**2. Relevance of This Work**

Most of the previous studies regarding EESS and HESS have focused on the characteristics of each energy storage system separately. Some studies have focused on the study of the performance of ultracapacitors [14–17] and others have focused on hydraulic accumulators. [18,19], but very little research has been done to compare the two storage systems (hydraulic accumulators and ultracapacitors) side by side. The main purpose of this study was to have a direct comparison of the performance characteristics of ultracapacitors and hydraulic accumulators when used as energy storage devices. Previous studies concerning EESS and HESS for hybrid applications have not compared the benefits and drawbacks of both systems under similar operating conditions; these studies have been focused on each system individually. For this work, an experimental procedure was developed for measuring the charging and discharging cycles of a hydraulic accumulator. Likewise, a test bench using ultracapacitors of similar energy storage capabilities was tested. The estimated energy capacity of each system was modeled with the equations shown in the following section. The experimental procedure and experimental equipment are also described in that section.

The results of this research can be used to stablish a control strategy to optimize systems that use hydraulic accumulators as energy storage systems or as a design strategy to select one over the other.

### **3. Test Bench Description**

The main purpose of the test benches developed in this study was to compare an electrical and a hydraulic energy storage system under similar operating conditions in order to determine the efficiency, power density, energy density, and cost by energy capacity. The main characteristics of the hydraulic accumulator and the ultracapacitor used are shown below in Tables 1 and 2.


**Table 1.** Characteristics of the hydraulic accumulator used in this study.

**Table 2.** Characteristics of the ultracapacitor used in this study.


To compare both devices in a similar way, two test benches were designed and built. Both test benches were designed to measure charge and discharge response of the systems, which in a hydraulic system are the pressure and flow rate and in the electric system are the voltage and current.
