My books about energy and economics
Contact: rogerkb at energyevolutionjournal dot com
Article_8
11/03/2024
I recently came across a web page[1] of the SolarPACES (Solar Power and Chemical Energy Systems) organization discussing a long term energy storage system which involves running generating turbines using elemental sulfur (S) as a fuel. Sulfur burns at a temperature of 1200°C and it produces sulfur dioxide (SO2) which is disproportionated back into S and O2 by a solar thermochemical process so that the sulfur is not a mined fuel but rather an energy storage medium.
This process was originally developed by General Atomics who published documents about this process more than a decade ago[1][2]. A central technology for carrying out this process is the extraction of sulfur from sulfuric acid (H2SO4). This extraction is three part process the net result of which can be summarized as follows:
H2SO4 ==> S + 3⁄2 O2 + H2O
You may be wondering how sulfuric acid got into the picture. It gets into the picture because the chemical process for SO2 disproportionation produces sulfuric acid as well as elemental sulfur:
2H2O(l) + 3SO2(g) ==> 2H2SO4(aq) + S(s,l) (115-170°C)
This process only extracts 1⁄3 of the sulfur from the input SO2. Using this process alone would require continually mining new sulfur and disposing of large quantities of sulfuric acid. Therefore to turn this process into a true system of energy storage it is necessary to have a process for extracting sulfur from H2SO4. The total three part process is given by:
The first two steps of this process require concentrated solar energy to drive the decomposition of the sulfuric acid. The third step of SO2 disproportionation is an exothermic reaction. That is the energy generated by producing sulfuric acid more than compensates the energy required to separate the sulfur from SO2.
The concentrated solar energy would be provided by an array of moveable mirrors which focus sunlight on a central receiver located on a so-called power tower. Such concentrated solar power (CSP) systems are currently used with molten salt as the transfer fluid and steam turbines as the electrical power production machinery. The temperature attained by such molten salt systems is between 500 and 600°C.
In the simplest version of the new technology proposed by General Atomics[1] sulfur is burned to run a gas turbine and the resulting SO2 is fed back into process 3 which recovers 1⁄3 of the sulfur. The other 2⁄3 is recovered by using the whole 3 step process.
If one considers the possibility of using this process for seasonal energy storage (i.e. using summer solar energy to produce electricity in the winter) then an interesting interpretation of the use of H2SO4 can be derived.
Suppose that instead of immediately disproportionating the SO2 you store it until summer, and only then begin the process of sulfur production. At the beginning of this process all of the sulfur you need for next winter is stored in SO2. 1⁄3 of the required sulfur comes from disproportionating the SO2 (reaction 3) while the other 2⁄3 comes from running the full 3 step cycle. Now we consider the required sulfuric acid inputs and outputs.
First we consider the output of H2SO4. The production of each mole of S is accompanied by the production 2 moles of H2SO4.
Now we consider the input of H2SO4. 2⁄3 of each mole comes from the whole 3 step process. From the formula for step one it is evident that the required input of H2SO4 is 3×2⁄3=2 moles. The 1⁄3 of each mole that comes from the stored SO2 requires zero input of H2SO4. Therefore the net input of H2SO4 per mole of sulfur is 2 moles
Since the input and the outputs are the same the amount of H2SO4 is not changed during this process. The process began with all of the required sulfur stored as SO2 and it ends with all of the required sulfur stored as elemental sulfur. Therefore from this point of view the H2SO4 is acting as a "catalyst" which itself is not consumed, but which allows the sulfur in the SO2 to be transformed into its pure elemental form. I put the word catalyst in quotes because H2SO4 is clearly not a catalyst in the traditional chemical sense of that word.
Of course in practice one might choose to disproportionate the SO2 as soon as it is produced, in which case 2⁄3 of the necessary sulfur for the next season in temporarily stored in the form of H2SO4.
General Atomics developed this concept more than a decade ago but did not move toward a practical implementation. One of the barriers to implementation was the lack of an economical solar concentrator/receiver system capable of achieving the high temperatures required for process 2. The recent development of falling particle receivers (see article_3) and gas based solar receivers (see article_7) has raised optimism about producing economical solar heat in the right temperature range. Therefore new development efforts are underway to realize a solar sulfur energy storage system.[4]
Of course economical solar heat is not the only requirement of such a system. Economical high performance catalysts are required for each of the three chemical processes. It remains to be seen whether an economical practical energy production system will emerge from this concept in the near future.
Looking at the three step thermochemical process we see than out of every three moles of SO2 extracted from H2SO4 only one mole of elemental sulfur is produced. The other two moles end up back in the form of H2SO4. This fact implies that for sulfur extracted from H2SO4 the maximum efficiency of for the conversion solar energy to chemical potential energy is 33.3%. However 1⁄3 of the total sulfur comes from the single step process of sulfur disproportionation which does not require solar energy input. Therefore if 6 units of solar energy produce 2 units sulfur in the three step process the total production of sulfur is 3 units so that the theoretical maximum efficiency is 50%. In reality the efficiency will not be this high since the conversion of solar energy to SO2 in the first two steps of the process will be less than 100% efficient.
In the end it is really cost which matters, though efficiency is undoubtedly part of the cost equation. General Atomics estimates a cost $2/kWht. The subscript "t" usually means "thermal" which in typical CSP plants means energy stored as heat (i.e. before conversion to electricity). In the case of a sulfur energy storage system I assume that this subscript refers to the chemical potential energy of the sulfur which will eventually be converted to electricity at substantially less than 100% efficiency. If you assume 50% efficiency for the electrical conversion then the cost is $4/kWhe. If you are hoping to use this technology for seasonal energy storage it will take several decades to get the cost down under $0.20/kWhe.
CSP plants require direct sunlight to operate unlike PV panels which can still work in diffuse sunlight (i.e. on cloudy days). Therefore CSP plants are located in geographic areas which have relatively low amounts of cloud cover. If one is hoping to use sulfur storage as a general solution to the problem of renewable energy variability in time, then one must propose a method of dealing with the geographical variability of suitable locations for this technology. I will further discuss this topic in a future post.
[1] SolarPACES solar sulfur cycle
[2] General Atomics solar sulfur cycle 2011
[3] General Atomics solar sulfur cycle 2014
[4] European Sulphurreal project
Article_7
10/19/2024
Recently I came across an article[1] on SolarPACES.org which describes a new solar receiver capable of achieving a heat transfer fluid temperature of 1500°C using existing solar mirror fields designed for use with solar power towers. This temperature is a huge increase from the 565°C temperature achieved by existing solar power tower plants using molten salts as the heating fluid.
The developer of this new solar receiver is German company called Synhelion. Their own web site contains very little information about the design and operation of this receiver. In the only information I found there is the following[2]:
Our solar technology converts concentrated sunlight into the hottest existing process heat on the market. State-of-the-art concentrated solar systems typically work in temperature ranges between 400 and 600°C. Synhelion has developed a completely novel technology to attain much higher temperatures. We can provide fully renewable solar process heat beyond 1,500°C, offering industries that traditionally rely on the burning of fossil fuels a sustainable alternative.
In the SolarPACES article they show a diagram of the solar receiver with a reference to theory paper. The following paragraph taken from the paper explains the principle of operation[3]:
The receiver is constituted by an absorbing cavity with a windowed aperture. The cavity is filled with the streaming gas flowing from the aperture towards the back of the cavity. The incoming solar radiation enters the aperture, passes with minimal absorption through the gas, and is successively absorbed by a highly absorptive surface at the back of the cavity. The solar-absorbing surface thermalizes and re-radiates with a blackbody spectrum. The gas absorbs to a large extent the thermal re-emission and is heated up, and, at the same time, shields the aperture from this re-emission.
The receiver gases that are modeled are water vapor, CO2, and various mixtures of these two gases. As it turns out pure water vapor (that is steam) is the best performer. Water is a poor absorber of sunlight but a good absorber of the black body radiation which is emitted by the back wall of the receiver cavity.
Note the reference to gas streaming toward the back of the receiver cavity. Low temperature steam enters the cavity and is heated to 1500°C as it streams towards the back and then passes through a hole in the back of the cavity.
The steam is only a heat transfer fluid. After it flows out of the cavity most of the heat is removed by a heat exchanger and transmitted to a working fluid which acts as an energy storage medium and as a source of heat for any applications which are using the solar energy collected by the receivers. Presumably the steam is not condensed all the way back to the liquid state but is left as low temperature steam which is then returned to the solar receiver for another round of energy collection.
I can find no discussion of the design and performance of the heat exchanger or the nature of the working fluid. The SolarPACES article does mention that some heat is lost during the heat exchange process so that the working fluid temperature is more like 1400°C. This heat exchange process may be a weak point with respect to practical applications of this technology. 1000°C is considered an extreme temperature with significant design challenges for cost effective heat exchanger performance. Until I see some information about the design and performance of the heat exchanger I will remain skeptical about how quickly this extremely high temperature solar heat technology can be scaled up for practical applications.
Of course if you can achieve temperatures of 1500°C temperatures in the range 800 to 1000°C could also be achieved. A lower solar flux would be needed to achieve this temperature. The sunlight in concentrated on the power tower by an array of mirrors which can track the sun on two independent axes. With lower concentration requirements the tracking accuracy of the mirror would not have to be as high thus leading to lower costs for the solar collection field
There are several interesting applications for solar heat in the 800 to 1000°C temperature range. One of these is the closed loop Brayton gas turbine cycle using supercritical CO2 as the working fluid. I have previously mention this cycle in Article_3. The SolarPACES organization has an interesting energy storage application in mind which also has need of solar heat in this temperature range. I will discuss this application in my next article. Well established designs for heat exchanger exist at temperatures below 1000°C, so that the practical barriers to the use of solar heat in this temperature range are smaller than for the much higher temperatures being targeted by Synhelion.
[1] SolarPACES Synhelion article
[2] Synhelion solar process heat statement
[3] Synhelion receiver design paper
Article_6
10/12/2024
Calcium oxide, CaO and calcium hydroxide [Ca(OH2)] are very useful compounds known respectively as quick lime and slaked lime. Slaked lime can be formed by reacting CaO with water. The calcium for these compounds comes from the abundant mineral limestone which has the chemical formula CaCO3. When CaCO3 is heated to high temperature in a lime kiln carbon dioxide gas is driven off and CaO is left behind.
Various forms of lime are used in agricultural, environmental, metallurgical, construction, chemical/industrial applications, and more. According the National Lime Association[1] the single largest use lime is as a flux for purifying steel.
Even if the energy for lime production came from carbon free sources the release of carbon from from limestone is unavoidable. Therefore zero carbon lime production from limestone will require carbon capture and storage (CCS) for as long as humanity continues to utilize CaCO3 as a source material for this product. Capturing CO2 from traditional lime kilns is economically difficult so that various new methods of producing lime are a subject of research and development efforts[2].
One of these proposed new methods captures CO2 as part of the stable solid compounds sodium carbonate (Na2CO3) and its hydrate Na2CO3.H2O. Sodium carbonate and its hydrates are also known as washing soda.
The chemical reaction which produces slaked lime and sodium carbonate from calcium carbonate (limestone) and sodium hydroxide (NaOH) is:
CaCO3 + NaOH + xH2O ==> Ca(OH)2 + Na2CO3.xH2O, x=(0,1)
This reaction is fast process which requires very little mixing energy. It avoids the need for high temperatures and therefore the need for fuel burning. Since sodium carbonate is a stable solid compound there is no need for pumping CO2 into underground reservoirs.
However new energy and product disposal issues are introduced by the need to produce NaOH. This compound is typically produced by the chlor-alkali process which produces NaOH and chlorine gas from salt (NaCl):
2NaCl + 2H2O ==> Cl2 + H2 + 2NaOH
This is an electrolytic process which requires a large input energy in the form of electrical current. Conceivably the electricity could come from carbon free renewable resources, but the cost of the electrolyzers and of the renewable electricity becomes part of the economic equation.
For each mole of NaOH produced 1/2 mole of Cl2 is produced. Since chlorine gas can not simply be released into the environment some use must be found for it. So the question arises of how much extra chlorine production would result from converting global lime production to this new method.
global lime 2024: 475MMT[3]
global chlorine: 90MMT[4]
CaO molecular weight is 56.1. Therefore the MMT moles of Cl2 produced would be: 0.5*475/56.1 = 4.24
Cl2 molecular weight is 70.9. Therefore the MMT moles of Cl2 currently produced is 90/70.9 = 1.27
Therefore producing lime by the new method would require increasing the global production of Cl2 by a factor 4.3. Since chlorine gas cannot simply be released into the environment this excess production represents a significant barrier to using this method of lime CCS at a large scale.
[1] https://www.lime.org/
[2] https://www.sciencedirect.com/science/article/pii/S1364032122006505#bbib45
[3] https://www.expertmarketresearch.com/reports/lime-market
[4] https://www.researchandmarkets.com/reports/5305192/global-chlorine-industry-outlook-to-2025
Article_5
08/26/2024
A paper on carbon capture in the form of bicarbonate ions (HCO3-) published 2023 prompted a flurry of headlines about storing captured carbon in the oceans as baking soda as the following to examples show.
From newscientist.com:
We can suck CO2 from the air and store it in the ocean as baking soda[2]
From theweek.com:
New carbon capture technology can turn carbon into baking soda[3]
The idea is that hydroxide compounds (a combination of positive ions with negative OH- ions) in an aqueous solution can interact with CO2 to form to form bicarbonate compounds. For example:
Ca(OH)2 + 2CO2 ==> Ca(HCO3)2
The HCO3- ion is the carbonate ion and the combination of Ca2+ with this ion is calcium bicarbonate Ca(HC03)2.
Bicarbonate compounds are a source of alkalinity which can be released into the ocean which thus become a store of carbon without the acidification which comes from dissolved CO2.
In the paper referenced above they do not use Ca or any other common metal ion for carbon capture. Instead they use an engineered chemical complex called polyamine-Cu(II) complex, Polyam-N-Cu2+. Explaining the nature of this chemical complex is above my pay grade as a chemist so I will just refer to it at the mystery complex M2+. This complex can absorb CO2 just like Ca:
M(OH)2 + 2CO2 ==> M(HCO3)2
This chemical reaction is reversible so that applying heat will liberate the CO2 and restore the M(OH)2. The total energy and the temperature required to liberate the CO2 are much less than for capture systems which store carbon in the form of carbonates (e.g. CaCO3). Therefore this system could be used for traditional carbon capture and storage (CCS). That is you could blow air by your vats or trays of aqueous M(OH)2 until the solution was relatively saturated with CO2. After which the solution could be heated (possibly using waste heat) and a relatively dense stream of CO2 would be given off which could be captured and injected underground.
The authors however present another possibility for transforming M(HCO3)2 back into M(OH)2. They propose running a stream of sea water contain NaCl (ordinary table salt) through the carbon capture solution and using a water electrolyzer to split H2O. These actions result in the following net reaction:
M(HCO3)2 = 2NaCl + 2H2O ==> M(OH)2 + 2Na(HCO3) +2HCl
The M(OH)2 has been restored and in addition for each mole of M(OH2)2 2 moles of sodium bicarbonate (baking soda) plus 2 moles of HCl (hydrochloric acid) have been produced. The sodium bicarbonate can be released into the ocean as a beneficial source of alkalinity. However, dumping the hydrochloric acid into the ocean is not a cool thing to do, so the proposal is to sell HCl which is in demand for various industrial purposes. The money obtained from these sales helps to offset the cost operating the electrolyzers that are required for this method of CO2 storage. How extensively this method of carbon storage could be used would depend on how much demand there is for HCl.
[1]bicarbonate ions for the storage of captured carbon
[2]New Scientist on bicarbonate carbon storage
[3]The Week on bicarbonate carbon storage
Article_4
06/10/2024
Quick lime or calcium oxide (CaO)
is used for a variety of
industrial purposes. Typically it
is hydrated (CaO + H2O ==>
Ca(OH)2) prior to use. Among the
uses of this compound are:
1. Construction: lime is a key
ingredient cement, plaster, and
mortar
2. Steel Industry: lime is used
in the extraction of iron from
its ore and in removing
impurities from steel
3. Glass industry: lime is added
to soda-lime glass which is the
most common type (90% of
production).
4. Agriculture: lime is added to
acidic soil to correct its pH.
5. Water treatment: lime is added
to waste water to neutralize its
acidity
6. Chemical Synthesis: Calcium
Oxide serves as a cheap and
durable base for the synthesis of
numerous organic compounds.
The cheapest way to obtain CaO is
to heat limestone (CaCO3)
resulting in thermal
decomposition:
CaO3 + heat ==> CaO + CO2.
The heat could be provided by
carbon free renewable energy
sources, but the only way get to
net zero emissions for the carbon
released from the limestone is
via carbon capture and storage
(CCS).
For a number of manufacturing
processes (e.g. glass and steel)
the carbon emissions caused by
production of lime are a
relatively small fraction of the
total emissions. If you are into
the "more sustainable" meme you
might choose not to worry about
these emissions. After all if you
eliminate over 90% of CO2
emissions who but a killjoy would
spend a lot of time agonizing
over the remainder?
The British glass industry has
published a roadmap for reducing
carbon emissions[1], and to their
credit their goal is not to make
their industry "more
sustainable". Their goal is to
achieve net zero carbon emissions
by 2050. They are counting on CCS
to achieve 7% of emissions
reductions.
This use of CCS must continue for
perpetuity. As long as humanity
is using limestone as a source of
CaO we must continue to practice
CCS if we want net zero carbon
emissions.
The earth is very big place and
there may be lots of suitable
geologic sites for CCS. On the
other hand the earth's atmosphere
and oceans are very large
reservoirs of matter and for a
long time we assumed that human
emissions into these reservoirs
would have insignificant effects.
I hope we do not get surprised in
the matter of CCS. Lower total
resource demand through lower per
capita standards of consumption,
emphasis on long lived products,
and a high degree of recycling
would seem to be in order if we
are hoping for a long run of
human civilization on this
planet.
British Glass Net Zero Roadmap
Article_3
05/17/2024
Recently (Jan 2024) a group of
Italian reasearcher published an
open access paper called "A
falling particle receiver thermal
model for system-level analysis
of solar tower plants". The
following three paragraphs are
taken from the introduction of
this paper:
"In the "Gen3 Roadmap" published
in 2017 by Sandia National
Laboratories (SNL), Mehos et al.
introduced the three most
interesting Solar Tower (ST)
technology pathways on which the
research effort should be focused
on (i.e., molten salts, falling
particle, gas phase). Following
an extensive analysis of those
three technologies carried out in
the frame of the Generation 3 CSP
Systems funding program, the U.S.
Department of Energy announced in
2021 that the most promising
pathway to achieve higher
temperatures in CSP plants and to
meet 2030 cost targets is the
one based on Particle Receiver
(PR).
In detail, the concept is to
adopt particles as a heat
transfer medium (HTM) instead of
liquid fluids; this leads to some
advantages including: i) the
possibility of HTM direct heating
that allows achievement of high
temperatures, and ii) the
possibility of cheaper Thermal
Energy Storage (TES) because heat
can be stored in a relatively
inexpensive medium (e.g. sand).
More precisely, the adoption of
direct heating allows avoiding
the exposure of metallic tubes to
the direct solar radiation
allowing achievement of high
temperatures and this leads in
turn to the possibility of
adopting power cycles with higher
efficiencies than the ones
adopted for conventional molten
salts receivers."
The high temperature power cycle
which I have seen mention most
often in connection concentrated
solar energy is a closed loop
Brayton cycle with carbon dioxide
as the working fluid. The typical
maximum temperature cited for a
practical implementation is in
the range of 700°C to
800°C. In the paper reference
above the particle outlet
temperature of the modeled solar
field is 750°C which sits in
the middle of the range targeted
by the Brayton cycle researchers.
The max temperature of existing
molten salt receivers is
600°C.
Article_2
05/06/2024
Green Car Congress recently
posted an article[1] about some
large commercal orders for
Rheinmetall's heat pump module
designed to work in large
commercial electrified vehicles
From Rheinmetall's web site[2]:
"These two new orders mark the
next milestone in the Group's
marketing and positioning
strategy following the market
launch of the heat pump module,
which was specifically designed
for the electrification of drive
systems in commercial vehicles,
construction machinery, and
boats. The intelligent cooling
and heating management of the
heat pump pre-filled with R1234yf
not only increases the range of
vehicles and battery service life
but also makes driving more
comfortable. "
These heat pumps are an
interesting development since
climate control for
passengers/operators is a
significant issue in determining
the battery life in EVs.
Rheinmetall is clearly focused on
larger commercial vehicles and
not smaller sized passenger cars.
Some premium passenger cars are
already fitted with heat
pumps[3]. As an option they cost
about $1,300.
[1] Rheinmetall on Green Car Congress
[3]Heat pumps for EVs explained
Article_1
03/25/2024
Steve Hanely recently published
and article[1] on
cleantechnical.com entitled: "AI
Has A Voracious Appetite For
Electricity, And That’s A
Problem". The whole article is
interesting. The concluding
paragraphs are:
"And yet, trying to limit the use
of AI to endeavors that are
socially beneficial is like
trying to get people to only
drive their cars when absolutely
necessary. Not gonna happen. In
order to maximize the limits of
computer technology, we will need
to vastly increase our
electricity generating
capabilities. We don’t have
enough renewable energy as it is.
How much of it should be diverted
to power AI, or mine Bitcoin, or
support online sports betting?
Those are the sorts of messy
questions that few people want to
answer and so they prefer not to
ask them in the first place. And
so we careen blissfully into the
future using more and more
resources to amuse ourselves. If
we continue to do what we always
have, we will continue to get
what we have always gotten — a
profligate waste of resources to
keep us entertained. Yes,
friends, we are that shallow."
My reponse to the paragraphs was:
I do not think the problem is so
much that we are shallow as that
we are committed to the
predominance of a specific social
institution: private credit
markets. The Mexican poet
Ocatavio Paz had this to say
about this institution:
"The market (by which he meant
private credit markets) is
efficient but it has no purpose.
Its only goal is to produce more
in order to consume more."
As long as the rate at which
money is being converted into
more money (which is to say the
rate at which consumption rights
are being converted into a larger
amount of consumption rights in
the future) is a central and
universal measurement of social
welfare it seems unlikely that
the fight against climate change
will gain serious traction. One
step forward will be followed by
one (or even two) steps
backwards.
I think that we need to create
community credit markets whose
goal is not to turn money into
more money but rather to create
and maintain infrastructure which
will serve the long term welfare
of humanity and of the biosphere
which supports us. If it is
impossible that we can create
such an institution because of
some aspect of the human psyche
which only allows short term
acquisitive greed to make
efficient decisions about credit,
then it seems likely that homo
economicus is an evolutionary
dead end that is unfit to survive
in the long term.