Flexible heat production from a micro gas turbine: design and experimental analysis of humidified air cycles

Student thesis: Doctoral Thesis

Abstract

Micro Gas Turbines (mGTs) offer several advantages for small-scale Combined
Heat and Power (CHP) production compared to Internal Combustion
Engines (ICEs) such as low vibration level, cleaner exhaust and lower maintenance.
The major drawback is their heat-driven use. In periods with
no or low heat demand, part of the heat should be discarded to keep the
mGT operated at high electric load. Compared to ICEs, the lower electric
efficiency of mGTs makes them less attractive in this case and economic
constraints could even lead to complete shutdown. In addition, the specific
capital cost of the mGT is still high, which involves any shutdown having a
severe negative effect on the economic performance of the mGT.

In order to increase the flexibility of the mGT and shift its use towards
various heat and power demand profiles, the waste heat in the exhaust gases
can be used to generate auto-raised steam or hot liquid water, which is then
re-injected in the cycle. Humidifying the mGT working fluid will increase
the electric performance, resulting in higher profitability during periods
with no or low heat demand.

The scope of this thesis is the development of such a humidified mGT
cycle for flexible heat production. This thesis covers the entire development
process of a micro Humid Air Turbine (mHAT) cycle based on a Turbec
T100 mGT, including the dry and wet characterisation, the development of
the optimal cycle layout and water injection system and finally the test rig
construction and experimental evaluation.

For the development of a humidifi ed mGT, the dry and wet operation of
the available Turbec T100 mGT needed to be fully characterized. For
accurate wet simulations of the mGT, correct compressor maps are essential.
The compressor map was reconstructed by performing dry and wet (with
steam injection) mGT tests. Additional steam injection experiments were
performed to characterize the behaviour of the mGT in wet conditions.
Steam injection experiments resulted in stable mGT operation at reduced
rotational speed and pressure ratio, and increased electric efficiency. Finally,
the effect of water on the combustion of natural gas (Lean Blowout (LBO)
limit) was experimentally studied in an atmospheric, premixed, variable swirl
burner.

In the next step towards the humidified mGT, the optimal cycle for water
introduction in the mGT could be determined by using black box analysis
in combination with composite curves. Direct injection of heated water was
identified as the optimal cycle, resulting in an absolute efficiency increase
of 4.4%. Transforming the existing T100 mGT into a mHAT results in a
lower electric efficiency increase of 2% absolute, but requires less changes
to the T100 mGT. Therefore, the mHAT was selected as final humidified
mGT layout.

Transforming the T100 mGT into a mHAT requires the development of a
saturation tower. To reduce the pressure drop, a new type of saturation
tower, a spray saturation tower, was developed, using two-phase
ow theory.

Cross-current injection was identified as the optimal injection method for a
spray saturation tower for mHAT applications. Based on these simulations,
a cross-current spray saturation tower was developed and integrated in the
T100 mGT cycle.

Experiments on the modified Turbec T100 mGT were performed to evaluate
the test rig and identify its shortcomings. Using a specific developed wet
start-up and shutdown procedure, the effect of humidifying the compressed
air on the mGT performances was studied experimentally. These tests
resulted in stable operation at constant power output and reduced fuel

ow rate, resulting in absolute electric efficiency increase of 1.0 1.8%,
1.1 1.8% and 0:8 1:8% at power levels of 80, 85 and 90 kWe. These
changes are however in the range of the accuracy on the measurements.
Additionally, the electric efficiency was below the predictions from the
simulations results (2%), which indicated that the compressed air was not
yet fully saturated. These experiments however indicated the beneficial
effect of compressed air humidification on the mGT performance. Further
optimizing the mHAT test rig operation will heighten the nal electric
efficiency increase.
To conclude, the proposed route for waste heat recovery through water
injection in a mGT cycle offers high potential for electric efficiency increase
without major changes to the mGT cycle.
Date of Award26 Nov 2014
Original languageEnglish
Awarding Institution
  • Vrije Universiteit Brussel
SupervisorJacques De Ruyck (Promotor) & Svend Bram (Co-promotor)

Keywords

  • gas turbine
  • CHP
  • evaporation
  • humidified
  • microturbine

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