In last month’s article, we described the rationale for using thermal energy storage (TES) to reduce peak electrical demand costs. In this month’s article, we will go further into the calculations required for sizing as well as some design considerations and heat transfer media.
There are two basic TES strategies: partial load shift and full load shift. There are three options for the storage medium: chilled water, ice storage or hybrid heat transfer.
Strategy Option #1:
Partial load shift is when your goal is to reduce a part of your peak load by running the chiller at nearly constant output for 24 hours per day. The idea is best illustrated by the graph below. In this example, , the chiller’s cooling output from midnight to 8 am is greater than the building’s load, so the excess cooling is stored in a tank. This cold energy is then used to supplement the chiller during the day whenever the building load is greater than the chiller’s output, which is indicated by the red line in the graph below. This strategy is also called load leveling.
To determine the constant load that the chiller will operate, we need to determine the total number of cooling ton-hours and then divide by 24 hours in a day. In the example above, there are 14,000 ton-hours of cooling required during an entire day. Thus, 14,000 ton-hours / 24 hours:
= 583 tons is the rate that the chiller runs constantly.
A load-leveling strategy is usually ideal for new construction because if you can estimate the building loads, you can purchase a smaller chiller (600 tons versus 1,000 tons).
Strategy Option #2:
Full load shift is when your goal is to eliminate your chiller’s peak load by running your chiller at high output during non-peak hours and storing its energy as cold water/ice. As the graphic below shows, during the daytime, when the utility rates are highest, you turn off the chiller and utilize the cold storage alone to cool the building.
Using the example above, if the peak electrical demand rate period is from noon to 8 pm, we would want to minimize electrical use during that time, so we would shut off the chiller during that window. To determine the load that the chiller will run during the storage periods, we must remember that we now have only 16 hours per day to run the chiller. During the storage periods, we must make enough cold storage (and probably a little more to have a surplus) to coast through the peak periods of the day. Thus, we divide the total number of cooling ton-hours by 16:
= 14,000 ton-hours / 16 hours:
= 875 tons is the rate that the chiller runs during the storage periods.
A load-shifting strategy is usually ideal for retrofits because if you already have a large chiller (1,000 tons in the example above), you can use the large chiller to obtain a greater peak savings.
Storage Media: Ice vs Chilled Water
Chilled water can store 1 BTU per pound of energy. Systems are easy to set up because most chillers are already good at making cold water.
There is a space-saving advantage of using ice storage because the phase change can store or release 144 BTUs per pound (when ice changes to water and vice versa). You have to weigh this advantage of smaller storage tanks against the chiller modifications required to actually make ice. Some companies choose to make “slush” or use eutectic salts and other media for different types of applications. For these options, you will want to consult with TES experts who can advise on the technical merits of each approach.
Sizing TES tanks
Let’s assume we chose chilled water as a storage media and we wanted to do use load shifting strategy, as illustrated by the graphic that has the building cooling load shaded as a green color. In that example, the green area represents the cooling load during the peak period amounts to:
= (1000 tons)*(4 hours) + (750 tons)*(4 hours)
= 7,000 ton-hours
Converting tons into BTU/hour (1 ton + 12,000 BTU/hour):
= (7,000 ton-hours)*(12,000 BTU/hour)
= 84,000,000 BTU
= 84 MMBTU needs to be stored
If we have a chiller that takes 55 degree F. water and makes it 40 degrees, then our delta T is 15 degrees. Remembering that a 1-degree water temperature change represents 1 BTU per pound of water, then a 15-degree delta T means that each pound of water has 15 BTUs of storage/release capacity. To determine the amount of water required, we simply divide the total BTUs required by the 15 BTUs/pound:
= (84 MMBTU) / (15 BTU/pound)
= 5.6 million pounds of water can store 84 MMBTU (when delta T = 15)
Converting from pounds to gallons (1 gallon of water weighs 8.34 pounds):
= 5.6 million pounds * (1 gallon / 8.34 pounds)
= 671,462 gallons of water
This volume of water is comparable to an Olympic-size swimming pool. It may be hard to imagine how you could install that volume on your premises. However, as the pictures above show, chilled water systems do exist in some high profile venues, such as Dallas/Fort Worth International Airport, which has a TES system that stores 6 million gallons and 15 MW.
For a comparison, you can find ice storage tanks as shown in the photo above (4,700 tons of storage), which is at the National Air and Space Museum. This is a much smaller footprint, but obviously is more complex, because it uses ice.
When investigating a TES, there are many factors to consider, including:
- The amount of space available in a basement, outside or below ground.
- The availability of contractors that could service the system.
- The availability of utility rebates in your area for TES systems.
The answers to these questions should help you to determine the feasibility and potential of TES for your facility.
Eric A. Woodroof, Ph.D., is the Chairman of the Board for the Certified Carbon Reduction Manager (CRM) program and he has been a board member of the Certified Energy Manager (CEM) Program since 1999. His clients include government agencies, airports, utilities, cities, universities and foreign governments. Private clients include IBM, Pepsi, GM, Verizon, Hertz, Visteon, JP Morgan-Chase, and Lockheed Martin. In August 2014, he was named to the Association of Energy Engineers (AEE) Energy Managers Hall of Fame.