A report on steam reformer and fuel cell cooling techniques and fuel cell testing process

 

STEAM REFORMER-

Steam reforming (SR), sometimes referred to as steam methane reforming (SMR) uses an external source of hot gas to heat tubes in which a catalytic reaction takes place that converts steam and lighter hydrocarbons such as methane, biogas or refinery feedstock into hydrogen and carbon monoxide (syngas). Syngas reacts further to give more hydrogen and carbon dioxide in the reactor. The carbon oxides are removed before use by means of pressure swing adsorption (PSA) with molecular sieves for the final purification. The PSA works by adsorbing all impurities from the syngas stream to leave a pure hydrogen gas.

Reforming  Reactions -
The reforming of any hydrocarbon is as follows:

Process  Scheme
Steam reforming can be performed for a wide range of fuels, but the process itself is similar in all cases.

Process for on-board steam reforming.

The fuel of the vehicle is pre-heated in a heat exchanger by the hot exhaust gas of the engine. Before entering the heat exchanger, hot water (steam) is separated from the exhaust gas and used in the steam reformer. Because steam reforming requires water, the usage of a water separator for the exhaust gas makes on-board water storage redundant. In the steam reformer the heated fuel and the steam are converted to syngas, which is then burned in the engine to produce duty.

Advantages-
Some advantages of steam reforming are:

1. The burning value of the fuel is increased, because steam reforming is an endothermic process, resulting in a more efficient fuel.

2. Steam reforming produces less exhaust emissions than burning the feedstock fuel

Disadvantages-
The disadvantages of steam reforming are:

1. Soot is formed in the reactor at high temperatures

2. Water sequestration from the exhaust is not easy to perform

FUEL CELL COOLING TECHNIQUES-

As we know that that the fuel cell efficiency is practically 50%. Which means the half of the energy produced is wasted as heat in the stack, which gives rise to increase in temperature. The increase in the temperature dries the membrane and reduces the conductivity so that fuel cell stops working. There are many types of fuel cell cooling techniques which depends on size and type of use of fuel cell-

1. AIR cooling- It is one of the best method of cooling. Air cooling is done by passing air at high velocity through the porous graphite flow field. Air cooling can be used for the stack of size 2KW. 

2. LIQUID cooling- liquid cooling technique basically involves the use of water. It is most used technique for cooling. In this method cold water is passed through the stack cooling plate. The cold water takes the heat produced within the stack with it and removes the heat in atmosphere and controls the temperature of fuel cell up to the desired level.

3. Heat plate cooling- Heat plate is very high heat conducting plate. Heat plate is placed after 2 -3 MEA in the stack. It absorbs the heat from the stack and releases in the atmosphere.

4. Heat pipe cooling technique- heat pipe is a also a very high thermal conducting plate. Heat pipe is placed in the contact with the stack. It takes the heat generated in the stack and put it outside. So that it controls the temperature of stack.

5. Evaporative cooling of fuel cell- the evaporation of any liquid decreases the temperature of that liquid. Evaporation is possible at any temperature above the absolute zero. In the process of evaporation the molecule of liquid leaves out the liquid by taking the internal energy (temperature dependent) of liquid. So the temperature decreases. it is latest method of fuel cell cooling which is in the research stage. This method of cooling involves the use of wicking material which has capillary action. As we know that at the cathode side water is produced so we can utilize this water for cooling of fuel cell and humidification of hydrogen.  We can put the wicking material in the flow field of fuel cell which will absorb the water produced in the cathode side and will carry it to the anode side so that it can be used for the humidification of H2.

FUEL CELL TESTING SYSTEM- 

We have seen the testing of 1 KW stack.

To test the stack we have an automatic testing system which is controlled by computer. The user can give input to the system like temperature, humidity and flow rate of different gases. The hydrated H2 is supplied from the anode side. And the hydrated O2 is supplied from the cathode side. The testing machine takes controls of flow and humidity of these gases.

As we know that the fuel cell has near about 40-50% efficiency. So half of the power produced is wasted as heat in the stack. So temperature of stack increases very much. To control the temperature rise water is circulated through the stack.

There are many temperature probes which constantly measures the temperature of different part of fuel cell stack. There is also facility to measure the potential of each cell. Each cell potential is measured by voltmeter and is displayed on the computer.

To start testing first we connect all gases connection to the fuel cell. And we connect cooling water pipe also. Fuel cell current collector terminal is connected to the testing machine which is capable to apply different load.

We supply heated H2 and O2 through humidifier. So that the fuel cell stack temperature increases and causes the activation of catalyst and the hydrated gas causes good conductivity of membrane. The PEMFC can be operated nearly 90 degree.

As the temperature of fuel cell increases the open circuit voltage of cell increases. Once the cell temperature reaches nearly 75-80 then we can apply load to take current from the fuel cell.

The fuel cell being tested had 36 cell, So that it is giving near about 35 V on open circuit. We can plot V-I characteristics (polarisation curve) of fuel cell. When we start taking current so many loses in the FC starts which increases the temperature of FC. So we have to start stack cooling system to stop the temperature increase of the stack. If we not stop the temperature rise then it will make the membrane dry and reduce the conductivity of fuel cell so FC stops working.

As we start taking current fro the fuel cell the voltage start decreasing. The rate of decrease in the voltage is not constant all over the curve.

As the current starts increasing the first loss that begin is activation loss after some more increment in the current the ohmic loos becomes significant.

Polarisation curve of Fuel cell

Further increase in the current leads to concentration polarization(mass transfer) losses.

Activation losses are caused by the slowness of the reactions taking place on the electrode surface. The voltage decreases somewhat due to the electrochemical reaction kinetics. This can be seen in the left-hand section of the current-voltage curve above.

The comic losses result from resistance to the flow of ions in the electrolyte and electrons through the cell hardware and various interconnections. The corresponding voltage drop is essentially proportional to current density, hence the term "ohm losses".

Mass transport losses result from the decrease in reactant concentration at the surface of the electrodes as fuel is used. At maximum (limiting) current, the concentration at the catalyst surface is practically zero, as the reactants are consumed as soon as they are supplied to the surface.

STATCOM

overview-
Electrical loads both generate and absorb reactive power. Since the transmitted load often varies considerably from one hour to another, the reactive power balance in a grid varies as well. The result can be unacceptable voltage amplitude variations, a voltage depression, or even a voltage collapse.

Similarly to the SVC the STATCOM can provide instantaneous and continuously variable reactive power in response to grid voltage transients, enhancing the grid voltage stability. The STATCOM operates according to voltage source principles, which together with unique PWM (Pulsed Width Modulation) switching of IGBTs (Insulated Gate Bipolar Transistors) gives it unequalled performance in terms of effective rating and response speed. This performance can be dedicated to active harmonic filtering and voltage flicker mitigation, but it also allows for a STATCOM to be comparatively downsized, its footprint can be extremely small. ABB has branded this high performance STATCOM concept SVC Light®.

Installing a STATCOM at one or more suitable points in the network will increase the grid transfer capability through enhanced voltage stability, while maintaining a smooth voltage profile under different network conditions. The STATCOM provides additional versatility in terms of power quality improvement capabilities.

STATCOM/SVC Light Technology

SVC Light is based on a technology platform also used for HVDC applications (HVDC Light). The most important building block is the Voltage Source Converter (VSC) equipped with Insulated Gate Bipolar Transistors (IGBTs) that are controlled by Pulse Width Modulation (PWM). A VSC is capable of both generating and consuming reactive power. If required, air core reactors and high voltage AC capacitors can be used along with the VSC as additional reactive power elements to achieve any desired range.

STATCOM/SVC Light Principle

SVC Light can be seen as a voltage source behind a reactance. Physically it is builtlas a three-level inverter operating on a constant DC-voltage. It provides reactive power generation as well as absorption purely by means of electronic processing of voltage and current waveforms in a voltage source converter (the grid will see it as a synchronous machine without inertia). This means that capacitor banks and shunt reactors are not needed for generation and absorption of reactive power, facilitating a compact design, a small footprint. The high switching frequency of IGBT allows extremely fast control, which can be used in areas such as mitigation of voltage flicker caused by electric arc furnaces, voltage balancing, harmonic filtering and robust grid voltage recovery support. A DC capacitor bank is utilized to support and stabilize the controlled DC voltage needed for the converter operation. Voltage source converters connected in "back-to-back configuration" between two AC busbars have the capability to operate with active power allowing a Dual Purpose scheme to be feasible. Using such a back-to-back configuration enables active power transfer between two AC grids (synchronous or asynchronous or even with different frequencies) while, simultaneously, the converters provide reactive power support to the AC networks. 

Urbanized environmentally friendly grid voltage support, a case story.

Urbanisation seems to be an ever-present force, more or less worldwide. But another trend in the opposite direction involves the relocation of electricity production to places far away from the city centers. Our favourite working and living places shall become cleaner and the aging machines that brought people together are now retired, often along with the factories they initially supported. Computers and air-conditioning follow people downtown. The result is that the growth of city centers increases the stress on the power transmission system, consuming considerable reactive power in a destabilising manner.

Given this helicopter perspective, it becomes apparent that it is not straightforward simple to retire old downtown workhorses. The authorities in Austin, Texas were faced with this problem as a down-town gas/oil fired plant was doomed due to its environmental impact. The introduction of extremely compact STATCOM (in the form of SVC Light®) technology greatly facilitated for the municipal utility Austin Energy to take a fast-track approach for the closing of the power plant, while maintaining adequate voltage stability margins in the grid operation. Within the timeframe of two years, all the pre-project activities (studies, specification, supplier selection), design, delivery, installation and commissioning of the approximately +100Mvar/138kV STATCOM was completed. It went into commercial service in December 2004. The Austin example shows how elegantly STATCOM technology can replace a generator, or more correctly, the voltage support capability of the generator. It does so cost efficiently and with a minimum of environmental impact.