Complimentary Battery Energy Storage In Inverter-Based Microturbines And Fuel Cell Systems
Abbas Akhil and Tom Byrd, Sandia National Laboratories, Albuquerque, NM1 Fuel cells and microturbines are emerging as technologies of interest for distributed generation applications. Plug Power (Latham, NY), manufacturer of 5-kW residential-sized PEM fuel cell systems, and Capstone Turbine (Chatsworth, CA), manufacturer of 30-kW and 60-kW microturbine generators, are two commercial suppliers that offer distributed generation systems. The prime power sources for the systems (the PEM fuel cell stack and its reformer and the microturbine permanent magnet generator) have finite output power ramp rates that are sometimes not sufficient meet large, step load changes. When operating in grid-connected mode, the grid supplies additional power as necessary to compensate for large changes in load and to provide power during the time it takes for the fuel cell or microturbine to come online (ramp time). When operating in standalone mode both technologies use on-board battery packs to contribute electrical energy to improve ramp rates and to provide energy for starting the prime power source. Presently a fuel cell system from PlugPower and a Capstone microturbine system (see Figure 1) are under test at Sandia National Laboratories’ Distributed Energy Technologies Lab (DETL) to characterize their performance in response to various step load changes when operating in standalone mode. This data will be compared to responses for similar load changes in grid-connected mode to isolate the contribution of the battery storage component in improving the response characteristics of the systems.
Figure 1. PlugPower 5-kW fuel cell (left) and Capstone 28-kW microturbine (right).
The Capstone system (see Figure 2) includes a Model 330 28-kW microturbine and an inverter that converts the system’s ~1100 Hz output current to 760 VDC, which is then rectified to a usable 480 VAC at 50/60 Hz. The system also includes, as mentioned above, a Hawker valve-regulated lead-acid (VRLA) battery pack (see Figure 3), which consists of 18 12-V batteries with a total storage capacity of 2.8 kWh (13 Ah × 216 V at a 10-hr rate). In normal usage, the battery operates at ~80% state-of-charge (SOC), or ~256 VDC, and shuts down automatically when the SOC is less than ~ 5 to 10% (approximately 20 minutes at full load). Battery equalization to 100% SOC is performed at pre-programmed intervals and takes ~4 hours. During periods of extended non-use, the battery can be placed in a sleep state that will skip the pre-programmed equalization. The
battery must be at 60% SOC to operate at its rated load and must be at 200 VDC to start the motor on the microturbine without assistance from the grid (‘black’ start).
Figure 2. Capstone 28-kW microturbine system.
Figure 3. Microturbine battery pack.
To evaluate and quantify the contribution of the on-board battery pack, the system was evaluated in both grid-connected and stand-alone modes. To determine the effect of the battery on response time and load management, various loads were applied to or removed from the system at six-minute intervals with the system in grid-connected mode (see Figure 4). The system responded as expected with a fairly fast ramp up from 0 to 24 kW. When operating in this mode, the average ramp time took less than 1 minute at a ramp rate of 580 W/sec (see Figure 5). For smaller load steps the response was almost instantaneous. The system was then evaluated in standalone mode using only the battery to maintain the output voltage. In this mode the ramp time was reduced to less than one second and the ramp rate was increased by a factor of 9, from 580 W/sec to
4700 W/sec (see Figure 6). Further, it took less than half a second for the output voltage and current (AC and DC) to achieve the necessary levels once the system was brought online (see Figure 7).
Figure 4. Microturbine response to various loads when in grid-connected mode.
Figure 5. Microturbine ramp up from 0 to 24 kW in grid-connected mode.
Figure 6. Microturbine ramp up from 0 to 24 kW in standalone mode.
Figure 7. AC and DC output voltage and current when microturbine generation is online in standalone mode.
The PlugPower fuel cell system (see Figure 8) is designed for residential use and includes a Model SU01 5-kW PEM fuel cell, an inverter that converts the fuel cell’s DC output to AC at 50/60 Hz, and the on-board battery pack (see Figure 9). The battery pack for this system is a Power Battery VRLA module consisting of 4 12-V cells with 4.4 kWh of storage (91 Ah × 48 V). Ramp time and step load response performance will be evaluated for this system using a test strategy similar to that used for the microturbine system. The system’s response time and load management will be evaluated for both grid-connected and standalone operating modes to determine
the component of the system power that the battery provides. It is expected that the use of battery storage will enhance the performance of the fuel-cell system in much the same way as it did for the microturbine system. Additional work will be done to characterize proper battery management for the system.
Figure 8. PlugPower fuel cell system.
Figure 9. Fuel cell system battery pack.
It seems evident that the addition of storage to distributed generation systems that use fuel cells or microturbines can greatly improve these systems’ ramp times. The addition of storage also allows black start and cool down of the generation resource if necessary. To effectively use these distributed generation resources in any standalone load-following application will require the use of storage in order to ensure that the response of the system to power fluctuations is transparent to the user.
