Stirling AirshipFrom SuperluminonThis article introduces and combines a few novel concepts on the design and operation of an unmanned cargo airship powered by a Stirling engine heated by hydrogen and solar energy. Since the airship is designed to travel relatively long distances, and a portion of its energy will be provided by solar power, it is reasonable to examine the efficiency of the system over a 24-hour period. The goal of this design is to produce a system that could become a viable replacement for semi-trucks in order to relieve strain on US highways as well as provide industry with more flexibility as it grows. 
LiftHydrogen will be used for the lifting gas, since it is less expensive than helium, can be burnt as a fuel. This also helps by reducing the complexity of operation, and in some cases, may increase the flexibility of the application. Hydrogen -- 1.2 Kg/m3 lift at STP Cell Size 25m x 100m = 200,000m3 62% of 200,000m3 for airship-shaped volume (see LZ-129 stats) ~120,000m3 HOWEVER, this is at sea level. Revised lift calculations:
V = (1mol * R * 273.15K) / 30148.66Pa
1g / 75.33L ? kg/m^3
0.0132749kg * (1m^3 - (0.459041kg/m^3 / 0.0132749kg/m^3))
Thus, when the airship is at high altitude, it only has a lift capacity of 52.8 metric tons. This is from an airship that is roughly 3/5ths the size of the LZ-129 Hindenberg. The useful lift of the LZ-129 was only 46% that of its gross lift from hydrogen, due to its structure. However, with new composite and nano-composite materials it could be taken under assumption that number could reach 25%. The useful lift of this cargo airship, then could be nearly 40 tons. TODO-- Stats for different, temperatures:
Atmosphere
PowerSolar Parabolic reflectors Although the parabolic reflectors would be concentrating sunlight within the gas membrane, it's important to note that the concentrated solar energy does not focus within the membrane, but outside it, within the Stirling engine pod. Reflector size 50m x 100m = 5,000m2 Assuming that high-altitude insolation is something like... 10kW/hr/m2/day 5,000m2 @ 10kW/hr/m2/day = 50,000 kW/hr/day = 180GJ Theoretical 50% efficiency of Stirling engine 90GJ/24hr = 3.75GJ/hr TODO-- Stats for variables: total area of reflector, insolation at altitude HydrogenHydrogen will not only be used as lifting gas, it will be carried as fuel. Solar energy can provide 30% of the heat energy required to propel the ship, however, it will require the remainder to be made up by burning hydrogen to maintain the Stirling engines at their most optimal temperature. Radiators will heat the lifting gas using waste heat from the stirling engine. Heating the lifting gas improves buoyancy, especially when the hydrogen is being burnt to propel the airship. Materials
Perhaps a self-healing membrane could be used, coated from the inside with a substance that 'heals' the membrane as it oxidizes. Materials Calculations(1.39 g/cm^3) / (5mil * cm^2) 80cm^2/g 1m^2 of Mylar weighs 125g? ConstructionThe parabolic reflector segments would be modular and could be assembled in mid-air. This makes a hangar for construction of these devices smaller, and lowers the start-up cost. OperationThe LZ-129 Hindenberg, during its first year of operation, flew nearly 200,000 miles around the world. The cargo airship will have widespread depots where it can dock and replenish its hydrogen stores. Unused gas cells with parabolic reflectors could be mounted on the ground and used to generate energy required to electrolyze water, as well as to store it within the membranes. IncomeConsider that domestic, scheduled, air carrier freight revenue is about $.85 per ton-mile.[1] At 40 metric tons and 200,000 miles, this airship could generate $6,100,000 per year. The ship could, theoretically, fly day and night, and travel up to 800,000 miles a year, however, with downtime factored in, even 500,000 miles could be possible for an unmanned vehicle. Operational ExpensesHydrogenIf natural gas is $7 per GJ, hydrogen could cost only $1 per kilogram. 120,000m3 * 0.0132749kg/m3 = 1592.988kg Thus, it could, theoretically, cost only $1600 to fill the entire airship with hydrogen. However, hydrogen is also used for propulsion. One kilogram of hydrogen can provide 120MJ of energy. For comparison, gasoline has an energy density of only 44MJ/kg. The LZ-129 used about 300GJ of energy per day from her diesel Mercedes engines. Since we wish to go faster than the 80+MPH of that day, that is a fair figure to use for an airship 60% of the LZ-129's size. Because of this, including an thermal to mechanical transformation efficiency of 50%, 5000kg of hydrogen would normally need to be carried to match the 300GJ output of the Hindenberg, or $5000 worth of fuel each day, or $1.8m per year. However, with solar energy providing 30% of that energy (90GJ/24hr from sunlight), that number could be cut to $3500/day, or $1.3m per year. Thus, the solar energy system would break even if it cost less than $500,000 over only one year. SafetyThe FAA is concerned with any use of hydrogen as a lifting gas. It's important to note that this will be an unmanned airship. If catastrophic failure is imminent, the hydrogen can be quickly vented and dispersed into the atmosphere by rupturing seals on the top of the gas cells. Damage should be limited to only that due to the weight of the ship, and not fire. Comparison to the LZ-129For comparison to a professionally-built zeppelin. Dimensions: 245m x 41m 323,462 m3 volume for cylinder the size of the LZ-129 200,000 m3 volume total 62% of cylinder for airship-shaped volume Engine Output890kW * hr ? J = 3.2GJ * 4 = 12.8GJ 12.8GJ per hour = 307.2GJ/day |
