solar cooling using ejectors - solar thermal group - anu

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Solar cooling using ejectors

The ejector is a thermally driven compressor that operates in a heat pump refrigeration cycle. In a heat

pump system, the ejector takes the place of the electrically driven compressor, but uses heat rather than

electricity to produce the compression effect

Ejectors – A Brief History

Ejectors have been in use prior to 1900 where they found use in evacuating air from leaky low pressure

steam condensers. An ejector in this application acts as a vacuum pump, driven by low pressure steam

which was readily available in such environments. The ejector’s role was characterised by steady state

conditions and empirical design. Efficiency was not as important as reliability.

Within 20 years, ejectors found widespread use as vacuum pumps in industrial settings. It was a small

step to form a vapour compression heat pump using the ejector as a heat driven compressor. Steam

driven ejector heat pumps became common in air conditioning, particularly of hotels and ships during the

early 20th century; wherever there was a ready supply of low pressure steam or a steam boiler. Ejector

systems were found to be low cost, very reliable and maintenance free.

During the 1930s, Freon refrigerants were developed and vapour compression heat pumps based on these

new refrigerants were far superior in performance to ejector systems. Ejector air conditioning fell from

favour for 50 years until the Montreal protocol of 1987 highlighted a link between Freon use and

atmospheric ozone depletion.

This rekindled an interest in ejector technology and at about this time, two important improvements in

ejector design were made. Firstly, refrigerants other than water were tested and found to perform better.

Secondly, researchers began to look at system integration issues and to compose systems incorporating

solar energy and hybrid designs.

The modern era of ejector research combines supersonic thermodynamics, computational fluid dynamics

and experimental work. Despite this effort, the inner workings of the apparently simple ejector are not fully

understood, but are reasonably well modelled.

Researchers are able to design ejectors with confidence and there are industrial ejectors ranging in size

form several hundred watts to huge multi-megawatt steam ejectors.

Description of the Ejector System

The ejector is a thermally driven compressor that operates in a heat pump refrigeration cycle. In a heat

pump system, the ejector takes the place of the electrically driven compressor, but uses heat rather than

electricity to produce the compression effect (figure 1).



Figure 1. Solar driven ejector heat pump compared to a conventional electric heat

pump

The ejector has no moving parts and is simple and reliable which make it attractive for commercial

production. However, the thermal efficiency of the ejector is low which implies that the ejector requires a

large solar collector and large condenser to operate in a heat pump application. Thus the savings in

electricity consumption must be compared with the additional cost of the solar collector. One is trading

capital cost for operating cost, as with most solar systems.

A liquid pump is required to generate a pressure difference for the ejector heat pump to operate, but since

liquid is being compressed, the amount of electricity required is relatively small. All other components in

the heat pump circuit are conventional.

The ejector cycle consists of high and low temperature sub cycles. In the high temperature sub cycle,

heat that is transferred to the ejector cycle from the heat source causes vapourisation of the ejector cycle

working fluid in the generator at a temperature slightly above the saturation temperature of the refrigerant.

Vapour then flows to the ejector where it is accelerated through a converging-diverging nozzle.



Figure 2. Typical ejector cross-section, pressure and velocity profiles along an ejector operating in critical

mode. Primary, secondary and discharge streams are indicated in red, blue and green respectively.

Adapted from Chunnamond & Aphornratana (2004)

Since much of the vapour enthalpy is converted to kinetic energy, conservation of energy suggests that

the vapour temperature and pressure will be very low. The low pressure at the exit of the nozzle which

acts to draw vapour flow from the evaporator (figure 2,c).

The generator and evaporator flows then mix in the ejector and the combined flow undergoes a transverse

compression shock (figure 2,f). Thus thermal compression replaces the electrical compressor in a

conventional heat pump. Further compression takes place in the diffuser such that a subsonic stream

emerging from the ejector then flows into the condenser (figure 2,h).

At the condensor, heat is rejected from the working fluid to the surroundings, resulting in a condensed

refrigerant liquid at the condenser exit. The ejector needs to provide sufficient exit pressure such that the

saturation temperature of the refrigerant at this point is greater than the condenser cooling medium,

otherwise heat cannot be rejected and the cycle ceases to operate. This is the malfunction mode of the

ejector, caused by excessive condensing backpressure. Malfunction can be overcome by suppling greater

generator pressure and temperature.

Liquid refrigerant leaving the condenser is then divided into two streams; one enters the evaporator after a

pressure reduction through the expansion valve, the other is routed back into the generator after

undergoing a pressure increase through the refrigerant pump. The fluid is evaporated in the evaporator,

absorbing heat from the air-conditioned environment, and then it is entrained back into the ejector

completing the cycle.

Although figure 1 indicates a direct connection of the generator and solar array, there is usually an

additional heat exchange circuit (figure 3) in the high pressure loop to eliminate the possibility of two



phase refrigerant flow in the solar collector. For readers familiar with p-h diagrams, figure 3 clearly shows

the high and low pressure ejector sub cycles.

Figure 3. Solar powered ejector based solar cooling system (Sokolov, 1993)

Despite the complexity of internal operation, the ejector may still be considered to be a compressor and

its performance may be defined conventionally by its compression ratio (Pc/Pe) and its isentropic

efficiency. The ejector heat pump cycle still benefits from subcooling prior to evaporation and from

minimising superheating through compression.

The advantages of simplicity and reliability of the ejector will be apparent. Additionally, the ejector

mechanism offers freedom of choice of refrigerant and is not complicated by the need for compressor

lubricant compatibility. Also, the ejector is tolerant of liquid slugging since both generator and evaporator

ports are essentially open tubes.

Operational Characteristics

The operational characteristics of fixed geometry ejectors are somewhat different to conventional

compressors. The ejector performance is typically very sensitive to gas properties and operation away

from the design point.

Ejector performance is noted by a constant capacity region, a critical operating point and a malfunction

region, for a given evaporation and condensing temperature (figure 4). Ideal operation of the ejector,

indicated by maximum entrainment of the evaporator flow, is indicated by the knee of each curve in the

figure. This point is very close to the malfunction condensing temperature where the entrainment falls to



zero so that there is no cooling effect. Indeed the ejector is so sensitive to backpressure (itself related to

ambient temperature), that complete malfunction occurs with several degrees of condensing temperature

from the optimum operating point.

Figure 4. Indicative operating characteristics of ejectors designed for fixed condensing temperatures

Figure 5. Fixed geometry ejector operating at a given evaporator temperature (8ºC) and generator

temperatures, but varying condenser temperature

A second important observation is that an increase in generator (solar) temperature will allow continued

operation at elevated condensing temperature but at the expense of COP (figure 5). This is because the

mass flow of the choked primary choked nozzle decreases with increasing driving temperature and thus,

there is less motive power to combat the increased condenser backpressure. This implies that a fixed

geometry ejector will not be able to take advantage of high collector temperatures during periods of high

insolation.

While most research papers expound the benefits of the coincidence of solar radiation and cooling

demand, close examination shows that this is not the panacea it is made out to be. Solar radiation

intensity on a horizontal plane peaks at noon. Thus the heat available to a solar driven ejector peaks at

the same time. Due to the thermal inertia of the earth, ambient temperature peaks around mid afternoon,

several hours after the peak in solar availability (figure 6). Furthermore, the inside of a building peaks in

temperature several hours later as its own thermal inertia reacts to ambient temperature and, to a lesser



extent, solar radiation. Thus the peak cooling demand of a building may occur late into the afternoon, well

after the solar cooling system is able to deliver its rated cooling capacity. Frustratingly, the solar cooling

system will have excess capacity in the morning when it may not be required.

Figure 6. Solar availability in not coincident with cooling demand

Some reprieve may be gained by orienting the solar collector to the west such that the peak solar

availability is less, but more coincident with demand. However, far greater effectiveness may be achieved

by storing the coolth produced early in the day for later use. This has benefits in reducing the peak

afternoon requirements (and thus collector size and cost), but also in allowing solar contribution to

evening cooling. An annual computer simulation readily reveals the tradeoff between collector size,

storage capacity and solar contribution to the cooling demand (figure 7), in this case for a small ejector

system for a residence. The cost of storage and solar collectors should be overlaid on this data in order to

make an ejector system as cost effective as it can be.



Figure 7. Computer modelling can assist in sizing of an ejector system based on solar fraction for a

residential ejector based system.

Key Research Challenges

Ejector based systems are characterised by their simplicity and high reliability, tolerance to a range of

working fluids, but also poor performance and range of operating conditions. The performance issues must

be addressed if ejectors are to become mainstream. In particular, the low thermal COP and poor off-

design performance must be improved.

Performance is usually interpreted relative to cost. The cost of the ejector system is dominated by the

cost of the heat source which can be high if solar collectors are required. Commonly temperatures around

70-100ºC are required to drive an ejector, making them suitable for use with non-concentrating solar

collectors or waste heat from cogeneration systems. Successful ejector based systems will seek to

maximise annual utilisation of the heat source by providing multiple services: summer space cooling,

winter space heating and water heating. Such systems do not exist although the technology is available.

The third set of challenges relate to system integration. There are challenges in design and sizing of

ejector system components such that the cooling system maximises the use of the (varying) solar

collector output to produce the highest solar contribution to the cooling load. This has implications for

including energy storage and smart control schemes into the system design. Little research has been

performed in this area to date. Indeed, there has been little research into real-time dynamic control of

ejector systems since most operate at steady state in a laboratory environment.

Ejector Modelling

There are a number of means to model the performance of an ejector as a component and an ejector

system. The ejector is often modelled in isolation in research reported in the literature. This modelling is

usually either directly based on thermodynamic compressible flow theory with minor corrections for non-



ideal behaviour, or numerically derived using computational fluid dynamics.

Thermodynamic Modelling

For over fifty years, ejectors were empirically designed based on rules of thumb and experience with

steam driven devices. Early attempts to thermodynamically model the steam ejector were carried out by

Keenan (1950). Keenan was able to reasonably predict the ejector performance characteristics firstly for

constant area mixing ejectors then also for constant pressure mixing ejectors. Further clarification of the

mixing mechanism was provided by Munday and Bagster (1977). Perhaps the most important

improvement in understanding was provided by Eames (1995) and Huang et al (1999) with the description

of a one dimensional design methodology. These advances provided researchers with a means to design

an ejector, including the effect of supersonic shock and three calibration constants such that model and

experimental data generally agreed within about 10%.

Based on this approach, ejector design maps for fixed and variable geometry ejectors may be produced

(figure 8a,b). Such images do not describe the operational characteristics, but are helpful in determining

the size of the mixing chamber for double choking mode of ejector operation.

Figure 8a. Design chart for a fixed geometry ejector

Figure 8b. Design chart for a variable geometry ejector

Since Huang’s method, there have been several modifications that give refined descriptions of ejector



operation. The first is the Shock circle method (Zhu, 2007) which better accounts for the boundary layer

secondary flow in the mixing chamber prior to mixing. This gives typical errors of less than 5% compared

to published experimental data and would be considered the current state of conventional thermodynamic

modelling of ejectors.

An unconventional approach to ejector design was suggested by Eames (2002) whereby the ejector was

shaped to produce a constant acceleration in the mixed refrigerant flow. This eliminated the entropic

shock in the ejector and so this design was able to produce greater compression effect for a given driving

power. However, the design is only suitable for steady state operation at the design point since the very

specific shape is does not allow correct operation at off-design conditions. Nevertheless, it is well suited

to a cogeneration system whereby the conditions would be reasonably controlled.

However, the ejector is but one component in a solar cooling system. A proper understanding on such a

system can be better obtained by modelling the dynamic behaviour of the ejector in response to changing

operating conditions (ambient temperature, solar radiation) and in response to a control strategy. The

ejector component has low thermal mass and short transport delays so that the dynamics of an ejector

circuit are generally dominated by other components, particularly the solar collector.

Furthermore, the ejector system should be modelled over an entire season to evaluate the effect of

component sizing and control strategies, especially if storage is included in the system design. Annual

modelling is readily accomplished using a program such as TRNSYS which captures local weather data,

dynamic loads and control responses at high time resolution.

Researchers widely acknowledge that thermodynamic modelling cannot precisely describe the mixing

process occurring inside the ejector. Further advances in this area through calibration constants may

provide improved matching to experimental data but are not likely to provide useful insights into ejector

processes. Recent advances in numerical modelling appear to be the best approach in this case.

Numerical Modelling (CFD)

Computational Fluid Dynamics has matured over the last decade with the advance in hardware

computational capability. This is allowing researchers to investigate the ejector processes in far greater

detail including supersonic shock effects, real gas behaviour, metastable refrigerant states, boundary

layer flow, flow separation and the like. Due to the complexity of highly turbulent supersonic compressible

flow involving a real gas model, only highly developed CFD packages are suitable for ejector modelling.

Most researchers use Fluent or ANSYS CFD.

Perhaps the most important choice in CFD analysis of ejectors is the selection of a turbulence model.

The standard ?-e turbulence model has found to be inadequate for describing expanding supersonic jets

and Bartosiewicz (2003) offers some recommendations. In particular, the hybrid ?-e-sst model seems to

offer good results.

The results of CFD studies are now producing close agreement in entrainment ratios with experimental

data, provided that the ejector is operating in double choking mode (figure 9). Despite this apparent

success, models with differing turbulence handling may agree on design point entrainment while

demonstrating very different flow phenomena within the ejector. Thus it may be too early to make useful

deductions on the inner workings of the ejector.

Insights into real ejector flows are provided by advanced visualisation techniques involving transparent

ejectors. Few researchers are involved in this activity and results are limited at this stage. This technique

will become important as a means to verify CFD predictions.



Figure 9. Typical CFD output, a mach number plot for an ejector

Combined Cycles

Hybrid Cycles

Despite recent advances in understanding of ejector operation, the COP of a simple ejector cooling

system remains stubbornly low. As a result, many researchers have proposed hybrid cooling systems

incorporating combinations of ejectors and another cooling system. The most common hybrid systems

are:

Multiple ejector hybrids

Mechanical vapour compression / ejector hybrids

Absorption / ejector hybrids

Multiple Ejector Hybrids

Several attempts have been made to combine several ejectors in a cooling system. The most obvious

configuration of a double effect ejector, whereby the condensing heat from the topping cycle ejector drives

the generator of the bottom cycle ejector, is yet to be demonstrated.

However, a compound ejector has been proposed by Dennis and Garzoli (2009). In this configuration,

several ejectors effectively work in series to build vapour compression. This configuration was found to

substantially relieve the constant capacity constraint at low condensing temperatures and provide

impressive performance gains at high condensing temperatures when compared to a conventional ejector

(figure 10). The two ejectors were found to require the same mixing chamber geometry and differ only in

the primary nozzle dimension. Design is possible using simple rules of thumb.



Figure 10.

Comparison of operating performance of reference and compound ejectors designed for condensing

temperatures 30ºC, 40ºC and 48ºC.

The critical design parameters are the setting of the intermediate pressure between the high and low

pressure ejectors and the division of solar motive energy between the two ejectors.

Yu et al (2006) proposed a two stage compound ejector whereby the second stage of compression was

performed by a liquid jet pump ejector. This configuration is also performing well using HFC refrigerants.

There appears to be an advantage with compound ejectors compared to multiple effect cooling systems

(eg absorption) in that an increase in generator temperature is not necessarily required for this design.

True multiple effect ejector systems are yet to be demonstrated. This would involve collecting heat from

the diffuser of one ejector to evaporate vapour in the generator of a downstream ejector. As with other

multiple effect cooling systems (eg absorption) this would require elevated collector temperatures.

Mechanical vapour compression / ejector hybrids

The use of multiple ejectors helps to address the off design performance constraints of ejectors. However,

the issue of intermittency of heat source can be troublesome, particularly when an ejector is coupled to a

solar collector for motive energy. Furthermore, in such cases, cooling services are commonly required into

the evening due to thermal inertia of buildings and so some kind of backup or storage system is required.

Since thermal storage is expensive, an elegant solution is to combine a mechanical vapour compression

cooling system with an ejector.

An ejector can assist the mechanical system in a number of ways; by reducing the compression work of

the mechanical compressor by directly pre-compressing vapour from the evaporator (Takeuchi, 2009) by

cooling the condenser of the vapour compression unit (figure 12)(Sokolov, 1993) and by pre-cooling the

liquid prior to its entry to the evaporator (Huang 2001). This design neatly takes care of the solar

intermittency but does not address the off design limitations of ejectors.

In each of these cases, peak load on the electrical vapour compressor is reduced. This has benefits for



the electricity grid and may allow the electrical compressor to be downsized. Also, the cooling system

reverts to conventional electrical operation when solar input is not available so that the addition of the

ejector is a peak lopping strategy.

Figure 12. The Sokolov hybrid ejector system (Lewin, 2008)

Several cascade configurations involving ejectors have also been proposed. In this configuration, the

ejector is driven by the heat of compression of the electrical compressor and acts to reduce the pressure

ratio of the electrical compressor (Yu, 2006). Such configurations are attractive since no external motive

force is required to drive the ejector and little additional complexity or cost is added to the refrigeration

system in order to realize the savings.

Absorption / ejector hybrids

A number of researchers have attempted to use ejectors to boost absorption cycle chillers.

In this case, the ejector compressor works in parallel or series with the absorption compressor and is

driven by the regeneration heat from the absorption cycle. In such cases, the COP can easily exceed 1.0

for a single stage absorption chiller but requires elevated generating temperatures.

This type of hybrid forgoes the advantages of ejectors – simplicity of implementation.

Ejector Research at ANU

At ANU, we are progressing knowledge in the field of solar ejector cooling systems in the following areas:

1. Hybrid designs

2. Systems integration

3. Advanced predictive control systems

4. Cool storage using warm ice

5. Advanced high performance ejector design including compound and variable

geometry designs

6. CFD analysis of ejectors

The ANU has a dedicated solar cooling laboratory with a variety of solar concentrating and non-

concentrating collectors. We are seeking suitably qualified PhD students with interests in the above



research areas. For further information or comments on this website, please contact:

Dr Mike Dennis College of Engineering and Computer Science The Australian National University Email:

[email protected]

References

Bartosiewicz Y., Aidoun Z., Desevaux P.,Mercadier Y., CFD experiments integration in the evaluation of

six turbulence models for supersonic ejector modelling, Proceedings of Integrating CFD and Experiments,

Glasgow, 2003.

Chunnamond, K & Aphornratana, S 2004, Ejectors: applications in refrigeration technology, Renewable

and Sustainable Energy Reviews, vol. 8, pp. 129-55.

Dennis M., Garzoli K., Compound Ejectors with improved off-design performance, School of Engineering,

The Australian National University, 2009.

Eames I., A new prescription for the design of supersonic jet pumps: the constant rate of momentum

change method, Applied Thermal Engineering, Vol 22, pp121-131, 2002.

Eames, IW, Aphornratana, S & Haider, H 1995, ‘A theoretical and experimental study of a small-scale

steam jet refrigerator’, International Journal of Refrigeration, vol. 18, no.6, pp. 378-86.

Huang B., Chang J., Wang C., Petrenko V., 1999, A 1-D analysis of ejector performance, International

Journal of Refrigeration, vol. 22, pp. 354-64.

Huang B., Petrenko V., Chang J, Lin C., Hu S., A combined cycle refrigeration system using ejector

cooling cycle as bottoming cycle, International Journal of Refrigeration 24 (2001) 391-399.

Keenan, H., Neumann, E.P., Lustwerk, F., An investigation of ejector design by analysis and experiment.,

J. Appl. Mech.Trans. ASME 72, 299–309.,1950.

Lewin S., Performance optimisation of a solar assisted air conditioning system, Honours Thesis, School

of Engineering, The Australian National University, 2008.

Munday J., Bagster D., A new ejector theory applied to steam jet refrigeration, Ind Eng Chem, Process

Des Dev, 1977, Vol 16 pp 442-449.

Sokolov M., Hershgal D., 1993, Optimal coupling and feasibility of a solar-powered year round ejector air

conditioner, Solar Energy, vol. 50, no. 6, pp. 507-16.

Takeuchi H., World’s first ejector cycle for mobile refrigerators to stop global warming, Proceedings

Eurotherm 85 International seminar on ejector/jet-pump technology and applications, Louvain La Neuve,

Belgium, 2009.

Yu J., Chen H., Ren Y., Li Y., A new ejector refrigeration system with an additional jet pump, Applied

Thermal Engineering 26 (2006) 312–319.

Zhu C., Wen L., Shock Circle method for ejector performance evaluation, Energy Conversion and

Management, Vol 48, pp 2533-2541, 2007

UPDATED: 22 December 2009/ RESPONSIBLE OFFICER: Head of School/ PAGE CONTACT: John Pye

mailto:[email protected]

mailto:[email protected]

http://cecs.anu.edu.au/user/3538

solar cooling using ejectors - solar thermal group - anu

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