The effects of the operating temperatures, such as heat output (TH), heat input (TM) and heat output (TL) temperatures on the coefficient of performance (COPHT), specific heating power (SHP) and second law efficiency (E) were presented. At the given operating conditions of heat output temperature 463 K, heat input temperature 403 K and heat discharge temperature 308 K; the experimentally estimated values of COPHT, SHP and E of the DS-MHHT were W/kg and 0.29, respectively.
Subscripts
1 INTRODUCTION
2 STATE OF THE ART
5 RESULTS AND DISCUSSION
NUMERICAL STUDIES
1.9(b) CO2 emissions from electricity and heat generation in 2009 and 2010. 3.1 Schematic representation of coupled metal hydride reactors.
INTRODUCTION
Sorption Systems
Furthermore, a large amount of waste heat is released directly into the atmosphere or into the surface water without rational reuse. This low-grade waste heat would become useful energy resources if they can be recycled efficiently.
Conversion losses from
About two-thirds of the primary energy used to produce electricity is rejected as "waste". Although the exploitation and utilization of primary energy sources promotes economic development, it also causes serious environmental problems, such as ozone depletion potential (ODP) and global warming potential (GWP).
Gross electricity
It can significantly reduce the consumption of primary energy and also to promote the economic and social sustainable development by developing some advanced energy systems, such as the distributed energy system and the advanced integrated energy technologies.
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Nuclear 8385
Net electricity production
Electricity delivered to
Metal Hydrides
Furthermore, the performance of a metal hydride-based hydrogen storage system can be easily controlled by adjusting the hydrogen supply pressure and varying the absorption temperature. A metal hydride-based hydrogen storage system offers several advantages over traditional hydrogen storage systems, such as a safe supply of hydrogen at a constant, higher pressure.
Thermodynamics of Metal Hydride Formation
Therefore, according to the Gibbs phase rule (Eq. 1.3), the number of degrees of freedom is two, i.e. If additional hydrogen is added, there would still be some hydrogen dissolved in the metal hydride.
Applications of Metal Hydrides
- Hydrogen storage
- Hydrogen compressor
- Thermal energy storage
- Metal hydride batteries
- Heating and cooling systems
At the same time, the hydride A absorbs this hydrogen by rejecting the heat of absorption to the reaction bed and the coolant at the medium temperature, TM. At the same time, the hydride A absorbs this hydrogen and rejects the heat of absorption to the reaction bed and the coolant at the low temperature, TL.
Motivation of the Present Work
Thus, the quality of the heat in heat transformer systems is upgraded to higher levels. Therefore, the main focus of this thesis is given on the development of metal hydride-based heat transformers for upgrading the heat input from C to C.
Structure of the Thesis
The metal hydride heat transformer (MHHT) falls into the category of dry absorption systems and can upgrade heat from medium temperature C) to high temperature C). It also provides higher heat storage capacity and wider operating temperature range compared to other conventional heat pumps / heat transformers.
STATE OF THE ART
Reaction Kinetics
The reaction rate was measured in the temperature range 15 - 80 C. The activation energy of 32.24 kJ/mol H2 was observed from the Arrhenius plot which showed good agreement with the data obtained from the energy formation. In the high temperature range 70 - 90 C, a second order kinetics was observed. 1989) studied the effects of aluminum and iron substitution on the reaction kinetics of MmNi5 (Mm = misch metal).
Heat and Mass Transfer Studies in Metal Hydride Devices
- Hydrogen storage (absorption and desorption)
- Heat pump
- Heat transformer
- Hydrogen compressor
The efficiency (rates of hydrogen absorption and desorption) of a hydrogen storage system depends on the heat transfer characteristics and mass of the hydride layer. The performance of the MHHC was predicted by solving the unsteady heat and mass transfer characteristics of coupled metal hydride beds of cylindrical configuration.
Experimental Studies on Metal Hydride Devices
- Hydrogen storage
- Heat pump
- Heat transformer
- Hydrogen compressor
Corrugated aluminum sheets were used to improve heat transfer. the system varied from operating temperatures of heat source temperature 158 °C, heat rejection 30 °C and cooling temperature 0 °C. 1995) • Metal hydride coolant performance was numerically predicted by solving the transient heat and hydrogen transport processes between LaNi4.5Al0.5 (regenerative alloy) and LaNi5. 1997) • Thermodynamic analysis and experimental study of a thermally driven hydride slurry heat pump was studied using the hydride pair Zr0.8Ti0.2Cr0.6Fe1.4 / Zr0.8Ti0.2MnFe.
1995) • Presented 1-D model to predict MHHT performance based on heat transfer and reaction kinetics.
Literature Closure
Considerable research has been done in the literature on the thermal modeling of metal hydride based heat pumps [Dantzer and Orgaz (1986), Gopal and Murthy (1995a & 1995b), Kim et al. There are few experimental studies on metal hydride based heat pump [Nagel et al. 2010)] have been reported in the literature. To develop a detailed mathematical model for predicting the performance of a single-stage metal hydride heat transformer.
To develop a detailed mathematical model for predicting the performance of a two-stage metal hydride heat transformer under different operating conditions.
HEAT AND MASS TRANSFER MODELS
Single-Stage Metal Hydride Based Heat Transformer (SS-MHHT)
- Physical model and principle of operation
- Selection of metal hydride pairs
- Mathematical model
- Performance analysis
- Solution methodology
At the bottom wall (next to the porous filter) gas pressure is uniform over the length of the reactor;. 3.16). The final state of process ab is taken as the initial state of process bc. During this process, the reactors A and B are sensibly heated to TM and TH respectively. 3.26), the temperature of the hydride bed is updated.
The final state of this process is the initial state of the process ab of second cycle.
Double-Stage Metal Hydride Based Heat Transformer (DS-MHHT)
- Physical model and principle of operation
- Selection of alloys
- Mathematical model
- Performance analysis
The next section explains the mathematical modeling of the hydrogen exchange process between reactors A1 and C1. As explained in the physical model, the desorption of hydrogen and the temperature of the hydride bed Al are estimated using the reaction kinetics (equation 3.7) and energy equations (equation 3.8). Initial and boundary conditions (absorption): The initial state of reactor C1 at time t = 0 is given as.
However, this temperature difference depends solely on the mass flow rate and specific heat capacity of the heat transfer fluid.
Details of Metal Hydride Reactor .1 Design of the reactor
- Assembling the reactor
The other end of the reactor is attached to an outer flange which can be fitted to an inner flange with stainless steel nuts and bolts. The pictorial views of the copper wire matrix and the sintered metal filter are given in Fig. respectively. The other end of the filter is closed by attaching it with a piece of stainless steel.
The filter is then carefully inserted into the copper tube of the copper filament matrix and the inner flange is attached to the outer flange with a Teflon washer.
Single-Stage Metal Hydride Based Heat Transformer .1 Experimental setup and procedure
- Selection and activation of metal hydride alloys
The amount of hydrogen transferred during absorption/desorption processes is measured using a Coriolis mass flow meter (sensitivity 0.001 g). Valves V1, V2 and V3 are now open and hydrogen gas is desorbed from Reactor A which is absorbed by Reactor B. This combined process of desorption and absorption stops when there is no change in the total flow observed in the mass flow meter.
This process stops when there is no change in the total flow observed in the mass flow meter.
Double-Stage Metal Hydride Based Hydrogen Compressor
- Operating principle
B Sensible
- Details of experimental set up and procedure
- Alloy selection
- Data reduction
- Double-Stage Metal Hydride Based Heat Transformer .1 Experimental set up and procedure
- Selection and activation of metal hydride alloys
- Performance parameters
Then all valves are closed and reactor A is heated to the heat source temperature (TH) and reactor B is cooled down to the low temperature (TC). Valves V1, V4 and V3 are opened and reactor A is allowed to desorb hydrogen which is absorbed by reactor B. The heat transfer fluid has temperatures. at the inlet and outlet of the reactors, and the hydrogen gas temperatures at the outlet of the supply cylinder and the storage cylinder are also recorded simultaneously.
Now valves V1, V2 and V4 are opened and reactor A is allowed to desorb hydrogen which is absorbed by reactor C.
COP Q
High purity hydrogen gas (99.99%) is used in the experiment and ultra high purity argon gas (99.9%) is used for bleeding purposes. The reactors are connected to thermostatic baths in the range of 20 – 250 ºC for supplying the heat transfer fluid to the reactor during the absorption and desorption process. A vacuum pumping system consisting of a rotary pump and a diffusion pump is used to create a vacuum in the installation.
Details of various instruments and sensors used in this experimental study are detailed in Appendix C.
RESULTS AND DISCUSSION
Numerical Results of Single-Stage Metal Hydride Based Heat Transformer
- Numerical validation
- Effect of convective boundary conditions on the variation of hydride concentrations
- Variation of average hydride bed temperature profiles over a cycle
- Variation of hydride gas pressures over a complete cycle
- Variation of heat transfer fluid temperature at different axial locations
- Heat interactions during the operation of SS-MHHT
- Performance investigation of SS-MHHT
Due to the heating of the hydride, there is a significant drop in the temperature of the heat transfer fluid when it reaches the right boundary. The change in the average temperature of the layer of reactors A and B in the cycle is shown in the figure. This is due to the higher heat of formation of hydride B than hydride A.
During the desorption process, it turns out that the heat transfer fluid temperature at the right boundary is lower than that of the left boundary.
Numerical Results of Double-Stage Metal Hydride Based Heat Transformer
- Transient variation of reaction bed temperature and HTF inlet and outlet temperatures
- Variation of hydrogen concentration over a complete cycle
- Variation of cumulative heat transfer over a complete cycle
- Effect of half cycle time on hydrogen transfer
- Performance investigation of DS-MHHT
It is observed that the amount of hydrogen transfer increases significantly with thf up to 14 min, after which the effect is negligible. It is observed that the heat output (QHT) increases with thf as a result of the increase in the amount of hydrogen transfer. The decrease in COPHT and SHP with TH is mainly due to the lower amount of hydrogen exchange between the connected reactors.
5.18, it is observed that THTF decreases as TH increases, which is due to decrease in driving potential and less amount of hydrogen exchange with TH.
Experimental Results of Single-Stage Metal Hydride Based Heat Transformer
- Variation of bed temperature of the reactors A and B
- Variation of hydrogen flow rate between coupled reactors
- Variation of heat interaction to/from the reactors A and B
The variation of the hydrogen flow rate and the cumulative amount of hydrogen exchanged between the coupled reactors is illustrated in Fig. The heat transfer strongly depends on the hydrogen flow rate between the coupled reactors. Initially, the hydrogen flow rate is high between the coupled reactors, due to this the heat transfer is also high.
The hydrogen flow rate is then gradually reduced, which also gradually reduces the heat transfer.
- Effect of heat source temperature (T M ) on HTF temperature difference
- Performance investigation of SS-MHHT
- Experimental Results of Double-Stage Metal Hydride Based Hydrogen Compressor
- Variation of hydride bed temperature with hot fluid temperature
- Effect of hydrogen supply pressure on delivery pressure
- Effect of hydrogen supply pressure and heat source temperature on delivery pressure and amount of hydrogen compressed
- Effect of hydrogen supply pressure and heat source temperature on compressor work
- Effect of hydrogen supply pressure and heat source temperature on compression ratio (CR)
- Effect of hydrogen supply pressure and heat source temperature on compressor efficiency ( C )
- Experimental Results of Double-Stage Metal Hydride Based Heat Transformer
- Variation of bed temperature of the reactors
- Variation of heat interaction to/from the reactors A, B and C
- Effect of heat source temperature (T M ) on HTF temperature difference
- Performance investigation of DS-MHHT
- Comparison of SS-MHHT and DS-MHHT Systems
- Feasibility of Commercialization of the MHHT
For a given heat source temperature, the delivery pressure is found to increase with the hydrogen supply pressure. The variations of compression ratio (CR; ratio between delivery pressure and hydrogen supply pressure) with hydrogen supply pressure and heat source temperature are shown in Fig. CR is found to increase with heat source temperature and decrease with pressurized hydrogen supply.
Effects of heat source temperature, TM and heat rejection temperature, TL on COPHT.
CONCLUSIONS AND FUTURE SCOPE
Numerical Studies
- Single-stage metal hydride based heat transformer (SS-MHHT)
- Double-stage metal hydride based heat transformer
The changes of reaction bed temperature, hydride concentration, coolant temperature and hydride equilibrium pressures of the reactors are shown during a complete cycle. The effects of thermal conductivity and bed thickness on cycle time and SHP of SS-MHHT are presented. The effect of half-cycle time (thf) on COPHT, SHP, total heat production, and the amount of hydrogen exchanged between the paired reactors are also presented.
The performance parameters of the DS-MHHT such as coefficient of performance (COPHT), specific heating power (SHP), second law efficiency (E) and HTF temperature difference (THTF) are predicted at different heat output (TH), heat input (TM) and heat rejection ( TL) temperatures.
Experimental Studies
- Single-stage metal hydride based heat transformer
- Double-stage metal hydride based hydrogen compressor
- Double-stage metal hydride based heat transformer
The effects of heat source and heat rejection temperatures on the COPHT, SHP and E were investigated. The effects of supply pressure and heat source (desorption) temperature on the delivery pressure, amount of hydrogen compressed and isentropic efficiency of the DS-MHHC were investigated. The CR was found to increase with heat source temperature but decrease with supply pressure.
At the given operating conditions of heat output temperature 463 K, heat input temperature 413 K and coolant temperature 308 K; experimentally estimated values.
Scope of Future Work
Gopal MR, Murthy SS (1992) Prediction of heat and mass transfer in cylindrical annular beds of metal hydride, Int. Gopal MR, Murthy SS (1993) Parametric studies on heat and mass transfer in metal hydride beds, J. Gopal MR, Murthy SS (1995d) Studies on heat and mass transfer in metal hydride beds, Int.
Kang BH, Yabe A (1995) Performance analysis of a metal-hydride heat transformer for waste heat recovery, Applied Thermal Engineering, Vol.
