Hot Oil Pump Control
The receiver oil temperature and boiler water temperature will control the oil pump.
The difference in temperature will either turn on and off the oil pump or control its speed.
For the on/off control, the Arduino will turn on the pump when the receiver pipe oil is a given temperature hotter then the water in the boiler. The pump will then be turned off when the difference in temperatures hits a minimum. Below the minimum temperature, circulating the oil will not significantly increase the water temperature as the return pipe cools the oil slightly. The Arduino will turn on the oil pump using a small transistor and a solenoid for on/off control similar to the sun tracking motor control.
Running the pump at low speed uses much more energy then turning the pump on and off but the constant flowing of the hot oil results in the heat exchanger being much more efficient. If speed control is needed for the pump, a larger solid state relay will be used with the Arduino's PWM function to quickly turn the pump on and off to regulate its speed. The hotter the receiver temperature is in comparison to the boiler water temperature, the faster the pump will run. There will be a minimum where the pump will shut off, but this system is meant to always have the pump running when the sun is strong.