A - C

Absorber: In Solar Thermal collectors the absorber is normally a selectively coated copper or aluminum surface attached to a pipe containing cooling liquid. Thermal surface absorbers convert all incident radiation (sunlight) to heat at a single local temperature.

Auxiliary Heat: Solar Thermal Domestic Hot Water systems should be designed with a Solar Fraction (SF) of no more than 80 percent. An auxiliary heat source is needed to supply the remaining heat required by the system.

Types of auxiliary heat include:

  • External bottom fired boiler, electric resistance elements.
  • Gas (condensating or not, standing pilot or electronic ignition)
  • Oil
  • On-demand (gas or electric)

Electronic ignition allows some solar thermal controllers to delay the ignition of the gas flame to maximize the amount of solar thermal gain. The quantity of energy contributed by the sun translates to a much higher gas savings because gas water heaters are only about 65% efficient.

Many electric water heaters have two heating elements. The top element (day tier) is used to provide fast heat on demand and comes on at any time heat is needed. The bottom element (night tier) is programmed to heat water at night when utility rates are lower. Two zone electric tanks should have the lower element disabled in order to minimize electric use and maximize solar gain. The top element should only heat the top third of the tank for optimal use in solar hot water.

On demand water heaters require an appropriately sized hot water storage tank in addition to the demand heater. The advantages include small size and no energy consumption unless heat is needed. This is the optimal combination.

Black Chrome: Black Chrome is an absorber coating. The ideal absorber coating is capable of converting all the light striking the absorber into heat. Once the sunlight is converted into heat the ideal absorber coating will retain the heat until it can be transferred out of the absorber. Black Chrome is the first step toward selective surfaces. Black Chrome has better absorption than black paint and significantly lower emissions.

Black Paint: Absorber Coating. The ideal absorber coating is capable of converting all the light striking the absorber into heat. Once the sunlight is converted into heat the ideal absorber coating will retain the heat until it can be transferred out of the absorber. Black Paint is a simple cheap coating with good absorption. The major drawback with black paint is that has very high emissions causing the captured heat to be lost back to the environment.

Blue Sputtered: Blue Sputtered - The newest absorber technology has the highest absorption and the lowest emissions. Blue sputter is similar to black chrome in absorption however blue sputter has significantly lower emissions. Blue Sputter is the best coating technology currently available.

BTU: Stands for British Thermal Units.

The amount of heat needed to raise 1 pound of water 1 degree F (equal to 252 calories).

This is an imperial unit of measurement for heat widely used in the US and also in the UK. The conversion to the metric unit kWh is: 1 kWh = 3412Btu, and for surface area values, 1kWh/m 2 /day = 314Btu/ft 2 /day

Closed Loop: This system operates by circulating water mixed with glycol, a nontoxic fluid engineered to prevent freezing, through collectors, then through a heat exchanger. The heat exchanger then heats the water in your storage tank, providing you with accessible hot water for your daily use. The solar collectors contain a built-in temperature sensor that halts heat transference when the water temperature reaches its peak, then it works to keep the temperature constant.

CO2: Carbon dioxide (CO2) is produced by the burning of fossil fuels and is one of the greenhouse gases responsible for global warming.

Conduction: In Solar Thermal an example of heat conduction is the loss of solar energy through the insulation in the collector, piping and storage tank.

A more general description of heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter, from a region of higher temperature to a region of lower temperature, and acts to equalize temperature differences. It is also described as heat energy transferred from one material to another by direct contact.

Controller: The controller is the brains of a solar hot water system.

In a solar hot water system using forced water circulation a controller is designed first and foremost to determine when to start and shut off the pump used to circulate water in the collectors. In a closed loop system the controller also controls a second pump circulating water through the heat exchanger.

Other controller functions include compensating for overheating during events that affect demand like vacations. Vacation mode is where the controller limits the temperature in the tank, far below boiling, then pulses the pump when the collectors become hot during the heat of the day in order to cool the collectors while not overheating the tank. This only works when the collector area is not over sized.

Some controllers have connections which allow sensing system temperatures and flow rates. A few very advanced controllers have the ability to be connected to the Internet, communicating system parameters in order to make remote system monitoring possible.

Convection: In Solar Thermal an example of convection is the movement (floating) of heated (less dense) liquid from a solar collector up into a storage tank while colder (and more dense) liquids sink to the bottom of the collector.

A more general description of convection refers to the movement of molecules within liquids, and gases. Convection is one of the major modes of heat transfer and mass transfer. In fluids, convective heat and mass transfer take place through both diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid. In the context of heat and mass transfer, the term "convection" is used to refer to the sum of advective and diffusive transfer.

CPC: abbreviation for Concentrating Solar Collectors

D - E

Dole Dribble Valve: Solar collectors in open loop systems are exposed to the night sky and occasionally below freezing weather. This causes the collectors to freeze. As the water in the collector freezes it expands destroying the collector.

Freeze protection valves like the Dole/Eaton Dribble Valve operate by using the volume change upon freeze of a wax-like material filling a small enclosure to open the valve. When the valve opens the relatively warm supply water is allowed to circulate. The valves require no electricity to operate. The set point temperature (Tset) is the temperature at which the valve starts to open, and is determined by the wax makeup. Tset is typically chosen to be either 35F (for protecting pipes) or 45F (for protecting collectors). The higher set point is used for collectors because they can freeze above 40F, due to infrared radiation from the collector to the cold sky. The fully open position is reached when the effective valve temperature drops a few degrees below the set point.

Dos Disc: A cast and machined brass coupling invented by Heliodyne using a EPDM "O" ring to provide a positive seal in Solar Hot Water (SHW) systems. A Dos Disc is a coupling used to connect piping systems to solar collectors and to connect solar connectors together in an array.

Drain Back: In this type of system a pump forces potable through the solar collector and related piping. At the end of the day the pump is turned off. The collector and related piping are designed to gravity drain eliminating the possibility of freezing.

Evacuated Tube Collector: Evacuated Tube Collectors are also called Vacuum Tube Collectors. These collectors have multiple evacuated glass tubes which heat up solar absorbers and, ultimately, solar working fluid (water or an antifreeze mix -- typically propylene glycol) in order to heat domestic hot water, or for hydronic space heating. The vacuum within the evacuated tubes reduce convection heat losses, allowing them to reach considerably higher temperatures than most flat-plate collectors. For these reasons they can perform well in extreme cold conditions. The advantage is largely lost in the warmer climates that exist in the United States, except in those cases where very hot water is desirable, for example, commercial process water. The high temperatures that can occur within the tubes may require special system design to avoid or mitigate overheating conditions though some have built in temperature limitation[2].

A claimed advantage this design has over the flat-plate type is that the constant profile of the round tube means that the collector is always perpendicular to the sun's rays and therefore the energy absorbed is approximately constant over the course of a day. Measured collector efficiency data does not support this claim. The question about what to do with the "lost" sun shining through the gaps between evacuated tubes (gaps which can be as wide as the tubes' absorptive surface themselves) can be addressed either by by adding specially curved metal reflectors under the evacuated tubes or by reverting to the use of flat plate collectors which are designed not to offer any gaps in the collector's light interception profile. When used for domestic hot water in the United States Heliodyne flat plate collectors consistently out perform evacuated tube collectors.

Evacuated tubes are fragile which makes them difficult to ship. Evacuated tubes have a tendency to lose vacuum over time. The many evacuated tubes which make up an array are lighter than a single flat plate collector making them easier to lift to a roof, however have the disadvantage that installation times are much greater due to the added complexity of many tubes, header and clamp assembly's. In the event that a tube is broken it can not be repaired where flat plates can be reglazed.

Expansion Tank: An expansion tank is a small tank used in closed water heating systems and domestic hot water systems to absorb excess water pressure. Excess pressure is caused by thermal expansion as water is heated. The expansion tank usually contains a rubber diaphragm and the bottom of the tank contains water, which is openly connected to the plumbing system. Anytime the water pressure rises, it will push against the diaphragm, and gently compress the air. The compressibility of the air cushions the pressure shock, and relieves pressure in the system that could otherwise damage the plumbing system.

When expansion tanks are used in domestic hot water systems, the tank and the diaphragm must conform to drinking water regulations and must be capable of accommodating the required volume of water.

Expansion Tank Types include:

  • Bladder – Full capacity is useful for expansion (These are less common in the United States however are used in Heliodyne domestic hot water systems)
  • Membrane – Cannot use its full volume, must be mounted vertically (more common in the United States).

F - I

Flat Plate Collector: A flat plate is the most common type of solar thermal collector, and is usually used as a solar hot water panel to generate solar hot water. A weatherproofed, insulated box containing a black metal absorber sheet with built in pipes is placed in the path of sunlight. Solar energy heats up water in the pipes causing it to circulate through the system by natural convection (thermosyphon) or can use forced circulation using a pump. The water is usually passed to a storage tank located above the collector in thermosyphon systems or in any remote location using forced circulation. This passive solar water heating system is generally used in hotels and homes in sunny climates such as those found in the United States and Europe.

For these purposes, the general practice is to use flat-plate collectors with a fixed orientation (position). The highest efficiency with a fixed flat-plate collector is obtained if it faces toward the sun and slopes at an angle to the horizon equal to the latitude plus about 10 degrees. In the United States this is about 45 degrees however since the incidence angle has a relatively small effect on solar gain it is normal to place collectors flat on south facing roofs.

There are many flat-plate collector designs but generally all consist of (1) a flat-plate absorber, which intercepts and absorbs the solar energy, (2) a transparent cover(s) that allows solar energy to pass through but reduces heat loss from the absorber, (3) a heat-transport fluid (antifreeze or water) flowing through tubes to remove heat from the absorber, and (4) a heat insulating backing.

Some types of collectors are specifically designed to contain potable water. In areas where freezing is a possibility, metal collectors must be carefully plumbed so they completely drain down using gravity before freezing can happen so that they do not crack.

Some collectors are part of a sealed heat exchange system, rather than having the potable water flow directly through the collectors. A mixture of water and propylene glycol (which is used in the food industry) can be used as a heat exchange fluid to protect against freeze damage, up to a temperature that depends on the proportion of propylene glycol in the mixture. The use of glycol lowers the water's heat carrying capacity only marginally, while the addition of an extra heat exchanger may lower system performance at low light levels.

The first accurate model of flat plate solar collectors were developed by Hottel and Whillier in the 1950s.

Galvanic Corrosion: Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when in electrical contact with a different type of metal and both metals are immersed in an electrolyte such as hard water. Plumbers use ann insulated section of pipe called a dielectric union to break the path for current flow in order to reduce galvanic corrosion.

Generic: A name that specifies a particular product but not a particular brand. Often training is done using a generic name such as "solar collector" where a specific proprietary name might be "GOBI" which refers to a specific Heliodyne solar collector containing advanced coatings, laser welded collector surfaces and an advanced extruded aluminum frame.

Glazing: Glazing refers to the glass panel covering flat plate collectors. Normally the glazing material in premium quality collectors is 5/32 of an inch thick, textured to minimize reflecting the suns energy and tempered for added strength. The glazing on a flat plate collector is highly resistant to damage from hail and is strong enough to walk on (although this is not recommended).

Glycol: Types of glycol

  • Food grade – Used at low temperatures.
  • Polypropylene – Used in high temperature applications. Requires the use of double wall heat exchangers with leak detection.
  • Ethanol – Toxic so is not a good choice for Domestic Hot Water. Ethylene glycol or antifreeze is used in automotive cooling systems and aircraft deicing. The use of Ethanol was common in domestic hot water systems in the 1980’s.

Heat Exchanger: A heat exchanger is a device built for efficient heat transfer from one medium to another. They are widely used in solar thermal, space heating and refrigeration. One common example of a heat exchanger is the heat exchanger in a domestic hot water system.. A glycol solution is circulated by a pump through a solar collector. The heat from the glycol solution is transferred to potable water in a storage tank. Advanced heat exchangers have a leak detection method combined with annular groves in the exchanger tubing to produce higher efficiency from turbulent flow.

Heat Loss: In solar hot water energy is stored as hot water in a large storage tank. Heat loss is normally expressed in degrees lost per hour. Heat loss is directly affected by tank design, tank insulation, piping insulation and pipe design.

Storage tank and pipe insulation values are normally expressed as an R-value.

Heat Transfer: Heat is transferred through three basic mechanisms in solar thermal systems. They are:

  • Radiation - In Solar Thermal an example of radiation is either the heat from the sun and to a smaller extent the energy radiated from a hot solar collector. Selective surfaces are used to maximize the absorbed radiant energy and minimize the radiant energy lost.

    A more general description of thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. Infrared radiation from a common household radiator or electric heater is an example of thermal radiation, as is the light emitted by a glowing incandescent light bulb. Thermal radiation is generated when heat from the movement of charged particles within atoms is converted to electromagnetic radiation. The emitted wave frequency of the thermal radiation is a probability distribution depending only on temperature, and for a genuine black body is given by Planck’s law of radiation. Wien's law gives the most likely frequency of the emitted radiation, and the Stefan–Boltzmann law gives the heat intensity.

  • Convection - In Solar Thermal an example of convection is the movement (floating) of heated (less dense) liquid from a solar collector up into a storage tank while colder (and more dense) liquids sink to the bottom of the collector. Another example is the separation of hot and cold water in a storage tank (called stagnation) with the hot water at the top and cold at the bottom.

    A more general description of convection refers to the movement of molecules within liquids, and gases. Convection is one of the major modes of heat transfer and mass transfer. In fluids, convective heat and mass transfer take place through both diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid. In the context of heat and mass transfer, the term "convection" is used to refer to the sum of advective and diffusive transfer.

Conduction - In Solar Thermal an example of heat conduction is the loss of solar energy through the insulation in the collector, piping and storage tank.

Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter, from a region of higher temperature to a region of lower temperature, and acts to equalize temperature differences. It is also described as heat energy transferred from one material to another by direct contact.

J - M

Latitude:..Coordinate lines for locating a position on Earth that run east and west and are parallel to each other, running from 0° to 90° (angle from the equator). The latitude as well as the time of year affects the optimal angle for solar thermal collectors. Normally solar thermal collectors are angled for maximum winter energy adsorption.

N - O

Open Loop: The sun rises in the morning and shines on both the solar collector and a small temperature sensor on your roof. An automatic control system [1] compares the temperature between the roof sensor and a second sensor in the bottom of your hot water storage tank; if heat is needed, the control system turns on a small circulating pump [2] that circulates water from the bottom of your hot water storage tank [3] up through the solar collector panel [4] on the roof. As the water is pumped through the solar collector, it absorbs the sun’s heat and flows back into the tank. Since hot water rises (through a natural process called thermosyphoning), a check valve [5] prevents heated water in the storage tank from rising into the solar collector at night. On average, a good day of sun will provide a two-day supply of hot water. During periods of rainy or extended cloudy weather, a backup electric heating element (6) automatically comes on, so you never run out of hot water.

P - R

Potable: Drinking water is water that is of sufficiently high quality so that it can be consumed or utilized without risk of immediate or long term harm. Such water is commonly called potable water. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard even though only a very small proportion is actually consumed or used in food preparation (often 5% or even less).

Hard water causes scaling, which is the left over mineral deposits that are formed after the hard water has evaporated. This is also known as limescale. The scale can clog pipes, ruin water heaters, coat the insides of tea and coffee pots, and decrease the life of toilet flushing units.

According to the United States Geological Survey, 89.3% of US homes have hard water. The softest waters occur in parts of the New England, South Atlantic-Gulf, Pacific Northwest, and Hawaii regions. Moderately hard waters are common in many of the rivers of the Tennessee, Great Lakes, Pacific Northwest, and Alaska regions. Hard and very hard waters are found in some of the streams in most of the regions throughout the country. Hardest waters (greater than 1,000 ppm) are in streams in Texas, New Mexico, Kansas, Arizona, and southern California.

Excessive mineral deposits in potable water can negatively affect the performance and lifespan of open loop solar thermal systems.

Preassure: The amount of force exerted per unit area. Normally expressed as pounds per square inch or psi. In solar hot water systems normal pressures are:
 30 psi as a cold pre-charge pressure on a glycol side of a closed loop system.
 40 to 80 psi for city potable water systems.
 150 psi rated operating pressure on every part of a solar thermal system.
 300 psi manufacturing test pressure.

Radiation: In Solar Thermal an example of radiation is either the heat from the sun and to a smaller extent the energy radiated from a hot solar collector. Selective surfaces are used to maximize the absorbed radiant energy and minimize the radiant energy lost.

A more general description of thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. Infrared radiation from a common household radiator or electric heater is an example of thermal radiation, as is the light emitted by a glowing incandescent light bulb. Thermal radiation is generated when heat from the movement of charged particles within atoms is converted to electromagnetic radiation. The emitted wave frequency of the thermal radiation is a probability distribution depending only on temperature, and for a genuine black body is given by Planck’s law of radiation. Wien's law gives the most likely frequency of the emitted radiation, and the Stefan–Boltzmann law gives the heat intensity.

R-value: The R value is a measure of insulation's heat loss retardation under specified test conditions. The primary mode of heat transfer impeded by insulation is convection but unavoidablably it also retards heat loss by all three heat transfer modes: conduction, convection, and radiation. The primary means of heat loss across an uninsulated space is by natural convection, which occurs because of changes in air density with temperature. Insulation greatly retards natural convection. Most insulations trap air so that significant convective heat loss is eliminated leaving only conduction and radiation transfer. The primarily role of such insulation is to make the thermal conductivity of the insulation that of trapped, stagnant air. However this cannot be realized fully because the glass wool or foam is needed to prevent convection and increases the heat conduction compared to still air. Radiation heat transfer is minimized by having many surfaces interrupting a "clear view" between the inner and outer surfaces of the insulation. Such multiple surfaces are abundant in batting and porous foam. Radiation is also minimized by low emissivity (highly reflective) surfaces. Lower thermal conductivity and, therefore, high R values can be achieved by replacing air with argon when practical such as between sealed double-glazed windows and special closed-pore foam insulation.

American customary units, used in the United States, measure R-value in degrees Fahrenheit, square feet hours per Btu, (ft²·°F·h/Btu). This is commonly written in the form R–## (eg. R–19). The conversion is 1 ft²·°F·h/Btu ≈ 0.1761 K·m²/W, or 1 K·m²/W ≈ 5.67446 ft²·°F·h/Btu.

S

Sacilability: The ability to use a specific design Solar Thermal system in a wide range of applications. These might range from a small single collector system through a larger 20 panel pool heating system. As an example Thermosyphon Solar Thermal systems do not scale well where forced circulation systems do.

Selective Surface: Selective surfaces a series of coatings applied to the surface of a solar collector. Selective coatings are used in solar thermal collectors to convert as much of the suns radiant energy to heat as possible while re-radiating as little of the captured heat as possible. Blue Sputtered coatings are currently the best available coating.

SHW: Abbreviation for Solar Hot Water also referred to as Solar Thermal.

Solar heating systems are generally composed of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage. The system may use electricity for pumping the fluid, and have a reservoir or tank for heat storage and subsequent use. The systems may be used to heat water for a wide variety of uses, including home, business and industrial uses. Heating swimming pools, underfloor heating or energy input for space heating or cooling are more specific examples.

In many climates, a solar heating system can provide up to 85% of domestic hot water energy.[1] This can include domestic non-electric concentrating solar thermal systems. In many northern European countries, combined hot water and space heating systems (solar combisystems) are used to provide 15 to 25% of home heating energy.

Solar Fraction: The Solar Fraction (SF) is the Solar Fraction (abbreviated SF) is:

(Solar Hot Water Energy Contribution) / (Total Hot Water Energy Consumption)

The design goal is normally a Solar Fraction of 60% to 80%.

A Solar Fraction above 80% causes summer stagnation. A Solar Fraction below 60% does not provide enough savings to offset the cost of the system. The same system design will result in a different Solar Fraction depending on the latitude and local weather patterns.

Solar Radiation: Solar Radiation is measured as the number kilowatt hours per square meter per day. This is a measure of the potential solar electricity or British Thermal Units (BTU) per square foot. These numbers indicate the potential for solar, water or space heating.

Solar Thermal: Solar Thermal includes all methods of collecting the suns energy in the form of heat. Large commercial scale systems exist using heat storage and exchange mediums other than water. In most cases Solar Thermal is synonymous with Solar Hot Water. Solar heating systems are generally composed of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage. The system may use electricity for pumping the fluid, and have a reservoir or tank for heat storage and subsequent use. The systems may be used to heat water for a wide variety of uses, including home, business and industrial uses. Heating swimming pools, underfloor heating or energy input for space heating or cooling are more specific examples.

In many climates, a solar heating system can provide up to 85% of domestic hot water energy.[1] This can include domestic non-electric concentrating solar thermal systems. In many northern European countries, combined hot water and space heating systems (solar combisystems) are used to provide 15 to 25% of home heating energy.

Stagnation: Selective absorber coatings have made modern domestic solar hot water collectors much more efficient. Stagnation occurs when a domestic solar hot water system becomes heat soaked in periods of low heat demand or periods of intense summer solar gain. Stagnation can cause damage ranging from the rapid destruction of glycol in closed loop systems to minerals plating the collector tubes in open loop systems.

In order to avoid stagnation domestic solar thermal system design must be done in a balanced way where the collector area is sized such that the heat absorbed can be dissipated by the hot water demands on the system. Many other system design choices can mitigate or eliminate the negative effects of stagnation. Examples include:

  • Controller vacation mode
  • High temperature glycol
  • Larger expansion tanks
  • Higher operating preassures

Stratification: Thermal stratification is based on a natural process: Since warm water is lighter than cold water, it will ascend until it reaches a layer of warmer water or the top of the tank. This process facilitates the efficient utilization of solar heat: The higher the temperature difference between Collector and Solar Storage and the longer such a difference exists, the higher the Efficiency of solar heating. Therefore the solar Heat Exchanger will be mounted near the bottom of the tank, where the water is relatively cold, so even small amounts of solar heat can be "harvested". And while the outlet will be near the top of the tank, where the temperatures are highest, the inlet feeding fresh cold water will be positioned near the bottom. The more stable the thermal stratification, the higher the efficiency of the solar heating system and the comfort for the user by providing reliably sufficient amounts of hot water.

In solar hot water a thin and tall design is used to enhance separation of hot and cold water due to hotter (less dense) water rising to the top and colder (more dense) water sinking to the bottom (stratification). Stratification is useful because it allows us to maintain high water supply temperatures longer during water use.

T - Z

Temperature: In solar hot water systems normal operating temperatures are:

  • 65 F normal potable water from the ground
  • 106 F a normal warm shower
  • 140 F maximum temperature considered safe for domestic hot water
  • above 150 F causes an undesirable increase in sediment in tanks an piping.

above 185 F causes cracking damage in glass lined hot water storage tanks and shortens the service life in glycol.

Tempering Valve: Solar hot water systems have the ability to produce dangerously hot water, often 180 F and greater. Tempering valves, also called anti-scald valves, and thermally actuated valves contain a thermal sensing component that opens or closes in response to temperature changes. They are used to regulate the temperature of hot water and prevent scalding. Thermally actuated valves and tempering valves consist of an operating valve and a thermostatically-controlled bypass valve. Water that enters a fixture at a desired temperature flows through the operating valve and then out the fixture. Water that enters a fixture at an excessively high temperature flows through a hot water tempering valve, bypassing the operating valve. Using a thermal actuator element, the bypass valve continuously senses the temperature of the water until a safe temperature is achieved. The bypass valve then closes, and water is routing to the operating fixture.

Thermal Storage: A tall thin well insulated metal tank used to store hot water for later use. The tank is designed to facilitate stratification. Tanks are commonly made of stainless steel or porcelain (glass) lined steel. Steel tanks normally have a service life of approximately 10 years, stainless steel tanks last at least twice as long. Normal tank sizes are larger than normal domestic hot water tanks. Typical sizes for Solar Thermal Domestic Hot Water tanks are 80 gallons and 120 gallons.

Thermosyphon: In a Thermosyphon system the storage tank is attached directly to the top of the solar collector. As water in the collector heats up, it becomes lighter and rises naturally into the tank above; at the same time water in the storage tank that has cooled flows down into the collector, replacing the heated water that has risen out of it. Thus a natural convection circulation occurs when as the sun heats water. Thermosyphon systems, which may be placed on locations other than rooftops, are reliable and relatively inexpensive, but allowance must be made for the weight of the water and for freeze protection.

Vacuum Tube Collector: Vacuum Tube Collectors are also called Evacuated Tube Collectors. These collectors have multiple evacuated glass tubes which heat up solar absorbers and, ultimately, solar working fluid (water or an antifreeze mix -- typically propylene glycol) in order to heat domestic hot water, or for hydronic space heating. The vacuum within the evacuated tubes reduce convection heat losses, allowing them to reach considerably higher temperatures than most flat-plate collectors. For these reasons they can perform well in extreme cold conditions. The advantage is largely lost in the warmer climates that exist in the United States, except in those cases where very hot water is desirable, for example, commercial process water. The high temperatures that can occur within the tubes may require special system design to avoid or mitigate overheating conditions though some have built in temperature limitation[2].

A claimed advantage this design has over the flat-plate type is that the constant profile of the round tube means that the collector is always perpendicular to the sun's rays and therefore the energy absorbed is approximately constant over the course of a day. Measured collector efficiency data does not support this claim. The question about what to do with the "lost" sun shining through the gaps between evacuated tubes (gaps which can be as wide as the tubes' absorptive surface themselves) can be addressed either by by adding specially curved metal reflectors under the evacuated tubes or by reverting to the use of flat plate collectors which are designed not to offer any gaps in the collector's light interception profile. Evacuated tubes are fragile which makes them difficult to ship. Evacuated tubes have a tendency to lose vacuum over time. The many evacuated tubes which make up an array are lighter than a single flat plate collector making them easier to lift to a roof, however have the disadvantage that installation times are much greater due to the added complexity of many tubes, header and clamp assembly's. In the event that a tube is broken it can not be repaired where flat plates can be reglazed.