Supercritical Carbon Dioxide and Its Application to Rankine Cycle

Introduction

The Global warming and climate change crisis have been a great concern in our human history. The main cause comes from the greenhouse gases (GHGs) emission produced by fossil fuel-powered generation facilities in such as the combustion process of a coal, oil or gas (Jacobson, 2009). The other significant cause also comes from burning of fossil fuels for driving vehicle, inefficient energy usage in homes and businesses operation. To deal with the problems, the prime importance is the greenhouse gas reduction with our highest efforts in science and technology.

1997 Kyoto Protocol, the world’s first climate change treaty, is a binding agreement for reducing greenhouse gas emissions with 192 parties’ agreement. The second commitment period (2013-2020) had been announced in 2012, and proposing the targeting cut of 20% GHG emission levels of 1990 by 2020. The Hydrofluorocarbons (HFCs) had been recommended and widely used in previous years as a working fluid in the industries, instead of other chemical working fluid, because HFC has no effect to the Ozone Layer Depletion (zero ODP). However, HFCs has very high Impact on Global Warming Potential (high GWP), which directly causes global warming crisis. On the other hand, HFCs is predicted approximately 2 – 4% increase of the overall climate forcing impact by 2050 (Marley, 2001). Due to the fact mentioned above, in 2013, the international agreement of industrial countries to phase down HFC use under the Montreal Protocol has started, targeting 80% reduction of HFC by 2030 (Su, Fang, Li, Wu, Zhang, Xu, & Hu, 2015).

In the 21^st session of the Conference of the Parties (COP), which was held in 2015, to the United Nation Framework Convention on Climate Change (UNFCCC), so called “Paris Agreement”, also suggests to reduce the carbon emission at the earliest and handle the increasing of global average temperature to well below 2 °C (Victor and Kennel, 2014). To achieve the objective, the natural working fluid such as carbon dioxide (R-744 or CO₂), which is also listed in the required collective control in the Kyoto Protocol has recommended to utilize and use instead of HFCs. Taking in account of these reasons CO₂ itself is environmentally friendly, which is by definition 0 and 1 of ODP and GWP, respectively.

In the meaning of recycling and utilizing of GHGs, the interest to use CO₂ as a natural working fluid in the thermodynamic cycle has been greatly increased due to its high vapor pressure and large volumetric refrigeration capacity when compared to other working fluids. Moreover, the critical pressure and critical temperature of CO₂ are 7.38 MPa and 31.1 °C, respectively, which are very much lower than other working fluids as well. Thus, CO₂ can become as supercritical state easily even when it is operated at comparatively low temperature. The characteristics and properties of various working fluids are compared in Table 1 (Kim, Pettersen and Bullard, 2004). The other point of interest in CO₂ properties is that classified as non-flammable, non-toxic working fluid and also chemical inactive, which are safe to human health and the environment. From these reasons, many attempts have been made to replace ordinary working fluids to CO₂ in power cycle, in which the Rankine cycle system is also one of the efforts.

Kim et al., 2004.

R-12: dichlorodifluoromethane; R-22: chlorodifluoromethane; R-134a: tetrafluoroethane; R-407C: ternary mixture of difluoromethane/pentafluoroethane/tetrafluoroethane (23/25/52, %); R-410A: binary mixture of difluoromethane/pentafluoroethane (50/50, %); R-717: ammonia; R-290: propane; R-744: carbon dioxide.

^a Ratio of saturation pressure at 0 °C to critical pressure.

^b Ratio of 273.15 K (0 °C) to critical temperature in Kelvin.

^c Volumetric refrigeration capacity at 0 °C.