Solar Thermal
By Kenton Shepard
Solar water heating first became popular in the 1970’s when federal, state and utility incentives encouraged their installation, as is happening now. Inspectors will see many of these older systems still in place, many no longer working. In practice, inspectors will encounter a wide variety of system configurations and components and recommending a specialist inspection is a good idea in order to pass on liability. These systems can be expensive.
The idea is fairly simple. Solar insolation heats a circulating fluid which transfers its heat to a storage tank from which home hot water can be drawn, either directly to plumbing fixtures or to supply preheated water to boilers or hot water heaters.
EVACUATED TUBE COLLECTORS
 
Evacuated tube collectors are more efficient than flat plate collectors, but also more expensive
 
Source
Each dark tube holds a heat pipe containing a fluid which turns to vapor as the tube warms. Hot vapor rises to the top of the tube, transferring its heat to another fluid which circulates through the header pipe and down to the storage tank.
Evacuated Tube Efficiency
When looking at the tubes from above (0o) each tube's surface is clearly visible, and is exposed to the maximum amount of sunlight. At this angle some light is lost between the tubes.
When the sun reaches an angle of 40o, the solar tubes are still fully visible with no gaps between and with no overlap. It is at this point that the collectors are most efficient. The tubes are exposed to all the sunlight shining towards them, and all the tubes are still perpendicular to the sun
As the angle increases, the tubes start to overlap, shading each other. They are still facing the sun, but the actual surface area of absorber exposed to the sunlight is reduced. Only a small amount of sunlight falls beyond 40o (early morning and late afternoon), and so this decrease in surface area has minimal influence on the total daily energy output of the collector.
This solar collector design actually provides more heat in a stationary position than if it physically tracked the sun throughout the day!
FLAT-PLATE COLLECTORS
 
Source Source
Flat-plate collectors consist of an insulated box with a dark-colored absorber plate placed in the bottom. Above the absorber plate lie a series of coils, typically copper tubing, through which a liquid circulates. After leaving the collector(s) the fluid transfers its heat to a storage tank.
Some systems are designed to drain fluid back into a tank when it stops circulating.
A common find… an old collector from the 1970’s, glazing fogged over. Look for abandoned equipment inthe basement. Electrical conductors serving abandoned equipment should be disconnected and terminated in a proper enclosure
PARABOLIC TROUGH COLLECTORS
Source
These collectors are more common in commercial structures. The reflective trough focuses the sun's energy on a single point, such a a pipe containing a circulating fluid.
SYSTEM OVERVIEW
No matter what type of collectors are used to heat the transfer fluid, heat from the fluid must be transferred into the home and stored or distributed. The following system is typical. Heat is stored in a material with high thermal mass, usually water in a large tank.
 
The photo above shows two insulation-wrapped pipes routed through a circulation pump housing (actually only the cold pipe connects to the pump). These are the hot fluid pipe (red temperature gauge on the left) bringing fluid from the collectors and the cold fluid pipe (blue gauge on the right) returning fluid to the collectors.
An air vent is visible at the top, and mounted on the wall to the right is the control box.
The control box is connected to one temperature sensor at the collectors and another at the storage tank. It contains circuitry which switches the circulation pump on and off.
The storage tank, filled with water. The system should have an expansion tank such as the red one visible here.
Inside the tank shown above…hot fluid from the collectors flows through the coils inside the tank, transferring heat to tank water. Two coils shown here are part of the collector loop. The third coil preheats home water supply.
 
Pre-heated water leaves the top of the big storage tank through an insulated copper pipe and replaces the cold supply line at an electric water heater. Hot water leaves the water heater through a partially-insulated red plastic supply pipe which, along with a blue cold water pipe, enters the top of the home plumbing fixture supply manifold (wall mounted at left).
All photos not cited are by Kenton Shepard |
