hydrogen tech

8/6/2019 Hydrogen Tech

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with respect to distribution is its low volumetric density. As a gas, hydrogen is about 14 timesgravimetrically lighter than air. Compressed gaseous hydrogen transport is only possible inheavy, expensive vessels that can withstand pressures up to 10,000 psi, or a system of pipelinesthat must either be constructed from the bottom up or retrofitted from existing natural gas pipelines. Cryogenic hydrogen can be transported more easily than gaseous hydrogen, but the

conversion from gaseous to liquid hydrogen is energy intensive, inefficient, and considerablyexpensive. At this time, pipelines are considered the most likely transport method for ahydrogen economy. Major concerns surrounding hydrogen distribution include high cost and a phenomenon known as hydrogen embrittlement that causes pipelines and storage vessels to crack and fail over time. Decentralized production of hydrogen eliminates losses associated with long-distance transport but increases the demand for effective hydrogen storage on-site.

The characteristic that best sets hydrogen apart from other energy carriers such as electricityis a higher capacity to be stored for use at a later time. Storage research is primarily focused oncompressed gas, cryogenic hydrogen, and metal hydrides, but a growing number of alternativemethods including carbon novel materials, chemical hydrides, and glass microspheres are also

being tested. Compressed gas is the most mature storage technology, but compression addsinefficiencies to the hydrogen life-cycle and requires stronger, costlier materials for tank construction. Extensive materials research is being conducted to improve compressed gasstorage technology; advancements have already been made in carbon-fiber wrapped tanks, whichare lighter and safer than traditional steel tanks. Cryogenic hydrogen is denser than compressedgaseous hydrogen, therefore requiring less storage volume. Energy and economic costsassociated with cryogenic hydrogen storage are higher than compressed gas storage costs.Between 10 and 30 percent of the fuel value of hydrogen is required for liquefaction, and tanksmust be super-insulated to maintain cryogenic temperatures near -250oC. Solid storage in metalhydrides is not yet feasible, but preliminary research suggests that metal hydrides will be prominent in the future hydrogen economy. Using the concept of temperature change, hydrogenis adsorbed within interstices of metal hydride lattices. The resulting granules can be storedmore safely than compressed gas or cryogenic liquid hydrogen. Hydrogen is released from themetal hydrides by applying heat. The high costs currently associated with adsorption makemetal hydride storage impractical, but economic feasibility will increase as technologicaladvances are made.

The electrochemical conversion of hydrogen and oxygen to electricity and water in a fuelcell is the most publicized aspect of a future hydrogen economy. The fuel cell works much likean electrolyzer in reverse: diatomic hydrogen is broken into electron and proton components atthe anode, electrons flow through an external circuit to be consumed as electricity, and hydrogenions (protons) pass through the electrolyte to the cathode, where they are combined with gaseousoxygen to produce water. Fuel cells are categorized by low or high temperature operation andare classified by the type of electrolyte they contain. Examples of low temperature fuel cellsinclude phosphoric acid (PAFC) and proton exchange membrane fuel cells (PEMFC); hightemperature models include molten carbonate (MCFC) and solid oxide fuel cells (SOFC). Thewide range of power outputs available make fuel cells suitable for a variety of applications.Relatively high fuel cell efficiencies are coupled with high material costs; research anddevelopment efforts will continue to focus on optimization until a feasible model is developed.



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Costs of a hydrogen-based economy are determined by production, delivery, storage, andelectrochemical conversion methods. Hydrogen produced will be sold by the kilogram, or energy equivalent to one gallon of gasoline. For hydrogen production by electrolysis, the targetcost is $3.00/kg. The price of electricity corresponds to 58-68% of production costs and islargely the reason why the target has not yet been hit. Current wind-to-hydrogen costs are

reported as low as $5/kg, and photovoltaic-to-hydrogen is approximately $30/kg. High capitalcosts for renewable energy systems are likely to blame for the higher price of electricity, andthese costs are expected to fall in the future. Hydrogen delivery by pipeline in a centralized production system is anticipated to be the most economical distribution method, but findingsreported throughout the literature are inconclusive regarding what the cost will be. One sourcelisted the expected cost of transport through pipelines at nearly $1/kg; this is about five timesmore expensive than the current gasoline transmission and dispensing system. Capital cost and product losses by leakage contribute the most to delivery costs. Hydrogen storage prices vary bythe expected turnover rate, system size, and physical state of stored hydrogen. Short-termstorage costs in above ground compressed gas tanks are reported to be as low as $0.18/kg, butmost findings are strictly theoretical and uncertain. Fuel cell capital costs are considered the

greatest barrier to a total hydrogen economy. Currently four times more expensive to install thaninternal combustion engines, fuel cells are hardly cost-effective today. Target capital costs for fuel cells are near $35/kW, and present capital costs are reported between $2,500 and$3,000/kW. The cost of a hydrogen economy must be lowered for a transition to occur.

Cradle-to-grave efficiencies of a hydrogen economy are determined by researchers using awide variety of assumptions. In an attempt to simplify conflicting reports between varioussources, we constructed an efficiency schematic for 1 kg of hydrogen and three possible pathways (gaseous, liquid, and solid storage). Efficiency ranges are based on a lower heatingvalue of 33 kWh and a higher heating value of 39 kWh. This can be seen in Figure 1 (next page).

Additional concerns raised about a possible hydrogen economy are primarily environmental.Though hydrogen leaks are considered by most to be harmless because hydrogen is common tothe universe, molecular hydrogen leaks may wreak havoc on the atmosphere. More hydrogenemissions may result in additional water vapor in the stratosphere, which can lead to climatechanges such as increased noctilucent clouds, destruction of the ozone layer, and changes introphospheric chemistry. However, some researchers report increased hydrogen concentration inthe atmosphere will likely have little effect on the climate. Life cycle analyses on the hydrogeneconomy show that construction of hydrogen production and conversion plants is energyintensive and not environmentally friendly.

A hydrogen-based economy is not expected to take root in the near future. Processequipment and methods must be vastly improved to optimize system efficiency and economics.Research and development of hydrogen technologies will continue in laboratories and casestudies until a practical solution is found or until an alternative energy economy is developed.



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Figure 1: Conversion efficiency of 1 kg hydrogen to electricity

This schematic summarizes literature findings of energy input, output, and efficiencies in

gaseous, liquid, and solid hydrogen cradle-to-grave systems for 1 kg of H2 assuming the 39

kWh higher heating value (HHV) found in literature. Energy and efficiency ranges reflect the

varying results found in case studies. Electrolyzer and fuel cell efficiencies ( η ) are taken

directly from case studies; energy losses associated with these are obtained by calculationsinvolving input energy and the given efficiencies. Storage and delivery figures are explicitly

listed in the case studies as kWh/kg-H2; values from the literature are shown on the schematic

and are used along with input energy to calculate efficiencies for the intermediate steps.

Overall efficiencies at the end of each pathway reflect the lowest possible (most E in , least E out )

and highest possible (least E in , most E out ) efficiency values.

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