The Power of Salt

Cambridge, Mass. -- Where the river meets the sea, there is the potential to harness a significant amount of renewable energy, according to a team of mechanical engineers at MIT.

Pressure retarded osmosis (PRO) is a method of producing renewable energy from two streams of a different salinity. Illustration: Jose-Luis Olivares/MIT.The researchers evaluated an emerging method of power generation called pressure retarded osmosis (PRO), in which two streams of different salinity are mixed to produce energy. In principle, a PRO system would take in river water and seawater on either side of a semi-permeable membrane. Through osmosis, water from the less-salty stream would cross the membrane to a pre-pressurized saltier side, creating a flow that can be sent through a turbine to recover power.

The MIT team has now developed a model to evaluate the performance and optimal dimensions of large PRO systems. In general, the researchers found that the larger a system’s membrane, the more power can be produced — but only up to a point. Interestingly, 95 percent of a system’s maximum power output can be generated using only half or less of the maximum membrane area.

Leonardo Banchik, a graduate student in MIT’s Department of Mechanical Engineering, says reducing the size of the membrane needed to generate power would, in turn, lower much of the upfront cost of building a PRO plant.

“People have been trying to figure out whether these systems would be viable at the intersection between the river and the sea,” Banchik says. “You can save money if you identify the membrane area beyond which there are rapidly diminishing returns.”

Banchik and his colleagues were also able to estimate the maximum amount of power produced, given the salt concentrations of two streams: The greater the ratio of salinities, the more power can be generated. For example, they found that a mix of brine, a byproduct of desalination, and treated waste water can produce twice as much power as a combination of seawater and river water.

n this simplified PRO system, permeate from a dilute feed stream enters a concentrated draw stream in a pressurized state via osmosis — after which useful power can be extracted from the draw-permeate mixture. Image: Leonardo Banchik

Based on his calculations, Banchik says that a PRO system could potentially power a coastal wastewater-treatment plant by taking in seawater and combining it with treated waste water to produce renewable energy.

“Here in Boston Harbor, at the Deer Island Waste Water Treatment Plant, where waste water meets the sea … PRO could theoretically supply all of the power required for treatment,” Banchik says.

He and John Lienhard, the Abdul Latif Jameel Professor of Water and Food at MIT, along with Mostafa Sharqawy of King Fahd University of Petroleum and Minerals in Saudi Arabia, report their results in the Journal of Membrane Science.

Finding Equilibrium in Nature

The team based its model on a simplified PRO system in which a large semi-permeable membrane divides a long rectangular tank. One side of the tank takes in pressurized salty seawater, while the other side takes in river water or wastewater. Through osmosis, the membrane lets through water, but not salt. As a result, freshwater is drawn through the membrane to balance the saltier side.

 

“Nature wants to find an equilibrium between these two streams,” Banchik explains.

As the freshwater enters the saltier side, it becomes pressurized while increasing the flow rate of the stream on the salty side of the membrane. This pressurized mixture exits the tank, and a turbine recovers energy from this flow.  

Banchik says that while others have modeled the power potential of PRO systems, these models are mostly valid for laboratory-scale systems that incorporate “coupon-sized” membranes. Such models assume that the salinity and flow of incoming streams is constant along a membrane. Given such stable conditions, these models predict a linear relationship: the bigger the membrane, the more power generated.

But in flowing through a system as large as a power plant, Banchik says, the streams’ salinity and flux will naturally change. To account for this variability, he and his colleagues developed a model based on an analogy with heat exchangers.

“Just as the radiator in your car exchanges heat between the air and a coolant, this system exchanges mass, or water, across a membrane,” Banchik says. “There’s a method in literature used for sizing heat exchangers, and we borrowed from that idea.”

The researchers came up with a model with which they could analyze a wide range of values for membrane size, permeability, and flow rate. With this model, they observed a nonlinear relationship between power and membrane size for large systems. Instead, as the area of a membrane increases, the power generated increases to a point, after which it gradually levels off. While a system may be able to produce the maximum amount of power at a certain membrane size, it could also produce 95 percent of the power with a membrane half as large.

Shown here is the maximum power that can be produced for a 4:1 seawater to river water combination. As the dimensionless area gets very large, the overall maximum power can be produced. Image: Leonardo Banchik

Still, if PRO systems were to supply power to Boston’s Deer Island treatment plant, the size of a plant’s membrane would be substantial — at least 2.5 million square meters, which Banchik notes is the membrane area of the largest operating reverse osmosis plant in the world.

“Even though this seems like a lot, clever people are figuring out how to pack a lot of membrane into a small volume,” Banchik says. “For example, some configurations are spiral-wound, with flat sheets rolled up like paper towels around a central tube. It’s still an active area of research to figure out what the modules would look like.”

“Say we’re in a place that could really use desalinated water, like California, which is going through a terrible drought,” Banchik adds. “They’re building a desalination plant that would sit right at the sea, which would take in seawater and give Californians water to drink. It would also produce a saltier brine, which you could mix with wastewater to produce power. More research needs to be done to see whether it can be economically viable, but the science is sound.”

This work was funded by the King Fahd University of Petroleum and Minerals through the Center for Clean Water and Clean Energy and by the National Science Foundation.

New Software Modeling Tool a Boon for Wind Industry

Wind energy is blowing away skeptics — it's so close to achieving cost parity with fossil fuels that just a little extra efficiency is all that is likely needed to push it into the mainstream and past the Energy Department's goal of 20 percent wind energy by 2030.

Wind energy is blowing away skeptics — it's so close to achieving cost parity with fossil fuels that just a little extra efficiency is all that is likely needed to push it into the mainstream and past the Energy Department's goal of 20 percent wind energy by 2030.

That extra efficiency may be realized with the help of a software tool built by the Energy Department's National Renewable Energy Laboratory (NREL). It's called Simulator for Wind Farm Applications (SOWFA), and it can calculate how undulating ground, whipping blades, surface temperatures, and other variables alter the air flow and energy production at wind farms.

In the Energy Systems Integration Facility (ESIF) Insight Center, NREL Senior Engineer Pat Moriarty, left, and NREL Senior Engineer Paul Fleming review velocity (blue) and turbulence (yellow) in a simulation of the Lillgrund Wind Farm in Denmark. The researchers have written the open-source software tool Simulator for Wind Farm Applications (SOWFA), which can calculate how undulating ground, whipping blades, surface temperatures, and other variables alter the air flow and energy production at wind farms. Photo by Dennis Schroeder, NREL

SOWFA's key innovation is that it simulates an extensive range of scales—from regional weather patterns down to the space between turbines and all the way to the movement of electrons. It gives a complete picture of an active plant, showing how controlling each turbine can influence the direction of the wakes, and detecting the effect on downwind turbines. And it helps researchers understand how local atmospheric conditions affect local wind around the wind farm.

SOWFA is the first tool that enables developers to improve the performance of not only one wind turbine, but the entire wind farm. And because it is an open-source "community" software platform available for free to academics, investors, wind developers, and manufacturers, users have the support of an online forum where they can discuss problems and solutions with other SOWFA users.

One application of SOWFA is the simulation and design of coordinated control of individual turbines in order to maximize plant-wide output. Using these controllers, upstream turbines can yaw their rotors to redirect their wakes away from downstream turbines, substantially improving power capture. Wake losses at wind farms can reduce total power production by 10%. If SOWFA can be used to design controllers that cut those losses in half, that's a huge benefit. If employed at wind farms across the country and the world, the additional revenue could add up to billions of dollars.

SOWFA Simulates Wake Effect for Turbines Downwind

NREL Senior Engineer Pat Moriarty, left, explains a wind turbine model at the ESIF Insight Center to Secretary of Energy Ernest Moniz, right, during a tour for the dedication of the ESIF last year. Photo by Dennis Schroeder, NREL

Across the Great Plains and at many spots offshore, if you see one wind turbine, you're likely to see a dozen or a hundred. A wind farm provides economies of scale—contract with a farmer or win a permit for offshore wind, and it makes sense to erect multiple turbines all tied to a single system.

However, those rows of giant turbines can be problematic. As the turbine's blades are turned by the wind, they also disrupt the wind, causing a wake similar to a calm spot behind a tree or building. If that wake heads straight for the next turbine downstream—maybe 800 meters away—its churn will mean weaker wind and less power produced by that second turbine.

But SOWFA shows turbine manufacturers, wind farm developers, or investors how a yaw can impart a thrust that curves that wake around the downwind turbine. "Wake, from a power perspective, is lower-energy wind," said NREL Senior Engineer Paul Fleming, one of the engineers using SOWFA in his research. "If you can move away that deficit of energy, you will have faster winds and more overall production at the wind farm."

"In the past, wind farms have relied on dissipation to control that energy loss—they just move the turbines farther away from each other," said NREL Senior Engineer Pat Moriarty, a leader of the SOWFA team. "Now, we can control it in a different way. And there are other ways to achieve more control."

SOWFA was created to model all the different variables at a wind farm—the topography, the air, surface and air temperatures, even the effects of the turbines themselves. Other models have looked at pieces of the whole, but now that the wind industry is maturing and the cost margins are decreasing, it's increasingly important to model the whole system and maximize energy production

Turbines and Central Controller Talk to Each Other

NREL Senior Engineer Paul Fleming, left, and NREL Senior Engineer Pat Moriarty review wind farm data in SOWFA. SOWFA can simulate and design coordinated control of individual turbines in order to maximize plant-wide output. Photo by Dennis Schroeder, NREL

With SOWFA, the central super-controller can receive information from individual turbines and send command messages to them. In other words, the central controller and the turbines can talk to each other. SOWFA's central controller can send a message asking a turbine to adjust its yaw—but the turbine can override that command if conditions at the turbine are such that adjusting the yaw would be unsafe. This architecture is designed to mimic as closely as possible how wind plant controls might be implemented at a real wind farm.

Another reason SOWFA is unique among wind modeling tools is that it has the capacity to incorporate the heat on the earth's surface into its calculations. The sun is the main reason that air moves. It causes different air temperatures and pressures, coaxing the wind in a particular direction at a particular speed and at different heights above the ground. The varying temperature of the ground and the hills and vales of the farm also influence how the wind moves through the atmosphere. In the daytime, when the ground is hot, the wind tends to be more turbulent, rising from the surface. At night, when the ground is cooler than the air, the wind becomes more horizontal and speeds up. The same rules apply offshore, except instead of the ground there is the water surface, and instead of hills and vales, there are the size and motion of the waves.

The software tool also can deliver the numbers to show a possible overall net benefit if that first row of turbines reduces the capture of wind so the second row can capture more.

Reducing Uncertainty Can Lower Financial Risk, Interest Rates

SOWFA has drawn acute interest from manufacturers, developers, and utilities. "One of the most important outputs of SOWFA is the reduced uncertainty," Moriarty said. "The finance community cares not just about energy production, but about the uncertainty of energy production over the next 20 years that wind farm would be operating. They particularly want to know what the worst-case energy production can be."

SOWFA's improved look at physics will help the community better understand the uncertainty gap between the average energy production and the worst-case energy production. That, in turn, should lower the interest rates for financing a wind farm, which can be a huge part of the total cost.

Supercomputers Make the Modeling Possible

Supercomputers made SOWFA possible, said Moriarty, noting that he and his NREL colleagues started work on the project in 2005, but that its success accelerated as computers reached the ability to do trillions of calculations per second. To accurately predict the motions of wind at a wind farm, computers need to simulate the dynamics in the space occupied by up to a hundred turbines — and model the flows, counterflows, cross flow, and temperatures in every square meter.

Of course, the goal is to arrive at the most accurate forecasts of wind energy production before the turbines are erected. Moriarty, Fleming, and the rest of the team use NREL's Peregrine high performance computer to simulate the movement of air through rows of turbines.

Understanding the dynamics of a wind farm means analyzing the effects of everything from clouds to rises and dips in the terrain, and even the height of surrounding  crops.

Researchers can use SOWFA to study how new control schemes can increase power production and reduce fatigue from wake effects, and developers can use SOWFA to move turbines like chessmen on a chess board to maximize output — all before construction begins.

The Energy Department, which provided funding for the SOWFA project, is in the process of developing experiments to test that the modeling predicted by SOWFA matches real-life conditions and energy output at the wind farms.

Offshore Wind Power Can Save US Billions On Electricity, Recent DOE Study Finds

Offshore wind power isn’t usually associated with lower-cost energy, at least not in the public imagination. But it turns out that installing 54 gigawatts of offshore wind power off America’s coasts can cut the cost of electricity in the U.S. by an astounding $7.68 billion a year. That’s right: $7.68 billion annually. (If utilities and grid operators pass those savings onto consumers, that’s about $100 a year per family of four.)

As the American Wind Energy Association’s Offshore WindPower 2014 conference begins today in Atlantic City, that’s just some of the good news to report about pollution-free, offshore wind power. This news is out in a recent U.S. Department of Energy study that proves offshore wind power’s potential in the U.S. is far more than theoretical: The National Offshore Wind Energy Grid Interconnection Study.

The study itself was designed to assess technical capacity and the possibility that offshore wind power can become a major component of the U.S. energy system — “to identify and help address the market barriers to the large-scale introduction of offshore wind energy into the U.S. energy portfolio,” its authors say. Just as other experts have noted, that potential is simply waiting to be realized, with about a dozen U.S. projects in some stage of development. The right state and federal policies can help move these projects off of their drawing boards and into the water, the study authors say, and with that, help create a promising clean-energy future for all of us. 

To begin with, there isn’t just 54 gigawatts of cost-effective, accessible wind power potential within 50 miles of U.S. coastlines. There’s more than 134 gigawatts of potential at 209 sites, the NOWEGIS authors conclude. The Atlantic coast, all on its own, has enormous wind resources. And there is substantial potential in the Gulf of Mexico, the Great Lakes, and the Pacific, too. In scouting these areas, the authors were sure to exclude important habitats and marine sanctuaries. (NRDC continues to work with both the U.S. Department of the Interior’s Bureau of Ocean Energy Management and with offshore wind power developers to ensure that one environmental good (pollution-free wind power) doesn’t come at the expense of another (important ocean wildlife and habitat protections.))

More good news from the study: Thanks to the popularity of offshore wind power in both Europe and Asia, the technology is evolving fast, meaning its becoming more powerful and less expensive simultaneously.

The report confirms what other researchers have discovered: Offshore wind power can be an especially important resource for densely populated coastal areas, like the Northeast, the Mid-Atlantic, and northern California, where energy prices are high and land available for generation and transmission is generally limited. All of this means that offshore wind power can help meet peak power needs while reducing grid congestion and fighting the risk of blackouts, especially on hot summer days and cold winter nights when energy demand is highest.

State and federal policies will be key to advancing this promising industry, the study found. State renewable energy standards and other state policies, particularly those that require that a certain percentage of a state’s electricity come from offshore wind power, are essential to making offshore wind power a reality here in the U.S. And the report praises innovations from both the DOI and the DOE — including BOEM’s “Smart from the Start” siting program that can protect vulnerable habitats while expediting permitting and leasing, and the DOE’s Offshore Wind Innovation and Demonstration Initiative, designed  to advance offshore wind power technology, remove market barriers, and demonstrate advanced technologies.

Federal tax policies will be pivotal, too. Conventional, fossil-fuel-powered electricity is highly subsidized by the federal government, NOWEGIS’s authors note. “Policy makers will need to determine whether offshore wind should likewise receive tax credit support to encourage its development….Enacting tax credit support that will extend through 2020 will help support the first generation of offshore wind projects.”

Of particular importance is extension of the now-expired Investment Tax Credit for Offshore Wind Power. It’s due for Congressional consideration as part of a package of so-called tax extenders during the post-elections lame duck session this November. It’s time this and other clean-energy provisions pass.

As an advocate, I’ve promoted many of offshore wind power’s benefits: It can provide reliable, pollution-free energy to the coastal states where a full three-quarters of America’s population lives. It can create tens of thousands of good-paying, permanent jobs. Now, the DOE’s NOWEGIS highlights the substantial amount of money offshore wind power can save all of us — if we enact policies that jumpstart this promising industry now. 

Renewable Energy Storage Gains Critical Mass

A host of U.S. renewable energy storage executives recently gathered here in the Silicon Valley are referring to the rapid expansion of the storage industry by comparing it to the solar power industry six or eight years ago, before that companion sector became the juggernaut it is now.

These storage advocates were assembled for the Energy Storage North America exposition, which doubled in size compared with last year, according to conference chairperson Janice Lin. Also co-founder of the California Energy Storage Association, Lin said that “energy storage is a game changer for the electric power system, and this year’s ESNA event truly represents that.” 

Storage Belatedly Following PV Growth Curve

A number of investors in energy storage touted the wisdom of their bets. “Energy storage is one of the holy grails of renewable energy; the main issue is that costs still have to come down,” commented Andrew Chung, a partner at the venture capital firm Khosla Ventures, based in Menlo Park. “Energy storage wasn’t invented for grid-scale applications, but with it getting acceptance in solar, more states and nations will start to move toward implementing policy,” he said. Khosla lists six energy storage start ups within its portfolio now.

Similarly, Peter Rive, the co-founder of SolarCity said, “In three to five years, storage will be a standard component of residential solar power, not optional, but deployed with every solar system.” He continued, “It won’t look that different to the customer, but the fundamental value proposition will be cleaner and cheaper.” Rive also said that electric vehicle (EV) charging capability was likely to become a feature of the energy storage rollout.

Standout Projects Raise Storage Profile

Among standout energy storage projects highlighted at the show was the City of Santa Clara’s Levi Stadium, which features over 1,000 solar panels and six electric vehicle charging stations. The stadium also has installed Green Charge Networks' GreenStationsystem to help curb its power demand and lower electric bills, which spike during certain hours on game days, compared with minimal power and energy consumption throughout the remainder of the year.

Developers like AES also are building a global network of pioneering energy storage projects associated with renewable energy. “We now have 200 MW of energy storage online, mostly in the United States and Chile, including four utility scale plants built, and 2,000 MW on order,” said John Zahurancik, the president of AES Energy Storage, based in Arlington, Va. “There is a demand for 30 GW of new peaker plants by 2024, and by that year, battery production capacity will exceed the peaker need by a factor of four,” he added, suggesting energy storage would be well positioned to compete.

Solar panel manufacturers now moving into energy storage also will drive adoption more rapidly. Kyocera, for example, recently has forged new strategic relationships with both Stem, the energy storage developers, and with Healthy Planet Partners, a clean energy solutions fund that finances, implements and maintains distributed energy and energy. Many energy storage companies have begun offering financing through their own balance sheets or through such partnerships with financial companies. 

Ontario a North American Storage Leader

Apart from the now well-know U.S. state programs in California and Hawaii, the current energy storage initiative for the Canadian Province of Ontario is “to purchase 35 MW of varying storage at a rough cost of $42 million, “which will show up in couple of years,” says Kim Warren, the vice president of operations for the Independent Electricity System Operator (IESO) of the province, based in Toronto.

“We received 431 applications for 12 projects, with differing technologies, and we are doing a three to five year study of the combined storage system to see how storage will value, like California is doing; we may even conduct it in consultation with California,” Warren said. Describing the limitations of pumped hydro storage, and provincial plans to shut down 13,000 MW of nuclear power within five years, He suggested that wind and solar fit the ISO’s needs better. “We already have 9,000 MW of wind and solar contracted,” he said.

The Ontario IESO is studying the use of renewable energy storage not only for the primary grid, but for microgrids and remote off-grid locations, which abound in Canada. “We have purposely put renewables and storage in both congested and uncongested areas,” Warren said. “It is inevitable that as we move toward microgrids, especially with solar parity, we will need more intelligent conversation between the grid operators and the microgrids. The regulators will have to decide who pays for what,” he said.

Does the Psychological Effect of Oil Prices Impact the Solar Industry?

NEW YORK -- Plunging oil prices are shifting investor sentiment away from renewable power and helping pull down shares for solar companies, said Gordon Johnson, an analyst at Axiom Capital Management in New York.

Oil prices have plunged as growing production and slowing demand trigger concern of a global oversupply. The Bloomberg Global Large Solar Index of 21 companies has slipped 25 percent over the past month, more than triple the 7.4 percent decline in the Nasdaq Composite Index over the same period.

“Countries that aggressively expand solar have significantly higher costs than their neighbors” for energy, said Johnson, who has sell ratings on five of the six solar manufacturers he covers.

Solar power doesn’t compete with oil as a source of electricity except on small islands and in some crude exporting countries. Oil generates less than 1 percent of U.S. power supplies and 5 percent globally.

That link may be more emotional than based on fundamentals, said Josh Baribeau, an analyst at Canaccord Genuity Inc. in Boston.

“We believe that part of the solar sell-off may also be driven by the fall in oil prices,” he said in a note to clients on Oct. 14. “We have never liked the psychological correlation between solar stocks and oil, but it exists to some degree.”

All but two companies in the solar index have declined in the past month. Yingli Green Energy Holding Co., the biggest panel maker, slipped 17 percent and Trina Solar Ltd., the No. 2 producer, declined 24 percent. The index rebounded to gain 2 percent at the close yesterday in New York.

“A drop in oil doesn’t affect our space,” David Crane, chief executive officer of NRG Energy Inc., said yesterday in an interview. This year, the largest U.S. independent power producer acquired three solar power supply companies.

European Commission Studies Renewable Costs

LONDON -- New analysis commissioned by the European Commission concludes that renewables such as onshore wind and solar PV are close to that of conventional technologies like coal, gas and nuclear in terms of costs a subsidy levels.

Furthermore, by including external factors, which are not currently not reflected in market prices, such as climate and health costs, renewables are comparable if not even.

Interim findings from the Ecofys study commissioned by the EC deliver the first full dataset on costs per megawatt-hour (MWh) as well as subsidies for all forms of electricity generation. According to this analysis the total value of public interventions in energy, aside from transport fuels, was between €120-140 billion in 2012 for the 28 countries of the European Union.

“Unsurprisingly, given the efforts to expand the share of renewable energy in the EU’s overall energy consumption, the largest amounts of current public support in 2012 went to renewables,” said the Commission, noting that in 2012 subsidies for solar power were €14.7 billion, onshore wind €10.1 billion, biomass €8.3 billion and hydropower at €5.2 billion.

Conventional power generation technologies received subsidies of a similar order though, with €10.1 billion for coal, nuclear at €7 billion and natural gas at € 5.2 billion.

Furthermore, the Commission notes, these subsidy figures do not reflect the free allocation of emission certificates nor tax support for energy consumption.  Including these elements would inevitably narrow the subsidy gap still further.

Presenting figures on levelized costs per MWh, electricity from coal is around €75, onshore wind is marginally higher while nuclear and natural gas are around €100. Solar power costs have fallen to about €100-€115 depending on the size of the specific installation.

In addition, the commission estimates the impact of external costs at between €150 and €310 billion, a figure which if included, say renewable energy trade groups, would put such technologies economically more beneficial than conventional fuels.

European Wind Energy Association (EWEA) analysis of the data in the report puts onshore wind at around €105/MWh and offshore wind at €186, compared with gas at up to €164, nuclear at €133 and coal at €162-€233.

Commenting on the figures Justin Wilkes, EWEA deputy CEO, said: “This report highlights the true cost of Europe's dependence on fossil fuels. Renewables are regularly denigrated for being too expensive and a drain on the taxpayer. Not only does the Commission's report show the alarming cost of coal but it also presents onshore wind as both cheaper and more environmentally-friendly."

He added: "We are heavily subsidizing the dirtiest form of electricity generation while proponents use coal's supposed affordability as a justification for its continued use. The irony is that coal is the most expensive form of energy in the European Union.”

The European Photovoltaic Industry Association (EPIA) noted that the Ecofys study “proves that solar energy is cost effective today, and is improving competitiveness at a rate that conventional technologies will never be able to achieve.”

"Despite decades of heavy subsidies, mature coal and nuclear energy technologies still rely on similar levels of public support as innovative solar energy is getting today. However, support to solar electricity is already coming down, in line with the rapid technology cost reduction, as opposed to coal and nuclear energy, which remain locked into subsidies as they have been for the last 40 years. With its increasing cost-effectiveness, solar is set to overtake conventional technologies in the short term," said Frauke Thies, EPIA Policy Director.