Fuel Cell Technology for Distributed Generation: An Overview

Xinhong Huang, Member, IEEE, Zhihao Zhang, Student Member, IEEE, and Jin Jiang, Senior Member, IEEE

Department of Electrical & Computer Engineering, The University of Western Ontario London, Ontario, Canada. N6A 5B9

E-mail: xhuang49@uwo.ca, zzhang6@uwo.ca, jjiang@eng.uwo.ca Abstract-Fuel cell technology for distributed generation (DG)

is attracting more and more attention due to its high efficiency and unique environmentally friendly features. In this paper, the system characteristics, state-of-the-art, prospective markets and applications of different fuel cells are discussed. The operational principles of these fuel cell systems are also presented and their distinct characteristics are summarized. As a new technology for DG, this paper focuses more attention on the DG application aspect of fuel cells. Comparison of different type of fuel cells in the framework of DG is also made. Furthermore, the power conditioning and control techniques for fuel cell based DG are also reviewed. Finally, future prospects of fuel cell based DG are predicted.

I. INTRODUCTION

According to the International Energy Outlook 2005 report [1] prepared by the Energy Information Administration of the USA, world net electricity consumption will nearly double from 14,275 TWh in 2002 to 26,018 TWh in 2025. In addition, environmental concerns have lead to reduce the greenhouse gas emissions. Under such circumstances, the traditional centralized power generation alone can no longer satisfy the requirement of the ever increasing in power demands, the power quality and safety. Distributed generation (DG) becomes an important supplement to centralized power generation. Moreover, deregulation and restructuring in the energy market also create incentive for small-scale power generation in distributed forms. The total capacity of DG is expected to grow rapidly in the near future. A proper combination of traditional centralized and newly developed distributed power generation will be less costly to consumers, can increase generation and transmission efficiency, improve the security and flexibility of the entire power system, and in the meantime reduce the greenhouse gas emissions.

Distributed generation relies on small-scale power generation technologies to provide electricity at or close to the load site. A DG system is generally not centrally planned and dispatched, and usually connected to a distribution network, directly to customer’s facilities, or both [2][3]. The rating of DG ranges can range from a few KW up to 100 MW depending on different applications [4]. Distributed generation is also capable of providing diversified energy options to both utilities and consumers, including standby generation, peak shaving capability, premium power, remote power, or cogeneration. There are various technologies available for DG,

including combustion gas turbines, micro-turbines, wind turbines, photovoltaic/solar panels, and fuel cells. Among these technologies, the use of fuel cells for distributed generation is attracting more and more attention due to its unique environmentally friendly features.

Fuel cells convert chemical energy directly into electrical energy by an electro-chemical reaction between hydrogen and oxygen. The by-products are water, heat, and carbon dioxide in very small amount (only when carbon-containing fuels other than pure hydrogen are used, such as CO and CH4). In the last decade, fuel cells have become one of the most promising technologies to meet the increasing energy demands for the 21 century. Fuel cell technology is well suited to distributed generation due to its high efficiency, scalability, modularity and negligible amount of harmful emissions. Moreover, the next generation nuclear power plants known as “Generation IV” will be able to produce the hydrogen directly from thermo cracking [5]. It is expected that this will greatly increase the popularity of fuel cell applications.

This paper provides an overview of the fuel cell technology for distributed generation. The current research and recent developments on this subject has been reviewed. In section II, the basic notion of fuel cell technology is introduced. The system characteristics, state-of-the-art, prospective markets and applications of several types of fuel cells are also covered. Section III compares fuel cell based DG with other DG technologies. A comparison of DG technologies based on different fuel cell types is also presented. In Section IV, the power conditioning and control techniques for fuel cell based DG are reviewed. Different power conditioning unit (PCU) topologies and control techniques are presented. Finally, the future prospects of fuel cell based DG is summarized in Section V.

II. DEVELOPMENT AND APPLICATION STATUS OF FUEL CELL TECHNOLOGIES

A. Principles The principle of fuel cells is not a new concept. The first fuel

cell was invented in 1839. It used hydrogen and oxygen on platinum electrodes with sulfuric acid as the electrolyte to generate electricity. However, like many of the early discoveries in electricity generation, fuel cell technology remained only as curiosity to general publics for more than a century until late 1950s [6][7].

A fuel cell is an energy conversion device that converts the chemical energy directly into electric energy and some heat without any intermediate thermal or mechanical processes [8]. Fuel cells are similar to batteries containing electrodes and electrolytic materials to generate electricity. The structure of a simplified fuel cell is shown in Fig. 1 [3]. A positive electrode (anode), a negative electrode (cathode), and an electrolyte are essential for the reaction to take place. In addition, the material of electrolyte is generally used to identify the type of fuel cells.

In principle, a fuel cell can operate with a variety of fuels and oxidants. Hydrogen has long been recognized as the most effective fuel for practical use because of its high electro-chemical reactivity, which minimizes the need for expensive catalysts. Oxygen is the preferred oxidant due to its abundance in the atmosphere. The chemical reactions in fuel cells can be described by (1) to (3).

Over all cell reaction: H2 + ½O2 → H2O (1) Anode reaction: H2 → 2H+ + 2e- (2) Cathode reaction: ½O2 + 2e- + 2H+ → H2O (3)

B. Fuel Cell Types and Characteristics Presently, there are five major types of fuel cells in different

stages of commercial availability or research/development. They are proton exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC). Since AFCs are nearly zero tolerance to CO2 and CO constituents in the fuel, therefore, they require high purity hydrogen and oxygen for reaction, which makes AFCs unsuitable for DG. Hence, AFCs will not be discussed hereafter.

Even though different fuel cells operate on similar chemical reactions, they are still very different in many aspects in terms of operating characteristics, materials used in construction, and suitability for potential applications [9].

Table I [6-9] shows the characteristics and requirements of four types of fuel cells, which are potentially suitable for DG applications.

TABLE I CHARACTERISTICS AND REQUIREMENTS OF MAJOR FUEL CELL TYPES

Fuel Cell Type Parameters

PEMFC PAFC MCFC SOFC

Electrolyte

Solid polymer membrane (Nafion)

Phosphoric acid (H3PO4)

Lithium and potassium carbonate

Stabilized solid oxide electrolyte (yttria, zirconia)

Operating temp. (˚C) 50-100 ~200 ~650 ~1000

Operating press. (psig) 15-30 ~15 15-150 ~15

Fuel Pure H2 (tolerates CO2)

Pure H2 (tolerates CO2, approx. 1% CO)

H2, CO,CH4, other hydrocarbons (tolerates CO2)

H2, CO,CH4, other hydrocarbons (tolerates CO2)

Oxidant O2 in air O2 in air O2 in air O2 in air

Efficiency 35-45% 40% >50% >50% Cell voltage

(VDC) 1.1 1.1 0.7-1.0 0.8-1.0

Install Cost (US$/KW) 1,400 2,100 2,600 3,000

Ideally, the theoretical maximum voltage produced by a

single cell as shown in Table I would be realized at all operating currents. In reality, the cell achieves its highest output voltage only at open circuit conditions and voltage drops off with increasing current drawn, similar as any voltage source with internal resistance. In fuel cell terminology, it is known as polarization and can be represented by a polarization curve as shown in Fig. 2 [3][7].

The polarization curve characterizes the cell voltage as a function of the current. The current, in turn, depends on the size of the external electrical load that the fuel cell powers. The polarization curve also shows the electrochemical efficiency of the fuel cell at any operating current since the efficiency is often defined as the ratio of the actual cell voltage over the theoretical maximum voltage.

Polarization is caused by chemical and physical factors associated with various elements in the fuel cell, such as temperature, pressure, gas composition, and fuel properties and reactant utilization. These factors limit the reaction processes

Fig. 1. Fuel cell operation diagram

Fig. 2. Typical fuel cell polarization curve

when the current flows through. There are three main types of polarizations – activation polarization, ohmic (or resistance) polarization, and concentration polarization. The deviation of the cell potential from the ideal behavior is a direct result of the sum of these factors over the entire load range. As can be seen from Fig. 2, fuel cells have the best performance in the ohmic polarization region.

C. State-of-the-Art in Development and Applications In the last 20 years, the fuel cell technology experienced

several stages of development from AFC, PAFC, MCFC, and SOFC to PEMFC. From inception in the 1970s to 2004, the market shares for each major type of fuel cells are: PAFC-6%, PEMFC-65%, MCFC-3%, and SOFC-4% respectively [10]. The total share is about 78% of the whole fuel cell applications market.

So far, among all fuel cell types, PAFC technology is the most advanced and matured, without any significant technical barriers. PAFC is also the first type of fuel cell to be commercially available. PAFC technology for power generation has been demonstrated by stationary power plants up to 11 MW in size. PS25, 200kW PAFC produced by ONSI Corporation in the USA since 1993, is the market entry unit targeted primarily for commercial cogeneration. Presently, there are over 200 units operating throughout the world. Many of them have now been operating for several years with some operating experience gained. The stack life of PAFC has exceeded five years and mean-time-between-forced-outage is longer than any other engine based systems [11]. Moreover, a new enhanced PAFC with an estimated 80,000 hours operating lifetime, the longest on the market, is scheduled to be released soon by the United Technologies Corporation (UTC) [12].

The electrode/electrolyte technology for PAFC has now reached a mature level where developers and users are focusing their resources on producing commercial capacity, multi-unit demonstrations and pre-prototype installations [13]. However, cost reduction is still the major hurdle in sustained development and commercialization. In order to achieve significant cost reduction at a commercialization level, both higher sales volumes and design improvements in the power plant itself are required. Costs associated with every element of the power plant, including fuel processor, stack design, power conditioning and control, and ancillary components and their assembly, must be reduced also.

MCFC is often considered as the second generation fuel cell as they are anticipated to be commercialized after PAFC [7]. It still has some technical and economic issues to be resolved. Many demonstrations have been carried out in the USA and Japan. Up to now, Energy Research Corporation (ERC) in the USA still leads the worldwide commercialization effort. It has demonstrated a large-scale MCFC technology with the construction and test of a 1.8MW power plant in Santa Clara, California, which is the largest fuel cell power plant in North America. MC-power has been developing MCFCs since 1987 and is slightly behind ERC in the timeframe for expected commercial product release. It plans to enter the power market

with 250kW and 1MW units. In Europe, the research of MCFC technology has been carried out mainly in Holland, Italy, Germany, Denmark, and Spain. Now the manufacturers are focusing on improving performance of the components, endurance, cost reduction, systems integration, and high volume manufacturing methods to speed up the pace of its commercialization. MCFCs are expected to locate their largest market share in industrial cogeneration once the cost target is achieved [7].

SOFCs, which are considered as a third generation fuel cell, are approaching commercialization in a big pace, especially in stationary applications and distributed generation market [7]. So far the SOFC technology has demonstrated satisfactory efficiency and life performance. There are two different SOFC designs: tubular and planar. To date, Siemens Power Generation, the world leader in SOFC technology, has developed the most advanced tubular SOFC, and successfully demonstrated 1kW-250kW SOFCs in field. Planar SOFC technology lags tubular development, but is making good progress in improving performance and reducing manufacturing cost. In addition, when integrated with a gas turbine (GT), SOFCs are expected to achieve system efficiencies over 70%, which is much higher than any of the current power generation technologies. Currently, significant amount of research has been devoted to hybrid systems coupling fuel cells with power generation device based on other technologies. Westinghouse has devoted their efforts to developing 1-5MW pressurized SOFC-GT units to achieve high system efficiency and to reduce capital and production cost of electricity. Ztek (Waltham, MA) has developed 1 kW and 25 kW stacks to date. Their 25 kW stack module will be the building block for larger applications. In addition, Ztek has planned to develop two main product lines. One combines a 15-100 kW SOFC module with an HVAC system. The other is a 250 kW packaged system that would integrate seven 25 kW SOFC stacks with a small (60-100 kW) micro-turbine. Meanwhile, the development efforts for SOFCs are also focused on reducing manufacturing cost, improving system integration, and lowering the operating temperature [9]. Overall, market analysis is favorable for SOFC based systems. If the cost targets are achieved, SOFCs will likely to be a very successful candidate for DG markets.

PEMFC has attracted increasing attention due to recent technical breakthroughs. In the last few years, the cell power density has been increased in an order of magnitude and the substantial cost reductions have been achieved [7]. The low operating temperature, rapid startup, high power density, and simplicity in operation make it the most flexible technology and the only one being tested for automotive applications. Furthermore, these properties also make PEMFCs attractive in remote, standby, and premium power onsite markets. In 2005, PEMFC has dominated the whole fuel cell market in their applications to transportation, small stationary, portable and some less well-defined niche usages [12].

Now many major automakers are devoting themselves to the

research on PEM fuel cell vehicles. These include Chrysler, Ford, GM, Honda, Nissan, and Volvo, named a few. Among them, Daimler-Benz and Ford Motor Company have committed $750 million US dollars for the PEMFC research, development, and cost reduction [7]. With this investment, PEMFCs are expected to overcome the numerous fuel processing, system, and cost reduction hurdles within the next few years. GM Canada Ltd. and Hydrogenics Corp. have demonstrated a hydrogen powered PEMFC forklift in early 2005. Ballard Power Systems in Canada, Plug Power in the USA are the two of those pioneering companies for the developing of PEMFC technologies. Ballard Power currently is pursuing distributed generation applications aimed at the commercial market with large systems of 250 kW. Plug Power has developed a 7kW DG system for residential applications. In brief, the potential dual market (transportation and stationary power) for PEMFCs will drive the rapid improvement of PEMFC technologies.

III. FUEL CELL BASED DISTRIBUTED GENERATION

A. Advantages and Disadvantages as Compared to Other Distributed Generation Technologies

Based on the source of energy utilized, distributed generation technologies can be divided into three categories: (1) DG based on fossil fuel (relying on such systems as: reciprocating engine, micro-turbine, combustion gas turbine, and fuel cells); (2) DG based on renewable resource energy including solar power photovoltaic (PV) system, wind power, or low-head hydropower production; and (3) mixed DG combined two or more DG technologies together with some energy storage devices [6][14].

Compared with other DG technologies, fuel cell based DG has a number of advantages as listed below [6][15]:

- High energy conversion efficiency, on the order of 17% to 33% higher than other fossil fuel based DG options.

- Low emission, when using hydrogen, the emission of fuel cell based DG is almost zero.

- No moving parts apart from the air and fuel compressors. Therefore, it is more reliable, less noisy and low vibration. It also has lower maintenance cost and longer operating life compared to an equivalent coal-fired power plant or an internal combustion engine.

- Modularity, scalability and quick installation. Fuel cells are manufactured as standard size units and can be easily combined to meet any amount of power demand without having to redesign and reconstruct the whole plant.

- Good opportunities for cogeneration. In addition to electrical power, medium to high temperature fuel cells, such as PAFC, MCFC, and SOFC, will also generate pure hot water and medium grade heat, both of which can potentially be used for domestic or industrial applications. This is known as combined heat and power (CHP) applications that could further increase

the overall system efficiency. Fuel cells are particularly well suited to distributed power

generation applications because of the above characteristics. However, they also have some drawbacks, such as [6][7]:

- High initial cost. Even though the cost of fuel cell systems has been dramatically reduced since their first application in space industries, however they still cost much more than other competing DG technologies. This is the major barrier for fuel cells to succeed in the already competitive market.

- Complex support and control systems. The complexity of fuel cell systems increases significantly when the fuel cells operate in conjunction with an on-board reformer.

- Fuel sensitivity. Many fuel cells are more sensitive to impurities in the fuel, such as PEMFC and PAFC, than reciprocation or turbine engines.

- Unproven track record. Since fuel cells do not have a long history of commercial usage, therefore they are often considered risky from a commercial application point of view.

B. Comparison of Distributed Generation Technologies based on Different Type of Fuel Cells

Comparison of DG technologies based on four different fuel cell types is shown in Table II [6][7][15].

TABLE II COMPARISON OF DIFFERENT FUEL CELL TYPES BASED DG TECHNOLOGIES Fuel cell

types Advantages Disadvantages Applications

PEMFC

-Solid non-corrosive electrolyte -Low temperature operation -Quick start-up -High power density -Higher safety

-Expensive platinum catalyst -Sensitive to fuel impurities -Lower efficiency -No ability for internal reforming -Expensive membrane

Residential, RP, PP, L&MT, portable power

PAFC

-Produce high grade waste heat for cogeneration application -Stable electrolyte characteristics

-Corrosive liquid electrolyte -Sensitive to fuel impurities -No ability for internal reforming

MT, PP, commercial cogeneration

MCFC

-High efficiency -No metal catalysts needed -Internal reforming ability

-Corrosive liquid electrolyte -Intolerance to sulfur -Slow start-up

HT, PP

SOFC

-Solid electrolyte -High efficiency -Generate high grade waste heat -Fuel flexible

-Moderate intolerance to sulfur -Slow start-up - No practical fabrication process yet - Technology is not mature yet

Residential, commercial cogeneration, PP, RP

Note: L&MT-Light and medium duty transportation applications (e.g., automobiles, trucks, buses) HT- Heavy duty transportation applications (e.g., rail, marine-ships, naval vessels) PP-Premium power; RP-Remote power

PEM fuel cells use a solid polymer electrolyte, and operate at relatively lower pressures and temperatures with higher power density as compared to other types of fuel cells. Therefore, they are more suitable for applications in small to medium power levels, such as potable applications, automobiles, small stationary applications, or micro-grid power generation. The major advantages of PEMFCs are the low internal temperature, high power density, and quick start-up. The disadvantages are relatively low efficiency, high cost, with no ability for internal fuel reforming, and a low tolerance to carbon monoxide.

PAFCs operate with efficiency comparable to PEMFCs but at lower power densities. The operating temperature of PAFC is high enough to facilitate the recovery of heat produced within the stack for hot water supplies or heating in building applications, but not high enough to overcome the need for noble metal catalysts. Similarly, PAFCs are also very sensitive to the fuel impurities.

MCFCs are typically designed for mid-size to large stationary power applications. Their operation temperatures are fairly high, which allow them to have both higher efficiency and more recoverable exhaust heat for cogeneration. Moreover, noble metal catalysts are not required. Furthermore, at this high temperature, fuels other than hydrogen can be used by reforming the fuel within the cell stack in a process called “internal reforming”. This makes MCFCs more fuel flexible. Unfortunately, the high temperature also contributes to the significant disadvantages for MCFCs: slow start-up and unsuitable for small modular manufacture.

SOFCs operate at the highest temperatures among all fuel cell types that provide very high efficiency, fuel flexibility, and produce considerable amount of reusable waste heat for CHP applications. This also simplifies the system configuration by permitting internal reforming and facilitates the development of cogeneration systems as well as hybrid power systems that use fuel cells as topping cycles for gas turbines and/or steam cycles [9].

IV. POWER CONDITIONING FOR FUEL CELL BASED DISTRIBUTED GENERATION

Since a fuel cell generates only unregulated DC power that varies with load, therefore it cannot be directly used in most applications. In this case, it is essential to use a power conditioning system to convert the unregulated power source into a more stable AC power source for DG applications. A power conditioning unit is a critical component for a fuel cell based distributed generation system. A generic fuel cell based DG system is illustrated in Fig. 3 [7].

DG systems can be grid independent or grid parallel as well as a combination of both. If the application involves interconnection with the grid, then the PCU must also be able to synchronize the frequency of the fuel cell power system with that of the utility grid. These issues are common to all types of DG technologies. In 2003 the IEEE has developed a standard

that establishes criteria and requirement for the interconnection of distributed resources with electric power systems, IEEE 1547: Standards for Distributed Resources Interconnection with Electric Power Systems [9].

The PCU often includes DC-DC converters and DC-AC inverters. In addition to the common requirements of low cost and high reliability, the electrochemical characteristics of fuel cells also demand the following stringent requirements for PCU [16-18]:

1) High efficiency in operation over a wide input voltage and load ranges. This is due to the fact that the unregulated DC voltage can vary widely with the load and aging of the stack. In addition, considering the low-voltage, high-current nature of the fuel cell, switching & conduction losses of PCU need to be minimized. A high efficiency PCU will contribute to improve the total system efficiency.

2) Sluggish power transients. The PCU cannot react to load changes faster than the fuel cell, otherwise it will demand more instantaneous power than actually available. A constant power loop must be employed to prevent an overload or fault condition. Bulk storage capacitors or batteries can be used for this purpose.

3) PCU is often required to convert the low input voltage to a higher level. Two-stage power converter is commonly used in such a circumstance. A front end DC/DC converter is used to boost the voltage to a required level, and then an inverter is used to generate the AC output.

As mentioned above, in the PCU, a DC-DC converter is used when the output voltage is at a low level. The electric characteristic of the DC-DC converter should be complementary to that of the fuel cell to match the load characteristics [19]. The ripple in the input current/voltage needs to be limited to a low level to reduce their effects on the output current/voltage of fuel cell. When a fuel cell is working under load current pulses, it should be able to adjust the output power to ensure high-efficiency and reliable operation. Furthermore, isolation may be required to meet the safety regulations and effective protection.

The typical DC-DC converter topologies considered for fuel cell based DG including: boost converter, buck-boost converter, half bridge converter, and full bridge converter, etc [23]. These converters can also be divided into hard-switching converters and soft-switching converters according to the switching method used [20]. Pulsewidth modulation (PWM) technique is often adopted in the control strategy for DC-DC converters. There are two types of control modes: voltage and current mode [21]. The voltage mode is based on the single loop, while the current control mode adopts two-loop configuration. Control unit for DC-DC converter includes digital control unit and analog control unit. Now digital signal processing (DSP) technique is widely adopted in implementation of the control unit [22].

A DC-AC inverter is needed when the fuel cell based DG

supplies the power to AC loads. Depending on phase requirements of the output AC voltage level, DC-AC inverter can be a single-phase inverter, or a three-phase inverter. The control techniques for an inverter interfaced fuel cell DG include: constant real and reactive power control, constant voltage and real power control, and frequency droop based real power control [17]. Currently, high-frequency link inverter based on multiple-carrier PWM is a new topology for the inverter design [24]. Moreover, fuzzy control strategy has also been adopted in the control of SOFC based DG [25].

V. CONCLUSIONS

Fuel cell is a promising and competent power generation technique. It is anticipated that the future power market will be a combination of centralized power generation and DG with small to medium scales. Fuel cell technologies will play an important role in the future development of distributed generation.

All fuel cell types currently still suffer from high capital costs. If fuel cells can realize their target cost reduction and continue to demonstrate their anticipated wide range applicability, they will become a successful candidate for different applications, such as stationary, space, portable and transportation applications including locomotive and marine.

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Fuel Processor Fuel Cell

Power Conditioning

Unit

Heat and Power

Recovery System

Depleted anode gas

H2 rich gas

Air

Air

Fuel

Power or Process Heat

Low-grade Heat

AC Power DC Power

Fig. 3. Generic fuel cell system