Abstract
Thermal energy storage using the latent heat of phase change materials (PCMs) is a promising technique to solve the time mismatch between the availability and usage of flue gas heat in distributed generation systems (DGSs). A diesel-engine-powered DGS integrated with two-stage tube-type PCM modules for exhaust gas heat recovery was developed and studied. Energy and exergy analysis for the PCM storage unit was carried out to verify the effectiveness of the PCM modules for heat recovery and to highlight the merits of the cascaded configuration through a practical engineering case. Furthermore, the performance of the DGS was evaluated to study the contribution of PCM storage to improving system efficiency. The results showed that 56.4% energy and 48.3% exergy of the input flue gas were stored by the two-stage storage unit. Additional integration of the low-temperature PCM module to the high-temperature module improved the average storage efficiency from 33.6% to 62.3% for energy and 33.1% to 50.8% for exergy. By utilizing the stored energy for heating water, the thermal efficiency of the diesel engine was increased from the original 35.8% to 41.9%, while the exergy efficiency was improved from 29.5% to 29.7%.
Highlights
Distributed generation systems (DGSs) with an internal combustion engine as the prime mover are recognized as an effective way to utilize fuel energy and reduce greenhouse gas emissions [1,2].In addition to electricity generation, the exhaust gas heat from the engine can be recovered by various techniques such as the Rankine cycle [3,4,5], heat exchangers [6,7,8], thermoelectric generators [9,10], and so forth
Of the gas which was assumed as 250 K; mpcm and mtube denote the mass of steel tube, respectively; mod denotes the temperature increase of the module, which was set as 300 the phase change materials (PCMs) and steel tube, respectively; cpcm and ctube denote the specific heat capacity of the PCMs and steel tube, respectively; ΔTmod denotes the temperature increase of the module, which was set as 300 and 100 K for the High-temperature module (HTM) and low-temperature storage module (LTM), respectively; ψ = 70% denotes the usage rate of the latent heat; and Nr = 1.1 denotes the redundant factor
Experimental investigations were performed according to the DGS operation scheme shown in measurement of PCM temperature (U1), heat transfer fluid (HTF) temperature (U2), flow rate (U3), the mass of PCMs temperature (U ), HTF temperature (U2 ), flow rate (U3 ), the mass of PCMs (U4 ), and phase change (U4), and phase 1change enthalpy (U5)
Summary
Distributed generation systems (DGSs) with an internal combustion engine as the prime mover are recognized as an effective way to utilize fuel energy and reduce greenhouse gas emissions [1,2]. To improve the performance of a latent thermal storage system, cascaded storage arranged in series according to the melting temperatures of PCMs has been further studied. A numerical investigation of a molten-salt packed-bed PCM system was carried out and indicated that the multi-PCM unit achieved a considerably higher heat transfer rate than the single configuration by maintaining the temperature difference between the heat transfer fluid (HTF) and PCM [19]. Considering the advantages of cascaded PCM storage, integration of multi-PCM modules to manage the exhaust gas heat in DGSs is beneficial to maintain the balance between the user demand and system output, and it optimizes the usage of fossil fuels. An overall efficiency investigation for the DGS was conducted to emphasize the contribution of latent heat storage to improving system performance
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