Replacing LPG with Compressed Hydrogen Storage

1) Intro: notes on Liquefied Petroleum Gas

  • Mostly liquified propane and butane
  • Density: 1 kg of LPG is 1.96 liters
  • Energy density: 20.7 MJ/kg
  • Liquid propane at room temperature (21° C), it has to be held in a tank at a pressure of about 850 kPa (8.5bars)
  • Liquefaction reduces volume about 250 times.

Heat Values of Various Fuels (here):

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From energipedia:

Liquefied Petroleum Gas (LPG) is a by-product of natural gas extraction and crude oil refining. LPG is a mixture of hydrocarbon gases, the most common being butane and propane.[1] 

At room temperature, LPG is a colorless and odorless non-toxic gas. Under modest pressure or cooler conditions, it transforms into a liquid state. This process leads to the reduction of the volume to 1/250 of the gaseous aggregate state and allows to store and transport LPG easily in cylinders.

LPG has an useful energy value of 20.7 MJ/kg. In comparison, air-dried firewood has an energy content of around 16 MJ/kg and charcoal of 27 - 33 MJ/kg. Depending on the type of woodfuel, charcoal production, and cook stove, between 7.3 and 29.7kg of woodfuel would be required to provide the same amount of useful cooking energy found in 1 kg of LPG.[2] 

LPG is heavier than air, e.g. propane is one and a half times heavier than air, and can therefore accumulate above the ground. This may lead to LPG-’lakes’ that potentially can causes explosions. A foul smelling odorant is added to help detect leaks and thus reduce the risk of explosion.

2) The “3KG LPG Canister (pic on the right)

a) Physical characteristics

  • Diameter: 210mm / Height: 270mm
  • Volume: 6.0L total, but we can only fill 80% ⇒ about 4.8L
  • Weight: 3kg liquid, 5kg metal (total when filled is about 8kg)
  • Working pressure: 18bar / Test pressure: 34bar

b) LPG content

Since 1kg of LPG is 1.96L (density 1/1.96=0.510kg/L) and steel vessels are filled to 80–85% of their capacity to allow for thermal expansion of the contained liquid, we have:

  • 4.8L of LPG (80% of 6L)
  • 2.45kg of LPG (4.8L/1.96kg = 2.45 kg).

Note: it may be more if we believe that the net content is 3kg (but this does not correspond to the rationale 80% liquid in 6L. Since 6L is a sure value, that means that 3kg may correspond to filling the canister without a gaseous phase, posing risks!

  • 3kg LPG corresponds to 3/0.510 = 5.88L, which is 6/5.88*100 = 98% filled with liquid…

c) Energy content

LPG has an useful energy value of 20.7 MJ/kg.

  • 2.45kg of LPG contains 50.7MJ
  • [What is the efficiency of the heat / electricity conversion? depending on the method ?]
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A typical LPG cooking system is made up of a steel cylinder filled with LPG, a pressure controller, a tube connecting the cylinder to the pressure controller and the burner, and finally the burner itself.

3) How much liquid hydrogen we can put in a LKG LPG canister?

  • Energy per kilogram of liquid hydrogen: 120MJ/kg. Note that this is a kg, so it’s irrelevant if this is liquid or gaseous.
    • 1 kg of hydrogen contains 33.33 kWh (about 120MJ/kg) of usable energy assuming perfect combustion (petrol and diesel only hold only about 12 kWh/kg. Hydrogen has an energy-density-per-unit mass that is approximately three times higher than traditional fuel (from here and here).
  • Density of liquid hydrogen (weight per unit volume): 71g/L

The density of LPG liquid is 0.493 g/cm³ at 25 °C (77 °F)

  • For comparison, water is 0.997g/cm3 or 997/L - about 1kg/L).
  • Hydrogen turns into a liquid when it is cooled to a temperature below -252,87 °C. At -252.87°C and 1.013 bar. Liquid hydrogen has a density of close to 71 kg/m3 or 71000g/10^6cm3 = 0.071g/cm3 or 71g/L)
  • Energy per liter of liquid hydrogen (usable assuming perfect combustion):
    • From the above, 1kg of liquid hydrogen corresponds to 1000g/(71g/L) = 14.1L
    • So the energy per liter of liquid hydrogen is 120MJ/14.1L = 8.57 MJ/L
    • In kWh this is: 8.57/ (1000*3600) = 2.38kWh/L. Also verified here: Pressurised hydrogen contains about 0.5 kWh/litre at 200 bar, 1.1 kWh/litre at 500 bar and 1.4 kWh/litre at 700 bar. The best way of transporting hydrogen in terms of energy density is liquid hydrogen, achieving more than 2.3 kWh/litre

Notes:

  • Hydrogen has nearly three times the energy content of gasoline—120 MJ/kg for hydrogen versus 44 MJ/kg for gasoline. On a volume basis, however, the situation is reversed; liquid hydrogen has a density of 8 MJ/L whereas gasoline has a density of 32 MJ/L, as shown in the figure comparing energy densities of fuels based on lower heating values. Onboard hydrogen storage capacities of 5–13 kg hydrogen will be required to meet the driving range for the full range of light-duty vehicle platforms (from here)
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Energy we could store in a 6L canister

1) Liquid and Cold/Cryo compressed hydrogen storage (see above)

Liquid hydrogen is made possible by cryogenically cooling it to below its boiling point, -253 ◦C.

  • At atmospheric pressure (about 1 bar): it requires cryogenic temperatures, as the boiling point for hydrogen is −252.8°C. At this pressure, density is close to 71 kg/m3. 1 kg of hydrogen can be stored in a 15-liter tank. In order to maintain liquid hydrogen at this temperature, tanks must be perfectly isolated. Liquid hydrogen tanks typically operate at pressures up to 850 kPa (that is 8.5 bar or 123 psi).
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  • Cryo/Compressed: Hydrogen (gaseous state) has to be kept under pressure, and at very low temperatures, typically at 350 bar (5,000 psi) and 700 bar (from here). This is about 690 bar, 10,000 psi, or 69MPa. “At present time, the most promising hydrogen storage options are compressed hydrogen at 35 MPa (5kpsi) and 70MPa (10kpsi), at the temperature of 20 degrees Kelvin or -253.15 ℃)” (from here)
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Maintaining cryogenic temperatures is beyond the scope of our project. So this option must be discarded.

2) Compressed hydrogen at ambient temperature (this is what we want to do, we are not going to add any cooling system!)

We plan to produce H2 at atmospheric pressure (sea level), and then compress manually (car tire pump).

A small-sized car are usually 30 psi, medium-sized cars are 36 psi, and large cars are 42 psi. Let’s assume then 35psi (this is 241kPa, or 2.4 atm or 2.1 bar ).

Note: Could we produce that underwater? yes, that would mean in a depth between 10 and 20 meters.

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MASS of H2 in the canister at this pressure and temperature

Avagadro's Law: The volume of 1 mole of any gas = 22.4 liters. Mass of 1 mole hydrogen gas (H2) = 2 grams, so the mass of 22.4 liters (stp) is 2 g.

  • One litre of hydrogen under standard conditions weighs: 2/22.4 =  0.0896 g
  • If pressure multiplies by 2.4 (car tire), this just means 0.0896*2.4 = 0.215g per litre, and in 6L it’s just six times that:
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Mass of compressed H2 in a 6L canister is about 1.26 grams
Check (using PV=nRT)
  • R (ideal gas constant) = 8.314 joules per kelvin per mol.
  • The chemical amount, n (in moles), is equal to total mass of the gas (m) (in kilograms) divided by the molar massM (in kilograms per mole). The molar mass of H2 is M = 2.016 g/mol, so the mass of the gas is: m = n *2.016 (in grams).

Since all the hydrogen is in gaseous phase, this means 6L in the canister, not 4.88L (80%). 6L of hydrogen at 241kPa and at a temperature of 25C (298.15K) contains n = P.V/(R.T) moles of H2, that is:

241k [Pa] * 6/1000 [m3] / 298[K] / 8.314 [J/K/mol] = 0.50 mols

VOLUME in the ballon (before compression):

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The volume (V2) of H2 that needs to be produced by electrolysis and at sea level (before compression) is:

P1.V1 = P2.V2 ⇒ V2 = 2.4atm * 6L / 1atm = 2.6*6 = 15.6 L

STORED ENERGY:

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Since the heat value of H2 (per kg) is about 140MJ/kg, the stored energy would be: 140MJ/1000*1.26 = 0.176MJ or 0.053kWh (1MJ is 0.277778 kWh)

Since a 3KG LPG canister stores 50.7MJ, this means the equivalent in compressed H2 canisters is:

  • 50.7MJ / 0.176MJ = 288 canisters (of 6L and at tire pressure)
  • Or an equivalent volume in balloons at ambient temperature and sea level of: 288*15L = 4320L
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1 LPG canister ↔ 288 H2 manually compressed canisters, or about 4 cubic meters of H2 at ambient temperature

Note: A volume of around 11 m3 (which is the volume of the trunk of a large utility or commercial vehicle) is needed to store just 1 kg of hydrogen, which is the quantity needed to drive 100 km.

4) How much energy does an average Indonesian household uses per month? (vehicle gas, electricity, gas for cooking)

Indonesia is total consumption is 256.74 billion kWh of electric energy per year. Per capita this is an average of 932 kWh, or 78kWh per month (from here). Since the average household size in Indonesia is 3.8 people, this means 20.5 kWh per household/month (calculated from here). By comparision, the EIA aggregates data for the entire U.S. In 2021, the average annual electricity consumption for a U.S. home was 10,632 kilowatt-hours (kWh). Or about 886 kWh per month. But this is not the average household consumption, it includes all the industry! Also, we need to consider demographics (the target population are not the richest.)

More realistically:

  • active usage: ~3-4 hours of cooking every day —> uses 3-4 of 3kg cannisters /month
  • light usage: ~1-2 hours of cooking every day —> 2 of 2kg cannisters /month
  • Regular household uses 1-2 of 12kg cannisters (blue cannister /month)

The average amount of natural gas per house is 12.39 m3/month (here).

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Let’s take as a base number 1 blue canister per month, this is: 4 x 3KG LPG canisters

SUMMARIZING

A) REQUIRED QUANTITY of H2 to substitute LPG

  • Average number of LPG canisters/month: 4 canisters
  • Energy in LPG canister: 50.7MJ
  • Average household energy consumption (from LPG ): 4 * 50.7MJ = 203MJ
  • Mass hydrogen needed per average household to replace LPG: 203MJ/(140MJ/kg) = 1.45 kg

B) PRODUCTION (Solar power + electrolysis)

Can we produce this using solar power and a commercial electrolyser?

  • The EL 4.0 electrolyser we plan to use produces 1.1kg/24h when powered at 2.4 kW (here). This requires 2.4kW*24 = 57kWh. Note: as of 2022, more efficient commercial electrolysis requires around 53 kWh of electricity to produce one kg of hydrogen, which holds 33.6 kWh of energy
  • From consultation with Agung and Dery (company Energyhub, @August 23, 2023 ): taking into account solar intensity variations, and their solar panels (size?), it would be possible to generate an average power of 2kW during the day. This is approximately what the EL4.0 electrolyser needs to produce one kg of H2 per day.
  • Our solar panel system produces 6kW, so we can run at least two electrolysers simultaneously, to produce 2.2kg/day.
  • In both cases, this is more than the 1.45kg needed for an average household, and this happens in a day or two. Assuming the conservative number 1kg/two day, in a month we could have 15kg.
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This means that in a month one could power 15/1.45 = 10.34 households!

B) STORAGE (at ambient temperature):

Ambient pressure:

  • 1kg of H2 occupies 11m3. Therefore we would need 1.45 x 11m3 = 16 m3 or 16000L. This is a cube of 2.22meters side.
  • Number of tire-pressurized (3.2 bar) H2 canister (6L): 16000/6/3.2 = 888 standard green canisters per month
  • Green balloons (75cm diameter) have a volume of V = 4/3 π r³ = 4/3*3.1415*(0.75/2)^3 = 0.22 m3 or 220 liters. That means we would need 16000/220 = 72 balloons.
  • Per day, in balloons: 72/30 = 2.4 balloons (220*2.4 = 528L)
  • At 35 bars, this is a volume of 528/35 = 15L
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Pressurized:

  • Scuba diving tanks: If we use a big diving tank (18L, 300 bar) instead, we need 16000/18/300 = 3 tanks. Of course this needs a compressor. How much energy for that? (to do, assuming adiabatic compression). The compressor could be powered by a battery charged all day by excess solar energy or human powered - bicycles/gym)
  • The commercial electrolysis EL 4.0 (this) already produces H2 already at 35bars. Scuba tanks sustain this pressure without problem. Using 10L tanks e can store the energy for a month in 16000/10/35 = 17 tanks.

Notes:

  • Cylinders used for scuba typically have an internal volume (known as water capacity) of between 3 and 18 litres (0.11 and 0.64 cu ft) and a maximum working pressure rating from 184 to 300 bars (2,670 to 4,350 psi).

C) And what about producing just for day consumption without the need of storage?

  • 21 balloons per month means less than one balloon per day / household (exactly 21/30 = 0.7 balloons). This means that much less efficient electrolysers can be used (DIY?), but more importantly, that pressurized hydrogen is not needed at all. If the production of H2 is faster, then that excess can be compressed.

How much do they pay for it?

  1. How much volume of hydrogen is that at 1 atmosphere (in liter or m3)? How small can we make this (at a standard compression)?
    • Density of hydrogen at STP: 0.9 g/L
    • Energy density: 120 MJ/kg

LPG:

Roughly 50 MJ/

calorific value: 46 MJ/m^3 (vaporized at STP)

Pressure in a tyre: 30 psi

However this is not the whole story! We should study the process of electrolysis underwater: I dont think it will produce liquid hydrogen directly (or not work at all).

Maintaining 8.5 bars is easy, we should produce it at about 20meters. This is ok, humans can even scuba dive at this depth.

Producing liquid hydrogen underwater?