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8 Tips for Optimizing Energy Storage Enabled Chiller Plant for Maximum Savings
August 25, 2017 - HVAC
By Evan Berger, Director of Energy Solutions at CALMAC
Summer is upon us, and it’s always good to run a quick tune-up of your cooling system to make sure your components are running efficiently. For commercial and institutional customers, like office buildings, hospitals, schools, and college campuses, cooling typically accounts for 30-50 percent of your total electricity spend, so you want to invest some time to ensure that your thermal energy storage system is operating smoothly.
However, before getting started with our checklist, it is important for operator and maintenance personnel to understand the overall concept of the thermal energy storage system. Thermal energy storage is used in order to save the building owners money during peak demand periods. Air-conditioning during summer daytime hours is the largest single contributor to electrical utility “peak demand” charges. As more cooling is needed to maintain comfortable temperatures, the increased demand for electricity adds to the grid’s load, causing the utility to use additional, costlier generating sources known as peaker plants to handle increased demand. This is why most commercial customers in the U.S. pay a demand charge: it reflects the cost of using power during peak times of electric consumption.
The thermal energy storage system consists of thermal storage tanks and standard chiller equipment and accessories. The most common thermal storage tanks used in the market today are ice tanks which contain water and a heat exchanger. At night, the chiller circulates a glycol water solution through the ice tank’s heat exchanger. As the system heat transfer fluid is circulated, the solution freezes the water inside the storage tanks, making a full block of ice. The next day, that ice is melted to deliver part or all of the building’s cooling needs.
Now that you have an understanding, here are a few tips to make sure you’re getting your money’s worth:
- Review your Building Automation System (BAS) settings. One of the biggest money-wasters is a cooling system that turns on at random intervals, that cycles inefficiently, or that fails to run correctly at night to charge your thermal storage tanks. Conduct a review of your BAS system’s schedules to make sure that all the key components are turning on and shutting off when they should. It’s very common – and very expensive – for BAS systems to be placed on manual override for one unusual day (like a graduation ceremony or a ribbon-cutting), and then never be set back to their normal operation. Make sure those schedules are correct, and are being properly applied.
- Analyze your utility rate and ice tank scheduling. Utility rates tends to change from year to year, typically with demand charges getting higher. Confirm your summer peak and mid-peak periods, when your utility charges the highest rate for energy consumption. Check for coincident peak days, normally five days during the summer when the utility will charge an extra demand charge that often lasts for the next 11 months. To optimize savings, the ice tanks should be scheduled to run during these expensive peak periods and turn off during cheaper off-peak hours. If solar is installed, the ice tanks should discharge when solar capacity drops. Depending on the system design and outside temperatures, you may be able to turn off the chiller completely when the tanks are on to save even more.
- Calibrate your sensors. As the old saying goes about data, “garbage in, garbage out”: if your BAS is getting bad data, it will make bad, and costly, decisions. Make sure the temperature sensors of your cooling system are correctly measuring the entering and leaving chiller and ice tank farm chilled fluid temperatures, as well as the outside and inside air temperatures. Chiller return fluid temperatures should be used as an indicator of a “full charge” of the storage tanks when in the ice making mode. The ice-making termination set point temperature should be checked every 6 months. This will ensure the system is running as efficiently as possible. If the BAS’s thermometer is off by 1 or 2 degrees in either direction, you’ll end up with a more expensive electricity bill – and a bunch of unhappy occupants as well!
- Inspect your ice tanks. Setting the ice termination set point lower than what is stated in the manufacturer’s performance data can cause the water in the expansion area above the heat exchanger to freeze. It is important this water does not freeze so that it is available to fill voids created by the melting ice during discharge. Therefore, twice a year the tank should be checked for excessive ice buildup above the top heat exchanger tube. For some manufacturers, damage caused by total freezing of ice tanks is not covered in the warranty. CALMAC, the manufacturer of the IceBank energy storage system, has a limited warranty that covers issues from complete frozen solid tanks. Still there is good reason to avoid a complete freeze, namely that the excess ice is not usable because it’s too far from the heat exchanger. Avoiding a complete freeze can save energy in the long run. Check to make sure the ice tanks have the proper amount of water and add it if necessary. Algae can and does grow in dark cold places so treat the water with a biocide as recommended by the manufacturer to eliminate biological growth.
- Make sure your air cooled chiller condenser coils are clean. Dirty condenser coils perform heat transfer far less effectively, reducing capacity, efficiency, taxing your system, and taking a bite out of your budget. Clean off dirt and grime, inspect the chiller evaporator barrel and insulation, and look for any puddling or oil stains that may indicate a bigger problem.
- For water-cooled chiller plants, clean the cooling tower. Dirty cooling towers are another source of inefficiency – these need to be cleaned every year. A dirty cooling tower results in warmer condenser water temperatures which increased energy usage from the chiller and ice tank system. Plus for health and safety purposes, make sure the cooling tower water is treated with the appropriate chemicals.
- Inspect your pumps and change your air handler filters. Dirty air-handler filters impede air flow and create tremendous inefficiencies. Check AHU coils for leaks, drive belts for wear and tear, and control valves for unwanted movement. And service your pumps as well to make sure they are not leaking; even the smallest leaks can be very costly.
- If your system uses glycol, take a sample and send it to the manufacturer for analysis. You want to ensure that you are circulating the right concentration of glycol: if the concentration is higher than it ought to be, it increases your pumping energy consumption, and if you use too little, it raises your freeze point which could put your chiller in danger of freezing the evaporator. Your glycol manufacturer will examine the amount and health of your glycol, and can suggest inhibitors and buffers to improve your system, as needed. Just ask them for a sample kit and mailer.
The thermal energy storage system is similar to an ordinary chiller plant; it is remarkably simple to operate, but is enhanced with more flexibility for greater control of energy costs. The ice tanks themselves have no moving parts so their maintenance is minimal. The trick is to look at the holistic maintenance and operation of the entire system. With these steps and insights, you will be on your way to cooler and cost-efficient summer.