Energy & Power

Cover Report

Fuel Cell Backup: The Third Option
AHM Zafrul Hossain

It is an undisputed fact that businesses lose revenue if they suffer an interruption to their prime power source, even for just a few seconds. It is not just the loss of an ability to work or receive customer calls. These are bad enough, but many service providers will lose revenue when systems based on automatic processes fail. Web-hosting, data centers, online catalogues and call centers are just a few examples. If customers can’t access a product or service someone will lose out.

Businesses deriving income from web-based information services are especially vulnerable. There are many other situations where loss of power causes widespread chaos to everyone -- such as emergency services & hospitals, research establishments and education.

The “standby power” industry has been addressing this issue for years. When main power goes off, the standby power system has to operate either by generating power from fuel or by using stored power. Most often, generated power comes from a diesel generator, and stored power from a UPS with lead-acid batteries.

But there are lots of options. Generators can also be powered by natural gas or petrol (or aviation fuel, if it’s a gas turbine). Stored power can also come from NiCad batteries and flywheel systems. Modern electronic equipment is susceptible to even momentary power loss. For that reason a conventional generator, on its own, is insufficient to support a computer-based system. For the same reason, the ‘average’ standby power system for a computer room incorporates both a battery-powered UPS (to bridge the gap whilst the generator starts) and a diesel generator to provide the extended runtime that may be required.

Ten years ago the debate about reliability would probably have centered on individual component parts; differing battery technologies and battery design life times, plus lengthy discussions over which UPS technology offered the best solution for a particular application. Other discussions may have centered on the fuel source of the generator, i.e. is gas more reliable than diesel?

More recently the debate is likely to have been on widely differing ‘redundant’ architecture i.e. can a system be designed so that it never totally fails? And can all single points of failure be eliminated? The arguments are lengthy.

Whilst it may have been acceptable 10 years ago to talk to the ups manufacturer when the UPS failed and/ or the generator supplier if that wouldn’t start, now it is less so. Most customers are now looking towards a single source for the numerous components that make up a modern standby power system. Much more emphasis is placed on integrating the individual components and removing the necessity for personal intervention in the event of mains failure.

Users often require a complete standby power solution, managed by a single provider with a single source of contact, leading to greater efficiency and total peace of mind. If that provider can be supplier –independent and impartial -- therefore removing any complexity from the purchasing cycle -- so much the better.

Sometimes when extended auxiliary power is required a choice needs to be made between installing either large banks of batteries to keep the UPS powered for longer, or a generator running on a separate fuel source
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There are advantages and disadvantages with each option although it is generally accepted that if all other factors are equal there is a financial advantage to a generator if the extended run following a power loss needs to be in excess of 4-8 hours. Some UPS –such as that manufactured by S &C and those utilizing flywheel technology-are designed specifically to provide power to bridge the 30-second gap between mains fail and generator start up. Both these are particularly attractive when comparing probable lifecycle costs as they can be significantly lower than conventional online UPS technology and now there is a real, viable alternative: the fuel cell.

Fuel cells can be regarded as generators. But whereas conventional generators use internal combustion engines to rotate an a alternator, fuel cells generate power by producing electrons directly, with no moving parts. As a result, they have the potential to be very efficient and reliable --although unproven as of yet? Moreover, they are comparatively quiet and other than electricity and heat, they produce only water vapor. This makes them ideal for indoor use. With the maturing market in fuel cell technology and increasing awareness of environmental issues, a standby power solution incorporating a fuel cell is now a definite third option.

Backup power based on fuel cells is most suitable for applications with a power demand lower than 10 kw and energy demand larger than 10 kWh. In such applications fuel cell power sources can provide long backup times at a lower cost than batteries. Fuel cells have a long lifetime, especially in standby mode. Fuel cell power sources are maintenance-free and they are insensitive to both high and low temperatures.

Technology

Fuel cell technology is often presented as the technology of the future for a variety of consumer applications. Currently, the technology has reached maturity and cost competitiveness for certain niche applications for professional use. Fuel cells can particularly compete against batteries and diesel generators in backup and reserve power applications. In this document, the characteristics and applications of fuel cell backup power are described briefly, and compared to those of batteries and diesel generators.

Basics

A fuel cell converts the chemical energy of the fuel (usually hydrogen) directly to electricity. Different types of fuel cells are named after the electrolyte used in them. The fuel cell that is most suitable for backup power is the polymer electrolyte fuel cell (PEFC).

Polymer electrolyte and electrodes are the parts where electricity is generated. PEFC is robust and has a short start-up time. In addition to the fuel cell stack, the PEFC system includes a number of balance-of-plant components.

Fuel cells, batteries and diesel generators

Fuel cells are direct current (DC) power sources, similar to batteries. The essential difference is that fuel is stored outside of the fuel cell, as compressed hydrogen gas or as metal hydride. A fuel cell can produce power as long as there is fuel available in the fuel storage. Fuel cells are “charged” simply by filling the tanks. In this sense, fuel cells resemble diesel generators. The capacity of a fuel cell system depends on the size of the gas storage. The continuous power depends on the sizing of the fuel cell stack.

A fuel cell is an electrochemical energy conversion device. It produces electricity from external supplies of fuel (on the anode side) and oxidant (on the cathode side). These react in the presence of an electrolyte. Generally, the reactants flow in and reaction products flow out while the electrolyte remains in the cell. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.

Fuel cells differ from batteries in that they consume reactants, which must be replenished, while batteries store electrical energy chemically in a closed system. Additionally, while the electrodes within a battery react and change as a battery is charged or discharged, a fuel cell's electrodes are catalytic and relatively stable.

Many combinations of fuel and oxidant are possible. A hydrogen cell uses hydrogen as fuel and oxygen as oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include air, chlorine and chlorine dioxide.[1]

Fuel Cell Design

In the archetypal example of a hydrogen/oxygen proton exchange membrane fuel cell (PEMFC), which used to be called solid polymer electrolyte fuel (SPEFC) around 1970 and now is polymer electrolyte membrane fuel cell (PEFC or PEMFC, same as the short writing of proton exchange membrane) while the proton exchange mechanism was doubted, a proton-conducting polymer membrane, (the electrolyte), separates the anode and cathode sides.

On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electrically insulating. On the cathode catalyst, oxygen molecules react with the electrons (which have traveled through the external circuit) and protons to form water. In this example, the only waste product is water vapor and/or liquid water.

In addition to pure hydrogen, there are hydrocarbon fuels for fuel cells, including diesel, methanol (see: direct-methanol fuel cells) and chemical hydrides. The waste products with these types of fuel are carbon dioxide and water.

The construction of the low temperature fuel cell PEMFC: Bipolar plate as electrode with in-milled gas channel structure, fabricated from conductive plastics (enhanced with carbon nanotubes for more conductivity); Porous carbon papers; Reactive layer, usually on the polymer membrane applied; polymer membrane.

The materials used in fuel cells differ by type. The electrode/bipolar plates are usually made of metal, nickel or carbon nanotubes, and are coated with a catalyst (like platinum, nano iron powders or palladium) for higher efficiency. Carbon paper separates them from the electrolyte. The electrolyte could be ceramic or a membrane.

A typical fuel cell produces about 0.86 volt. To create enough voltage, the cells are layered and combined in series and parallel circuits to form a fuel cell stack. The number of cells used is usually greater than 45 but varies with design.

System Architecture & Dimensioning
Power & Energy

The dimensioning of a fuel cell system starts by establishing the power and energy need. What is the average power? The fuel cell should have a power output that covers the need. Cellkraft fuel cells are available in different sizes from 50 W to 2000 W. Higher power can be obtained by connecting several units in series or in parallel. Power is usually measured in Watts (W)

How much energy must the system contain? How long time should the fuel cell be able to power the load? Energy is commonly expressed in terms of kilowatt hours (kWh). A standard gas cylinder (200 bar pressure, 50 liters volume) filled with hydrogen will correspond to 10 kWhe. That means the cylinder can feed a fuel cell that delivers 1 kW electric power for 10 hours.

Battery or Ultracapacitor

If the application has a need for high power for short times it is a good idea to dimension the fuel cell for the average need and then use a battery or an ultracapacitor to cover the peak power demand. The power buffer should then be dimensioned in the same way as the fuel cell. What is the difference between the peak power and the average power? This power need should be covered by the battery or ultra-capacitor. It is then necessary to consider the length of the peak power demand and the time between the peaks to understand how much energy the power buffer must contain. Ultra-capacitors could be an alternative to batteries if the load profile includes short periods or pulses of high power. Cellkraft offers systems based on different types of battery chemistries (Lead-acid, Li-Ion) or ultra-capacitors.

There is a need for a small energy source for powering the start up of the fuel cell. If seamless power is of importance it is also necessary that this source can feed the load during the startup time of the fuel cell. Fuel cell systems are often hybrid systems because of these reasons, even if the load has a fixed power demand.

DC or AC Voltage

The application might require DC or AC voltage in a specific interval. If it is possible to use the raw DC voltage from the fuel cell no additional conversion is necessary. This means lower cost and higher efficiency. The fuel cell stack in itself has a rather wide voltage range. Maximum power could be achieved at a voltage about 50 percent of the zero-current voltage. The effective working interval of the fuel cell system is however more narrow.

If the fuel cell is combined with a battery or ultra-capacitor it is possible to get a quite well defined voltage from the system. If the required voltage is different from the fuel cell voltage (higher, lower or more precise) a DC-DC converter is used. If AC power is required a DC-AC inverter is connected to the fuel cell output.

Efficiency & Hydrogen Consumption
Efficiency

The efficiency of a fuel cell system is defined as the percentage of the fuel that is converted to electric energy. This is made by comparing the output electric energy with the consumed chemical energy. The common value of chemical energy is the lower heating value (LHV) of the fuel.

Hydrogen Consumption

High efficiency means low hydrogen consumption. A fuel cell system that delivers 1000 W net power will consume hydrogen according to the table to the right. Why is the efficiency not 100 percent.

All energy conversions will lead to a certain amount degradation of energy quality. All input energy will not come out as output. Some of the energy will be lost as heat. Fuel cell development work aims to maximize the output by minimizing the losses. The losses could be traced to either the stack or the components of system. The fuel cell stack converts the chemical energy to electric energy. The electrochemical process and the conductance of current will however lead to losses and heat generation. These losses always increase at high current outtake. Stack design and operating conditions could be optimized to reduce the losses. There are two kinds of losses at system level: Fuel losses and electric losses. The fed hydrogen will be converted to electricity in the stack. Small amounts of hydrogen will however be purged out of the system. This fuel will represent a minor loss that must be considered when establishing the efficiency. The last thing to consider is the internal components of the system. They will consume some of the power delivered from the stack. Components like the air blower, cooling pump, cooling fans, valves and control circuits will support the stack and consume electric power. The net power delivered from the system will be slightly lower than the gross power from the stack, because of the internal consumption. The overall efficiency of the system takes all these losses into account.

Applications

Small power – Large energy demand Fuel cell power sources are best suited for the applications in which the power need is low (<10 kW) and the energy needs are high (>10 kWh). The reason for this is that the fuel cell, the size of which defines the power, is expensive while the energy storage is relatively inexpensive compared to batteries. In other words fuel cell power sources can compete in applications where required backup times are long and the effects are low. When backup times are short, batteries are usually the best solution. When both large effect and energy storage are required, diesel generators are the most competitive alternative.

Durability

Polymer fuel cells have excellent durability both in operation and in stand-by. For example, the expected lifetime of Cellkraft’s PEFC systems exceeds 20 000 hours of operation, two orders of magnitude longer than needed in typical backup applications. For backup applications the lifetime in stand-by mode is usually the most important parameter.

Compared to batteries there is no chemical degradation in the electrochemically active part of PEFC. Therefore, the stand-by lifetime of PEFC systems is practically unlimited. A combined lifetime of several tens of years can be expected (99 percent stand-by and 1 percent operation).

The energy storage, in the form of compressed hydrogen, does not have any significant “self discharge”. Using the maximum allowed leakage rate (6 cm3/h) 50% of “self discharge” would take 95 years for a standard hydrogen bottle. These values can be compared to those of those for lead acid batteries that, due to their inherent chemical instability, have a life-time less than ten years even in optimal conditions (ambient temperature 20° C or less). At higher temperature the degradation rate is significantly accelerated. A rule of thumb is that when ambient temperature is increased by 10° C, the lifetime is halved. Diesel generators are similar to fuel cells in a sense that, with proper maintenance, they can reach over 30 years of combined lifetime including up to 5000 hours of active operation.

Maintenance

Fuel cell backup power sources require practically no maintenance. In principle, batteries are also maintenance free. However, some charging is important to keep up the charge level of batteries due to self-discharge. Diesel generators have the greatest maintenance needs.

Maintenance includes changing the lubrication oil, filters, and start battery. Start-up tests must be done in certain intervals in order to secure availability and long lifetimes. Reliability Due to short field experience of fuel cell technology there is limited, but promising data about reliability. The technology has fundamental (properties) to reach high reliability: A fuel cell stack has simple reactions compared to batteries. In both fuel cells and batteries the cells are coupled in series. This means that the weakest cells define the performance and reliability. However, fuel cells stacks can work even if some cells are not providing full voltage. A major difference is also that that fuel in fuel cells is not stored in cell volume. Therefore unbalanced energy states cannot appear.

The reliability of fuel cells is dependent on many engineering choices: materials for gaskets, choice of blowers, pumps and so on. Another important factor for reliability is how well the fuel cell system is integrated, assembled and installed. In contrast, batteries have complicated chemistry but do not need complicated balance-of-plant components. The reliability of batteries is mostly defined by their age. Capacity and reliability degrade with time. Degradation is strongly dependent on temperature. Battery capacity is largely dependent on their thermal history. Diesel generators are complex systems and require a substantial amount of electrical energy for start-up. Typically the start-up is the single most critical item for the reliability of diesel generators, especially in cold conditions.

With proper thermal conditioning and maintenance, diesel generators have high reliability. Measurability Fuel cell systems are transparent and can be monitored remotely. The stored energy can easily be measured from the pressure of the gas cylinders. The efficiency (hydrogen to electricity) is predictable. Due to this, the energy content can be expressed using units kWh or Ah. The stored energy can be measured remotely. The status of batteries is significantly more difficult to measure.

The energy capacity can be measured by discharging and charging the battery, but in reality this is not often used. Usually the battery is simply replaced after a certain period of time. The uncertainties concerning capacity reduce reliability and increase costs. In order to secure high reliability, the batteries are replaced long before the problems may occur. In diesel generators the effect as well as the stored fuel can be easily measured.

Heat Tolerance

The fuel cells require cooling as they operate and produce electric power. With a properly dimensioned cooling system, the fuel cell system can operate at high ambient temperatures, at least up to 50° C. The cooling of a fuel cells system is organized using a radiator, placed possibly outside of the building. In stand-by mode cooling is not needed. In stand-by mode the fuel cell system can tolerate very high ambient temperatures. No degradation is expected, even with an ambient temperature up to 75° C. This all means that the requirement for cooling can be lowered in many applications, since it is often batteries, not electronics that require cooling. In some cases, the need for energy and a service-intensive air conditioning system is removed. Diesel generators work well at high ambient temperatures if the cooling system is properly dimensioned.

Cold Tolerance

Properly designed and operated fuel cell systems can tolerate very cold temperatures. Fuel cell systems can also start at low temperatures; for example Cellkraft’s system can start from -33° C. There is no reduction in energy capacity or power outtake in subzero conditions. The capacity and available effect of batteries is significantly reduced at cold temperatures. Diesel generators should be continuously heated by electrical heating to be ready for start-up in cold conditions.