Measures and Control
The ARETHA panel built in Milan (Italy) wants to demonstrate the feasibility of the ARETHA concept presented in the operating principle section of this website. For this purpose two prototypes have been built: a small one of about 1 m2 and a big one of about 10 m2; both use one car radiator each.
Once the small ARETHA was successfully built, the project focused on maximizing the panel extension which can be supported by one radiator, optimizing its shape and computing its energy transfer efficiency. For this reason a monitoring system was required. A low cost and customized system has been built using Arduino board and some commercial sensors. The measurements carried out are:
The system also controls the function of a pump, which moves the water through the primary circuit, and swith on/off the radiator fan. This is in order to have water circulation only during sunny hours.
One Arduino board is devoted to collect and store data, and to control the external devices. A second Arduino sends data to an LCD display and to Internet. The two Arduino boards are equipped with a data-logger shield and an ethernet shield respectively.
Temperatures have been measured by a chip sensor, the DS18B20 from Maxim IC. This sensor reports celsius degrees with up to 12 bits of digital resolution. Its precision is better than 0.5 C, and it measures temperatures from -55 to +125 C. Each sensor has a unique 64-Bit Serial number etched into it, which allows to use one single data bus for a huge number of sensors. The sensor uses 1-Wire interface, compatible with Arduino by means of the "DallasTemperature" free library. This sensor has three pins: power supply, data and the ground. The data line is used as input and output. In ARETHA, all the temperature sensors are connected to a two line shielded cable, with the shield connected to the ground.
The measure of the water flow through the primary circuit of ARETHA is very important for calculating the energy balance. A mean value of flux is obtained with this volumetric meter (see the picture). A magnetic relay inside the meter, switch on a circuit every 10 liter passed through it. Arduino counts how many switch-on/off there are in a certain time and computes the flow.
Global Solar Irradiance is measured by a Kipp & Zonen pyranometer CMP6.
The picture on the left shows the two Arduino boards with their shields on top(data-logger and ethernet) and a 16x2 character LCD display. On the LCD, temperatures are displayed in tenths of degree. After the temperatures, the LCD displays flux meter, pump and fan status. A 12V DC power supply provides energy for the radiator fan. The pump is powered by 220V AC.
All the parameters, updated every 10 s, are showed here by means of the WEB Server software loaded on Arduino.

A detailed description of the temperature sensor positions inside ARETHA, is presented
here.

The picture below shows the pump used for water circulation.