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Friday, February 28, 2014

Harnessing the wind to fight the storms of climate change

How Offshore wind turbines can win a david and goliath battle with hurricanes. Here is a lateral idea: build large offshore wind farms that provide electricity from day to day, but also substantially decrease damaging hurricane or tropical cyclone wind speed and storm surge.

With tropical cyclones forecast to grow stronger and more intense with climate change, if not in greater frequency, such an idea has definite merit to explore. Climate change is predicted to escalate Tropical Cyclone / hurricane damage costs particularly for US and China.

We wouldn't need to put offshore wind farms right along the coast, perhaps just where they can be most effective at lessening storm damage to human lives and infrastructure. And allow us to plant mangroves and cultivate coastal wetlands to store carbon, and provide fish nursery habitat, and absorb storm surges, rather than hugely expensive rock and concrete sea walls.

Professors Mark Z. Jacobson, Cristina L. Archer and Willett Kempton have done the modelling to say that offshore wind farms could play a major role in reducing storm winds and storm surge. Their paper - Taming hurricanes with arrays of offshore wind turbines, published in Nature Climate Change on 26 February 2014, argues that large turbine arrays in the order of 300+ GW installed capacity may diminish near surface wind speeds by 25 to 41 metres per second (56-92 mph) and storm surge by 6 to 79 per cent.

The study abstract says:

Benefits occur whether turbine arrays are placed immediately upstream of a city or along an expanse of coastline. The reduction in wind speed due to large arrays increases the probability of survival of even present turbine designs. The net cost of turbine arrays (capital plus operation cost less cost reduction from electricity generation and from health, climate, and hurricane damage avoidance) is estimated to be less than today’s fossil fuel electricity generation net cost in these regions and less than the net cost of sea walls used solely to avoid storm surge damage.

The study modelled US hurricanes Katrina which devastated New Orleans in 2005, Isaac which hit Lousianna in 2012, and Superstorm Sandy which hit New York in 2012. The modelling showed that if offshore wind turbines had been deployed peak wind speeds and storm surge would have been decreased. For Hurricane Katrina wind speeds were simulated to decrease by 36-44 meters per second (between 80 and 98 mph) and the storm surge to decrease by up to 79 percent.

For Hurricane Sandy the modelling simulation showed a wind speed reduction by 35-39 meters per second (between 78 and 87 mph) and as much as 34 percent decrease in storm surge. Offshore wind farms may have drastically reduced the damage to New York and New Jersey. New York Mayor launched a $20 billion climate adaptation strategy in June 2013. This study raises the issue that perhaps offshore wind farm arrays should also be considered as part of coastal defence and energy supply for New York.

“The little turbines can fight back the beast,” said study co-author Cristina Archer, associate professor in the University of Delaware’s College of Earth, Ocean, and Environment.

Co-author, Professor Mark Jacobson from Stanford University School of Engineering commented: "We found that when wind turbines are present, they slow down the outer rotation winds of a hurricane," Jacobson said. "This feeds back to decrease wave height, which reduces movement of air toward the center of the hurricane, increasing the central pressure, which in turn slows the winds of the entire hurricane and dissipates it faster."

In the simulation as the hurricane approaches a wind farm, energy is removed from the storm’s leading edge reducing the wind speed. This percolates from the perimeter to the eye effectively reducing the storms ferocity and wind speed. As the turbines reduce wind speeds, that also results in reducing the height of ocean waves, thereby reducing the winds that push water toward the coast as storm surge. “There is a feedback into the hurricane that is really fascinating to examine,” said Archer, an expert in both meteorology and engineering.

Watch Professor Mark Jacobson on Youtube explaining the computer modelling simulations for this study


Cost effective power and hurricane mitigation

The wind turbines would effectively pay for themselves over time with avoidance of hurricane damage and the provision of carbon free electricity reducing greenhouse gas emissions. Hurricane Sandy damage bill was in the region of $82 billion spread across three states.

Alternative forms of protection from storm surge are already being considered, such as massive seawalls and barriers. But these also come at a significant cost of $10 billion to $40 billion per installation. These sea walls while effective against storm surges, do little in combatting the damaging force of storm winds. While it may be cheaper to build seawalls, they do not provide the added benefits of reduced pollution, low carbon electricity provision, and reduction in damage from storm winds as well as long term climate mitigation.

The Conversation article detailed the potential costs per kilowatt per hour:

At a cost of US9.4c per kilowatt per hour, wind turbines avoided up to US0.68c per kilowatt per hour of damage in New Orleans, and US0.13c per kilowatt per hour along the US eastern coast. When other costs were factored in, building extra wind turbines cost US4c per kilowatt per hour — cheaper than costs for fossil fuels.

“This is a paradigm shift,” Professor Willett Kempton said. “We always think about hurricanes and wind turbines as incompatible. But we find that in large arrays, wind turbines have some ability to protect both themselves and coastal communities, from the strongest winds.”

Is it achievable?

The modelling involved wind farms containing hundreds of thousands of turbines (up to 543,000) which may not be practical. But even in reduced numbers of say 20,000 to 40,000 turbines, moderate to high reduction in wind speed and storm surge may be effective. But there is a long way to go to achieve these numbers. The largest offshore wind farm at present is the 175 turbine London array.

Wind turbines also have a high speed operational limit of about 112 mph. This is roughly the equivalent of a category 2 or 3 hurricane.

Dr Nigel Martin at the Australian National University, who has studied renewable energy in Queensland, said wind turbine maximum speed is likely to be the major limiting factor.

Martin also raised that there may be considerable social and political hurdles to building such large wind farms in places like the Queensland coast where aesthetics, tourism, and indigenous values may hinder such a development. Those same aesthetic, tourism and indigenous values haven't managed to stop the construction of port infrastructure to make the Queensland coast a major shipping highway for the export of coal and Natural gas driving the destruction of the Great Barrier Reef and causing further climate change.

Jacobson has already addressed Martin's comments that the presence of massive turbine arrays would prevent hurricanes from reaching speeds whereby they would damage the turbines. But the possibility is open for higher engineering specifications of wind turbines to be increased for stronger wind speed.

Building such large offshore wind farms may prove viable in defending large population centres as well as providing low carbon power. But it should probably take place in conjunction with a range of adaptation measures. Boosting mangrove and coastal wetland environments is a low cost environmentally pro-active storm surge defence of coastal land communities that provides a host of environmental ecosystem services. And seawall barrier defences certainly have a place in defending city infrastructure and lives.

Jacobson also co-authored a study in 2012 that showed there was more than enough wind energy to power our world. In 2011 he advised that based upon research on the urban heat island effect it was more effective to install solar panels on roofs than to paint them white.

Sources:

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