perm filename NUCLEA[1,VDS] blob sn#246484
filedate 1976-11-09 generic text, type C, neo UTF8
COMMENT ⊗ VALID 00002 PAGES
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Americans use an incredible amount of energy each year. In fact,
with only about 6% of the total world's population, the U.S. uses 35% of
the world's energy. In l970, the U.S. consumption of total energy per capita
was about 250 KWh per day, and of that, the electrical consumption per day was
about 21 KWh per person. Last year, in l975, the FEA's estimate of the year's
electrical production alone was 1.9 x l0 KWh per year. (That is l.9 TRILLION
KWh per year!) And, more importantly, the present doubling time of electrical
generation in the U.S. is l0 years, its present growth rate is 2%, which itself
is accelerating. If Americans continue their present energy utilization trends,
they can expect an astronomical amount of over three and a half trillion KWH of
electricity needed in the year l985!
Unfortunately, as America's electrical energy consumption steadily soars,
the amount of fossil fuel resources steadily declines. Now, as the fossil fuel
resources continue to become more and more scarec, the price correspondingly becomes
more and more expensive. (The beginnings of which were demonstrated in the "energy
crisis" of l973.) Americans are now being forced to seriously seek other energy
alternatives which can competitively compete with the rising fossil fuel costs.
And, the people must find another means for supplying these enormous amounts of
energy within the next two decades if they are planning to keep up their present
patterns of energy consumption. Although coal, solar, geothermal, and hydro power
are being considered as major contributors to the future energy needs, it looks as
if nuclear power will, by necessity, become the dominant source of electricity
generation before the year 2000.
In l975, nuclear power supplied only 11% of the total electrical energy
needs, but the FEA predicts a 27% share in l985, while the Exxon Company forecasts
a possible 50% share in l980. Even though nuclear power is presently being hotly
debated by a substantial number of concerned Americans, the fact remains that
there are 58 nuclear power plants on line today, with a projected number of 170 in
the year l985. With energy utilization rates increasing now, we can expect that
nuclear power use will most likely increase greatly within the next decade. This
new power source will bring along with it not only the problems such as those
involved with waste disposal, fuel processing, and plant safety, but also the
possible problems arising from thermal pollution.
Thermal pollution is caused when the heat that is wasted in industrial
processes is transferred to a body of cooling water, raising its temperature 10 to
20 C., and then dumped back into the waterways. This temeperature rise can cause
a great deal of harm to the water ecology and its dependent life, thus making this
a potentially serious pollution problem.
It must be stressed that thermal pollution is not a new problem which has
been caused only by nuclear power plants. ALL power plants reject this unusable heat,
which is known as waste heat, whether they are coal, oil, nuclear, solar, or any
type of heat source. The reason for this is summed up in the second law of thermo-
dynamics which says that all processes cannot be l00% efficient. There is simply a
thermodynamical limit on the conversion of random heat energy to organized energy
(work). This is a limit imposed upon us from nature, and cannot be argued with.
The efficiency of a power plant can easily be calculated by employing the
Carnot equation, since the majority of plants today operate by using the steam cycle.
In this cycle, pressurized hot steam drives a turbine, which turns fuel energy into
mechanical energy in the turbine. This energy is converted into electrical energy,
in the form of electricity, and is then distributed to the consumers. The Carnot
equation allows us to calculate the actual amounts of waste heat which is produced
in this cyclic process. Pictured below is a typical power plant using the Rankine
cycle (a type of steam cycle).
Above, 1→2 is the conversion of thermal energy to mechanical energy. 2→3 is
the heat rejection incurred by condensing. 3→4, the work to run the water pump,
is the only work input of the cycle. Finally 4→1 is the heat input provided by
the boiler. The maximum efficiency of the power plant's heat engine is determined
by the Carnot efficiency, which is simply: MAXIMUM EFFICIENCY = 1-T(2)/T(1).
By looking at this equation, we can see that we want a very high entering temper-
ature (T(1)), and a very low exit temperature (T(2)), in order to obtain a high
engine (cycle) efficiency. Thus, the efficiencies depend on the temperature and
pressure of the steam generated, which is limited by the fuel temperatures in the
reactor, or by the temperature capacity of the metal which must enclose this heat,
and also, the cooling water's temperature.
Let us look at some typical power plant efficiencies. A modern fossil fuel
plant has a thermal efficiency of 40%, if it uses a steam cycle with superheat.
This is quite a high temperature, most plants have efficiencies around 35%. The
efficiencies of nuclear plants are varied, it depends on the type of reactor. A
Light Water Reactor (LWR), with a T(1) pf 277 C and a T(2) of l7 C, has an efficiency
33%. A High Temperature Reactor, (HTR) is 40% efficient, because T(1) can be
increased to 530 C. Generally speaking, nuclear power plants using the steam cycle
have low efficiencies and reject low-grade heat (usually about 30 C), while gas
turbines can produce high quality heat with efficiencies on the order of 40%. High
grade heat can be obtained from the steam engine, by extracting the higher temperature
steam from the turbine, but only at the expense of reducing electricity production.
High-grade heat can be easily used, while low-grade cannot. Presently, the major
emphasis by the power stations is on producing maximum ELECTRICAL efficiency. Should
this emphasis be shifted towards an emphasis on maximum ENERGY efficiency ? (That is,
obtaining the high-grade heat from the steam engines in order to effectively use it.)
Would the benefits derived from the possible uses of the high-grade waste heat
outweigh the disadvantage of decreasing the electrical capacity? I think so, and
have included at the end of this paper two uses of waste heat which I think can become
greatly beneficial. ENERGY efficiencies of 80% have been agreed upon to be possible
if this waste heat is used effectively.
Before I go on, I must add that present research in areas such as fluid mechanics,
combustion, heat transfer and lubrication might improve the Carnot efficiencies.
Some research is now directed towards finding alternatives to the low efficient
Rankine cycle. Those being explored are magnetohydrodynamics (MHD) and fuel cells. However,
no major breakthroughs on them have been discovered as of now to make the replacement.
With an efficiency of 33%, a 3,000 MW LWR would dump 2,000 MW of heat into
the environment. With hundreds of nuclear power plants each dumping out amounts
such as these, we can see where the temperature of the receiving water could become
quite high if the waste heat was continually dumped into the rivers, lakes, and
estuaries. This is where the problem of thermal pollution comes in.
By a rough estimate, it is assumed that 1 GW of electrical capacity causes
a body of water to be raised 10 C, (Assuming an effiency of 33%), and requires 50
cubic meters per second of cooling water. The Evironmental Protection Agency (EPA)
has limited the maximum temperature increase in receiving waters to be 5 C. Thus,
we can see that l GW will require 100 cubic meters per second of cooling water to
meet these limits! These huge water withdrawals can cause water shortages in areas
where water supplies are already deficient.
The U.S. has a run-off of 53,000 cubic meters per second, making a limit
of only 530 GW of power then to be obtained from the nuclear power plants. With
our high energy consumption rates, this figure should be reached by around l980.
Therefore, it is obvious that this heat will have to be dissipated elsewhere
besides in the runoff water. This is why I feel that immediate emphasis should
be placed on obtaining the high-grade heat and effectively using it, instead of
just throwing it away into the rivers.
But, WHAT IF the waste heat is continued to be dumped into the rivers?
What are the effects of adding this heat to the water ecosystem and life?
Probably the most important effect in raising the water temperature of
river water is the decrease of oxygen solubility; the oxygen capacity of the
water is reduced, which always spells out trouble. Thus, the river re-aeration
rate is decreased, due to the reduced oxygen saturation deficit. Along with all
of this is, of course, and increase in Biological Oxygen Demand. This is why dumping
sewage into warm water can be very serious--the wastes are oxidized at a much
faster rate, which requires a very high BOD. If there is not enough oxygen in the
water, the decomposition of the wastes will be anaerobic, which is undesirable
because it leads to such end products as methane, ammonia, hydrogen sulfide, and
CO . Thus, there is a lower waste assimilation capacity.
Other effects of the temperature rise include increase in evaporation
rates (which causes increased consumption rates of water), a reduction in ice
formation in winter, and an increase in chemical reactions. The viscosity of the
water can also be decreased, which can result in increased sedimentatin, leading to
possible sludge problems. And, of course, raising the temperature also leads to a
qualitiative and quantitative change in the aquatic population, and an increase in
the undesirable aquatic flora.
When we look at the effects of thermal pollution, we must be sure to
not just look at the direct effects that it has on the fish, which has been done so
often in the past. We must remember that the surface waters which can be penetrated
by sunlight and that can receive nutrients can support an active ecosystem.