24-11-2012, 12:56 PM
The development of a solar thermal water purification, heating, and power generation system: A case study
The development of a solar.pdf (Size: 1.02 MB / Downloads: 267)
Abstract:
The study conducted by the staff of the Center for Innovation and Development, University of Wisconsin-Stout, reduced- to-practice and confirmed the potential of an invention directed to a water purification system that also recovers power from generated steam. Water was the working fluid and was pumped from a reservoir to an array of 2- 4 foot by 8 foot parabolic solar troughs. A flow control valve adjustable for temperature and pressure, allowed the pressure within the troughs to build, thus increasing the boiling point of the water. At a temperature greater than 100 degrees Celsius, a saturated liquid stream passed through the valve into a vessel that was positioned at the focal point of sunlight within an 8 foot, 9 inch parabolic dish. The flash evaporation occurred, caused by a reduction in pressure on the downstream side of the flow control valve.
The National Renewable Energy Lab (NREL) over a 30 year period determined spatial interpolations of solar radiation values derived from 1961-1990, which are stored in the National Solar Radiation Data Base (NSRDB) (http://rredc.nrel.gov/solar/pubs/redbook/). The values given in table 2 are the yearly average mean values for solar radiation of Direct Beam Solar Radiation for concentrating collectors for a 1-axis, north-south horizontal axis, which is the type of solar collector used for this study. The yearly solar radiation average for Eau Claire, WI (data with closest proximity of actual testing) was 3.1 kWh/m^2/day. The solar heat energy was derived by multiplying the square footage of the system used for this study (10.6m^2) by the yearly solar radiation average value to achieve 32.86kWh/day. When converted to British Thermal Units the value was 112,162.1btu/day. Since 1200 btu/pound are required to change the state of water to steam, the solar heat energy of 112,162.1 btu/day is divided by 1200 to determine pounds per day (93.47). Thus, when converted to gallons per day, the amount of water vapor that should be produced by the system in Eau Claire, WI, based on the yearly average mean values, was 11.29 gallons.
Introduction
A water purification system proposal was conveyed to the staff of Center for Innovation and Development at the University of Wisconsin-Stout in November 2006 by Mr. Diccon Fiore with the intent of determining feasibility. The desalination system originally combined the methods of evaporation and distillation, thus the term “evapodis.” The impetus for this method of desalination originated with the perceived need for economical and reliable means of purifying surface water based on climate change models that showed a worsening of drought conditions and a rise in sea levels (http://news.bbc.co.uk/2/hi/science/nature/2943946.stm). The aim of the proposed system was not to compete with large-scale, industrial desalination and purification facilities, which are predominate in wealthy countries, but rather to provide a means of water purification in isolated communities in underdeveloped and arid regions. The purpose of this study was to determine the feasibility of the proposed system and reduce-to-practice the embodiment as eventually designed.
The description of the system included a solar-powered, evaporation and distillation apparatus intended to purify sea water that included the following components: a glass dome, an evaporation chamber, a vacuum chamber, and a condensation chamber with a refrigeration unit. The glass dome was to cover the evaporation and vacuum chambers. The evaporation chamber was a metallic, black spherical chamber with a bowl to hold sea water. As sunlight heated the chamber, was would evaporate and hot-moisture laden air would evaporate through an outlet connected to the vacuum chamber. A drain at the bottom of the bowl would open periodically to flush the brine. The vacuum chamber had an inlet portal at the bottom of a sphere and an exit portal at the top that was larger in diameter than the inlet portal. The process of the hot air exiting the chamber through a larger portal would create a vacuum in this chamber as well as the evaporation chamber. The purpose for this desired drop in pressure would be to decrease the effective boiling point of water, thus increasing the efficiency of the water purification system. The air vapor flowed from the vacuum chamber to the condensation chamber, which was cooled to increase the efficiency of distilling water.
Research and Development
The Center staff determined that the proposed system for desalination of water had merit but was probably not feasible to construct and implement in a cost-effective manner in its current configuration. An investigation into a more effective manner of distilling water commenced by researching existing systems for concentrating solar energy. This became a compelling notion for several reasons, because most notably the heat generated by the solar concentrator could be used to distill water and also operate a conventional power cycle, for example through a steam turbine or a Sterling engine. Further, solar heat collected during the day could also be stored in liquid or solid media like molten salts, ceramics, and concrete.
Construction
The development of the components to the system commenced with construction of a ten foot long, parabolic trough. A sheet of 4 foot by 10 foot 16 gauge aluminum was held in place by a series of plywood forms (see Figure 2), each having a groove cut into the shape of a parabola. A mirror film was secured to the aluminum with spray adhesive. A 1 inch diameter copper pipe was inserted into a hole located at the focal point of the parabola on each form. The pipe was supported on the ends of the trough with a wooden brace. This brace supported the pipe, forms, and entire parabolic trough. The trough rotated about the pipe (see Figure 3).