The success of the offshore wind sector will depend on storage systems

There are various reasons why the focus on storage of electrical power is so intense today. Two aims high in the list of reasons for this intensity are the  levelling of power output from inconsistent power sources and the simplification of power transport from source to a central storage hub or end user. Being able to save energy during periods of high output to be released into the grid when output drops is the key to the success of a dependable, sustainable large-scale energy source. The success of the offshore wind sector will depend on storage systems if it is to replace fossil fuel alternatives on a large scale and an effective grid system is a necessary requirement to make any storage system successful.

Hydro-electric power has long been seen as an efficient and simple manageable power source which is fast and easy to start and stop when required, but it does have certain geographical requirements not universally available. Norway is leading Western Europe in this sector producing 139TWh in 2015, almost twice as much as Sweden in second place, but these two countries use the hydro-electricity for everyday consumption as a level source of energy. The hydro plants maintain most of their water levels from melt water and precipitation throughout the year and not from electric pumps powered by offshore wind generated electricity .

However, there is room for greater capacity in Scandinavia. It is estimated that Norway has a total hydro-electricity production potential of 300TWh and the 139TWh in 2015 represents only 46.4% of this total. A part of the potential hydro-electric plants can be used as a storage system in the future for the grid connected offshore wind farms, using offshore wind power during times of low demand and high production to pump water to fill the reservoirs. For the next part of the equation Norway already has an extensive interconnector grid in the North Sea planned or in operation with one or more subsea cables to the UK, Netherlands, Germany and Denmark. It is a start but the current individual capacity of the interconnectors would be insufficient for large scale grid supply.

In the past 10 years we have seen offshore wind power grow from almost nothing to the current level of about 11GW. Within the next 12 years we can expect that North Sea wind power will reach 60GW, and by 2050 as much as 250GW. A European target has been set at 75% for all energy consumed coming from renewable sources by 2050. The North Sea coastline countries, UK, Norway, Denmark, Germany, the Netherlands and Belgium currently consume at total of approximately 5,500TWh annually. Today’s figure of 500TWh, only 9%, coming from renewable sources leaves much to be done in the next 32 years.

“Within the next 12 years it is expected that North Sea wind power will reach 60GW”

The offshore wind industry thrives on innovation and new alternative systems for storage are being announced almost every week, but there is one method that stands out above all the others – Power to Gas. However it depends on changing the energy to another dimension.

Power to gas and improved grid connections are the routes needed to make this growth happen, with two routes follow different paths. Power to gas follows the molecular path while the grid follows the electron path, although a combination of both will eventually provide the complete answer.

The gas produced, either on –  or off – shore is hydrogen. On the molecular path there is much discussion on deciding where the path should start. The alternatives start offshore with wind powered electrolysis plants on either a man made offshore island, such as the Dogger Bank island, or on refurbished offshore platforms over depleted natural gas fields. The onshore alternative is to use the electron path to a gas hub onshore where larger electrolysis plants could be more effective. Tests in the Netherlands are planned to see if existing pipelines can be used for transporting hydrogen. The pipes, previously used for the natural gas grid, are to be tested with hydrogen. It is expected that the only problems will occur with flanges at pumping stations. Hydrogen has a finer molecular structure than natural gas and it is possible that the bolted joints will have to be replaced  to stop the escape of gas. This can be easily corrected to give the unused offshore gas pipelines a new purpose transporting the new hydrogen from platforms on the depleted gas fields to the onshore storage facilities. 

Hydrogen is already a marketable feedstock for the chemical industry. The Rotterdam industrial area alone currently uses 350 kilo tonnes of hydrogen per year. As a fuel it is now rapidly becoming established as a source for heating networks in cities. In the Leeds City Gate project in the UK an existing natural gas fuelled network to provide heat is being replaced with a network which is fuelled 100% by hydrogen gas. Rotterdam city transport currently has 2 hydrogen powered buses on trial since autumn 2017. Early in January 2018 the Hyundai Nexo, already their 4^th generation development, was revealed to compete with the Toyota Mirai and Honda Clarity Fuel Cell, hydrogen powered cars where the only emission is water! Finally, there are power plants providing electricity for the onshore grid which are currently fuelled by natural gas that are being tested for conversion to hydrogen fuel. Electricity generation fuelled by natural gas makes up the largest portion in the UK energy mix with usually between 20 and 40 percent, depending on demand and the input from renewables. This could all be fuelled by hydrogen from offshore wind in the future. There is a market for hydrogen!

Storage of hydrogen allows a power generation system that follows the load providing continuity of energy level 24/7. The systems delivers power that follows the load trend, with flexibility and at a relatively low cost. Underground storage caverns has been successfully used in the UK for several years for storing natural gas and other hydrocarbon products. Before being pumped to the caverns underground the hydrogen is pressurised to 270 bars above atmospheric pressure (270 barg) from the 50 bar pressure used for transport in pipelines.

The salt caverns are made by drilling into the salt bed or dome, pumping water into the salt layer and dissolving natural salt in mineral beds, creating empty caverns. There are already more than 30 of the salt caverns in use today in the UK. The largest of these caverns exceeds 600,000m³, and the deepest sometimes more than 2,000m underground. Some are used to store natural gas to provide a ‘strategic reserve’ able to keep the gas turbine generation of electricity fuelled for several days if usual source supplies are interrupted. The construction of a 300,000m³ salt cavern would cost about £200m, most of which is spent on the surface facilities, which is relatively cheap when compared to other elements in the energy supply chain.

“It is possible to store hydrogen underground”

Hydrogen has already been stored underground for a feedstock supply for the chemical industries based around Teesside in North East England where the current need for hydrogen, 161 kilo tonnes per year, is likely to exceed 250 kilo tonnes per year by 2030. The supply of hydrogen for the Leeds City Gate project has an underground buffer storage space in the salt beds under the countryside in Yorkshire north of the port of Hull.

There are similar underground storage caverns in use in the United States and Germany where underground salt layers are also present. The largest of these, is in the U.S.A. where the largest single store holds over 100GWh of hydrogen. There are estimations that 20% of the renewable power surplus in Germany in 2050 will be targeted for conversion into hydrogen. This will require a total of 30 million cubic metres underground storage in 60 salt caverns. There are already about 3 times as many caverns currently in available in Germany.   

Offshore wind farms are commonly described in the press as, for example, the one used for the Hornsea Project One offshore wind farm: ‘Powering 1 million UK homes with green electricity’. With 1.2GW of power generation to be available by 2020, it is currently the world’s largest offshore wind project under construction. Now with Power to Gas they can now add another option and be described as powering electrolysis plants to produce hydrogen for storage; able to provide industry with a feedstock and a fuel. Both of these options will replace unsustainable hydrocarbon fuels and help decarbonise industry.      

This article was previously published in the Offshore WIND magazine, issue 1, 2018.