A rarely publicized but promising renewable energy technology that is creating interesting challenges for valve and actuator engineers is ocean thermal energy conversion (OTEC). One of the leaders in the field is OTEC International LLC (OTI) and Barry R. Cole is the company’s Executive Vice President and Director of Technology Development. A veteran of the valve industry, Cole is actively involved in the development, planning and implementation of the intricate system that makes it possible to harness this energy source.
This technology harnesses the temperature difference between warm surface water and deep ocean cold water to power a Rankine cycle and generate electricity. OTI uses a closed-cycle where liquid ammonia is vaporized in a heat exchanger fueled by the warm surface water. The vapor powers turbines that turn generators to produce electricity. The ammonia is cooled back to liquid state using the deep-ocean cold water and the process begins again.
Cole explained. “The surface temperature of the ocean, at the equatorial belt, gets heated up by the sun. The water averages right around 80 degrees, enough to transform a refrigerant like ammonia from a liquid to a gas. The hot surface water passes over a heat exchanger, where ammonia is run through as a working fluid. The ammonia boils, turns into a gas, which is piped to a gas turbine which turns a generator which makes the electricity, which is sent through a power cable to shore.”
The ammonia gas then has to be condensed back into a liquid using cold ocean water, from 3000 feet down. Cole continues, “It’s a very large diameter pipe; for a 100 MW plant about 31 feet. It picks up the cold water which is pumped up and used in the condensers. The heat exchangers transfer that ammonia gas back into a liquid, which is pumped back into the system.
Once it’s started, this system just keeps going. It is low temperature, low pressure, so it doesn’t have the same corrosive effects on the equipment that a high pressure, high temperature fossil fuel plant has.”
OTI uses anhydrous ammonia, pure NH3, which is lighter than air compared to other refrigerants, which are heavier than air. “It disperses rapidly in air and in the water,” said Cole. “The concentration dissipates very rapidly, so when you’re 41/2 miles offshore, the danger to the shore is minimal if any at all.”
Valve engineering genius
However, OTI has gone to great lengths to insure that there are no chances for leaks, and many of the safety features owe their existence and efficiency to the valve industry.
Cole expounded. “A lot has happened in the valve industry, in particular, the triple offset butterfly valve. That was a stroke of engineering genius, resulting from geometry that was enabled by computer modeling. When you see that work and see how little space that takes and how much better it performs than a ball or gate valve which needs 3 to 7 times the space and weight requirements, it really is incredible.”
As can be imagined, there have been many engineering challenges for OTEC, but Cole was pleased to report that they are having great successes, in particular, with the heat exchangers that have been created specifically for OTI’s projects, initially for Hawaii and Grand Cayman. “The design we’ve done for the heat exchanger is innovative, and we’re very pleased with the performance. Not only do they perform 2.5 to 3 times better than what is commercially available, and therefore perfect for our use, we know that there is a market for them outside of OTEC power projects.”
OTI’s heat exchanger extracts the energy from the sea water and puts it via the evaporators into the working fluid, the ammonia. Cole explained. “The two working fluids are controlled by valves. The design has multiple chambers in the heat exchanger, and they can be isolated with water and ammonia shut off valves for each of the chambers.
“We’re working with some companies to design very efficient valves to minimize the parasitic pumping. These valves are a very promising technology for us. These triple offset butterfly valves are 100% seating, so they shut off positively, which is what we need for ammonia. With seawater we could tolerate leakage, but absolutely not with the ammonia.”
With respect to actuation, the speed, timing and sizing is still being worked on by OTI’s engineers. “They need to size everything very carefully so that we don’t get water hammer effects, which could be quite damaging.”
OTI has been chosen for a project at the Natural Energy Laboratory in Hawaii (NELHA) where they are involved in development of a pilot plant that will use approximately $2 million worth of valves. “The OTEC plant will be in every way possible an emulation of what we will put out at sea. The heat exchanger towers will be segmented into two chambers to demonstrate the operations and maintenance concepts, shut the water and ammonia off on one side and keep the other side going.
The issue of ammonia leaks has been carefully considered in planning a facility at NELHA that also supports mariculture operations. While algae and seaweed love it, an ammonia leak could be detrimental to animal life. “That’s why it’s absolutely critical to have the best shutoff valves, and actuation is very important,” said Cole. “We have a very elaborate system of valves and actuators and monitoring systems to monitor any ammonia leakage that might occur and then actuate the valves to divert water around the facility so that there’s no danger to the other tenants.”
“With respect to water exchange, all of the valves don’t need to be as elaborate as the triple offset butterflies; that’s a bit of overkill. But seawater is quite corrosive so 316L is the preferred alloy,” said Cole. OTEC has also discussed plastic valve bodies because they’re not as subject to corrosion and there are already some in existing systems. The company has been talking to valve suppliers and is looking at options.
Ocean thermal energy conversion is a promising technology, and valve, actuation and control engineers can take great pride in knowing they have played an important part in its future.