Uncle Bill's Homebuilt Solar Water Heater

This project was to design and construct a domestic solar water heater controlled by an Arduino. These pages describe the design, development and operation of the system.

reflector diagram

The system uses parabolic trough reflectors which reflect and concentrate sunlight onto water pipes which absorb the heat.

The collector faces south (here in the northern hemisphere) to obtain maximum heat at midday.

The water pipe is painted black and is mounted horizontally, running east-west.

The elevation angle is changed according to the position of the sun.

view of collector

The heat collector box and solar panel

The heat collector consists of a wooden box with a glazed front mounted on a south facing wall. It houses 3 parabolic trough reflectors. Each reflector is 1.5m long and 0.5m wide. The reflecting surface is made from mirror polystyrene sheet. The wooden box is about 1.8m wide and 1.8m high.

The elevation angle of the reflectors is automatically aligned with the sun. (The photo was taken in the morning - no heat is produced at this hour because the sun is in the east.)

Mounted above the heat collector is the 60W photovoltaic panel which provides electrical power for the system.

Water is circulated by a pump through the heat collectors and through a coil in the hot water tank (a heat exchanger). A closed system is used so that antifreeze can be used to prevent freezing in winter.

LCD photo

The display

Line 1 "tracking" means that the system is actively tracking the elevation angle of the sun.

Line 2 shows collector temperatures: inlet and outlet, followed by the 2 LED light sensor outputs.

Line 3 shows water tank temperatures: inlet, outlet and top.

Line 4 shows environment temperatures: outside and inside the box, then the elevation angle of the collector in degrees.

The whole system is controlled by an Arduino. The Arduino receives inputs from 7 temperature sensors, 2 LED light sensors and a gravity sensor (an accelerometer) which measures the elevation angle of the reflectors. The arduino controls the water pump and adjusts the elevation angle of the reflectors when necessary (see next paragraph). It also outputs to an LCD screen which monitors the status of the system. There is also a data logger, which can be set to monitor 14 system parameters when required.

The elevation angle of the sun varies according to the time of year (higher in summer and lower in winter), and also according to the time of day. The elevation angle of the reflectors needs to be aligned with the elevation angle of the sun. In this design, the water pipes are fixed, and the reflectors are rotated about the water pipes to the required angle. The elevation angle is controlled by a geared electric motor driving a winch drum and cable. The elevation angle of the reflectors is aligned with the sun using feedback from LED light sensors which are attached to the reflector and are positioned to detect the shadow from the water pipe.

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