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Out of Gas? Refuel with Mist lift Ocean Thermal Energy

by Stuart Ridgway 19 April 2005

With the vigorous recent increase in crude oil prices the old predictions of the exhaustion of the worlds supply of oil are beginning to gain some credibility and attention. The cries of wolf, wolf are becoming quite a chorus. The wolf may not be at the door, but he is getting closer. Hubbert's peak, the time when oil consumption exceeds production, seems to be here, or will be in the next five years or so.

We have several problems; our motor vehicle fleet uses too much fuel, and, much of this fuel is imported from a politically unstable region.

We [the US and western Europe] are now at war in an attempt to stabilize our fuel supply. We hope for success in this endeavor. If we could get along with less fuel, it would be a great help. Much money is presently being committed to achieving this improvement. We hear fuel cells, electric cars, gas-electric hybrids, hydrogen economy, and et cetera. New technology is hopefully to be developed to mitigate the problems.

As the gap between supply and demand widens more and more attempts to manage the problem will be developed. Will motor fuel be rationed, either by price, or perceived need? In WW II fuel deliveries to the Northeast from Texas were much reduced by German submarines torpedoing tankers enroute. The average motorist was restricted to 3 gallons a week.

The arab oil embargo of 1973-74 emptied the Los Angeles freeways, low speed limits were installed on the Interstate highway system, and long queues for fuel formed at the gas stations. There was a rush to buy locking lids for ones fuel tanks to frustrate the siphoners.

But then the shortage was political, and OPEC opened the spigots before the developement of energy alternatives got too far along.

We will not escape so easily the consequences of a shortage that is geologically imposed.

Airplanes are fuel hogs. Today's fuel prices ore forcing some airlines into bankruptcy. When fuel prices double again air travel will become an upper class luxury, and buses and trains will be pulled back into service.

What to do? Reduce consumption. Legislate required vehicle fuel economy. Burn more coal and suffer dirtier air and more Global Warming greenhouse gas carbon dioxide. More nuclear power plants. Double daylight saving time. Crisis delayed a decade.

Ocean Thermal Energy Conversion

Yet there is a potential resource that has been known for a century, Ocean Thermal Energy Conversion (OTEC).

The tropical oceans of the world are an enormous energy resource. Their surface water is a heat source typically at 25 to 27 °C, and the kilometer deep water below is available for a heat sink at a temperature of about 5 °C. Various attempts have been made to develop low cost heat engines that can exploit this small temperature differential to provide useful mechanical energy.

There was a substantial program in the 70's and early 80's to develop the technology to exploit this resource, but cheap oil returned. Anticipated difficulties in delivering the power to distant markets, and discouragingly high estimated capital costs of the machinery caused a termination of most of the work.

Now that the cheap oil is on the way out it's time to resume a national effort to make OTEC real and practical, and thus advance toward the goal of energy independence.

Today there is cause for increased interest in the OTEC concept. The resource is "renewable". OTEC power is most environmentally benign, emitting no pollutants.

The world has hesitated to press onward with the use of this resource. First, the costs of the conversion machinery have seemed too high. Second, the parts of the world most endowed with the resource have much smaller energy demands, and are poorer, and less able to pay for a highly capital intensive energy conversion system.

OTEC heat engine candidates

The possible efficiency of such heat engines is limited by the laws of thermodynamics to the ratio of the temperature difference across the cycle (typically 20 °C) to the absolute temperature of the heat source (which is about 300 K). This allows a maximum possible efficiency of about 6.7 percent. Practical considerations of equipment efficiencies, temperature drops required to drive the heat transfer, and power to drive the necessary auxiliaries reduce the thermal efficiency practically obtained to 3 percent.

One might hope that, since the heat resource is "free", such a low efficiency does not matter. But the necessary consequence of a low thermal efficiency is that a large amount of heat must be processed by the engine for each unit of useful work developed. The French physicist Jaques D'Arsonval suggested the use of this resource over a hundred years ago.

The Rankine (closed) cycle engines he suggested use a typical refrigerant such as ammonia as a working fluid. It is boiled by heat from the warm water, its vapor passed through a turbine accomplishing the desired work of the cycle, and then condensed in a condenser cooled by the cold water. The boiler and the condenser need very large heat exchange surfaces. Several demonstration plants using this cycle have been built and run successfully for brief periods of time, and subsequently decommissioned.Georges Claude, who had liquefied air and prospered separating neon for the "neon sign", attempted in the twenties and thirties to build OTEC plants that used water vapor flashed from warm surface water in a vacuum as a working fluid. The surfaces of the warm and cold water flowing through his apparatus were the essential heat exchange surfaces, saving heat exchanger costs.

However the very low density of water vapor at OTEC temperatures makes it a poor working fluid for a power extraction turbine. The size and cost of a turbine adapted to this very low density is a serious handicap to the Claude open cycle. Economic success eluded him. Then the supply of low cost fossil fuels was great, and the environmental consequences of uninhibited combustion did not loom large. Forests dying of acid rain were rare. Global warming due to carbon dioxide emissions to the atmosphere was not an issue.

The mist lift process

A new concept introduced in 1977, the Mist Lift Process, offers a way around the high cost difficulties of previous OTEC engines. It avoids the giant heat exchangers of the "closed cycle" originally proposed by D'Arsonval, and the enormous water vapor turbine required by Claude's "open cycle".

In the Mist Lift process warm ocean water is sprayed upward from the bottom into an evacuated vertical duct. The ambient pressure is of the order of 2,400 Pascals (0.348 psi). Vapor evaporates from the warm water. A mixture of water droplets and water vapor is formed, a mist. At a distance of 10 to 20 meters above the bottom, cold water is sprayed upward into the duct. It condenses the vapor, and establishes a pressure of 1,200 Pascals which is lower than the bottom pressure. Driven by the pressure difference the vapor flows upward from the bottom to the cold water spray-condensing region, dragging the warm water droplets with it. The mist is thus accelerated to substantial velocity. As the vapor condenses the mist and cold water merge, forming a single-phase fluid, which coasts to the top of the duct. The lifted water is then collected, and passed through a hydraulic turbine to provide the output power of the plant.

It has used the vapor flashed from a spray of very fine warm water droplets to lift these droplets to the height of Niagara Falls. (140 ft). Alternatively the water can be first dropped through a hydraulic turbine to provide the desired power output from the cycle, then mist lifted and merged with the condensing cold water, and returned to the ocean. The mechanical coupling between the droplets and the lifting vapor depends upon the viscosity of the vapor, which does not diminish with lowering pressure, whereas the coupling between vapor and the turbine blades of the Claude cycle depends on the unfortunately very low vapor density which requires large turbines.

By placing the warm water and cold water injectors sufficiently below sea level one may dispense with cold and warm water supply pumps, which gives the concept an additional cost advantage over closed or Claude cycle OTEC.

A cost estimate of a conceptual design of a 4 MW Mist Lift OTEC power plant was prepared and published in 1984. This design was based on a modest extrapolation of the mist transport data acquired in fresh water experiments in 1980-81 and ocean water experiments in 1983. It was optimized for minimum cold water use with a condenser effectiveness of 0.9 yielding an output of 450 kJ per cubic meter of cold water. Allowances for cold water pumping power, mist generator loss, filter loss, hydraulic turbine efficiency, exit loss and non-condensable removal power reduced this yield to a net value of 300 kJ per cubic meter of cold water. The projected cost was $10,000,000.

The two stage mist lift

The cold and warm waters emerged from that Mist Lift plant mixed. It used a larger flow of cold water than warm water. The emerging water was cool, and could accept more heat, and it seemed that improved performance and lower costs could achieved by adding a second stage that uses the cool water output from the first stage, and reduce the total cost. We call this the two stage mist lift.

A recent design analysis of this concept predicts that a two stage Mist Lift plant can provide net power of 800 kW per cubic meter per second of cold water.

For experimental data, theory, and analysis see C. K. B. Lee and S. L. Ridgway, Vapor/droplet coupling and the Mist Flow Cycle, Journal of Solar Energy Engineering, May 1983, vol 105, pp 181-186.

 

Characteristics of 1.6 Mw 2-stage Mist Lift OTEC
Cold water flow	2.0 T/s
Head loss cold water pipe	5.0 m
Warm water flow stage 1	1.8 T/s
Warm water flow stage 2	1.8 T/s
Warm temperature	25 deg Celsius
Cold temperature	5 deg Celsius
Transfer temperature	14 deg "
Output water temperature	17.8 deg "
Stage 1 power	900 kW
Stage 2 power	720 kW
Input turbine:	3.6 cubic meters/s at 10 m head
Output turbine:	5.6 cubic meters/s at 18 m head

 

Performance comparisons

 

item	warm/cold ratio	power/cold flow	reference
closed cycles:		MW/T/s
Nauru	1.03	0.257	Avery p293
GE 40 Mw	1.15	0.217	Avery p303
Johns Hopkins	0.97	0.292	Avery p350
open cycles:
NELH net power	1.5	0.246*
Mist Lift 1984	0.6	0.308
Two stage ML	1.80	.80

The theoretical maximum power given Twarm=298, Tcold=278, for warm/coldflow ratio=1.0 is 1.4 MW/T/s, for warm/cold = 1.8 is 1.9 MW/T/s. There is much possibility for substantial increases in OTEC performance.

*The maximum output was 200 kW; the waters were supplied by NELH pumps which are not optimized for the power plant; a 100 kW charge was rather arbitrarily taken for the pumping. Zero charge would make its power/cold = 0.5

Conclusion: The 2 stage Mist Lift can yield twice the output per unit cold water supply of present OTEC versions, and has many other potential economies. Further research and development in this direction promises large returns!

References:

"Renewable Energy from the Ocean", J. H. Avery, C. Wu

"Hubbert's Peak, The Impending Oil Shortage." Kenneth Deffeyes, Emeritus Professor Geology, Princeton University

"Out of Gas", David Goodstein, Provost California Institute of Technology and Professor of Physics

"The Party is Over", Richard Heinberg, New College of California

"Experimental Demonstration of the Feasibility of the Mist Flow Ocean Thermal Energy Process", Second Terrestrial Energy Systems Conference, 1981, Colorado Springs, Colorado, S. L. Ridgway, R. P. Hammond, C. K. B. Lee.

"Projected Capital Costs of a Mist Lift OTEC Power Plant", Stuart L. Ridgway, Winter meeting ASME 1984; New Orleans, 84-WA/Sol-33

Appendix

The maximum thermodynamically possible work out from a flow W of warm water at temperature T1 and flow C of cold water at temperature T0 is given by:

1) work/cp= W(T1-T2)-C(T2-T0), where cp is the fluid specific heat, and T2 is the common temperature of the exit waters. But what is T2. If the heat engine is ideal, and no entropy is created T2 can be found from:

2) (W + C)*ln(T2)= W*ln(T1) + C*ln(T0) and substituted back into 1 to obtain the possible work

 

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