# Automated compressor surge recovery with cold air bypass in gas turbine based hybrid systems

David Tucker, Lawrence Shadle, Nor Farida Harun

► To cite this version:

David Tucker, Lawrence Shadle, Nor Farida Harun. Automated compressor surge recovery with cold air bypass in gas turbine based hybrid systems. 17th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery (ISROMAC2017), Dec 2017, Maui, United States. hal-03004937

HAL Id: hal-03004937

https://hal.science/hal-03004937

Submitted on 13 Nov 2020

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Automated compressor surge recovery with cold air bypass in gas turbine based hybrid systems

David Tucker<fnref n="1" />*, Lawrence Shadle<fnref n="1" />, Nor Farida Harun<fnref n="1" />

<figure><img src="image_2.png" /></figure>

ISROMAC 2017

International
Symposium on
Transport

Phenomena and
Dynamics of

Rotating Machinery

Hawaii, Maui

December 16-21
2017

## Abstract

Compressor surge and stall is one of the main operational challenges in solid oxide fuel cell gas turbine (SOFC/GT) hybrid systems. The problem arises because of the added large volume between compressor and gas turbine and resultant changes to system fluid dynamics. The compressor stall and surge and its recovery were characterized at different transient states in the SOFC/GT hybrid system. In this work, the Hybrid Performance Project built at the U.S. Department of Energy, National Energy Technology Laboratory was used to perform the experimental tests. The bypass of cold air dramatically increased the compressor surge margin in the hybrid system. The effects of ramping the cold air valve from 25% to 50% open and from 50% to 30% on the compressor map were evaluated to show the resulting pressure and airmass flow dynamics. An automated compressor surge recovery was demonstrated using a cold air bypass strategy at nominal speed and for emergency shutdown. The study shows that the actuation of cold air bypass helps improve the compressor surge margin and prevents stall. The surge recovery in the hybrid system was successful because bypass of the compressed cold air into the turbine inlet modified the system pressure and reduced the air flow transients.

## Keywords

Compressor Surge, Hybrid Power, Gas Turbine, Cold Air Bypass, Surge Margin

## INTRODUCTION

The recuperated gas turbine cycle is currently the most efficient way to convert thermal energy to electric power. Advanced power systems of the future will require hybridization of gas turbine with other technologies to realize efficiency and flexibility goals [1]. The coupling of a solid oxide fuel cell (SOFC) and a micro gas turbine is an example of hybrid system where the fuel cell works as thermal source for the gas turbine, replacing the combustion chamber in a typical cycle. In this way, high-grade heat from the fuel cell can be converted to additional electric power by the gas turbine, reaching total electric efficiency around 70% on natural gas LHV [2, 3].

In this type of system, the air is extracted from the compressor to feed the cathode side of the SOFC. The volume of a hybrid requires two orders of magnitude more than the compressor plenum volume of a simple cycle, which results in a degradation of compressor surge margin and complicated compressor dynamics [4, 5]. When turbine rotational speed is reduced during transients, compressor inlet airflow and pressure ratio decrease at different rates because of the large volume of the system. In this case, the operating point on the compressor map can follow a path toward the stall line. Drastic changes in turbine load or fuel or sudden heat absorption in the fuel cell stack, for example, can thus lead the compressor operation close to stall and surge conditions if not mitigated [6].

Compressor surge is not only detrimental for the turbomachinery, but also extremely harmful for the fragile SOFC electrolyte. Hence, compressor dynamics need to be adequately controlled, and a safe emergency system needs to be implemented on this type of power plants to avoid surge during sudden maneuvers such as emergency shut down [6].

The challenges illustrated and discussed are applicable to any gas turbine based hybrid system where a substantial volume is added between compressor and expander, such as concentrated solar gas turbine hybrid plants, integrated thermal energy storage gas turbine system, and integrated geothermal gas turbine hybrids.

## 1. METHODS

An automated process has been developed at the U.S. Department of Energy, National Energy Technology Laboratory (NETL) to implement compressor stall/surge recovery or prevention in gas turbine hybrid power systems. This process makes use of a cold air (CA) bypass valve to divert compressor flow directly into the turbine inlet, and it was tested using the Hybrid Performance (Hyper) facility at NETL, shown in Figure 1 [7, 8]. The CA valve has the effect of simultaneously lowering both the turbine inlet temperature and system pressure drop. It is the sudden decrease in system pressure drop and the corresponding increase in compressor inlet flow that increase the surge margin.

<footnote n="1">¹U.S. Department of Energy, National Energy Technology Laboratory, Morgantown, WV, U.S.A.</footnote>

<footnote>*Corresponding author: David.Tucker@netl.doe.gov</footnote>
<figure><img src="image_3.png" /><figcaption>Figure 1. Hybrid Performance Project Facility Flow Diagram</figcaption></figure>

The Hyper facility features a unique combination of hardware and software to emulate an SOFC gas turbine hybrid system. The turbomachinery hardware consists of a 120kW Garrett Series 85 auxiliary power unit (APU). The APU consists of single shaft, direct coupled turbine, a two-stage radial compressor, and gear driven 120 kW generator. Two counter flow heat exchangers, Solar Turbines Primary Surface Recuperators (PSR33), are used to preheat the air going into the fuel cell cathode volume.

The radial compressor is resistant to stall and surge in its normal configuration. However, integration with the hardware emulating the fuel cell increases the plenum volume (normally used to distribute cooling air around the turbine scroll) by 200-fold, mainly due to the cathode, air distribution manifold, and post combustor volumes inserted between the compressor and turbine. The CA valve bypasses the exhaust gas recuperators and the SOFC stack, diverting air from compressor discharge to turbine inlet, highlighted by the yellow boxes in Figure 1.

In the illustration shown in Figure 1, the Hyper facility is divided into three regimes, physical hardware, cyber-physical systems, and virtual components. The combination of these three approaches emulates the cycle shown on the right side of Figure 1. The virtual components are simply distributed numeric models capable of resolving calculations in real-time. For the Hyper facility, real-time is defined as 5 ms. The physical hardware used is made up of the APU, the exhaust gas recuperators, the generator and load bank.

The cyber-physical system is used to emulate fuel cell performance using hardware so that coupling can be captured and control strategies can be tested for a fully integrated system. The cyber-physical fuel cell is made up of pressure vessels and

piping to physically represent cathode volume and associated flow impedence, and a burner to emulate the thermal effluent from the fuel cell system. The hardware of the cyber-physical system is shown with solid lines in Figure 1, and the computational models are shown with dashed lines.

As shown by “Real-Time SOFC System Models” in Figure 1, a 1D distributed SOFC model takes vital user or model inputs as well as sensor inputs from the hardware to calculate changes in the SOFC states at each time step. The calculated thermal effluent is then fed to a reverse fuel valve model for feed forward modulation of the fuel valve to provide coupling with the turbomachinery. The interface and middleware system indicated by the dashed yellow boxes is made up of a dSpace system and CAN bus that communicates with the control platforms.

Overall, any changes to the compressor or recuperator outlet flows directly affect the cathode inlet flow and the fuel cell performance calculations, which in turn affect the turbine and compressor. This creates a cycle in which system coupling can be physically represented and evaluated. For the work presented herein, only the surge margin degradation due to the increase volume between the compressor and turbine is considered.

The results from three experiments are shown in this work. In the first case, the Hyper facility was brought up to nominal turbine speed conditions; the cold air bypass was opened from 25% to 50% over a 10 second ramp period to avoid drastic dynamics associated with step changes; after three minutes, the cold air bypass was reduced from 50% to 30% over a 10 second ramp period, and the equipment was shutdown. The process responses to this case are illustrated in Figure 2, in which both the turbine speed and CA valve positions are shown as a funciton of run time.
The first case is used to illustrate the effect of cold air bypass manipulation on compressor surge margin without compressor stall and surge in a quasi-steady manner. In this work, surge margin is defined as the distance along the constant speed line of the operating point (generally constant pressure ratio) to the compressor surge line.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Cold Air Bypass Position and Turbine Rotational Speed during Startup and Shutdown</figcaption></figure>

The other two cases studied were recorded during actual surge events where the CA valve was opened in a step change via a “blue button” in order to recover from surge and avoid turbomachinery damage or destruction. The “blue button” is a manual activation of the cold air bypass from the current setting to 55% open. In both of these cases, the CA valve started at 40% and was opened to 55% in a single step. Actuation of the CA valve was automated through the Hyper facility control system as soon as the operator pressed the blue button.

Pressures were measured at a frequency of 12.5 Hz using pressure transducers. Turbine speed measurements were performed by using triple redundant optical sensors and a pulse train transmitted to the frequency input of the control system at a 200 Hz sampling rate. Compressor inlet flow measurements were made either by a mechanically averaged annubar flow element at 12.5 Hz or a hot-wire anemometer at 200 Hz. Because of the large effective compressor plenum volume, flow changes occur significantly faster than pressure changes.

## 2. RESULTS AND DISCUSSION

### 2.1 Case 1: CA Modulation without Stall

Figure 3 shows the process outlined in Figure 2 between the two vertical dashed lines, from 325 seconds to 565 seconds. The temporal process is indicated chronologically by the arrows and dashed lines in Figure 3. The green line indicates the startup ramp from about 31,000 RPM. A significant increase

in the compressor surge margin was observed when the CA valve was opened, as indicated by the black dashed line. Upon reducing the CA valve, surge margin was reduced by a similar magnitude.

Looking at a small portion of the compressor map in Figure 3 indicated by the black arrow, “CA Open,” the data points associated with the opening of the CA valve are plotted in Figure 4. The blue line with arrows represents the chronological sequence of the data. A reduction in the compressor inlet flow leads the transient. Although there is significant scatter in the data, this is followed by a much smaller but concurrent reduction in the compressor pressure ratio as flow increases. The net effect is a substantial increase in compressor surge margin.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Compressor Map of Outlined Section Figure 2 from 325 s to 565 s</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Compressor Map of CA Opening from 25% to 50%</figcaption></figure>

### 2.2 Case 2: Surge Recovery during Shutdown

A compressor surge event is shown during emergency shutdown in Figure 5. Compressor inlet flow and turbine speed fluctuate dramatically during the stall/surge cycle. The pre-stall compressor mass
flow is shown in green, while the flow after the initial stall is shown in orange. The data encompassing the surge cycles is shown in more detail in Figure 6.

The “blue button” was engaged by the operator at the peak of the fourth cycle, and recovery of compressor flow was accomplished. As the turbine speed reduced to almost half its initial velocity, the magnitude of the compressor flow transients during the surge remained quite strong. In this set of data, the fuel to the system was cut at around 834 s, and it was this event that initiated compressor stall and surge. In this case, the turbomachinery could have been easily destroyed.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Compressor Mass Flow and Turbine Rotational Speed During an Emergency Shutdown</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Compressor Mass Flow and Turbine Rotational Speed After Cutting Fuel</figcaption></figure>

The data in Figure 5 is plotted on the compressor map in Figure 7. Again, the data before the first stall is shown in green, while the rest is shown in orange. The point at which the CA valve was activated is indicated by the “blue button” point on the figure. After actuation of the valve, the compressor flow recovered to a normal ramp down. Since the fuel had already been cut, the cold air bypass and compressor bleed were the only options available. While compressor bleed can effectively increase surge margin, it also puts excessive torque on the shaft while this is avoided using the CA bypass valve.

<figure><img src="image_9.png" /><figcaption>**Figure 7.** Data Shown in Figure 5 Transposed on to the Compressor Map. The data of compressor mass flow was collected using a hot-wire anemometer at 200 Hz.</figcaption></figure>

## 2.3 Case 3: Surge Recovery at Nominal Speed

During an electrical load transient at nominal speed, the compressor went into a stall and surge. The operator was able to activate surge recovery at the first indication of stall. The results are shown in Figure 8. The compressor stalled when a load on the electric generator of the turbomachinery was increased from 40 kW to 50 kW.

The excursion across the stall line is shown in Figure 8 from point 1, the nominal operating condition, to point 2. This was followed by a surge to point 3. The automated system opened the CA bypass at 55% once the surge recovery button was hit at point 4 in Figure 8. As a result of the immediate reduction in system pressure drop and increase in compressor inlet flow, compressor flow and turbine speed were recovered and the new operating nominal point was achieved. Upon activation of the CA valve, compressor flow was increased, and the surge cycle was stopped, preventing the next stall. The dynamic action was observed as the point 5 was realized before the systems returned to a steady state again at point 6.

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Chronological Illustration of Surge Recovery on the Compressor Map. The compressor mass flow was measured using an annubar flow sensor at 12.5 Hz.</figcaption></figure>
## 2.4 Summary

The dynamic of the cold air valve can realize a two-fold increase in surge margin over the steady state gain. This means that only small, continuous changes need to be made. The result is that compressor stall can be avoided during dynamic changes in the system performance, enabling smooth transients and wide swings in load adjustments.

## ACKNOWLEDGMENTS

This work was funded by the U.S Department of Energy Crosscutting Research program in The Office of Fossil Energy.

## REFERENCES

[1] Traverso, A., Magistri, L., and Massardo, A. F., "Turbomachinery for the air management and energy recovery in fuel cell gas turbine hybrid systems," Energy, Volume 35, Issue 2, 2010, pp. 764-777.

[2] Wang, J., Yan, Z., Ma, S., and Dai, Y., "Thermodynamic analysis of an integrated power generation system driven by solid oxide fuel cell," International Journal of Hydrogen Energy, Volume 37, Issue 3, 2012, pp. 2535-2545.

[3] Bakalis, D. P. and Stamatis, A. G., "Incorporating available micro gas turbines and fuel cell: Matching considerations and performance evaluation," Applied Energy, Volume 103, 2013, pp. 607-617.

[4] Greitzer EM. Surge and Rotating Stall in Axial Flow Compressors—Part I: Theoretical Compression System Model. ASME. J. Eng. Power. 1976, Volume 98, Issue 2, pp. 190-198, doi:10.1115/1.3446138.

[5] Ferrari, M. L., Silvestri, P., Pascenti, M., Reggio, F., and Massardo, A. F., "Experimental dynamic analysis on a T100 microturbine connected with different volume sizes," Proceedings of ASME. Turbo Expo: Power for Land, Sea, and Air, Volume 3: Coal, Biomass and Alternative Fuels; Cycle Innovations; Electric Power; Industrial and Cogeneration Applications; Organic Rankine Cycle Power Systems, Charlotte, NC, USA, June 26 - 30, 2017, doi:10.1115/GT2017-63579.

[6] Pezzini, P., Tucker, D., and Traverso, A., "Avoiding compressor surge during emergency shutdown hybrid turbine systems," ASME. J. Eng. Gas Turbines Power, 2013, 135(10), pp. 102602-102602-10, doi:10.1115/1.4025036.

[7] Li, Y., Weng, Y., and Weng, S., "Part-load, startup, and shutdown strategies of a solid oxide fuel cell-gas turbine hybrid system," Front. Energy, Volume 5, Issue 2, pp. 181-194.

[8] Zaccaria, V., Tucker, D., and Traverso, A., "Cold-air bypass characterization for thermal management of fuel cell gas turbine hybrids," ASME. J. Eng. Gas Turbines Power. Volume 139, Issue 6, 2017, pp. 062701-062701-8, doi:10.1115/1.4035396.

[9] Pezzini, P., Celestin, S., and Tucker, D., "Control impacts of cold-air bypass on pressurized fuel cell turbine hybrids," ASME. J. Fuel Cell Sci. Technol., Volume 12, Issue 1, 2015, pp. 011006-011006-8, doi:10.1115/1.4029083.