Glen Sannox: A ‘green’ ferry has more emissions than a diesel ship.

The long-delayed new “green” ferry will have a much larger carbon footprint than the 31-year-old diesel vessel that normally plies the route between the Scottish mainland and the Aran Islands.

An emissions analysis by CalMac estimated that MV Glen Sannox would emit 10,391 equivalent tonnes of CO2 a year compared to 7,732 for MV Caledonian Isles.

A dual-fuel ferry has more car capacity but requires larger engines that also emit methane, a greenhouse gas with a greater global warming effect than CO2.

Ferries procurement agency CMAL, which owns the vessel, said the comparison was “incorrect” because the Glen Snooks is a larger vessel.

Glen Sannox’s size is a factor in its carbon footprint, but so is liquefied natural gas (LNG) fuel, which is less climate-friendly than previously claimed.

The ship’s business case, drawn up in 2014, predicted it would emit around 400 tonnes less CO2 per year than ships on the Arn route.

Instead, it is now expected to produce an additional 700 tonnes, rising to around 2500 tonnes more if methane is included in the calculation.

A transport emissions expert told BBC News that the new ship would be better off running on diesel if the “upstream” carbon cost of importing LNG from Qatar was factored into emissions calculations. can

Professor Tristan Smith, from University College London’s Energy Institute, said: “At best the use of LNG is of negligible benefit, and at worst a detriment.”

Glen Sannox is the first ferry built in the UK to be capable of running on both LNG and marine gas oil (MGO), a low-sulphur type of diesel.

At its launch in 2017, then First Minister Nicola Sturgeon said it would contribute to “Scotland’s global climate change targets”.

But LNG technology also added complexity. The Ferguson shipyard had to overcome a number of engineering and regulatory challenges before delivering the vessel last month, years late and over budget.

The ship’s size also means that unless a major rebuild takes place, it cannot go to the usual mainland port at Ardrossan.

When it enters service next month, Glen Sannox will significantly increase the car’s capacity. It can carry 127 cars compared to about 90 cars for the Caledonian Islands.

Air pollutants, known as NOx and SOx, will also be reduced, while its power and maneuverability should mean less weather-related cancellations.

But in terms of greenhouse emissions, the CalMac analysis – seen by BBC News – shows the benefits of LNG technology are minimal.

Once methane emissions are normalized, the benefit of running a ship on LNG instead of MGO alone is less than 9%.

And the increased fuel consumption of the heavy ship means its overall carbon footprint is about 35 per cent larger than that of the Caledonian Islands, which is due to return to Aran in March after repairs.

Carbon emissions have nothing to do with the design and construction of Ferguson Shipyard’s Glen Snooks – the yard only built the ships it was asked to build.

Instead they are due to decisions made by ferries procurement agency CMAL, ferry operator CalMac and Transport Scotland before the contract was put out to tender.

The old ferry, the Caledonian Isles, was designed to carry 110 cars, but modern cars are so wide that they can now only fit around 90.

The Glen Sannox was specified to carry 127 modern cars, or 16 HGVs, and her high speed (although not essential for earen sailing) resulted in a much heavier vessel with This requires large engines.

When running on LNG, CO2 emissions are reduced by up to 25% – but this is almost entirely offset by the larger engine size and higher fuel consumption.

Another reason is methane.

LNG fuel consists mostly of methane, a greenhouse gas with a global warming potential (GWP) 28 times more potent than CO2 over a 100-year period.

A small amount of methane always passes through the combustion engines, and escapes through the funnels – known as “methane slip”.

CalMac has calculated that methane seeps add more than 1,800 tonnes of CO2 equivalent per year. This was not considered in the commercial case of ships.

Scotland also has no local supply of LNG, so the gas has to be brought in via diesel road tankers from a terminal in Kent.

This 445-mile long road trip adds 140 tons of CO2 equivalent annually, compared to 19 tons for transporting MGO.

Ferries procurement agency CMAL, which owns Glen Snooks and its sister ship Glen Rosa, began recommending LNG as a low-emission ferry fuel long before 2012.

In a statement it said LNG was considered the “best option” at the time, and was always intended as a “transition fuel”.

A CMAL spokesman added: “Due to differences in vessel size, propulsion power and expected sailing time, it is incorrect to make direct comparisons between older vessels and MV Glen Sannox emissions.

“The latter is a much newer vessel that is larger and more powerful than the former, reflecting the increased demand for ferry services.”

Ferry operator CalMac said the number of passengers and vehicles on the Arn route had increased significantly over the past 30 years.

“Vessel sizes require greater strength to meet this demand, particularly those carrying cars, and must offer greater resilience against adverse weather,” a spokesman said.

“As a first in class ship, we will learn more about MV Glen Sannox and her fuel consumption as we sail her.”

CalMac’s emissions analysis, carried out at the start of the year at the request of Scottish ministers, almost certainly underestimates the ship’s carbon footprint.

The figures are based on ideal engine operating conditions that minimize methane leakage – and CalMac acknowledges that in real-life operations, emissions are likely to be higher.

The analysis also does not account for the so-called “blowdown” emissions of methane that occur each time a ship goes from LNG to MGO.

Any methane left in the engine’s crankcase and fuel lines is automatically vented into the atmosphere – and CalMac has not yet determined how much will be released.

The report also only considers the carbon cost of transporting the fuel from the LNG terminal in Kent – no allowance is made for the “upstream emissions” involved in extracting the gas in Qatar and transporting it to the UK.

CalMac argues that this is reasonable because it has no control over where the fuel is sourced.

But environmentalists and some academics say a full “fuel to burn” comparison would paint a more realistic picture of climate impacts.

“Upstream emissions are really important – especially if you’re importing gas from Qatar or the US,” Professor Smith said.

“The point of whether or not upstream should be included is irrelevant – it’s re-arranging the deck chairs on the deck of the Titanic to a point where we should go to zero emissions.”

It is possible to run Glen Sannox on biogas, which is produced from organic waste such as manure or by-products of the whiskey industry.

Because the carbon dioxide released when burning biomethane is produced in the atmosphere rather than underground, it is considered carbon neutral.

However, building such a plant in Scotland would require major investment and would include supercooling facilities to convert the gas into a liquid.

If the biogas facility is developed, one option would be to restart the Grangemouth oil refinery, where hundreds of jobs are currently at risk.

Four more large CalMac ferries under construction in Turkey will have conventional MGO engines with some limited battery power for maneuvering in port.

But they have a deeper draft (the amount of hull below the waterline) and lower cargo requirements – meaning a more fuel-efficient hull form is possible.

CMAL says they have also been designed with the possibility of retrofitting for potential future fuels such as ammonia.

An alternative option would be to consider catamarans, which are much more fuel efficient than single-hull vessels.

Chartered catamaran Alfred, which can carry 98 cars and cost just £14.5 million to build, has been operating successfully on the Arn route for the past 20 months.

Its owner describes it as the most environmentally friendly large ferry in Scotland due to its low fuel consumption.

CalMac believes that advances in battery technology mean that around 90% of its routes could potentially be serviced by all-electric ferries, rising to 100% if fast charging were available.

Professor Smith says this is currently the most climate-friendly option – although it will require significant upgrades to electricity infrastructure.

“Many other countries are putting battery electric ships on routes that are serviced by these ships,” he said.

“And it’s a solution that, if charged with renewable electricity, would have zero emissions at the point of operation.

“So it’s a much more sustainable and viable investment in the long term, and hopefully we’ll see more of these in the future.”