The Computer Corner Take II (#5) by Bill Kibler

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Solar Hot Air Collector - data collection

This corner is more about Solar Hot Air collection than computers, however I have been collecting data daily and using LAMP tables to show you what is going on. So there are two topics to cover, the solar hot air design and how to collect data. We will start with the collector.

The solar hot air collector is the simplest of designs as well as the simplest to build. Since I have a degree in solar energy, I know these designs and can build one with ease. Basically it work by the sun heating up a black body with air flowing over that black body and absorbing some of the heat. The air then is typically piped into a room to make it warmer. There are a few items to consider, such as direction the collector is facing.

One concept of solar that almost all normal people have wrong is the travel of the sun in the sky. Drive down any country road and you will find most houses oriented completely wrong to take advantage of the sun. I have seen way too many houses in which you can tell the design was done to take advantage of the sun, yet the home owners clearly oriented the house in such a way to spoil that design. I think what would surprise the average home owner is to realize that during the summer in North America, the sun will typically enter North facing windows both early and late in the day. At the summer equinox, the sun reaches it's most northernly travels and rises in the North East, while setting in the North West. Which will put sun into your Northern windows.

The sun's rays only enter south facing windows during the winter and depending on your latitude, will enter some during spring and fall. The spring and fall amount is midigated some by over hangs and window reflection. All this means, a south facing window or collector will not produce heat anytime except during the winter. Knowing that, I created my collector by building onto a south facing wall of my garage. The design uses a rather simple construction process. I took an insulated window that was 3 by 6 feet, and took it apart. The individual sections are spaced out from the back wall by 2x4s and form a 3 foot by 7 foot collector. I painted the T111 siding - a 5/8 inch plywood with groves, flat black before attaching the sheets of insulated glass to the outer frame. To make sure the air flow "collects" more heat, I have horizontal one by twos inside the collector such that the air must make a surpentine path.

A hot air collector works by air moving over a heated surface. The surface is heated by the sun's rays when they strike a black or darkened surface, often called a "body". There can be lots of math involved, but the simple take on this design has to do with thickness of the air space - too much or too wide a path and the transfer becomes less. There are plenty of books and web pages that will help you find an optimum soltuion if you want it. I did my design more as a simple "it can work without any math" approach. Thus if you look at the values returned, you can see, it is far from getting the most out the collector.

Before we get to the collection of data or LAMP side of things, let me explain the data collected and how you can tell if it is working well or not. I have four temperature sensors located in the solar collector. One sensor is in the air intake and reports incoming air temperature. Another sensor is in the air exhaust vent and shows how much heating of the intake air happened. I further check overall response to heating by having a sensor inside the collector, at the mid point that shows how hot the air is at mid point. Lastly I have a sensor located in the bottom and out of the air flow to give me the black body temperature, or my surface temperature of the painted surface.

What you see by looking at the collected data is two things - we never get to black body temperature and the gain in temperature is greatest in the lower section, while the upper section gains are less. Let us discuss the black body first. Black body will always be higher due to the fact that air is not moving past it and thus not pulling heat from surface. Black body represents the highest possible temperature assuming 100% efficenies which are never possible. Moving air, which is what you want to have happen will lower the temperature of the surface as the air removes the heat - the desired action you are after.

The losses of heat energy to other areas effect the other noted data collected. As the temperature increases, the difference between the air and the walls of glass increase and thus more loss through the glass happens. Thus a typical design will show quick and easy rises at the mid point, while the last bit is effected by greater losses and thus become harder to obtain. So a typical design will see 20 or 30 degrees of rise in temperature in the first half, while the second half will see only 30 to 40% as much. Designs that keep the overall temperature low work better than ones trying to get higher temperatures.

Collecting Data

I use LAMP to collect the data and display it on this web page. As you can see I take reading every half hour in centigrade and display both C and F. To help out with more backgound data, I have my office and some outside tmperatures as well. The collection uses a simple serial based temperature sensor using Dallas 1850 chips. These chips provide a ascii string and are easy to collect. The device I use makes it even easier, as the main work has already been done and there is plenty of code samples for you to use.

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