The Evaporator Fan VFD Initiative
During the last five years, a number of controlled atmosphere storage facilities in the Pacific Northwest have installed variable frequency drive (VFD) technology on evaporator fans. VFD in the application have the potential for very large energy savings during the low refrigeration loads of the holding season. Despite this potential, VFD installations in this application have been very limited due to concern regarding the impact of reduced air flow on fruit quality. Despite scattered qualitative reports of "good or better fruit quality" from those that have implemented the technology, no truly scientific evidence has been gathered.
This paper introduces the Evaporator Fan VFD Initiative, a research and market transformation program sponsored by the Northwest Energy Efficiency Alliance ("The Alliance"). Through a focused program of field trials, fruit quality assessment, and education, the Alliance hopes to prove the value of VFD in the refrigerated warehouse market, with significant effort in the controlled atmosphere storage sector.
Cascade Energy Engineering has been selected as the contractor to manage and implement this market transformation program.
Northwest Energy Efficiency Alliance
The Northwest Energy Efficiency Alliance is a non-profit consortium of utilities, governments, public-interest groups, and the private sector in the northwest (Washington, Oregon, Montana and Idaho). Over the next three years, the Alliance has committed approximately $65 million toward various energy efficiency market transformation projects in the region. The goals of a market transformation project are to affect the long-term industry standard patterns for design, purchase and operation of electrical energy consuming equipment.
Toward this goal, the Evaporator Fan VFD Initiative is focused on answering some fundamental fruit quality questions that over-shadow the potential energy savings for this application. With an estimated 80,000 hp in refrigerated warehouse evaporator fans, the potential regional energy savings is enormous.
All 3-phase induction motors are designed to operate at one speed; typically 1200, 1800 or 3600 rpm. Since the electric utility provides power at 60 cycles per second (Hertz), motor speed is determined by winding configurations within the motor. As a result, equipment driven by induction motors (including fans and compressors) are constrained to a single speed.
A VFD allows nearly any induction motor to vary its speed. The 60 Hertz sine-wave from the utility is converted to DC power. Modern pulse-width modulated VFDs then send pulses of varying width and polarity to the motor. Although the voltage waveform no longer resembles the original sine-wave, the current draw of the motor is remarkably sinusoidal. The result is a motor that can now be operated at any speed between 0 and 60 Hertz.
Fan Savings with VFD Control
Although VFDs can produce energy savings on many different equipment loads, evaporator fans offer some of the greatest potential savings. This is the result of the affinity laws, which govern the flow, pressure, and power requirements of a fan or pump in a closed, un-damped system:
- Air Flow - Fan Speed
- Pressure increase - Fan Speed2
- Power consumed - Fan Speed3
Evaporator fans in a refrigerated warehouse are a classic example of these laws that offer nearly perfect cubic power reduction with speed.
This means that a fan operating at 50% speed will ideally draw 12.5% shaft power. Although motor and drive efficiency might temper this ideal value to the range of 15%, the power reduction is tremendous. And to top off the fan energy reduction, any savings in fan energy results in reduced room heat loads. Compressors unload and condenser fans operate less as a result, adding another 20% to 30% to the direct fan energy savings.
Evaporator fan VFDs also offer an energy savings improvement over typical fan cycling operation.
Typical Installation Cost & Payback
A wide range of VFD electrical installations have been tried for evaporator fan applications. Through a number of established projects, it appears that a reasonable budgeting cost of $400/hp for VFD installation can be used. Thus, a sample room with (5) 2 hp motors should cost roughly $4,000 to retrofit to VFD control.
Ultimately, final economics depend on a number of variables, including hours of operation, current operating strategies (constant or cycling), fan horsepower and utility rate schedules. In regions with a higher energy cost (e.g., Yakima & Medford), the payback can be a few years. In regions with a lower energy rate (e.g., Wenatchee & Hood River), the payback can be as high as 8 to 20 years. As discussed later in this paper, without an improvement in fruit quality or utility incentives, regions of low energy cost may not realize acceptable economics from evaporator fan VFD installations.
A number of prominent companies have begun experimenting with VFD technology on evaporator coils. These include:
- Bear Creek (Harry & David)
- Washington Fruit
For all of these companies, the energy saving was dramatic.
- Limited fruit quality assessments are also encouraging: Broetje personnel weighed bins of fruit before and after storage and compared data for evaporator fan VFDs with fan cycling. They reported reduced mass loss with the evaporator fan VFDs.
- An analysis of pressure test readings after storage at Washington Fruit showed higher pressures in four of five rooms relative to similar fruit in other rooms where fan operation was continuous.
Simple Field Trials
Fruit quality research will encompass a several techniques of varying degrees of complexity. These tests can be divided into simple and detailed field trials.
Over the next two harvest seasons, fifteen simple field trials will be conducted. These tests will compare fruit mass loss over the course of a full holding season.
Participants will commit two CA rooms to the trial. VFDs will be installed in one room and a second room will be used as a "control" room for comparison. To the degree possible, the two rooms should be identical in terms of size, bin capacity, bin stacking pattern, evaporator model, fruit variety, and storage schedule. The rooms should also be filled within a day or two of each other.
As the rooms are filled, Cascade Energy Engineering will divert a bin of fruit from which small sample bags of fruit will be created for both rooms. Samples will be weighed prior to storage. Other basic fruit quality tests will also be conducted. These sample bags will then be placed in multiple locations in the stack as the room is loaded. Samples will be placed in both the VFD room and the control room.
When the rooms are opened for packing, Cascade will retrieve, weigh, and evaluate samples to determine the impact of the VFD operation on fruit quality.
Detailed Field Trials
Three more detailed field trials are planned that will investigate the influence of reduced fan speed on air velocities, temperature distribution, and humidity in the space. As in the simple field trials, this research will focus on comparing two identical rooms.
Other Field Trials
Another series of evaporator fan VFD field trials are geared toward non-CA applications. These tests could take place in common storage rooms or in refrigerated warehouses in other industries.
There will also be co-funding for two full scale evaporator fan VFD installations in the region. Participants for the full scale installations will be selected from those who participate in these early field trials.
Following review and assessment of candidate facilities and rooms, test sites and rooms will be selected. An electrical contractor will be employed to install the drive along with any necessary bypass and motor protection features. In addition, the vendor of the existing computer control system would be employed to modify the hardware and algorithms for speed control.
Project Cost Sharing
The evaporator fan and motor protection costs will be shared between the Northwest Energy Efficiency Alliance and the warehouse owner. All other costs, including data acquisition and fruit quality assessment are borne by the Cascade Energy Engineering, and ultimately by the Alliance.
Plans for the 1998-1999 Season
A total of twelve field trials (10 simple and 2 detailed) will be conducted during the 1998-99 storage season. Assessment of fruit quality will begin with the first room opened, likely in the spring of 1999. These results will be documented and shared with the owners, operators and contractors of the fruit storage industry. If possible, some results may be presented at the 1999 Post-Harvest Convention. Ultimately, all results will be available for review by the entire fruit industry.
Marcus H. Wilcox, P.E. President and Robert D. Morton P.E., Vice President
Cascade Energy Engineering, Inc.
6 1/2 N. 2 Ave; Suite 310, Walla Walla, WA 99362
14th Annual Postharvest Conference,
March 10-11, 1998