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Sun Jul 24, 2016, 02:18 PM

A potential use for waste heat to approach zero waste water discharge.

An important law of thermodynamics is that energy efficiency is related to the difference in temperature between the temperature of the generating device and the heat sink to which that heat is discharged. This is why all thermal power plants are more efficient in winter than in summer; a regrettable fact given that electricity demand, owing to the growing need for air conditioning as the atmosphere's destruction proceeds at a record pace, is higher in summer than in winter.

This is quantified in a very old thermodynamic law, Carnot's Theorem:

I am opposed to the use of the dangerous fossil fuel natural gas, and believe that it should be phased out, with all other dangerous fossil fuels, as soon as possible on an emergency basis. This said, the most fuel efficient power plants now operating on earth are, in fact, combined cycle dangerous natural gas plants which operate at very high temperatures, with the heated exhaust of a natural gas flame expanding against a superalloy heat resistant turbine, and exhausted to a boiler, in which the boiling water operates a second steam turbine, a combined Brayton and Rankine cycle. (There is no compelling reason that a similar approach could not be applied with certain kinds of nuclear power plants, albeit not those of the types most commonly built.)

This afternoon, before heading to the library to prepare for my work week, I've been reading for pleasure a very interesting paper on a seemingly unrelated topic, "zero discharge wastewater," which refers to the recovery of all of the water from things like sewage discharge, industrial use, including use by power plants.

Here is a link to the paper, published by two Yale scientists: The Global Rise of Zero Liquid Discharge for Wastewater Management: Drivers, Technologies, and Future Directions (Tiezheng Tong† and Menachem Elimelech*†‡, Environ. Sci. Technol., 2016, 50 (13), pp 6846–6855)

I'd like to point briefly to an excerpt in that paper:

The development of thermolytic draw solutes, such as the ammonia−carbon dioxide (NH3/CO2), paved the way for FO incorporated ZLD systems. The NH3/CO2 draw solution generates very high osmotic pressure-driving forces and can be regenerated by low-temperature distillation.61,62 A recent pilot study demonstrated the application of FO with NH3/CO2 draw solution to concentrate produced water from the Marcellus shale region to an average salinity of 180 000 mg/L.43

Because the thermolytic NH3/CO2 draw solution decomposes at moderate temperature (approximately 60 °C at atmospheric pressure),61 low-grade thermal energy, including industrial waste heat and geothermal energy, can be utilized to regenerate the concentrated draw solution. A recent study estimated that U.S. power plants produced 803 million GJ of waste heat at temperatures greater than 90 °C in 2012.63 This amount of heat, if utilized to power the NH3/CO2 FO, could potentially produce a maximum of 1.9 billion m3 of water annually, which would meet the treatment demands for boiler water makeup and FGD wastewater systems of all U.S. power plants.64 Also, geothermal energy is abundantly available in major ZLD markets such as the U.S. and China.42,65,66

(ZLD = Zero Liquid Discharge.)

Reference 63 in this paper, which reports 803 million GJ of waste heat being available, is this paper, also in the same journal:

Quantity, Quality, and Availability of Waste Heat from United States Thermal Power Generation, out of Carnegie Mellon. (Daniel B. Gingerich† and Meagan S. Mauter*†‡ Environ. Sci. Technol. 2015, 49, 8297−8306])

803 million GJ is about 8 exajoules, roughly 8% of US total energy consumption. It is notable that the discharge of this heat is generally, in most places, a serious environmental problem in its own right.

This paper reports a parameter, α, which represents the fraction of heat that is recoverable for useful purposes, and thus will not be discarded as waste. This parameter, given in table 1, is the highest for the form of energy that is most sustainable, safest, and cleanest, nuclear energy.

The discussion is offers a direction to move in order to turn a thermal power plant liability, waste heat rejection, into an asset.

Unfortunately, this is an academic exercise, and will be ignored entirely by the powers that be, whoever they are, as we proceed mindlessly and lemming like into the climate abyss, but it's always interesting to know what might have been in a more rational world.

Enjoy your Sunday afternoon.

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Reply A potential use for waste heat to approach zero waste water discharge. (Original post)
NNadir Jul 2016 OP
kristopher Jul 2016 #1
eppur_se_muova Jul 2016 #2
NNadir Jul 2016 #3

Response to NNadir (Original post)

Sun Jul 24, 2016, 02:37 PM

1. Or we can just skip the waste heat with something that is better.

Cheaper, cleaner, much faster to deploy, and safer all around.

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Response to NNadir (Original post)

Sun Jul 24, 2016, 05:10 PM

2. 803 million GJ is about 8 exajoules ?? Maybe 0.8 exajoules ?

Interesting developments, thanks. If it saves industry big bucks, it will likely be implemented. Politicians make big noises while producing little results, engineers produce big results, quietly.

Engineers produce the egg, politicians, the cackle.

This may be more important from the POV of water usage than energy savings, but water usage will soon be a topic that cannot be ignored any longer.

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Response to eppur_se_muova (Reply #2)

Sun Jul 24, 2016, 05:27 PM

3. Whoops. You're right.

Bad entry in Excel.


You're also right about water. It's the elephant on the environmental table, particularly in the era of run away climate change.

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