Sunday, May 21, 2006

waiting for the sun to shine again

By May 9th, we'd had three fabulous sunny days in a row and the pool temperature was up to almost 70 degrees F (starting from about 55).

Since then, the weather has turned cold and rainy and there is no sun until next Tuesday according to the current forecast. Almost two weeks without sun.

I've diverted to other projects temporarily.

Here is the current setup of six collectors waiting for the sun to shine again.

The current drive system (click on any image to enlarge it) consists of a motor driven screw which carries a traveller connected by push rod to the end of the metal shaft connecting the reflector arms.

The tracking sensors and drive electronics are not yet installed. The reflectors are moved manually by connecting a battery to the motor wires as long as it takes to move the reflectors toward the sun.

Here is another view of the drive mechanism. The springs at each collector arm help to absorb buffeting from the wind.

While the heat collector tubes are metal and opaque to light, I will add a glass tube at the focus of one or more of the collectors to allow the intensely focussed sunlight to travel through the water. While the heat absorption may not be as good as the black coated metal collector tubes, since the sunlight is concentrated about 20 times, I expect that the concentrated UV rays will sterilize microorganisms in the water.

I've pressure tested the seals of the one glass collector tube which I've assembled, shown here waiting for installation.

Sunday, May 07, 2006

the start of a new season

Spring seems early this year and I'm back to work on the tracking solar pool heater project. Yesterday I progressed a bit further than this picture and had the water flowing through six collector panels.

Here you can see the zig-zag path of the water pipes (click on the image to enlarge). The two left collector panels have not yet been mounted nor the flex hoses to attach the collectors to the inlet and outlet pipes.

The motor drive and tracking electronics have not yet been added either, but since I was working in the garden it was really no trouble to check the aiming of the panels every hour or so to keep them pointed at the sun. Because the collector pipes are fairly large (1.5 inch diameter copper), the focussed sunlight travels across the width of the tube over about an hour so the movement of the reflectors is not at all critical.

It was cloudy for the first part of the day and a steady breeze blew for most of the day. It seemed warm, but the air temperature didn't rise much above 60F in the shade. From about 1pm on, the clouds broke up and it was fairly steady sun for most of the afternoon.

I had been running the pump daily for a few hours over the last two weeks as we opened the pool and got the algae and winter debris cleaned up so the water was already pretty well mixed.

At about noon, when I had the water path through the collectors hooked up and the pool pump running for about an hour, the water temperature was about 55 degrees F. At 6pm, when I turned off the pump, the water temperature in the pool was about 60 degrees F, a heat rise of about 5 degrees F for the afternoon.

I was measuring the inlet and outlet temperature with some difficulty because of my primitive equipment. Because the full flow of the pump goes through the system, the water flows through the collector panels at a great speed. My temperature meter allows me to measure two temperatures and to take the difference between them, but with no decimal places on the measurement. Nevertheless, I was getting in excess of a one degree F rise in the water temperature through the system. I am using F because the degrees are smaller and if I use C, the meter only gives me zero, since the temperature rise is more than an F degree, but less than a C degree.

With the fast flow of water, even a one degree rise is fairly significant and within my expectations. The collectors feel cold to the touch although if you hold your hand in the focus you see very bright light on your hand and your fingers feel funny. I don't do that for more than a second or so, just to convince myself that heat is really flowing into the tubes!

I hope to improve my temperature measurement resolution and also to determine the flow rate accurately so that I can confirm the amount of heating. My estimate is that each panel should add the equivalent of about 500 watts of heat, making the six panels equivalent to a typical electrical clothes dryer (about 3KW). The frame I have built will hold seven more such panels.

This morning there is frost on the ground and the air temperature is about 32 degrees F. I can see that the water in the flex hoses is frozen, so I may be pushing it a bit, since the water does not drain from the system when the pump is off. The temperature last night didn't go much below freezing, so I hope that expansion of the ice didn't cause any damage. We'll see later today when I start it up again.