SolarWaterHeater

Objective

This solar water heater project employs electric commercially available solar panels to collect sun energy and heat water in a conventional electric hot water heater.

The solar panels are conventional solar panels generating DC electricity and the water heater is a commonly available unit normally powered by household AC electricity.

Design

Most electric hot water heaters for household use heat water using a resistive heating element immersed in the water and a simple bi-metallic thermostat. In the United States, they are usually powered by either 120 VAC or 240 VAC. The thermostat usually employs a bimetallic strip with electrical contacts to interrupt the electricity once the desired temperature has been reached and establish electricity flow when the water temperature drops below a threshold.

Tall water heaters often employ two heating elements and thermostats to heat the upper and lower sections of the tank. Shorter water heaters usually have only one heating element.

First, calculate the resistance of the hot water heater heating element, then the solar supply voltage to match this resistance and finally, the number of panels necessary for heating the water.

400px

Test Case

The Unit used for the initial testing is a Kenmore Power Miser 6 with the following specs:

Capacity: 40 gallon
Rated voltage: 240 VAC
Rated Power: 3800 Watts

The solar panels are Sanyo

24 VDC
230 Watts

The test design is as follows

Water heater resistance:

Power = Voltage * Current (P = E * I) => I = P/V
R = E/I = E/(P/E) = 240 Volts  / (3800 Watts / 240 Volts) = 15.1 Ohms

Max solar panel power output:

230 Watts
24 Volts
9.5 Amps

Calculate # series connected solar panels to match load at 9.5 Amps:

Supply voltage = R * I = 15.1 * 9.5 = 143.45 Volts
#panels = Supply voltage / Panel voltage = 143.45 / 24 = 5.9 Panels

Summary

It works!