume significant power. In the end nitrogen was used as a pilot fluid, with 4 watt pilot valves to control the opening.

This approach works well but the valves use up a lot of space, especially the actuators.

The intention for the future is to design a plate with spring-loaded clutches operating from a common motor drive. This should cut down on the volume and permit a much lower cost solution.

Actuation accounted for 70% of the cost and 70% of the volume of the valves. It was found to be a niche sector market where nobody has a comprehensive system of interchangeable valves, seals and actuators suitable for onerous conditions.

2.4.4 PROGRAMMABLE LOGIC CONTROLLER (PLC)

An 80 I/O PLC with interface modules cost $1500 in 1998. Quantity build could halve this price – but still nowhere near the objectives. A Mitsubishi F Series was chosen for development, Fig.

2.18. Production units are destined to use a custom-engineered microprocessor unit based on a Siemens/Thompson C167 CAN bus processor, which is becoming a standard in the European and American car industry. This unit must represent one of the toughest design challenges. To convert low voltage, heavy current into higher voltage with galvanic isolation, ultra-low current ripple and high efficiency. Many solutions have been analysed, although this one offers the best combination of characteristics. Figure 2.19 shows electronic system circuits.

Let us consider a square wave phase-shift chopper: at minimum volts input, we need full reinforcement to achieve 396 V output. However, at low load we have a 96 V DC link and perhaps 90 degrees phase shift between A and B. This is not too bad, except that we only draw output current for 50% of the time: this means that the DC link contains 100% current ripple at 2F switching frequency. Since the pulse width should always be 50% plus, the solution is to have three such choppers with 120° phase shift between them. This has the effect of overlapping the converters if the correct measures are taken. Consequently, the supply now only contains 30%

ripple worst case at 6F, but when the current is largest we have maximum overlap; see Fig. 2.19 (bottom).

The first attempt is to build this converter with each stage operating at 3 kHz, with torroidal 0.08 mm silicon steel cores. This is both silent and efficient. The edges are deliberately softened to reduce dv/dt (capacitive ripple). At low frequency this does not cost much in losses, with 10 microsecond edges, but reduces the spikes when the diodes reverse recover. The design is adaptable to different output voltages by rewinding the output transformers and chokes. It is believed 90%

efficiency can be achieved with this design at 60 V input, 7.2 kW. A double L/C Filter attenuates the current to the fuel-cell stack to ensure compliance with 0.01% ripple current rating. The reason for this is to prevent poisoning of the fuel-cell catalysts.

The cost of this unit is a problem. The silicon for the main switches are LAPT transistors, at 15 A and 200 V, using 2SA1302 and 2SC3276; the six switches cost $150 in parts (1998 prices). It is intended to have these parts integrated into a high power package. The chokes cost $150 and

The cost of this unit is a problem. The silicon for the main switches are LAPT transistors, at 15 A and 200 V, using 2SA1302 and 2SC3276; the six switches cost $150 in parts (1998 prices). It is intended to have these parts integrated into a high power package. The chokes cost $150 and