by David Ward, P.E.
Energy auditing plays a critical role in the efficient operation of many facilities. Buildings with large amounts of refrigeration are audited for such diverse purposes as planning capital investments, process analysis, energy use reduction, and establishing a baseline for performance contracting.
The presence of large amounts of refrigeration raises complex auditing issues that require complex evaluations. Since these buildings are difficult to compare on the basis of kWh/sq.ft./year or Btu/sq.ft./year, facility managers have no way of knowing whether or not they are being run efficiently, and what part the refrigeration systems play in the equation. This is troubling in that inefficient refrigeration systems can add thousands of dollars per year to operating costs.
The Auditing Process
The first step in the auditing process is to determine why the audit is being done and what is the desired outcome, since everything else is largely contingent upon these answers. If the purpose is a cursory energy use analysis, the process can be relatively short and simple: record the nameplate data, take a few measurements, note the run hours, and compare the calculated energy use to the current invoice. This type of audit is extremely useful in determining the percentage of the total energy being used by the lighting, HVAC systems, office equipment, production equipment, and refrigeration.
If, however, the purpose is to obtain baseline data for use in a performance contract, or to determine the efficiency of the refrigeration equipment, the audit will be much more complex and expensive. In this case, proper planning and preparation can save a great deal of time and money and result in much more accurate and valuable information.
Frequently throughout this discussion, we will consider an actual milk processing plant where the energy use has been increasing over time, while the production has remained constant. This has caused profit margins to steadily decline.
The plant manager has been trying to increase the maintenance budget to ensure that the equipment is in good working order, but the diminishing profit margins have resulted in tight constraints on maintenance spending. In addition, the strict requirements of the milk production process allow very little room for modifications in operating temperatures or run hours. Still, the plant manager has set a goal of cutting energy use by 10-15%, and finding a way to shed some of the load during the summer months to reduce demand charges.
In preparation for the audit, historical data are gathered for analysis. This includes energy use, energy costs, production amounts, and times, maintenance costs (including refrigerant losses), equipment type and age, operating parameters, facility load, product temperatures, humidity requirements, and local weather. The goal of gathering this data is to find a correlation between data sets that indicate a problem.
Preparation also requires defining the scope of the audit as much as possible. The administrative office space, for example, is served by an old, inefficient HVAC system. The windows leak and the lighting is poor. Still, this space only accounts for a small portion of the plant and the operating hours are relatively low. It is decided that the energy savings will not justify spending the time on a detailed audit, nor the investment required to generate any significant savings.
Phase 1: Initial Walk-through
In this phase, the equipment is reset to the original specifications, and we make certain that all the operating conditions are as they should be. Any maintenance issues that have been discovered are resolved. In a typical audit, this can result in a 3-5% energy savings by itself.
Often, the audit will end here, since this phase has a relatively low cost, provides a very fast payback, and should yield excellent information about the general operation of the facility. Such a limited audit, however, will not unearth deeper problems that may be causing significant operating inefficiencies.
Phase 2: Data Collection
This is where the real work begins and the biggest pitfalls lie. The main areas of focus here are measurement and trending of system operations.
It’s very easy here to go too far and collect more data than necessary. This can slow down the auditing process and increase costs. Therefore, metering and data acquisition must be balanced with accuracy.
In most cases, 15-minute interval data over the course of 7 days will suffice, but sometimes more frequent data are required. To capture the cycle frequency of a compressor or valve, for instance, 1-minute interval data over the course of a few hours or 30-second interval data for one hour may be necessary.
Other potential pitfalls include monitoring too many parameters on too much equipment, or worse, not getting coincident data on a key parameter, then having to do everything over again. This is where thorough preparation pays off.
It is important to understand that energy use is fundamentally a function of three variables: energy input, work output, and hours. Therefore, critical systems need to be identified and evaluated to determine the minimum number of points required to define the three variables. When in doubt, it’s better to monitor an unnecessary parameter than to miss a necessary one.
Assuming the hours of operation are known and fixed by the production process, two primary areas must be evaluated for potential energy savings: load reduction and system efficiency (kW/ton). Load reduction is relatively simple because it is mostly limited by process requirements. Sometimes though, opportunities for load reduction exist elsewhere.
In the milk plant, for instance, hot, humid summer air infiltrating into the space adds significantly to the refrigeration load. Temperature and relative humidity(RH) measurements taken every 15 minutes over the course of a few days identify this problem. Suggested solutions include automatic doors, better dock seals, changes in traffic patterns, and changes in airflow.
Other areas are examined for
load reduction: improvements in washdown procedures and ventilation of the humidity created, better incoming product temperature control, elimination of unnecessary lighting and motor loads, and modifications in packaging, preparation, or other practices where loads can be relocated elsewhere.
Load reductions at other types of facilities follow the same fundamental approach: a main load around which the refrigeration system is designed, and ancillary loads that are added on later. The main load is critically evaluated to determine if any adjustments can be made, but often, the ancillary loads offer the greatest potential energy savings.
Potential refrigeration efficiency improvements are tougher to identify and quantify. Those are defined by kilowatt (kW) input per ton of cooling output. KW measurements are relatively straightforward, but accurately determining tons of cooling delivered coincident with energy used is difficult. Here, more detail and accuracy are required.
Accurate capacity (tons) measurement invariably boils down to measuring temperature differential, which is relatively easy, and coincident measurement of flow rate, which is hard.
Potentially, there are two different types of flow to measure: chilled water, glycol, or a brine solution passing through a heat exchanger and circulating to the loads, and refrigerant circulated directly to the loads.
In the first case, pump specifications and pressure differentials are used to calculate the flow rate. Note though that pumps often operate outside the manufacturers' pump curves.
In the second case, to measure refrigerant flow, compressor specifications along with the existing operating conditions are used.
With detailed manufacturer's specifications and accurate operating conditions for the existing pumps and compressors, the flow rates of fluids and refrigerants can be determined to a high level of confidence.
It’s important that kW and ton measurements be simultaneous. Energy use and load vary greatly over the course of a day, and the refrigeration system responds to these changes. It is this constant fluctuation that provides the details of a system’s operating efficiency and the clues to the actions required for improvement. Look for temperature and pressure fluctuations and system response time. Take 1-minute data for 24 hours at least once during a production day, then again during the weekend, or on a non-production day, for comparison. This is a key requirement of a refrigeration system audit, and may reveal surprising results.
During this phase, the data are collected and reviewed after the first few hours or the first production cycle. They’re examined to verify that the meters are operating as they should, that the data being gathered are useful, and that there are no surprises requiring closer examination.
Commonly found problems include meters that are broken or out of calibration, time increments that are too long or too short, and data that is beyond the meter's range. A little time spent on an initial review can eliminate a great deal of lost time later. More information on this subject is available at the National Institute of Standards and Technology website at www.nist.gov.
It’s advisable to leave the metering equipment in place until the completion of this phase. Frequently, the analysis results in questions being raised about the data or facility operations that require modifications or more refined metering. This is greatly simplified if the meters have been left in place.
Phase 3, Data Analysis
This is the discovery and resolution phase of the audit process. The first step is to review Phase 1 in terms of the required outputs or deliverables. It may be necessary to modify these requirements based on what has been learned during the first two phases, so before any significant time is spent pouring through the data and information, stop to regroup and redefine the requirements.
The data and information are then organized into groups based on the required output. Possible data groups include energy use, HVAC system, refrigeration system, production, and loads. The data for each group are reduced and refined into a form that can be clearly compared with other groups. Graphs of hourly data for each day are very helpful. After the comparison, the information is refined and compared again.
Next, the data are examined one group at a time to determine where modifications can be made
to reduce energy use, and what the modifications should be. This is extremely subjective and the examination process will vary greatly based on what is discovered or who is doing the analysis.
From this, a list of specific energy saving measures is generated tailored to the facility's needs based on the information provided by the data. This process can be relatively imaginative, and energy saving measures should be generated with little regard for their expected costs since later, the list will be quickly reduced to the most cost-effective measures. Sometimes, measures that do not make sense on their own are grouped with others to generate a more comprehensive energy reduction strategy.
The milk plant data yields both expected and unexpected results. It’s determined that the suction pressure is lower than necessary, a new condenser is required, improved dock door seals are recommended, and compressor cycling is not properly following the load. The data also leads to examining the refrigerant for water contamination, and shows that one of the compressors is running continuously, but doing very little cooling.
The largest surprise is found in the warehouse, where a temperature meter reveals that a rear outside exit door is being left open continuously, except when the manager or the energy auditor walk through, because the covered stairway is being used as the unofficial smoking area. This is adding greatly to the system load.
In summary, an effective energy audit that has been developed to address the specific needs of the facility is a valuable tool that can keep it operating at peak efficiency.
David Ward is a licensed, professional engineer in private practice in Framingham, MA, and has served as chairman of ASHRAE's Technical Committee on Custom Engineered Refrigeration Systems. He can be reached at 508/875-2252.