Variable Geometry Ejector Research Articles

Seasonal energy performance assessment of a hybrid HVAC system driven by solar and biomass energy for space heating and cooling in residential buildings

The H2020 project Hybrid-BioVGE aims to develop a sustainable HVAC system concept for buildings’ climatisation entirely based on renewable energy sources. The Hybrid-BioVGE system provides space heating/cooling and DHW production of small- and medium-scale buildings and deploys solar and biomass heat as energy inputs. During the heating period, thermal energy is provided directly by solar thermal collectors and a biomass boiler. The cooling energy demand is met by a thermally-driven ejector cycle based on a Variable Geometry Ejector (VGE), which adjusts its geometry according to two degrees of freedom. A demonstrator of the Hybrid-BioVGE system was designed and installed in a residential building in Porto (Portugal). The demonstrator’s seasonal energy performance was assessed numerically by a simulation model implemented in TRNSYS. Simulation results show that up to 98% of the system energy input is provided by renewables during the heating season, with a solar fraction higher than 60%. Conversely, the system energy performance is lower than expected during the cooling season due primarily to the limited performance of the heat rejection loop.

Experimental Analysis of a Polygeneration System: Assessment of the Thermal Sub-System

In this paper, the experimental results of the thermal sub-system of a reliable and cost-effective polygeneration solar system are presented. This polygeneration system produces heating, cooling, and electricity from solar energy, which is used in an existing test building. Heat is generated in four evacuated tube solar collectors (ETCs). The heat may be used for space cooling through a variable geometry ejector (VGE) heat pump. In order to reduce the mismatches between generation and consumption, two thermal storage tanks were added. The performance of a new thermal storage, with 400 L, able to store both sensible and latent heat, was tested. The heating performances of the test building were assessed. Ejector cycle tests were also performed, and the variation of the cooling coefficient of performance (COP) was calculated for different flow rates. For heating, the results showed that the heat storage was capable of heating the test building for 8 h, with temperatures between 22 °C and 26 °C. All results showed that this polygeneration prototype could be capable of meeting the heating and cooling needs when applied to a real building.

Assessment of the energy performance of a solar/biomass hybrid system for the climatization of smalland medium-scale buildings

The H2020 EU-funded project Hybrid-BioVGE aims to develop an efficient system entirely based on solar energy and biomass for the climatization of buildings during both the heating and cooling seasons. The most innovative component of the concept is a solar-driven Variable Geometry Ejector (VGE) cooling cycle, which can adjust its internal geometry with two degrees of freedom, according to the effective operating conditions. This paper presents the components of the Hybrid-BioVGE concept and describes a demonstrator of the proposed solution installed in a single-family house in Porto (Portugal). The main characteristics of the building, the size and typology of thermal energy storages included in the system are presented as well. In order to demonstrate the energy-saving potential of the Hybrid-BioVGE concept, the numerical model of the demonstrator was implemented in the TRNSYS 18 environment. Outcomes of simulations include the system’s energy performance for both heating and cooling seasons, the achievable solar fraction and the VGE efficiency. Numerical results show that up to 98% of the system energy input derives from renewable sources, with a solar fraction of 61.5%, during the heating season. On the contrary, the seasonal performance factor in cooling operating mode is lower than expected. The VGE cooling cycle efficiency is mainly penalised by the heat rejection loop, which should be optimised to achieve the best performance.

Effect of spindle position on the performance and two-phase characteristics of controllable ejector for transcritical CO[formula omitted] refrigeration system

Spindle-controlled variable-geometry ejectors can adjust the system capacity to meet varying load requirements in transcritical-CO2 refrigeration systems. The present study numerically explores the effect of spindle position on the performance and flow characteristics of a controllable two-phase CO2 ejector. Spindle-controlled ejectors achieved entrainment ratios higher than that of fixed geometry ejector. Motive and suction flows exhibited high sensitivity to the spindle position near the critical operating range because of the transition between operating modes. The two-phase and shock characteristics associated with ejector flow and the variations observed for different spindle positions are discussed. Critical discharge pressures and entrainment ratios are estimated for various spindle positions. The critical compression and entrainment ratios correlate with the respective normalized spindle position in the ejector. The impact of the effective nozzle throat area on the flow entrainment range and achievable pressure lift is investigated. The results showed that the critical entrainment ratio bears a common linear relationship with the critical compression ratio for all positions of the spindle. The study relates the most significant performance and geometric parameters of the spindle-controlled two-phase CO2 ejector.

Experimental comparison of critical performance for variable geometry ejectors with different mixer structures

To further optimize the performance of Solar-driven Ejector Refrigeration system under variable driving conditions, ejectors design using conical-cylindrical mixers are more effective than using cylindrical mixers were elucidated. The effects of the nozzle exit position (NXP), area ratio (Ar), and expansion ratio (E) on the performance of the conical-cylindrical mixer ejector (CCME) and the cylindrical mixer ejector (CME) were investigated. It was revealed that compared with the performance of the CME at the optimal nozzle exit position, the variable geometry CCME exists a NXP range where both the critical entrainment ratio (Ercri) and critical compression ratio (Ccri) are larger. This NXP range can be widened by appropriately reducing the NXP or Ar of the CCME under a specified expansion ratio. Besides, under variable expansion ratios, the expansion ratio range in which both Ercri and Ccri of the CCME are better than those of the CME can be widened by appropriately reducing the NXP or Ar of the CCME. The results demonstrated that the CCME is more suitable for variable expansion ratios than the CME. This implies that using the variable geometry CCME has a higher energy utilization efficiency in a Solar-driven Ejector Refrigeration system than that of using the CME. This work provides a promising guide for achieving higher energy efficiency of an ejector refrigeration system under variable driving conditions.

The performance analysis of a variable geometry ejector utilizing CFD and artificial neural network

Currently, ejector-based refrigeration applications are attracting much attention as ways of improving refrigeration systems and energy conversion applications. The ejector performance is influenced mainly by the operating conditions and the geometry of the system, so the geometry parameters need to be optimized for optimal performance. However, due to the fluctuating operating conditions no fixed geometry will satisfy the whole range of performance requirements. Thus, a dynamic geometrical ejector is more feasible. A CFD model designed to test the functionality of a Variable Geometry Ejector with a movable spindle and variable nozzle exit position is presented, utilizing R1234yf due to its environmental advantages. In instances where the primary temperature falls below the ideal threshold, the optimal outcome was achieved by positioning the nozzle exit at 0 mm. In contrast, if the primary temperature is equal to or exceeds the optimal level, a negative value of the nozzle exit position can enhance the entrainment ratio, raising the critical temperatures by up to 11.8%. Not utilizing a spindle resulted in a higher entrainment ratio, but it also yielded the smallest stable operating range among the spindle-equipped scenarios. Further, adopting a spindle with a nozzle position of 0 mm led to a significant improvement in critical temperature, increasing it by 25.8%. A Design of Experiment analysis is conducted to show the most influential factors on the primary, and secondary mass flow rates and the entrainment ratio. An artificial neural network that merges the operating conditions and geometrical parameters succeeds in predicting the ejector entrainment ratio with R2 = 99.81%.

Experimental study of a R290 variable geometry ejector

Ejectors are classified as fluid-dynamics controlled devices where the “component-scale” performances are imposed by the local-scale fluid dynamic phenomena. For this reason, ejector performances (measured by the pressure-entrainment ratio coordinate of the critical point) are determined by the connection of operation conditions, working fluid and geometrical parameters. Given such a connection, variable geometry ejector represents a promising solution to increase the flexibility of ejector-based systems. The present study aims to extend knowledge on variable geometry systems, evaluating the local and global performances of the R290 ejector equipped with a spindle. The prototype ejector was installed at the R290 vapour compression test rig adapted and modified for the required experimental campaign. The test campaign considered global parameter measurements, such as the pressure and the temperature at inlets and outlet ports together with the mass flow rates at both inlet nozzles, and the local pressure drop measurements inside the ejector. In addition, the experimental data were gathered for different spindle positions starting from fully open position the spindle position limited by the mass flow rate inside the test rig with the step of 1.0 mm.

A novel control strategy with an anode variable geometry ejector for a SOFC-GT hybrid system

A novel control strategy was developed with an anode variable geometry ejector for a solid oxide fuel cell-gas turbine (SOFC-GT) system. The anode inlet temperature was controlled by combining two modes to achieve a greater controllable range: anode variable geometry ejector adjusting and after-burner fuel valve adjusting. Two tests were carried out under small-scale and large-scale load steps. The results indicate that all controlled variables can be effectively kept around set-point. Moreover, the critical parameters are kept within safe ranges, including steam-to-carbon ratio higher than 2.0, peak temperature gradient below 10 K/cm, peak time-dependent temperature gradient below 3 K/min, and surge margin higher than 15%. The developed novel control strategy can maintain almost constant SOFC spatial temperature. The spatial temperature variation is within 2.50 K under 5% load step, and within 14.30 K under 30% load step. Besides, the novel control strategy can keep the system efficiency at a high level (more than 63.21%) during the load tracking. Compared with the conventional control strategy with a fixed geometry ejector, the results demonstrated that the novel control strategy can significantly improve the system performance, especially the transient behaviors of after-burner fuel rate, turbine inlet temperature, and system efficiency.

The influence of Variable Geometry Control on a R290 Ejector Refrigeration System

The large-scale deployment of ejector refrigeration systems (i.e., solar-based ejector refrigeration systems), although representing a promising alternative compared with mechanical compressor ones, is hindered due to limitations regarding ejector control modes. Indeed, ejectors are fluid-dynamics controlled devices and, because of their fixed geometry, they operate at their highest efficiency in a narrow range of operating conditions, which is in contrast with the dynamic pressure and temperature levels characterizing real applications. In this context, variable geometry ejectors (VGE) represent a promising solution to increase the flexibility and operation range of this component. The present study aims to extend the present body of knowledge regarding VGE systems, evaluating the impact of a spindle-provided ejector operated with R290 on the performance of the refrigeration system. The analysis has been carried out using an integrated lumped parameter/CFD approach, thus linking the local flow properties and global performances. Different spindle positions have been tested to assess how the different nozzle area ratios affect both the entrainment ratio and the critical pressure. Results showed that increasing primary nozzle area ratio the system can effectively reduce the thermal input, increasing the average COP at the expanse of a lower critical pressure. In conclusion, using a moving spindle control system might ensure an improvement of the ejector performance.

Primary energy saving potential of a solar-driven ejector cooling system: a case study for a Portuguese residential building

The seasonal energy performance of a cooling system based on an innovative variable-geometry ejector (VGE) is numerically investigated by using TRNSYS. The VGE-based system is mainly driven by solar energy, collected through solar thermal collectors, and is coupled to a residential building located in Porto. A biomass boiler is used as back-up heater. The energy performance of the investigated cooling system is compared with that of a conventional solution, based on a commercial air-to-water chiller. Results point out that, almost 75% of the generator heat demand can be supplied by solar collectors and about 90% of the overall energy input of the ejector-based system is satisfied by renewables. Moreover, numerical simulations confirm how the capability to vary the ejector geometry on the basis of current operating conditions allows to strongly improve the ejector seasonal efficiency. A second series of simulations aimed to further enhance the system performance. A master control logic which extends the VGE operation time in correspondence of favourable ambient conditions was introduced, in order to store additional cooling energy in the cold buffer tank. This strategy has proved to be effective, since the energy consumption of the biomass boiler could be reduced up to 35%.

Hybrid Ejector-Absorption Refrigeration Systems: A Review

Absorption Refrigeration Systems (ARS) are potential alternatives to direct expansion (DX) refrigeration systems. This review focused on the incorporation of an ejector into absorption refrigeration cycles to constitute Hybrid Ejector-Absorption Refrigeration Systems (HEARS). The ejector adds several advantages to the absorption refrigeration systems depending on its location in the cycle. The two prevalent configurations of HEARS are Triple pressure level (TPL-HEARS), and Low Pressure Condenser (LPC-HEARS). Previous studies revealed the preference of the latter configuration as it allows lower circulation ratios, enhances the refrigeration effect, and could achieve a COP up to 1. Moreover, LPC configuration is suitable with single, double, and variable-effect absorption systems with a COP of above unity. In turn, the TPL-HEARS notably enhances the absorption process, particularly when a variable geometry ejector is utilized. This configuration could obtain a COP around 1.1, but only with high-density refrigerant vapor. Lately, to attain the advantages of both configurations, some studies investigated the viability of adding two ejectors to the cycle. This paper meticulously reviews investigations conducted on the emerging dual ejectors-absorption refrigeration technology. This paper reveals the general performance trend and the maximum attainable COP by each type of hybrid ejector-absorption refrigeration system. DEARS and Ejector-driven absorption refrigeration systems (ED-ARS) could achieve COP that ranges between 1.2 and 1.46. The use of a flash tank and a RHE is essential in NH3/H2O HEARS. At high generator temperatures (of 120–170 °C), DEARS was found to be the system with less complexity and best performance. Nevertheless, the performance of the DEARS might drop significantly if the heat source temperature is fluctuating. Thence, the variable-effect HEARS is considered the best alternative. The capability of HEARS to be integrated with different power generation cycles is also highlighted. Finally, the review presents possible future research opportunities to improve the absorption refrigeration technology.

Multi-scale evaluation of an R290 variable geometry ejector

Although representing a promising alternative to mechanical compressor driven systems, ejector refrigeration has not widely spread due to its limits due to the system operation's rigidity. Ejectors are fluid-dynamics controlled devices and because of their fixed geometry, operates at their best efficiency in a narrow range of operating conditions, which is often in contrast with the dynamic pressure and temperature levels of real refrigeration systems. Variable geometry ejector represents a promising solution to provide the ejector cycle with increased flexibility and an overall performance improvement. The present study aims to extend knowledge on VGE systems, evaluating the impact of a spindle-provided ejector operated with R290 on the refrigeration system's performance. The analysis has been carried out using an integrated model which allowed us to interpret the results by CFD visualization. Eight spindle positions (in the range of 0–7 mm) have been tested for different primary and secondary boundary conditions to assess how the different nozzle area ratio (in the range of 2.25–4.41) affects the entrainment ratio and the critical temperature. Results showed how increasing primary nozzle area ratio moving the spindle towards the mixing chamber can effectively reduce the thermal input, increasing the average COP and reducing the critical discharge temperature. For instance, a + 33% increase of the nozzle area ratio, enhanced the COP by an average + 57.1% but lowered the average critical temperature by −6.7 K. Fluid dynamics significantly affect the entrainment process and pump-effect phenomenon with particular attention to the primary flow expansion. In detail, the entrainment ratio benefitted from over-expanded jets, whilst the compression effect increased for an adapted jet. In the end, the performance curves have been obtained, which correlate the critical temperature conditions to the system performance.

Applying a variable geometry ejector in a solar ejector refrigeration system

Abstract This paper presents an experimental study of the influence of a variable geometry ejector (VGE) design on the performance of a small-scale, 1.5 kW nominal capacity solar heat driven ejector air-conditioning system under real-life working conditions. Under variable operating conditions (e.g. solar radiation, ambient temperature) fixed geometry ejector performs poorly, therefore the objective of the present work was to prove the benefit of the VGE concept. In the experimentally tested VGE the area ratio through a movable spindle (SP) and the nozzle exit position (NXP) can be adjusted in order to respond to the operating conditions. The results showed very stable operation of the cooling cycle during the experiments. Both NXP, SP had considerable influences on the cooling cycle performance. An optimal NXP was identified, which did not change with various operating conditions. Similarly, SP also showed an optimum; however, the optimum point depended strongly on the operating conditions. The results clearly indicated the benefit of using the VGE design over the fixed geometry. COP improvement was as high as 24% with a maximum of 0.29 corresponding to a cooling capacity of 1.6 kW when compared to a fixed geometry ejector. The study also confirmed the importance of using a variable-speed pump in the ejector cycle.

Energy and economic analysis of a variable-geometry ejector in solar cooling systems for residential buildings

Abstract In the present, study two 1-D ejector mathematical models are used to predict the performance of a solar ejector cooling system with fixed and variable geometry ejectors. Thes models are implemented using MATLAB and EES Software. The effect of the mixing efficiencies on the performance of the system was investigated, and two empirical correlations were obtained using experimental data for an ejector cooling cycle with R141b as a refrigerant. The results obtained with varying efficiencies were found to be significantly more accurate than those obtained with previous models, which assumed a constant efficiency. The results showed that the ejector with fixed-geometry was very sensitive to the variations of the operational conditions and could practically be functional only under specific conditions. It was also found that the solar ejector cooling system with a variable area ejector has higher coefficient of performance under a vast range of operational conditions. The new system could be easily implemented in residential building air conditioning systems with solar energy as the driving source of heat. The refrigerant R134a was found to give the best performance considering operational, safety, and environmental factors. The economic analysis revealed that the new system is profoundly better than the conventional systems in terms of payback period and net present value.

Thermal and electrical performance assessment of a solar polygeneration system

In this paper, a reliable and cost-effective polygeneration system for an existing test building, able to provide electricity, cooling and heating needs is presented, modelled and assessed. The system relies on solar energy as the primary source (PV and solar thermal collectors). Building cooling demand is achieved with a variable geometry ejector heat pump. To reduce the mismatches between generation and consumption, different types of storage are included (electrical and thermal). A system prototype is under development and will be installed as a demonstration site in Portugal.To assess the test building energy needs, the electricity consumption of all consumers was measured. For the thermal energy demand a numerical model using TRNSYS software was developed. The building thermal performance was validated using experimental data. The numerical model also includes the solar field, thermal energy storage and ejector cycle. To model and simulate yearly electricity production PVSyst software was used.

POLYSOL – Thermal and electrical performance assessment of a cost-effective polygeneration system

Buildings are responsible for a large portion of the energy consumption and harmful emissions to the environment. To change this situation, the European Performance of Buildings Directive requires new buildings to be nearly Zero Energy Buildings. To achieve this goal, energy saving measures combined with large use of renewables on-site must be applied. In this paper, a reliable and cost-effective polygeneration system for an existing test building, able to provide electricity, cooling and heating needs is presented, modelled and assessed. The system relies on solar energy as the primary source (PV and solar thermal collectors). Building heat and cooling demand is achieved with a variable geometry ejector heat pump. To reduce the mismatches between generation and consumption, it includes different types of storage (electrical and thermal). For the test building energy needs assessment, the electricity consumption of all test building consumers was measured. Thermal energy demand evaluation required the development of a dynamic numerical model using TRNSYS software. The building thermal performance was validated using experimental data. The numerical model also includes the solar field, thermal energy storage, ejector cycle subjected to know meteorological conditions and operation control. To model and simulate yearly electricity production PVSyst commercial software was applied. Regarding the thermal energy, it was found that on a yearly basis the energy supplied by the thermal collectors about six times the demand. Nevertheless, on the hourly and daily levels, there are thermal energy shortages, due to the lack of solar radiation or mismatch between production and demand. Regarding the electric energy consumption, the potential electric energy production is about 1.6 times the demand. Like for the thermal energy, on the hourly and daily levels, there are energy deficits mostly in winter, for which an electrical energy storage unit will be used. Additionally, the highest peak occurs in summer because of low solar radiation at the end of the day and high demand due to the high cooling load of the test room.

Parametric study of a Novel Hybrid Solar Variable Geometry Ejector cooling with Organic Rankine Cycles

The effect of ejector area variation to account for the unsteadiness of the driving low-grade solar energy source on the cycle performance of a combined variable geometry solar ejector cooling Organic Rankine Cycle under different operating conditions was studied. An Organic Rankine Cycle was combined with the ejector cooling cycle to utilize the excess solar heat in peak hours and to generate power. Simulation and modeling of the system were conducted using the Engineering Equation Solver and Ebsilon Software. Conservation principles and laws of mass and energy transfer were applied to all components of the system and a one-dimensional thermodynamic model was used to model the ejector. It was found that the effect of the ejector area ratio and evaporator pressure were more predominate on the cycle performance than that of the generator pressure. An improvement of 78% of the system coefficient of performance compared to the standard ejector cooling cycle could be achieved using the Variable Geometry Ejector. In addition, further enhancement in the system coefficient of performance could be achieved by regulating the generator and evaporator pressures. The Organic Rankine Cycle efficiency was found to be 5.8% with a total system efficiency of 11.35%. The system cooling capacity was improved by 81% compared to the standard ejector cooling cycle. The proposed system was found economically feasible in countries, where the electricity price is higher than 15 c$/kWh.

Performance investigation of an auto-tuning area ratio ejector for MED-TVC desalination system

In Multi-Effect Distillation with Thermal Vapor Compression (MED-TVC) desalination system driven by low-grade thermal energy, it is arduous for conventional fixed geometry ejectors to operate efficiently with unstable feeding motive steam. In this paper, a variable geometry ejector named auto-tuning Area Ratio (AR) ejector equipped with a bellows-driven spindle is proposed to enhance the performance of MED-TVC desalination system under varying operating conditions. A Computational Fluid Dynamics (CFD) model has been built and validated to investigate the influence of AR on the system performance. A correlation to calculate the optimal opening degree under varying primary pressure with different critical back pressures is established and identified by data regression method. Moreover, with this correlation, an optimization strategy for auto-tuning AR ejector in MED-TVC desalination system is presented. Simulation results show that by using the auto-tuning AR ejector, relatively higher entrainment ratios (Er) can be achieved under varying motive steam. When the pressure of motive steam fluctuates from 800 kPa to 2000 kPa, the average Er of auto-tuning AR ejector is 1.39, higher than fixed geometry ejector of 0.69 with a critical back pressure of 20 kPa. The proposed auto-tuning AR ejector would have wide applications in MED-TVC desalination systems and other systems driven by low-grade energy.

Ejector performance analysis under overall operating conditions considering adjustable nozzle structure

The variable-geometry ejector (VGE) is feasible for unstable heat-source utilization; the ejector can be adjusted to its design point to obtain high efficiency. Moreover, as the adjustable nozzle in the VGE significantly affects the performance of the ejector, a theoretical model is necessary to evaluate VGE performance. In this study, a two-dimensional theoretical model was proposed based on an adjustable-nozzle theory. Method of characteristics was employed to accurately predict the driving-flow development in the mixing section. In addition, the suction-flow velocity distribution on the effective area was considered. The proposed model was validated by employing the data from literature and additional experimental data obtained from a VGE test setup using R134a. The validation result shows that the proposed model predicts the ejector performance accurately; moreover, the model is more adaptive while the nozzle configuration changes. The theoretical model, proposed herein, is practical for the design and application of the VGE.

Effects of the nozzle configuration on solar-powered variable geometry ejectors

The variable geometry ejector (VGE) used in solar-powered ejector refrigeration systems enables performance regulation with unstable heat input, which is important in solar energy utilization; however, the ejector performance is significantly affected by the nozzle configuration. To study the effects of the nozzle configuration on the VGE and to optimize the VGE configuration, experimental investigations are conducted on a supersonic nozzle and a subsonic nozzle used in the VGE under a variety of operating conditions. The experimental results show a close relationship between the driving flow behavior and the ejector performance. The driving flow behavior is influenced by the occurrence of Mach waves, which is significantly influenced by the nozzle configuration. Numerical simulations are conducted that further explain the Mach wave behavior as well as the driving flow development inside the VGE. The results are significant for the optimization and application of the variable geometry ejector in solar-powered ejector refrigeration systems.