A few years ago I received a call from a frustrated client who was the head of facilities for a large telecom company. He had recently replaced an aging inefficient central chilled water plant that had outlived its useful life. Like many of you reading this article, my client prides himself as one who constantly keeps abreast of the latest technologies and this was an opportunity to really apply that knowledge. He had little doubt that this was a “can’t miss” return on investment. In my client’s mind, he had done everything right, he had hired one of the premiere engineering firms to do the design, replaced old inefficient centrifugal chillers with modular variable speed ones, upgraded the programming of his existing building automation control system to use the most modern optimization programs, put variable speed drives on the chilled water pumps, installed a water side economizer and then had the entire system commissioned by a reputable commissioning agency.

After the first three months of operation the results were disappointing but the client felt that this was just the “break in” period as there were ongoing issues in getting the fairly complex systems working together. After the fourth month, he came to the reality that the savings were just not appearing. Satisfied that the issue was not a change in building operation, he brought together the mechanical contractor, chiller and the controls vender, but try as they might, no one could figure out what was wrong.  The vendors said that their equipment was operating to specification; the control people checked their programs and no one could find the culprit. We were retained to find a solution.

Simple things often overlooked

You would think that after almost forty years of investigating these types of issues I would learn to start simple. Like those before me, I was drawn into reviewing the control algorithms and re-checking the balance of the system. As simple as it is, the solution begins with how this particular chiller manufacturer was applying a control algorithm to gain efficiency. Often called condenser water optimization, the goal is to control the tower motor speed to achieve an optimal condensing temperature given compressor load. If done properly, condenser water optimization has shown a documented result of as much as a 27% plant energy savings. 

In this case, condenser water optimization was prevised on knowing the current outdoor wet-bulb condition to determine the minimum condenser water supply temperature that can be achieved.  Since there was not an outdoor wet-bulb sensor at the site, a combination of outdoor air relative humidity and temperature were substituted using psychrometric calculations. This became the Achilles heel of the whole system because as it turned out, both the outdoor air relative humidity and temperature sensors were reading substantially higher than they should. With both sensors reading incorrectly, the compressor algorithm was completely off which made the tower fan run far more that it should have and the water side economizer to not operate when it should. The result was that thousands of dollars energy was wasted all because of two relatively inexpensive sensors (installed long before this upgrade). It will likely not shock anyone that nobody thought to check these sensors as they almost always go un-noticed. Had my client not been vigilant in watching the monthly energy bills this problem may have never been noticed.

Epidemic Proportions

[an error occurred while processing this directive]Very few buildings today have the optimized chiller program described above, but clearly every chiller manufacturer will make this function available very soon. The need to drive greater energy savings can only go so far if we rely solely on new technologies that produce more efficient prime movers (fans, compressors, pumps). Advanced algorithms for controlling all types of systems will increasingly be the norm. It is easy to see, for example, that controlling condenser fans on roof top units can be optimized similarly. Intuitively “Intelligent Buildings” wanting to take advantage of these algorithms will require many sensor inputs. The dilemma is, like my client, if those sensors are not measuring properly, bad things can happen. In some cases it can manifest in missed opportunities for maximum equipment optimization such as missed economizer operation or maintaining incorrect supply air temperatures, in others, such as oil and refrigerant pressures transducers it can cause catastrophic failure and unplanned down time. The white elephant in the room is how we address this issue in light of the enormous load on maintenance staffing.

Options

One popular solution is to retain a reputable commissioning company to periodically ensure overall systems performance. Although this is a really good idea which should always be encouraged, Murphy’s Law dictates that all too often something really bad will go unnoticed between commissioning periods. In the best case scenario, someone notices an odd behavior and the problem is resolved, unfortunately there are too many examples like the one I site here where several thousands of dollars of energy spend are lost before the issue is discovered and resolved.

New niche companies are appearing that use advanced algorithms to keep constant vigilance on facility operation on a daily basis. These algorithms are run on every piece of equipment and sensor available from the building automation system. Detection comes in the form of ensuring that not only sensors are working properly but that the equipment they control never strays too far from the design intent. The results are far fewer catastrophic failures and energy conservation measures maintained throughout the life of the building. This has proven to be effective for existing buildings which do not have great sophistication but will increasingly become more valuable as more advanced algorithms are implemented.