Lake Erie as a Thermal Energy Reservoir

1. Lake Erie as a Thermal Energy Reservoir Jordan Murray Advisor: Dr. Iwan AlexanderDepartment of Physics, Case Western Reserve University, Cleveland, OH ABSTRACT METHODS DISCUSSION • Designed an Excel model of a Rankine Cycle heat engine using ammonia as a working fluid, which was used for preliminary analysis. • Constructed a more refined MATLAB model to create a more detailed representation of system performance over the space spanned by reservoir temperature variables. • Applied appropriate temperature restrictions based on NOAA data for Lake Erie. • 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. 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. 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. 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. 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. 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. Ocean Thermal Energy Conversion (OTEC) Above: A logical diagram of a closed cycle (Rankine) heat engine. RANKINE CYCLE ANALYSIS RESULTS CONCLUSION 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. 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. Deep Lake Water Cooling 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. 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 REFERENCES TEST- The Expert System For Thermodynamics http://www.enwave.com/dlwc.php http://www.glerl.noaa.gov/data/pgs/hydrology.com http://www.et.web.mek.dtu.dk/Coolpack/UK/helpfiles.html