Makeshift Sunlight Sensor

Shortly after graduating college I had the opportunity to "test drive" a solar house. This was a house that had been designed and built by college students, but had not been lived in yet. Going into this I was aware that I would need to solve issues with the house as they popped up.

One of those problems was with the solar hot water heaters. As designed the solar hot water heaters would pump a heat transfer fluid (Ethylene Glycol) through a set of collectors mounted on the roof. When the sun shown, the water would be heated and then transfer the heat to the hot water tank by a coil immersed in the tank.

Solar CollectorsSolar Collectors

The problem is, if the system pumps while the sun is not out it will operate in reverse, cooling the tanks. It is critical for the system to know when to pump and when to not pump. The original design measured the temperature after the collector and the temperature in the tank. If the temperature after the collector was sufficiently higher than the temperature in the tank, the pump would run, if it was lower or not enough higher, it would stop. The problem was that the temperature sensors were not calibrated well to each other and the sensor after the collectors only read correctly when the pump was on.

There several possible solutions to this problem. The "correct" solution would probably have been to add an additional actuator valve that would allow the fluid to be circulated through the collectors without circulating through the tank. However this would have required fairly extensive system modifications, expensive parts and draining and repressurizing the system. Thus I strive for an easier to implement and test solution.

The first one I tried with limited success was to simply use time of day to determine whether or not to run the pump. This worked OK, but if the controller lost power it would lose its time reference. It also couldn't react to cloudy days.

The solution I eventually ended up with used a Radio Shack(tm) solar cell to detect daylight. In order to provide basic weather proofing I used the box that the solar cell was sold in to provide the enclosure. I drilled two holes in the back, one for each wire soldered to the solar cell. I then sealed the box and holes with epoxy. To allow for angling of the cell I built a structure using an l beam, two pieces of square stock and a u-bolt. The first square beam is attached vertically to the l-beam and has a 1/4-20 hole tapped near the end. The second square beam is tapped 1/4-20 through the end.

Sun Sensor InstalledSun Sensor Installed