Solar Hot Water Terminology
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Iwalani's Voyage Around the World
Weekly logs of Iwalani's three year circumnavigation written by Philip Shelton, Amy P. Wood and Stewart the Cat.

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The "World Voyagers" Book
A true story of the three year circumnavigation by Philip Shelton, Amy Wood and Stewart the cat. From designing and building a 42 foot wooden cutter "Iwalani" to sailing around the world— this is not a watered down, sugar coated tale, but a "no holds barred" account of just what it's like to live a "dream."

Absorber The blackened surface in a collector that absorbs the solar radiation and converts it to heat energy.

Active System A solar heating or cooling system that requires external mechanical power to move the collected heat.

Ambient Temperature The temperature of the surrounding air.

Auxiliary Heat The extra heat provided by a conventional heating system for periods of
cloudiness or when a solar heating system cannot provide enough.

British Thermal Unit (BTU) The quantity of heat needed to raise the temperature of one pound of water one degree Fahrenheit.

Check Valve Prevents reversed thermo-siphon circulation when the collector pump is not running.

Closed-Loop System Water, or heat transfer fluid HTF, heated by a solar collector remains seperate from the water going to the faucet through the use of a heat exchanger.

Collector A device that collects solar radiation and converts it to heat.

Concentrating Collector A device that concentrates the sun's rays on an absorber surface which is significantly smaller than the overall collector area.

Cover Plate A sheet of glass or transparent plastic placed above the absorber in a flat plate collector.

Delta-T Temperature difference, usually referred to in solar hot water systems as the temperature difference between the storage tank and the collector.

Differential Controller Device (powered by AC or Photovoltaic power) that controls the collector loop circulator pump, moving heat energy from the collector to the storage tank.

Direct System One in which the potable hot water in the water heater is used for space heating and domestic hot water for doing dishes, laundry, taking showers, etc. There is no separation between the two.

Draindown System Potable water is circulated from the storage tank through the collector loop. Freeze protection is provided by solenoid valves opening and dumping the water at a preset low temperature. Collectors and piping must be pitched so that the system can drain down, and must be assembled carefully to withstand 100 psi. city water line pressures. Pressure reducing valves are recommended when city water pressure is greater than the working pressure of the system. Because of these complications, draindown systems are no longer being installed.

Drain Back System The solar heat transfer fluid (usually water) automatically drains into a tank by gravity when the pump is not running. A heat exchanger is necessary, because the city inlet pressure would prevent draining. Collectors and piping must be pitched so that the system can drain back to a location that will not freeze.

Evacuated Tube Collector This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube inside a vacuum. The vacuum prevents heat loss to the outside air.

Flat Plate Collector A solar collection device in which sunlight is converted into heat on a plane surface without the aid of reflecting surfaces to concentrate the rays. Flat plate collectors have no vacuum, so some of the heat produced by the sun is lost to the outside air.

Galvanic Corrosion A condition caused as a result of a conducting liquid making contact with two different metal that are not properly isolated physically and/or electrically.

Gravity Convection The natural movement of heat that occurs when a warm fluid rises and a cool fluid sinks under the influence of gravity.

Headers The pipe that runs across the edge of an array of solar collectors, gathering or distributing the heat transfer fluid from, or to the risers in the individual collectors.

Heat Dump Used to prevent overheating. Depending on the type of system, it could be a finned radiator, radiant heat loop or an extra storage tank. Heat dump requirements can be minimized by tilting the collectors to the winter sun angle, your latitude plus 22.5 degrees.

Heat Exchanger A heat exchanger is a device that exchanges the heat from one fluid to another without a mixing of the fluids. A typical use would be to keep the potable domestic hot water separate from the solar collector heat transfer fluid HTF.

Heat Pump A mechanical device that transfers heat from one medium to another, thereby cooling the first and warming the second.

Heat Storage A device or medium that absorbs collected solar heat and stores it for use during periods of inclement or cold weather.

Heat Transfer Fluid HTF Liquid used in a closed solar collector system loop. This could be water (in climates that do not freeze) or polypropolyne glycol (a non-toxic antifreeze).

Hybrid Solar Energy System A system that uses both active and passive methods in its operation.

Insolation (INcidence SOLar radiATION) Not to be confused with in-sul-ation. The total amount of solar radiation direct, diffused and reflected-striking a surface exposed to the sky over a given time period. Insolation is expressed as watts per square meter per minute (or hour or day or year).

Irradiance Solar irradiance is the power of the sun reaching the surface earth and expressed as watts per square meter. The solar constant (maximum value) is expressed as 1,000 watts per square meter of exposed surface.

Open Loop System An open system is one that is open to the atmosphere. Water heated by a solar collector goes to the hot water faucet.

Passive System A solar heating or cooling system that uses no external mechanical power to move the collected solar heat.

Photovoltaic Cells Semi conductor devices that convert solar energy into electricity.

Polypropylene Glycol Non toxic antifreeze used as a heat transfer fluid.

Potable Fit to drink.

Risers The flow channels or pipes that distribute the heat transfer liquid across the face of an absorber.

Standby Heat Loss Heat lost though storage tank and piping walls.

Thermal Capacity The quantity of heat needed to warm a collector up to its operating temperature.

Thermistor Sensing device that changes its electrical resistance according to temperature. Used in the control system to generate input data on collector and storage temperatures.

Thermosyphoning The process that makes water circulate automatically between a warm collector and a cooler storage tank above it.

Tilt Angle The angle that a flat plate collector surface forms with the horizontal plane.

Trickle Type Collector A collector in which unpressurized heat transfer liquid flows over corrigated plates rather than through tubes.

Tube-in-Plate-Absorber A type of solar thermal collector where the heat transfer fluid flows through tubes formed in the absorber plate.

Tube-Type Collector A collector in which the heat transfer fluid flows through metal tubes that are fastened to the absorber plate with solder, clamps or other means.

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