Solar Hot Water Panels - the basics
I think solar hot water panels are probably the best place to start for those interested in alternative energy. They're relatively easy to understand, install and operate.
Let's look at the basics in this discussion just to familiarize ourselves with what they do and how they do it. Let's keep it simple, and save the complexities for later.
As the title implies, this is about flat plate panels, not evacuated tubes. Flat panels are old technology, but they have their place in the world.
For me, the appeal of solar hot water panels is their reliability, availability and price. There are any number of solar water heating systems being dismantled off of laundromat roofs, so your chances of getting some panels at a reasonable price are pretty good.
Solar hot water panels have a relatively simple design. Imagine a 4 foot by 8 foot panel mounted vertically with the bottom edge angled south and the top edge angled north - about 45 degrees from the level ground below. That's a good start.
There is a header at each of the narrow ends of the panel that run from side to side. The one at the bottom is the inflow and the one at the top is the outflow. Both the inflow and outflow headers are capped at one end and open at the other. Let's say each header is a one inch diameter copper tube.
Smaller tubes, say three eighths of an inch in diameter, run the length of the panel and they connect the headers together in perhaps 20 or more places, with an even amount of space between each of these risers. Between the risers is thick cooper foil connected to each riser and running the length of the panel.
All headers and risers are painted black or nearly so, in order to have maximum absorption of heat when the sun is shining.
The entire panel is sealed. The front of the panel is prismatic glass. The rear of the panel is sheet aluminum on the outside and thick rigid foam insulation with a reflective foil covering on the inside. The sides are made of aluminum as well.
Operation of Solar Hot Water Panels
Water flows into the bottom header and is forced up the multitude of risers to the top header. Of course, we're doing this during the day when the sun is shining, so the headers and risers are hot from being in the sun inside that glass box. More importantly, the copper foil between the risers is also hot from being in the sun.
With all the surface area from the foil and small riser tubes, the water gets hot as if rises and enters the header at the top where it exits the solar panel and returns to a reservoir to be recycled through the system.
Each passage of the water heats it more and more, within the limits of the panel. Prismatic glass helps maximize the exposure of the panel interior to the sunshine. Insulation in the panel helps keep heat from escaping. Maximum surface area from foil and small risers helps transfer heat effectively in the panels.
That's how the panel works.
Freeze Protection in Solar Hot Water Panels
There are two basic ways to offer freeze protection to solar hot water panels. The first is to circulate anti-freeze inside the system. If you're using the panels for domestic hot water, this will require use of a heat exchanger that keeps the anti-freeze from coming in contact with you domestic hot water.
The advantage to setting up your solar hot water panels in this manner is that they can be left completely full of water without the fear of freezing.
Otherwise, you'll have to set things up as a drain back system. In this arrangement, the panels aren't filled with water until there is demand. Demand can take the form of a temperature switch that turns on the circulating pump only when the panels reach an appropriately high temperature.
On demand, water circulates from a separate drain back system composed of a storage tank and heat exchanger. The system allows for recirculation, and provides sufficient storage capacity to allow the circulating water to drain back into the storage tank once the demand is terminated.
In this way, the solar hot water panels never have plain water in them during times when it might freeze.
I prefer the drain back design because it eliminates use of toxic material. There is no need to mess with anti-freeze if you don't have to. And, you don't have to be concerned about cross contamination should the anti-freeze to domestic water heat exchanger develop an internal leak that would cross contaminate the system.
Done with Solar Hot Water Panels, back to Alternative Energy Sources