Global climate change and fuel poverty are the two most pressing issues in energy production today. Globally 17% of the population, 1.2 billion people, are currently without electricity (IEA, 2016). In order to mitigate this problem we must significantly increase our global energy output. However, as climate change is an ever present and pressing issue, we cannot just increase our reliance on fossil fuels in order to solve this problem. This would continue to exacerbate the greenhouse effect that is warming our planet. For example in the UK alone, the last fourteen months have consecutively broken temperature records, the highest global temperature has been recorded over the first six months of 2016. The ten warmest years on record all follow after the year of 2000 (on track to become eleven as this is written in July of 2016) (See Figure 1) (NASA, 2016). This alarming increase in global temperature demonstrates that, whilst an increase in global energy output essential, this must be done in a clean and efficient manner and must not add further greenhouse gas emissions to our planet’s atmosphere, causing this warming effect.
Figure 1: Graph from NASA showing the average temperature anomaly for each year from 1880. Temperature anomaly is the deviation of that years global mean temperature from the global mean temperature for that 5 year period
Despite the importance of maintaining climate stability, globally in 2013 67% of electricity was still generated by burning fossil fuels (see Figure 2). The IEA predicts that globally renewable sources such as wind and solar power will rise from 22% to 26% by 2020. In the UK the power from renewable energy has increased six fold since the year 2000. However, in the same period electricity consumption increased globally by over 55% (IEA, 2013). It is vital that renewable sources of energy proliferate even more that they have over the past decade and replace fossil fuels as the providing the majority of energy, while also keeping up with increasing demands for electricity.
As wind and solar power production becomes more widespread the problem of intermittency becomes an increasingly significant (and expensive) issue. A 2003 report by Carbon Trust predicted that when 10% of electricity in the UK is produced by renewable sources it could cost up to £2.6 billion to build the infrastructure to mitigate this intermittency, and cost £40 million a year to maintain. As the UK approaches a state where 20% of our electricity is renewable, the cost of building infrastructure rises to £4.5 billion, and at 40% the yearly cost could reach up to £1 billion, potentially adding £390 on average to yearly household energy bills. Currently the best plan is to avoid the problem and simply to triple the number of wind turbines and solar farms built. The theory is that if a large number of facilities are spread over a wide enough area approximate reliability could be achieved. This is not only three times more expensive than is strictly necessary but it means a large proportion of the energy being produced will be wasted.
Figure 3: Graph showing every production in Giga Watts (GW) over a 24 hour period in Germany, 2013. Dark blue indicates energy produced by solar power, light blue indicated energy produced by wind turbines & grey indicated electricity produced from conventional sources such as coal, nuclear or natural gas.
In Germany for example, solar power capabilities have increased by 400% since 2008. With no effective storage methods however, they ran into problems with electricity production surge during the day and rapidly falling at night, known as ‘peaking’ (see Figure 3). This causes major disruption, not only from of the risk of overloading circuits but also because other methods of power generation must be decreased during the day and increased again at night (an expensive process). If the energy produced from renewable technologies could be stockpiled as a sustainable fuel it could be released when it is needed, thus making it far cheaper to manage. Our projects intention is to store this excess energy as diatomic hydrogen gas via a bacterial system.
The most prolific extant method for carbon neutral hydrogen production is the electrolysis of water. By running a current through water, H2O is split into O2 and 2H+ ions, which are then reduced to H2. This approach however has its issues, and in 2012 it only produced 0.25% of global hydrogen. In 2016 new approaches allowed for an increase in hydrogen production via electrolysis three-fold, but from steam reformation of natural gas remains the dominant technology. Currently electrolysis is not economically scalable due to the high cost of electricity. As renewables become more abundant, electrolysis will become cheaper but this is not enough to outcompete fossil fuels today.
Figure: 4: Shewanella oneidensis
Our team, BioWire, is exploring a new way of mitigating this energy storage problem. The key to this is the unique physiology of the gram negative bacteria Shewanella oneidensis MR-1. These ‘rock-breathing’ microbes possess the MtrCAB protein complex, termed a molecular nanowire (see Figure 4). This porin cytochrome complex can channel electrons across the outer periplasmic membrane using iron heme cofactors. Naturally S. oneidensis is found in anaerobic sediments at the bottom of lakes. This means for cellular respiration diatomic oxygen cannot be used as the terminal electron acceptor. Resultantly S. oneidensis uses the MtrCAB porin cytochrome complex to pass electrons through the outer periplasmic membrane and reduce metals in their environment such as iron and manganese, using them as the terminal electron acceptor in cellular respiration. Figure 5: Molecular mechanism of MtrCAB in conjunction with the FeFe or NiFe hydrogenases
Our intention is to drive this backwards, sending electrons into the cells via the MtrCAB complex. This is possible, as demonstrated by Ross et al. (2010). These electrons can then be transported across the periplasm and to hydrogenase enzymes (see Figure 5). These enzymes can use electrons to reduce two protons (2H+) to one diatomic hydrogen molecule (H2). Enzymes have multiple advantages over conventional hydrogen production methods as they consist of inexpensive natural materials, have higher reaction kinetics and are preserved throughout the process. One limitation of these hydrogenase enzymes is that they are highly oxygen sensitive, and exposure to oxygen irreversibly inhibits the catalytic function of the proteins. As S. oneidensis is an anaerobic organism they can function to protect these enzymes from oxygen damage.
We can use this molecular pathway in Shewanella oneidensis MR-1 (see Figure 5) to convert energy from renewable sources to a clean fuel, hydrogen. Theoretically this can be done on a large scale using the microbial bioreactors at wind and solar farms, so a higher proportion of the energy produced is captured and stored as Hydrogen. This Hydrogen can then be stockpiled and burnt in centralised facilities to generate electricity in line with demand. Our project aims to demonstrate the possibility of this concept.
There is no cheap, large scale (terawatt hour scale) method of storing electricity. It is difficult to draw comparisons between our approach and other emerging methods as the data does not yet exist because all of these technologies are in their early stages and none of them are economically scalable yet. Our project aims to perform some proof of concept experiments where our objective is to explore the potential of using bacterial systems to store energy as hydrogen. We are attempting to increase the yields of hydrogen produced from Shewanella oneidensis MR-1 through the use of an overexpression system, in comparison to the wild type strains. In this way we hope to demonstrate the potential microbial bioreactors have as an energy storage system (see Figure 6).
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Ross D.E, Flynn J.M, Baron D.B, Gralnick J.A. and Bond D.R. 2011. Towards Electrosynthesis in Shewanella: Energetics of reversing the Mtr Pathway for Reductive Metabolism. PLoS ONE. 6(2): 1 - 8.
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