• Energy Research

Abstract Membrane-based liquid desiccant dehumidification (MLDD), as a promising technology for temperature and humidity control, suffers from a degradation of effectiveness when handling large air flows in industrial sectors. Existing methods such as multi-stage dehumidifying and internally cooling provide a solution to solve this issue, but their implementation in the large-scale systems is somehow challenging due to the difficulty in sealing ducts, and their mechanisms for enhancing effectiveness have not yet been clarified in depth. This paper aims to apply a tractable multi-stage internally-cooled structure for improving the MLDD effectiveness at an acceptable cost and to uncover the underlying mechanisms from the perspective of the driving forces for heat and mass transfer. A dimensionless finite-difference model is first developed to capture the physical fields of the multi-stage internally-cooled membrane-based liquid desiccant dehumidifier (MI-MLDD). The MI-MLDD is then compared with the single-stage adiabatic one (SA-MLDD) in terms of the effectiveness, maldistribution and thermodynamic limits of driving forces. Four structural improvement methods including increasing stages and layers in the unlimited-size and fixed-size schemes are proposed to further improve the MI-MLDD effectiveness. Their enhancing mechanism is explained by introducing six dimensionless parameters that denote the intensity, distribution and maximum driving forces of heat and mass transfer respectively. Besides, the specific cooling capacity is defined to evaluate the energy efficiency variation of dehumidifiers caused by different effectiveness improvement methods. Compared to the SA-MLDD, the MI-MLDD increases the sensible and latent effectiveness by up to 64 % and 18 % due to the mitigated maldistribution and the increased minimum local driving forces, even as the air-to-solution flow ratio exceeds 2.4. The unlimited-size scheme improves the MI-MLDD effectiveness more significantly than the fixed-size one due to a linear increase in the number of heat and mass transfer units. The specific cooling capacity could reach the maximum value at the condition where the MI-MLDD operates without unfunctional contact areas. These findings highlight the potential of MI-MLDD for handling the large flow air with the improved performance.

Abstract Traveling-wave thermoacoustic electric generator has drawn increasing attention due to its great prospect in energy conversion. In this work, a traveling-wave thermoacoustic electric generator capable of generating about 500 W electric power is studied numerically and experimentally. The performances and the operating characteristics of the system under different working conditions are tested and analyzed. The maximum electric powers can be obtained with electric load resistance around 100–120 Ω, and the highest thermal-to-electric efficiencies can be achieved at much larger load resistances. The efficiency at low load resistance is relatively small due to the large pressure amplitudes inside the thermoacoustic system, which increases the dissipations. The variation trends of the electric power and the thermal-to-electric efficiency with the load resistance intrinsically result from the changes of the corresponding acoustic impedance of the linear alternators, which determines the output performance of the thermoacoustic engine meanwhile. The distributions of the acoustic power losses are then calculated and firstly illustrated quantitatively. It is shown that the resonator causes most of the acoustic power losses, and the losses in hot heat exchanger, thermal buffer tube, and feedback tube are also significant. The output performance of the system can be improved by increasing the heating temperature and the mean pressure. A maximum electric power of 473.6 W and a highest thermal-to-electric efficiency of 14.5% are achieved experimentally when the mean pressure is 2.48 MPa and the heating temperature is 650 °C. A pair of linear alternators with a larger swept volume and appropriate acoustic impedances is finally designed to couple with the thermoacoustic torus directly. Numerical results show that the maximum electric power can be increased to 718 W and 1005 W when the mean pressures are kept at 2.48 MPa and 3.20 MPa, corresponding to the improvements of 42.6% and 29.4% compared with those of the original system.

Abstract A modelling methodology is developed and used to investigate the technoeconomic performance of solar combined cooling, heating and power (S-CCHP) systems based on hybrid PVT collectors. The building energy demands are inputs to a transient system model, which couples PVT solar collectors via thermal store to commercial absorption chillers. The real energy demands of the University Campus of Bari, investment costs, relevant electricity and gas prices are used to estimate payback times. The results are compared to: evacuated tube collectors (ETCs) for heating and cooling provision; and a PV-system for electricity provision. A 1.68-MWp S-CCHP system can cover 20.9%, 55.1% and 16.3% of the space-heating, cooling and electrical demands of the Campus, respectively, with roof-space availability being a major limiting factor. The payback time is 16.7 years, 2.7 times higher than that of a PV system. The lack of electricity generation by the ETC-based system limits its profitability, and leads to 2.3 times longer payback time. The environmental benefits arising from the system’s operation are evaluated. The S-CCHP system can displace 911 tons CO2/year (16% and 1.4 × times more than the PV-system and the ETC-based system, respectively). The influence of utility prices on the systems’ economics is analysed. It is found that the sensitivity to these prices is significant.

Abstract To explore the effects of Gedeon streaming on the onset and damping behaviors, infrared imaging is applied for the first time in a traveling-wave thermoacoustic engine to observe the temperature evolution of the regenerator. Under conditions of with and without Gedeon streaming, the temperature distribution differences of the regenerator in the onset and damping processes are compared and analyzed. Based on the visual images, the dimensionless temperature distribution reveals some phenomena that have not been revealed by traditional measurement methods. Analysis of the thermal and mass flows is made to further understand the mechanism of the onset and damping processes.

Abstract Acoustic impedance matching is critical to the overall performances of a traveling-wave thermoacoustic electric generator. This paper presents an effective approach for matching the acoustic impedances of the thermoacoustic engine and the linear alternators for maximizing the output electric power and thermal-to-electric efficiency. The acoustic impedance characteristics of the engine and the linear alternators are analyzed separately, and the methods for modulating the acoustic impedances are investigated numerically. Specially, two different coupling locations including one at the resonator and the other one at the loop of the thermoacoustic engine are compared. It is found that the imaginary part of the load acoustic impedance should be near zero for a good output performance of the engine at either coupling location. The real part of the optimal acoustic impedance for the coupling location at the resonator is smaller than that for the one at the loop. The acoustic impedance of the linear alternator can be simply and effectively adjusted to the expected range by tuning the operating frequency, load resistance and the electric capacitance. Both the experiments and numerical simulations show that a better matched condition can be achieved when they are coupled at the location at the resonator. Maximum output electric power of 750.4 W and the highest thermal-to-electric efficiency of 0.163 have been achieved. When they are coupled at the loop, the maximum electric power and the thermal-to-electric efficiency become 506.4 W and 0.146 due to the lower quality of the acoustic matching. The acoustic matching approach presented in the paper would be helpful for guiding the designs of thermoacoustic/alternator and compressor/cryocooler systems.

Abstract A thermoacoustic compressor is capable of converting an alternating gas flow to a direct one with a large pumping rate on the basis of the pressure oscillation nature of thermoacoustic engines and the flow rectification effect of check valves. Theoretical calculations are first carried out to study the factors that affect the performance of the closed and open thermoacoustic compression systems. It is shown that the frequencies of directly connected thermoacoustic engines should avoid small integer multiple relationships to operate efficiently. Increasing the pressure amplitudes is beneficial for the pressure lift in a closed system as well as the pumping rate in an open system. A demonstrative closed thermoacoustic compressor was then experimentally studied. A maximum average gas pumping rate of 4.55 Nm3/h during the first 2 s of the compression process was achieved when all components were at the same initial mean pressure of 2.13 MPa. The maximum pressure lift reached 0.4 MPa when the initial mean pressure was 2.4 MPa. It was found that the pressure lifts were roughly proportional to the pressure amplitudes. Due to the superposition of alternating and direct gas flows, deformation of pressure waveforms which has a negative effect on the performance was observed.

Beating effects between a thermoacoustic source and its mechanical partner-a piston-spring oscillator are numerically predicted and experimentally observed in the free-decay process. Through analyzing the indicator diagram, periodic energy transfer characteristics between a thermoacoustic source and its mechanical partner during the beating oscillation are revealed and analyzed. The oscillation frequency is found to split into two modes intrinsically even when the resonance frequencies are initially tuned to be the same. The patterns and frequencies of the beating oscillations are sensitively affected by the characteristics of acoustic sources. The study sheds light on the underlying mechanisms of beating oscillations occurred in thermoacoustic systems with multiple resonant sub-units.