Solar Water Heater with Plastic Bottles

How to make a water heater with big plastic (PET) bottles. Inside each bottle there is a 3.5 meteres piece of 4 mm drip watering tubing. This heater takes advantage of the greenhouse effect inside the bottles. It is like the heat inside a car in the sun in summer. Since when the circuit is stopped the water heats more inside the tubing it woul be a good idea to use a timer with i. e. 1 minute ON and 1 minute OFF. This also would protect the pump motor from excessive heating. With a small water pump the