Urban Energy Futures
by Frank Zeman
People continue to move into urban areas resulting in more and more of us living in cities that are located near waterways, with many metropolitan areas bordering the ocean. As with Hurricane Sandy, the combination of rising sea levels and powerful storms poses a grave risk to people and their livelihoods.
Sandy’s economic cost to New York and New Jersey alone is estimated at more than $60 billion. Of New York’s share, only $9 billion (one-third) is allocated to preventing damage from future storms. This ratio highlights that it’s likely cheaper to prevent damage from the next storm rather than wait to pay for the clean-up, although the longer the interval between storms, the less valuable the investment. This is further amplified by the fact that economic damage is different from momentary happiness and life satisfaction. Not to be too metaphysical, but as we consider spending these large sums of money, it’s worth gauging their effects. Happiness craters immediately before and just after any such event but tends to bounce back on the order of weeks1, whereas life satisfaction rebounds more slowly but still within a couple of years2. The question then arises: If it takes years to get projects started, has the population moved on, and would the money be better spent elsewhere? Keep in mind that the storm surge in Manhattan was more than 13 feet, meaning that we are talking about large structures.
What does this have to do with energy? One of the major consequences of Sandy was the power failure, which left more than 8 million people in the dark. It’s not hard to see that strong winds will down power lines and that saltwater will spark live electrical outlets. Transformers and gas lines are a larger threat, and were likely behind the Breezy Point fire. These risks harken back to changes in urban energy brought about by the Industrial Revolution. Large coal-fired power stations became a public nuisance and were banished to the countryside, resulting in high-voltage lines into cities downgraded, using transformers and stations, to the block level. These are connected to the panels in our houses. All along these pathways are places to add resiliency to the system as well as opportunities to develop alternatives. Therein lies the question whether to use the money to build large-scale infrastructure (that either works or doesn’t), or spread it around to make more resilient neighborhoods.
The natural instinct is to build a barrier. There are many precedents, such as in London, Rotterdam, and Venice, but the benefits may be less clear. Although a large barrier will work, provided the surge doesn’t exceed its crest and sea-level rise stays slow, it should be noted that a barrier is a static structure that only prevents damage as long as water doesn’t work its way around the system. A barrier also limits the flow of inland water (usually precipitation runoff) from leaving the area. Alternatively, barriers on land, such as a sea wall or dune, are more local alternative. Dunes have the advantage of being cheaper and can be built up over time; they have lower capital costs but require more maintenance. A wetland/dune system was part of a re-design for Manhattan in the wake of Hurricane Irene that included floating neighborhoods3. Walls, on the other hand, must be built at once (higher costs) but allow for the maximum use of space (i.e., right up to the vertical surface). Though dunes are natural and can be nourished over the years, they are functionally inert. A seawall, on the other hand, could be outfitted with generators to provide electricity on most days, and more importantly in the aftermath of any storm. In particular, vertical axis wind turbines and wave generators could be built into the structure.
Large 500-foot-high wind turbines are not practical for boardwalks, but smaller vertical-axis turbines (spinning axis perpendicular to the earths surface) can be placed at lower elevations, e.g. on lampposts. Such a deployment would not be the most efficient but would spread the risk, both economically and meteorologically. Multiple smaller devices can be deployed over a longer time horizon with a higher fraction expected to survive storm damage. The turbines are very scalable, from the 100-watt range up to the kilowatt scale. The main advantage of the vertical-axis turbine is rapid response to changing wind directions. Given that the blades rotate 360 degrees, this may seem counterintuitive. However, the important direction is not the wind but rather the relative wind—that is, the vector addition of the wind and blade velocities. Provided the tip speed ratio (tip speed/wind speed) is more than three, the direction of the relative wind varies only slightly during rotation. In the event of a power outage after a storm, battery packs could be connected to these devices in order to power pumps, shelters, kitchens, and other critical functions. At other times, the energy generated could be used locally to reduce blackout risk.
Wave energy might also be incorporated into the design. This would similarly provide power during the interim periods as well as emergency power in a storm’s aftermath. Energy can also be generated as the floodwaters recede, provided they are channeled through the generator. Wave energy systems operate on the elevation difference between the crest and trough of waves. There are two basic mechanisms to extract this energy: moving water or air. Systems based on water are analogous to hydroelectric power, with the energy of the wave used to raise the water level of a small reservoir adjacent to the seawall. Often, a converging, inclined channel is used to provide additional energy beyond the average wave height. The water is then returned to the sea via a low head turbine, which generates the power. The air-based systems are more analogous to wind energy, with the structure used to create a confined space at the seawall. As the wave rises and falls, the air space shrinks and expands, pushing air in and out of a ventilation duct. A wind turbine in the duct generates energy from flow in both directions (e.g. the LIMPET device). The challenge with this device would be around existing sea level, and not at the storm surge height. These devices could be mounted on posts and raised along with sea level, which would also ease maintenance. One advantage of the wave system is that the seawall design and path could be planned in such a way as to optimize wave production at suitable locations, much like refraction tends to concentrate wave energy at coastal headlands.
The large-scale defense systems, such as the Thames Barrier, have not really been discussed because the benefits have yet to be borne out and the costs are large. The barrier must remain open the majority of the time for commercial reasons, and therefore water levels behind it will rise along with sea level. And while a storm rages, all the associated precipitation is trapped behind it. It also impedes navigation immediately after the storm. More simply, a large sea barrier is an attempt to fight nature with likely predictable results. It would be better to go with the flow, rather than try to turn back the tide.
Sandy has similarities to the Tohoku earthquake and tsunami that struck Japan in March 2011. The much larger devastation caused by that event, analogous to the Breezy Point fire, led to a reevaluation of housing. The result was presented at the 2012 architectural biennale in Venice at Japan House. Architects considered many solutions and settled on a vertical structure rooted on a dozen vertical pillars (imagine telephone poles). The overall structure contained multilevel indoor and outdoor spaces woven in and around the pillars. These provide many different living spaces but also safety features in a storm, the first of which is access to elevated outdoor areas for emergencies (a danger after Katrina) and enclosed areas above floodwaters. The structure also has two separate functions on the ground floor: the pillars that support the house and the walls that contain the living spaces. In the event of flooding, destruction of the walls would not affect the support structure, and people above would be safe. In this manner the ground flood could be readily remediated and reconstructed while living continues. Add to this modifications as elevated electrical connections and renewable energy systems (e.g., solar photovoltaic roof tiles) to provide a 21st-century home that will work with nature and is equipped to deal with the increasing number of extreme events.
The future is unknown and increasingly unpredictable. Resilient infrastructure would benefit society between storms, whereas megaprojects for every coastal city or rebuilding after every storm would eventually crush the economy. Much of our existing infrastructure needs replacing, and creating distributed renewable energy networks in dense urban areas will help alleviate blackout risks as well as aid recovery from any such events. In the end, there’s no reason to build walls that Mother Nature will eventually get around when housing and urban areas in general need to be upgraded. This may mean that it is more economical to abandon some areas (meaning provide no flood insurance) than to build a seawall.
Posted on: February 1st, 2013