Carbon Dioxide Could Replace Global-Warming Refrigerant

ScienceDaily (July 4, 2000) — WEST LAFAYETTE, Ind. – Researchers are making progress in perfecting automotive and portable air-conditioning systems that use environmentally friendly carbon dioxide as a refrigerant instead of conventional, synthetic global-warming and ozone-depleting chemicals.

It was the refrigerant of choice during the early 20th century but was later replaced with manmade chemicals. Now carbon dioxide may be on the verge of a comeback, thanks to technological advances that include the manufacture of extremely thin yet strong aluminum tubing.

Engineers will discuss their most recent findings from July 25 to 28, during the Gustav Lorentzen Conference on Natural Working Fluids, one of three international air-conditioning and refrigeration conferences to be held concurrently at Purdue University. Unlike the two other conferences, the biannual Gustav Lorentzen Conference, which is being held for the first time in the United States, focuses on natural refrigerants that are thought to be less harmful to the environment than synthetic chemical compounds.

"The Gustav Lorentzen Conference focuses on substances like carbon dioxide, ammonia, hydrocarbons, air and water, which are all naturally occurring in the biosphere," says James Braun, an associate professor of mechanical engineering at Purdue who heads the organizing committee for all three conferences. "Most of the existing refrigerants are manmade."

Purdue engineers will present several papers detailing new findings about carbon dioxide as a refrigerant, including:

• Creation of the first computer model that accurately simulates the performance of carbon-dioxide-based air conditioners. The model could be used by engineers to design air conditioners that use carbon dioxide as a refrigerant. A paper about the model will be presented on July 26 during a special session sponsored by the U.S. Army in which researchers from several universities will present new findings.

• The design of a portable carbon-dioxide-based air conditioner that works as well as conventional military "environmental control units." Thousands of the units, which now use environmentally harmful refrigerants, are currently in operation. The carbon dioxide unit was designed using the new computer model. A prototype has been built by Purdue engineers and is being tested.

• The development of a mathematical "correlation," a tool that will enable engineers to design heat exchangers – the radiator-like devices that release heat to the environment after it has been absorbed during cooling – for future carbon dioxide-based systems. The mathematical correlation developed at Purdue, which will be published in a popular engineering handbook, enables engineers to determine how large a heat exchanger needs to be to provide cooling for a given area.

• The development of a new method enabling engineers to predict the effects of lubricating oils on the changing pressure inside carbon dioxide-based air conditioners. Understanding the drop in pressure caused by the oil, which mixes with the refrigerant and lubricates the compressor, is vital to predicting how well an air conditioner will perform.

Although carbon dioxide is a global-warming gas, conventional refrigerants called hydrofluorocarbons cause about 1,400 times more global warming than the same quantity of carbon dioxide. Meanwhile, the tiny quantities of carbon dioxide that would be released from air conditioners would be insignificant, compared to the huge amounts produced from burning fossil fuels for energy and transportation, says Eckhard Groll, an associate professor of mechanical engineering at Purdue.

Carbon dioxide is promising for systems that must be small and light-weight, such as automotive or portable air conditioners. Various factors, including the high operating pressure required for carbon-dioxide systems, enable the refrigerant to flow through small-diameter tubing, which allows engineers to design more compact air conditioners.

More stringent environmental regulations now require that refrigerants removed during the maintenance and repair of air conditioners be captured with special equipment, instead of being released into the atmosphere as they have been in the past. The new "recovery" equipment is expensive and will require more training to operate, important considerations for the U.S. Army and Air Force, which together use about 40,000 portable field air conditioners. The units, which could be likened to large residential window-unit air conditioners, are hauled into the field for a variety of purposes, such as cooling troops and electronic equipment.

Simulation model of a low-temperature supermarket refrigeration system.

HVAC & R Research

| October 01, 2006 | Getu, Haile-Michael; Bansal, Pradeep Kumar | COPYRIGHT 2008 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. This material is published under license from the publisher through the Gale Group, Farmington Hills, Michigan. All inquiries regarding rights should be directed to the Gale Group. (Hide copyright information)Copyright

This paper presents a computer simulation model of a low-temperature supermarket refrigeration system whose main components include a compressor, a display cabinet, a condenser, and a thermostatic expansion valve. The energy consumption of the system was evaluated as a function of the store relative humidity, which was measured every 16 seconds for 15 days. The model results were validated with the measurements taken from a representative in-situ supermarket in Auckland, New Zealand. The simulation results indicated that the rate of heat transfer in the evaporator was reduced by nearly 10.5 W for each 1% increase in daily average store relative humidity due to the insulating effect of the frost. The frost thickness was found to grow at 0.0354 mm for each 1% increase in the daily average store relative humidity over a 12 hour operation period between the defrost cycles. The pressure drop of the air over the evaporators increased by around 12 Pa for every 10% rise in the daily average store relative humidity.

INTRODUCTION

Supermarkets, which contain heating, cooling, and ventilation (HVAC) as well as refrigeration systems, are the most energy-consuming commercial establishments (Mei et al. 2002). According to the real measurements in the current study, the average power consumption of compressors/rack systems for both the low- and medium-temperature refrigeration systems of the supermarket amounts to roughly 20% of the total energy use. The energy consumption due to auxiliary equipment (defrosting, anti-sweat resistance heaters, display cabinet lights, and fans) and the air-conditioning system of the building are, respectively, 14% and 17%, while the remaining 49% is attributed to water, heating, and lighting systems of the establishment. Defrosting of evaporator coils is one of the most energy-consuming processes in supermarket refrigeration systems. A large amount of frost accumulation decreases the performance of the coil due to reduced rate of heat transfer and airflow. This consequently reduces the refrigerating capacity of the evaporator. Hence, frosting needs attention and should be minimized to improve the energy efficiency of the system and temperature control.

The impact of store relative humidity on refrigerated display case performance has been studied by Howell (1993a, 1993b), Orphelin et al. (1997), and Henderson and Khattar (1999), where the possible energy savings were predicted by reducing the store relative humidity and the energy penalty realized by the air-conditioning system. Among other studies by Inlow and Groll (1996), McDowell et al. (1995), Horton (2002), and Walker and Baxter (2003) on supermarket refrigeration systems, Ge and Tassou (2000) have presented a mathematical model for direct expansion supermarket refrigeration systems using TRNSYS (2001). Their display cabinet model, however, was quite restricted and used loads only due to infiltration, conduction, convection, and radiation, and the model was validated only for medium-temperature display cabinets. The simulation and the experimental work carried out so far in the supermarket industry has mainly focused on medium-temperature refrigeration systems. Further, dynamic frosting, as a result of store relative humidity, is ignored except in the medium-temperature cabinet models of Ge and Tassou (2000). Therefore, there is a need to develop a complete simulation model for low-temperature supermarket refrigeration systems, since they are most affected by the store relative humidity. This paper, therefore, presents a numerical model to investigate the performance of in-situ low-temperature display cases as a function of store relative humidity. The model includes major components of the refrigeration system, such as compressors, display cabinets, condensers, and thermostatic expansion valves. Specific correlations were used to compute frost thickness, rate of heat transfer across the evaporators, and COP. The model results were validated against the measurements taken from an in-situ supermarket located in Auckland, New Zealand, from December 1, 2004, to January 10, 2005. The model was written in a software package called Engineering Equation Solver, or EES (2004), which has built-in properties of many refrigerants. The computer model can be used as a convenient tool to accurately calculate the energy savings of the low-temperature refrigeration systems as a function of store relative humidity.

THE LOW-TEMPERATURE SUPERMARKET REFRIGERATION SYSTEM

Supermarkets have three different types of cooling systems. The first one is the air-conditioning system, whose operating temperature is roughly 5[degrees]C, which regulates the relative humidity and temperature in the occupied store. The second one is the medium-temperature refrigeration system. It has an evaporation temperature of around -8[degrees]C and provides refrigeration for fresh food such as meats, vegetables, and dairy products. The third system has an evaporation temperature down to -40[degrees]C and is called the low-temperature refrigeration system. Nowadays, there are various types of refrigeration systems, such as direct expansion/multiplex, secondary-loop, distributed, and cascade refrigeration systems. However, the direct expansion refrigeration system is the most commonly employed configuration in most supermarkets for providing refrigeration to display cabinets located in the store.

The low-temperature direct expansion refrigeration system under consideration (see Figure 1) consists of display cabinets/freezer rooms of varying geometry, multiple compressors, air-cooled condensers, thermostatic expansion valves (type DANFOSS TUA/TUAE), and evaporator pressure regulating valves (EPRVs). The function of EPRVs, as presented by Tahir and Bansal (2005), is to keep the evaporating temperatures of the respective display cabinet/freezer room at the required levels. The pressure and temperature at the suction manifold of the compressors are also determined by these valves. The dotted line enclosure shown in Figure 1 contains the evaporator assemblies, including drain points for condensate collections during defrost periods. These components in the enclosure are located within the bottom parts of the display cabinets and on the walls of the freezer rooms.

The low-temperature refrigeration system (Figure 1) consists of three major circuits that feed refrigerant R-404A to three sections of the supermarket, such as frozen food cabinets, fish/meat cabinets, and freezer rooms. The geometrical parameters and the operating temperatures of each type of cabinet and freezer room are given in Table 1.

DEVELOPMENT OF SIMULATION MODEL

The development of the simulation model for a low-temperature supermarket refrigeration system, which includes major components such as compressors, display cabinets, condensers, and thermostatic expansion valves, is discussed in the following sections.

Compressor Model

Multiple compressors (rack) in a supermarket refrigeration system are mostly of the semi-hermetic reciprocating type. If the compressor performance data are known, the compressor model may be developed based on the philosophy presented by Popovic and Shapiro (1995). Their model requires inputs such as refrigerant inlet state, outlet refrigerant pressure, clearance volume, polytropic exponents for specific refrigerants, and motor speed to calculate refrigerant mass flow rate and refrigerant.