C.A. Isaza

Thermodynamics of Fuel Cells

In this chapter the basic thermodynamic and electrochemical principles behind fuel cell operation and technology are described. The basic electrochemistry principles determining the operation of the fuel cell, the kinetics of redox reactions during the fuel cell operation, the mass and energy transport in a fuel cell, etc., are described briefly to give an understanding of practical fuel cell systems. The ideal and practical operation of fuel cells and their efficiency are also described. This will provide the framework to understand the electrochemical and thermodynamic basics of the operation of fuel cells and how fuel cell performance can be influenced by the operating conditions. The influence of thermodynamic variables like pressure, temperature, and gas concentration, etc., on fuel cell performance has to be analyzed and understood to predict how fuel cells interact with the systems where it is applied. Understanding the impact of these variables allows system analysis studies of a specific fuel cell application.

State of the Art of Sorption Refrigeration Systems

Sorption cooling systems have been used commercially for some decades for different applications including air conditioning and refrigeration, using a diverse range of thermodynamic cycles and technologies for many size and capacities. However, their use has been limited mainly because of their low efficiency and high investment costs, at least compared with compression systems that are widely used all over the world. Because of this, sorption and desiccant systems have been used, in general, only when large amounts of waste thermal energy that can be used as the energy supplied to the system are available, and recently with, for example, solar and geothermal technologies.

Cogeneration Fuel Cells – Air Conditioning Systems

As already discussed in Chapter 1, energy is a finite resource and its rational use implies an increase in energy efficiency. The electric generation efficiency is always less than 100% due to resistive, transmission and distribution losses, which can be quantified as heat sent to the environment. This waste heat determines the quantity of energy that can be used by other systems in order to improve the process efficiency.

Energy and Cogeneration

The word energy is derived from the Greek in (in) and ergon (work). The accepted scientific energy concept has been used to reveal the common characteristics in diverse processes where a particular type of work is produced. At the most basic level, the diversity in energy forms can be limited to four: kinetics, gravitational, electric, and nuclear.

Potential Applications in Demonstration Projects

Fuel cells are devices where electrical energy is produced by redox electrochemical reactions of hydrogen or enriched hydrogen fuels and oxygen, or simply air. Powerful fuel cells or stacks have emerged as potential replacements for the internal combustion engine in automobile vehicles, because the use of fuel cells implies clean and efficient use of energy, and the unique by-products are water and heat when pure hydrogen is used as fuel.

Profitability Assessment of the Cogeneration System

From Chapter 1 it is clear that nowadays electricity and cooling demands are growing considerably worldwide, and that also there is a necessity to provide these by using more efficient and cleaner technologies, as the proposed in the present book.

Selected Fuel Cells for Cogeneration CHP Processes

Before going into the details of cogeneration combined heat and power (CHP) processes, it is necessary to establish the operation principles and performance characteristics of the most common fuel cells for consideration in CHP processes. According to the phenomena governing the performance of fuel cells, it is worth noting that fuel cells are electrochemical energy conversion devices, where redox reactions occur spontaneously and the fuel and oxidant are consumed, and the electrochemical energy is transformed into electricity to produce work.