Storing Energy in Air

Below are excerpts from several articles about CAES, or Compressed Air Energy Storage. The titles of each section are a link to the original article, if you want more information. Besides that, there's a fantastic selection of articles about 'outdated' energy technology used throughout the ages here.

Solving the intermittency issue of solar and wind generation
Electricity generated by fossil fuels is increasingly unsustainable and a shift towards renewable energy – principally from the sun and wind – is vital. Renewable generation is already less expensive per unit than its polluting counterparts, but the fact the sun doesn’t always shine and the wind doesn’t always blow presents an obstacle to a serious takeover of the energy sector.

Energy storage could overcome this pressing “intermittancy” issue. If storage was available at sufficiently low cost and high performance, renewable energy would rapidly displace all other generation forms.

Energy is already stored, of course, in batteries or various other technologies. Even reservoirs can act as huge stores of energy. However nothing that exists or is in development can store energy as well, and as cheaply, as compressed air.

CAES at its simplest involves storing air in tanks: you just suck in some air from the atmosphere, compress it using electrically-driven compressors and store the energy in the form of pressurised air. When you need that energy you just let the air out and pass it through a machine that takes the energy from the air and turns an electrical generator.

Why not batteries?
Over their lifetimes, chemical batteries store only two to ten times the energy needed to manufacture them. Small-scale CAES systems do much better than that, mainly because of their much longer lifespan.

Furthermore, they do not require rare or toxic materials, and the hardware is easily recyclable. In addition, decentralised compressed air energy storage doesn’t need high-tech production lines and can be manufactured, installed and maintained by local business, unlike an energy storage system based on chemical batteries. Finally, micro-CAES has no self-discharge, is tolerant of a wider range of environments, and promises to be cheaper than chemical batteries.

Problems: Heat and Pressure in Large-scale storage
While pumped hydropower storage has a charge/discharge efficiency of 70-85%, and chemical batteries reach 65-90%, the CAES plants in operation in Germany and the US have an electric-to-electric efficiency of only 40-42% and 51-54%, respectively.

The low energy conversion efficiency is mainly due to the fact that air increases in temperature when being compressed to high pressures (both CAES plants operate at 50-70 bar, which is 10 to 20 times the air pressure in a bicycle tyre). Because the energy density of air decreases with rising temperature, both CAES plants remove the heat prior to storage and dump it into the atmosphere. This implies a significant source of energy loss.

Furthermore, when air is decompressed from a high pressure, the temperature decreases to such an extent that the water vapour in the air can freeze, thereby damaging the valves and the expander of the storage system. To prevent this, and to increase power output, both CAES plants heat the air in combusters using natural gas fuel prior to expansion. Obviously, this further decreases the energy efficiency of the overall process, rendering the present CAES systems entirely dependent on fossil fuels for their operation.

A conversion efficiency of 40-50% means that wind or solar power generation capacity must be doubled to make up for that loss. Consequently, we need more energy, more materials, and more space for the same energy output. The environmental friendliness of CAES is thus at least partly negated by its low efficiency.

Moreover, CAES’s low energy conversion efficiency is inherently linked to its low energy density, which means it relies on very large storage reservoirs. In principle, the energy density of compressed air can be greatly improved by using higher air pressures, but as the air pressure increases, more energy is turned into waste heat and the efficiency of the whole process further deteriorates. Consequently, a CAES system – in its current configuration – is always a compromise between efficiency and energy density.

Problem: Avoiding Energy Conversions
What can be learned from comparing historical and current technologies based on compressed air? A first and crucial difference is the number of energy conversions involved. In historical systems, mechanical energy (for example, from a waterwheel or a steam engine) was directly converted to compressed air (using an air compressor), and then – most often – converted back to mechanical energy (for example, moving a pneumatic hammer). Consequently, there were only two sources of energy conversion loss: in the air compressor, and in the air expander.

Compressed air is still vital to the productivity of many industries and services around the globe, being used in thousands of applications – from food packaging and metal smelting to the manufacturing of microchips and plastics. However, compressed air is now produced by air compressors that run on electricity. This introduces two additional sources of energy loss: the electric generator (which converts mechanical energy from an energy source into electricity) and the electric motor (which converts electric energy back into mechanical energy to run the air compressor).

As a result, today’s industrial use of compressed air is very wasteful: assuming each converter is 75% efficient, and assuming no other energy losses, only 30% of the energy input is converted into useful output.

1 Solutions: Take advantage of heat and pressure
1.1 Store the heat to warm the out coming air
Today, most CAES engineers are focused on further improving efficiency by using the waste heat of compression to reheat the compressed air upon expansion. This method is called “Advanced Adiabatic CAES” (AA-CAES) or “fuelless CAES” and removes the need to reheat with natural gas as in the standard “diabatic” CAES. The technology is expected to reach an overall efficiency of roughly 70%, bringing it closer to the efficiency of chemical batteries and pumped hydropower storage plants.

1.2 Use the cooling to cool spaces people use.
However, in bars and restaurants, these reheaters were not used. Instead, the cold air was used for refrigeration, freezing, cooling or ventilation purposes. In 1892, F.E. Idell described a Paris restaurant where “the exhaust was carried through a brick flue into the beer cellar. In this flue the carafes were set to freeze, and large moulds of block ice were also being made for table use, while the air was still cold enough in passing away through the beer cellar to render the use of ice for cooling quite unnecessary, even in the hottest weather.” 18

1.3 Use the technology for cooling first, side effect, running lights.
The use of compressed air for cooling or freezing sometimes went together with the production of electricity for lighting, driving a dynamo. In these cases, the air motors were basically worked for their exhaust, with electric light being the by-product. Taking advantage of temperature differences also happened in the earlier mining applications, where the exhaust of the rock drills helped to cool (and ventilate) the mines.

A similar and promising idea today, is compressed air energy storage combined with thermal storage to provide electricity, heating, cooling, refrigeration and/or ventilation at the same time. In fact, this approach also avoids several energy conversions, as it could replace refrigerators, freezers, air-conditioners and heating systems running on electricity. The method could work at the level of a city district or an industrial area, but it is especially interesting for decentralised energy storage.

Solutions: Small-scale, Low Pressure
Below air pressures of roughly 10 bar, the compression and expansion of air exhibit insignificant temperature changes (“near-isothermal”), and the efficiency of the energy storage system can be close to 100%. There is no waste heat and consequently there is no need to reheat the air upon expansion.

2.1 Solutions: Scroll compressors
Another novelty is the use of scroll compressors, which are the types of compressors that are now used in refrigerators, air-conditioning systems, and heat pumps. Both fluid piston and scroll compressors have a high area-to-volume ratio, which minimizes heat production, and can easily handle two-phase flow, which means that they can also be used as expanders. They are also lighter and less noisy than typical reciprocating compressors.

2.2 Solution: Multiple Small Tanks
To give an idea of what a combination of the right components can achieve, let’s have a look at a last research project. It concerns a system that is based on a highly efficient, custom-made compressor/expander, which is directly coupled to a DC motor/generator. Apart from its efficient components, this CAES project also introduces an innovative system configuration. It doesn’t use one large air storage tank, but several smaller ones, which are interconnected and computer-controlled.

The setup consists of the compression/expansion unit coupled to three small (7L) cylinders, previously used as air extinguishers, and operates at low pressure (max 5 bar). The storage vessels are connected via PVC pipework and brass fittings. To control the air-flow, three computer-controlled air valves are installed at the inlet of each cylinder. The system can be extended by adding more pressure vessels.

A modular configuration results in a higher system efficiency and energy density for mainly two reasons. First, it helps more effective heat transfer to take place, because every air tank acts as an additional heat exchanger. Second, it allows better control over the discharge rate of the storage reservoir. The cylinders can be discharged either in unison to satisfy a demand for high power density (more power at the cost of a shorter discharge time), or they can be discharged sequentially to satisfy a demand for high energy density (longer discharge time at the cost of maximum power).

By discharging the cylinders sequentially, the discharge time can be greatly increased, making the system comparable to lead-acid batteries in terms of energy density. Based on their experimental set-up, the researchers calculated the efficiencies for different starting pressures and numbers of cylinders. They found that 57 interconnected cylinders of 10 litre each, operating at 5 bar, could fulfill the job of four 24V batteries for 20 consecutive hours, all while having a surprisingly small footprint of just 0.6 m3.

Interestingly, the storage capacity is 410 Wh, which is comparable to the 360 Wh rural system noted earlier, which requires an 18 m3 storage vessel – that’s thirty times larger than the modular storage system.

The electric-to-electric efficiency for the 3-cylinder set-up reached a peak of 85% at 3 bar pressure, while the estimated efficiency for the 57-cylinder set-up is 75%. These are values comparable to lithium-ion batteries, but adding more storage vessels or operating at higher pressures introduces larger losses due to compression, heat, friction and fittings.

Nevertheless, when I e-mailed Abdul Alami, the main author of the study, thinking that the results sounded too good to be true, he told me that the figures were actually overly conservative: “We stuck to low pressures to achieve near-isothermal compression and to ensure safe operation. Operating at pressures higher than 10 bar would create serious thermal losses, but a pressure of 7-8 bar may be beneficial in terms of energy and power density, though maybe not in terms of efficiency.”

Case Study, The Paris Compressed Air Network
Around the same time in Europe, the French took pneumatic power transmission one step further by setting up a city-wide power distribution network in Paris. It would remain in use for more than 100 years (from 1881 to 1994), distributing compressed air at a relatively low pressure of 5-6 bar over a network of (eventually) more than 900 km of mains, serving more than 10,000 customers.

Left: The Paris network in 1962
Black shows the ancien reseau (old network), while red shows the nouveau reseau. (new network)
This map is believed to show the compressed-air network at its greatest extent. There are now many mains extending outside the Thierry wall line.
In 1959 SUDAC had over 900 km of pipes.

The Paris compressed air network started as a system designed exclusively for regulating clocks by impulses of compressed air sent through subterranean pipes. By 1889, the network in Paris was regulating 8,000 clocks through 65 km of mains. The clock regulating service was retired in 1927, after it became clear that electricity was better suited for the job. However, by that time, the compressed air network in Paris had proved highly successful in small industrial and service establishments.

The French set up a city-wide power distribution network in Paris, which served more than 10,000 customers and remained in use for 100 years.

Already in 1892, F.E. Idell wrote that “among the smaller industrial purposes for which the air motors are used in Paris, I find the driving of lathes for metal and wood, of circular saws, drills, polishing machines, and many others. They are also used in the workshops of carpenters, joiners and cabinet-makers, of smiths, of umbrella makers, of collar-makers, of bookbinders, and naturally in a great many places where sewing machines are used, both by dressmakers, tailors, and shoemakers, from the smallest to the largest scale.”

Over the years, the share of commercial and domestic use of compressed air decreased, as electricity became more important. However, industrial consumption of compressed air kept growing, and many large factories in Paris – from car producers to glass manufacturers – were connected to the unique power distribution network until the very end. Dentists became new users during the 1970s and 1980s. 1218