Why Solar Hot Water?
Heating water requires a lot of energy. For example here in Brazil we use instant water heaters that are built into the shower head. To raise water by 45F degrees (from 60F to 105F) about 5,000 to 7,500 watts are required. That is equivalent to heating your shower water with four or five microwaves on at the same time! In America typical water heaters have 4,500 watt heating elements (think 45 100 watt light bulbs on at the same time). Needless to say with electricity costing 0.22 cents USD per kilowatt hour here in Brazil, there is a financial incentive to find an alternative way to heat water.
Before I get into how I built the heater here is some basic information about it:
# It heats 80 gallons (310 liters) of water to 135F (55C) on sunny days.
# It cost me $300 USD
# I used PVC instead of copper to reduce costs
# To increase efficiency I sheathed the pipes in the collector with glass tubes made from old florecent bulbs. Click here to read why I did this
# For $30 USD I installed an automatic electric backup for cloudy days.
# It is a passive solar heater, so there are no moving parts - it relies on a thermosyphon to circulate the water.
# The solar collector is approximately 27ft2 (2.5 m2)
Building the Heater
The solar water heater consists of two basic parts, the hot water tank and the solar collector. The tank is 310 liters (80 gallons), it sits in the attic and is insulated with spray foam insulation.
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 I tore off the metal parts on each end, then poked a large hole in both ends so that a 1/2 inch pipe could fit through. Then I pushed a piece of a sponge through a couple of times to clean out the powder. When I was done I had a long glass tube. While I was taking apart the bulbs I used a mask and was sure to wash my hands and cloths afterwards, since they contain mercury.
Close up of the bulbs and pipes, The entire array, The solar collector in the insulated box.
Click for larger images.
The 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 Heater installed (before glass I tried transparent plastic sheeting but it wilted in the heat), Installing the Water tank
Click for larger images.
The panel was installed at a 35 degree angle and about 1 **** 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.
How to prepare the florescent bulbs
There are two ways to break open the ends.
The simple, but tedious way is to puncture them with a screw driver. Here's how I did it:
First I tore off the metal caps, then I put a screw driver in the end and wrapped a rag around. The rag is dual purpose. One, it helped protect me from the mercury inside (I also wore a mask and did it in a ventilated area), the rag also helped slow down the air as it flowed into the bulb. The bulbs seem to have a near vacuum inside them, so when they are punctured the air enters pretty quickly, and can propel some of the debris from the puncturing into the other end and break it. The rag helps slow the air down. After the bulb was broken on both ends I tapped out just enough glass so that a ˝ inch pipe could fit in I found G.E. bulbs to be the easiest to work with. Afterwards be sure to wash your hands.
In the end the bulbs are just for insulation so they don’t have to be perfect. They just keep the hot air around the pipe, and the ends are sealed off with foil, which covers up a lot of the imperfections. Of the bulbs in my collector about ˝ of them have breaks or small imperfections. But it doesn’t seem to matter, it still increases efficiency.
Here is a link to the other way to break open the ends. It isn’t nearly as simple. I haven’t tried it, but it was published in a magazine so I am sure it works. I found it with an article about a florescent bulb solar air heater. It uses common items to build a glass cutter. From what the site says, it should make nice looking cuts.
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.
I suspended it inside the water tank so that it only heats the upper portion of the water. I assume that if it were resting on the bottom that the hot water would rise. By the time the hot thermostat measured 110, the water at the top of the tank would be well over that.
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.