Two Interesting Papers On the Utilization of Low Grade Heat.
It is an incontrovertible law of thermodynamics - the famous second law - that the system that can sustain the highest temperatures will be the most efficient.
In the dangerous natural gas industry, which is the fastest growing energy business in the world - if one is intelligent enough and educated enough to understand that the peak power of a so called "renewable energy" facility is nowhere near its average continuous power (intelligence and education which seem increasingly rare) - the most efficient systems are combined cycle system, which can demonstrate efficiency of close to 60%. In a combined cycle gas plant the gas is burned and expands against a turbine, its temperature actually exceeding the melting point of the turbine blades, were they not coated with thermal barrier coatings. The exhaust leaving the turbine is still hot enough to boil water even under pressure, and as such is used to power a standard Rankine steam cycle.
I discussed this state of affairs in a relatively recent post in this space (An interesting thesis on the utility of MAX phases in the manufacture of turbine blades.
Despite its use in high efficiency systems, and despite having the so called "renewable energy" industry function as a smokescreen for its use, natural gas is not sustainable. It's continued use is a crime against all future generations, humans who are babies today, and humans who will be babies 500 years from now, assuming that there will be humans in 500 years.
The only sustainable form of energy is nuclear energy, whether we admit it to ourselves or prefer going along stupidly claiming otherwise, and nuclear energy, and only nuclear energy is capable of continuously providing combustion free high temperatures.
The problem of course is that the laws of thermodynamics require that the heat must go somewhere, and usually that "somewhere" is most often a body of water. (This is true not only for nuclear plants, but is also true of gas plants and coal plants. This is the subject of considerable public ignorance which assumes, incorrectly, that only nuclear plants have cooling towers.) Where thermal electric plants operate away from ocean water, they are generally net water consumers. This point is made in the first of the two papers I will briefly discuss in this post: A Combined Heat- and Power-Driven Membrane Capacitive Deionization System (Hatzel, Hatzel and Zhang, Environ. Sci. Technol. Lett., Article ASAP Published online October 2, 2017)
The paper is open sourced, anyone can read it.
The introduction says it all:
Managing energy consumption during water treatment processes and water use during energy generation is a critical component of the water–energy nexus.(1) Thermoelectric power plants account for 38% of fresh water withdrawals in the United States, and a majority of this water is used for on-site cooling (?80%) and power generation (?10%).(2, 3) Technologically, dry cooling could aid in minimizing the demand for water during cooling, and low-energy water treatment technologies could reduce the amount of energy spent on boiler water treatment. Furthermore, improvements made in treating boiler water have a direct impact on improving plant thermal efficiencies, as high total dissolved solids (TDS) result in a low rate of heat transfer due to corrosion and fouling.(4) Boiler water treatment will become increasingly important because high-efficiency supercritical based power plants require more stringent water quality.(4)
Treating water to pure and ultrapure levels can be energy intensive and traditionally requires treatment strategies that combine softening with multiple passes through a reverse osmosis (RO) system. Most energy generation and industrial sites that require pure water also have access to an abundance of waste heat, which could act as an ideal free energy source for water treatment.(5) Therefore, developing synergistic approaches to use this “waste energy” source has become desirable. Currently, indirect and direct means for converting low-grade waste heat into deionized water do exist. Indirect approaches include those that convert heat to power through technologies such as thermoelectric devices and then use that power to operate a water treatment system.(6) While possible, undesirable energy conversion losses, larger system footprints, and cost typically limit their practical implementation.
Here is what the authors claim:
Here, we aim to detail a process for harvesting thermal energy within an electrochemically driven deionization system termed membrane capacitive deionization (MCDI). MCDI offers several advantages for effective brackish water treatment (low specific energy consumption), yet is purely driven by electrical energy.(14) We experimentally investigate the potential for harvesting thermal energy through exploiting the electrostatic and membrane potentials dependence on temperature. We also highlight the role heat plays in limiting losses that arise when moving MCDI toward high-water recovery operating conditions.
The details are in the paper, which is, again, open sourced.
Here's a nice cartoon that suggests what is going on:
This desalination system consumes electricity, but does so more efficiently than, for example, reverse osmosis, and it utilizes low grade heat in such a way as to eliminate its disposal to water.
Some comments on desalination. I'm not entirely sanguine about desalination, owing to a concern about changes to ocean currents deriving from saline gradients. This is probably not quite as serious as changing planetary weather patterns as the wind industry proposes to do - I've seen several very silly references in the the literature to using wind turbines to stop hurricanes, one proposed by the anti-nuke idiot Mark Z. Jacobsen at Stanford, which suggests (were it true, undoubtedly its not) that wind turbines can stop the, um, wind. This said, changing salt gradients in the ocean is certainly problematic. (Happily the wind industry is too expensive and useless to actually produce significant energy, so weather patterns are safe for the time being.)
However, desalination may be a risk that future generations may have to assume, since we have left them with nothing other than trillion ton quantities of dangerous fossil fuel waste.
I note that the concentration of carbon dioxide on a volumetric scale is much higher in seawater than in air, which suggests that if one were trying to remove dangerous fossil fuel waste from the atmosphere, the processing of seawater would be a key to accomplishing that task - if it can be achieved. I've written about that in this space elsewhere.
Another paper I've seen on waste heat, written by Mexican scientists (both of whom are far more intelligent than that orange excuse for a human being in the White House) proposes to add a third layer to a combined cycle system by using a working fluid that boils at temperatures much lower than that of water, 100C.
Here is the paper, which is regrettably not open sourced but must be obtained in a good science library: Thermo-Economic Multiobjective Optimization of a LOW Temperature Organic Rankine Cycle for Energy Recovery (Ruben Omar Bernal-Lara† and Antonio Flores-Tlacuahuac Ind. Eng. Chem. Res., 2017, 56 (40), pp 11477–11495)
Here is the introduction:
The high energy demand in the world in the last years has become a focus of attention for researchers due to the constant quest of alternative processes to produce power in a economic way and low environmental impact. The recovery of energy from waste heat streams is an example of an alternative energy process seeking to meet energy demand and taking care of sustainability issues.1,2 In fact, waste heat sources are commonly found in solar and geothermal sources, as well as in industrial process streams. Thermodynamic cycles have been widely studied to use waste heat sources and produce power.3?5 Most of those cycles use water as working fluid for low-temperature energy recovery. The most common thermodynamic cycle is the water Rankine cycle. However, the thermodynamic efficiency of the Rankine cycle is low at temperatures below 370 °C.6 To improve the performance of the Rankine cycle for lowtemperature energy recovery, the Organic Rankine Cycle (ORC), featuring organic compounds as working fluids, has been proposed. 1 Organic compounds are good candidates as working fluids because of their low boiling point, low medium vapor pressure, and high vaporization enthalpy at low temperature ranges.
Unfortunately several of the working fluids in these systems are in fact, HFC's (and even one banned CFC), all of which are potent greenhouse gases in their own right; a safer and more sustainable option would be to use flammable working fluids, and several are examined, including cyclopropane, n-butane, isobutane and n-pentane.
This is a math heavy paper - clearly over the head of an American President with tiny hands and an even tinier brain - but the point is well taken. No matter how one goes about this matter however, all these systems require a low temperature heat sink. It's not clear that, since we have elected to do absolutely nothing about climate change except to post ever more absurd fantasies about the so called "renewable energy" nirvana which did not come, is not here and will never come, there will be appropriate low temperature heat sinks in the future, almost certainly not in Mexico.
Here is a nice graphic that touches on the cost of these systems:
Here is the Pareto curve and a schematic cartoon of the system:
I have been thinking and reading about approaches to waste heat utilization for some time - these are just two examples - and note that there are many other useful things that might be done with it, but those are subjects for another day.
Have a nice day tomorrow.
