Method for Analyzing the Value of Distributed Energy Storage at the Facility Level – Step 4A: Estimating Energy and Power Savings from Thermal Energy Storage.

Method for Analyzing the Value of Distributed Energy Storage at the Facility Level – Step 4A: Estimating Energy and Power Savings from Thermal Energy Storage.

Now that we have gone through the entire evaluation method for electrical energy storage devices, let’s step back and look at how thermal energy storage devices could be deployed to help reduce energy and power consumption. This will allow for a side by side comparison of these very different technologies. Figure 1 shows the step in the evaluation process to consider thermal energy storage.

Figure 1 Showing the current step (step 4A: Estimating Energy and Power Savings from Thermal Energy Storage) in the methodology for evaluating a facility level energy storage deployment.

For this step in the analysis we will consider Ice Energy’s, Ice Bear (see previous posting on Thermal Energystorage for a brief overview of the Ice Bear). Though Ice Energy’s Ice Bear is a thermal energy storage device, Ice Energy provides a means of estimating the electrical power and energy reductions realized by deploying an Ice Bear. This is done so that facilities managers can consider the Ice Bear alongside electrical energy storage devices. The simple metric is that a single Ice Bear unit can be applied to a 3-5 Ton AC system or a single 5-ton stage of a 7.5-20 Ton system (Ice Energy, 2012). Each Ice Bear unit would provide a 7 kW reduction in peak power demand and a total of 35 kWh of energy shifted to off peak (Ice Energy, 2012).

The first step in evaluating the potential for an ice energy storage system to be retrofitted onto existing air-conditioning units is to understand the size and operating conditions of the existing air-conditioning units. This is first  done in order to size the deployment of thermal energy storage devices. Other information about the existing system can also be helpful. For instance, in the example of the software company, it was helpful to know that there were AC units dedicated to the mission critical data centers in the building. Beyond energy and power saving, deploying Ice Bear units might have the potential to improve the reliability of the AC units themselves, further adding value to a potential thermal energy storage project. It was also discovered that these AC units have the ability to “run free”, or  use cold outside air during the late fall, winter and early spring to provide cooling. Below a certain temperature the compressor and condenser are turned off and outside air is used for cooling. Understanding the temperature at which this switch to “running free” happens is another valuable piece of information. Though the software company is located in New England, the air conditioners are used to cool occupied space almost every day of the year. However, the compressors and condensors are used for cooling only on days when the outside temperature is above the “run free” temperature.

For the purpose of this exercise we will say that using outside air for cooling occurs when the outside specific base temperature is 62°F or less.  During the rest of the year, the energy intensive compressor and condenser are engaged to produce air that is cooler than the outside air. It is during this time (when the outside specific base temperature is above 62°F) that the Ice Bear unit would provide benefit. In estimating the annual benefit of an Ice Bear unit, the duration of the interval requiring the condenser and compressor must be understood. This interval was determined by counting the number of days when there is at least one cooling degree day above the specific base temperature of 62°F. “"Cooling degree days", or "CDD", are a measure of how much (in degrees), and for how long (in days), outside air temperature was higher than a specific base temperature” (BizEE, 2013). For the software company location, from 11/19/2011 to 11/19/2012 there were a total of 1410.3 cooling degree days. During this time, there were 158 days with one or more cooling degree days (Figure 32) (BizEE, 2013). See http://www.degreedays.net/ for one of the many tools out there for calculating cooling degree days.

Figure 2 showing the cooling degree days above the specific base temperature of 62°F (BizEE, 2013).

Using the above estimation, at the location of the software company, each Ice Bear would reduce the peak power by 7 kW and time shift 35 kWh of energy 158 times a year. Though there are some days from November to March that have one or more cooling degree days, it is unlikely that the Ice Bear unit would be engaged on those somewhat anomalous days. With this in mind, the peak power reduction would only occur from April to October (7 months). In those 7 months the peak power is reduced by 7 kW per Ice Bear. The demand charge savings is found using the following equation:

When combined with an AC unit, the Ice Bear surpasses the overall efficiency and performance of the AC unit alone. At night when temperatures are low and thermal efficiency is high, the integrated high-efficiency AC condensing unit is operated to produce ice. During the day, the unit offsets the operation of the energy-intensive commercial AC condensing unit when temperatures are high and efficiency of the AC unit is at its worst. For this reason Ice Energy declares the Ice Bear to be “loss-less”, allowing it to essentially shift 35 kWh of energy consumption from the daytime peak to nighttime off peak hours. For this reason, the Ice Bear essentially has 100% efficiency. Unlike the LightSail RAES V1 and the VRB-ESS®, the demand charge savings do not need to be corrected for additional charge phase energy consumption. The reduction in energy consumption and the associated savings is simply peak energy consumption – 35 kWh per day of operation. From this point LCC should be used to determine the value of deploying a thermal energy storage device. An evaluation of the environmental impacts would follow a methodology similar to what was done for electrical energy storage devices.

Works Cited

BizEE. (2013). Degree Days.net - Custom Degree Day Data. Retrieved February 17, 2012, from Degree Days.net: http://www.degreedays.net/

Ice Energy. (2012). Product Sheet; Ice Bear Energy Storage. Windsor, CO: Ice Energy.