Piston-cylinder work producing expansion device in a transcritical carbon dioxide cycle. Part I: experimental investigation

Abstract

Carbon dioxide is receiving strong consideration as an alternative refrigerant substituting hydroclorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs) due to its zero ozone depletion potential and negligible global warming potential. The system performance of CO₂ systems, however, is typically poor compared to the current conventional air conditioning systems using HCFC or CFC. One of the most effective ways to achieve parity with CFC and HCFC systems is to replace the expansion valve with an expansion device that minimizes entropy creation and allows for energy recovery during the expansion process.

A piston-cylinder type work output expansion device was designed, constructed and tested as part of the study reported here. The first-cut prototype device is based on a highly modified small four-cycle, two-piston engine that is commercially available. The work-producing expander replaced the expansion valve in an experimental transcritical CO₂ cycle and increased the system performance by up to 10% as characterized by COP. The prototype device was not meant to be a final product, but provided valuable insight and experimental results to validate a detailed simulation model of the device. The model and corresponding theoretical analysis are presented in a companion part II paper.

Introduction

Chlorofluorocarbons (CFCs) have been phased out and hydroclorofluorocarbons (HCFCs) are in the process of being phased out as refrigerants due to their potential to destroy the ozone layer of the earth's atmosphere. Although hydrofluorocarbons (HFCs) have zero ozone depletion potential, they have a significant global warming potential and their future use has been questioned as well. Therefore, substances with no ozone depletion potential and no or negligible global warming potential, e.g. natural fluids such as carbon dioxide, ammonia, hydrocarbons, and water, are being considered as alternative refrigerants.

Carbon dioxide (CO₂) is non-toxic, non-flammable, has zero ozone depletion potential, and negligible global warming potential as a refrigerant. It thus is close to being the ideal refrigerant except that its thermodynamic cycle characteristics result in system COPs that are typically lower than HFC vapor compression systems. Nevertheless, CO₂ systems are receiving strong consideration for automobile and military applications due to its high volumetric heat capacity, which translates to reduced weight and volume in packaged systems. Additionally, the benign nature of the gas allows the elimination of recovery and recycling procedures and equipment. For all of these reasons, CO₂ systems are an especially attractive candidate for weight critical systems. For example, the US Army is seeking a deployable air conditioning system with significant weight and size reduction, resulting in reducing tactical power generator sizes [7].

Fig. 1 presents a schematic of the typical transcritical CO₂ cycle and Fig. 2 illustrates the cycle in a temperature–entropy (T–s) diagram. As shown in Fig. 1, the basic transcritical CO₂ cycle consists of a compressor, a gas cooler, an expansion valve and an evaporator. The cycle is composed of four basic processes: compression (1–2), heat rejection (2–3), expansion (3–4h) and heat absorption (4–1) as shown in Fig. 2. In the expansion process, the paths 3–4s and 3–4h represent isentropic expansion and isenthalpic expansion, respectively. In the compression process, the paths 1–2s and 1–2 stand for the isentropic and actual processes, respectively.

The performance of the basic carbon dioxide based air conditioning system may be less than that of HFC- or HCFC-based air conditioning systems due to the thermodynamic characteristics of the transcritical cycle. Robinson conducted a parametric study which compared the performance of an existing HFC-134a based automobile air conditioner to the performance of a prototype carbon dioxide based automobile air conditioner [8]. His analysis showed that the prototype transcritical CO₂ cycle device will operate with a Coefficient of Performance (COP) which is 66–75% (average 70%) of the experimentally determined COP of the production HFC-134a unit while providing the same evaporator capacity.

Robinson and Groll compared the COP of a military packaged air conditioner using HCFC-22 as the refrigerant with that of a model carbon dioxide based packaged air conditioner by using computer simulation models [9]. Their analysis showed that for a packaged air conditioner application using the same evaporator size and capacity, the optimized transcritical CO₂ air conditioner would operate with similar values of COP to the system using HCFC-22 as the refrigerant. However, in order to take advantage of the system weight reductions offered by the CO₂ system, a real-world design would actually employ a reduced size evaporator. This will translate into lower thermodynamic performance when compared to the HCFC-22 unit.

To take advantage of the potential weight and volume reductions offered by the transcritical CO₂ cycle while at the same time achieving or surpassing the efficiency of conventional HFC cycles, system improvements are required. One of the methods proposed for increased performance is to modify the expansion process by replacing the classic throttling valve with a work-producing expander. It can be seen from Fig. 2 that as the carbon dioxide passes through the expander not only is energy recovered but also the enthalpy at the inlet to the evaporator is reduced resulting in an increase in evaporator capacity (process path 3-4w). As shown in this figure, the entropy generation will also decrease with the expander in the cycle. Further cycle improvement can be realized if the work produced by the expander is used in the compression process.

Maurer and Zinn experimentally investigated axial, swash-plate piston expanders and internal gear expanders in a specially constructed liquid-loop test stand [6]. Both expanders were first tested as off-the-shelf components. Based on the experiences gathered during the first round of testing the design of both expanders was modified. The suction port of the axial, swash-plate piston expander was redesigned and better piston rings were introduced. With respect to internal gear expander, the tolerance of the sealing gap between the gears and the housing was reduced. After these modifications were implemented, the isentropic and volumetric efficiencies that were achieved with both expanders were up to 50%.

Heyl et al. proposed a free piston expander–compressor unit [4]. To ensure continuous operation of the piston, the authors proposed to terminate the expansion by opening the exhaust valve in a position of the stroke where the resulting balance of forces stops the movement of the piston. In this case, the expansion remains incomplete resulting in unavoidable loss when compared to complete expansion. The authors calculated that the efficiency of the unit would be 78% relative to a complete expansion due to this loss.

To date no experimental data have been published for the transcritical CO₂ cycle that incorporates a work producing expander. This paper is focused on addressing this void by measuring the performance of such a cycle and making comparisons to a cycle which incorporates a more conventional throttle valve. Towards this end, an expander was designed and constructed for implementation in a prototype cycle. Labeled the ED-WOW (for Expansion Device-With Output Work), this device replaced the throttling valve in a typical conventional CO₂ cycle. The prototype cycle was based on the design specifications required for a field-deployable military air conditioning unit, with Table 1 providing the details of the design parameters. The prototype device was not meant to be a final product, but provided valuable insight and experimental results to validate a detailed simulation model of the device that was developed at the same time. The model and corresponding theoretical analysis are presented in a companion part II paper [3].