A first generation open cycle Brayton engine concept for use in full scale solar module testing was defined. The concept extended to include solar/fossil hybrid capability. The combustion system defined for hybrid operation consists of a wide range combustor liner, a single airblast atomizer, an ignitor and a high-voltage ignition unit. Wide range combustor operation would be achieved through combining pilot and primary zones. The hybrid control mode and the solar only control mode are both based on the concept of maintaining constant turbine inlet temperature and varying the engine speed for part-power operation. In addition, the hybrid control concept will allow the operator to set a minimum thermal power input to the engine by setting a corresponding minimum engine speed. When the solar thermal power input falls below this minimum, fossil fuel would be utilized to augment the solar thermal power input

Elkins, R.

Six, L. D.

English

NASA Technical Reports Server

ti . N t^ U 247 74
A HYBRID BRAYTON ENGINE CONCEPT
L. Six and R. Elkins
AiResearch Manufacturing Company of Arizona
Phoenix, Arizona
ABSTRACT
A first generation open cycle Brayton engine concept for use in full-scale solar
module testing, was defined in NASA/JPL study contract DEN 3-69. This concept
has been extended to include solar/fossil hybrid capability. The combustion
system defined for hybrid operation consists of a wide range combustor liner, a
single airblast atomizer, an ignitor and a high-voltage ignition unit. Elide
range combustor operation would be achieved through combining pilot and primary
zones.
The hybrid control mode and the solar only control mode are both based on the
concept of maintaining constant turbine inlet temperature and varying the
engine speed for part-power operation. In addition, the hybrid control concept
will allow the operator to set a minimum thermal power input to the engine by
setting a corresponding minimum engine speed. When the solar thermal power
input falls below this minimum, fossil fuel would be utilized to augment the
solar thermal power input.
INTRODUCTION
A concept definition study (NASA/JPL Contract Den 3-69) defined a first genera-
tion open cycle Brayton engine for use in full scale solar module testing origi-
nally scheduled to commence late in 1980. The engine concept has been presented
at previous DOE review meetings. It is also described along with other engine
concepts and supporting study results in a final report now being readied for
release (1). This initial engine concept includes the use of an existing turbo-
compressor from the Garrett/AiResearch (Phoenix) GTP36-51 gas turbine engine to
minimize the procurement duration and costs. The engine concept was designed to
operate with solar energy only, using the solar Brayton receiver being built at
Garrett/AiResearch (Torrance) under JPL contract.
Subsequent to initiating the NASA/JPL study contract, a new requirement was
added based on the fact that JPL's Second Engineering Experiment (EE2A) would be
a hybrid system. This required adding a combustor to the engine concept to
provide for solar/fossil hybrid capability. This presentation's purpose is to
describe the combustor and corresponding controls formulated to convert the
initial Brayton engine concept to the current MOD-0 Hybrid Brayton engine con-
cept.
COMBUSTION SYSTEM
The combustion concept is presented in Figure 1 and consists of a wide range
combustion liner, a single airblast atomizer, an ignitor and a high-voltage
ignition unit. This system was chosen because it would provide simple low-cost
design features and a proven wide operation range.
107 ` ` 	 °°^^ro^4
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https://ntrs.nasa.gov/search.jsp?R=19800016281 2020-03-21T18:59:17+00:00Z
ATOMIZING
.	 AIR
fuEt
1	 "— +
AIA room	 titpl
	
OIIUTION
	
SAGES TO
SOIAA RECEIVER	 ZONE	 ZONE	 TNAINNEINLET
JAIMAAM
	 O
ATOMIZER
FIGURE 1. COMBUSTOR CONCEPT
The wide range combustor system is considered a conventional ty ?e because it has
neither fuel staging nor variable geometry. The wide range would be achieved
through a combination of pilot and primary zones. The system would rely on
strong recirculation in the pilot zone for flame stabilization. At low fuel flow
rates, the flame front would be confined to the pilot zone. The flame front
would expand into the primary zone for operation at the higher fuel flow rates.
This burner would also take advantage of the airblast atomizer characteristics to
obtain satisfactory spray quality over an estimated eight to one fuel flow range.
Atomization would be achieved through the use of a high velocity air stream to
shear the fuel film into small droplets. This type of atomizer is currently in
use on several AiResearch gas turbines. Cooling to keep the maximum liner skin
temperature levels below operating limits would be accomplished by using conven-
tional film cooling skirts in the primary and dilution zones, and a combination
of impingement cooling and a thermal barrier in the pilot zone.
Fuel system design must also address the possibility of fuel thermal decom-
position in both the manifold and the atomizer due to the high inlet air
temperature to the burner. Operating temperature of these parts would Se
minimized by routing the fuel in the inner of the two concentric passages
leading to the fuel atomizer.. Low temperature air bled from the compressor
discharge for use by the airblast atomizer would be ducted through the outer
concentric passage. The outer surface of the outer concentric passage which is
exposed to the hot air from the solar receiver would be-treated with a thermal
barrier coating similar to that used in the combustor pilot zone.
The system can be designed to operate on a variety of fuels including unleaded
gasoline, jet-type kerosenes, diesel fuels and natural gas. It is currently
planned to design the first units to use JP-4 fuel.
108
CONTROL CONSIDERATIONS
Addition of a fossil fuel combustor will significantly increase the flexibility
of, and the opportunities for utilizing the solar Brayton module. For instance I
prior to installing the module on the concentrator, fuel could be used to start
and check out hybrid power conversion module operation without requiring a solar
receiver and the attendant radiant energy soUrce to supply the input thermal
power. Also, when little or no insolation is available such as for night-time
maintenance or for night-time and cloudy day power generation t
 the combustor
would allow the module to be fully operable. Again f
 the combustor could be used
to prevent module shutdown during cloud passage on an otherwise sunny day, or
the combustor could also be used to provide makeup input energy during periods
of low or intermittent insolation to maintain some pre-selected power delivery
rate. For example f
 the combustor could be used to supplement the energy from
insolation and provide rated power. ,
The control and power conditioning concept is shown in Figure L Dashed lines
indicate the additions to the solar only control concept required to describe
the hybrid control concept. The control concept will maintain the turbine inlet
temperature T 
7 
at a constant value and increase or decrease the engine speed N
as the insolation to the receiver increases or decreases respectively. Thw
engine speed will be varied by commanding the control rectifier to change the
electrical load on the engine generator. The desired engine speed is that speed
at which the engine air flow will be heated to a pre-selected value (15000F) at
the outlet of the receiver. For this control concept to function, the 60 Hz
utility grid or bus bar must be prepared to accept all of the electrical power
generated. If it cannot t
 excess power wasting must be accomplished until the
grid load can be increased, or the solar power module deactivated. A concept
for accomplishing this function is presented in the left side of Figure 2.
11 
NZ
OTILITV CN11) ON
FuEL usE
LOCIC
I	 INVENTER	 I
KRFil'o -
::::^M
CONTROL
VARIABLE FBI#. AC 116MAL CONIUSZQi^'
LHI—LITIN_!
SININA109 N
TURIO CONFUSION
PATIENT
CONINTION
BUFFER
;,TCN
11AITIly
BANK
EXCESS ORIGINAL PAGE, IS
IRAN or, POOR QUALM
Pak
SYSTEM -- ^IENGINE
FIGURE 2.	 CONTROL AND POWER CONDITIONING CONCEPT
109
In the hybrid control system concept, fossil fuel would be utilized to augment
solar thermal input. Figure 3 shows an approximate representation of how the
engine rotor speed and pertinent temperatures would behave if subjected to a
simplified insulation schedule shown in Figure 3(a). in this case fossil fuel
is used to complement the insolation during certain periods of the day identi-
fied along the aoscissa by the terms "fossil only", "hybrid" and "solar only".
The fuel use logic input picked for illustration is one in which the solar
module would be adjusted to operate continuously at rated output power starting
before sunrise and continuing at rated power until after sunset except when the
insolation exceeds rated intensity. This selected power level at which the
module would be required to operate when using fossil fuel would be established
by setting an adjustable speed reference called N set shown in Figure 3(b). Ns tin turn would be bounded by a small speed tolerance band in the control mods^t
identified by Nhi h and Nlow . When increased insolation would be experiencedduring hybrid operation it would first tend to cause the receiver outlet temper-
ature to increase and this would cause a fuel flow reduction until some previ-
ously established fuel flow would be reached, as at ►► d" in F$.pure 3. The engine
speed would then increase until it reached Nhi h as at "e" which would slut off
the fuel flow. The control would then behave n the same manner as a solar only
control in the region between "f" and "g".
When the engine speed falls to Nlow because of decreasinq insolation, fuel flow
would be resumed as at " h ►► in Figure 3. The control would be designed to avoid a
requirement for the fuel atomizer to operate over an excessively large fuel flow
range by appropriate choice of a minimum fuel flow. The effect of this minimum
fuel flow value is shown on the three curves of Figure 3 by small perturbations
in the regions identified by I'd-e-f", "h", "i", and "k-1". Figure 3(c) shows
that the turbine inlet temperature stays constant throughout the assumed daily
schedule of input thermal power as a result of the complementary action of the
receiver temperature rise and the combustor temperature rise. Through the
action of N set , the engine can be adjusted to operate at rated power as shown in
Figure 3 1 or it can be set at other values higher or lower, depending on the
application requirements and the fuel consumption economics. Choosing N et at
a value corresponding to maximum power level would involve the use of larger
fuel quantities. Setting N s t at a value slightly above self-sustaining would
minimize fuel consumption and still avoid restarts following cloud passage.
110
RCUR OUTLET TEMP
NT CONTROL
$AND	 I
WHICH
	 E	 IY	 W m
FUEL FLOW
	 $
MINIMUM H
l
DESIGN TUH$
INLET TEMP
MAXIMUM
b MATEO
.,. SELF-SUSTAINING
-
.a
FUEL F101N	
MAXIMUM
MINIMUM
---..........., .... RATED
(C(
v	 ^
ds
/I
.t
W
e,
r	 ^
	
(FOSSILI SOLAR
	
^\ FOSSIL "" SELF-SUSTAINING
FOSSILO1uw/e tivinlfl _
	
	fm In nu1Y I^wv 1 ANLV	 uynnln .^ AulyxNl
TIME OF DAY
FIGURE 3. HYBRID CONTROL CONCEPT-TYPICAL DAY
UI^,ICI^AI. Q A^I^^ITYPURRSUMMARY	 V
An isometric view of the hybrid MOD-0 engine is presented in Figure 4 which
shows the combustor assembly installed between the receiver outlet and the
turbine inlet. The combustor addition will cause thermodynamic performance to
drop by about 1.3 efficiency points at the engine shaft because of added pres-
sure drop, and a small percentage of bleed flow which bypasses the recuperator
in order to provide fuel line cooling and fuel atomization.
Features or advantages of adding hybrid capability to the Brayton engine concept
are identified as follows:
•
	
	 Retains constant turbine inlet temperature control for good part load
efficiency
•
	
	
Adds option for continuous operation at rated power independent of insola-
tion
•
	
	 Adds option for operation at turbine inlet temperature higher than the
receiver's capability
•	 Provides operation during cloud passage
•	 Adds option for operation at night for power generation or maintenance
Ill
Al
The combustion system and conti;ol concept deocribed will be subjected to anal-
ysis and design in the NASA/JPL contract for the MOD-0 engine.
FIGURE 4. BRAYTON ENGINE GENERATOR AND CAVITY RBCEIVER ASSEMBLY
REFERENCES
1.	 L. D. Six, et. al., Conq2RL Definition Study of Small Brayton Cycle Enginea
for Dip ersed Solar Electric PowerLems, NASA CR 159592, (Januaryy&
1980.)
112


A hybrid Brayton engine concept

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