Alejandro Avendaño Friday April 21, 2006

Transcription

1 FUEL CELLS AND DISTRIBUTED GENERATION Alejandro Avendaño Friday April 21, 2006

2 Introduction Distributed Generation The Electric Power Research Institute (EPRI) defines distributed generation as the integrated or standalone use of small modular resources by utilities, utility customers, and third parties in applications that benefit the electric system, specific customers, or both. (EPRI, 1998) It is synonymous with onsite generation and cogeneration.

3 Available Technologies Various technologies are available for DG, including: Turbine generators Internal combustion engine/generators Micro-turbines Photovoltaic/solar panels Wind turbines Fuel cells

4 Fuel Cell (FC) Basics A fuel cell is an electrochemical device that converts the chemical energy of a fuel directly into electrical energy. Intermediate conversions of the fuel to thermal and mechanical energy are not required. They operate much like a battery except that the reactants (and products) are not stored, but continuously fed to the cell. Fuel cells were first invented in 1839, but the technology largely remained dormant until the late 1950s. During the 1960s, NASA used precursors to today s fuel cell technology as power sources in spacecraft.

5 Fuel Cell (FC) Basics (continued) Figure 1: Schematic of an individual fuel cell [1]

6 Advantages of Fuel Cells Fuel cells have a number of advantages over conventional power generating equipment: High efficiency (see Figure 2) Low chemical, acoustic, and thermal emissions Siting flexibility Reliability Low maintenance Modularity Fuel flexibility

7 Advantages of Fuel Cells (cont) Figure 2: Comparison of power plant efficiency [4]

8 Advantages of Fuel Cells (cont) FCs emit less carbon dioxide and nitrogen oxides per kilowatt of power generated. Noise and vibration are practically nonexistent. Noise from fuel cell power plants is as low as 55 db at 90 feet [4]. The lack of moving parts also makes for high reliability and low maintenance.

9 Fuel Cell Stacks A single fuel cell will produce less than one volt of electrical potential. To produce higher voltages, fuel cells are stacked on top of each other and connected in series. Stacks range in size from a few (< 1 kw) to several hundred (250+ kw).

10 Fuel Cell Stacks (cont) Figure 3: Components of a fuel cell stack [4] Because not all of the reactants are consumed in the oxidation process, about 20 percent of the hydrogen delivered to the fuel cell stack is unused and is often burned downstream of the fuel cell module.

11 Fuel Cell Systems Figure 4: Diagram of a generic fuel cell system [4]

12 Fuel Cell Types Currently, there are at least six different fuel cell types in varying stages of development. Four of these are receiving the most development attention. In general, electrolyte and operating temperature differentiates the various fuel cells. Listed in order of increasing operating temperature, the four fuel cell technologies currently being developed are: A) Proton Exchange Membrane Fuel Cell (PEMFC) 175 F (80 C) B) Phosphoric Acid Fuel Cell (PAFC) 400 C (200 C) C) Molten Carbonate Fuel Cell (MCFC) 1250 F (650 C) D) Solid Oxide Fuel Cell (SOFC) 1800 F (1000 C)

13 Fuel Cell Types (cont) A. Proton Exchange Membrane Fuel Cells (PEMFC) Figure 5: A schematic diagram of PEMFC [2]

14 Fuel Cell Types (cont) The PEMFC provides a very high power density. Automotive fuel cell systems based on PEMFC technology have demonstrated a power density as high as 1.35 kw/liter, which is comparable to that of the IC engine. Operating temperature is reached quickly. The combination of high efficiency, high power density, and rapid start-up makes the PEMFC attractive as a replacement for conventional automobile engines.

15 Fuel Cell Types (cont) B. Phosphoric Acid Fuel Cells (PAFC) Phosphoric acid fuel cells operate with efficiencies that are comparable to PEMFCs The operating temperature of the PAFC is approximately 200 C (390 F).

16 Fuel Cell Types (cont) C. Molten Carbonate Fuel Cells (MCFC) Molten carbonate fuel cells are typically designed for mid-size to large stationary (or shipboard) power applications. As illustrated in Fig. 6, the MCFC consists of nickel and nickel-oxide electrodes surrounding a porous substrate which retains the molten carbonate electrolyte. Fig 6 MCFC [1]

17 Fuel Cell Types (cont) D. Solid Oxide Fuel Cells (SOFC) Solid oxide fuel cells operate at the highest temperatures, 800 C 1000 C (1500 F 1800 F), of all fuel cell systems. Since the electrolyte is solid, the cell can be formed in a variety of configurations. One of the most common cell configurations has a tubular geometry. Fig 7 SOFC [1]

18 Fuel Cell Types (cont) Table 1: Fuel cell characteristics [1]

19 Fuel Cell System Applications in DG Fig. 8 A block diagram of a grid-connected FC power system [2]

20 Modeling of a PEMFC Internal Voltage and Voltage Drops The internal voltage of an FC is a nonlinear function of the FC current, internal temperature, and pressure of oxygen and hydrogen gasses. Like in batteries, FC output voltage is the difference between its internal voltage and its internal voltage drops, namely the activation, ohmic, and concentration voltage drops. Fig. 9 The equivalent electrical circuit of an FC, considering the double-layer charging effect inside the FC [2]

21 Modeling of a PEMFC (cont) Dynamic Model of PEMFC Fig. 10 A diagram for building a dynamic model of PEMFC [2].

22 Modeling of a PEMFC (cont) Experimental setup Fig. 11 Experimental setup for the SR-12 PEMFC stack [2].

23 Modeling of a PEMFC (cont) Results Fig. 12 V-I characteristics of SR-12 and the model [2]. Fig. 13 P-I characteristics of SR-12 and the model [2].

24 Modeling of a PEMFC (cont) Fig. 14 The transient responses of SR-12 and the model [2].

25 Hybrid FC systems A hybrid power system consists of a combination of two or more power generation technologies to make best use of their operating characteristics and to obtain efficiencies higher than that could be obtained from a single power source. Hybrid fuel-cell systems are power generation systems in which a high-temperature fuel cell is combined with another power generation technology. The resulting system exhibits a synergism in which the combination has far greater efficiency than could be provided by either system operating alone.

26 Hybrid FC systems (cont) PEM Fuel Cell-Wind Power Hybrid System Fig. 15 Wind fuel cell hybrid power system [3].

27 Conclusions Current worldwide electric power production is based on a centralized, grid-dependent network structure. This system has several disadvantages such as high emissions, transmission losses, long lead times for plant construction, and large and long term financing requirements. Distributed generation is an alternative that is gathering momentum, and modern technologies, such as fuel cells, are likely to play an increasing role in meeting ever-increasing power demands.

28 Conclusions (cont) Fuel cells have many advantages over conventional power generating equipment: High efficiency Low emissions Siting flexibility High reliability Low maintenance Modularity Multi-fuel capability.

29 References [1] Fuel Cell Systems: Efficient, Flexible Energy Conversion for the 21st Century Michael W. Ellis, Michael R. Von Spakovsky, And Douglas J. Nelson [2] Fuel Cells: Promising Devices for Distributed Generation M.Hashem Nehrir, Caisheng Wang and Steven R. Shaw IEEE Power and Energy Magazine [3] Hybrid Fuel-Cell Strategies for Clean Power Generation Kaushik Rajashekara IEEE Transactions On Industry Applications, Vol. 41, No. 3, May/June 2005 [4] Fuel Cells for Distributed Generation A Technology and Marketing Summary March 2000

30 THANK YOU QUESTIONS?