Heating hot water can be one of your home biggest expense. Building a solar hot water heater could save you thousands of dollars worth of electricity or be last link in building your off grid home. In many rual areas central hot water isnt always an option, instant water heaters can be effective but also consume a lot of energy, which is a no no when running your off-grid home of batteries.
Building the Heater
The Solar Hot Water heater has two main components, a hot water tank and a solar collector. Solar collectors work by extracting energy from the suns rays. In this case the solar consists of black pipes contained in a glass covered housing.The black coating absorb the suns rays and heat up, the water is then pumped into the hot water tank for storage.
The solar collector is made up of 1/2 inch and 3/4 inch PVC pipe and florescent light bulbs. The collector is encased in an insulated box with glass on one side.
The solar collector is made up of 1/2 inch and 3/4 inch PVC pipe and florescent light bulbs. The collector is encased in an insulated box with glass on one side.
The solar collector is approximately 2.5 m2 (27 ft2), the pipes that are exposed to the sun have a total volume of about 18.5 liters (5 gallons), which is equivalent to 6% of the volume of the hot water tank. (Meaning that if the water circulates 16-17 times all the water in the tank will have been heated once, in theory).
To make the plumbing part of the heater I bought 64 T fittings and with them made 2 pipes, 32 T’s each with the T’s butt up against each other. Then between the two I put a pipe that was just a bit longer than a florescent bulb.
To prepare the bulbs remove the metal end caps on each end, then poked a large hole in both ends so that a 1/2 inch pipe could fit through. (See below for more on how to complete this step) Then push a piece of a sponge through a couple of times to clean out the powder. While working with florescent bulbs be sure to wear a mas and glove inorder to avoid contact with the dangerous chemicals inside.
bulbs were painted black on the back side, and the pipes were completely painted black. Then I closed off the ends of the bulbs with tin foil and a bit of spray foam, since they are just used as a basic insulator its not that important that they be air tight. The solar collector was lined with black plastic, underneath were some Styrofoam sheets to help insulate the heater.
The panel was installed at a 35 degree angle and about 1 foot below the bottom of the water tank. It is below the tank so that it does not act as a water cooler at night. Basically, the cold water is already at the bottom, so it should not circulate with the water in the hot water tank.
On sunny days when the outside temperature is in the mid 80's the water heats up to about 135 or more. Even on cloudy days the water heats up to the 90's. We installed it a month ago and since then we have only turned our electric shower head on full blast once. On cloudy days we turned it on only half way (2500 watts vs. 5000 watts).
Connecting it to the water tank -- I put 5 holes into the tank. They are listed here from highest to lowest.
# Over flow -- just a pipe that carries water out of the tank if the float were to malfunction and it were to over fill.
# Water Intake Valve. Just a pipe and float that fills up the tank with more water as we use it, similar to what is used in a toilet tank.
# Hot water pipe. This feeds the house with hot water. It is above the two pipes that feed the solar heater so that there is always water in the solar heater. This protects it from over heating if our water pump shuts off and no water comes into the tank to replace the water we have used.
# Return from Solar Heater. This pipe connects to the top of the solar heater, and to the top of the tank, just below the pipes above. As the water is heated it becomes less dense and rises. As it rises cold water takes its place. So the hot water flows slowly through the return to the top of the tank.
# Supply to the Solar Heater. This pipe connects to the bottom of the solar heater, it also connects to the bottom of the tank. It supplies the solar heater with the cooler water that settles to the bottom of the water tank.
Cheap $30 Backup System for Cloudy Days:
On mostly cloudy days the heater reaches about 90 degrees, on very cloudy and windy days it wont get above 80 and may only reach 70. So I came up with a backup. (Note: while I have this tested and installed, I actually have it unplugged to save electricity). I bought a ‘Bucket Heater’ that automatically turns on at 80 degrees and turns off at 110.
Why I built the solar water heater this way
After doing a lot of research on the internet, and a couple experiments I came to a few conclusions.
# While glass does block/reflect some of the light, it also creates an insulating buffer. The benefits of the insulating buffer out ways the lost light. This is essentially the same insulating concept used in double glazed windows.
# Since the temperature loss increases when the difference between the outside temperature and the temperate inside the water heater increases, it is better to heat a lot of water to 110-115 degrees, than to heat a little water to 150 degrees. For example, if the hot water tank is at 150 and the outside temperature is 80, there is a 70 degree difference and more heat will be lost through the insulation than if the temperature difference was 30 degrees, and the hot water was at 110.
# Passive solar heating requires (as the name implies) no moving parts. As water warms up it becomes less dense and moves upward, pulling cold water into the collector. This requires no pump, making it very simple and energy efficient, as well as there being less to go wrong.
# A solar heater would heat the water using two forms of heat transfer, radiation and convection. Radiation would reach at best 180 degrees of the pipe, convection would reach 360 degrees of the pipe, doubling the surface area. In other words, as well as trying to expose the pipes to as much sun as possible, I needed to also trap hot air produced by the radiation around the pipe to help heat it even more.
With this in mind I set out to design a solar collector. I priced copper but found it to be very expensive. PVC was much cheaper, but it does not withstand more that 135 degrees. This I found out not to be entirely true.
After some tests I found that the PVC pipe itself will withstand higher temperatures without losing rigidity, however I assume that the fittings would begin to fail at lower, but still hot, temperatures. I should also note that the water pressure in the collector is very low, just as much pressure as is created by about 2-4 feet of fall, others who had failures in their PVC fittings had put their heater inline before their water heater, as a primer. Unlike houses in the USA where the water enters the house already pressurized, houses in rural Brazil have water tanks in their attics to create pressure. So I figured that the max temp for my PVC would be higher than the standard since I have sub standard pressure and will subject the pipes to much less stress.
So decided to go with PVC and to insulate the fittings from the temperature inside the collector and protect them from sunlight. That way only the pipe itself would be exposed to the sun light and the highest temperatures.
Since I wanted to try to keep the heat close to the pipes I decided to use old florescent bulbs to act as a second glazing. The wind will cool the glass panels that cover the solar collector, which in turn cools the air inside the collector. The florescent bulbs act as another barrier. The trap in the hotter air around the pipes and keep it from circulating with the cooler air outside of the bulbs. In tests the pipes with bulbs heated up faster and to a higher temperature than those without.
For more details please see Jakes site at http://www.teaters.com/modules.php?name=News&file=article&sid=217
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