For the very beginning just a copy of the above mentioned Design Challenge application, see below. Further updates will follow soon.
Autonomous solar systems for hot water supply used in mild climate conditions are usually build from solar collectors, an accumulation tank, photo-voltaic panels providing electric power for a solar fluid pump and a controller switching the pump on and off (based on comparison of actual solar fluid temperature at collectors and hot water temperature inside the tank). One such a system was measured for several month ,  in different weather conditions and based on the results quite different control unit was proposed. As the original measurements were done on a Raspberry Pi connected to ADC Pi V2 interface board the same hardware platform is also used for the newly created design. Internet access for remote control and display is implemented through a browser interface.
Basic Functionality Description
Measurements described in ,  revealed that this particular solar system has its limitation residing in insufficient photo-voltaic power generated during corner cases (in particular during sun rise and sun set) and in cloudy weather. Though the controller switched the pump on that was in fact not running due to not enough electric power available. This finding led to the first design decision:
1. Photo-voltaic power output is directly (only via a 2 Amp Schottky diode, see below) connected to the pump.
This solution has two major advantages: System functionality remains autonomous (not dependent on external power source) and furthermore the pump controller is not needed at all. So if the controller is connected (for optimization purposes, see further) than in case of its breakdown the overall system still remains functional.
The second design decision was to improve solar energy intake by:
2. Connecting an external power supply (again via the Schottky diode, not to interfere with the photo-voltaic source) to the pump.
This power supply is of PC ATX type. On / off switching is provided via Power on signal by a normally opened contact of a controller relay (to ensure overall system functionality in case of a blackout).
For historical reasons the design is based on Raspberry Pi SBC equipped with ADC Pi Plus converter for temperature measurements with five external industrial thermistors connected. (Another option not requiring ADC converter is a set of five DS18B20 based one wire temperature sensors.) There is one temperature sensor connected to: the solar collector, the heat exchanger input, the heat exchanger output, the hot water tank and the hot water output.
Second expansion board used in the controller design is PIFACE DIGITAL 2. (Unfortunately this board, on the contrary to ADC Pi Plus, does not conform to PiHat specifications.) Only those two switching relays soldered on the board are utilized: The first one for switching the external power supply on / off by connecting / disconnecting its Power on signal to / from ground via normally opened contact. The second relay prevents tank overheating by switching on electromagnetic valve on hot water output when the temperature there reaches 95 DGC.
A touchscreen is connected to display graphics with measured values (and also voltage actually supplied by photo-voltaic panel is measured by a spare ADC channel for information purposes) and to eventually manually control on / off switching of the external power supply (to switch the heating fluid pump on and off). See the example picture of a web interface below (courtesy of Ladislav Lebeda).
This article describes a possibility of a controller replacement on a commercially available autonomous hot water solar system with Raspberry Pi based design. Usage of this SBC equipped with necessary expansion boards together with connection of the external power supply allows not only better available solar energy utilization but also more data collection, their retention in a database and remote Internet access. From the stored data solar power intake calculation is possible and also graphical presentation of measured values can be provided, please visit the above published web page to see different possibilities.
 MÍČKOVÁ, Petra. Kvantifikace energetického přínosu řídící jednotky solárního systému. Brno, 2013. Bakalářská práce. Vysoké učení technické v Brně.
 HAVLÍČEK, Lukáš. Kvantifikace energetických ztrát fototermického solárního systému ohřevu TUV při napájení fotovoltaikou. Brno, 2014. Diplomová práce. Vysoké učení technické v Brně.