• Energy Research

Abstract This work presents a detailed and generic dynamic modelling and simulation method of scroll expander for small scale power generation system. The geometric models of the scroll were built step by step, including the scroll involute, scroll dimensions, chamber areas and volumes, the scroll profile modifications and so on. The valve model, internal leakage model, motion equation, heat transfer equation and energy balance equation were combined with the geometric models to complete the scroll expander modelling. A mathematic model of a direct current generator or an experimentally determined correlation of generated power against rotational speed of the used generator was integrated to the expander model as the power output unit. To enhance the adaptability of the current model, an overall dynamic friction coefficient of the scroll expander and the generator was innovatively proposed and introduced as one of the key parameters in the present study. The accurate value of this coefficient should be experimentally determined for a specific expander – generator system; with the knowledge of such a parameter, the mechanical friction loss can be accurately and easily calculated in the simulation study. The present modelling and simulation method have been validated by several sets of experimental results based on different scroll expanders studied by different researchers, and the corresponding overall dynamic friction coefficient was found in the order of magnitude of 10 −3 N m s.

Abstract The present paper aimed at exploring absorption heat transformer (AHT) to upgrade ultralow grade waste heat in the temperature range of 40–60 °C. The performance of AHTs with different configurations, including single stage, double stage and double effect, were numerically analysed and compared in terms of temperature lift, coefficient of performance ( COP ) and exergy coefficient of performance ( COPe ). The most influential and crucial factor for the studied AHTs is the recirculation flow ratio ( FR ), the increase of which results in an increasing temperature lift but gradually declining COP . The COPe can achieve its maximum value with a certain FR , and such a state can be considered as the optimal working condition. Within the studied waste heat temperature range, the optimal FR in single stage AHT is in the range of 10–12, at which the system can deliver 17.1~34.7 °C temperature lift with COP at 0.471~0.475. The best configuration amid the studied four different double stage AHTs has a temperature lifting capacity of 31.8~68.6 °C with a COP around 0.30. The double effect AHT compromises its temperature lifting capacity for the highest COP among all the AHTs studied, which can reach about 0.65 though necessitates relatively higher waste heat temperature and higher strong solution concentration to drive the cycle; the double effect AHT is not recommended for the upgrading of ultralow grade waste heat.

Tetra-n-butyl ammonium bromide clathrate hydrate slurry (CHS) is very promising for cold storage and energy saving in air-conditioning applications because of its appropriate phase-change temperature range of 0–12°C, reasonably large latent heat, and good fluidity. In this study, we experimentally investigate the flow and heat transfer characteristics during CHS generation in both an in-house-made annular channel heat exchanger and a commercial double-tube heat exchanger. Meanwhile, the visualization of the growth of the hydrate crystals is also carried out and the thickness of the crystal layer is estimated from the visualization results. It is concluded from the experimental results that the pressure drop and heat transfer coefficient play very important roles in CHS generation and the commercial double-tube heat exchanger is more effective for CHS generation.

Abstract This work experimentally investigated adsorption equilibrium and reaction kinetics of ammonia adsorption/desorption on the composite of strontium chloride (SrCl2) impregnated into expanded graphite, and also discussed the potential influence of the addition of expanded graphite on the SrCl2-NH3 reaction characteristics. The measured and analysed results can be very useful information to design the system and operating conditions using the similar chemisorption composites. Equilibrium concentration characteristics of ammonia within the studied composite were measured using the heat sources at 90 °C, 100 °C and 110 °C for the decomposition process, where the degree of conversion achieved 50%, 78% and 96% respectively. Therefore, the equilibrium equation reflecting the relationship between temperature, pressure and concentration was developed, and a pseudo-equilibrium zone was found, which should be useful information to setup the system operating condition for the desired global transformation. It was suspected that the addition of expanded graphite altered the reaction equilibrium due to the pore effect and the salt-confinement. The concept of two-stage kinetic model was proposed and kinetic parameters were determined by fitting experimental data. The developed kinetic equations can predict dynamic cyclic performance of a reactive bed in similar geometric structure with reasonable accuracy. Such a chemisorption cycle using the SrCl2-expnaded graphite (mass ratio 2:1) composite can be used for cooling application, and the maximum SCP value can be achieved as high as 656 W/kg at t = 2.5 min, and the COP can be 0.3 after one hour of synthesis process under the condition of Tev = 0 °C, Tcon = 20 °C, Theat = 110 °C.

Abstract Phase change material slurry is widely used in such applications as thermal energy storage and thermal management. Thermal conductivity of phase change material slurry is one of the most important thermo-physical properties that are necessary for system design and performance evaluation. In the present study, thermal conductivity of a phase change material slurry, tetra-n-butylammonium bromide (TBAB) clathrate hydrate slurry (CHS), is experimentally measured by using transient hot-wire method. The theoretical model of thermal conductivity measurement of phase change material slurry under the influence of phase change is proposed and numerically analyzed to obtain real thermal conductivity. It is found that phase change significantly affects the measurement of thermal conductivity in that the real thermal conductivity is smaller than that obtained directly from the experiments because phase change enhances heat transfer during the measurement. The smaller the solid fraction of TBAB CHS, the larger the influence of phase change on thermal conductivity is. Such effect is apparently ubiquitous, which should also be taken into consideration in thermal conductivity measurement of other phase change material slurries.

Phase change material slurries (PCS) can serve as both the heat transfer fluids and energy storage media, consequently, they are potentially applicable to the thermal systems, e.g., the secondary refrigeration and air conditioning loops, so as to improve the energy efficiency and to reduce the quantity of refrigerant used in the system. The design of the system using PCS needs the quantitative information about the thermal and fluidic behaviors of PCS. Here we provide an overview of the characteristics of two big groups of PCS, namely microencapsulated phase change material slurry (MPCS) and semi-clathrate hydrate slurry (CHS). The focuses are placed on the flow and heat transfer features and thermal properties, such as specific heat, viscosity and thermal conductivity. The suitable materials for making PCS are also discussed and compared based on the available data in the literature, and some examples of the applications of PCS are summarized as well.

Abstract The performances of three chemisorption working pairs operating under the resorption cycle were studied gravimetrically by comparing the desorbed and adsorbed mass of refrigerant in different operation conditions. All pairs used NH 3 as refrigerant and MnCl 2 in the main reactor, but each one used a different salt for the cooling effect production in the secondary reactor. These salts were NH 4 Cl or NaBr or BaCl 2. The experimental results indicated that the degree of conversion in reaction between the NH 3 and BaCl 2 was inferior to 25% during cooling production at 0 °C or below, whereas the reactions with the other salts had conversions of at least 80%. When the systems operated with heat source temperature for the main reactor at 155 °C, heat sink temperature for both reactors at 30 °C, and cooling effect production temperature at 0 °C the coefficient of performance (COP) of the system using NH 4 Cl and the system using NaBr were similar and around 0.30; however, the former system had a specific cooling power (SCP) 5% higher than that of the latter system. Because the reaction in the system with NH 4 Cl was practically halted in a period much shorter than that used in the experiments, it is possible to expect that if the period of the cooling period was shortened, the difference between the SCP of those systems would be much higher.

Abstract A phase change material slurry – TBAB (tetra-n-butyl ammonium bromide) CHS (clathrate hydrate slurry) has received intensive attractions in recent years due to its dual-function as thermal energy storage and transport media simultaneously in air conditioning and refrigeration applications. In the present study, the rheological characteristics of TBAB CHS were measured using a rheometer at various solid fractions and in a shear rate range of smaller than 1000 s −1 . The results indicated that TBAB CHS was a pseudo-plastic non-Newtonian fluid which showed shear-thinning characteristics. The flow behaviour indices and fluid consistencies of type A and type B TBAB CHS were determined based on the power-law fluid model, which showed good consistency with the previous results obtained from the pressure drop measurements in straight tubes. The apparent viscosity of type A TBAB CHS was larger than that of type B TBAB CHS. The obtained apparent viscosities were compared to the calculated results by using the empirical equations, and the reason for the discrepancies was discussed. Based on the obtained rheological characteristics, the pumping power consumption of TBAB CHS as a secondary refrigerant was estimated and compared to that of chilled water at the same cooling capacity. The result showed a drastic reduction of pumping power when using TBAB CHS in lieu of chilled water.

Abstract Hydrate-based CO2 capture (HBCC) technology, which is a promising alternative method to CO2 capture, has received increasing attention in recent decades as it has mild operating conditions and unique separation mechanism. This paper summarises several available methods on improving the separation performance of HBCC technology, mainly including chemical additives and improvement of capture process. The chemical additives are generally divided into two classes: thermodynamic promoters (THF, TBAB, TBAF, CP, C3H8) and kinetic promoters (SDS, DTAC). In addition to the common single stage process, the multistage process and hybrid conceptual process coupled with membrane separation are developed to obtain more concentrated CO2. Then the evaluation indicators of separation performance are introduced: CO2 recovery and separation factor. Moreover, the separation performance of CO2 capture from either post-combustion flue gas or pre-combustion fuel gas is discussed and the development direction in the future is highlighted as well.