How Efficiency Enhancements in Conventional Energy Systems Translate into Greenhouse Gas Reductions: The Case for Hydrophobic Coatings
Fossil fuels such as coal, natural gas and oil, under the umbrella term “conventional energy sources”, have for decades addressed electricity generation needs: presently accounting for about 67% of the needs in the US and about 87% worldwide [1,2]. Owing to their limited supply and the ever-increasing greenhouse gas emissions, countries have been forced to re-evaluate the use of fossil fuels and consider alternative forms of energy such as solar, geothermal and wind energy. While modern research in conjunction with new incentive programs continue to bring alternative forms of energy to fruition, the basic conundrum remains: is the proposed renewable energy mix sufficient to completely end reliance on fossil fuels in the near future? Some studies have estimated it will take several decades to make this transition: the world’s energy reliance on fossil fuels barely changed from 88% to 87% between 1990 and 2012 [2]. This is a staggering reality, which suggests that conventional fossil fuel energy is here to stay at least in the near future and as such, any efficiency enhancements in conventional electricity generation that can minimize greenhouse gas emissions are highly desirable.
Recent research has demonstrated that improving efficiency in conventional power plants by simple yet viable changes in materials of construction may have tremendous impacts on fuel consumption and subsequent reduction in greenhouse gas emissions. Such efficiency enhancements might be necessary as part of long-term plans to control greenhouse gas emissions and mitigate climate change. Specifically, the use of hydrophobic materials in these upgrades is promising, and has been discussed below by analyzing recent scientific papers.
What are Hydrophobic Materials?
The word hydrophobic, derived from Ancient Greek, literally means, “water fearing”. A common example of a hydrophobic material is the lotus leaf: with a slippery, waxy coating on the surface it is capable of keeping itself remarkably dry. An interesting class of hydrophobic materials are “rare-earth oxides”, which are compounds of the lanthanide group of elements. Their name is a misnomer; rare-earth oxides are in-fact highly abundant in the earth’s crust and are naturally-occurring. The Varanasi group at MIT has demonstrated that all rare-earth oxides from cerium oxide to lutetium oxide have the extraordinary capability of repelling water and staying dry (See figure on the left) [3,4]. Unlike typical hydrophobic materials such as waxes, these materials can withstand harsh temperature and pressure conditions without compromising hydrophobicity.
How can hydrophobic materials improve energy efficiency in conventional plants?
For years, researchers have studied ways to improve energy efficiency in conventional power plants in order to maximize the energy generation output for a given fuel input. Succinctly put, in a conventional power plant, the heat of combustion from fossil fuels is utilized to boil water and convert to high-pressure steam, which then drives turbines to generate electricity. The steam then condenses to water, which is recycled back to the boiler. Traditionally, steam condensers have been constructed from metals which are typically hydrophilic i.e. “water loving”. As such, steam condenses in a film-wise manner on these metals, as seen in part (b) of the figure below. The presence of this hydrophilic film is very detrimental to process efficiency: using the analogy of an uncomfortable jacket on a hot summer day, this film adds significant resistance to heat transfer and prevents steam from condensing on the underlying surface, thereby reducing heat transfer performance.
If steam were instead made to condense on a hydrophobic material, it would do so in a “drop-wise” manner — as seen above in (c) and (d). These drops shed very quickly and expose more of the underlying surface to steam, thereby significantly reducing thermal resistance and improving heat transfer performance. Films of hydrophobic rare-earth oxides as thin as a few hundred nanometers coated on existing condenser materials improve heat transfer performance by over ten times, which in turn corresponds to a superior improvement in energy efficiency by a remarkable 5–7%. For a typical 500 MW coal-fired power plant with a typical efficiency of 32%, the improvement in performance by using hydrophobic rare-earth oxides can translate to a net greenhouse gas reduction of anywhere from 5% to 10% for the plant, which is very significant. Furthermore, these materials are readily available and cheap (cerium oxide, for example, currently sells at only $2.35/kg) and can be easily retrofitted in existing coal or natural gas power-plants as thin film coatings using industry-standard ceramic deposition techniques.
The main take-away message from this post is that potential improvements and efficiency upgrades in conventional forms of energy should not be discounted while considering the evolution of the future energy mix and planning targets for greenhouse gas emissions. Given the current widespread reliance on fossil fuel these enhancements (such as using hydrophobic coatings in steam condensers) can be a viable part of the broader technical framework to meet reduction targets and mitigate climate change.
References:
[1] U.S. Energy Information Administration (2015) Monthly Energy Review [Online] Available: http://www.eia.gov/totalenergy/data/monthly/pdf/sec7_5.pdf
[2] Smil V., A Global Transition to Renewable Energy Will Take Many Decades, Scientific American, Volume 310, Issue 1[Online] Available: http://www.scientificamerican.com/article/a-global-transition-to-renewable-energy-will-take-many-decades/
[3] Azimi, G., Dhiman, R., Kwon, H.M., Paxson, A.T. & Varanasi, K.K. “Hydrophobicity of rare-earth oxide ceramics,”Nature Materials, 12, 315–320 (2013).
[4] Khan, S. Azimi, G., Yildiz, B. & Varanasi, K.K. “Role of surface oxygen-to-metal ratio on the wettability of rare-earth oxides,” Appl. Phys. Lett., 106, 061601 (2015).
