Application of Supercritical Carbon Dioxide for Solar Water Heater

For supercritical CO2, a small change in temperature or pressure can result in large change in density, especially in the state close to the critical point. The large change in density can easily induce the natural convective flow. In this chapter, a solar water heater using supercritical CO2 which is originally designed and constructed will be introduced. The solar water heater is a closed loop system with main components of an evacuated solar collector and a heat exchanger. The working fluid of CO2 is naturally driven by the large change in density with absorbing and transporting heat in the solar collector. And the heat energy (hot water) is produced by exchanging the transferred heat with water in the heat exchanger. This chapter will describe the typical system operation and performance at different season and climates.

Energy Conversion Using the Supercritical Steam Cycle

A supercritical steam (or Rankine) cycle is used today for more most of the new coal-fired power plants. More recently, it has been proposed as well for future water-cooled nuclear reactors to enhance their efficiency and to reduce their costs. This chapter provides the technical background explaining this technology. Some criteria for boiler design and operation, like drum or once-through boiler design, fixed or sliding pressure operation and coolant mixing, are discussed in general to explain the particular challenges of supercritical steam cycles. Examples of technical solutions are given for two large-scale applications: a coal-fired power plant and a supercritical water-cooled reactor, both producing around 1000 MW electric power.

Application of Supercritical Pressures in Power Engineering

SuperCritical Fluids (SCFs) have unique thermophyscial properties and heat-transfer characteristics, which make them very attractive for use in power industry. In this chapter, specifics of thermophysical properties and heat transfer of SCFs such as water, carbon dioxide and helium are considered and discussed. Also, particularities of heat transfer at SuperCritical Pressures (SCPs) are presented, and the most accurate heat-transfer correlations are listed. SuperCritical Water (SCW) is widely used as the working fluid in the SCP Rankine “steam”-turbine cycle in fossil-fuel thermal power plants. This increase in thermal efficiency is possible by application of high-temperature reactors and power cycles. Currently, six concepts of Generation-IV reactors are being developed, with coolant outlet temperatures of 500°C~1000°C. SCFs will be used as coolants (helium in GFRs and VHTRs; and SCW in SCWRs) and/or working fluids in power cycles (helium; mixture of nitrogen (80%) and helium [20%]; nitrogen, and carbon dioxide in Brayton gas-turbine cycles; and SCW “steam” in Rankine cycle).

Thermal Effects in Near-Critical Fluids

In this chapter is addressed the very particular thermal behavior that supercritical fluids exhibit when nearing their critical point. In this region, supercritical fluids exhibit strong anomalies in their thermodynamic and transport properties. Pressure change associated to a temperature variation leads to a nearly isentropic thermalization of the fluid, the “piston effect”, which leads to a paradoxical “critical speeding-up”. Bulk fluid temperature is uniform, temperature gradients are confined in thermal boundary layers, making the bulk fluid a thermal short-circuit. It follows very particular behavior, as dynamic heat pipes or heat going seemingly backward, in apparent contradiction with the 2nd principle of thermodynamics. Under an acceleration field, thermal convection occurs only in the boundary layers, which paradoxically can enhance the fluid stability or even cool the fluid after a heat pulse. These effects can deeply modify the supercritical fluids thermal behavior in space and energy activities, giving to these effects socio-economic relevance.

The Basic Thermal Hydraulic Issues of Applying Supercritical Fluid to Nuclear Reactors

This chapter is mainly focused on illustrating some introductory progress on thermal hydraulic issues of supercritical water, including heat transfer characteristics, pressure loss characteristics, flow stability issues and numerical method. These works are mainly performed in Nuclear Power Institute of China (NPIC) these years, to give a basic idea of elementary but important topics in this area. An analytical method was proposed up to predict the heat transfer coefficient and friction coefficient based on the two-layer wall function. Flow instability experiments have been carried out in a two-parallel-channel system with supercritical water, aiming to provide an up-to-date knowledge of supercritical flow instability phenomena and initial validation data for numerical analysis. An in-house code has been developed in NPIC in order to better utilize and further expand the experimental results on supercritical flow instability. At last, some future research directions are suggested for reference.

Heat Transfer in Supercritical Fluids

Results of experimental study of non-stationary heat transfer in supercritical fluids, which were obtained using the method of controlled pulse heating of low-inertia wire probe, are discussed. The aim of this study was to clarify the peculiarities of heat conduction mode at significant heat loads. A threshold decrease in the “instant” heat transfer coefficient, the more pronounced the closer the pressure value to critical pressure, has been found, as well as the absence of impact of the isobaric heat capacity peak known from stationary measurements on the experimental results. These results give new insights into selection of the operating pressure of supercritical heat transfer agent. Small time and spatial scale in the experiments (units of millisecond and units of micrometer) in combination with high-power heat release (up to 20 MW/m2) makes it possible to associate the results with the behavior of boundary layer region of heat transfer agent.

Heat Transfer and Fluid Flow of Supercritical Fluids in Advanced Energy Systems

This chapter aims to clarify the supercritical fluids thermal hydraulics characteristics including heat transfer, pressure drops and flow instabilities for the purpose of accurate design and efficient safe operation of advanced energy systems. At first, the convection heat transfer models considering the effect of nonlinear properties and the effect of buoyancy and acceleration have been provided and discussed. Secondly, the hydraulic resistance models for supercritical fluids have been selected and suggested for different conditions. Thirdly, the published models for supercritical flow instabilities based on four different regional partitions are summarized and clarified. At last, two typical case studies have been provided to further intuitively elaborate the thermal hydraulics of supercritical fluids within the advanced energy systems.

Principles, Experiments, and Numerical Studies of Supercritical Fluid Natural Circulation System

Due to the unique thermal and transport properties, Supercritical natural circulation loop (NCL, or thermosyphon) has been proposed in many energy conversion systems. This chapter presents the principals of supercritical natural circulation loop system and its application challenges. A specially designed experimental prototype system is introduced and compared with numerical findings. The system is operated in wide range of high pressures in the critical region. It is found that in a supercritical NCL system, very high Reynolds number natural convection flow can be achieved only by simple heating and cooling. Thermal performance analysis and parameter effects are carried out along with the experimental development. The heat transfer dependency on operation and its mechanisms are also explained and summarized in this chapter. The comparison of experimental and numerical results contributes to better understanding of NCL stability phenomena and applications in energy systems.

Supercritical Natural Circulation Loop

Supercritical natural circulation loop is a compelling technology for cooling of modern nuclear reactors, which promises enhanced thermalhydraulic performance in a simple design. Being a new concept, related knowledge base is relatively thin and involves several conflicting theories and controversies. Present chapter summarizes the observation till date, starting from the very fundamentals. The phenomenon of natural circulation and suitability of supercritical medium as working fluid are discussed in details. Different methods of analyses, including analytical, simple 1-D numerical and multidimensional computational codes, as well as experimental, are elucidated. A comprehensive discussion is presented about the effect of various geometric and operating parameters on the system behavior, from both thermalhydraulic and stability point of view. Finally, a few recommendations are included about the operation of such loops and future direction of research.

X-Ray Scattering Studies of Expanded Fluid Metals

Fluid metals exhibit significant properties of thermodynamic-state dependence, since the inter-particle interaction among the constituents (electrons and ions) considerably changes depending on their thermodynamic conditions. The authors have thus far carried out X-ray scattering experiments of fluid metals in the expanded state, which have enabled us to gain insight into microscopic understanding of the structural and electronic properties of fluid metals. The purpose of this chapter is to provide intriguing aspects of fluid metals originated from the existence of conduction electrons, which distinguishes fluid metals from non-conducting fluids, through the results of fluid rubidium and mercury.