Topic

Rankine Cycle Desalination Plant


Team Members:

Ravi Sahai

Spencer Herrick


Description:

Project Overview

In order to reduce the cost of desalination and produce power at the same time, our project proposes to combine a standard Rankine cycle power plant with a desalination plant. Desalination plants are used to convert salt water to fresh water and typically have been very expensive and energy intensive. The cost of desalination can range from 49 to 81 cents per cubic meter, which is about 5 times the cost of producing tap water from a fresh water source (the average cost to produce tap water from a freshwater source is about 8 to 13 cents per cubic meter). Instead of recycling the water that is generated as steam (this is the process in a standard nuclear power plant), we will condense it and collect it as fresh water and constantly pump in new ocean water. In addition to generating electricity, our power plant will also produce fresh water. The goal of this project is to determine the feasibility of such a power plant and perform a comprehensive cost benefit analysis to evaluate this proposal.

Desalination

Desalination refers to the process of removing salt and other particulates from seawater, brackish water, or wastewater, making it suitable for industrial and residential use. Through distillation and reverse osmosis, water treatment plants can remove most of the salt and other impurities from saline water, making it clean and safe to drink. Desalination plants are used to produce potable water around the world, especially in countries that have a limited supply of fresh water. It is one of the only non rainfall dependent water sources, making it a valuable source of water in countries like Australia, Saudi Arabia, Israel, and Singapore who typically rely heavily on rainfall in dams to provide their drinking water supplies. However, the process is extremely energy intensive and requires highly specialized and expensive infrastructure. The cost to desalinate water depends on the technology and location, but on average ranges from 49 cents to 81 cents per cubic meter of water. In comparison, treatment of freshwater sources to produce drinking water cost about 8 cents per cubic meter, or 15% the cost of desalination

The table below summarizes the energy costs of conventional water sources (Warragamba dam and other water storages, deep storage) with reusable and recyclable wastewater and desalination.

Energy_intensity_water.jpg
The table above illustrates the energy intensity of desalination. Desalination is about 20 times more energy intensive than conventional water sources and 2 times more intensive than wastewater recycling/reuse.
Overview of the Rankine Cycle

The animation below shows the basic water flow system for a pressurized water reactor. This type of system uses a rankine cycle to convert heat into electrical power by converting water into steam. This type of cycle is utilized for many different types of power plants, including coal, natural gas, and nuclear. These various reactions produce massive amounts of heat; this heat is collected by pumping a primary water feed though the reaction. This water is pressurized so that does not turn into steam even at incredibly high temperatures; in the case of nuclear reactors, this water is separated from the rest of the system to prevent the spread of radiation. This high temperature water is then flown through a heat exchanger along with a secondary feed water system. Here in the steam generator, the heat is transfered from the primary to the secondary water, and the secondary water is boiled into steam. When the water turns to steam there is a large increase in pressure that pushes the steam through a turbine. The steam turns the turbine as it pushes by, and this spins a generator that produces electricity. In a normal cycle this steam is then cooled back into water, often by pumping sea water though, and this water is then pumped back into the steam generator to be boiled again.
Standard Nuclear Steam Plant
The Pressurized Water Reactor (PWR)
The Pressurized Water Reactor (PWR)

















How this cycle could be modified to produce fresh water as well as power

One of the most simple ways to desalinize water is to boil saltwater and condense the pure steam that comes off. Though this method is simple, it is an energy intensive process that requires several mega joules of energy to heat a single kilogram of water, for this reason it is not utilized as a method for desalination plants. The Rankine cycle boils water to create steam in order to produce electricity, because this is the same process to purify water, these two process could be combined. The resulting system would use the same fuel to produce water and power. To alter the Rankine cycle for this purpose, it would require that sea water be pumped directly into the steam generator. Creating such a system would require several mechanical systems that would have to be added to the original Rankine cycle, and some alterations that would have to be made to prevent losses in efficiency. The steam produced by this system would generate power, just as in the normal Rankine cycle, but the steam would then be collected and condensed as fresh water. This water would be almost completely pure, except for a very small amount of volatile organic material that may have have to be filtered out before human consumption. This system also allows the for the implementation of a cogeneration facility; the heat from the steam could be used to heat homes and other buildings. A power plant that implements this desalination system could provide heat, water, and electricity, cheaply and efficiently.

Modified Cycle That Desalinizes Seawater While Producing Electricity

Real_Final_PWR.jpg

Efficiency Losses


Increased boiling temperature

This alteration to the Rankine cycle does represent some engineering challenges and efficiency losses. From a theoretical viewpoint there is going to be an extra amount of energy required to separate the salt from the pure water. This extra energy manifests itself as the higher boiling temperature that seawater has then fresh water.

The increased boiling point can be calculated using the chemistry equations:

ΔTb equals Kb · mB and Kb equals RTb2M/ΔHv where :
R(gas constant) (J/K*mol)
8.314
Tb (Boiling point of water) (K)
373
M (molar mass of water) (kg/mol)
0.018015
DeltaH (Heat of vaporization)(J/mol)
40650
mb (molality of sea)(mol(salt)/kg(water))
0.59890486

The result of this calculation is an increased boiling temperature of 0.307 degrees K.

This number can be used to calculate the power loss for the cycle by finding out how much extra energy is required to heat this water and applying this number to the amount of water that is being heated by the Rankine cycle.

specific heat seawater(J/kgK)
3993
Water flow for 1 GW reactor (lbs/hr)
1.59E+08


The extra power that it would take to boil seawater instead of pure water is 24.6 MW.
This calculation is for a 1 GW reactor, so this represents a power loss of 2.5%. This is the lowest theoretical power loss for the desalination system from an energy standpoint.

Salt Extraction

The biggest engineering obstacle for this enterprise is to find a way to filter the salt out of the system. With the amount of salt that this cycle would produce, there has to be a continuous way to pump it out. One of the most simple, yet most inefficient, ways to deal with the salt is to pump it out while the salt is still dissolved in the water. This method would be mechanically simple, yet a significant portion of the water would be lost, and with it energy.

Water, at the 260 degrees Celsius that it experiences in the steam generator, has a NaCl solubility limit of 50grams salt per 100 grams water. The seawater going into the generator has a salinity of 35 grams salt per kg of water. This means that 93% of the water could boil away before any solid salt forms. But to get rid of this salt, this means that 7% of the water would have to be lost to clear away the salt in its aqueous form. This water would not generate steam, so it represents a 7% loss in efficiency. The efficiency of the salt removal process could be greatly improved through engineering innovation. If there were a way to extract the salt in its solid form then this loss in efficiency could approach zero.

This saturated salt water could be dealt with in several ways. The water could be pumped back into the ocean, but although this would be easy, it may have an adverse effect on the local sea life. A better solution would be to put this water into a large open vat. This way, the remaining water would evaporate, leaving solid salt. This salt could then be sold, or given away without causing any aquatic harm.

Pressure Difference

There is also a loss in efficiency due to the different pressure that this system would have across the electricity-producing turbine. The power that is generated comes from the pressure difference between the high pressure steam coming out of the reactor and the cooler low pressure steam on the other side of the turbine. In a normal Rankine cycle that has a closed water loop, the low pressure side of the turbine would have a pressure that is actually less then atmospheric pressure. The desalination cycle would have a low pressure that is equal to the atmospheric pressure, because the water would be connected to the non-pressurized water distribution system for the region in question. But there are some engineering methods that could make this loss in power approach zero.

One way to reduce this inefficiency would be to use a pump to keep the the low pressure side of the turbine at the less-then atmospheric level that it would be in the closed loop Rankine cycle. The energy that it would take to run this pump would not actually represent a loss of efficiency because the desalination system is saving this amount of energy in the pump that pressurizes the seawater going into the team generator. This pump, in the closed cycle, would have to pressurise water that starts below atmospheric pressure, but in the desalination system this water would be a atmospheric pressure, which means less work for the pump. This saved energy would be the same amount required to run a pump that keeps the low pressure side of the turbine below atmospheric pressure. The problem is that the disolved gases in the sea water would make it impossible to run such a pump. To solve this problem it would be necessary to remove these gases before the seawater enters the steam generator. By heating up the seawater with waste heat from the steam, as seen in the heat exchanger in the design proposal, the seawater is near boiling before being pressurized. This means that the dissolved gases in the sea water will separate from the water, and they can be removed.

According to Henry's Law:

p = kh c


The concentration of the gases in the water is propotional to the pressure which is proportional to the temperature. This means that there is a linear relationship between the amount of gas and the temperature the gass concentration at near-boiling temperature can be projected.

Nitrogen_in_seawater.pngOxgen_in_Seawater.png
These two plots show that by 65 degrees Celsius all of the disolved gases should separate from the water.

At the near 100 degrees Celsius temperature that the seawater will be at, all of the dissolved gases will be separated. This means that once these gases are removed the low pressure side of the turbine can be kept at below atmospheric pressure, so no efficiency will be lost do to pressure changes across the turbine. There is a possibility that when removing the separated gases from the seawater some water vapor may also be lost. This would mean a very slight decrease in efficiency, because the pump would have to pump in that much more seawater to compensate. This would be a very small loss in overall efficiency. With this loss and allowances for other unforeseen slight losses in this gas extraction system, the maximum efficiency loss for the desalination cycle would be 0.5%.

Total Efficiency Losses

With the theoretical loss of 2.5% due to the increased boiling temperature, the 7% from the salt extraction, and the 0.5% from the gas extraction, the total loss in the efficiency of the original Rankine comes to 10%. Because the original Rankine cycle was only about 30% efficient to begin with, this means that this desalination system represents a 3% loss in the overall efficiency of the power plant. With engineering improvements in the salt and gas extraction process, the losses in efficiency could drop even lower.


Cost Analysis



Cost comparison for different fuels

To implement this system into the design of a new power plant, the increased installation costs would be similar, regardless of the fuel the plant will use. Because of this, the best way to compare these plants is to calculate the cost of increased fuel usage. To deal with the overall 3% loss in the efficiency of the cycle, these plants would have to run longer, using 3% more fuel to produce the original amount of power they were designed for. This means that these plants can be compared by the cost of this extra fuel, which is proportional to the cost to produce fresh water.

Screen_shot_2011-05-23_at_5.58.55_PM.png
This shows the cost trends for fuel for various types of power plants, and shows the current rates.


Cost of Fuel
Cost to run for one year
Cost if 3% more fuel
Ratio of increased fuel cost

($/kWh)
($/year)
($/year)
(Uranium as base)
Uranium
0.005
43800000
1314000
1
Coal
0.022
192720000
5781600
4.4
Natural Gas
0.051
446760000
13402800
10.2
This table shows that the nuclear power plant would face a much smaller cost for extra fuel, compared to coal and natural gas. This means that for a coal or natural gas burning plant it would cost 4.4 and 10.2 times as much to produce fresh water, respectively, as a nuclear power plant. Though implementing this this system would represent different costs depending on the fuel source, all of these methods would still be profitable, and even if the price of this fuel increases, the price of water is likely to increase as well. This means that over time all of these fuel types would likely be a profitable option.

Installation


Based on the costs that have been required to retrofit and replace parts in nuclear power plants in the past, the estimated cost to implement the desalination system for a nuclear plant would be about 1 billion dollars. These costs would be much lower if this system were to be implemented in a coal or natural gas plant, because the lack of radiation makes it safer and cheaper to open and change these plant. If this desalination system were to be incorporated into the design for a new power plant being build the costs would be very low. This desalination system is mostly an alteration of the original design, rather then adding new components, but of the of the heat exchanger and the pump that this system adds would be under 500,000.

Assuming a 1GW Nuclear Power plant, the water flow-rate is 159 million lbs/ hour, which is equivalent to 20,000litres/second, resulting in 630 million cubic meters of fresh water produced per year. The current cost of water is about $0.40 per cubic meter, and this results in a profit of $252,288,000 per year. However, we must take into account the decrease in electricity production due to the 10% decrease of the efficiency cause in mechanical and theoretical efficiency. If the current efficiency of our Rankine cycle power plant is 30%, a 3% loss in efficiency, due to the desalination process, yields an overall efficiency of 27%. A standard 1 GW power plant produces 7.89 GJ (Do you mean TWh?) of energy and generates $1.18 billion. The decrease in efficiency would lead to a $35,478,000 decrease in revenue. The difference between the revenue generated by our production of fresh water and the loss in revenue due to the decreased efficiency gives us a net profit of $216 million. We anticipate the cost of water to increase in the next few decades as a result of global water shortages, resulting in an increasing profit over time.

Assuming a 5% annual increase in the cost of water, over a 20 year life, the fresh water from our desalination plant would generate $7.63 billion over that time.

Screen_shot_2011-05-26_at_4.17.09_PM.png
This plot shows the long term profits our this desalination system.

Energy Savings

A coal powered desalination plant producing 630 million cubic meters of fresh water annually would require approximately 3,400 GWh of energy and release 3.7 million tons of CO2 into the atmosphere. By producing this amount of water through our desalination rankine cycle, we are able to save 3,165 GWh of electricity and 3.4 million tons of CO2 in emissions annually. To put these numbers in perspective, reducing CO2 emissions by 3.4 million tons is the equivalent of preventing 7 million barrels of oil from being consumed, taking 615,000 vehicles off the road, or 3/4 the annual emissions of a coal fired power plant.


Conclusion

This innovation to power production would be a major advance in desalination technology, and could mitigate future potential water shortages. There are many engineering challenges that would have to be overcome to implement this combined Rankine and desalination cycle. The water production would create minimal efficiency losses in power production; that means that fresh water would be produced at low costs. As with many types of new technology, there may be environmental impacts that are not anticipated; these problems may include impacts to local aquatic life. But with these potential problems there are definite solutions to social problems. Water is necessary for human existence, a truth that is cause strife around the world, in arid climates, and where the water supply has been contaminated. This system could provide fresh water and electricity to the areas that need it most, at affordable prices. It could also be an affordable solution for any country that faces increased water demand from increased population and agricultural needs. My rough calculation is that one cubic meter of water in boiling absorbs about 630 kWh of heat, and could produce about 300 kWh hours of electricity. If this innovation has a cost of 5% efficiency, then we "lose" 15 kWh of electricity which is more than would have been used to desalinate the water by conventional means, and at $0.10 per kWh, this is $1.50, much more than desalination presently costs...

To see a presentation that is an overview of this information, click here:

http://prezi.com/nxqiukkom58w/nuclear-desalination-plant/


Sources

Pressurized Water Reactor
https://netfiles.uiuc.edu/mragheb/www/NPRE%20402%20ME%20405%20Nuclear%20Power%20Engineering/Pressurized%20Water%20Reactors.pdf

Desalination
http://www.water-technology.net/projects/shuaiba/

Cost of Water
http://www.ct.gov/dph/lib/dph/drinking_water/pdf/dwc_fed_fund.pdf

Cost of Power Plants
http://losangeles.cbslocal.com/2011/02/18/san-onofre-nuke-plant-completes-costly-retrofit/
http://www.nei.org/resourcesandstats/nuclear_statistics/costs/

Composition of Sea water
http://publishing.cdlib.org/ucpressebooks/view?docId=kt167nb66r&chunk.id=d3_6_ch06&toc.id=ch06&brand=eschol
http://www.seafriends.org.nz/oceano/seawater.htm

Carbon Equivalencies
http://www.epa.gov/cleanenergy/energy-resources/calculator.html