Intermediate Temperature Solid Oxide Fuel Cell

The intermediate temperature range for a Solid Oxide Fuel Cell (SOFC) can be arbitrarily defined as 550* C to 800* C. Reducing the operating temperature of the SOFC stack is one of the significant directions being pursued to reduce the cost of SOFC stacks and balance of plant. There are both beneficial effects of reducing stack temperature and detrimental effects. In-the-balance, in most situations, the net effect is beneficial particularly in regard to cost. The most significant obstacle is that a set of fully compatible materials have not been developed for operation in this temperature range.

Beneficial effects:

• Reforming and Sulfur Removal - A better thermal match exists with existing reforming and sulfur removal processes. However, it should be noted the optimal nominal reformer temperature depends on the composition of the fuel.

• Sintering and Creep - Less sintering and creep of the stack materials. This helps maintain geometrical tolerances and high surface area of reaction.

• Thermal Stress - Lower operating temperature can in general improve material properties allowing greater geometrical flexibility and use of less material which in turn can improve overall area-specific-resistance and allow a wider range of sealing options.

• Material Flexibility - The type and range of materials is greater at lower temperatures. In particular metallics may be incorporated into SOFC stack designs.

• Balance of Plant - The balance of plant in general should cost less if the stack fuel and oxidant exit temperature is less than 800* C for the same reasons the stack should cost less.

• Heat Loss - Less heat loss from the stack for similar levels of insulation. In particular radiation losses can be significantly less since these are a function of T4. This can have a significant effect on the self-sustainability of the stack.

• Time to Reach Operating Temperature - Lower nominal operating temperature is obviously a benefit. Potential use of metallics can also impact this in a beneficial way.

• Thermally Activated Processes - Any detrimental thermally activated processes can be affected beneficially such as chromium vaporization, elemental interdiffusion and migration, metallic corrosion affects and some ceramic aging affects.

There are some negative affects from lowering the nominal operating temperature of the SOFC.

• Cell Voltage - For an identical stack the overall cell voltage will be lower as temperature decreases due to the decreased kinetics, diffusion , and ionic conductivity versus the improved electrical conductivity which typically does not dominate the cell polarizations. This is partially but not fully offset by the increased theoretical open circuit voltage of the electrochemical reaction at the lower temperature.

• Stack Materials - The only proven stack material set is functional between approximately 800* C and 1100* C. A proven material set in the intermediate temperature range does not yet exist.

Given the large number of potential beneficial effects of lowering the nominal operating temperature of the SOFC stack and their corollary affect on system cost, intermediate temperature SOFC concepts are being pursued by many organizations throughout the U.S. and the World.

In closure it is well recognized that significant research has occurred in this important technology area. To accurately present this research efforts will continue to compile and summarize all relevant information. This information will be presented as an addendum to this handbook or in the next addition.

S. Solid Oxide Fuel Cell

Solid oxide fuel cells36 (SOFCs) have grown in recognition as a viable high temperature fuel cell technology. There is no liquid electrolyte with its attendant material corrosion and electrolyte management problems. The operating temperature of >800°C allows internal reforming, promotes rapid kinetics with nonprecious materials, and produces high quality byproduct heat for cogeneration or for use in a bottoming cycle, similar to the MCFC. The high temperature of the SOFC, however, places stringent requirements on its materials. The development of suitable low cost materials and the low cost fabrication of ceramic structures are presently the key technical challenges facing SOFCs (1).

The solid state character of all SOFC components means that, in principle, there is no restriction on the cell configuration. Instead, it is possible to shape the cell according to criteria such as overcoming design or application issues. Cells are being developed in two different configurations, as shown in Figure 8-1. One of these approaches, the tubular cell, has undergone development at Siemens Westinghouse Corporation and its predecessor since the late 1950s. During recent years, Siemens Westinghouse developed the tubular concept to the status where it is now being demonstrated at user sites in a complete, operating fuel cell power unit of nominal 100 kW (net AC) capacity.

Figure 8-1 Solid Oxide Fuel Cell Designs at the Cathode

36. A broader, more generic name for fuel cells operating at the temperatures described in this section would be "ceramic" fuel cells. The electrolyte of these cells is made primarily from solid ceramic material to survive the high temperature environment. The electrolyte of present SOFCs is oxygen ion conducting. Ceramic cells could also be proton conducting.

The electrochemical reactions (Figure 8-2) occurring in SOFCs utilizing H2 and O2 are based on Equations (8-1) and (8-2):

at the anode, and

at the cathode. The overall cell reaction is

Figure 8-2 Solid Oxide Fuel Cell Operating Principle (2)

The corresponding Nernst equation, Equation (8-4), for the reaction in Equation (8-3) is



Carbon monoxide (CO) and hydrocarbons such as methane (CH4) can be used as fuels in SOFCs. It is feasible that the water gas shift involving CO (CO + H2O ^ H2 + CO2) and the steam reforming of CH4 (CH4 + H2O ^ 3H2 + CO) occur at the high temperature environment of SOFCs to produce H2 that is easily oxidized at the anode. The direct oxidation of CO in fuel cells also is well established. It appears that the reforming of CH4 to hydrogen predominates in

the present SOFCs. SOFC designs for the direct oxidation of CH4 have not been thoroughly investigated in SOFCs in the past (3,4) nor lately (no significant work was found). For reasons of simplicity in this handbook, the reaction of CO is considered as a water gas shift rather than an oxidation. Similarly, the favored reaction of H2 production from steam reforming is retained. Hydrogen produced by the water gas shift and the reforming of methane is included in the amount of hydrogen subject to reaction in Equations (8-1), (8-3), and (8-4).

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook

Post a comment