Experimental and theoretical analysis of the operation of a natural gas cogeneration system using a polymer exchange membrane fuel cell

doi:10.1016/j.ces.2005.07.033

Chemical Engineering Science

Volume 61, Issue 2, January 2006, Pages 743-752

https://doi.org/10.1016/j.ces.2005.07.033 Get rights and content

Abstract

This paper reports experimental and numerical results of an investigation of five identical cogeneration systems using PEM (Polymer Exchange Membrane) fuel cells and running on natural gas. The natural gas is reformed locally to produce hydrogen. The accuracy of numerical results is validated by comparison with experimental data and the system performances are analysed in terms of electrical, thermal and total efficiencies. It appears that the energetic performances are low, particularly at low current. Simple solutions for enhancing the system electrical performances by modifying control laws are proposed.

Introduction

The fuel cells (FC) convert directly the chemical energy of a fuel into electricity and heat. When hydrogen is used, which is usually the case, no pollutants are generated. However, hydrogen must be stored or produced locally, for instance by reforming of natural gas, which has a low greenhouse effect in terms of carbon dioxide emissions (compared to other fossil fuels); since it is mainly composed of methane, the number of hydrogen atoms per carbon is close to 4.

Fuel cells are under consideration for domestic micro-CHP (Cogeneration Heat and Power) and other small scale applications. Nevertheless, significant progress regarding their cost and lifespan are required. From an energetic point of view, electrical efficiency is not the only important issue and the thermal (or total) efficiency must also be considered.

According to the authors, 5 to 6 types of fuel cells can by distinguished, depending on their electrolyte. The most promising fuel cell for cogeneration are Solid Oxide Fuel Cells (SOFC), Molten Carbonate Fuel Cells (MCFC) or Protons Exchange Membrane Fuel Cell (PEMFC) (Dufour, 1998; Onovwiona and Ugursal, 2004; Sammes and Boersma, 1999). The electrolyte of PEMFC is a polymeric membrane with good proton conductivity. One of the most important characteristics of PEM fuel cells is their low operating temperature (50– $80^{\circ} C$ ), which makes them suitable to power vehicles or portable electronic devices, but which can be a drawback for cogeneration. Nevertheless, PEMFC are considered also for low or mid-power cogeneration systems (1 to 100 kW) (Dicks, 2000), keeping in mind that efficient low temperature heat recovery remains an important issue. PEMFC operation requires the continuous availability of hydrogen that can be produced by reforming of natural gas.

This paper reports experimental and numerical results of an investigation of five identical CHP units. These units (RCU 4500 V2) are designed and built by H-Power (Canada, now merged with Plug Power) and were put in operation in France between November 2002 and June 2003 (Le Doze et al., 2004). The sites (Dunkerque, Limoges, Nancy, Sophia-Antipolis) were chosen to offer various operating conditions. The net electric power is 5 kWe and the installations are designed for low temperature heat recovery (6 kW at $60^{\circ} C$ ). They run on natural gas reformed locally to produce hydrogen. The steady-state numerical model of these systems is developed under Matlab environment. This work was conducted within the EPACOP project (Expérimentation de 5 piles à combustible de petite taille sur sites opérationnels—testing 5 small fuel cells in real operating conditions) led by Gaz de France and co-funded by ADEME, the French agency for energy and environment protection.

Section 2 of the paper gives a detailed description of the CHP units and presents the main modelling hypotheses. In Section 3, the relevance of numerical results is validated by comparison with experimental data; then, the performances of the system are analysed in term of energetic efficiency. Some solutions for performance improvement are proposed in Section 4.

Section snippets

System description and modelling hypotheses

The natural gas must be transformed following various stages until its composition and temperature are suitable for feeding the fuel cell. There are three types of reforming processes for producing hydrogen from methane: partial oxydation (POX), auto-thermal reforming (ATR), and steam reforming (SR) (Kamarudin et al., 2004). Currently, most of the industrial hydrogen production is based on steam reforming of methane (Muradov and Veziroglu, 2005). This is also the case of the cogeneration system

Results

The accuracy of the model is validated by comparison of the gross and net electrical efficiency $η_{e, system}^{gross}$ (23) with experimental data (Fig. 3 and Table 2). The experimental and numerical results in Fig. 3 are close to each other at high currents (the maximum absolute difference is about 1 percentage point between 60 and 100 A) but diverge at low currents (20–30 A) where the difference can reach up to 50% of the numerical results. This is probably due to an increase of the relative importance

Solutions for performances improvement

The following proposals are based on technical considerations only, and focus on the electrical efficiency. They do not result from energy or entropy analysis, which would imply to choose optimisation criteria. The first modification consists in using the minimum values of S/C ratio $(S / C = 6.5)$ and excess of reformed natural gas $(λ_{NG} = 1.7)$ whatever the current intensity. The reformer should operate perfectly in steady with this S/C ratio but it would be necessary to check whether the steam flow

Concluding remarks

The accuracy (in terms of prediction of electrical efficiency) of the results of the simulation software is satisfying. It allows forecasting the behaviour of the system using the steady state model and proposing solution for enhancing its performances.

The first conclusion of this work is that the electric performances are low, all the more since the prototypes are not designed to operate efficiently at low current. It may be relatively simple to correct this point by modifying control laws. On

Notation

c	mole fraction, $mol {mol}^{- 1}$
E	potential, V
F	Faraday constant, $F = 96485 C {mol}^{- 1}$
g	Gibbs free energy, $J {mol}^{- 1}$
h	molar enthalpy, $J {mol}^{- 1}$
I	current intensity, A
$\dot{n}$	molar flow rate, $mol s^{- 1}$
N	number of cells in fuel cell stack, dimensionless
$\dot{Q}$	thermal power, kW
T	temperature, $^{\circ} C$
${\dot{W}}_{e}$	electric power, kW
x	moles of hydrogen per mole of natural gas, dimensionless

Greek letters

$ε$	reaction advancement, dimensionless
$η$	efficiency, dimensionless
$λ$	excess coefficient of reformed natural gas, dimensionless

Subscripts and superscripts

amb	ambient

References (18)

A.K. Avcı et al.
Quantitative investigation of catalytic natural gas conversion for hydrogen fuel cell applications
Chemical Engineering Journal
(2002)
P. Costamagna
Transport phenomena in polymeric membrane fuel cells
Chemical Engineering Science
(2001)
N. Djilali et al.
Influence of heat transfer on gas and water transport in fuel cells
International Journal of Thermal Sciences
(2002)
A.U. Dufour
Fuel cells—a new contributor to stationary power
Journal of Power Sources
(1998)
S. Dutta et al.
Numerical prediction of mass-exchange between cathode and anode channels in a PEM fuel cell
International Journal of Heat and Mass Transfer
(2001)
G. Gigliucci et al.
Demonstration of a residential CHP system based on PEM fuel cells
Journal of Power Sources
(2004)
S.K. Kamarudin et al.
Design of a fuel processor unit for PEM fuel cell via shortcut design method
Chemical Engineering Journal
(2004)
N.Z. Muradov et al.
From hydrocarbon to hydrogen-carbon to hydrogen economy
International Journal of Hydrogen Energy
(2005)
E. Newson et al.
Low-temperature catalytic partial oxidation of hydrocarbons (C1–C10) for hydrogen production
International Journal of Hydrogen Energy
(2003)

There are more references available in the full text version of this article.

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