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• Article 23 - May-June, 1997


Sensors, 2©


This month, I will finish the article I started in the last newsletter that dealt with a review of the SERI (now NREL) technical report entitled An Analysis of the Effects of Active Solar Energy System Control Sensor Degradation on System Performance by R. B. Farrington and W. Short (SERI/TR-253-2185).


Last time I ended noting that additional operating costs could result from sensor drift that could cause a controller to keep a solar DHW system operating after it was supposed to shut down. Here the cost of the electrical energy required to collect the solar energy exceeds the value of the solar energy collected. The question now becomes, when does a negative turn-off differential temperature become significant? Certainly, if a sensor has failed, such that the controller never turns the pump(s) off, not only is electrical energy wasted running the pump 24 hours a day, but solar energy collected during the day will be extracted from the storage tank and lost to the ambient air at night. Also under very cold conditions, a catastrophic freeze up of the system could result.


Using the TRNSYS 10.0 computer program, Farrington and Short simulated a typical solar DHW system for four locations: Albuquerque , Fort Worth , Madison , and Washington , D.C. The results show a decrease in the amount of net annual collected energy to range from about 2% at Albuquerque to 14% at Washington , D.C. , for a -3°F turn-off differential temperature. At a -5°F turnoff differential temperature the net collected energy is reduced by as much as 50%. At this point, the pump runs just about 100% of the time in Fort Worth and Washington , DC . The negative impact of having a turn-off differential temperature that is too high is minimal. For instance, a +10°F turn-off differential temperature results in a net annual collected energy decrease from about 1% at Madison to about 8% at Fort Worth. From this you can conclude that it is better to have a turn-off differential temperature that is too high than too low. Since the thermistors generally degrade from exposure to high temperatures, the collector sensor, due to exposure to high temperatures, is more susceptible to degradation than the storage tank sensor. Thus, it cannot be concluded that the collector and storage sensors degrade at the same rate and therefore effectively cancel any changes in sensor response. If the drift of the collector sensor is such that the measured resistance indicates a lower than actual temperature then the pump will turn off earlier than it should. But this will not lead to the problem of continued operation of the pump. If the drift of the collector sensor is such that the measured resistance indicates a higher than actual temperature, then the pump will continue to run and may cause large losses of the collected and stored energy.


In addition to those facts stated above, the report concludes that control sensor degradation may be one of the major reliability problems as well as a prime reason why systems are delivering less energy than expected.


What follows are my recommendations, not those of the SERI report authors.


1. Replace the collector sensor on systems that have been down for at least one to two months or more.


2. If the heat collection fluid is Bray oil or a water/glycol mixture and it is greatly discolored, this is a good indication it has been exposed to high temperatures over a long period of time. Again, as above, replace the collector sensor.


3. When you buy a group of new sensors, place them together on the bench and bring out their leads so that you can measure their resistances. Place a good quality mercury thermometer with the group. Let the sensor and thermometer temperatures stabilize for at least one half hour. Measure and record the resistance of each sensor making sure you touch only the sensor leads and not the sensor itself. Based on each sensor's resistance and an appropriate temperature versus resistance chart, assign the appropriate temperature to each sensor. Then compare each sensor's temperature to the temperature reading as noted on the thermometer. If you assume the thermometer is an accurate reference point, you can then see which sensors are reading high and which are low. You may consider pairing sensors (matched pair with less than 1°F difference between them) and replacing both collector and storage sensors as a pair when either needs replacement. Without testing an old storage sensor, there is no assurance that it also hasn't degraded somewhat. Degradation can go in either direction.


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