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APPLICATION OF ELECTROLYTIC HYDROGEN:
Use of off-peak or otherwise constrained non-fossil fuel electricity for production of electrolytic hydrogen is critical for reduction of overall CO2 emissions to the atmosphere and for electricity system power and voltage stabilization in the absence of fossil fuel generation. A primary application for electrolytic hydrogen is for conversion of biomass into methanol and then upgrading methanol into synthetic gasoline. A seconday application for electrolytic hydrogen is for production of ammonia and ammonia based fertilizers. In certain situations it makes engineering sense to use electrolytic hydrogen as a wind power energy transport medium or as a vehicle fuel. In order to financially enable use of electrolytic hydrogen the purchase price of an off-peak electrcal kWh must be small compared to the purchase price of an on-peak electrical kWh. As of November 2016 stupid Ontario government policy is preventing constructive use of surplus non-fossil electrical kWh for production of electrolytic hydrogen in Ontario. Any fix to this governmental problem involves apportioning the Global Adjustment over kW instead of over kWh.
This web page examines the production of hydrogen gas by electrolysis of water, with no emission of CO2 to the atmosphere and use of electrolytic hydrogen for limited period energy storage. A technology limiting issue with electrolysis equipment is use of platinum at the positive electrode.
ELECTROLYSIS OF WATER:
Electrolysis of water yields three products, hydrogen gas, oxygen gas and heat. This web page relies on data published by Hydrogenics with respect to the practically realizable electrolysis efficiency.
The company Hydrogenics manufactures a product known as HySTAT type V which electrolyzes water to form hydrogen gas. Hydrogenics claims that when its equipment is operated at full design capacity the practical conversion ratio is in the range 4.9 kWh / Nm^3 to 5.2 kWh / Nm^3 of hydrogen at a pressure of 10 barg. For unit comparison, according to Hydrogenics:
1 Nm^3 = 38 SCF.
1 m^3 = [(1 m / (.0254 m / inch)) X (1 foot / 12 inch)]^3
= 35.315 feet^3,
so the absolute temperatures at which a Nm^3 and a SCF are defined are different. It appears that hydrogenics is defining a Nm^3 at 0 degrees C and a standard cubic foot (SCF) at 21 degrees C.
Thus the hydrogen production rate is in the range:
(1 Nm^3 / 5.2 kWh) X (1000 Nlit / Nm^3) X (1 mole / 22.4 lit) X (2 gm / mole) X (1 kg / 1000 gm)
= 0.01717 kg / kWh
(1 Nm^3 / 4.9 kWh) X (1000 Nlit / Nm^3) X (1 mole / 22.4 lit) X (2 gm / mole) X (1 kg / 1000 gm)
= .01822 kg / kWh
The required input electrical energy per hydrogen molecule formed is:
(1 Wh / 0.01717 g ) X (3600 s / h) X (2 g / 6.023 X 10^23 molecules) X (1 j / w-s) X (1 eV / 1.602 X 10^-19 j) = 4.34597 eV / hydrogen (H2) molecule
One way of producing liquid methanol (CH3OH) is via the chemical reaction:
CO + 2 H2 = CH3OH
One litre of methanol at room temperature has a mass of about 0.7866 kg. The mass of contained hydrogen is:
(4 / 32) X 0.7866 kg = 0.098325 kg
The electical energy required to separate that hydrogen from water using a Hydrogenics electrolyzer is:
0.098325 kg X 1 kWh / 0.01717 kg = 5.726558 kWh
If the required electrical energy can be purchased at $0.055 / kWh the electricity cost of the hydrogen in the litre of methanol is:
5.726558 kWh X $0.055 / kWh = $.31496
Clearly the electrical energy cost per kWh must be very low and/or the waste heat from the electrolysis process must have value for displacing purchased heating fuel in order for production of methanol from electrolytic hydrogen to make economic sense.
HYDROGEN ENERGY STORAGE:
In certain geographically favorable locations it may make sense to use large diameter high pressure steel pipelines filled with hydrogen to both store and transport energy. The issue is that locations favorable for wind energy generation tend to be along sea coasts. Canada has one of the longest sea coasts in the world. However, much of the Canadian population does not live near the sea coast. Hence moving wind energy from locations where it is economic to generate to major population centers requires long energy transmission corridors. If the energy is transmitted via a high voltage electric transmission line the energy losses are low but the transmission line provides almost no energy storage. If the energy is transmitted via hydrogen gas in a pipeline the energy losses at each end of the circuit are relatively high but the pipe itself provides energy storage that is essential to give value to wind generated energy. A further advantage of this strategy is that part of the hydrogen can be directly used to displace natural gas in residential heating.
The success of the hydrogen gas energy transmission scheme rests on the existence of sufficient seasonal hydraulic energy storage elsewhere in the electricity system to balance out seasonality in the wind generation. In British Columbia and Washington State there are immense amounts of seasonal hydraulic energy storage on the Columbia and Peace Rivers. There is favorable wind generation along the west coast of Alaska. There are existing energy transmission corridors to Prince Rupert and to Kidimat. Thus if the governments of Alaska, British Columbia and Washington State act together and resolve the necessary treaty issues there is potential for wind generation to play a major role in the Pacific North West.
The situation for wind generation in Ontario is not nearly so rosy. In Northern Ontario close to Hudson's Bay and James Bay there is good natural wind generation. A hydrogen pipeline could be built from northern Ontario to southern Ontario. However, Ontario is missing a critical ingrediant which is seasonal hydraulic energy storage to balance seasonality in wind generation. While access to some of Quebec's hydraulic energy storage might be achieved via a transmission intertie, the costs of that intertie are very high and the amount of seasonal hydraulic energy storage that Quebec might provide is not sufficient to meet Ontario's projected requirements as existing nuclear reactors reach the ends of their working lives.
To understand the importance of seasonal hydraulic energy storage in this matter it is illuminating to consider the amount of energy that can be stored in a practical large diameter high pressure steel pipeline. The technology of such pipelines is highly developed because steel pipelines are currently used for interprovincial and interstate transmission of oil and natural gas.
In a confined space hydrogen gas is quite dangerous because it will naturally rise in air and form an explosive air-hydrogen mixture at ceiling level. However, in outside locations hydrogen is less dangerous because it will naturally rise in the atmosphere rather than pooling near ground level.
A major issue in using steel pipelines for containing hydrogen is that the range of air-hydrogen volumetric ratio mixtures over which explosive combustion occurs is much wider than for methane. The resulting safety issues require large pipeline design safety margins. A pipeline may have to be evacuated and/or filled with pure nitrogen prior to filling it with hydrogen. Thus the pipeline and its instrumentation, in addition to being rated for positive pressure must also be rated for a negative pressure. Also the hydrogen temperature everywhere in the system, particularly near pipeline compressor discharge ports, must be kept low to avoid hydrogen induced pipe cracking. The hydrogen (H2) molecule is small compared to pipeline metal molecules, so hydrogen will slowly diffuse into pipeline metals, particularly along grain boundaries. The rate of hydrogen diffusion into the metal increases as the temperature increases. The effect of this intersticial hydrogen is to increase the propensity for stress related cracking. It may be possible to minimize this hydrogen cracking through use of a suitable internal pipe coating.
HYDROGEN CHEMICAL ENERGY STORAGE IN A PIPE:
Ec = hydrogen chemical energy stored in pipeline
Ee = electrical energy recoverable from pipeline
K = hydrogen chemical energy per mole
P = absolute pressure in pipeline
Po = atmospheric pressure
V = contained gas volume of pipeline
Vs = steel volume in pipeline
S = hoop stress in pipeline wall
W = pipeline wall thickness
R = pipe inside radius
L = length of pipeline
To = freezing point of water = 273.15 degrees K
T = operating gas temperature in pipeline in degrees K
Rho = density of pipe steel
Ms = mass of pipe steel
Ec = K (number of moles of H2 in pipeline)
= K (contained volume of pipeline) X (number of moles per unit volume)
= K (contained volume of pipeline) X (number of moles per unit volume at pressure Po, temperature To ) X (P / Po) X (To / T)
V = Pi R^2 L
From ideal gas theory:
(number of moles per unit volume at pressure Po, temperature To ) = 1 mole / 22.4 lit
= 1 mole / (22.4 X 10^-3 m^3)
= 44.643 moles / m^3
Ec = K (contained volume of pipeline) X (number of moles per unit volume at pressure Po, temperature To) X (P / Po) X (To / T)
= K (Pi R^2 L) X (44.643 moles / m^3) X (P / Po) X (To / T)
The pipeline operating pressure P expressed in terms of wall hoop stress S is given by:
P 2 R L = 2 W S L
P = (2 W S L) / (2 R L)
= W S / R
Ec = K (Pi R^2 L) X (44.643 moles / m^3) X (P / Po) X (To / T)
= K (Pi R^2 L) X (44.643 moles / m^3) X (W S / R Po) X (To / T)
= K S (Pi R W L) X (44.643 moles / m^3) X (1 / Po) X (To / T)
The steel volume Vs in the pipeline is given by:
Vs = 2 Pi R W L
Ec = K (S / Po) (Pi R W L) X (44.643 moles / m^3) X (To / T)
= K (S / Po) (Vs / 2) X (44.643 moles / m^3) X (To / T)
Vs Rho = Ms
Vs = Ms / Rho
Ec = K (S / Po) (Vs / 2) X (44.643 moles / m^3) X (To / T)
= K (S / Po) (Ms / 2 Rho) X (44.643 moles / m^3) X (To / T)
Note that the stored chemical energy is directly proportional to the volume of steel or mass of steel in the pipeline.
RECOVERABLE ELECTRICAL ENERGY:
If we assume that the chemical energy is to be converted into electrical energy via a combined cycle gas turbine (CCGT) or a fuel cell at about a 50% recovery efficiency, then:
Ee = Ec / 2
Ee = K (S / Po) (Ms / 4 Rho) X (44.643 moles / m^3) X (To / T)
The Gibbs free energy for converting H2 gas to H2O vapor is:
K = 228.61 kJ / mole
An important parameter for energy storage evaluation purposes is:
Ee / Ms = (S / Po)(K / 4 Rho) X (44.643 moles / m^3) X (To / T)
which gives the mass of pipe steel necessary to store a unit of recoverable electrical energy.
Such a pipeline would likely be made of high strength steel with a specified minimum yield stress (SMYS) of 52,000 psi and for safety reasons would likely be operated at a maximum wall hoop stress of:
S = 52,000 psi / 3
= 17,333 psi
Atmospheric pressure Po is:
Po = 14.7 pounds / inch^2
(S / Po) = 17,333 psi / 14.7 psi
The density of iron is:
Rho = 7850 kg / m^3
The typical contained gas temperature is 15 degrees C = 288.15 degrees k, giving:
(To / T) = (273.15 / 288.15)
Ee / Ms = (S / Po) (K / 4 Rho) X (44.643 moles / m^3) X (To / T)
= (1179.1) [(228.61 kJ / mole) / (4 X 7850 kg / m^3)] X (44.643 moles / m^3) X (0.9479)
= (1179.1) [(228.61 kJ) / (4 X 7850 kg)] X (44.643) X (0.9479)
= 363.27 kJ / kg
In terms of kWh:
1 kWh = 1 kJ / s X 3600 s
1 kJ = 1 kWh / 3600
Ee / Ms = 363.27 kJ / kg X (1 kWh / 3600 kJ)
= 0.1009 kWh / kg
Thus one tonne of pipe steel can safely store:
1000 kg X 0.1001 kWh / kg = 100.1 kWh.
In Ontario the electricity requirement in the summer is about 2 kW / person. Hence if there was one tonne of hydrogen containing pipe steel for every one of the 12,000,000 men, woman and children in the province of Ontario the system could store sufficient electrical energy for about
(1 tonne / person X 100.1 kWh / tonne) / (2 kW / person) = 50 hours.
That is nowhere near the length of the province wide low wind periods that occur during the summer in Ontario.
The fundamental problem that Ontario faces is seasonal wind energy and almost no seasonal hydraulic energy storage. That combination makes nuclear reactors and some hydraulic generation the only sources of firm non-fossil electrical energy.
Another issue that should be considered is the number of tonnes of CO2 which are emitted during production, transport, field welding and burial of large amounts of pipe steel.
An alternative means of storing energy with hydrogen is liquid hydrogen. In 1969 rocket engines powered by liquid hydrogen took men to the moon, so liquid hydrogen is a proven large scale energy technology. One of the issues with liquid hydrogen is that the storage tanks must be very large so that their surface area to volume ratio is small, to minimize storage losses via heat conduction through the tank walls which causes unintended evaporation of the liquid hydrogen. Due to the required storage tank sizes liquid hydrogen is not viable for propelling private cars but might be viable for propelling railway trains.
This web page last updated November 24, 2016.
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