New energy storage system

Calliban · 2024-02-15 19:39:01

Louis' original topic was about a crushed rock, sensible heat thermal storage system. The advantage it has is in being a really simple system, using victorian era technology to raise steam and constructed from cheap materials. Crushed rock and alloy steels. These systems are conceptually easy to build.

The downside is poor cycle efficiency. Despite what the proponents of this system claim, realistic round trip efficiency will be 30 - 40%. But that is efficiency of electrical energy recovery. The other 60 - 70% is going to be low grade heat, with a temperature of around 30°C. But the heat is by no means useless. If we can integrate this system for combined heat and power, its efficiency will be improved. We can calculate what is known as exergy efficiency. Whereas energy is always conserved, exergy is degraded by entropy. We can think of exergy as the work potential of energy. The work potential of electricity is 1, or 100%. The ideal work potential of heat can be calculated from the carnot equation:

W = (Th - Tc)/Th = (303 - 283)/303 = 0.066

In reality, a real heat engine gets half to two thirds of ideal efficiency. So W = 0.033 - 0.044.

Adding this to a 30 - 40% electricity recovery, gives an effective exergy recovery of 33.3 - 44.4%. Actual energy recovery may be close to 100%. The exergy recovery is better than it sounds if the alternative is generate heat by resistance heaters.

Calliban · 2024-02-16 06:20:44

A hot rock energy storage system could be scaled down to levels useful for offgrid applications.

A tank would containing rock and gravel bonded by concrete. It would be insulated with rockwool and sand and heated to around 300°C using resistance heaters. Steam pipes would run through it. During power recovery, water would boil in the tubes, producing wet steam which wouod be dried yielding dry steam and condensate, which woukd return to the feed pump charge vessel. Dry steam would generate power using a small turbine. The condenser water would have a temperature of about 60°C.

For each unit of electricity used to charge the system, some 25% would be recovered as mechanical power by the turbine and 75% as hot water, which can be used as washwater.

Concrete has a density of about 2000kg/m3 and a specific heat of 1KJ/kg.K. A cubic metre of concrete heated from 20 to 320°C, would store some 600MJ of heat, some 150MJ (41.7kWh) of which is recovered electrical energy. That is sufficient electrical energy for 2 -4 days of power for a household and sufficient heat for all washwater and much of the space heating needed over the same period.

The overall exergy performance of this concept isn't great, but has a heat equivelent of about 1.46, whereas a pure resistance heater would be 1.0. How did I calculate that?

Some 75% of the initial electrical energy is recovered as heat, with a temperature of 60°C. The remainder is recovered as electricity. Assume that the 25% electricity is used to run a heat pump extracting heat from an ambient 10°C temperature, and outputting water at 60°C. Ideal COP = 283/50 = 5.66.

N = 5.66 x 0.25 + 0.75 = 2.115. But real heat pumps might get only half the carnot efficiency. So adjusting for this gives an exergy equivelant of 1.46. Compared to a resistance heater providing pure heat for direct use, steam recovery gives us more value because of the high work potential of that 25% of recovered electricity.

Last edited by Calliban (2024-02-16 06:36:11)

Mars_B4_Moon · 2024-06-01 14:06:58

Some long-duration energy storage systems now cheaper than Lithium says BNEF

https://www.pv-magazine.com/2024/05/31/ … batteries/

Numerous long-duration energy storage systems have promised to offer energy storage cheaper than lithium ion, but the rapidly falling prices of lithium batteries and the early stage in the learning curve of these technologies have often left them chasing a moving target they can not catch.

However while most long-duration energy storage (LDES) technologies are still early-stage and costly compared to lithium-ion batteries, some have already or are set to achieve lower costs for longer durations, finds BloombergNEF.

BNEF has surveyed seven LDES technology groups and 20 technology types in its report and found that the least expensive technologies are already providing cheaper storage than lithium-ion batteries for durations over eight hours.

Thermal energy storage and compressed air storage had an average capital expenditure, or capex, of $232/kWh and $293/kWh, respectively. For comparison, lithium-ion systems had an average capex of $304/kWh for four-hour duration systems in 2023, so generally shorter-term storage.

Storage duration, project size, and location are key factors affecting LDES capex. Gravity energy storage systems, which elevate weights when charging and controllably drop them when discharging, have the highest average capex, at $643/kWh.

The cost reduction rate of LDES technologies will largely depend on the expansion of deployment and the development of routes to market in major regions, BNEF notes.

Terraformer · 2024-06-02 04:09:56

Hmm. Approximately $0.3/W-hr for compressed air. That's $300B/TW-hr. But plenty of scope for reductions when it comes to grid scale (underwater storage).

Calliban · 2024-06-02 09:57:56

Terraformer wrote:

Hmm. Approximately $0.3/W-hr for compressed air. That's $300B/TW-hr. But plenty of scope for reductions when it comes to grid scale (underwater storage).

One thing I like about the idea of home scale CAES is that the system can be quite simple, if we are able to take advantage of the heat generated by compression and cold generated by expansion. The compression heat can be used to supply domestic hot water. The expansion cold is useful for refrigeration. An abundant cold source can be used for desalination as well. There are all sorts of ways of integrating thermal energy storage into this system as well. Ice can be used to store the cold from expansion and used to pre-cool air before and during compression cycles. Paraffin wax or water can store compression heat, which will then warm compressed air prior to expansion.

I think something like this can be made to work powering short range vehicles as well. We use a mechanical wind turbine to generate compressed air and heat. The heat gets used in a town district heat system. The compressed air is piped to vehicle charging points. We could even fit vehicles with water tanks that freeze as the vehicle operates.

tahanson43206 · 2024-06-02 10:37:59

For Calliban re #30

Thanks for adding the suggestion of freezing water as a way of dealing with expansion of air as it pulls energy from the environment in order to expand to do work.

Your thinking seems to be evolving over time.

The water is an alternative to drawing thermal energy from the outside air, which would be the default.

(th)

Terraformer · 2024-06-02 10:51:23

The thing about water is, it has a high latent heat of fusion. 90kWh/m^3 roundabout. It's ideal as a heat store for CAES, especially since you can potentially use ambient heat to make up for losses. Still exhausting cold air of course, but nothing like as cold as it would be without heating. I suspect it's got strong advantages over sensible heat storage.

Calliban · 2024-06-02 11:43:08

If we can be satisfied with a shorter range between recharges, then a compressed air vehicle is a more sustainable solution than either ICE or batteries. There are no imminent resource shortages for iron and steel can be recycled.

Whilst steel is quite heavy, it is the most durable metal we have. Provided stresses remain under the endurance limit and corrosion is kept under control, there appears to be no limit to the working life of a steel pressure vessel.
https://en.m.wikipedia.org/wiki/Fatigue_limit
https://www.engineeringtoolbox.com/stee … _1781.html

The graph in the wiki page extends to 1E9 cycles. If you recharge once a day, that is over 2 million years! So a compressed air car coukd have a long operational life.

Weight is a problem for compressed air vehicles using steel vessels. I used the spherical thin walled pressure vessel equation for a 1m3 spherical pressure vessel, with a radius 0.62m and endurance limit of 325MPa. For a 20MPa storage pressure, wall thickness is 2cm. This gives a vessel mass of 763kg. That is about 40% of the curb weight of a Tesla 3. The air in the tank weighs another 250kg at 0°C (I am assuming we chill it). Assuming isothermal expansion at 0°C, the energy stored in the tank is 106MJ. I used this to work that out.
https://tribology-abc.com/abc/thermodynamics.htm

A Tesla 3 gets 7km range out of each kWh of battery. If an air vehicle can do the same, then a compressed air car could get 206km range. But I think that the weight of the vehicle would tend to increase energy consumption. So to compete with a Tesla 3 on a mile/kWh basis, a compressed air car would likely have to sacrifice some of that range. But the upside is that a compressed air vehicle can scale as a mass market solution. The EV cannot.

As a stationary energy store, weight is less important. A steel pressure vessel for energy storage would be expensive to build and procure. But if well cared for, it could last for many decades or even centuries.

Last edited by Calliban (2024-06-02 12:03:13)

SpaceNut · 2024-06-02 11:47:50

Compressed air energy storage (CAES)
Advanced Compressed Air Energy Storage Systems: Fundamentals and Applications
Compressed-air energy storage

Of course we need energy to do the compresion....

Terraformer · 2024-06-02 11:58:02

Calliban,

Perhaps it would be worth adding a paraffin (or other material) tank to store the latent heat for expansion? Extracting enough heat from the air could be tricky.

Calliban · 2024-06-02 12:18:22

For a stationary energy storage system, the ground, warm water or a phase change material like wax can provide a heat store for compression heat. There are some problems with storing air in steel vessels if we wanted to scale these systems up. For example, a 50m diameter steel vessel at 20MPa, could store 2000MWh of energy. The problem is that wall thickness would then be 0.77m (2.5'). It would be almost impossible to fabricate a pressure vessel with wall thickness that large. It is impractical welding steel at that thickness. One solution would be to use pre-stressed concrete pressure vessels, with the stress absorbed by steel tendons. There is no theoretical limit to the size of such a vessel. Doing this allows economy of scale.

But small systems are quite practical as well. In fact, air would seem to work well as a home based energy storage system. Since a home needs both heat and cold for different applications, we could achieve a high aggregate efficiency in a small system that makes use of hot and cold. The compressed air can be generated by entirely mechanical energy devices. A small wind turbine driving a staged piston compressor, would not need the rare metals, copper or aluminium of an electric generator. All components are made from low embodied energy materials. So this is something that can be scaled without stretching the Earth's resources.

Last edited by Calliban (2024-06-02 12:25:03)

Calliban · 2024-06-04 08:27:50

Liquid CO2 energy storage is interesting as a home energy storage solution.
https://www.sciencedirect.com/science/a … 0421010852

It works well if you need to store modest amounts of energy (a few kWh) and don't have a lot of space. The expansion tank can sit outside in a yard or could even be put on the roof. The CO2 bottle is compact enough to fit in a cupboard. If CO2 is chilled prior to compression, it is unlikely to require intercooling, because the gas is beneath its critical temperature of 31°C. But any heat that is generated has immiediate use in a domestic situation for hot water. Any cold generated on expansion, can be stored within an ice tank and used to pre-chill CO2 prior to compression. The ice tank could be kept within a cool room, in which food is stored.

An integrated system like this in which hot and cold are useful in themselves, will always be more efficient than a system in which electricity is the only output. Such a system also avoids the need for an inverter. The power needed to drive the compressor can be DC from a solar panel or direct mechanical from a wind turbine. The expander turbine drives an alternator, which produces AC power at 50Hz.

Liquid CO2 would work well in large power applications as well. A wind turbine could be fitted with a compressor, rather than an electricity generator. The compressor would take in gaseous CO2 that is piped up the tower at atmospheric pressure and would generate liquid CO2, which would be piped down the tower. A CO2 gas main and liquid main, would be installed underground, providing each turbine with gas and taking off liquid CO2. A central generating station would store liquid CO2 and generate power by expanding liquid CO2 to gas through a turbine. Expanded gaseous CO2 would be stored in a telescopic gasometer tank. This provides a buffer between the turbines and the electricity generator.

In large solar PV applications, L-CO2 avoids the need for inverters and transformers. Instead, a group of panels power a DC motor, compressing CO2 gas to a liquid in a pipe. Individual pipes connect to a liquid main. That main supplies a large expansion powerplant like the one described above. It is even possible to have both wind and solar infrastructure supplying L-CO2 to a single generating station. In this way, we get smoother power output and more consistant power year round, with solar generating more in summer and wind producing more in winter. But importantly, there is only one centralised powerplant generating AC for the grid.

Last edited by Calliban (2024-06-04 09:05:25)

Mars_B4_Moon · 2024-06-13 08:55:47

Work starts on claimed world's largest cryogenic compressed air storage facility

https://www.bbc.co.uk/news/articles/crgg81j2xdpo

Work has begun on a £300m energy plant which will store surplus electricity from wind and solar farms in the form of liquid air.

The facility at Carrington near Manchester, designed by Highview Power, will create more than 700 jobs in the north-west of England, the firm said.

The energy stored at the site, which is expected to be operational by 2028, will then be put back into the grid at times of high demand.

tahanson43206 · 2024-06-13 10:18:40

Thanks to Mars_B4_Moon for finding and showing the report on a liquid air energy storage plant in the UK.

Here is a snippet ...

Work is beginning on what is thought to be the world's first major plant to store energy in the form of liquid air.
It will use surplus electricity from wind farms at night to compress air so hard that it becomes a liquid at -196 Celsius.
Then when there is a peak in demand in a day or a month, the liquid air will be warmed so it expands.

The method of warming the stored air is not revealed. I note that thermal energy will be released as the compression cycle proceeds.

I think the reporters covering this idea may be missing the point. Liquid Air is the absence of energy. The energy is actually "stored" in the environment.

When the stored energy is needed, it must be retrieved from the environment.

Changing the point of view wouldn't change the value of the technology.

However, if we are feeding thermal energy into the environment while storing the air, then the environment will become warmer.

Likewise, if we are pulling thermal energy from the environment while releasing the air, then the environment will become colder.

If this concept is applied on an annual cycle, then energy invested in creating liquid air would heat a building during the winter, and energy collected from a building during summer would provide for cooling.

I note that we are in the Home Improvements Category.

Perhaps this concept might be applied to the home heating and cooling market?

(th)

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