Before we explain what Advanced Adiabatic Compressed Air Energy Storage is, we need to talk about storage.
A very basic question! Why store energy? Storage must cost, and according to laws of physics operating in this universe no operation made by humans can be perfectly efficient. So why not generate energy when you need it. But that is precisely the problem. Our life style is such that our demands for power have variation patterns- daily, seasonal, and random. The daily demand peaks at certain hours of day and night, and goes to a minimum at certain periods. There may be sudden unexpected demands for energy caused by special events. Large power machines working at high power cannot suddenly cut back on their output, and small machines cannot cope up with large increases in demands. This had necessitated the development of energy storage even before the advent of renewable energy.
Renewable Energy Complicates the Picture
Solar and other forms of renewable energy are very desirable because of their green nature. But the incorporation of renewable energy into our power complex has made storage capability even more important. For example, solar energy is not something we can harness when we need it. Our ancestors have been tailoring their life-styles to comply with the sun hours. In the modern, industrialized world, that is not possible. Further, even sunshine does not follow a strict clock. Again there are daily, and seasonal changes complicated with severe unpredictability. Even if you can predict rainy weather days in advance, what can you do to make the sun shine through clouds? You just have to find ways of living with bad weather, and hours of darkness.
The Patterns Don’t Match
It would be very useful if the daily variations in energy demand and those in sunshine matched. Unfortunately, the two are quite uncorrelated. There is high demand for energy in the early hours of the night when the sun has already gone down. And solar energy availability peaks in the afternoon on a clear day. So energy storage is not only important but essential.
Methods of Energy Storage
Scientists and engineers have investigated and are continuously investigating various methods of storing excess energy. They are exploring all possibilities offered by laws of nature to store energy. Mechanical, thermal, chemical, and electrical storage systems are possible. And recently, engineered electroactive microbes have joined the list of possible energy storage mechanisms.
Considerations in Energy Storage
What considerations are important in developing an energy storage system? There are quite a few:
Cost is always an important factor in any operation. Can we afford the level of initial and running cost for the system we are looking for? Will it pay back the initial cost, and in how much time? Will it give us net profit and how much?
A simple system is always preferable to a complex system. Complex systems tend to be less reliable, and more difficulty to maintain. Staff need better training and there may be need for a bigger spares inventory and complex maintenance equipment.
This is an important parameter. How much energy will it store in a given volume. Obviously, space is always at a premium even in a desert. A large dispersed system poses problems of monitoring, maintenance, and security in addition to raising the cost.
Can the system be up-scaled? That means if we can make a model to store W units of energy in volume V, can we make model which can store n xW units of energy in approximately nx V units of volume. Or can we connect n such units together to increased the storage capacity n times?
This the ratio of energy usefully retrieved to the energy consumed. The energy consumed includes the energy consumed by the apparatus used in storing. Mechanical and thermal losses in the storage and retrieval process reduce this efficiency. Additionally, if energy has to be converted into a different form, eg, from electric to thermal and vice versa, there will be conversion losses also, which reduce the cycle efficiency further.
Different systems have different maintenance problems. Generally, simpler systems will need less maintenance effort and cost for a given amount of energy storage. The level of expertise demanded of the maintenance staff and their number will also be lower.
This is a very important factor. The very need for storage arose because we could not match availability of solar energy with the changes in demand. Hence, a storage and retrieval system must able to react to fast changes in demand. Purely electrical and chemical systems have little inertia and can respond to demand changes fast. Thermo-mechanical systems involving furnaces and turbines will normally increase the reaction time.
How long can energy be stored in a system without loosing a significant amount? Dynamic mechanical systems will invariable lose energy over time. Similarly, electrical and chemical storage systems will tend to lose energy as time passes due to internal leakages.
Mechanical Energy Storage
Mechanical energy storage can be either dynamic or static. A flywheel is an example of a dynamic mechanical energy storage system. Dynamic systems involve movement, and hence friction, leading to loss of energy and inefficiency. The efficiency decreases with storage time. Dynamic systems also involve wear and tear and hence, greater need for monitoring and maintenance. Maintenance may demand downtime. Dynamic systems may also have limitations in upscaling. Static systems will involve no friction, and friction loss during storage period will be minimized. Energy conversion losses however may be present.
An example of static (or more precisely, pseudo-static mechanical energy storage is compressed air energy storage (CAES). Available excess energy is used to compress air into a much lower volume. Allowing the air to expand will release energy which be gainfully utilized to drive a turbine as in conventional thermal power plants. The storage is similar to storing energy in a compressed spring which will hold the energy as long as it is not allowed to expand. Energy can be recovered from the spring when it is allowed to expand.
Conventional Compressed Air Energy Storage Vs. Advanced Adiabatic Compressed Air Energy Storage
In principle energy is stored in the air when it is compressed. This air can remain stored till we need it and then allowed to run a turbine- just like the one in a thermal power system- to recover the compression energy. However, there are practical difficulties. According to the well-known gas laws air at ambient temperature will heat up when compressed and cool when allowed to expand back. While in storage the heated air will lose energy to the huge mass of the storage system. The stored compressed gas will further cool when allowed to expand for the energy recovery when required. Efficiency of turbines depends on the inlet gas temperature. The cool gas must be provided additional heat energy to bring it to a suitable temperature. This reduces the round-trip efficiency of the storage system.
Diabatic and Adiabatic Processes
A thermal process (flow of thermal energy) in which no external energy is added or extracted is called a DIABATIC process. The conventional compressed air energy storage described above uses a DIABATIC thermal process. (Please note the spelling: it is diabAtic and not diabEtic as for sugar patients). If we can manage to store away safely the heat produced during compression, and return it to the air during the expansion process at the time of energy recovery, the process will become an adiabatic or non-diabatic process. The adiabatic compressed air energy storage is there more efficient because little or no reheating energy is required before feeding to the turbine during energy recovery.
The heat energy during compression is extracted through heat exchange with suitable structures which can store the heat in an insulated environment till required. These could be oil or molten salt systems in well-insulated enclosures. During energy recovery from the compressed air, these same TES give up heat to the expanding air thus pre=warming it for the turbine input. Little or no heating is required from outside. This makes the adiabatic compressed energy storage system very efficient with a theoretical limit of 100%. Practical values achievable are estimated to be about 70% which is much better than the diabatic storage systems.
Large volume structures are required for the compressed air energy storage systems. Creating such structures is costly. Desirable options are hollow underground geological structures like abandoned salt mines, mines created artificially by dissolving and pumping out underground salt deposits, and even abandoned gas pipelines. Underground aquifers have also been considered. Similarly, undersea porous rocks could be possible candidates for the ACAES Systems.