1. TO: Dunnough McJack
FROM: Ernest Moniz
RE: Off-Shore Energy Storage Analysis
DATE: March 7th
, 2015
Regarding Ethereal Engineering Enterprise’s inquiry into the feasibility of an air bag energy
storage system, we designed a power cycle and analyzed the utility of each working fluid/lake
combination. We found that several systems using a variety of combinations could work; our focus is on
one of the most efficient, and we will use it to demonstrate the process through which we analyzed the
rest. See Appendix A for cost and volume analysis of the lake/working fluid permutations. We found that
a system using oxygen at Lake Clark can generate the required .001% of US household energy
consumption. In a single submerged balloon system, the required volume is 33,690 m3
, which falls below
0.01% – 0.025% of Lake Clark’s 1.64 1011
m3
total volume. The cost of running this system would
include an $18,567,720 fixed cost and a running cost of $3,786 per day. The gross revenue generated by
the turbine is $6,854 per day. Selling at peaks hours, 8 hours a day, would result in a cost recovery time of
6,145 days. The system will require a molar flow rate of 1546.38 mol/s. For a complete table of
thermodynamic data relating to each gas species, see appendix D.
Working Fluid/Lake
Regarding the cost/volume Analysis in Appendix A, we have disqualified location/working fluid
permutations due to volumetric restrictions or economic infeasibility arguments. Some working fluid/lake
permutations were disqualified because of a balloon volume that exceeded the project restrictions. Others
require a running cost that is greater than the gross earnings for that system; these have been disqualified
on the grounds of economic infeasibility. We found that Lake Clark with Oxygen as a working fluid is a
promising permutation because of low balloon volume, positive revenue, and low heat loss through
temperature equilibration. We assumed that the temperature at depth for each lake is 4° C, and that their
maximum depth was constant. For the relevant data associated with each location, see Appendix B.
Turbine
We assumed that the turbine is perfectly adiabatic such that there is no heat transfer associated
with the gas expansion. The shaft work generated by the turbine is 96.2 kJ/kg. Running for eight hours a

2. day, during peak hours, this will generate the prerequisite 1.39 107
kwh. We set the output of the
pressure of the turbine to be atmospheric pressure, such that we can convert the maximum amount of
energy from the working fluid into shaft work without involving a system that experiences vacuum
conditions. For a complete set of turbine calculations, see Appendix C.1.
Compressor
As with the turbine, we assumed that the compressor is perfectly adiabatic. The compressor will
run for eight off peak hours a day, when energy is cheapest, requiring a power input of 106.3 kJ/kg. The
output pressure of the compressor is equivalent to the pressure of the working fluid in the submerged
balloon. The temperature difference between the working fluid at the compressor output and the lake
generates a Qc as heat is transferred through the balloon into the lake. The heat lost through temperature
equilibration is 10.1 kJ/kg (Appendix C.3). For compressor calculations see Appendix C.2.
Balloon
Our system includes two balloons; one surface balloon to store the working fluid at atmospheric
pressure during on peak hours and one submerged balloon to store the working fluid at maximum
pressure during off peak hours. No calculations are included for the low pressure storage balloon at the
surface. We are assuming an insulated pipe/balloon system for the turbine to compressor step. Therefore,
there will be no heat transfer between balloon and the atmosphere. The working fluid will be in the same
state inside the surface balloon and at the output of the turbine. See Appendix C.4 for balloon
calculations.
Safety and Environmental Considerations
Implementing this system requires a balloon to be secured to the bottom of a lake, which could
potentially interfere with local ecosystems through its large physical volume. When placing the balloon,
areas of high ecological importance will have to be avoided. Several of the available locations are within
state parks which may interfere with the approval of this system. A safety concern is the potential stability
of the balloon due to strong buoyancy effects. This could be very problematic because the amount of
working fluid inside of the balloon would cause it to rise very rapidly and anyone working nearby could
be in danger. A solution may be to include a mechanism designed to release all of the working fluid in the
balloon in the event that the balloon detaches from the bottom of the lake. The available working fluids
should not cause any environmental harm, even if released into the lake due to the fact that the molar

3. amounts will be insignificant relative to the molar amount of water in each lake. None of the working
fluids will react dangerously with water. Hydrogen is explosive in a mixture with as low as 4% air and
would be a severe safety hazard when stored in the surface balloon. We strongly recommend against
using hydrogen as the working fluid, even with stringent safety precautions.
Conclusion
The Ethereal Engineering Enterprise energy storage system will be able to store the required
amount of energy by implementing a system similar to the one laid out in this analysis. Once
implemented, the system will be able to run safely and efficiently. The long cost recovery time of the
project will have to be considered when establishing funding for the system. Please feel free to contact us
with further inquiries.