ejectors

canmeteNerGY | july 2009

EJECTORS 1

ejectors

INtroductIoNEjectors 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 inter-nal 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 5% in performance of the heating systems in the building sector in Canada would lead to a saving of 25 PJ and a reduction of 2.5 MT eq-CO

2 per year. In the industrial sector, a

5% increase in heat recovery would save 84 PJ and 5.6 MT eq-CO2 per year.

tecHNoLoGY oVerVIeWThe principle of operation of ejectors is based on the Venturi effect of a converging-diverging nozzle to con-vert 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).

Primaryflow

Secondary flow

EJECTOR a b c

Figure 1 Operation of an ejector

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.

Generator

Pump

Evaporator

ExpansionValve

WP

QE

QG

QC

1 2

3

45

6

QC

4Condenser

Benefits:

Thermally activated compressor

Tem

pera

ture

Pres

sure

P3

P2

P1

Enthalpy

1 2

3

45

6

Figure 2 Ejector based heat pump system

EJECTORS 3

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 com-ing from a low temperature energy source such as waste heat) supplies vapour at a high pressure P3 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 cHaLLeNGesSince 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 po-tential 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 sev-eral 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 3 is another phenomenon still far from understood. The en-trainment ratio relates the secondary mass flow rate m

s to the primary mass flow rate m

p in the ratio

ms/m

p. With the exit pressure (represented in the figure by the condenser pressure), they are the main perform-

ance 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 eject-or, 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.

4 EJECTORS

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 expan- sion device, in order to increase the system efficiency (Figure 4)

As a condensing ejector for heating applications, in order to reduce the compressor work, and therefore increase the system capacity and performance (Figure 5)

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 6) 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.

Condenser pressure

Critical pressure

Pc

Entr

ainm

ent

Rat

io -

.

Figure 3 - Typical performance curve

EJECTORS 5

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 15%

This ejector is driven by high temperature and pressure condensate which is used to draw low pressure va-pour 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 15% as a result.

Evaporator

Ejector

ExpansionValve

Separator

Compressor

3 4

56

7

89

WC

QE

QC

Evaporator

8

56Condenser

1 2

Tem

pera

ture

Pres

sure

Enthalpy

Ejector Basic Cycle

QC = Q1C

Wc

Ejector operation

Cooling effect

Wc

1 2

3

5

4

6

7

8 9

Benefits:

Reduced compression work 4 to 5 instead of 9 to 5

Figure 4 Ejector as an expansion device in a heat pump system

6 EJECTORS

B. COndEnSIng EJECTOR FOR HEATIng ApplICATIOnS ExpECTEd pERFORmAnCE ImpROvEmEnT: COp up TO 30%

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 30%, depending on the operating conditions.