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| Fig. 1: An example of a typical HTSE Cell. The gas tight electrolyte helps dispense the high heat advantageously at incremental states. (Courtesy of the DOE. Source: Wikimedia Commons) |
Society today is the most progressive it ever been in terms of innovation. Ranging from Moore's Law on the exponential growth of technology to the life-changing advancements in the medical field, scientists constantly push the question: how can this further be better? One paramount example is the evolutionary need for more efficient vehicles. Indisputably, our Greenhouse Gas (GHG) emissions have skyrocketed these past few years making this endeavor imperative for the future. Consequentially, the energy economy has been desperately looking for alternate solutions with H2 being the most notable candidate as a clean gas. The only caveat with the H2 gas alternative is that it cannot be found naturally forcing researchers to implore methods of separating it from water (H2O) into it constituent elements. Thus, to maximize the pollutant-free aspect of H2, separation through fossil fuels is overlooked and rather directed towards the effects of nuclear power plants with either electrolysis or a thermodynamic process.
Essentially, H2 can be produced when a system generates enough electricity to heat water from liquid to steam which is achieved through nuclear fission - the separation of certain atoms for electrical energy. Because nuclear fission coincides perfectly with the needs of electrolysis, investigation on the economic efficiency is currently underway for the three current processes: Polymer Electrode Membrane (PEM) electrolysis, low-temperature alkaline electrolysis, and high temperature steam electrolysis (HTSE). Both PEM and alkaline electrolysis effectively work as the inverse of each other by utilizing the attractive properties of ions and anions in relation to their respective electrodes. For example, alkaline electrolysis will take in the electricity generated from nuclear fission to break down highly ionized water into H2 gas for extraction and usage. However, both of these methods have proven to operate with an equally lower efficiency than HTSE. As a compensation factor, the electricity demand from nuclear fissions can be significantly reduced by increasing the temperature and reaching the heat of vaporization through thermal processes (which are much more cost effective than an electrical one). By increasing the temperature, scientists are able to optimize both heat and electrical energy which maxes out at about 33% with our current nuclear power. [1] As depicted in Fig. 1, a feedback mechanism occurs with the feedgas stream pumping a combination of 10% H2 and steam into the HTSE cell (to prevent the oxidation of Nickel at such a high temperature) which passes through a separator extracting about 90% of H2 for use.
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| Fig. 2:The solar power (a nuclear reactor as the heat source in our case) drives this process which is using more than 2 steps. Mn2O3 is reduced to MnO which goes on to react with NaOH, eventually creating NaMnO2 and releasing the favored hydrogen. The last step is a regeneration step. This cycle is controversial in its potential complexity and is thus being investigated with other cycles for significant H2 production. (Source: Wikimedia Commons) |
Noting the efficiency of a system at high temperatures has also shed light on the idea of a thermochemical cycle: a series of thermally driven chemical processes which decompose water into oxygen and hydrogen at rather moderate temperatures as the reverse is a direct one-step reaction requiring temperatures greater than 2800°K for for a substantial hydrogen conversion yield. [1] Supporting chemical compounds are infused internally to help recycle the system through its products with the only inputs being water and high temperature heat to help catalyze these chemical reactions. Due to the variation of usable chemical reactions to run this system, efficiency yields are still under heavy consideration. As can be seen in Fig. 2, one investigated example is the MnO cycle - this cycle risks the complexity of a more than 2+ reaction cycle but is compensated with the tradeoff of lower temperature requirements. Ideally, scientists are working to determine which cycle (the particular chemical substances it will consist of) will provide the maximum amount of H2 without as many excess reagents.
A nuclear reactor operates inherently by taking as much heat as possible from nuclear fission and supplying that to generate electricity for various American activities. On average, a substantial amount of H2 production will take about 1600 megawatt thermal (MWt) from a nuclear fission process that produces about 3000 MWt. Directly taking the thermal energy needed to power electricity for other events will have considerable transition issues as the nuclear power plant takes on new roles and intentions for society. [2]
Currently, the future of hydrogen is far from here - there are various factors which must be investigated thoroughly considering the implications of such an endeavor. The efficiencies are still not where they need to be in some cases for the following to be practiced immediately, however pure running fuel is inevitably on the horizon with these evolving mechanisms.
© William Mangram. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. Abanades et al., "Screening of Water-Splitting Thermochemical Cycles," Energy 31, 2805 (2006).
[2] G. Rothwell, E. Bertel, and K. Williams, "Can Nuclear Power Compete in the Hydrogen Economy?" in Nuclear Production of Hydrogen: Third Information Exchange Meeting, Oarai, Japan, 5-7 October 2005 (OECD, 2006).
