Flat plate vs. evacuated tube collectors.
There is an inherent mis-match between the efficiency of PV panels and solar thermal collectors such that the PV will have enough power to run a pump when the collector is not hot enough to be useful. This is most pronounced in cold climates. This is less true of evacuated tube collectors which are more efficient and do not radiate heat the way that flat plate collectors can.
Late in the day when your storage tank has accumulated a lot of heat on a cold sunny day is the point at which you may need to shut off your pump. The collectors are not getting enough sun to generate a higher temperature than the stored water. What happens if the pump continues running is that your stored heat is radiated out from flat plate collectors. With evacuated tubes you are likely to be pumping cooler water into the tank. The controller is designed to prevent this from happening.
Delta-T (aka hysteresis)
What does this mean? Delta is Greek symbol used to denote Difference, and T = temperature. Many other (AC powered) DTC's on the market have an adjustable Delta-T that sets the difference beween the sensor temperatures before the pump is activated. The controller does not, it
simply switches the pump on the moment one sensor (S1) is hotter than the other (S2) and turns it off the moment that S1 is cooler than S2.
This makes the design simpleand guarantees that you are never lowering the temperature
of your stored water - even by a fraction of a degree.
Placement of the sensors must be carefully considered toaccount for temperature drops across both sides of a heat exchanger.
Sensor location
Pressurized glycol systems.
On single pumped systems (where the heat exchenager is inside the storage tank) the hot (S1) sensor should be mounted to the pipe within 6" of the exit at top of the collector. This ensures a rapid response.
On double pumped systems where one pump circulates the collector to HX and another circulates from HX tostorage, the hot (S1) sensor should be attached to the pipe that comes from the collectors about 2-3 feet before it enters the heat exchanger. The controller should then be
used to switch the secondary pump.
The cool sensor (S2) should be located where it measures the lowest temperature of the stored water. This can be the pipe that returns to the heat exchanger from the storage tank, or if you can access the surface of the tank, then attach the sensor to the tank wall about 1/4 from the bottom.
Be sure the sensors are insulated from exposure to ambient air, since this will affect the reading. On pipe runs the sensors can be attached with a pipe clamp and wrapped with insulation.
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