Where the sun shines

Enhance conservation efforts by harvesting the energy cascading onto your rooftop

By Russ Kratowicz, Executive Editor

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.