Forums Forums Wirral Information Ask - Wiki Wirral Mersey Barrage

Why is Steve Rotheram so keen to spend an estimated £6 billion damming the Mersey, causing massive disruption, to generate the same amount of energy that a medium sized wind arm in the Irish sea would produce at only £2 billion? This would involve no disruption and would be built in a fraction of teh time.

Has he lost his mind?

Because it generates employment and income for the region which is his job. I'm totally against it, the Mersey will silt up much more than it is now, I don't believe their dredging plan will be anywhere near enough.

They need to put tidal turbines in the area of the existing offshore wind turbines instead of destroying the Mersey.

But most of all they need to create mechanical storage, we already have the facilities to produce more than enough power, we just can't distribute it to the right places at the right time. Cheap mechanical storage in the correct locations will get energy to the right places at the right time without have to rebuild much of the the national grid which is part of the current plan.

We are in the ridiculous position of paying electricity generators huge sums of money not to generate electricity, storage will reduce that waste.

I asked the chief engineer of the last barrage project how much siltation he expected . His answer was that they'd lose 30% in ten years, even with constant dredging.

Storage is needed, but it needs to be in huge quantities. Terawatt hours, and many of them.

Mechanic storage is useless on this scale . The biggest pumped scheme in the world is in China and it manages only 40 GW. about one twenty fifth of just one TWh.

Hydrogen is the ONLY way to store that quantity, held at high pressure in solution mined caverns in deep salt strata. This is widely used for natural gas A single caver of a million cubic metres capacity is possible and at 350 bar it would hold the best part of a TWh.

There is good information on one near us at stublack in Cheshire. You'll find details of it on the internet easily enough.

I'm quite a fan of hydrogen but while its energy density by weight is brilliant, its energy density by volume is pretty poor. Also hydrogen molecules are a much smaller than natural gas so leakage in salt mines would be far worse. Distribution facilities are expensive because of the high pressure and high volume required.

Stublack will hold two days worth of natural gas, the same size facility for hydrogen would only be something like 6 hours if they are the same pressure.

By mechanical storage I was thinking of gravity storage using rock, its cheap, scaleable, simple, highly efficient and can be located anywhere. It is also the probably easiest and cheapest to maintain.

High temperature thermal storage using dry sand is also taking off, any reasonably deep old quarry can be filled with sand.

There is a lot of push for cryogenic storage of liquid air but I can't see it being that efficient, its going to be throwing thermal energy away at every stage unless you store and recycle the hot & cold. Also extreme cold and high pressures are not good bedfellows, things crack too easily.

The most obvious one cost wise is synfuels - synthetic petrol or other hydrocarbons. Utilising the existing storage and distribution infrastructure is a massive saving. Its carbon neutral to keep everyone happy but as yet not as efficient as other storage methods.

Liquid sulpher batteries look good apart from the requirement to try and keep the sulpher liquid, its a lot of downtime if they solidify.

If we are basing our renewable energy supplies on tidal, solar and wind power it is essential to build the storage facilities otherwise we are literally throwing both money and energy away.

Mechanical storage is not really on. From the top of my head, a tonne falling 35 metres stores 1 kWh. A TWh is a thousand million kWh .

I'll leave you to work out what a TWh mechanical store would look like.

A million cubic metres at 350 Bar of hydrogen deep underground will store a day's electricity fot teh UK

ERRATUM

I should have said a tonne falling 360 metres generates 1 KWh. The Beetham tower is about half that height. Apologies.

Without storage all our solar, tidal, wind and solar based electricity is going to be pretty useless and we are running out of options.

Lithium is politically-economically unsafe, the price could rise greatly at any moment in time, the only reason the price has been dropping is to draw the world into its dependence for the financial trap.

Hydrogen is almost totally produced from carbon fuels at the moment, mass generation from water or air has not been resolved, disposal of waste oxygen could be a problem if produced on massive scales. Grid level storage is limited in locations.

Hydro is not feasible.

Thermal sand pits aren't being looked at in this country and would probably meat a lot of environmental barriers. Some parts of the country haven't got suitable locations.

Cryo-storage seems far fetched in massive scales.

Mechanical storage would be large in physical volume and number of sites.

That basically leaves synfuels based on CO2 conversion, I can't find the efficiency figures. Liquid carbon fuels have a lot of advantages, energy density being the main one followed closely by convenience.

The only other thoughts I've been having is undersea/underwater storage of gases, below a certain depth the gas becomes neutral then negatively buoyant so would only require weak containerisation such as bladders around those depths.

"Mechanical storage would be large in physical volume and number of sites"

The UK uses about 1 TWh a day of electrical energy. To run 100% on intermittent renewable energy we probably need to store ten times that 10 TWh

Now a 360 metre tower with a a hundred tonne weight will store 100 kWh Let's assume its 10 metres by 100 metres Ten of those taking 1,000 square metres would store 1 MWh. A GWh is 1000, times a MWh so to store a GW would take a million square metres. But we are not there yet.

A TWh is a thousand GWh so to store a TWh in this way requires a thousand million square metres and to reach our required storage capability we need 10,thousand million square metres of land.

So in a single building we are talking about a structure taller than the tallest building in the UK, but covering an area 100 km by 100 km. Even split into ten thousand units this would involve 10,000 buildings, each covering a square kilometre of land.

I don't believe mechanical storage is even close to being feasible.

Contrast this with hydrogen storage. Electricity should be converted to hydrogen as close to turbines / solar farms as possible. This can now be done at 95% efficiency - comparable with the efficiency of an electrical transformer. This hydrogen could be stored in solution mined caverns in deep salt strata at high pressure. Caverns of up to a million cubic metres are feasible and at 350 bar pressure could store a TWh. Ten of these would meet our required 10 TWh storage.

Pumping to high pressure takes about 4% of the energy in the gas, assuming industrial compressors are reasonably efficient, and this energy is in theory recoverable, although it probably won't be worth the effort.

The technology is widely used for storing natural gas and in much lower quantities for hydrogen so it is proven to work. The caverns are quick to create, and involve very little use of land - merely a valve head building. It is also possible to use exhausted natural gas fields to store hydrogen. The Rough gas field off the west cost of the UK has been proposed for this.

Conversion back to electricity can be done at about 60% efficiency, the remaining 40% appearing as heat, but a better solution is to pipe pure hydrogen to the user where this 'waste heat' can be used for space or water heating. We use about five times as much energy in heating as in electrical uses so using a Fuel an efficient fuel-cell based CHP unit would have to convert ost of the electrical output back to heat in resistance heaters. However, a more practical solution would be to use less expensive much less efficient fuel cells in the CHP unit. It is worth noting that moving large amounts of energy in electrical form is not very efficient. Even at high voltages, we lose about 10% of the electricity in transmission losses on average. Gas, bu contrast is far more efficient and transmission losses are negligible.

Fears of explosions and leakages are ludicrously exaggerated. We in the UK used 60% or more hydrogen for 150 years or more in coal gas. We have experience of it and it worked even without modern additions like flame failure detectors and excess flow shut off valves etc.

I can't see any other technology that would meet our needs when you do the necessary sums to this sort of level.

The current Government estimate is that by 2035 we require 10 billion cubic metres of hydrogen storage. We currently have around 1.3 billion cubic metres with another one billion in progress and a further billion committed. I believe that is not including anything extra for electrical storage but includes electricity generation.

Coincidentally we currently have around 1.3 billion cubic metres of salt mine natural gas storage.

Conversion of water to hydrogen using electricity is difficult. Straight electrolysis is highly inefficient and doesn't cope with brine, some other electroliser technologies are more efficient up to around 80% but again don't work well with brine, we are already short of clean water.

Storing electricity as hydrogen gives round-trip efficiencies of less than 50%, the accepted figure at the moment is be around 40%, if you include desalination you lose more.

Additionally transportation and local storage of hydrogen it is compressed to nearer 10,000 psi and while the turbofan compressors are incredibly efficient, the heat and cold created in the compression and expansion process would have to be recycled to retain the efficiency.

I'm leaning towards synfuels, effectively storing hydrogen as liquid hydrocarbons or alcohols, there are air plus electricity processes available which are carbon neutral and reasonably efficient.

Your figures for gravity storage are a bit misleading, a 10m X 100m X 1m (short!) block would weigh in the order of 3000 tonnes, thirty times what you imply, realistically a block would be more like 3m tall..

I am looking only at the electricity supply, but the same considerations apply generally.

When talking about billions of cubic metres of gas one should really give the pressure and I suspect that the figures given for cubic metres are at STP. In energy terms a cubic metre of hydrogen at STP contains abour 2kWh of energy. However, compressed to 350 bar it contains over a megawatt hour.

Electrolysis is now possible at high efficiency. 95% and above. There are now several methods of doing this. H2Pro in Israel is one worth looking at but a better one IMHO is due to Hysata in Australia. This uses a capillary fed electrolyte eliminating bubbling which cuts the efficiency to about 75% in conventional electrolysers. It was developed by teh University of Wolangong, and Hysata is a spin-off company now starting production. You can look it up here. https://hysata.com/. Bothe the University of Adelaide and a chinese group in Beijing university have systems they claim work from untreated seawater, the Adelaide group claiming 'near 100%' efficiency. The Beiging group have demonstrated their system in real world use for months. I don't know how efficient it is though.

As pointed out the reverse process is to date less efficient but if used sensibly in a fuel cell CHP system the 'waste' heat is no longer waste and efficiencies of around 90 to 95% are possible.

Transport of gas is no problem we do it now across the country in the gas grid. The highest pressures they use are no bigger than 80 bar which is done in order to use the mains for storage. With salt cavern storage, this can be greatly reduced meaning that these pipes would be suitable for hydrogen. There is simply no need for the high pressures you mention except of course for cars which use 700 bar. This could be done by a compressor at the filling station however. Going from STP to 700 bar requires about a kWh per kg of cas so it uses about 2.5% of teh energy the gas contains. Assume 50% efficiency in the compressor and we lose 5% of the energy which is acceptably low.

Finally, I envisaged a 100 ton weight being wound up and down a 360 metre tower to store 100kWh. The dimensions were for the tower. 10 metres square and 360 high. You would need ten such towers to store a MWh.

You mention synfuels as an alternative. I see two objections to this. First they all require carbon in teh molecule to hold the hydrogen. This would appear as CO2 when burnt, and the object of the exercise is to NOT release this gas. Even if it originally came from the atmosphere, it could only be carbon neutral and doing this would require additional energy. Which brings me to my second point which is there would inevitably be energy required to make the fuel and this would make it an inefficient process. Also internal combustion engines are inefficient and polluting.

As a final general remark, however, I believe we need not be TOO worried about efficiency. The primary energy is free, and it comes in copious quantities. Far more than we need. he main consideration is the cost of harvesting it. Basically, the cost of electricity from a solar panel is the capital cost of teh panel divided by the number of KWhs it produces, minus a negligible maintenance cost. Thus two 200 watt panels are a better bet than a 400Watt one if together they cost less.

Part of the electrolysis problem is the very low voltage, often between 1 or 2 volts which means you need high currents to get high power, the high currents increase the i2R losses.

H2Pro is around 80% efficiency, their 98.7% claim is a play on terminology https://en.wikipedia.org/wiki/H2Pro

Any claim of bubble free production of gas would appear to be far fetched, does the Hydrogen come as cubes? The bubbles are as essential as the conductive electrodes, surface characteristics of the electrodes and vibration can reduce the time the gas is blocking the electrode.

Solid batteries are different, there is no need for gas in the process, in electrolysis there is, its the intended product. Pressure would increase the weight of Hydrogen produced relative to the volume of bubbling but that is so low tech I presume that investigations into that was exhausted many years ago and the ideal pressures and temperatures are already known for multiple electrolytes and electrodes.

I was wondering about the cavity volumes too but found statements about the created volume of the cavities so took the volumes as dimensions rather than storage-equivalence. Properly written material rather than investor targeted hype is difficult to come across.

The depth of the cavern governs the pressure that it can support (unless lined), Cheshire's caverns are around 500 feet deep so around 350psi, some salt caverns in the states are 4000ft deep and can support up to 3000psi. I think the (UK) Portland caverns are deeper than Cheshire, then we also have a much larger salt area around Yorkshire which may have some deep levels in places.

Agreed on efficiency, I have often stated that who cares about loses when they are cheap, however in our capitalist world we don't pay for electricity based on the cost of production, we pay a similar order of cost for electricity whether it comes from a free source of not.

Synfuels are carbon neutral, I'm not sure there are any carbon-negative storage processes so is as good as it gets. The carbon is absorbed from air before the synfuel is consumed so actually the stocks of synfuel could be seen as also adding to carbon storage. I think the efficiency is around 80% using the single step methods which avoid the hydrogen production step but I've lost the details of the company in the states I was thinking about, they claimed they could produce virtually any octane rating, they might have been taken over by one of the Hydrogen producers and been sidetracked. If I remember rightly most of the process is cleaning up the CO2 which is in two or three stages of the process.

To make things clear. Hysata claim to produce a kg of hydrogen with about 41kWh. As the gas contains around 39kWh of energy, this works out to about 95% efficiency. As the have now started production, I assume it works. Both they and H2Pro published in Nature and their claims have not been challenged so I assume it works as advertised.

I don't know so much about H2Pro, but there are other companies making claims similar to Hysata's.

Extracting CO2 from the atmosphere costs energy. Lots of it if you use fractional distillation of liquid air. I would guess that Syn fuels would take the effluent from big CO2 emitters like cement manufacturers, to save the cost of this energy, but that means it isn't even carbon neutral.

The pressure you can use in salt caverns depends on the depth. The deeper you go the higher pressure you can use as what you aim at is roughly the same pressie in the cavern as the ambient pressure in the rock surrounding it. To low and the cavern would collapse. To high and the gas would probably find a way out.

There is a lot of information online on the installation at Stublach in Cheshire which is used for natural gas and is run by Storengy Ltd. Another one is being constructed at Portland I believe. Much bigger if I remember correctly.

A small point about high currents. That's really not a problem. You step down from mains or DC or whatever locally with a switch mode supply.

I designed and built one for recharging supercapacitors that will happily deliver 100A - even into a short circuit - and will charge up to 16Volts at that current. Modern MOSFETS make it easy. Its only 6" by 3" by 2". Works a treat.

Electrolysers are specified in current per squcm and long runs of heavy copper cabling is not really necessary anyway.

But that rating is in the order of 1A/cm2 at less than 2V you need a massive surface area to get the power, or huge numbers of smaller cells. You can put cells in series to reduce current/I2R losses and maybe the amount of copper needed but you will still need balancing circuits which in themselves are usually lossy, I guess at large scale the imbalances are less.

I've not had the fortune to visit a grid level inverter, obviously there are many of them around these days but I've been retired 20 years, I might have an ask around to see one. I'd be interested what frequency they operate at, the higher the current then the more capacitance in the active components, which limits the maximum frequency but as you reduce frequency you lose the efficiency of the inductors and I guess there is a minimum frequency of ripple that can be applied to the cells which may be governed largely by the capacitance of the cells.

At low voltages there aren't any efficient diodes but there are zero-switching designs using mosfets as you imply. Also as mentioned above, using series configurations to avoid low voltages alleviate the problems..

I was wondering what electrodes they use as most metals are lossy, for low current cheap cells they use stainless steel but it appears that higher currents use copper plated with rubidium or similar which will be efficient because of the copper core.

On a sidetrack I see Hyundai are producing commercial plastic-to-hydrogen systems - not directly an electricity storage process in the common perception but could be utilised as such. This is similar to the plastic-to-oil/petrol steam process. While the efficiency won't be there, the combined effect of recycling plastic could make environmental/economic sense.

Its not my field, having worked with picowatts rather than Terawatts, but I believe grid level converters - at least the big ones used on HVDC links - do not run at high speed. They are quite crude and some even simply chop DC at Mains frequency to produce a square wave and rely on subsequent L-C filtering to get a sinusoid. More sophisticated ones manage a sort of multilevel boxcar waveform to approximate a sinewave more closely. They too operate at low speed, a few multiples of Mains frequency.