A chemical heat pump using hydration of mgo particles in a three-phase reactor

A chemical heat pump using hydration of magnesium oxide in a three-phase reactor is proposed. Magnesium oxide particles suspended in the triethylene glycol are hydrated exothermally by introducing water vapour. The hydration rate was measured under the temperatures ranging from 383 K to 523 K. It was found that the reaction rate was proportional to the amount of adsorbed water molecules, and correlated in an equation.     

Magnesium oxide/water chemical heat pump to enhance energy utilization of a cogeneration system

A chemical heat pump using a magnesium oxide/water reaction system is expected to be applicable to cogeneration systems using gas engine, diesel engine, and fuel cells. The operability of the heat pump was examined experimentally under hydration operation pressures between 30 and 203 kPa. In the experiment, a reactant having high durability for repetitive operation was packed in a cylindrical reactor. The cycle of operation was repeated under various thermally driven operation conditions. The forward and reverse reactions were studied by measuring the reactor bed temperature distribution and the reacted fraction changes. The reactor bed stored heat at around 300–400 °C by the dehydration reaction and released heat at around 100–200 °C by the hydration reaction under the heat amplification mode operation. The practical possibility of the reactor bed was discussed based on the experimental results. The heat pump is expected to be applicable for load leveling in a cogeneration system by chemically storing surplus heat during low heat demand and supplying heat during peak demand. It was shown that the chemical heat pump would be able to improve the efficiency of energy utilization in cogeneration systems while also helping to reduce energy consumption and global carbon dioxide emissions.     

纳米多孔碳对MgO/Mg(OH)2化学蓄热性能影响的实验研究

为提高Mg O/Mg(OH)₂的热化学蓄/放热性能,采用焙烧法将氧化镁(Mg O)负载在纳米多孔碳(NCP)材料上制备纳米碳基氧化镁(NCP-Mg O)复合材料。研究结果表明,NCP载体使MgO在其表面形成粒径为10～30 nm大小的颗粒,复合材料NCP-MgO具有较高的导热系数,负载80%MgO后导热系数是纯MgO的2.6倍。在反应温度110℃、水蒸气压力57.8 kPa的实验工况下,发现水合速率的大幅提升是强化Mg O/Mg(OH)₂蓄热性能的主要原因,在水合反应60 min和120 min时,NCP-MgO复合材料的水合转化率分别是纯MgO的2.25倍和1.6倍。在水合反应120 min后,MgO负载率为80%的NCP-MgO复合材料的蓄热密度可达1 053 kJ/kg,是纯MgO的1.4倍。该研究可为MgO/Mg(OH)₂在化学蓄热系统的应用提供一定的参考。     

Thermal analysis of a magnesium oxide/water chemical heat pump for cogeneration

A chemical heat pump is examined experimentally as a chemical heat storage system in order to evaluate the contribution of the chemical heat pump to decentralised cogeneration. A new system that combines cogeneration with a chemical heat pump that uses a magnesium oxide/water reaction is proposed, and the feasibility of the combined system is discussed. A packed bed reactor of a magnesium oxide/water chemical heat pump was examined experimentally under various operation conditions. Thermal performance of the heat pump was analysed using the experimental results. The heat pump containing the reactor is expected to enhance the energy utilisation efficiency of the cogeneration system by storing and utilising surplus exhaust heat generated by the cogeneration system.     

Application of a chemical heat pump to a cogeneration system

The feasibility of a proposed system that combines a magnesium oxide/water chemical heat pump and a diesel engine as a cogeneration system is discussed based on experimental results. The combined system is intended to utilize the waste heat discharge from the engine by means of the chemical heat pump and to level the heat supply load of the engine, allowing enhanced energy utilization. The thermal performance of the chemical heat pump in the cogeneration system is estimated based on the results of a packed‐bed experiment. The estimation indicates that by storing the waste heat from the engine during low demand periods, the cogeneration system can produce more than several times the standard thermal output of the diesel engine during peak demand periods. Copyright © 2001 John Wiley & Sons, Ltd.     

Kinetic measurement on the isobutene/water/tert-butanol chemical heat pump; dehydration of tert-butanol

For this heat pump kinetic study, the dehydration rate of tert-butanol (an endothermic reaction) was measured under conditions suitable for operating a chemical heat pump. The proposed Langmuir-Hinshelwood rate equation agreed well with the experimental rate data. The heat pump kinetic study was examined in a batchwise operation by using both the endothermic reaction rate equation and a previously proposed rate equation for the exothermic reaction. Although the endothermic reaction rate was suitable for a heat pump, an improvement of the endothermic reaction rate was desired since it was found to be too slow. The endothermic reaction was about 5 to 20 times slower than the exothermic reaction. The absorbed and released heat rates per batch cycle were almost the same as for conventional heat pumps.     

适用于船用镁基脱硫技术的氧化镁水化率试验研究

基于船用低速柴油机脱硫系统试验平台，以氧化镁为原料，进行水合制备氢氧化镁试验研究。主要研究了水化温度、水化时间等参数对氧化镁转换成氢氧化镁水化率的影响；通过加入水化剂优化反应条件达到缩短水化周期及提高水化率的目的。试验结果表明:加入水化剂及提高反应温度对氧化镁水化有良好的促进作用。     

A Facile Method to Construct Graphene Oxide–Based Magnesium Hydroxide for Chemical Heat Storage

Mg(OH)₂ nanoparticles anchored on graphene oxide (GO) were facilely prepared by a hydrothermal method. The main diameter scale of nanoparticles on the graphene sheet was about 25–50 nm shown by transmission electron microscopy characterization results. X-ray diffraction results indicated that the nanoparticles are in accordance with the data on magnesium hydroxide. This material exhibited significantly improved heat storage capacity and a higher hydration rate than pure magnesium oxide, and the introduction of GO leads to greatly increased thermal conductivity of the nanocomposites. As a novel thermochemical heat storage material, Mg(OH)₂/GO has huge potential for high-efficiency energy systems.     

Thermal performance of a packed bed reactor for a high‐temperature chemical heat pump

The thermal performance of a chemical heat pump that uses the reaction system of calcium oxide/lead oxide/carbon dioxide, which is developed for utilization of high‐temperature heat above 800°C, is studied experimentally. The thermal performance of a packed‐bed reactor of a calcium oxide/carbon dioxide reaction system, which stores and transforms a high‐temperature heat source in the heat pump operation, is examined under various heat pump operation conditions. The energy analysis based on the experiment shows that it is possible to utilize high‐temperature heat with this heat pump. This heat pump can store heat above 850°C and then transform it into a heat above 900°C under an approximate atmospheric pressure. An applied system that combines the heat pump and a high‐temperature process is proposed for high‐efficiency heat utilization. The scale of the heat pump in the combined system is estimated from the experimental results. Copyright © 2001 John Wiley & Sons, Ltd.     

Optimization of magnesium hydroxide composite material mixed with expanded graphite and calcium chloride for chemical heat pumps

A composite chemical heat storage material (EMC) comprising a mixture of expanded graphite (EG), magnesium hydroxide (Mg(OH)₂), and calcium chloride (CaCl₂) was developed as a magnesium oxide/water chemical heat pump reactant. The optimization of a mixing weight ratio between the Mg(OH)₂ content of EMC and EMC itself was discussed from the viewpoints of both heat storage capacity and reactivity by considering the reaction rate constants from a kinetic analysis. It was confirmed that the dehydration reactivity of EMC increased as the mixing weight ratio decreased; however, the heat capacity of the EMC unit mass decreased. A multiplied factor consisting of the multiplied value dehydration rate constant and mixing weight ratio was introduced. It was suggested that a weight ratio of approximately 0.80 was the optimized value when the mixing molar ratio between CaCl₂ and Mg(OH)₂ was 0.10. Dehydration of EMC with an optimized mixing weight ratio and dehydration of pure Mg(OH)₂ were conducted under various temperatures to compare the reaction rate constants of each material. From this study, it was demonstrated that EMC performed better on dehydration than pure Mg(OH)₂.     

Dehydration and hydration behavior of metal-salt-modified materials for chemical heat pumps

Lithium chloride (LiCl) modified magnesium hydroxide (Mg(OH)₂) is a potential new material for chemical heat pumps. However, there is insufficient information concerning its dehydration and hydration behavior. In this study, the dehydration and hydration reactions, corresponding to the heat storage and the heat output operations, respectively, of authentic Mg(OH)₂ and LiCl-modified Mg(OH)₂ were investigated by thermogravimetric methods and near infrared spectroscopy. The dehydration of authentic Mg(OH)₂ proceeded as a one-step reaction. In contrast, the dehydration of LiCl-modified Mg(OH)₂ occurred in two steps. The dehydration reaction rates were increased by LiCl modification of the Mg(OH)₂ surface, while the activation energy for the first-order dehydration reaction was lowered. The mechanism for the hydration reaction of magnesium oxide (MgO) was different to that for the hydration of LiCl-modified MgO. This difference was explained by the effect of the LiCl on the MgO particle surface.     

Upgrading waste heat from a cement plant for thermochemical hydrogen production

A calcium oxide/steam chemical heat pump (CHP) is presented in the study as a means to upgrade waste heat from industrial processes for thermochemical hydrogen production. The CHP is used to upgrade waste heat for the decomposition of copper oxychloride (CuO.CuCl₂) in a copper–chlorine (Cu–Cl) thermochemical cycle. A formulation is presented for high temperature steam electrolysis and thermochemical splitting of water using waste heat of a cement plant. Numerical models are presented for verifying the availability of energy for potential waste heat upgrading in cement plants. The optimal hydration and decomposition temperatures for the calcium oxide/steam reversible reaction of 485 K and 565 K respectively are obtained for the combined heat pump and thermochemical cycle. The coefficient of performance and overall efficiency of 4.6 and 47.8% respectively are presented and discussed for the CHP and hydrogen production from the cement plant.     

Spherical expanding flames in H2–N2O–Ar mixtures: flame speed measurements and kinetic modeling

Although ignition of hydrogen–nitrous oxide mixtures is a serious issue for nuclear waste storage and semi-conductor manufacturing, available flame speed data have not been recently updated and thermodiffusive stability is not known. In order to palliate this, the flame speed of a hydrogen–nitrous oxide mixture diluted in Ar (60% mol) was measured in a spherical bomb as a function of equivalence ratio. The initial pressure and temperature were held constant around ambient conditions. It is shown that the unstretched flame speed of the hydrogen–nitrous oxide mixture is relatively low for a hydrogen-based mixture, with a maximum of 56 cm/s for the stoichiometric condition. Further, hydrogen–nitrous oxide–argon flames appear unstable with respect to thermodiffusive effects at an equivalence ratio of 1. The downward flammability limit of hydrogen–nitrous oxide–argon was observed for hydrogen content of 8 mol%. The modeling of these experimental data has been performed with three recently developed models. All kinetic schemes give satisfactory predictions of the experimentally observed data. Sensitivity and reaction pathway analysis have demonstrated that the dynamic of the system is dominated by the reaction N₂O + H = N₂ + OH which governs the rate of energy release.     

Optimal analysis of the exothermic process of a Mg/MgH2 thermochemical heat storage system

基于镁/氢化镁热化学储热系统,建立了二维非稳态数学模型.对吸氢放热过程中的传热传质现象进行了数值模拟,主要研究了壁面温度和反应床当量导热系数对系统反应速率的影响.结果表明,放热过程中存在最佳的壁面温度使反应速率达到最快,过高或者过低的壁面温度都将使反应床的温度偏离理论上的最佳值,从而降低反应速率.针对不同当量导热系数的反应床,最佳壁面温度也不相同;反应床的当量导热系数并非越大越好,应该根据具体的边界温度以及氢气压力情况进行合理的选择以获得最佳的反应速率.     

Dynamic simulation of CaO/Ca(OH)2 chemical heat pump systems

Using energy and exergy analyses, a dynamic simulation is carried out with a CaO/Ca(OH)₂ chemical heat pump system for heating and cooling applications. The system consists of hydration/dehydration reactor connected to condenser/evaporator with a control valve in between. During the dehydration process, heat is supplied at 700 K for dehydration of Ca(OH)₂ and steam is condensed at 293 K. During evaporation/hydration process, heat is supplied at 290 K for evaporation of water at 273 K and heat of hydration is supplied to a load at 353 K. Duration of one cycle takes about 12 hours. Two subsystems are used to provide for heating/cooling demands in a continuous manner. Using synthetic demands of a residential dwelling, various performance parameters have been calculated for a 24 hour period. The results showed that CaO/Ca(OH)₂ chemical heat pump system could satisfy heating and cooling demands of a typical dwelling. Its energy and exergy efficiencies were 58.7% and 61.6% for heating and 12.7% and 4.5% for cooling respectively.     

Numerical simulation of the performance of a solar-earth source heat pump system

Solar-earth source heat pump (SESHP) is a new type of energy saving air conditioner. In this paper, numerical simulation of the performance of a solar-earth source heat pump system (SESHPS) operated at alternate or combined mode is carried out respectively. The results indicate that a resuming-rate of 30–60% of the earth temperature near buried coil can be preferable when SESHPS is operated alternately at a period of 24 h, and the proportion of the operation time of solar-assisted heat pump (SAHP ) should be confined to 42–58%. When SESHPS is operated at combined modes 2, the energy-saving rate with and without heat storage water tank is 14.5% and 10.4%, respectively, compared with ground source heat pump (GSHP). As for the overall effect, the combined operation mode with water tank in which the heated water flows through the solar collector first and then through the buried coil is preferable. The results are significant for the engineering design, operation and management of SESHPS.     

Effect of load pattern on solar-boosted heat pump water heater performance

The performance of a solar-boosted heat pump water heater (HPWH) operating under full load and part load conditions was determined in an outdoor experimental study. The system utilised flat unglazed aluminium solar evaporator panels to absorb solar and ambient energy. Absorbed energy was transferred to the water tank by means of the heat pump and a wrap around condenser coil on the outside of the tank. The system COP was found to be in the range of 5–7 under clear daytime conditions and 3–5 under clear night-time conditions. Using part load testing of the HPWH system it was found that concentrating the coils in the lower portion of the tank could increase the efficiency of the condenser coil. It was also shown that there exists a generalised linear relationship that can be used to describe the system COP in terms of the temperature difference between the water in the storage tank and the ambient air.     

Behavior of Ca(OH)2/CaO pellet under dehydration and hydration

With construction of a thermochemical energy conversion prototype system to store solar heat, thermal dissociation of pellets of Ca(OH)₂ and hydration of CaO have been investigated in some detail for its application to the system. The inorganic substance is very attractive as a material for long term heat storage, but molar density changes associated with the reaction are fairly large. Therefore, this factor has been taken into account in the kinetic equation. The importance of additives and pellet size has been discussed considering reactivity and strength of pellets. An analysis has been attempted when chemical reaction is important. The deformation of pellets was observed during hydration.     

On the hydrogenation mechanism in magnesium I

The first time hydriding of spherical magnesium particles covered by a thin oxide layer and sieve-fractionated into narrow size distributions within the range 40–90 μm was followed by microgravimetry. The size distributions of the fractions were determined by semiautomatic image analysis. The hydridings were run at 402°C and 3 MPa hydrogen pressure after heating in helium. A dependence of the rate of hydriding on the heat treatment prior to reaction was observed and it is proposed that the heat treatment causes oxygen atoms to diffuse into the bulk metal and thereby break up the protective oxide layer. Based on the observed hydride propagation in the metal particles, a statistical model for the hydriding of a particle is applied to the hydriding curves for a series of samples. The data are found to be in fine agreement with the proposed model. It is concluded that care must be taken when generalizing results from the hydriding of magnesium powders.     

Experimental study of horizontal ground source heat pump performance for mild climate in Turkey

The aim of this study is to evaluate the performance of horizontal GSHP by considering various system parameters for winter climatic condition of Bursa, Turkey. For this purpose, a previously used experimental facility on cooling cycle [Coskun S, Pulat E, Unlu K, Yamankaradeniz R. Experimental performance investigation of a horizontal ground source compression refrigeration machine. International Journal of Energy Research 2008; 32: 44–56] was modified for the heating cycle. Soil thermal conductivity estimation was expanded and discussed. Preliminary numerical temperature distribution around GHE pipes was obtained. Tests were performed under laboratory conditions for space heating from December 2004 to March 2005. Variations of entering and leaving antifreeze solution temperatures, extracted heat from ground and rejected heat to the test room, super heat rate in evaporator and subcooling rate in condenser, total power consumption and coefficient of performance (COP) values for both the entire system and only heat pump unit (HPU) were obtained. Effect of outdoor temperature on system capacities and COP values with respect to outdoor air and mean soil temperatures were also presented. The COP of the entire system and HPU lie between 2.46–2.58 and 4.03–4.18, respectively. GSHP system was compared to conventional heating methods in the economical analysis and it was shown that the GSHP system is more cost effective than the all other conventional heating systems.