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Mist lift OTEC

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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