and Testing of an Ultra low Piston Heat EnginePiston Heat Engine Joshua Schmitt, Jordan Nielson,...
Transcript of and Testing of an Ultra low Piston Heat EnginePiston Heat Engine Joshua Schmitt, Jordan Nielson,...
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Modelling and Testing of an Ultra‐low Temperature sCO2 Opposing
Piston Heat Engine
Joshua Schmitt, Jordan Nielson, Nathan PoernerSouthwest Research Institute
Partnered with: Thermal Tech Holdings and PwrCor, Inc.
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Background• SwRI has a long history of understanding and
developing CO2 compressors and engines– Design and testing of radial and axial
turbomachinery and reciprocating machinery with CO2 as operating fluid
– Multi‐million dollar projects with industry and DOE to develop the technology
• SwRI contracted to conceptualize, model, design, assemble, and test a novel CO2 engine for low temperature heat inputs– Opposing piston design with expansive
properties of CO2 as the driving force
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Working Principles• CO2 experiences dramatic changes in
density above the critical point for small changes in temperature
• Trapped CO2 in heat exchanger is heated with water which builds pressure that pushes a piston
• Opposing piston design compresses CO2 into a separate heat exchanger that is cooled
• Processes is rapidly reversed by switching water sources on heat exchangers with trapped CO2
• A central piston cylinder communicates pressurized hydraulic oil to a motor
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Engine Cycle• Typically operates above the critical point
of CO2– High pressures over 1100 psi– Low temperatures over 88 F– Can operate under the dome if enough
cooling is available• P‐v diagram demonstrates how work is
produced– Area between high and low pressure is
available work– How fast this cycle repeats determines
power output• T‐s diagram demonstrates the efficiency
of the cycle– Area under heating line gives cycle heat load
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Cycle Modelling• CO2 properties pulled directly from
REFPROP which is published by NIST• Excel partially time dependent
model– Estimates of piston movement and
subsequent P‐v– Work derived from P‐v is path
dependent which means that the value of pressure affects resulting work
• Matlab fully time dependent model– Control volume approach– Trading of CO2 between different
volumes over small time steps– CO2 properties from REFPROP
derived lookup tables for speed
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Force Balance Model in Matlab• Piston balance of forces
and acceleration determined by pressures and seal friction– Piston movement changes
volume of each cylinder– Acceleration and velocity
over small time step determines the piston displacement
– 5 millisecond time steps used by the model
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Modeled System in Matlab• A network of possible exchanges
developed based on connections– Volumes can exchange mass
• Mass heat addition or removal to a volume is tracked by enthalpy values– Table lookup developed to speed
up code– 2‐D property tables based on
temperature and pressure• Temperature ranged from 20°F
to 400°F, • Pressure from 500 to 5000 psi• The ranges were divided into
1000 equally spaced points
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Matlab Heat Exchanger Model• Water transfer of heat
through metal body to CO2 in the heat exchanger
• HX CO2 volume is fixed but mass can be communicated to the piston CO2 volume
• The new mass and volume creates a new density at each time step
• Using density and enthalpy from heat transfer, a new temperature and pressure is calculated
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Matlab Model Cylinder CO2 Control Volume
• Moving piston head changes volume• Mass transfer to and from cylinder
based on balance of pressures between piston and HX CO2 volumes– Pressure loss of the lines communicating
the CO2 is accounted for– Velocity driven by pressure difference at
time step gives volume flow rate and mass flow rate
• Temperatures and pressures are recalculated as enthalpy and density change
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Matlab Model versus Recorded Data
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Piston
Position
(in)
Presure (psi)
Time (s)HX2 CO2 Pressure HX1 CO2 PressurePiston Position
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Component Overview (1)
Cold Water Pump
Hot WaterPump
Switching Water Valves (x8)
Heat Exchangers CO2 Tubing
Panel
Water Flows
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Component Overview (2)
Generator
Opposing Piston
CO2 Tubing
Oil Reservoir
Hydraulic Motor
Hydraulic Manifold
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How Power is Calculated• Pressure of oil
– Direct measure of sensor at motor inlet• Good for constant supply of pressurized fluid (2 pistons or a pressurized reservoir)• Delay in input if pressure drops below a threshold when operation mode adjusted during
testing– Calculation of oil pressure from CO2 pressure (Preferred)
• CO2 response is responsive to abrupt changes because they are always pressurized• Calculated from static pressure ratio with dynamic pressure loss
• RPM of motor– Number of pulses over time– Time between pulses (Preferred)
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Accounting for Engine Losses and Inefficiencies
• Dynamic pressure loss from observed behavior when hydraulic pressure sensor “catches up” with CO2‐based calculation– Hydraulic flow kept constant with regulator, so this loss can be applied
to the entire stroke• Motor efficiency provided by manufacturer applied to calculate
shaft power• Calculating thermal efficiency ( )
– Heat flow rate from CO2 T‐s diagram and power from motor shaft power out
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Best Case P‐V (190°F hot, 45°F cold, 80 GPM)
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Best Case T‐s (190°F hot, 45°F cold, 80 GPM)
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Power and Efficiency with Changes in Hot Water Temperature
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130 150 170 190Po
wer (kW)
Hot Water Inlet Temperature (°F)
CO2 CyclePower (kW)
Motor ShaftPower (kW)
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45%
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60%
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1%
2%
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5%
6%
130 150 170 190
Cycle Ra
tio to
Carno
t
Cycle Efficiency (kW)
Hot Water Inlet Temperature (°F)
CO2 CycleEfficiency
Cycle Ratio toCarnot
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Power and Efficiency with Changes in Cold Water Temperature and Flow Rates
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1%
2%
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45 55 65 75
Efficiency
Power (kW)
Cold Water Inlet Temperature (°F)
CO2 CyclePower (kW)
Motor ShaftPower (kW)
CO2 CycleEfficiency
0%
1%
2%
3%
4%
5%
6%
7%
8%
9%
10%
0
2
4
6
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60 65 70 75 80
Efficiency
Power (kW)
Water Flow Rate (GPM)
CO2 CyclePower (kW)
Motor ShaftPower (kW)
CO2 CycleEfficiency
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Conclusions• Conceptual cycle development showed the potential for utilizing
ultra‐low temperature heat sources to produce power• The first models were moderately successful in predicting the
behavior of the prototype but are in need of adjustments• Tests of the prototype successfully demonstrated an engine
producing power with hot inputs ranging from 130°F to 190°F• Warmer cooling temperatures or less water flow decreases power
output and efficiency• Future engines will introduce cycle modifications to push the
efficiency above 80% of the Carnot limit– Avoid the alternating of heat sources on the same heat exchanger
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Thank You