An environmental and economic comparison of cooling system designs for steam-electric power plants

Originally presented as a thesis (M.S.), M.I.T., Dept. of Civil Engineering, 1978, by Kenneth F. Najjar.The selection of waste heat rejection systems for steam-electric power plants involves a trade-off among environmental, energy and water conservation, and economic factors. This study compares four general types of cooling systems on the basis of these factors. The cooling systems chosen for study are: once-through systems including surface canals and submerged multiport diffusers; shallow closed cycle cooling ponds; mechanical and natural draft evaporative cooling towers; and mechanical draft dry towers. The cooling system comparison involves, first, an optimization of each cooling system and then a comparison among optimal systems. Comparison is made for an 800 MWe fossil unit and a 1200 MWe nuclear unit located at a hypothetical midwestern river site. A set of models has been developed to optimize the components of each cooling system based on the local meteorological and hydrological conditions at the site in accordance with a fixed demand, scalable plant concept. This concept allows one to compare the costs of producing the same net power from each plant/cooling system. Base case economic parameters were used to evaluate the optimum system for each of the four general cooling systems followed by a sensitivity study for each parameter. Comparison of energy and water consumption follows from the results of the performance model, while comparison of environmental impacts is mostly qualitative. Some quantitative modelling was performed for the environmental effects of thermal discharges from once-through systems, fogging from wet cooling towers and water consumption from the ponds, wet towers and once-through. The results of the optimization models of each of the systems are compared on the basis of: performance - discrete distributions of environmental conditions and transient simulation; economics - using base case scenarios and sensitivity values to arrive at costs expressed in terms of production costs, annualized costs and present value costs; energy and water consumption; and environmental effects. The once-through systems were found to be the least expensive of the four systems, the most energy efficient, but potentially the most environmentally damaging. On the other extreme, dry cooling towers are the most environmentally sound while being the most expensive and least energy efficient. Finally, the results of the economic optimization are compared with results from previous comparative studies

Ocean thermal energy conversion plants : experimental and analytical study of mixing and recirculation

Also issued as Massachusetts Institute of Technology. Ralph M. Parsons Laboratory for Water Resources and Hydrodynamics. Report no.231. Prepared by the Energy Laboratory in association with Ralph M. Parsons Laboratory for Water Resources and Hydrodynamics.Ocean thermal energy conversion (OTEC) is a method of generating power using the vertical temperature gradient of the tropical ocean as an energy source. Experimental and analytical studies have been carried out to determine the characteristics of the temperature and velocity fields induced in the surrounding ocean by the operation of an OTEC plant. The condition of recirculation, i.e. the reentering of mixed discharge water back into the plant intake, was of particular interest because of its adverse effect on plant efficiency. The studies were directed at the mixed discharge concept, in which the evaporator and condenser water flows are exhausted jointly at the approximate level of the ambient ocean thermocline. The OTEC plant was of the symmetric spar-buoy type with radial or separate discharge configurations. A distinctly stratified ocean with uniform, ambient current velocity was assumed. The following conclusions are obtained: The recirculation potential of an OTEC plant in a stagnant ocean is determined by the interaction of the jet discharge zone and a double sink return flow (one sink being the evaporator intake, the other the jet entrainment). This process occurs in the near-field of an OTEC plant up to a distance of about three times the ocean mixed layer depth. The stratified internal flow beyond this zone has little effect on recirculation, as have small ocean current velocities (up to 0.10 m/s prototype). Conditions which are conducive to recirculation are characterized by high discharge velocities and large plant flow rates. A design formula is proposed which determines whether recirculation would occur or not as a function of plant design and ocean conditions. On the basis of these results, it can be concluded that a 100 MW OTEC plant with the mixed discharge mode can operate at a typical candidate ocean site without incurring any discharge recirculation.Prepared under the support of Division of Solar Energy, U.S. Energy Research and Development Administration, Contract no. EY-76-S-02-2909.M001

Model Study of a Flood-control Pumping Station at the Charles River Dam

Prepared under contract with the Metropolitan District Commission of the Commonwealth of Massachusetts.This report describes the design, construction and testing of a Froudian model of a proposed 8400-cfs capacity pumping station, Site restrictions require that the flow approach the high specific-speed pumps asymmetrically from an existing ship lock through the Charles River Dam (Boston, Massachusetts). The model included a portion of the Charles River Basin, the existing navigation lock and the pump forebay at the exit of the lock. A single recirculating pump and a suction manifold was used in the model to withdraw water from the forebay through six intakes simulating the prototype pumping station. Flow patterns were obtained by photographs of floating confetti and subsurface streamers. Water surface measurements were made with a point gage read through a surveyor's level. The majority of tests were run with the maximum design discharge and the minimum basin pumping elevation. This provided the most severe forebay conditions of high velocity and low intake submergence. Tests were made to investigate: (1) the improvement of flow conditions at the entrance to the lock; (2) the performance of a single intake in uniform approach flow; and (3) the performance of several forebay and pumping station arrangements. The tests showed that: (1) an 18 ft diameter semi-cylindrical pier was needed at the lock entrance to reduce flow contraction and entrance loss; (2) the intake performed very well when the approach flow was uniform; and (3) the most satisfactory forebay arrangement, within the design restrictions imposed by the site, was with equal lengths of intake chambers. The center line of the pumps and the straight portion of the intake chamber walls were deflected 200 toward the approach flow. The. straight portion of the intake chamber walls were 51 ft in length and thence curved upstream in a circular arc. The circular arc terminated six ft from the lock line and the chord of the are forced an angle of 400 with the line of the lock. Vertical struts placed behind the intakes retarded circulation in the intake chambers and improved the flow into the intakes

Ocean Thermal Energy Conversion Plants: Experimental and Analytical Study of Mixing and Recirculation

Prepared under the support of Division of Solar Energy, U.S. Energy Research and Development Administration, Contract no. EY-76-S-02-2909.M001Ocean thermal energy conversion (OTEC) is a method of generating power using the vertical temperature gradient of the tropical ocean as an energy source. Experimental and analytical studies have been carried out to determine the characteristics of the temperature and velocity fields induced in the surrounding ocean by the operation of an OTEC plant. The condition of recirculation, i.e. the re-entering of mixed discharge water back into the plant intake, was of particular interest because of its adverse effect on plant efficiency. The studies were directed at the mixed discharge concept, in which the evaporator and condenser water flows are exhausted jointly at the approximate level of the ambient ocean thermocline. The OTEC plant was of the symmetric spar-buoy type with radial or separate discharge configurations. A distinctly stratified ocean with uniform, ambient current velocity was assumed. The following conclusions are obtained: The recirculation potential of an OTEC plant in a stagnant ocean is determined by the interaction of the jet discharge zone and a double sink return flow (one sink being the evaporator intake, the other the jet entrainment). This process occurs in the near-field of an OTEC plant up to a distance of about three times the ocean mixed layer depth. The stratified internal flow beyond this zone has little effect on recirculation, as have small ocean current velocities (up to 0.10 m/s prototype). Conditions which are conducive to recirculation are characterized by high discharge velocities and large plant flow rates. A design formula is proposed which determines whether recirculation would occur or not as a function of plant design and ocean conditions. On the basis of these results, it can be concluded that a 100 MW OTEC plant with the mixed discharge mode can operate at a typical candidate ocean site without incurring any discharge recirculation

Medical Students' Perceptions of Surgeons: Implications for Teaching and Recruitment.

ObjectiveThe purpose of this study was to assess first-year medical students' implicit perceptions of surgeons, focusing on the roles of gender and demeanor (communal = supportive, associated with women; agentic = assertive, associated with men).DesignSurvey study. Each survey had 1 of 8 possible scenarios; all began with a short description of a surgeon who was described as accomplished and well trained, then varied by surgeon gender (male/female), surgeon demeanor (agentic/communal), and type of surgery (breast cancer/lung cancer). Using a 0 to 5 scale, respondents rated their perception of the surgeon through 5 questions. These 5 items were averaged to create a composite perception score scaled from 0 to 5.SettingSurveys were administered at the University of California, San Francisco, and the University of California, Los Angeles.ParticipantsWe administered surveys to 333 first-year medical students who could read English and voluntarily agreed to participate.ResultsA total of 238 students responded (71.5%). They preferred the communal vs agentic surgeon (4.2 ± 0.7 vs 3.9 ± 0.7, p = 0.002) and male medical students perceived surgeons more favorably than female medical students did (4.2 ± 0.6 vs 4.0 ± 0.8, p = 0.036). The preference score did not differ according to surgeon gender (female 4.12 vs male 3.98, p = 0.087). There were no significant interactions between the factors of student gender, surgeon gender, or demeanor. Students who reported an interest in surgery as a career did not perceive surgeons more favorably than the students interested in other fields (4.3 ± 0.7 vs 4.0 ± 0.7 respectively, p = 0.066).ConclusionsBased on our findings, surgeon educators would likely find success in teaching and recruiting medical students by employing a communal demeanor in their interactions with all students, regardless of the students' gender or stated interest in surgery