What Has Worked to Slow Global Warming

by Emil Morhardt

Last week, in anticipation of the United Nations climate conference in New York, The Economist concluded that the single most important action to slow global warming so far has been enactment of the Montreal protocol. Say what? This isn’t on most environmentalists’ radar as an important factor. The Montreal protocol is the 1987 international agreement to save the ozone layer by phasing out Freon and other chlorofluorocarbons used in refrigeration. But these substances are powerful greenhouse gases as well as destroyers of stratospheric ozone, and the protocol caused millions of tonnes of them not to be released into the atmosphere. The article concludes that this avoided release of the greenhouse gas equivalent of 5.6 billion tonnes (bt) of CO2. This is about twice as much avoided CO2 as the next two most effective actions, global use of nuclear power (2.8 bt) and hydroelectricity (2.2 bt), and four times that of the fourth most effective action, China’s one-child policy (1.3 bt). I’m guessing that most of the 300,000 demonstrators in New York last week are not proposing an expansion of these latter three items, but their past effectiveness does make one think. The most effective actions taken specifically to reduce energy usage and CO2 emissions have been worldwide adoption of renewables (0.6 bt), US vehicle emissions standards (0.5 bt), and Brazil forest preservation (0.4 bt). The remaining 11 items on The Economist’s list are small potatoes, totaling less than 1 bt collectively. Continue reading

Liquid Fuels from Carbon and Hydrogen

Most technology development of alternative fuels production focuses on photosynthetic routes due to the endless supply of light and straightforward extraction of energy from it. However, there is also a wide range of processes and products for non-photosynthetic pathways in renewable fuel production. A class of these, electrofuels, can utilize an extensive range of microorganism and energy sources to produce variety of fuel types. Hawkins et al. (2013) discussed the key process and elements of carbon fixation for electrofuel production. The current blueprint of electrofuel projects focuses on autotrophic microorganisms using carbon fixation pathways to consume CO2 directly for the production of energy-dense liquid fuels. Electrofuel production requires complementary expertise of multiple fields such as synthetic biology, metabolic engineering, and microbiology to produce the desire type of fuel from the CO2-fixing species. Detailed biochemical characterization of each autotrophic species with cycle type, target product, and enzyme varieties can help to improve the efficiency of fuel production.  Chieh-Hsin Chen

Hawkins, A.S., McTernan, P.M., Lian, H., Kelly, R.M., Adams, M.W., 2013. Biological conversion of carbon dioxide and hydrogen into liquid fuels and industrial chemicals. Current opinion in biotechnology 24, 376−384

                  Electrofuel production is the process of using non-photosynthetic microorganisms to convert CO2 directly to energy-dense fuel. There are multiple sources of the electrons needed to power electrofuel process, including H2, formate, carbon monoxide, and electricity. In the paper, the authors focus on the use of hydrogen gas as source of reducing power for CO2 fixation, with hydrogen acting as a reducing agent for electron carriers and production of pyruvate. Though both aerobic and anaerobic process the microorganisms produce storable fuel.
                  There are currently six different biological pathways occurs spontaneously for carbon fixation. Each pathway has unique features arising from its molecular and biological context, and they differ in efficiency. One of the most common types of CO2-fixation is Calvin-Benson-Bassham (CBB) cycle found in plants, algae, and many bacteria. For example, Ralstonia eutropha is a metabolically diverse autotrophic bacterium that can grow on CO2 and H2; it is able to store excess carbon as polyhydroxyalkanoates (PHA). Wood-Ljundahl (W-L), 3-hudroxypropionate (3HP), 4-hydroxybutyrate (4HB), reductive tricarboxylic acid(rTCA) and dicarboxylates (DC) are also various pathways of CO2-fixation. One important characteristic to the various CO2-fixation pathways is the tolerance of the enzyme and redox carrier molecules to oxygen. Some pathways are found in anaerobic organisms, and these pathways utilize oxygen-sensitive reduced ferredoxins as electron carriers.
                  The efficiency of CO2-fixation depends on multiple factors. On average, it is found that W-L pathway and rTCA cycle were the most energy efficient routes, while CBB comes in as least efficient. One of the analyses of carbon fixation reaction is examining the ATP requirement and the reduction potential of electron carriers. For example, ferredoxin has much lower reduction potential than NADPH, but the extra energetic contribution from the difference is insignificant compared to the large difference in ATP cost for pyruvate formation between cycles. Another analysis looked into the individual reactions in carbon fixation pathways. There are many half-reactions in the process of carbon fixation, and some of the processes like carboxylation and carboxyl reduction reaction are energetically unfavorable. To increase the efficiency and reduce ATP requirements, some pathways manage to couple unfavorable reactions to exergonic reaction other than ATP hydrolysis. One of the faster ways to speed up a reaction is the use of enzymes; combining enzymes in novel ways may improve the efficiency of the pathway or reduce the energetic cost. The conversion of CO2and hydrogen to electrofuel is also dependent on the use of enzymes, carbonic anhydrase and hydrogenase.
                  Hydrogenase catalyzes the reversible conversion of molecular hydrogen and protons in the presence of electron carrier. Based on the molecular structure, hydrogenase can be categorized: [NiFe]-hydrogenase, [FeFe]-hydrogenase, [Fe]-hydrogenase. Different microorganisms appear to utilize different type; [NiFe]-hydrogenase is widespread among bacteria and archaea, [FeFe]-hydrogenase is found in anaerobic bacteric and eukaryote, and [Fe]-hydrogenase is found only in certain archaea. In CBB cycle hydrogenase helps catalyzing the oxidation of hydrogen for the reduction of NADP+, where in 3Hp and 4HB cycle hydrogenase catalyze the oxidation of reduced ferredoxin and generates an in gradiet that is used for ATP synthase. Other than the use of hydrogenase, enzyme that enhances the concentration of CO2 also plays a huge role in carbon fixation.
                  Any fuel or organic product from CO2 that relies on carbon fixation depends on the concentration of CO2 in the environment. Thus the mechanism of increasing the concentration of CO2 has developed to compensate the low ambient CO2 concentration. CBB cycles utilizing RubisCO, 2 CO2-fixing enzyme that has low attraction for CO2 and does not differentiate well between CO2 and the competing substrate O2, especially need this distinct mechanism to avoid unfavorable lost of carbon in photorespiration. Carbon concentrating mechanisms have only been described for CBB cycle because 3HP/4HB uses bicarbonate (HCO3) instead of directly using CO2. However, exploitation of carbon fixing enzyme for microbial eletrofuels can improve the engineering of enzyme.

                  The key to carbon fixation is the electrofuel-processing microorganism. The host development requires genetic engineer as well as knowledge of gene regulation and metabolism in the target host cell. Electrofuel is promising relatively to poor efficient photosynthetic biofuels, but only a handful of electrofuel organism are developed or reported.

Oceanographic Parameters to Explore the Environmental Impacts of OTEC Installations

Before field trials of ocean thermal technology (OTEC) become operational, researchers need a solid understanding of the environmental impacts of these installations. Due to the large thermal gradient and irregular bathymetry off the coast of Hawaii, the archipelago has several potential OTEC sites in the works. A pilot plant is under construction south of Barber’s Point, Oahu, and a commercial plant may be constructed off of Kahe Point, Oahu. A Final Environmental Impact Statement was conducted in 1981 for the Barber’s Point site, but this report needs to be brought up to current oceanographic and engineering standards. Comfort and Vega (2011) suggest a protocol for environmental baseline monitoring, which focuses on ten chemical oceanographic parameters, and addresses existing gaps in knowledge of ecology and oceanography near the two OTEC sites. In the operation of an OTEC plant, seawater intake pipes draw warm water from a depth of 20 m and cold water from a depth of approximately 1000 m. The water masses are mixed and discharged at 60 m or deeper. An environmental impact analysis can help to determine the optimal mixed seawater discharge level. —Meredith Reisfield
Comfort CM, Vega L. 2011. Environmental Assessment of Ocean Thermal Energy Conversion in Hawaii. Hawaii National Marine Renewable Energy Center, Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, HI, p 1-8.

During the operation of an OTEC plant, large water masses are redistributed. A 5 MW OTEC plant requires 25 m3/s of both cold and warm water flow. The daily flow tops 2 million cubic meters of water. The redistribution of large volumes of water could significantly impact an ecosystem, affecting primary production, nutrient concentrations, and densities of larval fish and other plankton. Comfort and Vega propose using existing data sets as baseline environmental information for OTEC. They recommend an additional year-long, directed, baseline monitoring program to address gaps in existing knowledge. Fortunately, as researchers at the Hawaii Natural Energy Institute acknowledge, significant data are already available to describe current circulation, and oceanographic parameters such as temperature, salinity, nutrients, and primary production off the coast of Oahu. The OTEC plume’s trajectory has also been modeled. OTEC operation raises several concerns about biological impacts. The redistribution of water on a large scale will affect the temperature stratification, salinity, oxygen and nutrient levels near the site. A primary concern in OTEC installations is the potential for upwelled nutrients to fertilize surface waters and prompt phytoplankton blooms. To avoid altering the primary productivity in surrounding waters, it is crucial that the plumes discharged from OTEC facilities settle at a sufficiently low depth so that the potential for functional biomass increase is reduced. Small organisms, including plankton and fish larva, can easily be trapped in the intake pipes with high mortality due to sudden and significant temperature and pressure changes. Many organisms migrate vertically throughout the water column on a daily basis, so understanding which organisms may be entrapped requires further knowledge of the ecosystem. Since floating objects in the ocean tend to accumulate large groups of fish and seabirds, larger organisms are likely to interact with the OTEC installations.  These organisms are less likely to be entrained in the system due to their larger size and swimming abilities which will allow them to easily manage the current flow. Vibration of the deep water pipe will create a signal that could be detected by marine mammals and fish, creating a risk of disruptions in marine mammal communication and navigation. The researchers propose additional monitoring of seasonal oceanographic parameters at relevant locations, further plankton sampling across multiple depths and time periods, and acoustical monitoring at the installation site to quantify baseline noise levels both before and after installation of the OTEC facility. Comfort and Vega note that given the wide availability of current data, gaps in knowledge could be quickly and efficiently addressed with one year of directed baseline monitoring. Studying the effects of an OTEC facility in operation with sufficient baseline data, rather than simply modeling these outcomes, could ensure that commercial OTEC plants have a minimal environmental impact. 

Evaluating Taiwan’s Solar Energy Potential

Taiwan is an island country off the coast of mainland China that uses fossil fuels to supply 90% of its energy needs.  Taiwan has no domestic fuel production and demand for energy is expected to increase by 37.4% between 2005 and 2025.  As such, it is important for the country to find alternative sources of energy.  Solar power is one option that has become increasingly attractive with the rapid improvement in that field of technology.  Two types of solar technology are mainly discussed in Yue and Huang’s (2011) paper: photovoltaics and solar water heating.  The researchers study the potential of these technologies in Taiwan, taking into consideration factors such as the area available for developing solar technologies, local laws and regulations, and the cost of implementing these technologies.  The study concludes that only 0.02% of Taiwan’s solar energy potential was realized in 2009.  Adopting PV and solar thermal technologies have the potential to reduce carbon emissions by 20.9 and 2.1 million tons each year, respectively.  However, due to high population density and tall buildings, the amount of area that can be covered with solar energy harnessing technology is small compared to the number of households it must support.  As such, solar technologies will have an important, but limited effect on Taiwan’s energy sources.—Alan Hu

CD Yue, GR, Huang. 2011. An evaluation of domestic solar energy potential in Taiwan incorporating land use analysis. Energy Policy 39, 7988–8002.

            Yue and Huang at the University of Kang Ning use a mathematical expression to calculate the annual heat output of solar water heaters by multiplying the total collector area, the annual solar radiation, and average heat efficiency of the solar collector system.  The average electricity output from PV systems can be modeled with another equation that multiplies the total photovoltaic module areas, the annual solar radiation, the module efficiency, and the aggregative coefficient.
            The researchers use the city of Tainan as a case study to apply the models described above.  Yue and Huang consider Tainan’s city laws concerning protruding structures on rooftops and geologic information on the region to estimate the energy potential.  The study determines that solar water heaters can provide 369.1 GWh per year and 628.5 GWh per year through PV systems.  Analysis of buildings depending on height allowed researchers to determine that PV systems and solar water heaters can respectively provide 22% and 44% of energy demand for electricity and hot water for a 12 story building.  In contrast, PV systems and solar water heaters can provide 109% and 217%, respecitvely, of energy demand for electricity and hot water for a 4 story building.
            Yue and Huang apply the same analysis to Taiwan as a whole to determine the potential of solar energy.  PV systems are determined to be able to provide 16.3% of electricity demand whereas solar water heating systems can account for 127.5% of energy demand from heating water.  In 2009, PV systems only provided 0.02% of electricity and solar water heating systems only provided 11.6% of hot water.  As such, both technologies have plenty of room to grow.
            An economic analysis is provided to determine the economic feasibility of the technologies.  Solar water heaters have a lifetime of 20 years and PV systems have an operating lifetime of 25 years.  Using market information and a discounted cash flow, Huang and Yue determined that solar water heaters would recoup their cost in 4 years while the costs of a PV system would not ever be recouped under the current tax regime.
            Widespread adoption of solar energy technologies could have a large impact on Taiwan’s energy policy.  For example, the country would be less prone to natural disasters that disrupt energy distribution infrastructure such as the 1999 Chi-chi earthquake which halted the supply of solar energy from south Taiwan to north Taiwan.  Also, peak load energy consumption occurs over summer in Taiwan due to intense use of air conditioning, which is increasing.  This summer load peak corresponds nicely with the increase in solar energy output over summer due to increased solar irradiance. 
            The study concludes that current exploitation of solar energy potential in Taiwan is far below the maximum potential.  Energy policy needs to be modified to make PV systems economically feasible, as currently, the benefits of the systems do not recoup their costs, but energy autonomy based purely on solar energy is improbable due to high population density and the prevalence of high-rise buildings.  Nevertheless, adoption of solar energies could reduce up to 9% of the country’s carbon emissions.  Yue and Huang recommend further study using land use analysis and believe that despite the many limitations, solar energy can have a sizeable impact on the energy landscape of Taiwan.

Generating Hydrogen Fuel for Electric Vehicles

Fuel cell electric vehicles (FCEVs) are automobiles that use hydrogen fuel instead of carbon-based fuels.  Because the byproduct of burning hydrogen fuel is water, it is a much cleaner form of energy at the vehicle level.  Large companies such as General Motors plan to begin selling FCEVs by 2015, and over 100 FCEVs Chevrolets have collectively driven over one million miles.  But the widespread adoption of FCEVs is hampered by the current cheap prices of carbon fuels and the general lack of hydrogen fuel infrastructure.  Researchers believe, however, that solar energy hydrogen generation systems based in single homes are a viable system for fueling FCEVs.  Kelly et al. (2011) at General Motors have already built a photovoltaic (PV) powered electrolyzing/storage/dispensing (ESD) system for use as a single FCEV home fueling system.  However, the system has only been tested for 14 days, and the widespread effect of day to day operation of the system on its efficiency is unknown.  The next step is to measure the efficiency and other characteristics of such systems.—Alan Hu
N.A., Kelly, T.L., Gibson, D.B., Ouwerkerk. 2011. Generation of high-pressure hydrogen for fuel cell electric vehicles using photovoltaic-powered water electrolysis. International Journal of Hydrogen Energy 36, 15803–15825.

            Kelly et al. at General Motors previously built PV-ESD system consisting of a set of solar arrays and an ESD system.  Four solar arrays were used, each having 10 Sanyo HIP-190BA3 modules.  The modules were wired parallel in each array, and each array was wired parallel to the electrolyzer.  As such, the PV system voltage output was equal to that of one module whereas the system current output was 40 times that.  The solar array tilt angle could be significantly altered to maximize solar irradiance depending on the season.  The second part of the PV-ESD system is the electrolyzer/storage/dispenser system.  The Avalence electrolyzer used was cylindrical and could contain hydrogen and oxygen produced at high pressures up to 6500 psi.  Due to problems in past experiments, however, the system was not run at 6500 psi but rather at 2000 psi.  As such, the system could store only 2kg of hydrogen as opposed to 6kg.
            In order to evaluate the performance of the PV-ESD system, Kelly et al. used the Sandia Photovoltaic Array Performance Model (SPAPM) to measure the voltage, current, and power values of the PV-ESD system.  The SPAPM uses a set of outdoor performance measurements and output current at two other voltage values to determine the I-V curve of the PV module.  Researchers analyzed the I-V curves of the PV and electrolyzer separately.  A resistive load bank was used to measure the I-V curve of the PV-ESD on a sunny day while a Sorensen DC power supply was used to measure the I-V curve of the electrolyzer.  These figures were used to calculate important PV figures through the SPAPM model including module efficiency and maximum power.
            After constructing a theoretical model, researchers began actual testing of the PV-ESD system.  The study was run from November 2008 to October 2009 with a period of inactivity between December 16, 2008 and March 24, 2009 due to the inability of the ESD to run in sub-freezing temperatures.  The study was run for a total of 109 days between sunrise and sunset, though for about 10% of the days, less than one hour of data was collected due to problems in the electrolyzer system. Researchers found that on sunny days, system efficiency started low, peaked, and then dipped at around noon.  In the afternoons, after the noon dip, efficiency rose until the sunset drop.  On cloudy days, the PV efficiency showed sensitivity to short increases in solar irradiance caused by passing clouds and the associated increase in temperature.  In general, researchers found that PV efficiency was tied to solar irradiance, the temperature of the PV system, and the impedance of the load that it is connected to.  The ESD efficiency was dependent on the operating voltage and the electrolysis cell temperature.  Electrolyzer operative voltage depended on its impedance and electrolysis cell temperature affected the electrolyte conductivity.
            Based on the experience of the researchers various possibilities for improvement were considered.  The experimenters recommended that the anode and cathode be reversed, that compression energy stored in high pressure hydrogen be captured and used to do work, that the membrane of the electrolyzer be made alkaline, that excess heat from the PV, which decreases its efficiency, be transported to the ESD, which increases its efficiency.  In general, the experiment was a proof-of-concept for a single FCEV fueling system.  The first phase of the experiment concerned the design and construction of the PV-ESD system while the second tested the built PV-ESD system.  It was found that the electrolyzer responded well to the constantly changing solar irradiance caused by passing clouds and that day to day temperature variations did not decrease its efficiency.  The coupling factor of the combined PV-ESD system was calculated by comparing maximum power and efficiency of the PV and the actual power and efficiency of the PV.  The researchers concluded that 1) the system operated without any major failures of the high-pressure electrolysis system, which had previously been a problem, 2) solar energy to hydrogen efficiency averaged 8.2%, 3) coupling factor averaged 0.91, 4) the system produced 0.67 kg of hydrogen over a full day of operation, and 5) solar to hydrogen efficiency is less than a third as efficient on an energy utilization per mile basis as solar battery charging.

The Hydrokinetic Power Resource in a Tidal Estuary: The Kennebec River of the Central Maine Coast

As we look to the world for sources of renewable energy, every bit we can harness is important.  Unlike our current system of high volume power generators, the grid of the future increasingly seems to be composed of many small generators working together in harmony.  One such source we have the technology to take advantage of is tidal power.  The power level available from the kinetic energy of tidal flows, in areas with relatively mild tidal ranges along coast or estuaries, can still be significant.  Using a numerical circulation model, David A. Brooks calculates available power peaks in one such region, the central Maine coast.  In recent history, the implementation of tidal power plants has been blocked by issues such as: cost of construction and maintenance of the requisite dams, gates, fishways and locks, as well as concerns about impacts on fisheries, navigation and a host of other environmental issues.  However, with dwindling fossil fuel supplies and global warming concerns looming, and turbine technology improving, tidal power has rekindled society’s interest.  Also, tidal power, as opposed to other hydrological power sources, does not require large impoundments such as dams, resulting in reduced installation and operational costs and minimal environmental impacts.—Donald Hamnett
Brooks, David A., The hydrokinetic power resource in a tidal estuary: The Kennebec River of the central Maine coast. Renewable Energy 36 (2011) 1492e1501.

            Brooks examined the central Maine coast because the Gulf of Maine and the adjoining Bay of Fundy are known for resonant semi-diurnal tides.  The range of these tides exceeds 15 meters at the head of the bay, but the mean for the entire coast is a smaller 3 meters.  In the nearby confined parts of river estuaries, narrow interconnecting passages, and between nearshore islands the tidal currents can exceed 2 meters per second.  To simulate the tidal, riverine, and wind-driven circulation of the coast, Brooks used a three-dimensional hydrodynamic model known as MECCA, or Model for Estuarine and Coastal Circulation Assessment.  On a three-dimensional grid the fields of velocity, temperature, and salinity are calculated using conservation of mass and momentum equations.  The forcing is specified at ocean boundaries, inshore points, and the free surface by tides.  The bathymetric data was provided by the National Geophysical Data Center, along with the model coastline data.  Velocities above 2 meters/second were found at the mouth of the Kennebec River, Bluff Head, and a few other points.  This is the velocity at which the power produced from the tide’s kinetic energy is 3 kilowatts per square meter.
            The kinetic tidal power widely available in regions with moderate tidal ranges, and is dispersed throughout these areas.  This attribute contrasts tidal with traditional forms of energy, which is focuses high quantity dependable generation in a few small sites.  The centralized nature of traditional power requires extensive distribution grids, and is a security threat as the power for a wide area could be taken out by problems with just one power plant.  Though tidal power has this advantage over conventional generation, it would require electrical grids that can accept and blend multiple power pulses.  From the model’s calculations, hundreds of megawatts of peak power are associated with the central Maine coast’s tidal systems, some of which could be practically harnessed.  In fact, the most promising site had a maximum power density of 6.5 kilowatts per square meter, in the lower Kennebec estuary near Bluff Head.  Were this resource harnessed and connected to the grid using a 500 square meter sub-region of the channel, this could supply the energy needs of about 150 typical homes, consuming 1.5 megawatt hours a month.  Further study into the monthly vertical and horizontal structure of the tides would be required to fully grasp the power potential of these coastal regions.  Tidal power as a small power generation tool has the potential to be another piece of the smart grid of the future.

Hydrogen Fueled Homes

A hydrogen economy is often seen as a greener alternative to the current carbon economy.  Hydrogen is the most common element found on earth and can be easily transported and burned as a fuel.  However, the earth’s hydrogen is trapped in its vast seas and in other compounds.  As such, before hydrogen gas can be utilized, it must be extracted and separated from its source.  One method of generating hydrogen gas is by splitting water molecules into their oxygen and hydrogen components.  The process involves running an electrical current through the water, which commonly requires the burning of fossil fuels, but the current needed to split water can be generated in another fashion—through photovoltaics.  The combination of solar cell and hydrogen fuel technology solves a fundamental problem for solar energy: energy can now be stored for use even when the sun is absent.  While widespread adoption of hydrogen fuel technology is currently economically infeasible, homes powered by photovoltaic generated hydrogen fuel are a possible future.  Shah et al. (2011) describe and analyze a hypothetical hydrogen home built in Wallingford, Connecticut.
Alan Hu
Shah, A., Mohan, V., Sheffield, J., Martin, K. 2011. Solar powered residential hydrogen fueling station. International Journal of Hydrogen Energy 36, 13132–13137.

            Wallingford was chosen as the site of the hydrogen home due to its existing hydrogen infrastructure developed by Proton Energy Systems and Hydrogen Highway.  It was also believed that due to the presence of existing infrastructure, there would be higher public acceptance of hydrogen homes.  The home, in two levels, includes bedrooms, bathrooms, a closet, a kitchen, a dining room, a living room, and a garage.  Built with the architectural heritage of the region in mind, the home has steeply sloped roofs.
            The home is to be powered by a total of 60 PV panels arranged in two arrays.  The first, containing 18 panels, is directed 19º off the East-west axis and is designed to capture maximum solar energy in the mornings.  The second array, containing 42 cells, is pointed due south to maximize energy output in the afternoon and evenings.  The specific orientation of the solar arrays can be adjusted to keep output high across different seasons.  Researchers assumed 4.74 hours of daylight per day and an energy utilization of 32.8 MWh/yr.
            The hydrogen system of the home comprises a high pressure hydrogen electrolyzer and three storage tanks.  The electrolyzer turns on when the pressure inside the storage tanks drops below 138 bar and off when the pressure reaches 165 bar.  Hydrogen fuel stored in the tanks is then piped to the hydrogen vehicle.  The vehicle is assumed to commute 56 km per day at a fuel mileage of 71 km per kg hydrogen, thus requiring 0.8 kg of hydrogen per day.
            Safety precautions of the hydrogen power system include the use of hydrogen detectors, an emergency shutoff button, fire extinguishers, remote emergency stops, and a pressure relief system.  The power is disconnected to prevent ignition from electrical sources.  Regular inspection is advised and warning signs are placed around the hydrogen fueling station.  Significant failure scenarios such as the vehicle colliding with the storage tank, fueling nozzle, leakage of hydrogen gas, or hydrogen overfill are all taken into account.
            The researchers developed a wheel-to-wheel analysis of the hydrogen home.  The Wallingford hydrogen home is estimated to require 95 kWh per day assuming a house consumption rate of 21 kWh per day and a vehicle consumption rate of 74 kWh per day.  The average daily output by the PV cells is 90 kWh per day which means that 5 kWh per day will need to be drawn from the power grid.  A comparison of power consumption between the hydrogen car and a normal car shows that the hydrogen car produces 34.8 grams of carbon per mile whereas a normal vehicle produces 272.2 grams of carbon per mile.
            The researchers calculate that 16.5 metric tons of carbon dioxide are saved each year with the use of the hydrogen home.  The energy efficiency improvement for the Wallingford home is calculated to be 23% and the hydrogen vehicle uses 13% of the carbon of a normal vehicle.