Difference between revisions of "Team:Reading/FuelCell"

Fuel Cell

A major aspect of our project is of course the design and production of the fuel cell itself. We set out to design our fuel cell with three important properties in mind; efficiency, complexity, and cost.
We designed our fuel cells design to:

• Facilitate greater voltage output from the bacteria
• Be simple and easy to set up and maintain
• Be cheap to manufacture and require little expertise
• Be easily affordable to the poorest of communities
• Safely house the bacteria

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Background

The main source of electricity for the longest time has been through batteries, first invented by Alessandro Volta in 1800, and then through generators which used steam or momentum to power turbines. The batteries relied upon an electrochemical reaction between two electrodes to create an electrical charge which could be passed through an external circuit. But this had an issue in that the batteries would eventually cease production of charge as the ions of the two electrodes were depleted. Rechargeable batteries were eventually designed like the small lithium ion batteries seen in circulation and the lead acid batteries used in car, however these would still eventually wear out due to the constant usage.

In the 1970’s research gained traction in developing fuel cell technology. Fuel cells work by providing an environment where once fuel is introduced chemical reactions can take place causing electrons to flow through an external system back into the cell producing a by-product and charge. The most prevalent example of this is the hydrogen fuel cell currently used in hybrid and zero-carbon cars and buses, hydrogen and oxygen are the fuel which creates electricity and water as the main by-products. Hydrogen atoms are sent to a platinum catalyst at the anode where they are stripped of their electron then; passing through an electrolyte, move to be then combined with electrons and oxygen at the cathode to form water. The electron once freed from its proton flows through an external circuit where it powers a device, then flows on to the cathode.

Also in the 1970s research on microbial fuel cells (MFCs) began focusing on two main areas, mediator based MFCs and mediator-less MFCs. Mediator-less MFCs rely on electron transport complexes, such as cytochromes, on the outer membrane of the bacteria releasing electron on to the anode. Mediator based MFCs are much more efficient at producing charge however the mediators used are often expensive, difficult to maintain and can be toxic to the microbe that is being used. MFCs rely on different combinations of heterotroph or autotroph: a singular species of heterotroph, singular species of autotroph, multiple species of heterotrophs, multiple species of autotrophs, mixture of multiple hetero and autotrophs, and even plant involvement. The wastewater industry has found MFCs to be useful in removing compounds and halving power usage of the treatment process.

Biological Photovoltaics (BVP) are a type of MFC where only oxygenic photoautotrophic bacterial species are used, such as Synechocystis sp. PCC 6803, earning them the moniker “Living solar cells”. Unlike in a Heterotroph based MFC where the electrons would solely come from the electron transport chain (ETC), in BPVs the electrons can be lost from both the ETC and the thylakoid membrane. Furthermore unlike inorganic photovoltaics BPVs continue producing electricity in the dark phase as the bacteria are still respiring, providing a possible avenue into 24 hour solar based energy. However the research into this untapped source is ongoing since current BPVs cannot compete with the energy output of conventional photovoltaics despite the environmental advantages of such a system.

Design

Our fuel cells design was created with modularity in mind. Multiple cells can be made and slotted together in batteries to increase overall power output. We also tried to keep the costs down to allow people in need a cheap sustainable energy able to afford a cell; this does mean it is not built using the most efficient components.

Carbon has been known to work effectively as an electrode for quite some time, just look at your standard C type Li ion battery and you will see a core carbon rod which a cellulose membrane help-ing create charge. Also as shown by Logan et al (2006) carbon rods work as effective cation exchange pathways that creates micro ohms of internal resistance, which would hinder charge production. I have combined these two concepts in the form of our carbon rod electrode by having it as the connecting bridge of each fuel cell and as the cathode on either side of the fuel cell. I believe this helps simplify the whole design as you just need to remove the crocodile clip from one of the cathodes and slot two cells together to create a more powerful cell. This method is not without losses of course, it may save space but there will be a charge loss due to internal resistance of whatever material connects the two cells. But as previously mentioned carbon rods produce very low internal resistance so I hope the charge loss is not too great.

After spending some time looking at standard photovoltaics I began noticing they have quite large cathodes and small anodes, this relies on there being a current in place and an area of no charge between anode and cathode. The cathode in this construct is where the electrons flow to and then on into the circuit, this is the opposite of how our fuel will function. In our fuel the electrons enter the circuit at the anode, leading me to design a cell with a greater size of anode than cathode.

Glass or Perspex? I have chosen Perspex (acrylic) over glass for a list of these reasons cheap, pres-sure resistant, inert, higher light transmission (92% light transmission, glass has a transmission factor of 87-90% as well as having a high refraction index.) however glass is very easily autoclaved and the results are well known, acrylic, although research has shown becomes stronger after autoclaving, is not as well documented. To increase the surface area of the fuel cell I have added a “ribbed” structure in the form of pillars of acrylic along the bottom increasing the surface area of the cell by 240mm2.

This works out to around £210 for 10 cells. £2,100 to make it comparable to modern day photovolta-ic cells in size which cost £5000, our cell costs 42% of low range solar panel. If we use bg-11 to fill these fuel cells we are looking at a price around the £5.500 mark to pay for enough medium, however with the SODIS purification method all the cells could be filled with river water.

Our Fuel Cell

This works out to around £210 for 10 cells. £2,100 to make it comparable to modern day photovoltaic cells in size which cost £5000, our cell costs 42% of your low range solar panel.

If we use bg-11 to fill these fuel cells we are looking at a price around the £5.500 mark to pay for another medium.

Carbon has been known to work effectively as an electrode for quite some time, just look at your standard C type Li ion battery and you will see a core carbon rod which a cellulose membrane helping create charge. Also as shown by Logan et al carbon rods work as effective cation exchange pathways that create micro ohms of internal resistance, which would hinder charge production. I have combined these two concepts in the form of our carbon rod electrode by having it as the connecting bridge of each fuel cell and as the cathode on either side of the fuel cell. I believe this helps simplify the whole design as you just need to remove the crocodile clip from one of the cathodes and slot two cells together to create a more powerful cell. This method is not without losses of course, it may save space but there will be a charge loss due to internal resistance of whatever material connects the two cells. But as previously mentioned carbon rods produce very low internal resistance so I hope the charge loss is not too great.

After spending sometime looking at standard photovoltaics I began noticing they have quite large cathodes and small anodes, this relies on there being a current in place and an area of no charge between anode and cathode. The cathode in this construct is where the electrons flow to and then on into the circuit, this is the opposite of how our fuel will function. In our fuel the electrons enter the circuit at the anode, leading me to design a cell with a greater size of anode than cathode (If you’re interested in the basic “how stuff works” explanation, a photovoltaic cell produces AC so current can go either way, our cell produces DC.)

Glass or Perspex? I have chosen perspex (acrylic) over glass for a list of these reasons cheap, pressure resistant, inert, higher light transmission (92% light transmission, glass has a transmission factor of 87-90% as well as having a high refraction index.) however glass is very easily autoclaved and the results are well known, acrylic, although research has shown becomes stronger after autoclaving, is not as well documented.

To increase the surface area of the fuel cell I have added a “ribbed” structure in the form of pillars of acrylic along the bottom.

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Large scale application of our design

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References