Excerpts from Contracting Business Volume 55, February 1998
This new supermarket uses ice storage for air
Nino Salvaggio International Market Place, Troy, MI is a recently constructed, upscale, 45,000-sq.ft. fruit and vegetable market that also includes fresh meat, dairy, fish, extensive wine selections, frozen food, a deli, bakery, flower shop, and coffee bar. In addition to normally available fruit and vegetables, we offer many exotic and ethnic products.
We use a variety of energy strategies, including ice storage, liquid pressure amplification (LPA), and variable frequency drives on condenser fan motors and glycol pump motors. All of these combine to address both todays high
time-of-day electric rates and the unknowns of future electric utility deregulation. Peak-time energy use charges now represent more than 50% of our electrical use costs.
We decided, along with the system designer Miles Carney, to use HCFC-22 in all the refrigeration and chiller systems. Carney pointed out that service technicians are familiar with this refrigerant, and that its easy for them to work with. He also stressed thatHCFC-22 is a proven alternative, that its less expensive than other options, and that its expected to be around for the next 20 or 30 years.
Our decision to use an ice storage system allowed us to useHCFC-22 in the low-temperature system, without having to worry about high-discharge temperature and compression ratios. By using part of the 50F glycol from the ice storage system for final condensing, we avoid this problem.
With proper controls, ice storage can be used for comfort cooling, refrigerant subcooling, or refrigerant condensing. The use of ice storage to reduce comfort cooling cost peaks in northern supermarket applications isnt normally cost-effective, because it can only be used for the three summer months. However, expanding its operation from April to November - eight months - by using it to achieve 60F condensing on the low and medium-temperature refrigeration systems during the on-peak energy hours, justified this approach.
How It Works
On a typical day, three of the six compressors on the split-suction glycol chiller system charge the five ice storage modules, totaling 490 ton-hours of cooling capacity during the 7 p.m. to 11 a.m. off-peak utility time period. The other three compressors are available for comfort cooling and refrigeration condensing loads. These shares vary with the weather and, on the hottest days, we may be making ice right up until 11 a.m., when the cost per kWh escalates upward.
The six chiller compressors all semi-hermetic are equipped with a pair of evaporative condensers fitted with variable speed fans. This system is designed to operate at low day and night-time condensing temperatures and pressures.
To ensure that we operate at peak performance, we employed an LPA refrigerant delivery system and its associated discharge desuperheating feature. The liquid delivery system assures us constant quality subcooled refrigerant at our thermostatic expansion valves. With discharge desuperheating, we can hold the entering condenser discharge refrigerant temperature under 140F, which helps us avoid much of the scale formation associated with water-assisted condensing. The cleaner condensers also put us in the best position to handle peak load conditions with lower energy usage.
Our refrigerant systems are designed to operate at low condensing temperature and pressure by transferring part of the refrigeration condensing to work from lower temperature suction groups to the highest suction temperature chiller compressor group. Because we didnt want to burden the high-suction temperature chiller with the total mechanical heat of rejection from the refrigeration systems, we directed the discharge gas through potable water heat exchangers and liberally sized air-cooled condensers.
By using these heat exchanges, we come close to transferring only the useful portion of the refrigeration evaporator loads to the higher suction temperature chiller.
For example, we expect to shift about 40% of the peak summer suction load from both the low- and medium-temperature compressors to the 4OF suction chiller system. Although the medium temperature system operates at 15F suction, the load is much greater than the low-temperature system, so it actually has a greater kWh load shifting potential. What we wanted to create was a stored "cascade" system that requires no compressors for the upper-stage condensing load during on-peak energy hours.
Our condensers are designed to condense at 10 to 15F above ambient. Typically, a condensing system operating at 5F suction temperature and 10OF condensing would produce 35.5 tons of refrigeration with 70.1 brake horsepower (BHP), or 1.97 BHP/ton. By using LPA and ice storage, its possible to reduce the saturated condensing temperature to 6OF (down 50F); produce 51.3 tons of refrigeration (up 15.8 tons); and reduce BHP from 70.1 to 52.3 (down 17.8 BHP). The BHP/ton is reduced by 0.95 BHP/ton, or from 1.97 to 1.02. Compressor capacity increases about 6% for every 1OF drop in condensing temperature (8% with semi-hermetic compressors).
Computerize control systems are used to direct the chiller and thermal storage system operation. The building control system is responsible for temperature, lighting, and energy management. The system can be checked and adjusted from a remote location.
Other benefits of the system design include:
-Longer compressor life and less refrigeration loss because of lower head
pressure and compression ratio
- BecauseHCFC-22 is currently less than half the cost of most refrigerants, any necessary replacement will be 1ess expensive
-The evaporative condensers wont fail because of scale build-up and excess chemical treatment, conditions which could shorten their operating lives.
The Detroit Edison Energy Group has set up a complete energy-use monitoring system, and will be tracking energy use from their offices. Because of this systems flexibility, we will be able to use this information to attain optimum performance.
Our system designer Miles Carney says, "If we couldnt make a major impact on peak energy usage, we would just be spinning our wheels. The heart of this project is the LPA refrigerant delivery system. This common link allowed us to make each major component more energy efficient and reliable. LPA also allowed us to rethink how we could use low ambient conditions and structured secondary heat exchanges to not only save money, but to save energy at specific times of the day. I believe this strategy is a viable means for facilities to shift energy use or create more capacity for peak load conditions."
System with LPA
"I spent many years developing the ideas that would make this structure a real marketplace," adds Nino Salvaggio, my associate partner. "To create the ambiance we wanted for our customers, we did some things that we knew werent energy efficient. For instance, most of the sales area walls are glass roll-up doors, which will be open on moderate-temperature days. The goal was to use the energy savings from these specially-designed mechanical systems to balance the total building energy use. The system has provided reliable refrigeration and a comfortable environment, even with our unique operations."
Kirk Taylor is associate partner, Salvaggio Markets, Troy, MI.
-excerpts from Contracting Business Volume 55,