Lake Erie as a Thermal Energy Reservoir
Jordan Murray
Advisor: Dr. Iwan Alexander
Department of Physics, Case Western Reserve University, Cleveland, OH
ABSTRACT
METHODS
The abundance and high specific heat capacity of
water enable large bodies of water to absorb and store
enormous amounts of energy. Bodies of water have
long been used as heat sinks for high-temperature
industrial applications, but pressure to conserve energy
has resulted in greater interest in rejecting the
unwanted heat in homes and offices into the cooler
bodies of water as well. The thermal inhomogeneity of
large bodies of water permits them to function as
energy reservoirs for an electricity generating heat
engine. This project examines the feasibility of the
application of two technologies which take advantage
of water’s energy storage capacity in Lake Erie.
DISCUSSION
OTEC is a technology which is capable of filling only a niche
role in the global power supply, namely, serving
developing tropical island nations. Temperature conditions
are optimal year round, enabling an OTEC installation to
operate up to the practical limit of ~100MW, at which
point the mass flow rate of circulated water becomes
more difficult to manage than is likely to be worth doing in
any case. The low power demands of such nations would
not make this limitation an obstacle. Open cycle systems
produce fresh water as a byproduct. The self-contained, if
not entirely self-sustaining (piping can become corroded
or obstructed by algal growth or other marine life e.g.
zebra mussels), nature of the technology is another
advantage for remote locations. As a major source of
alternative energy, there seems to be little hope for the
technology. None of the technology’s ancillary benefits
are important in a first world setting, temperate climates
severely hamper efficiency, and environmental concerns
would limit the capability to generate power outside of the
ocean.
Greenhouse gas emissions are negligible,
conventional low-emission energy sources such as
windmills would be a better investment.
1. Designed an Excel model of a Rankine Cycle heat
engine using ammonia as a working fluid, which
was used for preliminary analysis.
2. Constructed a more refined MATLAB model to
create a more detailed representation of system
performance over the space spanned by reservoir
temperature variables.
3. Applied appropriate temperature
based on NOAA data for Lake Erie.
Ocean Thermal Energy
Conversion (OTEC)
restrictions
4. Estimated the heat storage capacity of the Eastern
Basin of Lake Erie for use as a thermal reservoir for
a deep lake water cooling system.
RANKINE CYCLE ANALYSIS
Deep Lake Water Cooling, however , could enable a
significant reduction in the amount of power used for
climate control during summer. While the western and
central basins of Lake Erie are too shallow and
consequently warm too early in the season to be very
useful, but the Eastern basin is deep enough to contain
cold water through August. The deepest part of the basin
is small but still shallow enough that the temperature
strata inverts yearly, thus recharging the heat storage
capacity.
Above: A logical diagram of a closed cycle (Rankine) heat engine.
Deep Lake Water Cooling bypasses the inefficient step
of electrical power generation, instead using the heat
capacity of cold reservoir to absorb unwanted heat
from buildings, thus conserving electrical power which
would otherwise have been used for this purpose. The
technology has already been implemented successfully,
perhaps most notably in the system serving Toronto’s
financial district, which is capable of providing up to
207 MW of cooling power using cold water drawn from
270 feet below the surface of Lake Ontario. Water is
pumped from depth through large HDPE pipes to a
central heat exchanger. At this point, numerous closed
loop circulation lines transfer absorbed heat from
buildings to the cold lake water. The cooling power
available from a body of water is limited by
environmental concerns. Algal blooms and resultant
oxygen depletion can be triggered by the disturbance
of thermal stratification due to the discharge of large
amounts of heat into a body of water.
Power Output per Heat Exchanger Area
for h=4 W/Km2 and 1 C between reservoir and operating temperatures
-5
x 10
9
8
7
6
5
4
3
2
1
0
12
11
10
22
9
8
18
7
6
14
5
Lower Operating Temperature (C)
12
15
16
19
20
23
21
17
13
Upper Operating Temperature (C)
As was determined at an early stage by the rough model, the
performance of OTEC technology is limited by heat transfer
requirements. The low thermal efficiencies attainable while
operating across temperature differentials as low as those
found in lakes and oceans necessitate high rates of heat
transfer to develop even modest amounts of power.
Consequently, heat exchangers with large amounts of surface
area are necessary to meet this demand. The plot above gives
net power per heat exchanger surface area as a function of
high and low operating temperatures. Conditions on the lake,
of course, vary dramatically on seasonal timescales and in
different regions of the lake. The optimum conditions
attainable are represented by the deep red region in the upper
right corner. More commonly available conditions fall in the
yellow to orange regions. The area specific net power
varies roughly linearly with temperature in the region
of interest. OTEC systems operating in Lake Erie under
good conditions during summer could be expected to
perform only about half as well as a similar system
operating in an equatorial environment. While a
putative lake-based OTEC installation would already
suffer from low thermal efficiency, further sacrifices
would have to be made to due to heat exchanger
considerations. In order to take maximum advantage
of the temperature difference, it would be advisable to
constrain the working fluid to undergo phase changes
close to the reservoir temperatures. However, heat
transfer at constant temperature close to ambient
further inflates heat exchanger area requirements.
Therefore, it is necessary to trade thermal efficiency in
the interest of reducing heat exchanger size to increase
the specific net power. Below, is the result of
optimization for optimum lake conditions, with thermal
reservoirs at 6 and 22 C. To maximize specific net
power, the temperature differential must be halved.
1.6
CONCLUSION
OTEC is entirely unsuitable for use in Lake Erie. Heat
exchanger requirements pose engineering challenges
which are incongruent with the marginal power
generation capacities that are achievable. Additionally,
the system would be all but inoperable for most of the
year. Deep Lake Water Cooling appears to be quite
promising for the Eastern Basin region of Lake Erie, where
it could offset heating energy costs for densely populated
lakefront areas like Erie, Pennsylvania.
ACKNOWLEDGEMENTS
Thanks to Dr. Alexander for his advice and The Great
Lakes Institute for Energy Innovation for defraying the
cost of analytical tools, and SOOS Innovations for the
proposal on which the project was based.
REFERENCES
Operating Temperature Optimization
-4
x 10
TEST- The Expert System For Thermodynamics
1.4
Net Power per Heat Exchanger Surface Area
Deep Lake Water Cooling
RESULTS
Net Power / Heat Exchanger Surface Area (kW/Km2)
OTEC uses the naturally occurring temperature
stratification which occurs in large bodies of water to
generate energy. Solar-radiation keeps surface water
warm and buoyant which, in combination with other
factors, results in the formation of a separate
convective layer from cold water at depth. These layers
exchange heat at sufficiently low rates that they can be
considered distinct thermal reservoirs. Heat engines of
numerous types could be operated using the
temperature difference. Most commonly, the Rankine
cycle is used with ammonia as the working fluid. OTEC
systems function most efficiently when the
temperature differential is largest, as in equatorial
oceanic environments, where there is direct sunlight
and sufficient depth to guarantee the presence of
water at 4 C.
1.2
http://www.enwave.com/dlwc.php
1
0.8
http://www.glerl.noaa.gov/data/pgs/hydrology.com
0.6
0.4
0.2
0
0
1
2
3
4
5
6
Gap Between Reservoir and Operating Temperatures (K)
7
8
http://www.et.web.mek.dtu.dk/Coolpack/UK/helpfiles.
html