Renewable energy has its advocates and detractors but the debate is usually framed with reference to a number of key technologies – wind, wave, hydroelectric, tidal and solar power. In the UK, the large-scale adoption of these is viewed as vital to meeting our targets for reducing greenhouse gas emissions, or, if you’re sceptical about climate change, as an expensive mistake that could constitute a chasm in a looming energy gap.
But some companies further afield believe there are other sources of renewable energy that could be exploited by harnessing the power of the oceans and rivers in a different way, and that might offer another option in generating non-fossil-fuel-based electricity.
One of these methods is known as osmotic power, the other as ocean thermal energy conversion (OTEC). Both rely on natural principles in their operation and both are being explored through the development of pilot plants in the US and Europe.
In Norway, renewable energy firm Statkraft opened what it claims is the world’s first osmotic power plant in November 2009 at Tofte, just outside Oslo (see diagram). Osmotic power relies on the natural phenomenon of osmosis, or the passage of water through a semi-permeable membrane – the process by which plants absorb water into their leaves. According to Statkraft, when fresh water meets sea water, the “salinity gradient” between the two has the potential to generate enormous amounts of energy.
Statkraft has been involved in the development of osmotic power for 15 years or so, says Stein Erik Skilhagen, head of osmotic power at the company. The plant at Tofte is viewed as critical as a development project which demonstrates that the concept is viable, he says, although the Tofte plant is only generating tiny amounts of power. “We started working on osmotic power in the mid-1990s.
“There is a large degree of trial and error in developing a pilot plant. Our infrastructure now uses more power than the 2kW generated by the plant – but eventually this will be different. It was very important to prove that the concept works.”
Skilhagen admits that there have been considerable barriers to its adoption, such as the cost of the membranes used in the osmotic process. “We spent a lot of time with the technology,” he says.
“Initially the cost and technology of the membrane was not appropriate. Solar and wind power have been around for years and have had time to mature.
We now expect this process to happen with osmotic power. It is destined to become more prominent.”
The Statkraft plant uses polyimide as a membrane, and is able to produce 1W/m² of membrane. This amount of power is obtained with 10 litres of water flowing through the membrane each second at a pressure of 10bar. Both increasing the pressure as well as the flow rate of the water would make it possible to increase the power output.
Hypothetically, the output could easily be doubled. Skilhagen says incentives are needed to develop the technology with a view to building commercial-scale plant with an output of 1MW or so. “Predictability and longterm planning in the political sphere could provide the right framework for osmotic power to be developed,” he says. “The more predictable the support, the more likely investment is to take place.” Statkraft is currently in discussions about osmotic power with a number of utilities around the world.
OTEC, meanwhile, relies on natural phenomena within the oceans. It’s not salt versus fresh water in this case but rather the difference in temperature between water near the surface and deep water, or the “thermal gradient”, that the technology uses to generate power. It is being developed by Lockheed Martin in the US, which has been looking at exploiting the thermal gradient to generate electricity since the late 1970s.
Rob Varley, of the defence giant’s New Ventures division in Virginia, argues that there is an underexploited resource in the oceans, which are the largest natural collector of solar energy on the planet. He says: “There is a temperature gradient that goes from warm temperatures on the surface down to cold temperatures at the depths. In the tropics, the temperature delta might be as much as 20°C.
“According to thermodynamics, you can use that temperature delta to run a power cycle.”
Lockheed Martin developed its first OTEC plant in the 1970s and used it to generate 50kW of electricity and is currently developing a new pilot plant with funding from the US Navy, which is interested in the potential of OTEC to power its bases. OTEC works by running warm sea water through a heat exchanger, where it vaporises a liquid with a low boiling point, in this case ammonia. The vaporised ammonia is then use to drive a turbine, generating electricity. Colder water from the depths is sucked up through a long 1,000m pipe to recondense the ammonia vapour that has been used to drive the turbine. The ammonia, returned to a liquid state, is then pumped back into the evaporator and vaporised again, so the plant has a closed cycle.
The original OTEC plant was built and mounted on a barge off Hawaii. Any OTEC plant would use a significant amount of power to run. Varley estimates that a 100MW system would use about 40MW to operate on a commercial scale with components such as water pumps requiring a substantial amount of electricity. Roughly 1,000 gallons of water a second would have to pass through the system per megawatt of electricity.
Other uses for the technology are possible. For example, it could be employed to make fresh water in place of desalination plants or to create ammonia and hydrogen. Varley explains: “You could use warm sea water, introduce it into a vacuum chamber and evaporate some percentage of the water and use that water vapour to drive a very low pressure turbine, and condense it back to a liquid using cold sea water. There the side product is fresh water: that’s an open cycle because you’re continuing to bring in new warm water.”
Much like Statkraft, the desire at Lockheed is to convince investors, utilities and governments of the value of the technology so that commercial-scale plant can be built. Varley says: “Our roadmap forward is to field a power plant of sufficient scale so that an investor in a utility-scale plant has confidence in the risk and the cost associated with making that investment.
“For a 100MW plant, the capital cost is going to be in the order of $1 billion-plus. Nobody is going to write a cheque for that kind of money until they see some tangible demonstration.” Both Statkraft and Lockheed are also keeping an eye on the environmental implications of developing osmotic power and OTEC. Varley points out: “We have to do the environmental studies and look at impact on the thermal dynamics.”
OTEC could prove attractive in regions that rely heavily on imported oil and other fossil fuels for power generation, but sufficient temperature deltas would be required for it to work (see map). Hawaii took an interest in developing a system because of its heavy dependence on imports of fossil fuels. OTEC is also attractive because it generates baseload rather than intermittent power unlike some renewables such as wind and solar. Given that the technology has been understood for a considerable amount of time, Varley, unlike his counterpart at Statkraft, believes it has been unfairly passed over. He says: “For whatever reason – it may be politics, part of it may be fossil fuel or nuclear industry competition – OTEC has not been considered as a serious power resource. One of the main reasons back in the 1970s was that the capital cost was very expensive.
“When fossil fuels are cheap it’s cheaper to burn them if you aren’t worried about the environment. But the advantage of OTEC is that fuel costs are zero. Once you get past the capital cost, it should be an attractive power supply for that reason and in terms of energy security.”
The capital costs, although expensive initially, will not require a linear level of increase in costs to increase capacity, says Varley. “One attribute of OTEC is the larger the plant, the cheaper it is per MW,” he says. “That’s because not all the capital equipment scales with capacity. If I want 10 times the amount of power, I don’t need 10 times the amount of platform.” For instance, while it would be necessary to scale up the heat exchangers, the cold water pipe, pumps and turbines would not necessarily have to increase in size.
Varley says that despite the promise of technology such as OTEC he fears that the transition to low-carbon energy will be very difficult. “If you believe in global warming, something has to change,” he says. “My fear is that it’s already too late to transfer from a fossil-fuel-based market to a renewable market without a lot of pain.
“The pain goes by several names. One of those is the price of fuel and the gas price. Europe pays much more for gasoline than the US does. When we see gasoline prices hit $4 a gallon, people will panic.
“But Lockheed is very interested in applying our technology capability to help solve issues that are of national concern. And energy security is one of those.”