SHEM Glossary

Matt [SHEM] Mon Nov 29, 2010 1:36 pm

Here is a glossary of common solar hot water, and solar radiant heating terms.

BTU: Stands for British Thermal Unit, a measure of energy. A BTU is approximately the amount of energy needed to raise the temperature of one pound of water one degree Fahrenheit.
Differential Controller: The device that controls the solar fluid pump, and if applicable, the heating element, or solar radiant pump. It uses temperature differences to turn on and off.
Drainback: A style of solar water heating system. A typical regular drainback system uses an external heat exchanger, one or two pumps, and stores the potable water in the tank. The advantage of drainback over the other main style of system, glycol, is that the solar fluid drains back completely into the tank so that the solar fluid does not have to be freeze protected. SHEM systems are a style of drainback called Simple Drainback. See: Simple Drainback for more information.
Evacuated Tube: A style of solar collector. Evacuated tubes use selective coatings (black coatings) contained in a vacuum that minimizes heat loss to the ambient. Evacuated tube collectors can either use heat pipes, or U-Tubes to remove heat from the collector. Evacuated tubes also achieve much higher temperatures than flat plate collectors and are more efficient during the winter than flat plate collectors.
Flat Plate: A style of solar collector. Flat plates use copper tubes and fins coated in selective coatings (black coatings). Fluid flows through the copper tubes removing heat from the collector.
Flow Rate: The amount of water flowing through a point in a certain amount of time. Usually measured in gallons per minute (gpm), or liters per minute (lpm).
Glycol System: A type of solar system that uses glycol to freeze protect the collectors rather than draining back the solar fluid. In a glycol system the pipes and collectors always contain solar fluid. These systems also use pressurized fluid. Care has to be taken in the summer to prevent the glycol from "burning" in the collectors and becoming too acidic. If the fluid becomes too acidic, it will cause corrosion of the copper in the piping and the collectors.
Heat Exchanger: A device used to transfer heat from a higher temperature fluid, into a lower temperature fluid.
Radiant Heat: A type of heating for homes, garages, and other buildings. It uses copper or PEX tubing buried in the floor of the room to be heated. The system pumps warm water from the storage tank into the tubing which causes heat to be radiated through the floor producing a nice, gentle heat.
Simple Drainback: This is the style of system SHEM uses. Instead of an external heat exchanger, multiple pumps, and storing potable water, the SHEM Simple Drainback uses an internal heat exchanger, one pump, and doesn't store the potable water. The tank is used to store 80, or 120 gallons (depending on the model) of solar fluid (most commonly water). When hot water is needed, the cold potable water flows through 100' of corrugated copper heat exchanger submerged in the solar fluid which heats the water to near the temperature of the solar fluid.

This design has several advantages. Since the water in the tank is captive, no new sediment and corrosion is introduced into the tank, the life of the tank increases dramatically. Sediment and corrosion is the leading cause of hot water tank failure, so by eliminating the amount of sediment in the tank the life of the tank can be increased greatly. Another advantage is that a simple drainback tank reduces the amount of plumbing connections needed, which reduces cost, installation time, and hassle.
Stagnate: Stagnation is when a collector is sitting exposed to the sun without the system pump activated.
Stagnation Temperature: This is the temperature a collector reaches when it is stagnating. Since no fluid is running through it, a collector can reach incredibly high temperatures when it is stagnating. Flat plate collector stagnate anywhere in the range of 190-250°F (90-120°C), while evacuated tube collectors stagnate anywhere in the range of 220-370°F (105-190°C).
Therm: A therm is a unit of energy equivalent to 100,000 BTUs.
Thermistor: A temperature sensor. These are used by the controller to measure the temperature of the tank, collector, and if applicable, the floor in a radiant heat configuration, or an external storage tank.
Thermosiphon: A configuration which transfers fluid using density differences due to temperature differences, and gravity.