Why the Green Energy Transition Won’t Happen

Calliban · 2023-10-03 20:33:01

This ancient Persian freezer, preserved food all year round.
https://m.youtube.com/results?sp=mAEA&s … an+freezer

Similar to the old ice houses in European countries. During winter, ice would be loaded into the ice houses. The large size resulted in a low surface area to volume ratio for the structure, reducing heat transfer rate into the ice. Some ice houses were built underground. Others were above ground and surrounded by thick earth berms. In both cases, soil provided insulation and the ice remained frozen year round.

If there is to be a renewable energy transition, we need to be prepared to enact solutions that are outside of most peoples mindset. An ice house, could store the food needed by a town of people. Individual households and restaurants would remove the food that they intended to use that day. Individual homes would have insulated larders, which would maintain cold temperatures. The large size and soil insulation of the cold store, would mean that it would only need cooling intermittently. In fact, the heat pump doing the cooling might only need to be activated during the depths of winter when outside temperatures are at their lowest. The actual power needed to maintain cold conditions should therefore be quite low.

tahanson43206 · 2023-10-03 21:41:59

For Calliban re #176

Just so you know someone reads your posts...

In fact, the heat pump doing the cooling might only need to be activated during the depths of winter when outside temperatures are at their lowest.

(th)

kbd512 · 2023-10-04 02:30:23

tahanson43206,

I've actually been revisiting some of Calliban's posts about trompes and using waste heat from compression.

Calliban · 2023-10-04 03:14:35

Thanks both.

I have always been sceptical that a renewable energy transition is possible with anything resembling high living standards. But given that we are being forced down that path anyway, I try to examine the art of the possible in terms of solution sets.

It is remarkable how much of the energy we consume as a society is heat in one form or another. Either as hot or cold. Most of our electricity is generated from heat and a lot of it ends up being consumed as heat. Of course, all work energy ends up as heat in the end as entropy disperses it. In the northern hemisphere, where most people live, natural energy flows are highly seasonal. The wind blows strongest in autumn and winter and sunshine is most abundant in summer. Biomass grows in spring and summer and is harvested in autumn. If we are going to find ways of living on these energy sources, then the solutions must either incorporate large amounts of long-term storage or accept the fact that we can only use energy when it is available.

For heat, long-term storage is possible, but the solutions are constrained (I will come back to this). For electricity or mechanical power harvested from the sun, wind, waves and tides, short term storage may be possible, but long-term storage is impractically expensive. Even short term storage is expensive, because an energy store is in effect another powerplant, with its own embodied energy and operating costs, which is needed entirely because of the intermittency problem with renewable energy. In fact we need three powerplants if we want renewable electricity on demand with any reasonable reliability: A solar plant that generated in summer, a wind powerplant that generates in fall and winter and an energy store that covers lulls in the combined output from both powerplants. This ends up being impossibly resource intensive. For mechanical and electrical power, I suspect that much of society simply needs to adjust its working arrangements around an energy source that is only available intermittently. I will explore that later.

Heat is a different case. Heat can be stored in very cheap materials at a relatively high energy density. The energy storage density of water heated to 100°C, is comparable to an electrochemical battery. But water is cheap, both in financial and resource terms, compared to any electrochemical battery system. So we could store large amounts of energy in heat very affordably. But we run into another problem. If we store heat in a container at a temperature much higher than its surroundings, the heat will conduct, convect and radiate away into the surroundings. To store heat for long periods, the container needs to be either very large, very well insulated, or both. Insulation has practical limits. The insulation required to an interseasonal heat storage tank for say a single house, woukd be bulky and expensive. Interseasonal heat storage tanks tend to be built for large heat systems scaled for towns or city districts. This has been done successfully in Denmark. But to heat large numbers of individual buildings from a single large heat store, requires insulated distribution pipework, which is expensive. It is only really practical in dense urban settings.

The other option for making use of long-term heat storage would be to accept the fact that it is a centralised solution. Instead of attempting to distribute the heat through pipe networks, we could focus heat loads around a central store. Instead of heating houses within the town, we could have a single heated community centre, containing a large sitting room, library, bath house, swimming pool, cafe, etc, where people would accumulate to keep warm in their leisure hours. A laundry would be built here to take advantage of the hot water. Low temperature cooking could be carried out for the whole town at centralised heat stores. This allows many of the functions of society to continue using stored solar heat. But it would clearly represent a departure from the decentralised solutions that people have grown accustomed to. It would be a different way of life. But in American towns, which are very much more spread out compared to their European counterparts, this may be the only solution that works. It would have the side benefit of rebuilding a strong local community.

For electricity, we face a problem. An electric grid must instantaneously balance the demand between consumers and suppliers. Elements of both demand and supply are variable. A mismatch between the two creates frequency swings which can rapidly crash the grid. This is why grid operators much prefer controllable power sources provided by fossil and nuclear fuels. In some places, solar thermal can generate electric power year round. Some 12-24 hours of heat storage could allow these plants to generate at close to 100% capacity factor. But most locations cannot achieve that. Practical solar power is available only in summer in most locations where most people live and in places like Northern Europe, it may not be practical to use solar energy to generate large amounts of grid power longterm. The wind could provide substantial power, but is highly variable and seasonal. The present solution of using wind and solar backed up by natural gas CCGT plants with curtailment, is highly resource intensive and is unlikely to be sustainable. Long term, demand needs to adapt to supply, not the other way around. If we are sourcing electrical and mechanical power from intermittent sources, then we must adjust our demand patterns to use the energy when it is available and curtail our use when it is not. This will not be an easy adjustment to make. In a later post, I will attempt to explore it in more detail.

Transportation on an intermittent energy base presents problems of its own. We have discussed before using hot water, compressed air and batteries to power short range vehicles. Rail provides an excellent solution for people and freight. The wind can provide direct mechanical power to ships for ocean, sea and coastal transportation. Stored heat could power marine transport over short distances. The wind can power capsule pipeline transportation over land, by pumping water through pipelines and carrying neutdally bouyant capsules along. This would be slow and speed would be variable. But this may be a good way of moving heavy freight that can take its time. Next day delivery will be a thing of the past in a renewable energy powered economy. Long distance distribution may still be possible, but it will take time and forward planning. Say goodbye to JIT.

Last edited by Calliban (2023-10-04 04:25:41)

Calliban · 2023-10-04 05:38:58

On the topic of the trompe hydraulic air compressor: This could have useful applications for wave power.
https://en.m.wikipedia.org/wiki/Wave_power

Wave power has lagged behind development of wind and solar power, partly because it is a more niche resource and partly because of the difficulty of engineering systems that can extract power efficiently, whilst also surviving storms and corrosion in the marine environment with a reasonable design life. Maintenance of moving parts on a moving platform at or beneath the surface of the sea, is also difficult.

None the less, wave power is more promissing in some respects, because the power density of waves is much greater than wind or sunlight. Ocean waves carry an average of 20-50kW/m which compares to a time-averaged power output of 100 - 150W/m2 swept area for a large wind turbine. This means that wave power could eventually produce mechanical and electrical power with a lower investment of materials, with a commensurate improvement in EROI compared to wind power.

Wave power devices tend to be deployed far offshore, still on the continental shelf but in water depth of around 100 - 200m. At this depth, the continental shelf has funnelled the energy, increasing the wave amplitude, but friction and turbulence have not dissipated much energy. So the continental shelf, several miles from land is the optimum position to harvest wave energy. It affords us with another opportunity as well.

Human beings have built many machines and structures over their history. Generally, the products that last the longest are those without moving parts. Our oldest structures are compressive masonry, which do not experience flexural loads. Wind turbine towers are steel tubes, subject to repeated flexural loads in an aggressive marine environment. The resultant corrosion fatigue limits their operational lifetime to 20 years. The same is true for oil rigs, whose design life is approximately 20 years. Most container ships manage about this level of life expectancy as well, as steel hulls experience repetitive flexing as ships ride over waves. This offers lessons in how we should apply engineering to the development of wave energy. Many dynamic wave harvesting technologies, are deliberately designed to flex with waves in order to extract energy. Floating devices must similarly be tethered to the bottom and the tether will experience repeated tensile cycles. Given that the energy density of the waves is fixed, the overall system EROI and materials efficiency will be proportional to the operating lifetime of the device. This suggests that we should design with three principles in mind: (1) Devices should avoid tensile loads, especially cyclic tensile loads. (2) Devices should avoid use of moving parts, which will wear out and need replacing under difficult conditions. (3) Masonry, such as brick, concrete and stone, has much lower embodied energy than steel and polymers. Humans have built very extensive infrastructure from masonry. We could never have afforded modern cities if all structures had to be steel.

This suggests to me, that wave energy harvesting devices should be compressive, masonry structures without moving parts. This is where the trompe comes into its own. Wave energy is concentrated in surface water and energy density drops rapidly with increasing depth. So our harvesting device needs to be on the surface of the water, but needs also to be anchored to the sea bottom. The easiest solution would be a concrete funnel type structure, that funnels incoming water into a taper. The funnel will sit on concrete legs, that transfer load to the seabed. The funnel will be ballasted with rock and sand, such that all forces acting on the legs remain compressive, against the worst dynamic load imparted by storm waves. To avoid life limiting corrosion, we should not use steel reinforcement. Cast basalt and glass fibres can provide reinforcement for the concrete legs.

Back to the funnel and how it will work. As waves crash into the funnel, the taper accelerates the incoming water, resulting in a pressure drop that can be used to suck in air through a venturi. The pipe carrying the bubbly water then bends downward, and travels down to a concrete receiver tank on the sea bed. As water is pushed into the funnel and down the pipe, the air bubbles compress. When they eventually reach the receiver tank at the bottom, pressure will be 10 bar(g) at a depth of 100m. Multiple devices would form a line along the coast. Flexible hoses would connect multiple receiver tanks to an air main, carrying the compressed air to shore. The air can be used to generate electric power, or alternatively, direct mechanical power for various applications. The undersea tanks will provide a store of air. The entire offshore device will have zero moving parts. Although the input to the trompe is pulsed, air will not be able to escape in appreciable amounts between pulses, because the driving velocity of the incoming water, will exceed the terminal velocity of bubbles between pulses. Incoming water would drive bubbles down the pipe a distance of X units. Between pulses, the bubbles would rise back up the pipe, say, 0.5X, before the next pulse drives them down again. So it is unlikely that any valving arrangement would be needed on the trompe.

Could we store energy in compressed air under the sea? At 10bar(g), 1m3 of air will store some 0.73kWh of energy. Let us assume we can recover that energy with 80% efficiency. So 0.58kWh/m3 recoverable energy. Let us assume storage tanks are 10m high. Given four decades of deindustrialisation, UK baseload electricity consumption is about 30GWe. To store 24 hours of power, we must store some 720 million kWh, requiring some 1.24 billion m3 of air. Stored in ballasted 10m tall concrete tanks, this would occupy a seabed area of 124 million m2 or 124km2. If we assume that the air tanks have some space between them, lets say their packing density on the seabed is about 50%. Air tanks capable of storing 1 day of UK baseload piwer, would cover an area 248km2. That is asquare about 10 miles aside, about the size of a city centre. The thing that makes this idea practical is the fact that air tanks could be thin walled concrete, hemispherical end cap cylinders. These would be partially flooded and manouevered into position on the sea bed. They would then be ballasted with dredged sand and gravel. This would ensure that as the tanks fill with air, all wall stresses remains compressive. This would allow even a thin walled concrete structure to function as an air store for decades or centuries, without age related degredation. Even if installation is expensive, long system life would allow undersea compressed air storage to be added incrementally.

Last edited by Calliban (2023-10-04 07:00:19)

Calliban · 2023-10-04 07:19:54

Could undersea CAES be used to store energy in the North American Great Lakes?
https://en.m.wikipedia.org/wiki/Great_Lakes

In the previous example, I calculated that to store 300GW-days of energy at a depth of ~100m, some 1.24 billion m2 of air must be stored undersea at a pressure of 10bar(g). The great lakes have a surface area of 244,000km2. If storage was apportioned in each of them according to surface area, then storing this much air would raise lake level by 5mm. This is almost negligible. I note that four of the lakes have maximum depth greater than 200m and Lake Superior maximum depth exceeds 400m. Air stored at 400m, would contain about 5x more energy per unit volume than air stored at 100m. Lake Superior alone, could store enough air to meet the electrical requirements of North America for about 5 hours, raising its level by only half an inch.

I conclude that the Great Lakes could provide an extremely useful energy storage function for North America. The lakes have no use as a site for wave energy. But as a way of storing energy generated by other means in compressed air, they could be very valuable.

A wind energy map of North America indicates that the Great Lakes and the areas around them have relatively good wind resources at 100m height.
https://www.nrel.gov/gis/assets/images/ … -nm-01.jpg

In this area, both onshore and within the lakes, large mechanical wind turbines could be built. Instead of driving electric generators, these turbines would pump water into trompes, generating compressed air. Compressed air would be directed into pipelines, which would charge the undersea air tanks. Large generating stations (say, 2GWe each) would be built onshore, converting compressed air into electrical power supplying the grid.

Last edited by Calliban (2023-10-04 07:33:11)

kbd512 · 2023-10-04 13:15:46

Calliban,

It seems as if storing compressed air at the scale required would be easier to accomplish at greater depths, up to a point.

How thick does the steel need to be if we're using A36?

At 1.5" for a 60" diameter vessel / 63" outer diameter, 588psi, I get about a third of the yield strength of A36, 12,350psi, as the hoop stress.

My concept of operations is that tanks are filled and emptied, pumped into a shore-based supply system every day.

I thought you might like this:
HK Porter - Modern Compressed Air Locomotives - 1914

Last but certainly not least, since we're going to store compressed air in the rooms of her ice water mansion...

Will the gales of November be remembered?

Sorry, I couldn't resist. Seriously, though, setting this up will be dangerous.

There needs to be a pipeline built back to shore and compressed air transportation infrastructure created. It would seem that HK Porter indicates that re-heating with hot water also extracts significantly more work from the expanding air.

Calliban · 2023-10-04 16:01:05

According to Wiki, which isn't always a reliable source, A36 has minimum yield strength of 36,000psi, which is 250MPa.
https://en.m.wikipedia.org/wiki/A36_steel

Assuming a pipe with diameter 60" (1.52m):

t = Pr/Ys = (4,000,000 x 0.76) / (250,000,000) = 4 x 0.76/250 = 1.216cm

Assuming a design factor of 3, gives a wall thickness of 3.65cm, or 1.44". So 1.5" wall thickness A36 tube would do the job. Welds could be problematic, especially in cold temperatures. That is something I hadn't considered before. Deploying the piping and air tanks will require a lot of diving work, which will not be cheap. For this to work as a sensible investment, I think we would need to be assured of long system life. Ideally, we want systems that can be installed once and used forever, or at least for several decades.
*******************************************************************

Additional 1: A bit of problem with my undersea CAES idea. At 40bar(g), 1m3 of compressed air contains some 15.23MJ of potential energy. At the same pressure, natural gas would have density of 27.87kg/m3 and would contain some 1494MJ/m3. So compressed air at 40bar(g), has only 1% of the volumetric energy density of natural gas. This means that for the same amount of delivered energy, the compressed air pipeline would be 100x more expensive than an NG pipeline of the same length.

Whilst I cannot say that this renders the project impractical without a full cost study, it does suggest that air pipelines would be much lower power density than NG pipelines. If air has to be piped a long distance under water or on land, then the capital cost of the pipeline could become a significant cost driver. So keeping pipelines as short as possible will be important. I don't think it will be practical to pipe air over long distances in the way that natural gas is piped. The volumetric energy density is too low for this to make much sense.

Additional 2: Regarding the 1918 locomotive. It is reassuring to learn that practical systems were developed at WW1 technology levels.

Last edited by Calliban (2023-10-04 16:40:15)

kbd512 · 2023-10-05 10:22:35

Calliban,

If we can't obtain the natural gas at a reasonable price, then what matters most is having a workable alternative energy solution that can truly last forever. As you and the people you read have pointed out, hydrocarbon fuels won't last forever. An increasing percentage of our total energy supply needs to be sourced from truly sustainable alternatives to combustion. Air, water, steel, and concrete seem like a better starting point than unsustainable electric / electronic alternatives. As compared to all hydrocarbon fuels, alternative energy solutions that don't start with the word "nuclear" in their title are going to have very low energy density. For most of our modern conveniences, hot water and refrigerant loops or compressed air are adequate to the task, even though they're not ideal.

So... About that train. I don't know if I did the math correctly, but...

GE AC6000W Diesel-Electric Locomotive (prototypical of the existing prime mover solution for rail freight)
Lenth: 76ft
Width: 10ft 3in
Height: 16ft
Weight: 432,000lbs / 195,952kg
Fuel Capacity: 5,500 US gallons
Engine: 7FDL16, 4,400hp / 3,283kW, 200 - 1,050rpm
Tractive Effort: 188,000lbf (start) 166,000lbf (continuous) at 11.6mph
Adhesion Factor: 2.16 - 2.35
Max Speed: 75mph on worn wheels

In 2022, Union Pacific's trains reached an average speed of about 23 miles per hour. The average speed is influenced by network fluidity.

We will never compete with diesels for range and speed, but maybe we can compete on total operational cost over time, which includes all costs related to extraction of energy. If the prices for gas and diesel become untenable, then we should have a backup solution fully developed and ready for implementation at scale.

Eglin ES-5 steel yields at 216ksi, so rather than AAR TC128 Grade B steel, we need ES-5 steel for compressed air and hot water tanks. ES-5 contains little to no strategic alloying metals. It's similar in composition to garden variety scrap steel, with a special heat treatment, and does very well on notch tests down to very cold temperatures. ES-1 steel, which retains much of its strength up to 950F, might be the alloy of choice for the hot water tank, even though it does contain a small amount of Tungsten.

Compressed Air Tank Design
3" thick ES-5 steel cylinder, 42inD x 600inL
weight 156,200lbs
1,924ft^3 internal volume
200atm working pressure
384,800ft^3 of compressed air
720cfm per 1hp-hr, so 534hp-hr (534hp for 1hr)
hoop stress is 45ksi for a 4.8X safety factor
In conjunction with a hot water storage tank at 300C, we can expand the air volume by a factor of 12, so 6,408hp-hr

We have around 25,000 in-service locomotives here in the US. We need 100,000 of these new compressed air / hot water locomotives to complete a full day's work, though. It's an economic stimulus opportunity. Water at 300C is about 1,246psi, so far below the hoop stress imposed on the compressed air tanks. If the hot water and compressed air tanks are both ES-5 and the same thickness, then the hot water tanks remain well within the steel's fatigue limits, even with the loss of strength from elevated temperatures. Water starting at 300C with thermal energy transferred to the expanding air until the water temperature reaches 100C, should provide 6,408hp-hr.

To run the train at 25mph, we need a constant driving force of around 3,000hp, so each air / hot water locomotive provides 2hrs of run time. To obtain 8 full hours of runtime, we need 4 of these air/hot water locomotives per train. This is not ideal, but workable. We'll have a pair of these locomotives on each end of the train, which is what we typically use now. Our engineers can then run their locomotives for a full 8-hour work-day, pull into the next station to take on fresh loads of compressed air and hot water from an air and hot water services pipeline network, and then the locomotives are ready to run for another 8 hours.

After 100 years of continuous operation in the described manner, we've accumulated 36,500 stress cycles from filling and depleting the energy contained in the air and hot water tanks, or 109,500 (10^5) stress cycles if we ran them 24 hours per day. We'll use Silicon-based aerospace CVD coatings on the steel tanks / valves / fittings / tubing, as the oil and gas industry does, to inhibit corrosion. I think we can make a case that the operators will get their money's worth from these trains. Apart from the power turbine, the valves, wheels, and working fluids are the only other moving parts. Our oil consumption would be limited to lubricants.

Since there are no electronics required, no non-recyclable metals, drastically less complexity, and little in the way of major maintenance, I think it's safe to say that there's a permanence-based business case to be made here. If you follow the manufacturer's instructions, diesel-electric locomotives required 20,000hr overhauls that take about 9 weeks to complete. The US Army overhauls about 3 locomotives per year and it takes their people about 10 weeks per machine. Our overhaul would likely be much shorter and less frequent, unless corrosion becomes a problem. There are no combustion engines or electric motors to rebuild, only the power turbine and transmission.

Calliban · 2023-10-08 17:02:02

Kbd512, I decided to run the numbers on the idea of a compressed air powered train. It looks like it would work. But the weight penalty of the compressed air tanks suggests to me that distance between stops should be no greater than 200 miles. My assumption is a train powered by compressed air tanks charged 40bar(g). This would allow the deeper parts of Lake Superior to provide an undersea air store.

To drive a train at 25mph requires 3000HP, which is 197.5MJ/km. 1m3 of air at 41bar(a) contains some 15.225MJ. Therefore, 1m3 of air at 40bar(g) will propel a train some 0.077km in some 6.9 seconds. To drive for 8 hours (200 miles), would require some 4174m3 of air. A single DOT-111 tank car has a tank capacity of 131,000 litres. Therefore, some 32 tank cars would be needed to carry the compressed air. The tare weight of the DOT-111 is 29.5 tonnes. The weight of 32 cars would be 944 tonnes. The compressed air itself would weight 6.553 tonnes per car, or 210 tonnes total. So total air car mass would be 1154 tonnes when fully charged.

DOT-111 cars are only pressure tested to 100psi (6.8bar(g), 7.8bar(a)). So a standard DOT-111 could not be used to store compressed air at 40bar(g). We would need specifically designed air tank cars. The pressure vessels needed to carry the air would be substantially heavier than than the ordinary tanks. But the air itself has almost negligible weight compared to liquids. I am going to assume that we use high alloy steel for these tanks and that any weight addition added by the pressure tank can be balanced by mass savings in the load supporting trailer. So the air car weighs the same as a DOT-111 car. I may refine the study later to test that assumption.
https://en.m.wikipedia.org/wiki/DOT-111_tank_car

As air expands, it cools, because expansion energy is created by converting internal energy into work. To expand the air efficiently, it must be expanded in stages, with interstage reheating. My assumption is that this heat will be provided by water, heated to 169°C. This hot water will be heated prior to transfer to the train using either stored solar heat, in a hot rock energy store, or a small modular reactor. At this temperature, the vapour pressure of water is 7.8bar(a), or 100psi(g). To store this water on the train, a standard DOT-111 will be used. The change in internal energy between 300K and 442.5K is 600KJ/kg.
https://webbook.nist.gov/cgi/fluid.cgi? … fState=DEF

To reheat the air between expansion stages, we must replace all of the energy that is removed from the gas as mechanical work. The total mechanical work done by the train between 200 mile (322km) stops is equal to: 197.5 x 322 = 63,600MJ. To replace that expansion energy, the mass of hot water needed would be:

M = 63,600,000/600 = 105,926kg (106 tonnes)

The density of water at 169°C is 898.1kg/m3. So the water will occupy some 117,944 litres. The DOT-111 has a tank capacity of 131,000 litres. So a single DOT-111 can carry the hot water and will have a full weight of 135.5 tonnes. The total mass of the air cars and water car will be: 1154 + 135.5 = 1,289.5 tonnes.

Ignoring air resistance and assuming a zero gradient, the energy consumption of the train will be dominated by rolling resistance. We can calculate the extra propulsive work needed to haul the air cars and the water car, which are much heavier than the diesel they replace. The following equations apply:

Energy (Qd) = Driving force (Fd) x Distance (D)

Fd = M x g x Crr

The rolling resistance coefficient, Crr, of steel wheels on steel rails is 0.001 - 0.002. I will assume the higher value for conservatism.
https://www.engineeringtoolbox.com/roll … _1303.html

To move 1,289.5 tonnes, the additional energy cost per km would be:

Qd = F x D = (1,289,500 x 9.81 x 0.002) x 1000 = 25.3MJ/km

The added weight of the compressed air and water cars increases work energy consumption per km, from 197.5MJ/km (for the diesel train) to 222.8 MJ/km (for the air train). This is a 12.8% increase in energy consumption. We can adjust the required number of compressed air tank cars accordingly:

N = 32 x (222.8/197.5) = 36.1

Applying the same factor to the hot water, gives a required volume of 133,000 litres. In reality, the system would need to carry additional energy (and mass) to account for air resistance and the effects of gradients. If we scale up the capacity of the system by 30%, the total energy consumption of our train will be:

Qt = 1.128 x 1.3 = 1.47x diesel train work-energy consumption.

But as the train exhausts its stored air, its mass will reduce by: 210 x (222.8/197.5) = 237 tonnes. If we assume that cold water is also allowed to drain out of the train after its thermal energy is extracted, then the train will lose another 135.5 x 1.128 = 133 tonnes of weight. So the average weight of the air energy storage system, i.e the weight halfway through its journey, will be 87% of its starting mass, because air and water are exhausted and dumped overboard. So actual work energy consumption will be 1.28x that of the diesel train.

I conclude that using compressed air at 40bar(g) to propel a freight train between 200 miles stops is practical. Total work energy consumption of the train will be ~30% greater than a diesel, because of the much greater mass of the air propulsion system. However, the air train requires both compressed air and hot water, so total energy consumption will be twice as high as the work energy. For comparison, when the ~40% efficiency of a diesel locomotive engine is factored in, the energy consumption of the air train is almost identical to a diesel train, if we consider compressed air and hot water to be manufactured fuels and compare their internal energy to the higher heating value of the diesel. This is not that surprising, as both systems are heat engines, using a hot pressurised gas to drive the train. The air powered system weighs slightly more, but work energy efficiency is slightly greater. However, the air system energy stores are produced directly from the mechanical energy of the wind and the thermal heat of the sun. If we had to manufacture synthetic fuel, it would need to be made from hydrogen produced from electricity and then reacted with either CO2 or nitrogen to produce a storable liquid fuel. An air train may have some energy efficiency advantages if we get to the point where liquid fuels must be manufactured from hydrogen, CO2 and N2, rather than being refined from a liquid that we can pump out of the ground. But for these efficiency advantages to be realised, the distance between stops cannot be much greater than 200 miles, as beyond this the weight of the compressed air storage system would push up the weight of the train excessively.

Last edited by Calliban (2023-10-08 17:32:15)

Calliban · 2023-10-08 18:52:47

The compressed air energy storage system powering the train in Post 185, has an energy density of 49.3KJ work energy per kg of weight. Suppose we live in a country that has no domestic oil supplies and faces the risk of imports being cut off in the near future. Could we build vehicles capable of transporting people and goods within towns and cities using compressed air? I am going to assume a driving speed imposed by speed limits of 20mph and vehicles limited to a speed of 30mph. This makes engineering a car for crash protection easy.

I am going to use the Tesla 3 as a model for the body shape of the vehicle, because it is an extremely optimised shape. The Tesla 3 has a height of 1.443 and a width of 1.849m. The cross section area is therefore 2.67m2. Drag coefficient is reported as 0.02.
https://en.m.wikipedia.org/wiki/Tesla_Model_3

The drag equation is:

Fd = 0.5 x rho x v^2 × Cd x A

Calculating for 20mph (8.94m/s):

Fd = 0.5 x 1.2 x 8.94^2 × 0.02 x 2.67 = 2.56N.

Assumption: Car mass is 1000kg, minus payload. Rolling coefficient for the wheels is 0.005 for low resistance tubeless tires on concrete. This makes for a bumpy ride, but driving speed is only 20mph, so maybe we can tolerate it.
https://www.engineeringtoolbox.com/roll … _1303.html

Frr = m x g x Crr = 1000 x 9.81 x 0.005 = 49.05N

Total resistance = Fd + Frr = 51.61N

Suppose the car is carrying 2 passengers, each massing 80kg, with 40kg shopping in the boot. In this case, total vehicle weight increases by 20% and total reistance increases to 61.42N. I am going to assume that the engine and compressed air system account for half the empty mass, or 500kg. If the engine accounts for 100kg, then the weight of the energy store woukd be 400kg. The approximate range, R, on flat ground without braking, is:

R = (400 x 49,300)/61.42 = 321,000m

This amounts to 321km, or 200 miles.

From this example, I conclude that it is possible to build a basic air powered car that will provide transportation around towns a cities. The low speed assumed here (20mph) is only 5x walking speed. But limiting the vehicle to this speed allows a very light vehicle to remain safe in a crash and minimises energy losses due to air resistance.

The actual energy consumption of the car would be 61.42KJ/km of work energy, or 122.84KJ/km combined work and thermal energy. In the UK, cars drive 20 miles per day, on average. If this car were an air car, total energy consumption would be 3.96MJ. Assuming each air car carries an average of two people, energy consumption would be 2MJ/capita-day
https://www.nimblefins.co.uk/cheap-car- … mileage-uk

A city of 1 million people would require 1TJ compressed air and 1TJ hot water per day. Spread over 24 hours, that is only 11.6W of air power and the same amount of heat per capita. That is small compared to the 500W per capita average electric power consumption in Britain. So small, low speed air cars could provide sustainable transportation at an affordable energy cost.

kbd512 · 2023-10-08 23:20:52

Calliban,

A few questions, if I may:

1. Why not increase the train's storage tank pressure from 40bar to 200bar?

This reduces the required volume of the compressed air by a factor of 5. That's 7 tanks vs 32 tanks. If you switch to a 200bar air tank, then you may as well heat the hot water to 300C, because the same tank material contains less then half as much pressure to keep the hot water liquid. Both nuclear thermal and solar thermal can produce 300C hot water without issue. Insulation of storage tanks of the size we're talking about will ensure that heat losses remain low.

My envisioning of how this would work, is that the Great Lakes would compress the air to 40bar using trompes, so that's the service / supply pressure pumped around the country via pipeline and tank infrastructure, and then at the point of use at a train station or gas station for motor vehicles, the 40bar supply is further compressed to 200bar, the excess heat from compression is used to "top off" the temperature of hot water supplied by nuclear and solar thermal power plants, in order to raise the hot water working temperature in the vehicle's hot water tank to 300C. In places with lots of wind, wind turbines can supply the input power to further compress the air. You previously posted something about this to use combined heat and mechanical power from wind turbines.

2. As near as I can tell from the scientific literature, solid tires do very little to reduce rolling resistance unless both surfaces are smooth (train wheels on train tracks), or the speed is higher than the legal speed limit. Radial and bias-ply pneumatic tires can either be made fairly efficient or very inefficient dependent upon tread pattern, number of plies, tire geometry, and load.

3. Even running with the solutions you've proposed, is it not painfully obvious how wildly more efficient this proposed compressed air / hot water system is, relative to generating electricity or using combustion for everything?

There is no possible way that attempting to electrify everything or running everything off combustion is anywhere near as efficient as this would be. 4.64% of the total electric power people are presently consuming is a rounding error. We already have power galore. Available power is not the problem. We're trying to go about solving this problem by making everything electric, which has already run into a brick wall of energy and materials economics, and that's a real problem. What we don't presently have is a practical way to transport hundreds of millions of people without burning something.

A practical car doesn't need to go 400 to 500 miles to be useful. If a personal transport machine, powered by compressed air and hot water, rather than electricity or gasoline, represents a measly 4.64% of the UK's current electrical power consumption, then maybe we're onto something and we should present the facts of life to the decision makers. If people could buy practical compressed air / hot water cars for $10,000, that would represent the single greatest economic boom since cars first became widely affordable and available following WWII.

Direct heat consumption turns 1GWe nuclear power plants into 3GWt to 4GWt power plants. We already have enough heat energy coming out of our existing nuclear power plants that we don't need any additional infrastructure to power a fleet of 200 million passenger cars.

Professor Michaux thinks we're going to become a 2TW civilization, as compared to the 15TW civilization we have right now, and that people are going to have to "do without". If we can go to 2TW while all of our trains, trucks, and cars are still moving down the road, what powers them is wildly irrelevant to the fact that all of them are still running, without electricity and without combustion.

Calliban · 2023-10-09 01:44:47

Storing air at 200bar(g) is more efficient from a weight perspective. The pressure vessel needed to contain air at this pressure will be 5x heavier, but the air contains 7x as much energy per unit volume. Also, a high pressure air engine has superior power density, because the working fluid is denser on average. Fewer cars means reduced capital cost all else being equal. So higher pressure is better. But higher pressure either leads to more compression losses, or a more complex compression process, with more compression stages, each with their own interstage cooling. High pressure air can be stored at rail yards in PCRVs, with steel stressing tendons taking stress within a concrete pressure vessel. The tendons can be replaced and the material endlessly recycled. In this way, PCRVs could be made to last centuries. I suspect that the benefits of a higher air pressure still win out overall.

If I were to play Devil's advocate, I suspect that direct-electric using catenaries would outperform compressed air trains. Power is transmitted at 30,000V, allowing a thin cable to transmit many MW of power over a hundred miles between transformer stations, with very little line loss. The catenaries are an additional material and energy investment, requiring structural steel for the pylons and aluminium alloy for the cable. But an electrical train is much lighter than even a diesel train, because it doesn't have to carry its energy source. Power is transferred conductively. This is why the idea of a battery powered train is rather silly. The air storage system can be stationary in a direct-electric system and part of a CAES system supporting electric power supply. Weight then becomes less important. In neither case are battery materials needed. A cost study would determine whether the compressed air powered train or direct-electric train is the optimal solution. An electric train is a COTS solution, though I doubt an air train would require very much development work, as it is a simple concept.

A third alternative would be some kind of stored heat engine. We could use solar heated water at 300°C and allow it to boil to power an engine on the train. Between 300°C and 30°C, hot water stores about 1180KJ/kg. If our engine can capture about one quarter of this as mechanical work, then work energy density of hot water would be about 300KJ/kg.
https://webbook.nist.gov/cgi/fluid.cgi? … fState=DEF

This is comparable to an electrochemical battery, if one ignores the weight of pressure vessels. Stored hot water at 300°C may achieve a better overall energy density than stored air, because it can be stored as liquid, with density 735kg/m3, whereas air density at 200 bar is 240kg/m3. I will need to crunch the numbers on this one to be sure.

Stored hot water does simplify the system overall. But it only makes sense if that hot water can be provided by direct solar thermal or a nuclear reactor. If it has to be electrically heated then there is no point, as we effectively waste most of the work potential of electric power converting it to heat and back into mechanical power again. This is why we tend not to use stored heat engines for small applications. Though they could in principle be used to power large vehicles like trains and ships, which we have discussed before. A large molten silicon thermal energy store would be electrically heated, but the high temperature allows efficient energy recovery. If we can convert this heat into mechanical power with 40% efficiency using steam generators, then a molten silicon stored heat engine could rival a Li-ion batrery in terms of work energy density. But it would only really be a workable transport energy source for a large ship, because of the high insulation requirements of molten silicon at 1400°C.

For a train, we might use insulated containers of quartz to store heat. Quartz has specific heat of 1100J/kg. If we can heat it to 300°C and convert 25% of heat into work energy, then work-energy density would be 82.5KJ/kg. That is about the same mass energy density as our compressed air system. The bonus is that hot quartz doesn't need to be stored in a pressure vessel and it has a high density of 2700kg/m3. It can have steam generating tubes running through it. Something like this might even work for vehicles as small buses, short range trucks, or even cars. The insulation adds bulk, but not necessarily much weight.

Suffice to say, we have options. The discussion here is about which option fits best for specific applications. But there are ways of making society works using 'less' fossil fuels than we use today, without having to commit to an unsustainable PV, battery-electric future. Few things beat diesel in terms of overall energy density and cost effectiveness, so long as we can extract it from the ground and can live with its emissions problems. In fact, if anything else coukd, we woukd be using it already. There are things that can replace it sustainably in specific applications, but they tend to be more cumbersome and a little less energy efficient. But these systems can clearly be made to work.

Last edited by Calliban (2023-10-09 02:58:44)

Calliban · 2023-10-09 04:12:06

Additional. One option for a city car, would be to build an air compressor into a heat powered car with a hot storage tank containing quartz. The air compressor generates hot, pressurised air with a temperature of 300°C. We store the heat in the car and store the compressed air in a stationary CAES plant, that generates power for a house. District heat can be used to preheat the air in a stationary air engine. Or air can be used to generate direct mechanical power using air motors.

A stored heat car with a 50km range would need to carry a minimum of 37kg of hot quartz, assuming a work energy consumption of 61.42KJ/km and a 25% conversion of heat to mechanical power. Silicon dioxide has density 2700kg/m3. So the heat store would be a cube some 24cm aside. A 100kg block would be 33cm aside and would allow a 135km range on flat concrete at 20mph. It would still be small enough to fit under a car bonnet with insulation. I seem to remember discussing this idea before and there was some discussion of building a prototype. That is probably not practical for a lone engineer. But we can discuss the requirements and limitations here.

Last edited by Calliban (2023-10-09 04:17:44)

SpaceNut · 2023-10-18 17:47:20

The heat-pump nightmare is far from over

The trick is to store energy but not from the use of electrical to do it.

kbd512 · 2023-10-19 09:03:33

SpaceNut,

Speaking as someone who makes his living writing software for computers, there's an unhealthy obsession with disposable electronic devices that simply doesn't register with me. I see electronics as tools for sophisticated computation, rather than a suitable replacement for human civilization scale energy systems. The insistence on using electronics to generate electricity is as inappropriate a tool for powering entire nations as a hammer is for turning screws. Screwdrivers were invented for the express purpose of turning screws, whereas hammers were not.

There is nothing clean or renewable or recyclable about integrated circuit or photovoltaic semiconductors, wind turbine blades, or batteries. Sunshine and wind are renewable. The electronic devices being created to use that renewable energy are not themselves "renewable" in any way, shape, or form. All such electronic devices were made by burning absurd quantities of hydrocarbon fuels. These electronic devices contain a myriad of acutely toxic substances and the proceses for making them are equally toxic. Worst of all, we're not going to mine the quantities of metals required to create enough of them to replace hydrocarbon fuels unless the metals are sourced from other planets or asteroids. That recent discovery of a "large" Lithium deposit on the Oregon-Nevada border is equivalent to fighting a forest fire with a child's squirt gun.

Calliban · 2023-10-19 15:29:24

Britain is not the US. Most Britons live in densely populated towns. This makes ground source heat pumps impractical for most households. Use of piped hydrogen would essentially be a return to town gas, which was mostly hydrogen. Volumetric energy density of H2 is about one third that of methane. And most hydrogen is produced from steam reforming of NG. There would be no point transitioning from heating based on NG to NG derived H2.

Most towns in the UK should bite the bullet and begin installing district heat systems. Without fossil fuels, this is the only sustainable option.

Calliban · 2023-10-25 05:45:29

Compressed CO2 energy storage.
https://en.m.wikipedia.org/wiki/Compres … gy_storage

The advantage this has over compressed air, is that CO2 liquefies at room temperature under pressure, so your pressure vessel can be far more compact for the amount of energy stored. The downside on Earth, is that CO2 is a trace gas in the atmosphere. So you need a gas reciever tank to capture the expanded CO2 for reuse. The CO2 expands out of the bottle through an engine or turbine, producing work, and exhausts into the tank, where it is stored at atmospheric pressure. Energy storage is the reverse of that process, with the compressor sucking CO2 out of the tank and compressing it into the bottle.

On Mars, we wouldn't need a reciever tank, as the air is mostly CO2. We could power vehicles with compressed CO2. Each cubic metre of liquid CO2, can raise 66kWh of mechanical energy according to the link. This might work well for construction vehicles that can refill regularly from a trailer tank.

On Earth, this form of energy storage would not be good for vehicles. But it could be useful for offgrid energy systems. The need for a gas receiver adds capital cost. But the smaller size of the pressure vessel would more than compensate for this. One thing it has going for it is that electricity does not have to be part of the energy system. A small wind turbine driving a positive displacement pump would compress CO2 gas to 70 bar. The gas would be cooled through a heat exchanger and stored in a pressure cylinder as liquid. Mechanical power would be raised by reheating the liquid and boiling it. Appliances could be run either directly from the compressed CO2 gas or by hydraulics, driven by a gas-hydraulic pump. So a system like this could displace electricity use in a house. Residual electrical loads would be smaller, covering things like lighting and computing and could be met by a home solar system. It is much easier to provide a solar battery system for a 1000kWh/yr demand than for a 10,000kWh demand.

Last edited by Calliban (2023-10-25 05:57:09)

kbd512 · 2023-10-25 23:35:46

Calliban,

If you combine a nuclear decay heat source to heat up a tank of water, how much additional volumetric expansion of the CO2 do you think we could reasonably obtain from the hot water?

What do you think the sweet spot is for the size of the heat source and water tank, for a 40t fully loaded (15.2t on Mars) low-speed (20km/hr) off-road cargo vehicle?

At what point does adding more LCO2 / hot water / RTG start to become counter-productive (detract from payload or cause other operational problems)?

I want it to carry a 10t payload of collected materials in this (chunks of ice, Sulfur, Iron ore, Nickel-Iron meteorites, basalt, tanks of LCO2, etc). It's either an open top dump truck or flat bed type truck, unpressurized cab or no cab if it will be part of GW's land train concept.

Could we apply a similar concept here on Earth, using a nuclear reactor purely as a heat source, compressed air from offshore trompes, and hot water, not to power a vehicle, but instead to power a conveyor device to move ore from a mine to a smelter?

I'm trying to figure out how to replace mining trucks here on Earth, if such a thing is even possible. On Mars, a long enough conveyor is probably a non-starter, hence the use of a similar minor variation technology set for powering vehicles.

Calliban · 2023-10-26 00:15:27

I will look into this. An additional heat source will definitely add to the expansion energy of the CO2. A work energy of 66kWh/m3 works out to 300KJ/kg of CO2, or 240KJ/litre. A closed cycle diesel engine on Mars burning a hydrocarbon and liquid oxygen, would get work energy 4000KJ/kg bipropellant, assuming 40% engine efficiency. So the L-CO2 has much lower energy density than hydrocarbon, even if we have to carry the mass of the oxidiser. But if overall energy efficiency of compressed CO2 is better than a synthesised hydrocarbon, then the L-CO2 option may still come out ahead. It is worth examining in detail.

tahanson43206 · 2023-10-26 06:03:57

For Calliban re #195

Just lightly following the discussion between you and kbd512 ... wondered why your concept involves compressing CO2 on Mars?

There is a ready supply of solid CO2 and more is provided by the natural conditions there.

The solid form is easy to transport, it is compact and easy to work with (compared to the compressed gas). All that's needed is thermal energy to liberate the gas to do work, and at first glance, it would seem as though kbd512's low grade thermal energy supply would be sufficient for the task,

(th)

kbd512 · 2023-10-26 14:50:59

tahanson43206,

Thermodynamics of CO2 at different temperatures and pressures?

Liquefied CO2 will be in the range of 1,000-1,200kg/m3, which is over 2x more dense than LNG. Moreover, the density of heated CO2 at 300-bar and 600C is 170kg/m3, which is 2x higher than the equivalent density of steam.

That 2X density advantage makes the turbomachinery 10X smaller for sCO2 than for steam.

Solid CO2 / "dry ice" is theoretically 1.6g/cm^3, but actual bulk density is 1g/cm^3, just like water. One way or another, after solid CO2 is collected, it must then be used in liquid, super-critical, or gaseous form.

Edit: Apologies, I wasn't done adding to the post.

Last edited by kbd512 (2023-10-26 14:57:33)

kbd512 · 2023-10-26 17:57:01

tahanson43206,

I totally forgot to state my point. The isobaric heat capacity chart is why I want to use a hot water tank to heat our relatively "cold" LCO2 or SCO2 to operating temperature. If we had a hot and cold CO2 tank, a heat source for thermal power energy transfer, and the idea was to endlessly run a heat exchange loop with multiple tanks, we'd actually run out of thermal energy on the hot side before the cold side. It could be used in a closed loop with a heat source, but then you need more heavy CO2 tanks. As far as collection goes, yes, by all means, let's collect frozen CO2 and then compress it.

I want to "charge up" a hot water tank like a battery, or maybe a molten salt or low-melting point metal if that makes more sense (SR-90 gets hot enough to melt Lead), and then I want to use this hot tank "battery" to rapidly expand the CO2 (or compressed air here on Earth) for low-speed torque, with the ultimate intent of releasing the CO2 back into atmosphere (open cycle, to minimize weight and complexity), because obtaining more frozen CO2 at night and compressing it into a liquid is not a problem when you have a heat source.

tahanson43206 · 2023-10-26 20:19:51

For kbd512 re two posts ...

Thanks for those interesting (and informative) charts, and your explanation.

One detail that ** really ** caught my eye was this:

obtaining more frozen CO2 at night and compressing it into a liquid is not a problem when you have a heat source.

Not questioning your text here ... it's just that this process is not one I'd thought of before.

I remember that SpaceNut posted a chart showing the phase states of CO2, with a relatively small triangular shaped region where CO2 can be liquid. I'll have to try to find that again. it will probably help to show how adding pressure can liquefy CO2 directly from the solid phase.

(th)

Calliban · 2023-11-04 05:27:35

The electric car fad is now unravelling before our eyes.
https://m.youtube.com/watch?v=8P95NFlAnmY

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