Optimal High Pressure Research Articles

Analysis of a carbon dioxide transcritical power cycle using a low temperature source

A detailed analysis of a carbon dioxide transcritical power cycle using an industrial low-grade stream of process gases as its heat source is presented. The methodology is divided in four steps: energy analysis, exergy analysis, finite size thermodynamics and calculation of the heat exchangers’ surface. The results have been calculated for fixed temperature and mass flow rate of the heat source, fixed maximum and minimum temperatures in the cycle and a fixed sink temperature by varying the high pressure of the cycle and its net power output. The main results show the existence of an optimum high pressure for each of the four steps; in the first two steps, the optimum pressure maximises the thermal or exergetic efficiency while in the last two steps it minimises the product UA or the heat exchangers’ surface. These high pressures are very similar for the energy and exergy analyses. The last two steps also have nearly identical optimizing high pressures that are significantly lower that the ones for the first two steps. In addition, the results show that the augmentation of the net power output produced from the limited energy source has no influence on the results of the energy analysis, decreases the exergetic efficiency and increases the heat exchangers’ surface. Changing the net power output has no significant impact on the high pressures optimizing each of the four steps.

THEORETICAL ANALYSIS OF TRANSCRITICAL CARBON DIOXIDE VAPOR COMPRESSION CYCLE

In the transcritical carbon dioxide vapor compression cycle heat is rejected in a gas cooler at supercritical pressure. At supercritical heat rejection the refrigerant temperature and pressure are independent on each other and one of the most important parameters affect on cycle performance is the pressure on the heat rejection side called the high pressure. A theoretical analysis shows the effect of high pressure, superheat, evaporator and gas cooler exit temperature, and gas cooler exit temperature approach on performance of a transcritical carbon dioxide vapor compression cycle was investigated in this study. The study defines an optimal high pressure which gives a maximum coefficient of performance and suggests that the cycle should operate at or near its optimal high pressure in order to keep its maximum coefficient of performance. The high pressure will also affect the cooling capacity and this effect can be used for boosting capacity at high ambient temperatures. The effect of gas cooler exit temperature and compressor inlet temperature on the optimal high pressure also was investigated. The study shows that the optimum high pressure is mainly a function of gas cooler exit temperature and compressor inlet temperature, while the value of the maximum coefficient of performance is dependent on the gas cooler exit and evaporating temperature. The study also indicates that the superheat of the gas at compressor inlet will have a positive effect on the coefficient of performance when operating with high heat rejection pressure at high ambient temperatures with high evaporating temperatures.

The optimum high pressure for CO 2 transcritical refrigeration systems with internal heat exchangers

The system performance of a CO 2 refrigeration system is greatly affected by the compressor discharge pressure. An internal heat exchanger (IHX) with high effectiveness is an important factor to achieve high system performance. The expression traditionally used to describe the heat exchange effectiveness is not suitable for CO 2 systems. As a result a practical effectiveness expression for IHX, based on enthalpy difference, has been derived and is reported in this paper. Detailed analysis on the relationship between the optimum high pressure P k,opt and other systematic parameters was performed. Evaporating temperature has little influence on P k,opt; and IHX can minimize the sensitivity of the system to the refrigerant quality x at the evaporator outlet. Based on simulation data, a correlation of P k,opt was developed that predicts the simulation values with a deviation of less than 3.6% in the whole range and 0.94% when the evaporating temperature t 1=5.3 °C. The results reported in this paper can be used in optimum control and performance evaluation of the whole system.

Equilibrium low pressure generator temperatures for double-effect series flow absorption refrigeration systems

The equilibrium temperatures at the low pressure (LP) generator for double-effect series flow lithium bromide–water vapour absorption chiller are evaluated and the system performance is estimated at these temperatures. Influence of temperatures at high pressure (HP) generator, evaporator, condenser and absorber, and the effectiveness of heat exchangers on equilibrium temperature and internal heat transfer at LP generator, circulation ratio, and coefficient of performance are studied. Dual-heat mode of operation of the system is also investigated utilising low grade waste heat at the LP generator. Correlations are presented for equilibrium LP generator temperature, internal heat transfer at the low pressure generator, circulation ratio, coefficient of performance, optimum HP generator temperature and optimum circulation ratio for maximum coefficient of performance in terms of operating temperatures, which are useful in the design and control of absorption system even at the off-design conditions.

Determination of the optimum high pressure for transcritical CO 2-refrigeration cycles

Modern refrigeration systems have to fulfill many requirements, which include a rapidly growing number of environmental aspects. For compression systems, this entails demands for low energy for propulsion of the systems and a high coefficient of performance ( COP) respectively. Compression systems using the ecological refrigerant carbon dioxide (CO 2) are often run at transcritical conditions. This means that the evaporation of the refrigerant takes place at subcritical conditions, whereas the heat rejection into the environment occurs at supercritical pressures and temperatures. For such transcritical CO 2-systems with the main components compressor, gas cooler, expansion unit and evaporator, a maximum COP exists, depending on system, environmental conditions and operating conditions. The COP is influenced mainly by high pressure. A graphical method from the literature to determine the optimum high pressure is too time-consuming if several operating conditions are investigated. With a simulation model, the optimum high pressure can easily be determined for different operating conditions as a function of the refrigerant temperature at the gas cooler outlet. With these data it is possible to determine a control function. This function is used to adjust the high pressure so that the system can be run with a COP that deviates from the maximum values by less than 5.8%.