Part B | 12.5
318 Part B Autonomous Ocean Vehicles, Subsystems and Control
35:6 cm=s. The low speed characteristics at maximum
L=D are largely a consequence of the triplane’s relatively high wetted surface area.
In summary, the ZRay glider becomes more energy
efficient in horizontal transport of large cargo and payload than when flying by itself, although it does so at
slower speeds. More generally, vehicles with wings can
reduce their propulsion energy consumption by flying
cooperatively with other wings, in contrast to propdriven vehicles. Cooperative flight is enabled by the
simplicity of the wing’s trailing edge vortex system –
Sect. 12.2.
12.5 Thermal Glider
This vehicle is applicable for ultra-long range, depthunlimited roaming and for high endurance station keeping [12.4] (Table 12.1). The concept gives the glider
the ability of renewing its onboard energy stores by
harvesting environmental energy from the heat reservoir of the ocean, specifically from the temperature
differences of the cold deep water and the warmer surface water (available in 80% of the world’s oceans).
Ranges of 30 000 to 40 000 km, circumnavigating the
world, then become conceivable. Harvesting this thermal energy depends on the volume change associated
with the state (phase) change of a material with a melting/freezing point in the range of ocean temperatures.
Heat is absorbed from the warm surface water, causing a change of state from solid to fluid (melting),
and released to the cooler, deeper water during the
vehicle’s transit through the thermocline, resulting in
a state change back to the solid state (freezing). The
heat exchange volume occurs inside tubes that run the
vehicle’s length and provide a large surface area for
rapid heat flow. Almost all materials have a positive
thermal expansion coefficient so that melting causes
an increase in volume and freezing results in a decrease in volume (water below 4
ı C is an exception).
Because these volume changes are opposite to those
required of a glider’s buoyancy engine, they cannot
be used directly for forward propulsion. Rather, the
thermal buoyancy engine must include unique design
features to account for the positive thermal expansion
coefficient (as in Fig. 12.19) or the energy from expansion/contraction must be stored onboard over a half
Table 12.1 Analysis results for the net transport economy of various sizes of a thermal glider designed for the ultra-long
range, depth-unlimited roaming application (after [12.1])
Scale [L]
Vol. [L]
Net buoyancy [L]
u
w [m=s] Range [km]
NTE
Re vol
.1=3/
Cd vol
.2=3/
0:5
62
0:09
0:17
0:11
35 000
0:61
3:2 10 4
0:062
1
64
0:18
0:24
0:16
35 000
0:22
4:6 10 4
0:062
2
67
0:36
0:33
0:22
35 000
0:08
6:6 10 4
0:062
5
76
0:9
0:51
0:33
35 000
0:02
1:1 10 5
0:06
10
91
1:8
0:69
0:45
35 000
7:5 10 3 1:7 10 5
0:059
100
358
18
1:46
0:95
35 000
3:6 10 4 8:8 10 5
0:049
1000
3036
180
2:39
1:55
35 000
2:2 10 5 5:9 10 6
0:036
10 000
29 809
1:80 10
3
3:63
2:36
35 000
1:4 10
6
4:2 10
7
0:032
dive cycle to be useable for propulsion. The four stages
of the thermodynamic cycle are shown Fig. 12.19. Environmental energy is harvested by heat flowing into
and out of the working fluid in chamber 1, which contracts on freezing and expands on melting. The resulting
work is transmitted around the system by the transfer
fluid, typically mineral oil. Chamber 2 is an energy storage accumulator, with the transfer fluid pressurized by
nitrogen at a pressure greater than the maximum external ocean pressure. In Fig. 12.19a, the vehicle is in
stable thermal equilibrium in the warm surface water,
N 2 is compressed, the external bladder is inflated, and
working fluid is expanded. Descent begins by opening
the three-way valve (Fig. 12.19b), venting the external
bladder to the internal bladder. Maintaining the hull interior slightly below atmospheric pressure creates the
pressure differential for this flow. As the vehicle reaches
cold water, heat flows out of the working fluid, which
freezes and contracts, and draws in mineral oil from the
internal reservoir. The beginning of ascent (Fig. 12.19c)
results from opening the three-way valve, the pressurized oil in the accumulator moves to the external
bladder and the vehicle changes from negative buoyancy to positive buoyancy. During ascent (Fig. 12.19d),
the vehicle ascends to warm waters, heat flows into the
working fluid, which melts and expands, and oil flows
to recharge the accumulator.
Because the thermal glider is nearly identical in
shape and dimensions to the winged body of revolution
for single payloads, the glide polar data from Fig. 12.14
can be used for hydrodynamic input to an analysis of
318 Part B Autonomous Ocean Vehicles, Subsystems and Control
35:6 cm=s. The low speed characteristics at maximum
L=D are largely a consequence of the triplane’s relatively high wetted surface area.
In summary, the ZRay glider becomes more energy
efficient in horizontal transport of large cargo and payload than when flying by itself, although it does so at
slower speeds. More generally, vehicles with wings can
reduce their propulsion energy consumption by flying
cooperatively with other wings, in contrast to propdriven vehicles. Cooperative flight is enabled by the
simplicity of the wing’s trailing edge vortex system –
Sect. 12.2.
12.5 Thermal Glider
This vehicle is applicable for ultra-long range, depthunlimited roaming and for high endurance station keeping [12.4] (Table 12.1). The concept gives the glider
the ability of renewing its onboard energy stores by
harvesting environmental energy from the heat reservoir of the ocean, specifically from the temperature
differences of the cold deep water and the warmer surface water (available in 80% of the world’s oceans).
Ranges of 30 000 to 40 000 km, circumnavigating the
world, then become conceivable. Harvesting this thermal energy depends on the volume change associated
with the state (phase) change of a material with a melting/freezing point in the range of ocean temperatures.
Heat is absorbed from the warm surface water, causing a change of state from solid to fluid (melting),
and released to the cooler, deeper water during the
vehicle’s transit through the thermocline, resulting in
a state change back to the solid state (freezing). The
heat exchange volume occurs inside tubes that run the
vehicle’s length and provide a large surface area for
rapid heat flow. Almost all materials have a positive
thermal expansion coefficient so that melting causes
an increase in volume and freezing results in a decrease in volume (water below 4
ı C is an exception).
Because these volume changes are opposite to those
required of a glider’s buoyancy engine, they cannot
be used directly for forward propulsion. Rather, the
thermal buoyancy engine must include unique design
features to account for the positive thermal expansion
coefficient (as in Fig. 12.19) or the energy from expansion/contraction must be stored onboard over a half
Table 12.1 Analysis results for the net transport economy of various sizes of a thermal glider designed for the ultra-long
range, depth-unlimited roaming application (after [12.1])
Scale [L]
Vol. [L]
Net buoyancy [L]
u
w [m=s] Range [km]
NTE
Re vol
.1=3/
Cd vol
.2=3/
0:5
62
0:09
0:17
0:11
35 000
0:61
3:2 10 4
0:062
1
64
0:18
0:24
0:16
35 000
0:22
4:6 10 4
0:062
2
67
0:36
0:33
0:22
35 000
0:08
6:6 10 4
0:062
5
76
0:9
0:51
0:33
35 000
0:02
1:1 10 5
0:06
10
91
1:8
0:69
0:45
35 000
7:5 10 3 1:7 10 5
0:059
100
358
18
1:46
0:95
35 000
3:6 10 4 8:8 10 5
0:049
1000
3036
180
2:39
1:55
35 000
2:2 10 5 5:9 10 6
0:036
10 000
29 809
1:80 10
3
3:63
2:36
35 000
1:4 10
6
4:2 10
7
0:032
dive cycle to be useable for propulsion. The four stages
of the thermodynamic cycle are shown Fig. 12.19. Environmental energy is harvested by heat flowing into
and out of the working fluid in chamber 1, which contracts on freezing and expands on melting. The resulting
work is transmitted around the system by the transfer
fluid, typically mineral oil. Chamber 2 is an energy storage accumulator, with the transfer fluid pressurized by
nitrogen at a pressure greater than the maximum external ocean pressure. In Fig. 12.19a, the vehicle is in
stable thermal equilibrium in the warm surface water,
N 2 is compressed, the external bladder is inflated, and
working fluid is expanded. Descent begins by opening
the three-way valve (Fig. 12.19b), venting the external
bladder to the internal bladder. Maintaining the hull interior slightly below atmospheric pressure creates the
pressure differential for this flow. As the vehicle reaches
cold water, heat flows out of the working fluid, which
freezes and contracts, and draws in mineral oil from the
internal reservoir. The beginning of ascent (Fig. 12.19c)
results from opening the three-way valve, the pressurized oil in the accumulator moves to the external
bladder and the vehicle changes from negative buoyancy to positive buoyancy. During ascent (Fig. 12.19d),
the vehicle ascends to warm waters, heat flows into the
working fluid, which melts and expands, and oil flows
to recharge the accumulator.
Because the thermal glider is nearly identical in
shape and dimensions to the winged body of revolution
for single payloads, the glide polar data from Fig. 12.14
can be used for hydrodynamic input to an analysis of