Feasibility of small-scale cold energy storage (CES) through carbon dioxide based Rankine cycle

• Cheap electricity of 3.2–4.8 kWh was converted into cold in a low temperature (−50 to −40 °C) in off-peak hours.
• Experimental results indicated a sufficient cold energy supply to the working fluid (CO2) in the Rankine cycle.
• Practical tests of the large engine achieved a net electricity output, showing the feasibility of the CES technology.
• The CES efficiency of the small scale CES system achieved 24–44% with the large engine.

The CES technology stores cheap electricity in off-peak time as cold energy and utilises the cold exergy for electricity re-generation in peak time, showing its significant value in peak-shifting. This study is in the aim of validating the feasibility of a small-scale (lab-scale) CES system as a fundamental step in the development of the CES technology. A small-scale CES system with a power capacity of ∼5 kW and total electricity storage capacity of approximately 10 kWh was developed. The experimental results of the CES system showed the feasibilities of the electricity-to-cold storage in the deep refrigerator, the cold energy exchange process for CO2 cooling in the Rankine cycle, and the cold-to-electricity conversion by the piston based engine system. The Practical tests of the large engine achieved a maximum net electricity output of −160 W in the case study of using compressed air of 10 bar to expand in the engine with a motor speed of 60 rpm, resulting in a practical CES efficiency of 24–44%. However, due to the technical issues such as the gas leakage and blocking in the engine system, effective approaches for improving the engine performance need to be further investigated.