Influence of the Ambient Temperature, to the
Hydrogen Fuel Cell Functioning
POPOVICI Ovidiu and HOBLE Dorel Anton
University of Oradea, Romania, Faculty of Electrical Engineering andInformation Technology, Universitatii street, No 1, Oradea, Romania E-mail: popovici@uoradea.ro; dhoble@uoradea.ro
Abstract –The reversible fuel cell can be used to produce hydrogen. The hydrogen is further the chemical energy source to produce electrical energy using the fuel cell. The ambient temperature will influence the parameters of the hydrogen fuel cell.
Keywords: hydrogen; membrane; fuel cell
I. INTRODUCTION
The best solution to the air pollution problem can be the hydrogen fuel cell power, because we produce energy and water or vapour water, using only water and electricity. It is the most spectacular process, already used, with best results.
II. THE ELECTROLYSIS
This paper is focused on the process of obtain energy with nonconventional systems, in particular with the reversible fuel cell. The experimental system is produced by Heliocentris Company, and is used to obtain hydrogen and after that, in combination with oxygen to obtain electrical energy.
The principle of the electrolyze device, is very known and it means breaking the water molecules, using a special membrane, who containing a substance that allows hydrogen ions to flow in one direction, through the membrane.
This method is different comparing to the simple classic electrolysis, which need to have salt dissolved in the water. Using the electrolyze device we must not contaminate the membrane by letting in contact damaging chemicals. In the experimental module it is necessary to use distillated water, otherwise there is an important risk to damage the membrane.
The chemical reaction, in wich the water is broken down by the electrolyser to a ratio of 2 parts hydrogen, 1 part oxygen , is well known in techical literature, and it is shown below [5] :
Reduction Anode: 4 H+ + 4 e- ĺ 2 H2 Oxidation Cathode: 2 H20 ĺ O2 + 4 H+ + 4 e- The reaction: 2 H20 ĺ 2 H2 + O2
The membrane which it is utilised by Heliocentris Company in the construction of the fuel cell contains a nofion film between two layers of carbon. The carbon layers contain small quantities of platinum, added on one side and platinum + iridium on the other side.
The platinum and iridium which are inside the carbon films are the catalysis, their scope are to accelerating the chemical reaction.
The oxygen and the hydrogen is collected in the top of each cylinder of the fuel cell and. The gas who it is accumulate in the cylinders, by electrolyse effect pushing the water up in the cylinders. To obtain this electrolyse chemical reaction the system it is supply to a source of energy, 1,7V. The current who was measured is max. 150mA
The duration of this process of electrolyse, in this case was eight minutes, and the volume of the gas who was colected in the cylinder is 10ml. The gas volume/time variation, in the experimental water dissociation, are shown in Fig. 1.
Fig.1. The variation in time of the volume of hydrogen
III. THE FUEL CELL SOURCE OF ELECTRICITY
Using the same reversible fuel cell, if we supply hydrogen on one side and oxygen on the other, we obtain electrical energy.
The principial structure of the fuel cell with the Polymer Electrolyte Membrane (PEM) are show in picture number 2.
Fig.2. The fuel cell\
Operating at a internal temperature range between 65-95°C[3], the fuel cell has a solid polymer membrane, used as its electrolyte. The hydrogen ions H+, moves from anode to the cathode.
The principle of the fuel cell is the oxidation and reduction reaction between hydrogen and oxygen in the fuel cell, resulting water and electricity [5] :
Oxidation Anode: 2 H2 ĺ 4 H+ + 4 e- Reduction Cathode: O2 + 4H+ + 4 e- ĺ H20
The reaction: 2 H2 + O2 ĺ 2 H20
A single cell consists of two gas field flow plates, one for hydrogen and one for oxygen, separated by the membrane electrode assembly.
The field flow plates contain channels to ensure that the gases are in contact with as much of the membrane, as possible[6].
The big advantage using a solid polymer , is that is no necesary a compartment for the liquid electrolyte and no apear corosion effect.
The catalyst, the platinum, is deposited on a carbon support 0,7-1 ȝm , very thin, 3-5nm called nano cluster.[1,3]. They are applied on the both sides of the membrane, forming the catalyst for the anode and cathode. In a fuel cell, the electrolyte (the separating layer ), is also used to separate the gases oxygen and hydrogen in the cathode and anode compartments.
Oxygen enters on the cathode side and hydrogen molecules enter on the anode side of the cell,. Based on a reduction reaction the hydrogen molecule releases it’s electrons, which flows from the anode electrode through the external electric circuit to the cathode electrode and we obtain electricity. The hydrogen protons flows through the ion-conducting membrane to the cathode, there they combine with the oxygen molecules and the free electrons to form water. The hydrogen fuel cell is electrochemically converting hydrogen and oxygen gases into electricity and water[1],[5]. The principle it is shown in picture number 3.
Fig.3. Principle of the fuel cell
The molecular hydrogen, as is well known, is transformed in ionic hydrogen on the first carbon surface, and on the second carbon surface, the ionic hydrogen will combine with molecular oxygen, and results water.[2].
The membrane of a fuel cell is between 50 and 175 microns thick. It is produced in sheets which can be cut to the required size[3]. The electrolyte membrane consists of Polytetrafluoroethylene (PTFE) chains, commonly called Teflon. On the Teflon chains are side chains ending with sulphonic acid, HSO3. The teflon used in the fuel cell, has an important caracteristic: the sulphonate side attract water, is hydrophylic[2], wich is verry usefull.
The exchange protons membrane, based on nafion, is functionning usually under 85 degC. A low functionning temperature helps for an easy start. Aprox 50% from the maximum power is available on the ambient temperature 20 degC. The maximum power is riched in about 4 minutes[3].
IV. FUNCTIONNING OF THE FUEL CELL AT DIFFERENT TEMPERATURES OF THE AMBIENT
The fuel cell used for measurements was delivered by Heliocentris Energiesysteme GmbH, shown in Fig.4.
Fig.4. Reversible fuel cell, by Heliocentris The measurements were made in Producing energy Lab and Materials Lab, from the Department of Electrical Engineering , University Oradea.
Fig.5. Furnace with fuel cell
The functionning of the fuel cell can be fallowed by monitoring the electrical parameters, voltage and current, consumed by a load.
The device used for monitoring the ambient temperature is shown in Fig.5.
In tab.1 we have the data regarding the current and voltage, also the power, at different loads but for a constant ambient temperature 20oC.
Table 1. Determination at 20oC Load
Ohm
Current A
Tension V
Power W
Temp degC
Obs
10 0,082 0,840 0,068 20 8 0,107 0,820 0,088 20
7 0,120 0,790 0,095 20
5 0,140 0,770 0,107 20
4 0,200 0,750 0,150 20
3 0,230 0,740 0,170 20
2 0,385 0,690 0,266 20
1 0,492 0,642 0,315 20
0,7 0,240 0,742 0,178 20
For the same 9 values of the load, we did measurements of the electrical power delivered by the fuel cell, increasing the ambient temperature.
It was made same measurements at 40oC, 60oC and 80oC. The graph power/load depending on the ambient temperature is shown in Fig.6. for each four cases.
Fig.6. Power performance curve
V. CONCLUSIONS
The ambient temperature can be an important factor that influence the power delivered by the hydrogen fuel cell. The power is lower, as we increase the ambient temperature.
The power delivered by a fuel cell is depending on the external load, so the power density and the efficiency of a fuel cell, can not be achieved in the same time.
The maximum amount of energy that can be obtained from a fuel cell is called Gibbs energy and that energy must be compared with the calorific energy. The efficiency of a fuel cell is the report between the energy obtained end the calorific energy of the fuel.
For a hydrogen fuel cell, wich is extracting all energy, the efficiency is 83%, if the fuel cell is operating at 25°C [1].
Because the Gibbs energy decreases if the temperature is higher, it means that the efficiency of the fuel cell is also decreasing by increasing the temperature.
In practice, is not like in an ideal-theoretical situation and the efficiency of a fuel cell is lower at low temperature, that means that the variation of Gibbs energy is not important.
The fuel cell must have a good insulation when is functioning in an ambient with variable temperatures. If the temperatures are higher than 60oC, a cooler system is required, in order to have a constant power delivered by the fuel cell.
Evan at zero external load current, there are electrons delivered to the cathode.
Due to the ionization-deionization reaction, the efficiency of a real fuel cell, is about 9-15% lower than the theoretical efficiency.
REFERENCES
[1] EG&GServices, I.Parson, Fuel Cell Handbook,U.S. Department of Energy,Morgantown, Wv,2000 [2] Kordesch, Karl, and Günter Simader. Fuel Cells and
Their Applications. New York: VCH, 1996. [3] Larmanie.J, and Dicks.A, 2000,Fuel Cell Systems
Explained,John Wiley&Sons [4] www.heliocentris.com [5] www.drfuelcell.com
[6] http://education.lanl.gov/resources/fuelcells
