As part of my quest to reduce my carbon footprint and because I love technology and tinkering with systems, I decided to install a solar hot water system in my home. Several years ago, I helped start a solar company here in Georgia and as part of that startup, I was responsible for researching all of the different types of solar energy. I found that the most efficient solar energy for a residence is solar thermal or what is also known as a solar hot water system.
Solar hot water systems absorb the radiant heat from the Sun and transfer it to the hot water in your home. Solar thermal systems that use the Sun's radiant energy to produce heat are multiple times more efficient than Solar PV systems that use the Sun's photons to generate electricity.
There are many types of solar thermal systems. In tropical climates where temperatures almost never reach the freezing point of water, direct solar hot water systems are used that circulate the potable water in the home into exterior collectors. In some cases, these collectors are as simple as plastic water barrels painted black. Direct systems do not work well where temperatures drop below freezing. In these cases, the water in the collectors may freeze and cause the collectors to fracture when the water expands.
In temperate climates or colder, indirect systems are used where a water-based heat transfer fluid diluted with a propylene glycol mixture flows through the collectors. This lowers the freezing point of the fluid and protects the system from potential damage. The heat carried by the fluid is transferred to the potable water using a heat exchanger. Some of the most common types of heat exchangers are copper coils and flat-plate exchangers. We recently featured a post about a solar powered differential temperature controller which controls the flow of water from the collectors to the hot water storage tank or through the heat exchanger.
In the southeastern United States, we have lots of sun. For me, installing this system was an easy decision. I installed a solar thermal pre-heat system since I have a family and need to make sure we have hot water even on cold, cloudy days. With a pre-heat system, the solar hot water is pre-heated by the Sun and fed into an existing hot water heater. The solar thermal system is only part of the total hot water system; the rest is made up by a natural gas or electric hot water heater. When the pre-heated water reaches the temperature on the hot water heater's thermostat, the heating elements do not turn on. However, if the water isn't hot enough, the heating elements provide supplemental heating. This allows our home to always have hot water available when it's needed.
When deciding to install a solar hot water system, there are many types to choose from. Usually, there are only a few choices that will work well for your area and application. In my case, I have direct sunlight for 4-5 hours of the day. During the summer, the sun gets very hot and temperatures on the roof can reach well over 100 degrees. Since I'm in a subtropical to temperate zone, radiant heating would not fit well for my area. There would be too much waste heat to dispose of in the summer months. I chose glazed, flat-plate collectors with thermofin absorbers.
The system is an indirect, glycol/water mixture in a drainback configuration. This means that when the system shuts off, the glycol/water heat transfer fluid drains back into the reservoir in the interior of the home. This prevents stagnation and breakdown of the fluid during the hottest part of the year. When the differential temperature controller triggers the system to turn on, pumps push the heat transfer fluid from a reservoir up into the collectors. Once the fluid travels through the collectors, it falls back down to the reservoir carrying the heat collected on the roof back to the water in the tank through a heat exchanger.
If my home was partially shaded by trees, another system configuration common in our region is an indirect, pressurized glycol loop. This system keeps fluid pressurized in a glycol loop through the collectors at all times. When the temperature difference between the collectors and the hot water storage tank reaches the configured temperature difference, the pumps turn on and begin to circulate the fluid through the glycol loop. When the shutoff differential is reached, the pumps shut off and the fluid stops moving. This system tends to be more efficient than a drainback system but is subject to stagnation if you have lots of direct sunlight during the day with no obstructions or shading.
I have had the system active in my home now for over a year and have been enjoying solar-pre-heated water in my home year-round. I have also noticed a significant drop in my electricity bills ever since the system was activated. I'm able to track this since I log my energy bills and graph my usage. More details on how I do this here.
I am including a link to a Flickr slideshow of the system installation. Enjoy!