JANUARY 9, 2021
Guest Post By Joe Public
Drax’s latest Quarterly Bulletin has a section on storing excess wind power:
I have picked out these particular claims for observation:
1. Why no mention of those electricity storage systems’ capacity and discharge rates? Britain’s 4x pumped hydro stations have a total storage capacity of 26.7 GWh and discharge rate of 2.86GW.
2. “…28 TWh of storage … comparable to the total natural gas storage the UK has in the form of underground salt caverns.”
2.1 Not quite correct. It is comparable to the total natural gas storage Gt Britain has in the form of underground salt caverns plus LNG storage facilities. We have ~18,000GWh of conventional storage plus ~13,000GWh of LNG storage.
2.2 The Gross Calorific Value of hydrogen is just 3.3kWh/m^3 vs approx 11.1kWh/m^3 for Nat Gas, so low energy-density H2 has less than 30% the energy content of Nat Gas per unit volume at STP.
Consequently our energy storage capacity for hydrogen is not 28TWh, but just 8.4TWh at the same pressures.
3. “….Perhaps some of these wind farms should produce something other than electricity. Electrolysers can be used to turn electricity and water into hydrogen. The excess electricity production in 2030 could be used to make 670 million kg of hydrogen. That would be enough to fill 133 million fuel tanks in fuel cell vehicles such as the Toyota Mirai, or to heat nearly 2 million homes.”
3.1 It’s disappointing to see the deliberate obfuscation caused by mixing units – (the weight of hydrogen produced) denying readers the opportunity to easily compare relative figures. Why did the report’s authors not continue to use electricity-industry units of TWh/GWh they’ve already used in their article?
1 kg of hydrogen contains 33.33 kWh of usable energy.
3.2 The “37 TWh of excess electricity” production in 2030 could be used to make 670 million kg of hydrogen.”
So the “37 TWh of excess electricity” produces 670,000,000 kg hydrogen. But that mass of hydrogen has only 22.3 TWh of usable energy. i.e. it takes 66% more electrical energy input to make one unit of energy available via hydrogen output.
3.3 We’re told “…. (670,000 tonnes of H2) would be enough to fill 133 million fuel tanks in fuel cell vehicles…”
No mention is made of the energy needed to compress low energy-density H2 into those 133 million fuel tanks.
Approx 4kWh of energy would needed to compress 1 kg (33.3kWh) of H2 to 700bar.
This means (4/33.3) 12% of the available “37 TWh of excess electricity” is needed simply to compress the H2 into those 133 million fuel tanks. That then means that just 33TWh available to produce the H2, so only (22.3 TWh x 88% =) 19.6TWh of useable H2 is available when stored at 700bar.
Consequently, 37TWh of initial electrical energy input results in just 19.6TWh of hydrogen being available at the input of the vehicles’ fuel cells.
1.89 units of energy input to obtain 1 unit of energy into the fuel cell.
The fuel cell is then only 40% – 60% electrically efficient. This means end-to-end process efficiency requires approx 3.78 units of energy input to obtain 1 unit of energy OUTPUT from the fuel cell.
4. The authors consider “Hydrogen could potentially be hauled to shore at lower cost, piggy backing off the existing oil and gas pipelines, which will see limited use as the North Sea fields start to wind down.” yet don’t explain what they consider to be ‘piggy-backing’.
4.1 Do they consider it to be feasible to inject hydrogen into an operational oil pipeline? Do they realise that natural gas imports have to comply with National Grid’s strict quality-control standards that hydrogen doesn’t meet?
4.2 Regarding steel pipelines – both those to be ‘piggy-backed’ undersea, and on land – the authors might not be aware of hydrogen’s chemical effect:
“Conversion of the UK gas system to transport hydrogen” explains:
“At ambient temperature and pressures below 100 bar, the principal integrity concern for high-strength steel is hydrogen embrittlement. Hydrogen will diffuse into any surface flaws that occur due to material defects, construction defects or corrosion, resulting in a loss of ductility, increased crack growth or initiation of new cracks. These will ultimately lead to material failure. Higher pressures are thought to increase the likelihood of material failure although no threshold value has been defined independently of other factors …”
Paul’s Additional Comments.
- Their calculation that we need 1000 times more storage than we currently have sums up why storage can never be the answer to long term intermittency ( as opposed to intra-day needs), particularly since pumped storage accounts for about 95% of current storage, something which cannot be easily increased.
- These projections are based on 40 GW of offshore wind, so the problem of surpluses will become much more acute as more wind capacity is added later.
- The surplus wind power, 37 TWh, equates to about a quarter of total wind power. If this excess had to be thrown away, it would effectively increase the costs of wind power by a third.
- As Joe rightly points out, the capacity of salt caverns is not 28 TWh, as far as hydrogen is concerned. It is less than 8 TWh, meaning that most of the surplus cannot electricity cannot converted to hydrogen and stored.
- Claims of enough hydrogen to fill 133m fuel tanks, would imply maybe 3 million hydrogen cars. In reality, there are unlikely to be more than a few thousand on the road in 2030, and little prospect of many more in 2050. There may be a market for fuel cells in lorries and buses, but that will in all likelihood be decades away. (Apart from anything else, where will cars and lorries get their hydrogen from in winter, when there is no surplus wind power?)
- It is good to see they confirm that when wind power is in surplus here, it will also be on the continent, and equally so when wind power is low.
But I’ll leave the final comment to Drax!