Low Tech Magazine on Compressed Air Storage
Contents
Article
- Low Tech Magazine article - [1]
Notes
- 40-50% efficiency of air storage, compared to 70-90% for batteries.
- Pumping and air engines are 60-70% efficient.
- Scroll Compressor is nearly 100% efficient, but expensive.
- Highest ESOI of any energy source - [2]
- Conclusion of Reference [7] - The sizing of storage tanks for a SHS-CAES has been determine by modeling all the components downstream of the storage tank. To operate the system with SHS load of 29.65W for 12 hours requires a tank size of 18 m3, with an initial pressure of 8 bar and regulator setting 3.511 bar.
- Does this calculation reconcile with the 65 cubic meter figure below? Here we have 0.4 kWhr with 18m3 - so with ~3x that volume we have about 1.4 kWhr - so about 2x worse than the experimental extrapolation of the 65 cubic meter volume (see experimental below).
- 8 bar system operates at around 60% roundtrip efficiency - that is impressive.
- P. 60 of thesis [3] shows the size of tank required for 3kWhr storage - 65 cubic meter. This is based on 5% overall efficiency - abysmally low, but real for off-the-shelf compressor and air tool.
- P. 54 of reference [8] [4] shows only 5% efficiency of overall system was obtained, calculating the total wattage of output to the total wattage of input. This is about 10x less than a claimed 50% efficiency of air storage. What gives?
- P. 60 of Ref [8] states that tank cost would be $25k and overall $30k.
- Summary: efficiency of small scale prototype was 5% using an off-shelf tool motor (3Whr of usable power extracted from a 65l tank at 8 bar). This translated to a 3kW system being 65k liters. However, this is no-where near the predicted 50% roundtrip efficiency? Thus, indicates that the 65k liter figure is off by a factor of 10 for real, easily achievable results? Why was the efficiency so particularly low here?
Marcin Comment
The article seems a bit faulty.
It appearst that your analysis of the 65 cubic meter tank is faulty. You are implying that the efficiency there is decent (such as the 40-50% promised for 8-bar systems). Wading through Reference [8] - a master's thesis produced only 5% overall efficiency - so the 65 cubic meter figure is based on the 5% overall efficiency. You mention that the 200 bar system was 11-17% efficient. Can you reconcile this? It appears that the truth is - the low pressure systems that utilize off-the-shelf components are extremely inefficient.
