Solar Thermal Hot Water Heaters

A Solar Thermal Collector: is a device designed to receive solar radiation and convert it into thermal energy. Normally, a solar thermal collector includes a frame, glazing, and an absorber, together with the appropriate insulation. The heat collected by the solar thermal collector may be used immediately or stored for later use. Solar thermal water heaters in 2010 equal the energy equivalent of the electrical capacity of 40 large nuclear power plants. The energy production of solar thermal energy is 30 times that of photoelectric solar energy. By 2005 solar water heaters had saved the consumption of almost 70 million barrels of oil and cut carbon emissions by 29,000,000 tons.
Flat Plate Air Solar CollectorFlat Plate Air Solar Collector

Types of Collectors

1) Flat Plate Air Collectors - They are mainly used in solar space heating, and may be tied to forced air systems. They have some advantages over solar liquid-flat panels (they produce heat earlier and later in the day than liquid systems and they do not freeze.) They aren’t as flexible in their applications. Heat evaporates from air faster than liquid, making air collectors less efficient than the liquid models.
Flat Plate Liquid Solar CollectorFlat Plate Liquid Solar Collector
2) Flat Plate Liquid Collectors – durable, weatherproof boxes which contain a dark absorber plate located under a transparent cover – are still the most common type of collector used for water heating in many countries although not as efficient as evacuated tube collectors in many ways.
3) Unglazed Collector - A solar collector that consists of an absorber without the glass covering (glazed) of a flat-plate collector. Unglazed collectors are usually made of black plasticUnGlazed Solar CollectorUnGlazed Solar Collector that has been stabilized to withstand ultraviolet light. Because they are not insulated, these collectors are best suited for low temperature applications where the demand temperature is below 30°C. By far, the primary market is for heating outdoor swimming pools, but other markets exist including heating seasonal indoor swimming pools, pre-heating water for car washes, and heating water used in fish farming operations. There is also a market potential for these collectors for water heating at remote, seasonal locations such as summer camps.
Unglazed collectors are usually made of black plastic that has been stabilized to withstand ultraviolet light. Since these collectors have no glazing, a larger portion of the Sun's energy is absorbed. However, because they are not insulated a large portion of the heat absorbed is lost, particularly when it is windy and not warm outside.
Batch Solar CollectorBatch Solar Collector
4) Batch Integral collector-storage systems - Also known as ICS systems, they feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector (black tank), which preheats the water. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. They should be installed only in mild-freeze climates because the outdoor pipes could freeze in severe, cold weather.

5) Vacuum Tube Collectors - solar water heaters are made up of rows of parallel, glass tubes. There are several types of evacuated tubes (sometimes referred to as Solar Tubes). The vacuum that surrounds the outside of the inner tube greatly reduces convection and conduction heat loss to the outside, therefore achieving greater efficiency than flat-plate collectors, especially in colder conditions. This advantage is largely lost in warmer climates, except in those cases where very hot water is desirable, for example commercial process water.Solar Vacuum Tube Collector IndirectSolar Vacuum Tube Collector Indirect Thermal diode effect is a name given for this vacuum insulation. The high temperatures that can occur may require special system design to avoid or mitigate overheating conditions. Evacuated tubes offer the advantage that they work efficiently with high absorber temperatures and with low radiation. Higher temperatures also may be obtained for applications such as hot water heating, steam production, and air conditioning. Incidence Angle Modifier (IAM) is the variance in output performance of a solar collector as the angle of the sun changes in relation to the surface of the collector. Evacuated tubes provide an important and measurable increase in efficiency in the morning and afternoon when the sun’s angle is between 40 and 80 degrees from perpendicular. The result is a more constant heat output for the better part of the day. Flat Panels are affected by outside air temp and cold wind, and Evacuated Panels are not so much.

Types of vacuum tube collectors: Glass-Glass U Pipe, Glass-Glass Evacuated Collector, Glass-Metal and Glass-Glass Water Flow Path

--- a) Glass-Glass - tubes consists of two glass tubes which are fused together at one end. The inner tube is coated with a selective surface that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn ("evacuated") from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss. These tubes perform very well in overcast conditions as well as low temperatures. Because the tube is 100% glass, the problem with loss of vacuum due to a broken seal is greatly minimized. Glass-glass solar tubes may be used in a number of different ways, including direct flow, heat pipe, or U pipe configuration.
Solar Vacuum Tube U Pipe With HeaterSolar Vacuum Tube U Pipe With Heater
U Pipe: 1.Outer glass   2.Vacuum   3.Selective coating inner glass  4.Aluminum heat transfer fin  5.U pipe(Hot terminal)  6.U pipe(Cold terminal)U Pipe: 1.Outer glass 2.Vacuum 3.Selective coating inner glass 4.Aluminum heat transfer fin 5.U pipe(Hot terminal) 6.U pipe(Cold terminal)

--- --- i) U Pipe – (U Tube) (direct flow) U-Pipe Collectors can be installed perfectly vertical or horizontal, allowing for a wider variety of installation options, which allow these solar collectors to be used where other collectors cannot be used, such as on the top of a flat roof building where tilt mounting is not an option due to space limitations, as an overhang from buildings where there is no roof space to mount the solar collectors, or flush vertically on a wall. The coolant in the top tank is circulated into each of the u-pipe tubes.

Evacuated Tube, How it WorksEvacuated Tube, How it Works Evacuated Tube PartsEvacuated Tube Parts
--- --- ii) Heat pipe evacuated collector tube - This single hollow copper tube glass glass vacuum tube system are more efficient than the u-pipe type. In this type of collector, a heat pipe collector incorporates a special fluid which begins to vaporize even at low temperatures. The steam rises in the individual heat pipe and warms up the carrier fluid in the top tube by means of a heat exchanger. The condensed liquid then flows back into the base of the heat pipe. The pipes must be angled at a specific degree above horizontal so that the process of vaporizing and condensing occurs. This type of collector is especially good in northern climates where it can get very cold, and where water can be of poor quality. In time the poorer quality of water can cause blockages in smaller pipes. Another advantage is if one tube is damaged, the main solar system still works and its coolant is not lost, but with the u-pipe, the whole system would have to be refilled.

Solar Vacuum Tube Glass to MetalSolar Vacuum Tube Glass to Metal
--- b) Glass-Metal - tubes consist of a single glass tube. Inside the tube is a flat or curved aluminum plate which is attached to a copper heat pipe or water flow pipe. The aluminum plate is generally coated with Tinox, or similar selective coating. These type of tubes are very efficient but can have problems relating to loss of vacuum. This is primarily due to the fact that their seal is glass to metal. The heat expansion rates of these two materials. Glass-Metal tubes are more efficient but glass-glass is generally more reliable and much cheaper.
Solar Vacuum Tube Water in TubeSolar Vacuum Tube Water in Tube

--- c) Glass-Glass Water Flow Path – (water in inner tube) tubes incorporate a water flow path into the inner glass tube itself. The problem with these tubes is that if a tube is ever damaged water will pour from the collector onto the roof and the collector must be "shut-down" until the tube is replaced. Also these systems most run with low pressures to keep from breaking the inner glass.

6) Parabolic Mirror or Concentrating Collectors - are usually parabolic troughs that use mirrored surfaces to concentrate the sun's energy on an absorber tube (called a receiver) containing a heat-transfer fluid, or heat the water itself. This type of solar collector is generally only used for commercial power production applications, because very high temperatures can be achieved. It is however reliant on direct sunlight and therefore does not perform well in overcast conditions.

Parabolic Solar Dish CollectorParabolic Solar Dish Collector
--- a) Parabolic Solar Dish Collector - A concentrating solar collector that is similar in appearance to a large satellite dish, but has mirror-like reflectors and an absorber at the focal point. A parabolic dish system uses a computer and dual-axis tracking to follow the Sun across the sky and concentrate the Sun's rays onto the receiver located at the focal point in front of the dish. In some systems, a heat engine, such as a Stirling engine, is linked to the receiver to generate electricity. Parabolic dish systems can reach 1000°C at the receiver, and achieve the highest efficiencies for converting solar energy to electricity in the small-power capacity range.

Parabolic Trough Solar CollectorParabolic Trough Solar Collector
--- b) Parabolic trough collector - A type of concentrating solar collector that uses U-shaped troughs to concentrate sunlight onto a receiver tube, containing a working fluid such as water, which is positioned along the focal line of the trough. Sometimes a transparent glass tube envelops the receiver tube to reduce heat loss. Parabolic troughs often use single-axis or dual-axis tracking. In rare instances, they may be stationary. Temperatures at the receiver can reach 400°C. The heated working fluid may be used for medium temperature space or process heat, or to operate a steam turbine for power or electricity generation. In California, multi-megawatt power plants were built using parabolic troughs combined with gas turbines.

Concentrating Solar TowerConcentrating Solar Tower
--- c) Power solar tower - Also known as a central receiver solar power plant, a type of concentrating collector system that employs a field of large mirrors that follow the Sun's path across the sky. The mirrors concentrate sunlight onto a receiver on top of a high tower. A computer keeps the mirrors aligned so the reflected rays of the Sun are always aimed at the receiver, where temperatures well above 1000°C can be reached. High-pressure steam is generated to produce electricity.

--- d) Stationary concentrating collector - A type of concentrating collector that uses compound parabolic reflectors and flat reflectors for directing solar energy to an accompanying absorber or aperture through a wide acceptance angle. The wide acceptance angle for these reflectors eliminates the need for a sun tracker. This class of collector includes parabolic trough flat-plate collectors, flat-plate collectors with parabolic boosting reflectors, and solar cookers.


System Configurations

Active or Passive: Active systems utilize a circulating pump and some type of temperature control. Passive systems do not have any moving parts and rely on the basic principle of physics - that hot water rises & cold water falls.
Direct or Indirect: In direct, potable (drinking) water for use directly flows through the solar collector. With indirect, the potable water is isolated from the collector’s heated fluids and a heat exchanger warms the water.
Pressurized or Non-pressurized: Some systems use atmospheric pressure and others pressurize the water loop or loops. This makes a difference in boiling- freezing points and in potable water flow.

Passive

Batch Solar Water Heater CollectorBatch Solar Water Heater Collector
Batch (ICS Integral collector storage system) combine the hot water storage tank and the solar collector surface into a single component, eliminating the need for circulating pumps or automatic control systems. In its most simple implementation, a water storage tank painted black and sitting out in the sunlight is a rudimentary ICS system.
This type of system works best as a preheater for a conventional or tankless water heater. The cold water line that feeds the conventional water heater is diverted [1] and sent first through the ICS solar module [2]. Circulation is provided by utility mains pressure. In other words, when hot water is drawn out for use from the conventional water heater [3], the storage tank is replenished with solar heated water instead of cold water. This allows the electric or gas heater to work substantially less.
The biggest advantage is simplicity: the system has no pumps, no temperature sensors, no electronic controls and no heat exchanger. When combined with a tankless water heater, the system can free up five to six square feet of floor space by eliminating the conventional water heater storage tank. The biggest disadvantage is nighttime heat loss. Stored heat is lost through the glass cover plate at night, which of necessity cannot be insulated to prevent heat loss. However, this heat loss is reduced in advanced ICS systems by stretching a thin clear film just underneath the glass cover plate, which creates an insulating air gap. Also, while the greater thermal mass of stored hot water within an ICS solar module makes this type of system more freeze resistant than the direct system (above), ICS systems are not appropriate for climates that experience more than four to five freezing nights per year.

Thermosyphon Circulation is provided by the thermosyphoning principle: Hot water rises and cold drops. There is no pump or electronics. The hot water storage tank is located higher than the solar collector and circulation flow is induced when the coldest water in the bottom of the storage tank falls by gravity through a circulation line into the bottom of the solar collector panel, where it is heated and rises. Water heated in the solar collector panel rises into a circulation line to a high point in the water storage tank. Advantages. Unlike the ICS system, which combines hot water storage with energy collection and so can lose heat at night through its glass cover plate, the thermosyphon system optimizes system efficiency by fully insulating a separate storage tank. The thermosyphon system’s primary drawbacks are freezing, weight on roof and appearance. The hot water storage tank must be higher than the solar collector, so it becomes a bulky protrusion on the roof. In the 1940’s and 50’s some systems placed the tank under the roof in the attic peak and the collector at a slightly lower place on the roof top. Modern thermosyphon systems place the tank on its side, along the top edge of the solar collector panel. While modern solar water heating collectors look like skylights, many homeowners are resistant to the idea of a tank on their roof. While the ICS system spreads its hot water storage over a greater roof area, the weight in a thermosyphon system storage tank is usually more concentrated. An older roof structure may not be able to support the added weight of a hot water storage tank.
Bubble Pump Is a version of the indirect system except the closed loop fluid is moved by a bubble pump that utilizes the pumping action of water boiling from the solar collectors. This system uses low or no pressure and has to use antifreeze in cold climates which may affect the bubbling action. The bubble pump produces a pressure difference of approximately 3 feet of water.

Active

Active direct or “open loop” system, this is the type of system most often installed in non-freezing sunbelt climates within the United States. In the direct system, an electronic control system compares the temperature of a sensor located at the solar collector with the temperature of a sensor located in the bottom of the hot water storage tank, where the coldest water is located. When the solar collector temperature is warmer than the water in the bottom of the tank by some predetermined difference (four degrees, for example), the electronic control turns on a small pump, that draws cold water from the bottom of the hot water storage tank and circulates it through the solar collector. Solar heated water is returned to the top of the tank. Some versions of this system use a small photovoltaic (solar electric) panel to operate a direct current (DC) circulating pump.
The direct system typically produces the highest operating efficiency because there is no nighttime heat loss from hot water stored on the roof; nor is any efficiency lost through a heat exchange process. Potable water from the hot water storage tank is circulated directly through the collector. The only disadvantage of this system is that freeze protection is provided by circulating warm tank water through the collector. This is not a desirable method of freeze protection in climates that experience more than a day or two of freezing weather each year, because energy loss during freezing weather could be significant. Even more important, freezing weather can coincide with a power outage, preventing the pump from circulating warm water through the solar collectors.

Drainback systems are usually indirect with a heat exchanger but can be direct That is, it may pump the water for use directly into the solar collector or it may use a heat transfer liquid. If a direct system it is called draindown. Either way, the process is the same. Water is pumped into the solar collector where it is heated. With the aid of gravity, it drains back into the storage tank to be used. A sensor on the solar collector can detect when no further heat comes from the sun at which point the water is allowed to drain. Because no water stays in the collector after nightfall, it is protected against freezing. This system uses very little pressure when the pump is turned off, atmospheric pressure only. If used as direct then 2 pumps are required or gravity is utilized for the tap (Drinking or potable) water. The solar collector and control system are the same in either circulation system. However, with indirect an antifreeze solution can be circulated through the solar collector and back into a heat exchanger in the hot water storage tank. The addition of a heat exchanger adds to the cost of the system and creates some degree of heat transfer energy loss. Drainback systems are specifically designed to provide fail-safe operation in climates with frequent freezing during the coldest winter weather. Like the direct system, an electronic control system compares the temperature of a sensor located at the solar collector with the temperature of a sensor located in the bottom of the hot water storage tank (where the coldest water is located). When the solar collector temperature is warmer than the water in the bottom of the tank by some predetermined difference (four degrees, for example), the electronic control turns on a small pump. By allowing heated water to drain back into the storage tank after heat sensors detect no further solar radiation, the system is protected from the threat of freezing in cold weather. A drain back system is one type of solar hot water heating, but it can be one of the more complicated means of doing so.

Active Indirect or Split or “closed loop” system, uses a pump to circulate a pressurized non-freezing (usually propylene glycol and water), heat-transfer fluid through the collectors and a heat exchanger. Usually this heat exchanger is inside the water heater (storage tank) but can be installed in the incoming cold water before it arrives at the standard electric or gas water heater. This heats the water that then flows into the home. They are the most popular system in climates prone to freezing temperatures. This system does not drain the fluid during cold cloudy days like a unpressurized drainback system. A controller is used to activate a circulation pump to ensure heat transfer from collector to storage tank. Automatic control starts the pump as soon as the collector is warmer than the tank.
The 3 main components are a solar collector, pump and a heat exchanger. These units contain various devices such as a controller, gauges, manometer, pressure relief valve(s), tempering valve, non-return check valve, air purge valve, temperature sensors, relay, manual shutoff valve, drain-cock, filling loop with double check valve(check local codes), pipes, joints, insulation and an expansion vessel. The expansion tank(s) account for the differences in volume resulting from changing temperatures in the heat transfer liquid. The water heater usually contains a backup electric heating coil for cloudy periods. This backup electric coil can be operated by the controller or use its own thermo sensor that is set much lower than the solar control.


The History of Solar Water Heating

Thermal solar water heating history goes along with other solar machines and advances including solar photoelectric advances. But the thermal part of the story goes way pre-electric age.
The story of solar water heating began in the 1760s in Geneva, Switzerland, where Horace-Bénédict de Saussure, a Swiss naturalist, observed that it is always hotter when sun rays pass through a glass-covered structure, whether in a coach or a building, than into a site unprotected by such material. To put his hypothesis to scientific scrutiny, in 1767 he built an insulated box, its bottom painted black to absorb as much sun energy as possible, with two panes of glass covering the top — the prototype for all solar water heaters. De Saussure found that when he exposed the box perpendicular to the sun, the inside heated to temperatures far above the boiling point of water. The Swiss scientist had demonstrated, for the first time, the greenhouse effect. De Saussure speculated, “Someday some usefulness might be drawn from this device for it is actually quite small, inexpensive and easy to make.”

In 1891, Clarence Kemp, an American plumbing and heating manufacturer, placed a black-painted water tank inside a glass-covered box with a similar design to de Saussure’s. As the bottom of the box heated, the colder water inside the tank absorbed the heat and became hot enough to be drawn for bathing or dishwashing. Here was the first commercial solar water heater. Kemp called it The Climax. As California had lots of sunshine, in the late 19th century thousands of citizens affluent enough to be willing to pay for hot water but who had no local or cheap source of fuel were willing to spend $15 for Kemp’s invention. By 1897, a third of the homes in Pasadena, California had water heated by the sun.
The Climax had one drawback: The water was heated and stored in the tanks, which were exposed to the elements at night and during bad weather. Under such conditions, they cooled down sometimes to such a degree that customers would have to forego a morning bath or washing up for breakfast with hot water.

In 1909, William J. Bailey found a way out of the dilemma: separating the solar heating of the water from its storage. His solar collector consisted of water pipes attached to a black-painted metal plate inside a glass-covered box and connected to an insulated remote storage tank located above the collector. As the sun heated the water, it became lighter than the heavier, cooler water entering from the bottom, forcing the hotter water to naturally rise into the storage tank and remain warm during the night and the following morning. Bailey called his company the Day and Night Solar Water Heater Company to emphasize his product’s advantage. Day and Night solar water heaters soon drove the Climax out of business to dominate the burgeoning solar water heater business in California, Arizona and Hawaii.
The discovery of plentiful oil and natural gas in southern California in the 1920s killed the local solar water heater business.
Every energy crisis and price increase has renewed the interest in solar energy including the solar thermal water heaters.
In 1969 there were 4 million solar water heating tanks on the roofs of Japanese homes. Today there are 10 million solar thermal systems at work in Japan.
Solar-water heating systems got a real boost in the 1970s with the oil embargo and then when tax credits were offered by state and federal programs to help people make the investment.

Single walled vacuum tubes (evacuated tubes) were developed in Germany by Daimler-Benz Aerospace mid 1980's, with support from Sunda Solar SEIDO in 1986.

From 1984 to 1990 Luz International builds nine commercial-scale thermal solar energy installation in the United States. They produce a total of 270,000 kilowatts. These plants are in California's Mojave Desert & comprise the Solar Energy Generating Systems (SEGS), SEGS VIII and IX (each 80 megawatts), located in Harper Lake, are, individually and collectively, the largest solar power generating plants in the world. SEGS plants are concentrating solar thermal plants.
In China more than 30 million households rely on the sun to heat their water. A solar water heater in China costs less than $200, about equal to an electric one but without the electric blackouts and the $120 annual electric cost.
Israel has no oil supply of its own. In the 1960's, a lack of oil resulted in a boom in solar thermal systems there. When the price of oil dropped, so did solar thermal sales, but today, 90% of Israeli homes use solar hot water heating.