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This paper presents an analysis of single-stage scroll expander performance for pure fluids and their zeotropic mixtures in supercritical conditions using dimensionless parameters, namely specific speed (N s ) and specific diameter (D s ). Scroll geometries with different aspect ratios were modeled for a range of expander inlet temperatures. The expander efficiency was modeled accounting for losses due to leakage, friction, and over- and under-expansion. The expander efficiency was plotted as a function of two dimensionless parameters to analyze the applicability domain of scroll expanders. For a particular expander inlet temperature (T in ), zeotropic mixtures result in more compact scroll geometries than pure fluids, leading to reduced losses and better expander efficiency. Any increase in T in results in larger scrolls leading to higher leakage and thereby lower efficiency. At suitable operating conditions, an optimized scroll expander design can achieve expansion efficiency as high as 75%. Sub-optimal scroll designs for a given application lead to lower expansion efficiency due to over- or under-expansion losses.

An operational strategy to optimize building operating energy costs for suppliers and consumers is an important challenge for electrical power utilities. There are various supply-side measures that utilities have to take to ensure continuous energy supply for building heating and air-conditioning. During peak energy demand, utilities are often forced to use more expensive and less efficient generation, thereby increasing the cost of energy. However, some demand-side management practices behind the consumer meter can help in meeting this challenge. One such measure is the use of thermal storage for heating, ventilation, and air-conditioning applications in commercial buildings. There is a gap of adequate knowledge of an optimal control strategy of cold storage operation in buildings adapting to applicable time of day tariffs to minimize annual energy use and annual energy cost of operation. There is also a need to use commercially available tools to avoid the use of complex mathematical models. This study demonstrates strategic controls with six operating modes for using thermal energy storage to shift peak electricity demand, using the time of day tariffs as a decision variable, and reducing operating costs, while also minimizing the size of the system. EnergyPlus was used to model a standard reference large office building for three thermal energy storage system cases: mixed chilled water storage, stratified chilled water storage, and ice storage. An annual average shifting of 25–78% of peak electricity was achieved from the simulation results. The strategy was able to achieve an annual 10–17% cost reduction for consumers using the time of use rates available from a local utility.

A novel approach involving the use of ceramic materials was investigated for encapsulating chloride based PCMs with melting points higher than 650 °C. Low-cost ceramics with excellent thermal and chemical stability under molten-salt conditions were identified as the encapsulants. The processing procedure for these materials was discerned by systematic porosity distribution and materials compatibility studies. The influence of sintering temperature on the reactivity of feldspar, ball clay, kaolin and the mixture thereof with molten sodium chloride was investigated by IR spectroscopy. The porosity of the sintered ceramic samples was analyzed through water and dye absorption tests, as well as SEM analysis. The results were used for developing an optimum ceramic capsule fabrication procedure, which involved the use of a green ceramic body followed by sintering at 1190 °C. Sodium chloride and its eutectic with potassium chloride were used as the PCMs. The selected PCM was poured into and filled the fabricated ceramic capsule through a hole which was sealed at a temperature close to or higher than the melting temperature of the PCM. The fabricated capsules have been tested and have survived more than 150 thermal cycles without showing degradation in their thermo-physical properties.

Inorganic salts are potential phase-change materials for medium- and high-temperature thermal applications. It is essential to acquire knowledge of their behavior in latent heat thermal energy storage for the design of storage devices and the construction of an energy conversion and utilization system. In this study, a eutectic mixture of NaNO3 (mass ratio of 46%) and KNO3 was selected as phase change material, and a nitrate salt-based latent heat thermal energy storage unit was built to experimentally investigate its operating characteristics during charging and discharging. The thermophysical properties of the eutectic nitrate salt were measured and presented. The air outlet temperature remained almost unchanged during the melting of salt in the charging process, but it decreased gradually during the discharging process because the thermal resistance increased with the salt solidification. The melting time was shortened by 31.0% and 38.1% when the air inlet temperature was increased from 260 °C to 270 and 280°C, respectively. The solidification time was shortened by 22.2% and 33.3% when the air inlet temperature was reduced from 210 °C to 200 and 190°C, respectively. When the air mass flow rate was increased from 0.964 g/s to 1.446 and 1.962 g/s, the melting time was shortened by 32.4% and 57.4%, respectively, while the solidification was only shortened by 8% and 16%, respectively. The charging ratio or discharging ratio can be calculated through heat-loss evaluation to depict the thermal energy change in a charging or discharging process. The results indicated that charging ratio increased almost linearly with the melting of the salt. The air mass flow rate had a significant impact on this parameter. The influence of the air inlet temperature was gradually weakened with the increasing air inlet temperature. Owing to the coupling effect of heat loss and airflow, the influence of air parameters on the discharging ratio was weak. The effectiveness of storage indicates the extent to which the latent heat of salt can be utilized. It increased from 29.7% to 52.8% when the air inlet temperature was reduced from 210 to 190°C. This study provides insights into the phase-change characteristics of the nitrate salt and the nitrate salt-based latent heat thermal energy storage unit.

The aim of this study is to evaluate the field performance of gas engine driven heat pumps (GEHP) used in a commercial building. Four, 8 TR (cooling capacity) GEHP units were employed to provide air-conditioning to seven thermal zones in a commercial building. The performance of these GEHPs was assessed over a period of 10 months and compared with prior results reported in the literature. Additionally, this study provides a comparative economic analysis of GEHPs and conventional electrical heat pumps. During normal operation, the cooling capacity ranged between 12.3 kW and 19.3 kW. The average COPunit of these systems varied from 0.15 to 0.85 during field operation. The gas engines were found to operate at significantly lower loads than their design capacity, and therefore, produced overall lower efficiencies. Oversized GEHP systems were deployed to meet the low cooling/heating demands attributed to the lower overall field performance. This study demonstrates the importance of sizing the GEHP/HVAC system similar to the load in buildings.

The outstanding properties of single-layer graphene sheets for energy storage are hindered as agglomeration or restacking leads to the formation of graphite. The implications of the aforementioned arise on the difficulties associated with the aqueous processing of graphene sheets: from large-scale production to its utilization in solvent-assisted techniques like spin coating or layer-by-layer deposition. To overcome this, aqueous dispersions of graphene were stabilized by cellulose nanocrystals colloids. Aqueous cellulose nanocrystals dispersion highlights as a low-cost and environmentally friendly stabilizer towards graphene large-scale processing. Colloids of cellulose nanocrystals are formed by electrostatic repulsion of fibrils due to de-protonated carboxyl or sulfate half-ester functional groups. Graphene dispersions are obtained by hydrothermal reduction of electrochemically exfoliated graphene oxide in the presence of cellulose nanocrystals. This approach allows the preservation of the intrinsic properties of the nano-sheets by promoting non-covalent interactions between cellulose and graphene. The dispersions could be cast to form free-standing flexible conducting films or freeze-dried to form foams and aerogels for capacitive energy storage.

This work focuses on the design of a low cost utility scale thermal storage, consisted of vertically placed cylindrical phase change material capsules, for next-generation power plants. In this paper a simple model for the design of a storage unit is presented. This model includes both latent and sensible heat stored in phase change material. The exergetic efficiency and the operation time of the storage is considered as the design criterion in this model and the influence of various design parameters on the exergetic efficiency and operation time is investigated. Results show that under study conditions exergetic efficiency varies between 98.991% and 98.692% and operation time changes between 4 h and 21 h . Higher efficiency values obtained for lower number of capsules, lower heat transfer fluid flow rates and lower distance between capsules. Decreasing number of capsules and the distance between capsules also decreases the operation time. However, the decrease in the flow rate increases the operation time.

This work is a part of a project aimed to develop low-cost utility scale thermal storage consisting of vertically placed cylindrical PCM capsules for next-generation power plants. This paper focuses on the energy transport mechanism during the phase change process. In this study an experiment was designed and constructed to investigate the charging and discharging in a vertical cylinder with enhanced radiative absorption. Charging and discharging was done in a tube furnace and time variant boundary conditions were used to negate the furnace regularities. A CFD model using the enthalpy-porosity method was created and validated based on the experimental data. The behavior of liquid fraction with the inclusion of radiation absorbing particles was analyzed. The usual present melting/solidification front was replaced by a much wider melting zone as a result of radiation absorbing particles.

This paper presents the organic Rankine cycle performance comparison of several working fluids with low global warming potential and low ozone depletion potential at several heat source temperatures. At an evaporating temperature of 80 degrees C, maximum first law efficiency of 5.8% was achieved with ammonia, while at 145 degrees C and 180 degrees C, diethyl ether provides the maximum cycle efficiencies of 11.4% and 13% respectively. For the best operating conditions of the ORC model, a suitable two-stage scroll geometry was modeled and its performance was evaluated. Stage I and stage II scroll geometries with volume ratios of 5.3 each were modeled for the supply conditions of 180 C and 2.6 MPa. The geometries provided a combined shaft work of 8.7 kW at combined expander efficiency of 89% accounting for the losses due to the leakage in the expander.

This study was conducted to improve the flow distribution of air after a single 90-degree bend for temperatures above ambient conditions. In the literature, no study has been performed in the literature testing a flow conditioner for heated compressible flow. This study used an existing Thermal Energy Storage (TES) system for flowrates above 100 m3/h with Reynolds numbers over 10,000 was used. Due to a single 90-degree bend in the inlet flow a distance of three diameters from the entrance to the tank, the flow distribution was considerably shifted to the side of the outer bend. The constrained systemwas tested with various flow conditioners with fluid temperatures between the ambient temperature and 60°C. The Spearman and the Mitsubishi Heavy Industries (MHI) plates were the most effective at improving the flow under all tested conditions. The system was modeled for different flow conditioners and validated using a computational fluid dynamic (CFD) model.Copyright © 2016 by ASME

A lab-scale packed-bed system was used to test an industry scalable latent heat thermal energy storage (TES) system suitable for supplementing heat to a steam powered cycle. Metallic cylindrical capsules were filled with a eutectic of sodium sulfate (Na2SO4) and potassium chloride (KCl) as the phase change material (PCM) for energy storage. The packed bed system consisted of a cylindrical storage tank with the cylindrical capsules arranged vertically. This system was modeled and validated. The charging and discharging efficiencies of different operating parameters were analyzed. The calculated ratio of exergy to energy efficiency was around 90%. The overall efficiency of the bed, however, was found to be low due to the losses through the packed bed. Possible losses and limitations of the system were analyzed, and recommendations were made for improved designs.