National lab works on improving latest refrigeration technologies

July 11, 2000
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OAK RIDGE, Tenn. — Nestled in the verdant hills of Tennessee is the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL). The largest of DOE’s multiprogram research laboratories, ORNL does research on everything from basic science and chemistry to materials research, nuclear safety, and energy conservation.

ORNL is also home to the Building Technology Center (BTC), which works on building energy efficiency improvements. Under that umbrella exists the Advanced Heating and Cooling Equipment Program which, among other things, focuses on improving commercial refrigeration equipment and systems. The major sponsor of ORNL’s BTC research activities is DOE’s Office of Energy Efficiency and Renewable Energy (EERE).

Researchers carry out their tests in lab facilities that would make just about any scientist or engineer green with envy. More than 4,000 sq ft of lab space is devoted to hvacr testing and research. With state-of-the-art equipment and facilities, researchers have been able to come up with many commercial refrigeration improvements over the years.

These, in turn, have been passed on to contractors and system designers.

Testing facilities

Working in the extensive hvacr labs at ORNL is Van Baxter, P.E., who has researched air conditioning, heating, and refrigeration equipment at the lab for more than 20 years.

He explains that the lab is very much a collaborative effort between the DOE, industry groups, and the researchers at the lab. “Sometimes we generate the ideas to research, and sometimes we get proposals from groups outside the lab that sound interesting.”

Wherever the idea comes from, researchers at ORNL test the merits of the suggestion in their extensive labs. Three environmental chambers are available for the testing of various types of heating, cooling, and refrigeration systems.

The largest of the chambers consists of a two-room psychrometric test stand, which features both natural gas and electric utilities to support testing of thermally activated heat pumps, electric vapor compressor heat pumps, refrigerators, desiccant heat pump systems, and hydronic systems. The refrigeration system can support 20 tons of load at an ambient temperature of 95°F, while the refrigeration plant can hold the ambient temperature at -20° while supporting a 3-ton load.

An air recirculation loop is available at ORNL to test air-to-water heat exchangers and air-to-refrigerant evaporators and condensers. The loop has a variable-speed blower that is capable of delivering 7,500 cfm of air against a pressure drop of 4 in. wc.

It is equipped with an air heater, a steam injector, and a chilled water coil to control the temperature and humidity of air entering the test heat exchangers over a range of about 40° to 120° and 40% to 90%, respectively.

A vapor-compression refrigeration cycle test loop at the lab is equipped with a variable-speed compressor and can handle capacities up to 3 or 4 tons. The unit features a relatively low refrigerant charge (about 5 lb) and sample chambers located in the compressor discharge line and the liquid line.

This unit can be used to compare cycle performance of different refrigerants and refrigerant mixtures under the same operating conditions.

And those are just the major components of the lab.

Why all the research?

With these extensive — and expensive — labs, the question arises of why does all this research need to be done in the first place?

According to Baxter, there are many reasons, but the biggest one is that the DOE would like to see refrigeration applications — including supermarkets — use less energy. The supermarkets would like to see a reduction as well.

“If you look at a large supermarket, its annual energy bill is about the same as its annual profit. If they can take a dollar off of their energy costs, that adds a dollar directly to their profit.”

Baxter adds that about half of the energy used in a supermarket is consumed by food refrigeration. That’s followed by energy used for lighting and finally the hvac system. More energy-efficient equipment would mean supermarket owners would save money but more importantly, there would be less energy consumption, which generates CO2 emissions, which may contribute to global warming.

Reducing the total equivalent warming impact (TEWI) is a major goal at ORNL and DOE. Baxter says they’re looking at reducing both parts of the TEWI equation: refrigerant leaks and reducing electricity consumption to run the refrigeration systems. “For supermarket systems, the refrigerant emissions are a fairly large chunk of the total TEWI.”

Refrigerant leakage became more of an issue since supermarkets started placing compressors in machine rooms at the back of the store (see boxed item, page 15). Or sometimes the compressors were placed in pre-fabricated machine rooms on the roof.

In either case, it became necessary to distribute the large amount of refrigerant (between 3,000 and 5,000 lb) to the display cases all over the store via long refrigerant lines. With so much piping and so many connections, there are many more opportunities for leaks to develop.

Fortunately, much work has been done in this area and Baxter estimates that new systems today lose “only” 15% of their refrigerant charge each year, as opposed to the 30%-plus annual loss supermarkets used to experience 10 or 15 years ago.

“We’re looking at systems that use less refrigerant to begin with and are also tighter and less subject to leak, so it reduces the direct TEWI effect substantially.”

What's in the works?

To reduce TEWI, Baxter says they’re evaluating a number of low-charge options to current state-of-the-art equipment, but two methods are of particular interest right now.

“We’re investigating the secondary coolant-type approach and the distributed compressor approach in order to reduce the amount of charge and hopefully increase the energy efficiency of supermarkets.”

The secondary loop system would involve locating the compressors in the back room, but instead of running long refrigerant lines, a self-contained chiller would refrigerate a secondary coolant, some sort of a brine or antifreeze-type fluid.

That fluid would be pumped to the display cases instead of refrigerant. Baxter says this system can reduce the amount of refrigerant by more than 90%.

The distributed compressor method is another approach ORNL is working on. In this system, the compressors would be positioned closer to the display cases or storage coolers-freezers they serve. This could reduce the charge by up to 43%.

A field test of this system type is underway in collaboration with Price Chopper Stores, Massachusetts Electric, Foster-Miller Inc., and the Electric Power Research Institute (see article on page 13, “DOE sponsors advanced supermarket refrigeration and hvac systems field test.”)

Instead of rejecting heat into the store space, however, a water loop would be run around the store to take away the condenser heat from the refrigeration equipment. The water loop would then go to a cooling tower for heat rejection.

“You can also have your heat pumps for store air conditioning and heating use that same water loop to take the waste heat from the refrigeration and use that as a source for providing heat.”

Baxter adds that people are also experimenting with distributed systems that use air-cooled condensing, but the condensers are mounted on the roof of the store. “Sometimes they’re looking at even mounting the refrigeration compressors and condensers up on the roof and then send the pipes down to the display cases.

“That approach doesn’t cut refrigerant charge as much as the water-cooled, but it’s still a savings over having all the compressors in one place.”

What the future may bring

Even with all the research ORNL does to refine refrigeration technologies, there’s always something else somebody would like to have.

The next “something” to supermarket owners is to have increased flexibility with their refrigeration equipment.

“Store owners would like to be able to take an ice cream case and put it in one corner of the store today and then be able to move it tomorrow to a different location for merchandising flexibility.”

That would mean developing a system where it would be possible move the cases around and have water and electrical connections distributed around the store. Owners could then move the cases and just plug in a couple of pipes and be ready to go.

These self-contained units may well be the next “wave of the future.” And you can be sure that researchers at ORNL and DOE will be working to make these systems energy efficient and environmentally friendly.

Sidebar: Supermarket success story

While the refrigeration labs at ORNL frequently contribute important new findings to the industry, one of their most prominent successes occurred almost 20 years ago.

At that time, ORNL and DOE formed a partnership with industry to improve the efficiency of supermarket refrigeration systems. From 1980 through 1982, Foster-Miller Associates (FMA), H.E. Butt Grocery Co., and Friedrich Commercial Refrigeration participated in the partnership.

The primary components of the improved system are an unequal parallel compressor rack system, a microprocessor controller that modulates the compressor capacity to meet the refrigeration load, and a condenser with floating head pressure control. Field trials of the advanced system at a supermarket in San Antonio, Texas demonstrated a 16% energy savings over conventional systems.

Subsequent to the DOE/ORNL work, Electric Power Research Institute (EPRI) sponsored studies with FMA and other manufacturers that resulted in further improvements in the compressor and control system. Supermarket energy use reductions of up to 35% were reported through use of the advanced systems.

The control algorithms developed under the DOE/ORNL project provided the foundation for the microprocessor controls uniformly installed in new and retrofitted supermarket systems since the mid-1980s.

As of 1995, 80% of supermarkets reported using the advanced system. With an average of 30% energy savings per store, total cumulative energy savings was approximately 0.6 quadrillion Btu since market entry.

Assuming that the control strategy pioneered by DOE/ORNL can take credit for half of the total savings, about 26 billion kWh or $2 billion at $0.08/kWh in energy and cost savings have been realized. DOE’s investment in the development program was approximately $1.2 million, resulting in a benefit/cost ratio exceeding 1600:1.

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