How to build a DIY Passive Solar Thermal Water Heater. Simple and Efficient!!

    This project goes into the build of a passive solar thermal water heater using pex pipes and 4 X 8 plywood piece. The pex pipes are sturdy and tough , doesn’t easily leak. This passive solar heater can reach upto 120 – 150 F on a typical sunny day. Instead of pex pipes, you can use cheap irrigation pipe for this project. The pex pipes are more stronger and durable . The heater can be used for heating your domestic water, space or room heating or just heating a swimming pool.

    The box that contains the pipes are made of 4 X 4 plywood . Four pieces of 2 X 4 pressure treated lumber are joined along the sides using screws to make the frame. We staple in some bubble insulation along the dimensions of the box . Since the insulation material is silver, it will reflect heat. To avoid this ,we coat it with a flat black paint to attract the solar heat.

    Two holes are drilled on the sides of the frame for the inlet end of the pipe where the cold water comes in and outlet end where the hot water comes out. The inside and the outside of the heater is painted flat back using rust-oleum high heat paint to absorb maximum heat from the sun.

    The first layer of half inch pex tubes or irrigation pipes are secured inside the solar heater box using half inch pex talon clamps. The clamps are installed on four sides of the box securing each loop of the pipe. The second or upper layer of pipes are secured in using zip ties. The total length of the pipe is 200ft.

    The pipe comes in through the inlet hole and goes to the outside on the first layer , all the way around and work itself inside .It then goes through the top layer and all the way to the outside and then exit through the outlet hole.

    The bottom layer pipes aren’t going to be exposed to the sun as much but they still will be warmed up because the whole box is covered with lexan polycarbonate sheet. The top layer with the pipe that goes outside through the outlet hole will have the highest thermal BTU.

    A very inexpensive reed thermometer with a 4 inch stem is installed on the side of the heater using a half inch to three eighths bushing reducer .

    A 4 X 8 Makrolon Polycarbonate Sheet is placed on top of the heater box and secured down in place using a No 8 One and one fourth sheet metal screws ,finishing washers and rubber grommets. Silicon adhesives are used to seal the gap formed between the sheet and the box frame.

    The Solar thermal heater is placed at an angle of 20 degrees. This is done with the help of leg supports with dimensions 16 and 8 inches 2 X 4 pieces at both the sides. A 50 watt Renogy Solar Panel is also installed adjacent to the heater. This Solar Panel is for powering the bilge water pump.

    A 500GPH 12V bilge pump is used to pump the cold water through the pipes into the heater . In order to control the flow of water through the pump , it is connected to an speed control electric circuit box which has a relay, a buck boost converter,a motor pump speed controller, potentiometer and a switch. The pump is powered by a 50W Solar Panel .The negative connection from the panel is connected to the relay, the positive goes to the switch.

    The relay determines the voltage for the buck boost converter .It activates on a certain voltage we set and then powers the buck boost converter. The buck boost converter will keep a constant voltage no matter what the voltage the solar panel is putting out. It is then connected to a 15 amp motor pump speed controller and a potentiometer which is used to control the voltage of the bilge pump motor. The 12V 500GPH bilge pump is connected to the motor pump speed controller.

    In order to test the unit, we place the heater near a pool to heat it. The bilge pump is submerged into the pool which is then connected to the heater with help of a PVC hose. The output pvc hose is returned with heated water back to the pool. The water reaches upto 140 F based on our test.



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