Method for Analyzing the Value of Distributed Energy Storage at the Facility Level – Step 5: Accounting for Round Trip Efficiency

Figure 1 shows the current step in the evaluation methodology…

Figure 1 Showing the current step (step 5: accounting for round trip efficiency) in the methodology for evaluating a facility level energy storage deployment.

Because the round trip efficiencies of each storage device is less than 100%, more energy will be consumed during the charge phase than will be delivered during discharge. This will increase the overall energy consumed in a day. This increase in overall energy must be taken into account when determining the cost benefit of each energy storage system. The additional energy required, per day, is found using the following equation:

For the LightSail RAES V1 a round trip efficiency of 70% was used while a round trip efficiency of 85% was used for the VRB-ESS®. The additional energy costs resulting from the round-trip efficiency of the energy storage system were found using the following equation:

These additional, round trip efficiency related energy costs were subtracted from the demand charge management savings for each energy storage device to provide a more accurate estimate of overall savings. The extent to which the round trip efficiency reduces the demand charge savings depends heavily on the facility’s energy billing scheme. If peak energy costs are significantly greater than the off-peak energy costs (which is common for larger facilities) it may be possible that the demand charge savings reduction may be eliminated or reversed by the fact that inexpensive energy is being used to charge the device. Discharging the device during peak periods could, in this case, allow the device to also be profitably deployed in an arbitrage or Time Of Use (TOU) energy cost management application. The facility that housed the software company had a relatively unique billing scheme, for a facility of its size, because the same energy rate was paid during peak and off-peak periods. In that example the increased energy to overcome the round trip efficiency directly reduced the demand charge management savings.

Figure 2 showing the Load profile on peak usage day at the facility. The dashed line represents the peak that would be realized if the energy storage device were operated at full power.

Many modern energy storage devices require a charging time that is equal to or only slightly greater than the discharge interval. There are some devices which can actually charge faster than they discharge. In Figure 2, the time interval for the area below the dashed line and above the load curve represents the charging interval for the energy storage device. By visual inspection it is seen that the charging interval area is more than twice the discharge area. If the charging interval area were not clearly at least twice the discharge area it would be important to pay more attention to the duration of the charging interval. It would not be an effective energy storage device deployment if charge interval were so large that it impacted the devices ability to reduce the peak power.