High Altitude Brayton Cycle Generators
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High altitude LTAs are also being considered for commercial communications to complement or supplement satellites. Both missions require unmanned, long-endurance operations well above the Troposphere to avoid large amplitude atmospheric disturbances such as jet stream winds. To achieve long endurance, solar-powered, regenerative energy systems have to be used. Power during night hours and for unusual peak daytime events is obtained from stored energy. The best approach is deemed to use hydrogen as night-time fuel that can be replenished by electrolysis during daytime hours. The challenge is how to generate power using ambient air at altitudes where the density has declined from 14.7 psia at sea level, to 1.05 psia at 60,000', 0.649 at 70.000', and 0.403 at 80,000'. A system arranged to operate at 60,000' may not be able to operate in an atmosphere that is less than half as dense at 80,000'. The solution most commonly assumed to fit these requirements best is a regenerative hydrogen system based on solar-powered electrolysis to store energy for night-use in a fuel-cell. However, this approach overlooks the weight of the amount of stored oxygen required. DARPA is conducting a feasibility demonstration program to power and propel a low earth orbit satellite with a stored hydrogen and oxygen regenerative system employing photovoltaics in combination with an electrolyzer and fuel-cell. However, since it is an orbiting satellite the cycles in darkness are short in comparison with those of a stationary airship and the weight of stored oxygen is, therefore, not as critical. An LTA design based on stored hydrogen and oxygen can suffer severe oxygen weight penalties. Depending on reserve power requirements deemed necessary to hold station in wind storms the stored oxygen concept can be rendered unfeasible. For example, an airship designed to carry a payload of 12,000 lb up to 80,000' with the capability of withstanding a severe wind storm would need to accommodate a weight of oxygen three times greater than the entire payload. Much of this weight penalty can be eliminated by using outside air instead of oxygen as oxidant during wind storms. The problem with a PEM fuel cell is that to use ambient air at altitude it has to be pressurized. This creates a parasitic load which escalates sharply at high altitudes. At 80,000' the pressurization system effectively overwhelms the fuel cell.
The generation system is based on
a unique form of closed Brayton cycle consisting of a two-stage turbo
compressor system with intercooler, recuperator, external combustion
to heat the high pressure turbine air and also to re-heat the air into the low pressure turbine.
This system offers thermal efficiencies in excess of 45% over a range of
power outputs from 20% to full power. This is accomplished by controlling
the air density (pressure level) in the closed loop.
The external combustion system is proprietary
and includes water recovery in the event that hydrogen regeneration is
required. The system
design provides air for
combustion, re-circulates a portion of combustion gas and provides air to
pressurize the closed loop.
Since fuel cells are projected to reach comparable thermal efficiencies to
those of this system, the size and weight of the sink heat exchanger would
be about the same for both. Excluding the weight of the heat sink exchanger,
the specific weight for this form of Brayton cycle system capable of meeting
the full power requirement at 80,000' would be in the order of 3.17 lb/kW
(1.44 kg/Kw). With the heat sink exchanger included, comparable figures
would be 4.47 lb/kW (2.03 kg/kW). At 75 kW this translates into a system
weight of around 325 lb.
The system as described utilizes high speed rotating elements of reliability
proven in extensive field tests of up to 10,000 hrs without failure.
The two heat rejection heat exchangers are well within the state-of-the-art
of aluminum manufacture, the recuperator follows design criteria that have
been proven in commercial micro turbines as does the high-speed, wild
frequency alternator. The novelty of the system lies in its ceramic
combustion and combustion heat transfer elements and the method employed to
induce air into the combustor and provide a source of air to pressurize the
closed-loop.
System Comparison:
The following is a comparison of a fuel cell and a Brayton cycle system.
The problem is that the system has to cope with occasional storms that
include winds as high as 100 knots. This introduces a peak load requirement
of close to 800 kilowatts. Based on an average overnight level of 700
kilowatts, the fuel cell system has a hydrogen capacity of 2,000 lb per
night and corresponding oxygen consumption of 16,000 lb. At this level of
energy, the complete system with solar cells included would weigh as much as
100,00 lb. Keep in mind that
13.18 cubic feet of displaced air is required to lift one pound of
weight at sea level. At 80,000' that figure jumps to 480 cubic feet per
pound of extra weight. Hence, to lift 100,000 pounds of system weight would
require 48 million cubic feet of additional airship volume!
The solution to this problem must include reducing the stored oxygen weight
which can be
done by using atmospheric oxygen. The other key element in system weight
reduction is to regenerate only enough hydrogen meet all but the power
required to counter
the most severe winds and to meet the peak storm conditions with the
addition of stored compressed hydrogen.
This approach is illustrated in the adjacent system diagram.
To meet these conditions, this system
could be designed with a
regenerative hydrogen capacity of 660 lb per night. This cuts the size, weight
and power demand of the electrolyzer about in half. The weight penalty for
this system is the 1,300 lb of reserve hydrogen required to counter the
worst parts of two wind events. Including the containment weight of the
hydrogen, the penalty amounts to close to 13,000 lb.
The complete systems weight for this approach is estimated to be around 50,000 lb, which
at 80,000' altitude
would reduce the extra airship volume from 48 to 26 million cubic feet
. High Altitude (80,000') Airship Application
System Weight Comparison Brayton Cycle Fuel Cell
** The same rationale when applied to very high altitude airplanes produces similar results. Instead of airship volume, the battle becomes wingspan and since an airplane has greater forward velocity there is a beneficial level of dynamic pressure to make things a bit easier. Basically, the approach described for the airship helps to solve the power problem for any high altitude platform with long endurance requirements. |
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The
Missile Defense Agency (MDA) wants to demonstrate the practicability of a lighter
than air (LTA) high altitude airship ("HAA") that is capable of staying aloft for
more than a year at 70,000' to 80,000' carrying a multi-mission payload of
up to 12,000 lb. with a nominal power requirement of 75 
The
fuel cell system consists of a hydrogen/oxygen fueled, solar regenerative
arrangement as illustrated in the diagram. It consists of
surface-mounted photovoltaic cells, a multiple stack electrolyzer, a
multiple stack fuel cell, oxygen, hydrogen and water storage tanks and a
power conditioning system. In this instance the system is sized for
sustained operation at 80,000' with a constant power demand of 140 kilowatts
which includes 75 kilowatts for the payload and enough thruster power to
cruise at 45 knots. This part of the operation would require an electrolysis
rate of roughly 400 lb of H2 per night and 3,200 lb of O2,
which implies a corresponding need for water storage of 3,600 lb at the end
of each night. 