Formation temperature range expansion and energy storage properties of CO2 hydrates

A hybrid thermal cycle is developed by storing liquid CO2 using an intermittent heat pump cycle (IHP) and forming CO2 hydrates (CHRs) using a CHR thermal cycle (CHT) with a high charge/discharge efficiency at low temperatures. The proposed technology for physical batteries operates the IHP in summer, when outside temperatures are high, and the CHT in winter. The operating period of a CHT with a high charge/discharge efficiency can be extended and the annual efficiency of IHP/CHT hybrid systems can be improved by increasing the temperature of the low-temperature heat source of the CHT. Therefore, cyclopentane, the crystal structure of which differs from that of CHRs, is added to the host fluid in this study. Then, the CHR formation temperature range is expanded based on the results of the experiment. When the CHR formation temperature (outdoor air temperature as low-temperature heat source) is increased from the conventional 0 °C–15 °C, the amount of generated CHR slightly exceeds the amount generated by a conventional system. These findings show that a CHT system, which conventionally requires a low-temperature heat source (approximately 0 °C), can now be operated at a moderate temperature of 15 °C. When CHR are formed at 0 °C for 18 h and then dissociated at 25 °C, the prototype reactor produces a high-pressure dissociated gas of 20.5 MPa at an initial formation pressure of 5 MPa. When cyclopentane is added to water in a 3500 cm3 vessel at the appropriate concentration, CO2 hydrates large enough to be produced even at outdoor temperatures of about 15 °C or less. The proposed system can be operated at outdoor temperatures below 15 °C. These results represent the close realization of the proposed CHT.