Posted by OilVoice Press - OilVoice
By Euan Mearns
The Scottish Government has launched a consultation on their new draft energy strategy and targets (links are top right on the page linked to). The new energy strategy in general terms has four main strands:
This post will focus on the thermodynamics, efficiency and costs of making H2 via electrolysis of water and steam methane reforming. I calculate costs of £142 / MWh for electrolysis and £100 / MWh for steam methane reforming (SMR). These compare with industrial methane prices of £15 / MWh at odds with multiple claims made by the Scottish Government that hydrogen is a low cost option.
Let me begin with a look at some of the claims made in the Scottish Government consultation report:
On Page 33
On Page 35
A good starting point for this discussion is to take a look at current gas and electricity prices that are shown in Table 1 as reported in BEIS Table 3.4.2. In my calculations I use the tariffs for very large industrial consumers with values of £95 per MW hour for electricity and £15 per MWh for natural gas. The wholesale price of electricity is oft quoted at around £50 per MWh the difference being transmission charges that make up about 50% of the total.
Table 1 Recent UK gas an electricity prices for various classes of non-domestic consumers.
Natural gas, therefore, is dirt cheap compared to electricity and a central plank of our economy's current business plan is to take cheap natural gas to make more valuable electricity from it. Making H2 via electrolysis from water turns this thermodynamic and economic plank of our economy on its head. It should be abundantly clear that it will be impossible to make cheap hydrogen gas from expensive electricity. In fact, the plan calls for this electricity to come from renewable sources that will make it significantly more expensive than the £95 used in this analysis!
I imagine that this is the reason the proposals appear to call for methane to be used as the feedstock. But this has two major drawbacks. The first of course being that using methane for heating and transport is hardly a renewable source, and given the ban on fracking, there has to be concern about the security and cost of future methane supplies. The second is that steam methane reforming produces CO2 and to achieve the objective of CO2 free energy, carbon capture and storage (CCS) needs to be added to the process and this ruins the economics.
The thermodynamics and cost of H2 production
And so to the meat of the calculations. I want to stress that I am unsure about the accuracy and reliability of the calculations for both H2 production and CCS. Complex process engineering is involved. The H2 production numbers for SMR are based on a scheme published by the Colorado School of Mines (Figure 1). Engineer Alex Terrel simplified this scheme and my figures are based on those derived by Alex. I also sent my calculations to Robert Rapier who raised no objections. But if any commenter wishes to correct or elaborate upon what I present then this would be most welcome.
Figure 1 Energy requirements for SMR as presented by the Colorado School of Mines.
Figure 2 Schematic showing the energy and costs for the production of H2 gas via the electrolysis of water.
In order to make CO2 free H2 fuel by electrolysis, one needs to start with a CO2 free source of electricity. I am satisfied that Scotland's electricity supply is largely decarbonised for most of the time thanks to our 2 GW of nuclear power. Imports from England will be heavily polluted with fossil fuel but that is a story for another day. It must be noted that after the Scottish nuclear stations close (2023 and 2030) the level of imported electricity will likely rise significantly.
Electrolysis is quite simple and is a process that disassociates the H2 from the O in H2O by simply passing an electric current through water. The method is roughly 67% efficient, hence if we put in 1 MWh of electricity @ £95, we get out 0.67 MWh of H2 at a cost of £142 / MWh. This compares with an industrial cost of methane at £15 / MWh. At a stroke we have uplifted the cost of industrial gas for heating by a factor of 9.5.
This is madness. It would make more sense to simply use the electricity at £95 / MWh and endure a 6.3 fold uplift in costs (95/15 = 6.3). It has to be noted that the £142 / MWh compares more favourably with domestic gas prices of £45 / MWh, but I daresay there would need to be price adjustments to maintain the ratio of 3:1 between very large industrial and domestic users.
For fun I've shown what would happen if you converted the H2 back to electricity in a CCGT at 60% efficiency. We manage to more than double the price and gain absolutely nothing. Thermodynamics is unyielding.
If we do not have a C free source of electricity then we need to add CCS to the FF production and end up with an input cost of roughly £212 / MWh and therefore need to more than double the cost of H2 produced. It should be clear why the Scottish Government has a preference for using SMR as a more cost-effective route. But I should note that Aberdeen boasts an electrolysis based H2 production system to power its fleet of hydrogen busses. Another story for another day.
Figure 3 Schematic showing H2 gas production by SMR. High temperature steam is reacted with CH4 under pressure to produce CO and H2 (see equations lower left). The energy required to raise steam is one of the main energy inputs and costs. Note that Alex Terrell has a proposal to use heat from nuclear reactors as the source which is yet another story for another day.
Figure 3 shows the process energies and costs going via the SMR route. The SMR process is described lower left on the graphic and occurs in two stages. First steam methane reforming where a significant amount of energy is required by way of heat to raise steam. This produces a mixture of CO and H2. The CO is then reacted with water that produces CO2 and more H2 via the water-gas shift reaction. In addition to the process heat required, a small amount of electricity is required to run the plant and of course natural gas is required. We see that 1 MWh of natural gas provides 1.21 MWh of H2. Its not magic since the total energy added is 1.525 MWh (CH4+electricity) and the process is efficient at 79%.
What we see is that we could make 1 MWh of H2 for (£15+£2.4+£47.5)/1.21 = £53.63, much cheaper than electrolysis. But the problem is that the process produces CO2 that must be captured transported and stored in order to fulfil the objectives of this exercise. I have used a UK government report as the basis of costing CCS (see Figure 2). The costs are given for MWh of electricity produced, and if it is gas that is being burned it will produce approximately twice as much CO2 as the SMR process since with a CCGT 1 MWh of gas will produce 0.5 MWh of electricity. SMR is much more efficient. Adding the cost of CCS @£56 / MWh brings us to £100 / MWh of H2 produced. Cheaper than electrolysis, but still 6.7 times more expensive than methane.
Figure 4 Similar scheme to that shown in Figure 3 apart from the heat source to raise steam is derived from combusting a portion of the H2 produced.
Figure 4 shows a variant of Figure 3 where instead of using expensive electricity as the heat input source some of the H2 produced from cheap methane is diverted back to provide the 0.5 MWht heat input to raise steam. It was anticipated that this may lower costs while in fact a marginally higher cost of £103 / MWh of H2 is calculated. The reason for this lies in the cost of CCS. In this scheme 1 MWh of methane is combusted to produce 0.71 MWh of H2 instead of 1.21MWh as before. The ratio of CO2 produced / H2 produced has shot up increasing the cost of this expensive part of the process.
Once we have hydrogen we can convert this to motion relatively efficiently in a fuel cell powered vehicle that will be prohibitively expensive to buy or to provide heat in a domestic condensing boiler (furnace) that is about 90% efficient. But as detailed below hydrogen in the gas mains may present a plethora of problems that were raised in comments in my earlier post on New Renewable Energy Targets for Scotland.
Problems of Hydrogen in Gas Mains Pipes
This from Burnsider:
As an aside, hydrogen is almost as bad as helium in terms of its propensity to leak, so putting it directly into current gas pipelines might be difficult. Also, it burns with an almost invisible flame and forms explosive mixtures over a very wide range in air.
And Joe Public:
And at a stroke, the heat-carrying capacity of the entire transmission & distribution grid, all storage capacities (including line-pack), measurement capacity of all meters AND customers' own pipework is reduced by over ⅔rds.
Nat gas ~40MJ/m^3
Hydrogen ~ 12.7MJ/m^3
And again from Joe Public:
As ‘Burnsider' above mentions, it has much, much wider explosive limits.
Nat gas: 5.3% – 15% v/v
Hydrogen: 4% – 75%
And from Singletonengineer:
My experience with hydrogen is limited but includes the immediate aftermaths of 3 hydrogen fires. All nasty. Two with serious explosions due to pure hydrogen escaping to air. They were probably ignited by static electricity at the site of the leaks.
Hydrogen is seriously difficult stuff to control, especially pure hydrogen. What were those explosive limits in air again? From memory, LEL 4% and HEL 75%.
Is the proposal to dilute all H2 to, say, 20:80 blend H2:Air? In which case, there will be 5 times the volume to pump and to meter.
Speaking of meters, like the gas jets and mains, they will also need to be replaced.
In summary, the smaller atomic radius of hydrogen makes it easier for it to escape from pipes, it has a much larger explosive range mixed with air from 4% to 75% and a lower energy density meaning that the operating pressure would need to be increased, increasing the risk of escape and explosion. There were some dissenting voices in the earlier debate but my view is why would anyone want to bother or take the risk of all this when the starting point is an uplift by a factor of 6.7 times the cost?
Alternatives for a Low C Future
Since unabated combustion of coal to make cheap electricity seems to be off the menu for the foreseeable future, I want to conclude with a look into alternatives, both of which use electricity and heat pumps for space heating. The first is a natural gas system combined with CCS and the second is nuclear electricity for heat.
Figure 5 The starting point is a CCGT that is 50% efficient producing 1 MWhe with CCS and an overall cost of £212 / MWe taking into account generation and ancillary CCS and distribution costs. This scheme is marginally more cost competitive than H2 when using a heat pump as the heat source.
Figure 5 shows a scheme for a CCGT operating at 50% efficiency. 2MWh of methane go in and 1 MWh of electricity comes out. One fifth of the power produced goes to CCS and I apply a blanket rate of £212 / MWh produced that encompasses generation, capture, transport, storage and transmission (as per earlier UK government source). As a rule, electricity can be converted to heat in a resistance heater at 100% efficiency. And so our remaining 0.8 MWe is converted to 0.8 MWt at a cost of £265 / MWht – still very expensive.
Using electricity for heat we can use to good effect the positive efficiency offered by a heat pump. Heat pumps extract heat from air and doing so can provide a 300% energy gain (COP 3). We can therefore turn our 0.8 MWe into 2.4 MWt at a cost of £88 / MWt. Marginally though not significantly better than the hydrogen MSR route. But we do need to go to the bother of buying and installing the heat pumps that are not cheap.
Figure 6 Using low C electricity from a new Gen III nuclear power station combined with a heat pump to convert the electricity to space heat, we end up with a solution that is cost-competitive with cheap natural gas today.
Finally, Figure 6 shows the schematic for a nuclear electric heating system. Power is produced at £92 / MWh based on the inflated cfd for Hinkley Point C to which £50 / MWh distribution costs are added giving a price of £142 / MWhe supplied. Converting this to heat using a COP 3 heat pump yields £47 / MWht. At last we have a solution that is remotely plausible, and one that doesn't involve building turbines, power lines and CCS pipelines and pumping stations everywhere. What's not to like?
The Scottish Government is embarking upon a consultation exercise, seeking views from the general public, on a subject that is far too complex for them to understand. Pose the question:
Do you want to have a low carbon, secure and cheap energy supply?
and the response will undoubtedly be yes. Ask instead:
Do you want to have one of the least reliable, hazardous and most expensive energy systems ever?
and I believe the response would be no thank you.
The approximate costs of delivering 1 MWht (heat) for four different low carbon system configurations are as follows:
None of these options are currently renewable since they either use nuclear electricity, natural gas or both. The electric heat options are only viable by either bringing the price of new nuclear down and /or overhauling home heating to heat pump systems. This may seem dramatic, but it is feasible. Conversion to H2 for home heat at high cost will die a certain death when the first tower block explodes like a Fukushima reactor building where the explosions were caused by hydrogen gas and not nuclear fission.
By Euan Mearns
Visit source siteEnergy MattersEuan MearnsHydrogenScotland