Closed Brayton Cycle

Gas-Cycle Systems. In principle, any permanent gas can be used for the closed gas-cycle refrigeration system however, the prevailing gas that is used is air. In the gas-cycle system operating on the Brayton cycle, all of the heat-transfer operations involve only sensible heat of the gas. Efficiencies are low because of the large volume of gas that must be handled for a relatively small refrigera tion effect. The advantage of air is that it is safe and inexpensive. 

This cycle also uses continuous counterflow heat exchanger and is closely related to the Joule-Thomson and Claude cycles as shown in Fig. 5.15(a) [60], The cryocooling or reverse Brayton cycle derives from a reciprocating gas engine patented by G. B. Brayton in... 

The gas turbine cycle may be either closed or open. The more common cycle is the open one, in which atmospheric air is continuously drawn into the compressor, heat is added to the air by the combustion of fuel in the combustion chamber, and the working fluid expands through the turbine and exhausts to the atmosphere. A schematic diagram of an open Brayton cycle, which is assumed to operate steadily as an open system, is shown in Fig. 4.2. 

In the closed cycle, the heat is added to the fluid in a heat exchanger from an external heat source, such as a nuclear reactor, and the fluid is cooled in another heat exchanger after it leaves the turbine and before it enters the compressor. A schematic diagram of a closed Brayton cycle is shown in Fig. 4.3. 

An engine operates on the closed Brayton cycle (Fig 4.8) and has a compression ratio of 8. Helium enters the engine at 47°C and 200 kPa. The mass flow rate of helium is 1.2 kg/sec and the amount of heat addition is 1 MJ/kg. Determine the highest temperature of the cycle, the turbine power produced, the compressor power required, the back-work ratio, the rate of heat added, and the cycle efficiency. 

Chen, L., Sun, F., and Wu, C., Performance analysis for a real closed regenerated Brayton cycle via methods of finite-time thermodynamics. International Journal of Ambient Energy, 20(2), 95-104, 1999. 

The HTR-IOGT project is featured by a closed recuperated and inter-cooled Brayton Cycle consisting of the reactor core, a helium turbine, a recuperator, a pre-cooler, a low pressure compressor, an inter-cooler and a high pressure compressor, as shown in Figure 2. 

The R D program included two experimental setups both demonstrating helium as a closed-loop Brayton cycle working fluid [44] ... 

For purposes of limiting the scope of the thesis, the reactor will use a directly coupled closed Brayton cycle for power conversion and will use highly enriched Uranium Nitride fuel. The sections of the thesis will be ... 

A gas-cooled nuclear reactor was chosen due to its simplicity and suitability for the space environment. There are numerous alternatives for cooling a reactor core, but gas cooling is one of the simpler methods and most attractive when used with a closed Brayton cycle (CBC) power conversion system. The use of other reactor coolants would necessitate the inclusion of a heat exchanger and introduce complicated freeze/thaw problems, increasing the complexity and the weight of the reactor. 

A Closed Brayton Cycle will be used for the power conversion system. This cycle has the advantage of being a well understood and robust power conversion cycle. Extensive testing of similar systems gives confidence in the long-term durability of this system... 

There are two primary variants of the Brayton cycle the open cycle and the closed cycle [El-Wakil, 1984]. In an open cycle system, the coolant is drawn in from the outside environment, heated, run through a turbine, and discharged back to the outside atmosphere. In a closed cycle, the gas is in a closed loop and used repeatedly. There are two variants of closed Brayton cycles direct and indirect. In the direct system, the heat source is directly coupled to the gas flow system while in an indirect system, the coolant passes through an intermediary heat exchanger. A block diagram of a direct closed Brayton cycle is shown below ... 

In the NPPS based on the NPP technology, the reactor is successfully combined with a dynamic power conversion system operating in a closed gas turbine Brayton cycle (helium and xenon) or in a potassium steam turbine Ranldne cycle. 

The GT-MHR design directly couples the reactor with a turbogenerator in a closed helium Brayton cycle to produce electricity with 48% net plant efficiency. This high efficiency and the expansion of the power output to 600 MW(t) within the existing GT-MHR physical envelope results in a substantial reduction in the busbar power costs compared to the steam cycle modular helium reactors. The power generation costs are forther reduced by the simplified operation and maintenance required of the gas turbine plant, as compared to the steam cycle plant with its much more complicated balance of plant. 

There are several types of heat pumps that are currently feasible for industrial applications, including mechanical vapor-compression systems, closed-cycle mechanical heat pumps, absorption heat prnnps, heat transformers, and reverse Brayton-cycle heat prnnps. 

The SSTAR reactor is coupled to a supercritical carbon dioxide (S-CO2) Brayton cycle power converter. It provides higher cycle efficiency than a helium ideal gas Brayton cycle or a Rankine saturated steam cycle operating at the same core outlet temperature. A key contributor to the high efficiency is the low amount of work (PdV work) to compress S-CO2 immediately above its critical temperature - due to the high S-CO2 density. Table XXII-6 compares the densities of S-CO2 at cycle conditions versus those for helium in the Eskom Pebble Bed Modular Reactor (PBMR) as well as typical liquid coolants the S-CO2 density is more like that of an ordinary liquid. Thus, the S-CO2 temperature and pressure at the low end of the cycle are designed close to but slightly greater than the critical temperature (30.98°C) and pressure (7.373 MPa) to exploit the small PdV work of compression. 

There is a strong incentive to operate as closely as possible to the critical temperature to raise the cycle efficiency. The dependency of the S-CO2 Brayton cycle efficiency upon the turbine inlet temperature is presented in Fig. XXIII-12. 

XXni-4] SIENICKI, J.J. et al.. The STAR-LM lead-cooled closed fuel cycle fast reactor coupled to a supercritical carbon dioxide Brayton cycle advanced power converter, GLOBAL 2003 (Proc. Int. Meeting, New Orleans, LA, USA, November 16-20, 2003), ANS/ENS. 

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