ejectors canmetenergy july 2009

Transcription

1 ejectors CanmetENERGY July 2009

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3 EJECTORS 1 EJECTORS Introduction Ejectors are driven by waste heat or heat from renewable sources, and directly activated by a thermal source to produce heating, cooling or refrigeration. The potential impacts of ejectors are huge as they may be utilized in a variety of applications. They can be directly integrated in a heat pump-cooling-refrigeration system as an internal component to increase the system efficiency. They can also be used in hybrid systems (cascade, subcooling) as an ejecto-compression or ejecto-absorption systems to increase the system global performance. The applications are numerous. They can be installed in HVAC systems for buildings, houses and communities, and particularly in combination with renewable energy systems or distributed generation systems for meeting near- or net-zero energy houses, buildings and communities. They help industry reuse waste heat by upgrading the temperature of the waste heat and increasing cooling and refrigeration system performance. It is important to note that an increase of % in performance of the heating systems in the building sector in Canada would lead to a saving of 2 PJ and a reduction of 2. MT eq-co 2 per year. In the industrial sector, a % increase in heat recovery would save 8 PJ and. MT eq-co 2 per year. TECHNOLOGY OVERVIEW The principle of operation of ejectors is based on the Venturi effect of a converging-diverging nozzle to convert the pressure energy of a motive fluid (Primary flow) to kinetic energy to entrain a suction fluid (Secondary flow), and then recompress the mixed fluids by converting kinetic energy back into pressure energy (Figure 1). EJECTOR a b c Primary flow Secondary flow Figure 1 Operation of an ejector

4 2 EJECTORS Ejectors are thermally activated static compressors and consist of a nozzle (a primary convergent-divergent nozzle) embedded in a main, generally cylindrical, body. The compression effect results from the interaction of the two fluid streams. The motive stream is at high pressure and is produced in a generator using a heat source. This heat source can come from low grade temperature heat. Ejectors thus have the advantage that they can be driven with waste heat, and used as heat pumps in appropriate cycles to produce heat upgrading, cooling or refrigeration effects, provided that a thermal source is available. Figure 2 illustrates the principle of operation of an ejector based heat pump system. Q G Pump W P Generator Condenser Q C Evaporator Q E Temperature Pressure P Benefits: Thermally activated compressor P 2 P 1 Enthalpy Figure 2 Ejector based heat pump system

5 EJECTORS The ejector based heat pump system presents the same components of a typical vapour compression system except for the compressor which is replaced by an ejector, a pump and a generator. The generator (heat coming from a low temperature energy source such as waste heat) supplies vapour at a high pressure P to the primary inlet of the ejector. This motive flow is accelerated in the primary nozzle (Fig. 1, a) where it reaches supersonic velocity, creating a depression at the nozzle outlet, drawing in the flow coming from the evaporator at a lower pressure P1. Both flows enter in contact in the mixing chamber (Fig. 1, b), where the two velocities equalize at a constant pressure and eventually a chock wave takes place, resulting in a pressure raise and lower velocity (subsonic). The diffuser (Fig. 1, c) allows the conversion of the remainder velocity into static pressure and the mixed flow reaches the intermediate pressure P2, which is the condenser pressure. After condensation part of the flow is expanded to the evaporation pressure P1 while the remaining flow is pumped back to the generator. Overall efficiency of ejectors is generally lower than competitive technologies such as mechanical compression or absorption but it has the very valuable advantages of simplicity, low cost and low maintenance over these technologies. It also has the unique advantage that it can use low temperature waste heat to operate. R&D Challenges Since the use of refrigerants in ejectors is very recent, there is very scarce data and information readily available on their design and operation. The available information is sometimes even contradictory. As a consequence, current design information is based on air and water data, thereby confining ejector applications to very limited cases. Recent efforts in cooling and heating design optimization have identified ejectors as offering a great application potential but operational and design knowledge with fluids other than air or water, or off design operation assessment have yet to be developed. For two-phase (gas-liquid) ejectors, the situation is even worse, knowledge is minimal. Ejector operation is conditioned by the complex interaction of a number of mechanisms.traditional design methods always introduce many simplifying assumptions and rely on empirical techniques. In practice ejector flow is neither one dimensional, nor is it in thermodynamic equilibrium. The state of non equilibrium complicates considerably the analysis process so that ejector design continues to be empirical or semi empirical despite the existence of several models, generally single phase and based on one dimensional gas dynamics. In all modeling cases the basic principles of mass, momentum and energy conservation have to be respected. It is the assumptions, the boundary conditions and the computations procedures that make the difference between the different approaches. The typical ejector behaviour presented in Figure is another phenomenon still far from understood. The entrainment ratio ω relates the secondary mass flow rate m s to the primary mass flow rate m p in the ratio m s /m p. With the exit pressure (represented in the figure by the condenser pressure), they are the main performance parameters characterizing ejector operation. For fixed ejector geometry and conditions, the entrainment ratio has a maximum value which remains constant when increasing the exit pressure. This remains so up to a maximum pressure called the critical pressure beyond which the entrainment ration starts declining. When designing an ejector, it is this point that is determined and beyond which the sharp decline in performance indicates the off design operation zone. The critical point corresponds to the optimal condition for this geometry, where both the primary and secondary streams are choked, i.e. they have both reached sonic conditions at their respective throats.

6 EJECTORS Entrainment Ratio - ω. Critical pressure Condenser pressure P c Figure - Typical performance curve APPLICATIONS The ejectors may be utilized: Inside a heat pump cycle»» In replacement of the expansion device, to recover the compressor work usually lost in the expansion device, in order to increase the system efficiency (Figure )»» As a condensing ejector for heating applications, in order to reduce the compressor work, and therefore increase the system capacity and performance (Figure ) In both cases, the ejector works in two-phase mode (two-phase flow). The same configurations may be applied to absorption heat pumps. In cascade with a heat pump system:»» The ejector is activated by a heat source and is used to sub cool the condenser outlet (Figure ) or to cool the heat pump condenser (Figure 7) In both cases, the ejector works in single phase mode (one phase flow) and helps improve the heat pump system performance for heating, cooling or refrigeration applications. Ejectors can also be used for absorption heat pumps.

7 EJECTORS 1. Hybrid ejector-vapour compression systems (Two-phase-flow ejectors) A. Ejector as expansion device for Cooling/Refrigeration applications Expected performance improvement: COP = 10 to 1% This ejector is driven by high temperature and pressure condensate which is used to draw low pressure vapour refrigerant from the evaporator and reject it to a medium pressure and temperature in the separator. The ejector is used in this case as an expander and reduces expansion losses of the cycle with an increase in COP of the order of 10% to 1% as a result. Q C Condenser Compressor W C Ejector Separator 7 9 Evaporator 8 Q E Temperature Pressure Benefits: Reduced compression work to instead of 9 to Cooling effect Ejector operation Enthalpy Figure Ejector as an expansion device in a heat pump system

8 EJECTORS B. Condensing Ejector for Heating Applications Expected performance improvement: COP up to 0% In this case the two-phase ejector is still driven by the condensate but prior to being sent to the ejector its pressure is raised through a booster pump so that the ejector is enabled to draw vapour refrigerant from the compressor. Such a cycle can be used in heat pump applications. Expected COP improvement over an ordinary heat pump can be as high as 0%, depending on the operating conditions. Q C 8 Condenser Ejector Pump W P 7 Evaporator Compressor W C Q E Temperature Pressure 7 Benefits: Reduced compressor work 2 to instead of 2 to 8 Enthalpy Figure Ejector as an additional compressor in a heat pump system

9 EJECTORS 7 2. multi-stage ejector heat pumping systems (One-phase flow ejectors) Expected performance improvements: (A) COP = to 20% (B) COP up to 0% Q G Q G Generator Generator Pump W P Q C Pump W P Q C Condenser Ejector Condenser Ejector V Heat Exchanger IV Condenser III D Heat Exchanger C Evaporator Compressor Evaporator Compressor I II A B Q E Q E Temperature Pressure Temperature Pressure V IV III D C D C I I II A A B Enthalpy Enthalpy Benefits: Refrigeration capacity increased: I to II instead of I to II Benefits: Compressor work reduced: B to C instead of B to C = Refrigeration cycle with ejector system = Refrigeration cycle without ejector system = Refrigeration cycle with ejector system = Refrigeration cycle without ejector system Figure - Subcooling (A) Figure 7 - Cascade System (B) Combination Ejector System/Mechanical System Ejector activated by external heat

10 8 EJECTORS The ejector system is activated with external heat. It is used: In A) to sub cool the liquid at the condenser outlet. In B) to cool down the condenser of the main refrigeration system. Heat for activation may come from industrial processes, heat rejection, solar collectors, distributed generation systems, and compressor superheat. The first application (A) consists in sub cooling the condensate of the primary system before expansion. The COP improvement ranges from to 20 %. The second application (B) of one phase ejector represented above is a cascade type of a mechanical refrigeration system. Expected COP improvement in this case is up to 0 %. CanmetEnergy S R&D Demonstration Program CanmetENERGY s intent is to position itself as a centre of expertise in ejectors and closely work with refrigeration system manufacturers for system integration. The objective is to become experts in the design and the optimization of ejector based systems. The strategy consists of activities on the following items: Ejector component»» One-dimension model»» CFD modeling»» Geometry optimization»» Operation fluid selection»» Test bench for model validation and practical experience development Ejector system»» System modeling for different ejector applications in one phase or two-phase modes of operation.»» Laboratory prototypes for model validation and development of practical experience»» The first targeted applications are: heat recovery from boiler exhaust to increase the capacity of cooling systems (district heating systems, industries); and ejector in replacement of an expansion device in a refrigeration system. Ejector demonstrations Demonstration projects in collaboration with manufacturing companies for the two above men-»» tioned applications

11 EJECTORS 9 The research performed consists in developing a model (one dimensional incremental procedure), adapted to monitor the relevant parameters all along the ejector and provide their distributions. At the same time using the properties of real fluids, it provides the axial dimensions and off design behaviour. This tool can be used both for design and simulation. For design, it allows to determine the appropriate dimensions for given operating conditions. For simulation, it provides all the information related to the operation (in terms of performance and controlling parameter distributions) for fixed parameters. In parallel, CFD analysis is also being performed for the cases of air and other refrigerants. This approach allows studying locally the different fluid phenomena implied and their interactions in an even more realistic way since we can simulate more complex geometries, different conditions of turbulence, different operating conditions etc The fully instrumented test bench for testing single phase ejectors (components only) provides a valuable asset for model validations and performance data collection, and give confidence in the modelling results. Simulation models of different systems using an ejector are also developed, in order to predict the system performance for different operating conditions (refrigerants, temperatures). It is planned to build two prototypes: a heat pump including an ejector as expansion device (two phase); and a hybrid refrigeration system where the ejector, activated by an external source, cools down the condenser. These prototypes will provide experimental data to validate the models and provide hands-on experience. It is expected that this unique combination of simulation-modelling and experimental tools will provide CanmetENERGY with a unique expertise in understanding of the phenomena occurring during ejector operation, in order to design more performing systems and increase the number of applications for ejectors. Demonstration projects are needed to gain practical experience, build the credibility and the confidence required for broader applications. Partners identified for this R&D activity have already expressed their interest to perform and support demonstration projects.

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