Background

Enlarge this picture Fig. 1. Principle of the condensing ejector.

Vapor-compression refrigeration systems typically utilize expansion valves or capillary tubes to lower the pressure of liquid refrigerant and deliver it to the evaporator. In a typical refrigeration cycle, the expansion valve lowers the refrigerant pressure by 5 to 7 times. The reason for lowering the pressure is to allow the refrigerant to evaporate at certain desired low temperature. The process of throttling is isenthalpic, which means that the energy produced during the pressure reduction is dissipated and eventually wasted. Numerous efforts were directed in the past in order to utilize this energy and to increase the efficiency close to the ideal Carnot cycle.

One such solution is to use this energy to help compress a part of a refrigerant vapor in a special two-phase, screw compressor. The efficiency of the cycle increased by almost 20 percent, but the incremental cost did not justify these savings. The other method to improve the efficiency of vapor-compression refrigeration systems in the past was to use the ejector in various arrangements. Through the action of an ejector, the compressor suction pressure was higher than it would be in a standard cycle. This resulted in less compression work, thus improving cycle efficiency. The theoretically attainable 20 percent efficiency improvement with such an ejector had never been confirmed in practice.

In the above discussion, the term “ejector” refers to a traditional Venturi nozzle where the outlet pressure is the intermediate between pressures of the working and transporting media. The application discussed here concerns a novel design for an ejector called a “condensing ejector.” The condensing ejector is a two-phase jet device in which a sub-cooled working medium in a liquid state is mixed with its vapor phase, producing a liquid stream with a pressure that is higher than the pressure of either of the two inlet streams.

The principle

Enlarge this picture Fig. 3. Schematic of the refrigeration system with the condensing ejector as a second-stage compressor.

The principle of condensing ejector operation is shown in Fig. 1 with a corresponding diagram of pressure distribution along the length of an ejector [3]. In general, a condensing ejector is a form of a jet pump operating with two-phase flow that can produce a discharge liquid stream with a pressure higher than either of the two inlet pressures. Two separate, concentric nozzles are used to accelerate the liquid and vapor streams. The high velocity streams enter a converging mixing section where the high relative velocity between vapor and liquid streams produces a high value of heat transfer coefficient, causing a sudden condensation of the vapor phase onto the liquid in the constant area section. This condensed vapor adds considerably to the momentum flux of liquid stream. Such rapid condensation process, which result in a single-phase liquid, has been called a “condensation shock.” The stream, now completely liquid, flows through the diffuser where the pressure is further increased.

While a theoretical basis for the condensing ejector principle has been reasonably established in the past, only one practical application has been reported — for underwater propulsion in deep-running torpedoes [1]. In such systems, the turbine exhaust has to be kept as low as possible. In order to equalize this low exhaust pressure with higher hydrostatic pressure at certain depth under the water, the salt water was injected to the condensing ejector, where it mixed with turbine exhaust gases. The steam from exhaust was rapidly condensing onto the salt water and the resulting condensation shock increased the pressure to the required level. All previous research has been concentrated on water-steam mixtures, and this is what is believed to be the first attempt of using the condensing ejector principle with refrigerant as a working medium. The major improvement described here is a second-step compression by an ejector device.

New cycle

Enlarge this picture Fig. 4. Comparison of p-h diagrams of the new refrigeration cycle with a two-phase ejector, Cycle 1 (points: 1-2-3-4-5-6-1 and 6-7-8-4) and the traditional cycle Cycle 2 (points: 1-2-3’-6-1). 1-2: evaporation of a part of the working fluid. 2-3: compression of vapor in the compressor (the first step). 3-4-8: mixing of vapor and liquid in the ejector. 4-5: compression in the ejector (the second step). 5-6: isobaric cooling of the liquid. 6-7: compression of a liquid in a pump. 7-8: expansion of part of the cooled liquid in the ejector. 6-1: throttling of the evaporating part of the working fluid.

The principle of the condensing ejector was utilized to construct the refrigeration system shown in Fig. 3. In this new system, the mechanical compressor compresses the vapor to approximately 2/3 of the final pressure. Additional compression is provided by the condensing ejector, therefore, the amount of mechanical energy required by a compressor is significantly reduced. The system includes main piping circuits (1) and (7), containing the evaporator (2), a compressor (3), a condensing ejector (4), a condenser (5), a separator tank (6), and an expansion valve (8). The circulation of the liquid phase of the refrigerant is provided by the additional liquid line (9), and a pump (10). The refrigerant, kept at low pressure, vaporizes in the evaporator using the heat energy of a low-temperature source (11). The vapor is then compressed in the compressor and sent to the ejector where it mixes with the liquid flow coming from the separator (6) via the pump (10). The flow of refrigerant is then directed to the condenser where it is cooled by transferring the heat to the high-temperature receiver (12). The ejector improves the efficiency of the cycle by decreasing the need for energy to run the compressor. The device described can be used also for heating and be operated as a heat pump.

The theoretical energy savings for the new system can be established by analyzing the thermodynamic cycles. Both new and traditional cycles are presented in Fig. 4. Calculations, made for R22 refrigerant, showed that a cycle with the condensing ejector may achieve a 38 percent theoretical efficiency improvement as compared to traditional cycle.

There are two major differences between the new cycle and existing applications of ejectors in refrigeration cycles:

1. 1. The cycle is characterized by the location of the ejector device after the compressor discharge in order to increase the final cycle pressure (pressure at the inlet to the condenser), while all to-date designs used ejectors for increasing the suction pressure of the compressor.
   
2. 2. All previous designs of the ejector relied on the increase of the pressure in the mixing chamber by the process of equalizing the velocities of both motive and suction streams. Consequently, the value of outlet pressure was intermediate between the pressures of motive and suction streams. The design described here produces the outlet pressure higher than the pressure of any of the stream components. This is achieved by the creation of a condensation shock, which was previously described in literature, but never used in refrigeration cycles.

Experiments

Fig. 5. Overall view of the experimental stand.

A laboratory stand with 10 kW capacity was fabricated under funding from the U.S. Department of Energy according to the schematic of Fig. 3 and by observing basic principles of design and assembly of refrigeration systems. Its overall view is shown in Fig. 5. The preliminary experiments were carried out with two main objectives:

1. To investigate the possibility of the pressure jump on the ejector.

2. To determine the energy savings for the new cycle with the ejector vs. the traditional cycle with single-step compression. In order to operate in those two modes, the stand was designed with appropriate valves to by-pass the ejector and to operate in the standard vapor-compression regime.

When running in the ejector mode, a pressure jump of 0.35 MPa was observed on the ejector at certain vapor-to-liquid ratio. This resulted in approximately 16 percent efficiency improvement compared to a traditional, single-stage compression cycle [2]. The experiments are continuing under second-phase funding from the DOE and NSF. The objective is to further improve efficiency to a point between the theoretically achievable (38 percent) and the improvement achieved in our first attempt (16 percent). Towards this goal, the geometry of the condensing ejector has been redesigned based on a computer simulation model developed at the University of Massachusetts [4]. It is therefore reasonable to assume that the efficiency can be increased by 25 percent. Such an increase can result in enormous economical benefits. The side bar on page 34 shows the calculation of savings under the assumption that all central air-conditioner units in the U.S. are using the new technology. Similar savings can be achieved for refrigeration units and heat pumps.

(Acknowledgement: This material is based upon work supported by the National Science Foundation and the Department of Energy, under Award Number DE-FG36-06GO16049.)

For more information, email: mark@mdienergy.com

SIDEBAR: Potenial Energy Savings

The following is a calculation of potential energy savings if all central A/C units in the U.S. used the condensing ejector technology. It is based on the assumption of 70 million installed central A/C units in U.S. of average size of 5 Tons, operating at 1,200 full load hrs/yr, and assuming average SEER of 11 (COP=3.2). Annual use of electrical energy would then be figured as:

70 million units x 5 Ton x 12,000 BTU/hr x 1,200 full load hrs/yr, divided by COP of 3.2, divided by 3,410,000 BTU/MWhr, which equals 0.46x 109 MWhr. Therefore, by increasing efficiency by 25 percent, the possible annual savings in U.S. is 120 million  MWHr, which at $50 per MWhr results in total cost savings of $6 billion per year.