DESIGN, SIMULATION, AND TEST RESULTS OF A HEAT-ASSISTED THREE-CYLINDER STIRLING HEAT PUMP (C-3).pdf

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DESIGN, SIMULATION, AND TEST RESULTS OF

A HEAT-ASSISTED THREE-CYLINDER

STIRLING HEAT PUMP (C-3)

KU 1 OTA Corporation

(+81)-6-494-7561, faX (+81)-6-494-7694

1-1, Hama 1-Chome, Amagasaki, Hyogo 661, Japan

John Corey

Clever Fellows Innovation Consortium, Inc.

302 Tenth St., Troy, NY 12180

518-272-3565, fax 518-272-3582

Naotsugu lsshiki & lsao Satoh

Tokyo Institute of Technology

12-1, 0-Okayama 2-Chome, Meguro-ku, Tokyo 152, Japan

(+81)-3-5734-3238, fax (+81)-3-3729-0587

ABSTRACT

This paper describes recent results in aproject at KUBOTA to

develop a gas engine-driven Stirling heat pump using both

engine shaft power and engine exhaust heat source. The

design, simulation, and test results of the third prototype

three-cylinder machine (C-3) are presented. The three-

cylindermachine is modeled as a combination of two Stirling

sub-systems, one a power producer and one a heat pump.

These have been separately optimized, then joined into the

three-cylinder heat-assisted heat pump case. Shaft power is

augmented by thermal power. Performance is effectively

controlled by the phase shifting of the third piston to adjust

the absorbing of thermal power. The test results of the C-3

prototype machine are given and are shown to compare well

with predictions made in the Sage simulation code.

or thermal power. In this heat-assistedStirling heat pump, by

proportioning the two energy sources to match the

characteristics of the driving engine, the heat pump is supplied

with the maxinium share of the original energy fueling the

engine. ?his freon-free system is expected to produce cooling

and heating water at high COP.

Otomo and et al. (1996) have been performing experiments

on a similar three-cylinder machine, mostly for validation of

the vector method of cycle analysis, using the Schmidt

assumption. Yagyu and et al. (1996) presented a third-order

method for analysis and optimization of multi-cylinder

regenerative machines and applied it to a three-cylinder heat-

assistedStirling heat pump case. The three-cylinder machine

is modeled as two Stirling sub-systems, one a power producer

andone a heat pump, using the Sage simulation code (Gedeon

1994). The heat pump sub-system, a two-cylinder Stirling

cycle device, was reported at the first prototype (A-type) level

(Yagyu and et al 1995), the second prototype (B-type) level

(Yagyuand et al 1996), and the third prototype (C-type) level

which incorporates improved heat exchanger sizing and

arrangement (Yagyu and et al 1997). Test results of the C-

type heat pump sub-system were shown and the predictions

made in the Sage simulation code were also validated. Tnis

paper describes the design andsimulation of the complete third

prototype (C-type) three-cylinder machine. The test results of

the C-3 machine are given and are shown to compare well with

predictions made in the Sage simulation code.

INTRODUCTION

Under the sponsorship of the New Energy and Industrial

Technology Development Organization (NEDO), through a

contract with the Energy Conservation Center (ECC), KUBOTA

is developing the Multi-Tcmperature Heat Supply System (ECC

1995). This system consists of a gas-fueled internal

combustion engine and a novel Stirling heat pump utilizing

shaft power and thermal power in a hybridof several cylinders.

The heat pump is mainly driven by engine shaft power and is

partially assisted by thermal power from engine exhaust heat

source. The heat pump includes a specialized cylinder, which

is heated by the exhaust gas of the driving engine and phased

to produce added mechanical power. This arrangement

recaptures some of the waste heat in the exhaust to reduce shaft

power needed for driving the heat pump. The system is

controlled by phase shifting of the cylinder to match the

engine heat balance and to match heat demand characteristics.

The system simultaneous 1 y supp 1 ies four-temperature heat

sources (263 K, 280 K, 3 18 K, and 353 K) for air-conditioning,

hot water supply, and refrigeration. ?his compares with other

heat pumps using helium in agas cycle, including Stirling and

Vuilleumier (VM) machines. They utilize either shaft power

PROTOTYPE OF THE 3-CYLINDER STIRLING

HEAT PUMP

The three-cylinder machine is a kind of heat pump which is

driven by an engine and assisted by its exhaust heat.

Simulating combined inputs of shaft power and thermal power,

the prototype was planned to use a motor and electric heaters.

CONCEPT OF C-3 PROTOTYPE MACHINE

Figure I shows a concept of the C-type prototype

This machine employs three single acting

machine(C-3).

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Sumio Ya yu & lchiro Fujishima

pistons not displacer pistons, which is different from VM

machines, and utilizes both shaft power and thermal power.

The prototype consists of a two-cylinder on one crank and a

third cylinder on another crank. The two-cylinder set is for

cooling and heating outputs and the third cylinder is for

thermal power input. The two crank shafts are connected by a

phase shifter and the phase of the third cylinder can be

arbitrarily set. Shaft power is supplied by a motor and

thermal power is supplied by a DC electric power device and

resistance heaters.

MOTOR

TORQUE

,/ (263- 280K)

METER

COLD

DC POWER

SUPPLY

L - HEX

H - HEX

COOLING

REGENERATOR

DEVICE (318K)

WITH PIPING

FIGURE 1 3-CYLINDER STIRLING HEAT PUMP

HEAT FLOW

Figure 2 shows a heat flow diagram of the three-cylinder heat

pump. The heat pump comprises two 2-cylinder Stirling sub-

systems: one between high andmedium temperature (H-M); one

between medium and low temperature (M-L). The M-L sub-

system acts as a heat pump andthe H-M sub-system serves as a

power producer which assists with extra shaft power. The

downstream heat of the H-M sub-system, which is waste heat of

the sub-system, is also utilizedas an additional heating output.

Taking both shaft power and thermal power, the three-cylinder

produces coolingirefrigeration and heating water.

TESTED C-3 PROTOTYPE MACHINE

Figure 3 shows a configuration of the tested C-type three-

cylinder prototype machine. The machine was a kinematic

type and the crank case was pressurized. Helium as a working

gas was sealed at the end of each crank shaft by mechanical

seals. The M-L sub-system was constructed as a Stirling cycle,

whose cylinders were set in 130degree phase difference. me

phase of the third cylinder can be arbitrarily set by using a

phase shifter located between crank shafts. Four low friction

side thrust rollers made of hard plastic were employed on the

piston and the side force was effectively sustained With

careful consideration for minimizing flow loss in the helium, a

regenerator of stacked stainless steel meshes was installed

between opposing bayonet heat exchangers and was connected

by smoothly bent pipes from the cylinders. The bayonet heat

exchanger consisted of 16 annular flow paths with extended

surfaces. These were employedfor coldand hot water outputs.

The hot heat exchanger was an annual type which contained

electric heaters.

The performance tests were conducted in various conditions

at mean pressures ranging from 0.8 to 2.4 MPa and at

revolution speeds ranging from 400 to 1,000rpm. Each inlet

water stream was controlled by cooling or heating device and

was regulated in outlet temperature at (280 K in cold side and

3 18 K in hot side). The thermal power was supplied by DC

electric heaters and shaft power was supplied by the motor and

measured by the torque meter. Encoders were equipped with

each crank shaft and the difference of the crank angles was

precisely measured. RTDs were used for water temperature

measuring and TCs were used for gas temperature measuring.

Dynamic gas pressures were measured by quarts pressure

transducers.

3 - CYLINDER

STIRLING

REFRIGERATION

FIGURE 2 HEAT FLOW DIAGRAM

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HEATING DEVICE

HEAT PUMP

COOLING /

FIGURE 3 C TYPE THREE-CYLINDER PROTOTYPE MACHINE

SIMULATION

The three-cylinder heat pump was modeled as shown in Figure

4. The C-type three-cylinder model was modified from the

two-cylinder machine by adding the thirdcylinder (H-cylinder),

the second regenerator (H-M regenerator), and heaters (H-hex).

The M-Lsub-system was constructed as a Stirling cycle, whose

pistons were set in constant phase difference. The third

piston phase can be arbitrarily set. The H-M part was

connected to the M-L sub-system using a common mid-

temperature cylinder (M-cylinder) and a heat exchanger (M-

hex).

The Sage simulation code has been applied to analyze and

optimize this model. The two sub-systems were separately

optimized, then joined into the three-cy 1 inder heat-ass i s ted

heat pump case subjected to M-cylinder pressure constraints.

The characteristics of engine heat balance limit the heat

available to the hot third cylinder and the power contribution

from H-M subsystem, so the performance of the M-L cycle

dominates the overall performance of the system. Therefore

the M-L cycle was first optimized, without prior constraints.

Then the H-M cycle was optimized subject to a complex

pressure constraint that forces its mid-temperature pressure and

phase to match those in the M-L mid-temperature cylinder.

The cylinder temperatures were set for an engine driven heat

pump for air-conditioning application. That is, 278 K at L-

heat exchanger wall, 3 18 K at M-heat exchanger wall, and 500

Kat H-heat exchanger wall individually.

HOT HEAT FLOW

M-L REGENERATOR

COLD HEAT WOW

H-M

REGENERATOR

.INDER

-”@

..........

H-

ABSORPTIONOF

CYLINDER

THERMAL POWER

FIGURE 4 3-CYLINDER SIMULATION MODEL

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The optimization of the M-L cycle was critical to total

system performance and sets the main geometry of the machine.

The optimized H-M cycle was scaled andcombined to match the

heat and shaft output balance of the driving engine. This

produced the first 3-cylinder simulation. Since some practical

adjustments must be made to construct areal 3-cylinder device,

that first 3-cylinder model was similarly adjusted. Then a

sensitivity analybis was done by varying key parameters in

repeated Sage runs. This provided guidance as to which

parameters might respond to further optimization and which

might be ignored in improving the 3-cylinder model.

Regenerator characteristics and dead volume distribution

proved the most important. Optimization in Sage of the full

3-cylinder nioclcl was coniplctcd by rcfining thesc parameters.

power PVh, and the indicated shaft power PVnet arc plotted

there. The simulation results arc shown by continuous curves

in the same figure as well. PVnet is summation of PVs at C-

cylinder, M-cylinder, and H-cylinder and it means net shaft

power. The indicated work from thermal power input had the

maximum value at the phase of 170 deg. At that phase, the

shaft power was most assisted by the thermal power. The

COPind-c derinedas the ratio of PVc to PVnet and the maximum

value was around the same phase.

The machine has two major features. The first is that the

assistedpower is varied by altering hot piston phase, in other

words, input ratio of shaft power to thermal power can be

changed. When the system uses an engine, it can be operated

to accommodate with engine heat balance. The secondis that

the phase has a large effect on the cooling andheating capacity

as well. Capacity control of the heat pump is available by

changing the H-phase. The cooling andheating capacity can

be changedaround 30 % in upward anddownward from the best

efficiency operating point in this machine. Capacity control

of the heat pump by phasing is different from the conventional

capacity control by changing speed The simulation results

agree with the test results within 10% accuracy. PVc and PVh

in the simulations are nearly coincident with test results,

however PVm deviates from them. Then PVnet and COPind-c

as calculated results are not so good lhe PVm in the test may

have phasing effects from local, unmodeled turbulence or

restrictions, especially near the three way mixing zone,

PV diagrams of C-cylinder, M-cylinder, and H-cylinder at the

mean pressure of 1.6 MPa and 600 rpm are shown in Figure 6.

The indicated work at each cylinder had a goodoval shape and

the presstire ratio is around 1. I. The simulation results were

agreed well with the test results.

RESULTS AND DISCUSSIONS ON 3-CYLINDER

TESTS AND SIMULATIONS

EFFECT OF H-CYLINDER PHASE

Figure 5 shows primary characteristics of the C-3 prototype

machine on a PV basis. The heat exchanger (H-hex) in the

engine driven three-cylinder Stirling heat pump was planned to

be a gas to gas heat exchanger. Therefore, the heating

capacity Qh has not been evaluated in the C-3 tests and a

comparison based on indicated works was appropriate to this

case. Changing the H-cylinder phase, the tests were

performed at various phases at the mean pressure of 1.6 MPa

and a speed of 600 rpm. The wall temperature of the H-hex

was maintained at 500 K in every test run. The independent

paramctcr, Phasc, was dcfincd as thc phase difference between

H-cylinder and M-cylinder. The cooling indicated work PVc,

the hcating indicated work PVm, the indicated work of thermal

800

5.0

700

4.0

600

PVh

PVnet

> e 400

500

3.0 a

300

2.0

200

1.0

100

80 100 120 140 160 180 200 220

0.0

PHASE [DEG]

FIGURE 5 C-3 PROTOTYPE PERFORMANCEVS. PHASE

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1.75

1200,

- H QC

0 PVC

A-

1000

PVm

m a

800

0 PVh

0 0

a a

6 400

600

200

100

200

300

400

500

I I I

400 600 800 1000

REVOLUTION SPEED [rpm]

VOLUME [Xl04m3]

FIGURE 6 PV DIAGRAMS

FIGURE 7 PERFORMANCE AT VARIOUS SPEEDS

1000

I 0 PVC

6.0

- o PVm

0 PVh

800

5.0

3 600

a 400

- A PVnet

4.0

3.0

2.0

200

- 0.5

l.O

o COPind-m

CoPind-c

1 .o

1.5

2.0

2.5

0.5

1 .o

1.5

2.0

2.5

PRESSURE [MPa]

FIGURE 8 PERFORMANCE AT VARIOUS MEAN PRESS.

FIGURE 9 COP VS. MEAN PRESS

EFFECT OF REVOLUTION SPEED

Figure 7 shows the speed dependence of the machine at the

mean pressure of 1.6 MPa, keeping 170deg for the H-cylinder

phase. Qc, Qm, and PVh were almost proportional to the

revolution speed at this test range. The simulation results

shown by continuous or dottedlines agree with the test results.

The maximum cooling capacity was 500 Wand the maximum

heating capacity was 900 W at the revolution speed of 1,000

rpm. The bayonet heat exchangers in cold and hot spaces

showed good performance and the efficiency defined as Q to PV

exceeded 85 YO.

the test results andespecially deviate at higher mean pressures.

It seems that the pressure effect has not been fully reflected in

the simulation model.

COP characteristics on a PV basis are shown in Figure 9.

IndicatedCOP was defined as a ratio of cooling indicated work

(PVc) or heating indicated work (PVm) to net shaft power

(PVnet), excluding mechanical loss from actual shaft power.

COP characteristics also showed pressure depcndence in this

machine. The maximum COPind-c was 3.0 andCOPind-m was

5.0 in this test range. In the simulations, the indicated work

in M-cylinder tendedto be largerthan measured in test, so the

indicated shaft power (simulated) became large. This led to

deviation in the indicated COP, especially in higher mean

pressure region.

EFFECT OF MEAN PRESSURE OF WORKING GAS

Figure 8 shows the mean pressure dependence of the

performance at the speed of 600 rpm. Qc, Qm, and PVh were

almost proportional to the mean pressure at this test range.

Qm and PVm of the simulation results are higher than these of

The actual COPc andCOPm are not shown there and they were

decreased by the large mechanical loss in the prototype

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