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Harvesting solar energy from rooftops can improve sustainability

July 15, 2004
Enhance conservation efforts by harvesting the energy cascading onto your rooftop
Felix Trombe, a French engineer, is credited with the simple idea of making a solar collector out of a south-facing wall, the key feature of which is an air space between a sheet of glass and a blackened concrete wall. The resulting device, a Trombe wall, functions as intended, but a lack of windows renders the interior living spaces as dark and somber as a tomb. And its energy collection process is reversible. When incident sunlight is insufficient, the Trombe wall moves energy from the warm interior to the great outdoors. Also, it's obviously not very good at tracking the sun. Surely there's got to be a better route to solar energy.Please join me for a dive into the morass we call the Web in search of zero-cost, non-commercial, registration-free resources that offer practical information about collecting energy from that thermonuclear orb hovering 93 million miles above our heads. Remember, we search the Web so you don't have to.

Sun 101

Let's start basking in the sun at the University of Hong Kong. The school offers "Seminars in Building Technology, Section One , Energy Efficiency in Buildings," which makes this site a good place to get an overview. The large collection of course notes introduces the basic concepts of energy efficiency in buildings, provides practical information about energy-efficient technologies, and explains the common methods used for building energy analysis, which is valuable in any case. You can find it at www.arch.hku.hk/teaching/SBT99/.

"Power From the Sun" is an online book by William B. Stine and Michael Geyer at the J.T. Lyle Center for Regenerative Studies, California State Polytechnic University, Pomona. It's a work in progress, which explains why every chapter hasn't been posted yet. Nevertheless, what is available represents a solid entry into the mathematics and theoretical aspects of solar energy. The equations presented there make it possible for you to model the precise geometry and interaction between the sun and your plant. Especially interesting is the material in chapter two that discusses the remarkable physical and chemical properties of the sun. You'll find this and more at www.powerfromthesun.net/book.htm.

HVAC and process apps

The popular press seems to mention solar energy only in conjunction with residential applications. Let me assure you that industrial applications exist. The most important of them are heat for industrial processes, solar cooling and air conditioning, solar drying, distillation and desalination, each of which requires solar-generated temperatures between 180°F and 480°F.

Regardless of application details, a solar collector is a mandatory piece of hardware. For an overview of suitable collector types and reference material, go to "Solar Heat for Industrial Processes," a Web page by The Potential of Solar Heat in Industrial Processes (POSHIP), a program of the European Commission Directorate for Energy and Transport. Mouse over to www.aiguasol.com/poship/poshipBO.htm for a good read. Before you leave the page, scroll to the bottom to access "The potential of solar heat in industrial processes , a state-of-the-art review for Spain and Portugal," a paper presented at the EuroSun 2000 Congress, Copenhagen, Denmark, 2000. The document summarizes recent developments in the field of medium- and high-temperature solar collectors, and gives an overview of efficiency and cost of existing technologies, It also outlines future trends resulting from current research and development projects. "POSHIP Final Report," another document you can access, is a 174-page opus the organization published on Oct. 15, 2001. It contains case studies and technical material that should be must-reads for anyone thinking about collecting sunbeams for industrial use.

Collector design

Exploiting the energy beaming down upon us requires capturing it in a collector, of which there are many designs. The most common solar device, a flat plate collector, consists of a conduit to transport some fluid through a flat, insulated, double-glazed box oriented to face the sun. Considering the large number of variables involved, predicting the unit's performance requires significant mathematical prowess. The good folks at the University of Wisconsin-Madison College of Engineering's Solar Energy Lab, the oldest of its kind in the world, offer the Collector Design Program (CoDePro). It's a free software download that allows you to design your own solar collector and compute efficiency curves for comparison with experimental data. Shine your light on http://sel.me.wisc.edu/codepro/new_codepro.html to start designing.

Your daily dose

Denver claims about 300 days of sunlight each year; however, for the residents of Seattle, seeing bright sun is a rare event. It's clear that the details of effective solar system design are a function of location and a few other variables, such as the quality, extent and duration of sunlight.

A good source of such sunlit information is the Solar Radiation Resource Information page, found at http://rredc.nrel.gov/solar/. For example, the archived files characterize solar radiation at daily and hourly intervals. Another file, "Solar Radiation Data Manual for Buildings," published by our National Renewable Energy Laboratory, gives hourly values of measured or modeled solar radiation and meteorological data for 239 data collection stations during the 30-year period from 1961-1990. These are only a few examples of what's available at the site for someone trying to develop a rigorous feasibility study for a proposed solar installation.

Tax money at work

A common attitude among rational taxpayers is that any research and development funded by the money revenuers take from us should be placed in the public domain. To some extent, our hired hands in Washington are playing along with that sentiment. For example, consider www.eere.energy.gov/buildings/tools_directory/, the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy's Building Energy Software Tools Directory. Entering "+free +solar" in the search box will return at least 20 solar- and energy-related software packages, of which several are free, others are free but cease working after a few days, and some are shareware. This is where you can learn about water heating, 3-D modeling to analyze collector shading, solar planning and design, thermal properties of building elements, skylights and more.

Design in detail

SOLAR-5.7 is a software package that displays 3-D plots of hourly energy performance. It uses hour-by-hour weather data, and can calculate detailed electricity costs and show a detailed picture of energy costs. To collect energy from the sun, one must first be able to predict the location of that glowing orb relative to a collection device. SOLAR-2 plots sunlight penetrating through a window and creates an hour-by-hour movie using a 3-D sun's-eye view of your building, which should be useful for siting a collector on the property. These programs and others are available from Murray Milne, Department of Architecture and Urban Design at the University of California, Los Angeles. His offerings are found at www.2.aud.ucla.edu/energy-design-tools/.
Use this free software at http://sel.me.wisc.edu/codepro/new_codepro.html to design your own solar collector.

The sun's path

From our frame of reference, the sun appears to traverse a slightly different path across the sky each day. The reason, of course, is that the earth both rotates on its inclined axis and moves laterally in its orbit. The result is a continuous shifting of our collective astral frame of reference. But being able to foretell when and where Ol' Sol will be shining is an essential element of solar design. The predictive calculations are an exercise in trigonometry that can be performed with a pocket calculator, a blunt pencil and a ream of paper. On the other hand, the same work can be done more easily on a PC, and easier yet online.

For example, consider Sunpath 3.2, a package from the Florida Solar Energy Center, an institute of the University of Central Florida. The software calculates the sun's position in the sky for any day of the year and time of day, or for a sequence of days and times. It's a good program if you're planning to do some number crunching because the output is an ASCII file that can be ported to spreadsheets and other numerical workhorses. Let your mouse scurry to http://fsec.ucf.edu/download/br/fenestration/software/Software_Download.htm to get the goods.

If you confine yourself to Excel, however, Robert L. Bleidt of Santa Clara, Calif., has a spreadsheet posted at www.hdtv.com/rb/sunpath.htm. It exploits the charting feature of Excel to generate a graph of the sun's position for a given time of day and month of the year for a point on the earth's surface. This resulting plot of azimuth versus bearing is commonly used for designing solar energy installations. To make it all work, however, you're going to need to know your latitude and longitude, as well as the magnetic variation in your location. With the plot in hand, you can stand at the proposed location for your collector, read your compass and visualize how high the sun will appear in any direction at any time of year. You can find your geographic coordinates using some of the Web sites highlighted in "GPS and GIS," the Internet column that appeared here in September 2003, but is now safely ensconced on www.plantservices. com. To determine your declination, go to www.ngdc.noaa.gov/seg/potfld/declination.shtml and enter either your ZIP code or coordinates.

The reason magnetic declination is so important is that you can plot the location of possible obstructions that might prevent full sunlight from reaching your collector system. An example showing a practical application of a sun chart is given in "Passive solar design using computer generated suncharts," an article by Thomas M. Crawford, an optical design consultant. Send mousie to www.srv.net/opt/sunchrt.html with explicit instructions to fetch Figure 3 from the document. Use your declination-corrected compass readings to superimpose site-specific obstructions on the chart to optimize collector siting.

Those who are a bit leery about downloading unknown software from the Web should visit the site maintained by Sustainable By Design, a company in Seattle that provides scientific, design, multimedia and communications services to the environmental community. There you will find several online Java-based computations, of which the most relevant are SunAngle, SunPosition and Sun Path. While you're visiting www.susdesign.com/design-tools.html, take a look at another program, Window Overhang Design, that represents the antithesis of solar collection. This interactive program aims at limiting solar heating by means of a window overhang, the dimensions and orientation of which admit maximum sunlight in winter and minimum in summer. 

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