M. Ram Gopal

Transcritical CO2 Heat Pump Dryer: Part 1. Mathematical Model and Simulation

In this study, a mathematical model and simulation code has been developed to investigate the performance of a transcritical CO2 heat pump dryer. The model takes into account detailed heat and mass transfer and pressure drop phenomena occurring in each component of the system. To take care of the variable heat transfer properties, the heat exchanger components were divided into several infinitesimal segments to examine the state, heat and mass balance and pressure drop for both refrigerant and air, and hence accurate results are expected. In Part 2 of the article, the model developed has been validated with experimental data and then the model was used to investigate effects of important operating parameters on the performance.

Transcritical CO2 Heat Pump Dryer: Part 2. Validation and Simulation Results

The simulation model of a transcritical CO2 heat pump dryer presented in Part 1 has been first validated with available experimental data in this part and then used to simulate the heat pump dryer to study the variation of performance parameters such as heating COP, moisture extraction rate, and specific moisture extraction rate. The validation with experimental data shows that the model slightly over predicts the system performance. The possible reasons for the difference between experimental and numerical results are explained. Simulation results show the effect of key operating parameters such as bypass air ratio, re-circulation air ratio, dryer efficiency, ambient condition (temperature and relative humidity), and air mass flow rate. Results show that unlike bypass air ratio and ambient relative humidity, the effect of dryer efficiency, recirculation air ratio, ambient temperature, and air mass flow rate are very significant as far as the system performance is concerned.

Studies on a metal hydride based solar water pump

Abstract A metal hydride based solar water pump has been simulated and effects of various design and operating parameters have been studied. The study shows that if the cost of metal hydride is much higher than the collector and/or land cost, then one should use more number of thin hydride bed solar collectors for best performance. On the other hand, if the solar collector and/or land cost is much higher than hydride material cost then thick bed solar collectors are preferred. It is possible to optimize the system performance for different discharge heads by varying the enthalpy of formation of metal hydrides, which is a unique characteristic of the hydride based solar water pump. The study shows that depending on the design and operating conditions, it is possible to pump as much as 3000 l of water in a day over a height of 15 m using 1-m2 collector area. The maximum overall thermal efficiency of the metal hydride based solar water pump was found to be about 1.5%.

Dynamic Performance Analysis of a Compressor Driven Metal Hydride Cooling System

Though several studies have been reported to show that compressor driven metal hydride cooling systems are competitive with conventional vapor compression systems, an elaborate computational model that takes into account the transient nature of the compressor and the conditioned space is still unreported. The results presented here are obtained for a room air conditioner with Zr0.9Ti0.1Cr0.55Fe1.45 as the hydrogen absorbing material and employing standard heat exchanger configurations. Though previous thermodynamic and transient studies predicted attainment of significant coefficients of performance, the present results indicate that even with the optimal design the maximum coefficient of performance and specific power will be around 2.38 and 750kJ∕kg-alloy.h, respectively. This indicates a need for better materials and effective control strategies so that these systems can become commercially viable.

Carbon dioxide as a secondary fluid in natural circulation loops

Abstract In recent years there is growing interest in the use of carbon dioxide as a secondary fluid in various cooling and heating applications. In the present work a comparative study is made between carbon dioxide and conventional secondary fluids that can be used in natural circulation loops for various refrigeration and air conditioning applications. For the same heat input and total loop length, the results are presented in terms of the ratios of pipe diameter required when a conventional fluid is used to the required pipe diameter with carbon dioxide as the secondary fluid. Results are presented for both laminar and turbulent flow conditions. It is observed that due to its excellent thermo-physical and transport properties and near critical point operation, carbon dioxide gives rise to very compact systems when compared to most of the conventional secondary fluids. The difference in size is much greater for laminar flow conditions compared to turbulent flow conditions. Also, for a given system size, the temperature rise/drop across the heat exchangers is much smaller in case of carbon dioxide in comparison to other fluids.

Optimum Operating Conditions for Subcritical/Supercritical Fluid-Based Natural Circulation Loops

Natural circulation loop (NCL) is simple and reliable due to absence of moving components and is preferred in applications where safety is of foremost concern such as nuclear power plants, high pressure thermal power plants, etc. In the present study, optimum operating conditions based on maximum heat transfer rate in NCLs have been obtained for subcritical as well as supercritical fluids. In recent years there is a growing interest in the use of carbon dioxide as loop fluid in NCLs for a variety of heat transfer applications due to its excellent thermophysical environmentally benign properties. In the present study, three dimensional CFD analysis of a carbon dioxide based NCL with isothermal source and sink has been carried out. Results show that heat transfer rate is much higher in the case of supercritical phase (if operated near pseudocritical region) than subcritical phase. In the subcritical option, higher heat transfer rate is obtained in case of liquid operated near saturation condition. Correlations for optimum operating condition are obtained for supercritical a CO2 based NCL in terms of reduced temperature and reduced pressure so that they can be employed for a wide variety of fluids operating in supercritical region. Correlations are also validated with different loop fluids. These results are expected to help design superior optimal NCLs for critical applications.

Effect of Tilt Angle on Subcritical/Supercritical Carbon Dioxide-Based Natural Circulation Loop With Isothermal Source and Sink

In recent years, a growing popularity of carbon dioxide (CO2) as a secondary fluid has been witnessed in both forced as well as in natural circulation loops (NCLs). This may be attributed to the favorable thermophysical properties of CO2 in addition to the environmental benignity of the fluid. However, an extensive literature review shows that studies on CO2-based NCLs are very limited. Also, most of the studies on NCLs do not consider the three-dimensional variation of the field variables. In the present work, three-dimensional computational fluid dynamics (CFD) models of a NCL with isothermal source and sink have been developed to study the effect of tilt angle in different planes. Studies have been carried out employing subcritical (liquid and vapor) as well as supercritical phase of CO2 as loop fluid at different operating pressures and temperatures. Results are obtained for a range of tilt angles of the loop, and a significant effect is observed on heat transfer, mass flow rate, and stability of the loop. It was also found that changing the orientation of the loop could be an elegant and effective solution to the flow instability problem of NCLs.

Compressor Driven Metal Hydride Cooling Systems: Simulation and System Dynamics

A comprehensive mathematical model to study the performance parameters of compressor driven metal hydride systems is presented. The dynamics of the compressor and the conditioned space has been taken into account. The model also accommodates the transient nature of the external fins. The effect of size and geometry of the heat exchanger has been given due importance while calculating the external heat transfer coefficient of the metal hydride reactors. A 1 TR system is designed using MmNi4.5 Al0.5 as the metal hydride and its operating characteristics are studied. The average COP and SP power obtained are around 3.1 and 125 W/kg respectively. It is seen that compressor remains idle for 30% of the time during stable cycling. The problem of large initial compressor loads during cut-in as is encountered in vapour compression systems are not faced in such systems. Moreover the compressor is not overloaded during sudden load changes.Copyright