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Home Energy Magazine Online May/June 2000
Other Methods of Shortening Refrigerator Monitoring Time
by John Proctor
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| The usage pattern of a typical refrigerator has a low in the morning and rises to a peak in the evening.
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| Table A. Time of Day Adjustment |
| Hour | Fraction |
| 1:00 | 0.975 |
| 2:00 | 0.952 |
| 3:00 | 0.929 |
| 4:00 | 0.912 |
| 5:00 | 0.897 |
| 6:00 | 0.882 |
| 7:00 | 0.89 |
| 8:00 | 0.922 |
| 9:00 | 0.934 |
| 10:00 | 0.94 |
| 11:00 | 0.969 |
| 12:00 | 0.993 |
| 13:00 | 1.03 |
| 14:00 | 1.033 |
| 15:00 | 1.039 |
| 16:00 | 1.064 |
| 17:00 | 1.091 |
| 18:00 | 1.113 |
| 19:00 | 1.12 |
| 20:00 | 1.108 |
| 21:00 | 1.082 |
| 22:00 | 1.06 |
| 23:00 | 1.045 |
| 24:00 | 1.022 |
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While kitchen temperatures are a major determinant of the short-term metered energy consumption of a refrigerator, there is another factor that will skew short-term monitoring results. This factor is the time of day. Metered use reaches a low during the hour beginning at 5 am and rises to a high during the hour starting at 6 pm. The average consumption pattern over a 24-hour period is shown in the graph. A metering time as short as one hour may be adequate to arrive at a fairly accurate estimate of the annualized energy use of a given unit, when the metered data are adjusted for kitchen temperature and time of day, as in the following equation:
(See Table A to select the appropriate coefficient.)
Once the metered data are annualized, the kWh for each unit can be compared against replacement criteria developed by the program administrators of the replacement project. Program economics and the local price of electricity will dictate what the replacement cutoff should be. Accepting that a few units that are misdiagnosed as high consumption refrigerators will inevitably be replaced will reduce the need for extremely precise annualized estimates and will also shorten metering times. The cost of longer-term monitoring may well exceed the cost of a few unnecessary replacements.
For example, in one program design, the replacement refrigerator had an annual consumption of 800 kWh. The cutoff for replacement was set at 1,800 kWh. With those parameters, and a one-hour metering time, it turned out that 40% of the refrigerators would be replaced and the average savings would be 1,310 kWh. With the project testing 1,000 refrigerators and replacing 40% of them, the annual savings would be 524,000 kWh.
We extended the metering time to two hours and found that slightly fewer refrigerators (38.4%) would be replaced, but the average savings would increase only to 1,404 kWh. For 1,000 refrigerators tested, the annual savings would be 539,000 kWh. For some programs the additional 15,000 kWh annual savings and the lower capital cost (17 fewer refrigerators replaced per 1,000 tested) may be worth the 1,000 additional hours of testing. However, for others the cost of the extended metering may exceed the combined additional annual kWh savings and the lower capital outlay.
Keeping accurate records of the makes and models of refrigerators metered--both those removed and those not removed--can eliminate metering time entirely for "dog" models. Models of refrigerators that statistics show will frequently be removed can be replaced without metering. However, with no monitoring, broken refrigerators will not be identified.
John Proctor is president of Proctor Engineering Group, Ltd., which is based in San Rafael, California.
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