28-06-2010, 07:50 AM
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SOLAR WATER HEATER
We are blessed with Solar Energy in abundance at no cost. The solar radiation
incident on the surface of the earth can be conveniently utilized for the benefit of human
society. One of the popular devices that harness the solar energy is solar hot water
system.
Solar water heating or solar hot water is water heated by the use of solar energy. 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, under floor 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. This can include domestic non-electric concentrating solar thermal systems
Residential solar thermal installations can be subdivided into two kinds of systems: compact and pumped systems. Both typically include an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) that is activated when the water in the tank falls below a minimum temperature setting such as 50 °C. Hence, hot water is always available. The combination of solar water heating and using the back-up heat from a wood stove chimney to heat water can enable a hot water system to work all year round in cooler climates, without the supplemental heat requirement of a solar water heating system being met with fossil fuels or electricity.
Among pumped options, there is an important distinction to be made regarding the sustainability of the design of the system. This relates to what source of energy powers the pump and its controls. The type of pumped solar thermal systems which use mains electricity to pump the fluid through the panels.
History
Flat-plate collectors for solar water heating were popular in Florida and Southern California in the 1920s. Levi Yissar built the first prototype Israeli solar water heater and in 1953 he started NerYah Company, Israel's first commercial manufacturer of solar water heaters. Despite the abundance of sunlight in Israel, solar water heaters were used by only 20% of the population by 1967. Following the energy crisis in the 1970s, in 1980 the Israeli Knesset passed a law requiring the installation of solar water heaters in all new homes (except high towers with insufficient roof area). As a result, Israel is now the world leader in the use of solar energy per capita with 85% of the households today using solar thermal systems (3% of the primary national energy consumption).
During this time, there was some resurgence of interest in solar heating in North America. Technical innovation has improved performance, life expectancy and ease of use of these systems. Installation of solar water heating has become the norm in countries with an abundance of solar radiation, like Cyprus, Israel and Greece, as well as in Japan and Austria, where there is less.
In 2005, Spain became the first country in the world to require the installation of photovoltaic electricity generation in new buildings, and the second (after Israel) to require the installation of solar water heating systems in 2006. Australia adopted the mandatory regulation for solar thermal for new construction in 2006 as well.
Types
Solar water heating systems can be classified in different ways:
The type of collector used (see below)
The location of the collector - roof mount, ground mount, wall mount[29]
The location of the storage tank in relation to the collector
The requirement for a pump - active vs. passive
The method of heat transfer - open-loop or closed-loop (via heat exchanger)
Compact systems (passive systems)
A compact system also known as a monobloc has a tank for the heated water and a solar collector mounted on the same chasis. Typically these systems will function by natural convection (thermosiphon) or heatpipes to transfer the heat energy from the collector to the tank.
A integrated collector storage system
A special type of compact system is the Integrated Collector Storage (ICS or Batch Heater) where the tank acts as both storage and solar collector. Batch heaters are basically thin rectilinear tanks with glass in front of it generally in or on house wall or roof. They are seldom pressurised and usually depend on gravity flow to deliver their water. They are simple, efficient and less costly than intense plate and tube collectors but only suitable in moderate climates with good sunshine. [30][31] A step up from the ICS is the Convection Heat Storage Unit. These are plate type intense collectors with built-in insulated tanks. The unit uses convection (movement of hot water upward) to move the water from heater to tank. Neither pumps or electricity are used. It is more efficient than an ICS as the intense collector heats a small(er) amount of water that is constantly rising to the tank. It can be used in areas with less sunshine than the ICS.
A passive open loop system
Direct ('open loop') compact systems, if made of metals are not suitable for cold climates. At night the remaining water can freeze and damage the panels, and the storage tank is exposed to the outdoor temperatures that will cause excessive heat losses on cold days. Some compact systems have a primary circuit. The primary circuit includes the collectors and the external part of the tank. Instead of water, a non-toxic antifreeze is used. When this liquid is heated up, it flows to the external part of the tank and transfers the heat to the water placed inside. ('closed loop'). Open loop (direct) systems have the disadvantage that during the night-time, where the temperature of the solar panel starts to drop below that of the water tank, the system starts working in reverse heating the water in the panel and cooling the water inside the tank. This problem is least noticeable in closed loop system using a heat exchanger as only the water in the heat exchanger and not the whole tank is affected by it. Zhuhai Tianke Energy Saving Equipment Manufacture Co., Ltd. managed to solve this problem using a patented design in their solar water heating systems heat exchanger which forces the flow of the water in the in the heat exchangers inlet pipe in an upward flow, thus restricting cold water flowing down to the panel.The force of the flow is made through a driver inside the heat exchanger (jacket). The jacket and the driver part is made of the same heat conducting material therefore sharing the same temperature on same levels of the heat exchanger.
A compact system can save up to 4.5 tonnes annually of greenhouse gas emissions. In order to achieve the aims of the Kyoto Protocol, several countries are offering subsidies to the end user. Some systems can work for up to 25 years with minimum maintenance. These kinds of systems can be redeemed in six years, and achieve a positive balance of energy (energy they save minus energy used to build them) of 1.5 years. Most part of the year, when the electric heating element is not working, these systems do not use any external source for power (as water flows due to thermosiphon principle).
Flat solar thermal collectors are usually used, but compact systems using vacuum tube collectors are available on the market. These generally give a higher heat yield per square meter in colder climates but cost more than flat plate collector systems.
Pumped systems (active systems)
Schematic of an pasive solar heating system
Schematic of an active solar heating system
An active solar heating system uses a pump for the circulation of water
How the solar water heating system is pumped and controlled determines whether it is a zero carbon or a low carbon system. Low carbon systems principally use electricity to circulate the fluid through the collector. The use of electricity typically reduces the carbon savings of a system by 10% to 20%.
Conventional low carbon system designs use a mains powered circulation pump whenever the hot water tank is positioned below the solar panels. The storage tank is placed inside the building, and thus requires a controller that measures when the water is hotter in the panels than in the tank. The system also requires a pump for transferring the fluid between the parts.
The electronic controllers used by some systems permit a wide range of functionality such as measurement of the energy produced; more sophisticated safety functions; thermostatic and time-clock control of auxiliary heat, hot water circulation loops, or others; display or transfer of error messages or alarms; remote display panels; and remote or local datalogging.
The direct pumped system, has one or more solar energy collectors installed on the roof and a storage tank somewhere below, usually in a garage or utility room. A pump circulates the water from the tank up to the collector and back again. This is called a direct (or open loop) system because the sun's heat is transferred directly to the potable water circulating through the collector tubing and storage tank; no anti-freeze solution or heat exchanger is involved.
This system has a differential controller that senses temperature differences between water leaving the solar collector and the coldest water in the storage tank. When the water in the collector is about 15-20° F warmer than the water in the tank, the pump is turned on by the controller. When the temperature difference drops to about 3-5° F, the pump is turned off.
In this way, the water always gains heat from the collector when the pump operates.
A flush-type freeze protection valve installed near the collector provides freeze protection. Whenever temperatures approach freezing, the valve opens to let warm water flow through the collector.
The collector should also allow for manual draining by closing the isolation valves (located above the storage tank) and opening the drain valves.
Automatic recirculation is another means of freeze protection. When the water in the collector reaches a temperature near freezing, the controller turns the pump on for a few minutes to warm the collector with water from the tank.
The dc pump and PV panel must be suitably matched to ensure proper performance. The pump starts when there is sufficient solar radiation available to heat the solar collector. It shuts off later in the day when the available solar energy diminishes. As in the previous systems, a thermally operated valve provides freeze protection.
Types of thermal collector
There are three main kinds of solar thermal collectors in common use. In order of increasing cost they are: Formed Plastic Collectors, Flat Collectors, and Evacuated Tube Collectors. The efficiency of the system is directly related to heat losses from the collector surface (efficiency being defined as the proportion of heating energy that can be usefully obtained from insulation). Heat losses are predominantly governed by the thermal gradient between the temperature of the collector surface and the ambient temperature. Efficiency decreases when either the ambient temperature falls or as the collector temperature increases. This decrease in efficiency can be mitigated by increasing the insulation of the unit by sealing the unit in glass e.g. flat collectors or providing a vacuum seal e.g. evacuated tube collector. The choice of collector is determined by the heating requirements and environmental conditions in which it is employed.
Formed plastic collector
Formed plastic collectors (such as polypropylene, EPDM or PET plastics) consist of tubes or formed panels through which water is circulated and heated by the sun's radiation. These are often used for extending the swimming season in swimming pools. In some countries, heating an open-air swimming pool with non-renewable energy sources is not allowed, and then these inexpensive systems offer a good solution. This panel is not suitable for year-round uses like providing hot water for home use, primarily due to its lack of insulation which reduces its effectiveness greatly when the ambient air temperature is lower than the temperature of the fluid being heated.
Flat plate collector
A flat plate collector
A flat plate collector consists of a thin absorber sheet (of thermally stable polymers, aluminum, steel or copper, to which a black or selective coating is applied) backed by a grid or coil of fluid tubing and placed in an insulated casing with a glass or polycarbonate cover.
Fluid is circulated, using either mains or solar electricity, through the tubing to remove the heat from the absorber and to transport it to an insulated water tank, sometimes directly or otherwise to a heat exchanger or to some other device for using the heated fluid. Some fabricants have a completely flooded absorber consisting of 2 sheets of metal stamped to produce a circulation zone. Because the heat exchange area is greater they may be marginally more efficient than traditional absorbers.
As an alternative to metal collectors, new polymer flat plate collectors are now being produced in Europe. These may be wholly polymer, or they may be metal plates behind which are freeze-tolerant water channels made of silicone rubber instead of metal. Polymers, being flexible and therefore freeze-tolerant, are able to contain plain water instead of antifreeze, so that in some cases they are able to plumb directly into existing water tanks instead of needing the tank to be replaced with one using heat exchangers. By dispensing with a heat exchanger in these flat plate panel, temperatures need not be quite so high for the circulation system to be switched on, so such direct circulation panels, whether polymer or otherwise, can be somewhat more efficient, particularly at low light levels.
As with evacuated tubes, most flat plate collectors have a life expectancy of over 25 years.
Evacuated tube collector
Evacuated (or vacuum) tubes panel.
Evacuated tube collectors are made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change. This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flat-plate collector). In some cases, the tubes are covered with a special light-modulating coating. In an evacuated tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it. The absorber can either consist of copper (glass-metal) or specially-coated glass tubing (glass-glass). The glass-metal evacuated tubes are typically sealed at the manifold end, and the absorber is actually sealed in the vacuum, thus the fact that the absorber and heat pipe are dissimilar metals creates no corrosion problems. Some systems use foam insulation in the manifold. Soda-lime glass is used in the higher quality evacuated tubes manufacture.
Later technology evacuated tube systems use the glass coated absorber. The glass is a boron silicate material and the aluminum absorber and copper heat pipe are slid down inside the open top end of the tube. In lower quality systems moisture can enter the manifold around the sheet metal casing is eventually absorbed by the glass fibre insulation and then finds its way down into the tubes. This leads to corrosion at the absorber/heat pipe interface area, also freeze ruptures of the tube itself if the tube fills sufficiently with water.
Two types of tube collectors are distinguished by their heat transfer method: the simplest pumps a heat transfer fluid (water or antifreeze) through a U-shaped copper tube placed in each of the glass collector tubes. The second type uses a sealed heat pipe that contains a liquid that vapourises as it is heated. The vapour rises to a heat-transfer bulb that is positioned outside the collector tube in a pipe through which a second heat transfer liquid (the water or antifreeze) is pumped. For both types, the heated liquid then circulates through a heat exchanger and gives off its heat to water that is stored in a storage tank (which itself may be kept warm partially by sunlight). Evacuated tube collectors heat to higher temperatures, with some models providing considerably more solar yield per square meter than flat panels. However, they are more expensive than flat panels, but generally of a less cost to repair in the event of damage. Evacuated heat tubes perform better than flat plate collectors in cold climates because they only rely on the light they receive and not the outside temperature. The high stagnation temperatures can cause antifreeze to break down, so careful consideration must be used if selecting this type of system in temperate climates. Tubes come in different levels of quality so the different kinds have to be examined as well. High quality units can efficiently absorb diffuse solar radiation present in cloudy conditions and are unaffected by wind. They also have the same performance in similar light conditions summer and winter.
For a given absorber area, evacuated tubes can maintain their efficiency over a wide range of ambient temperatures and heating requirements. The absorber area only occupied about 50% of the collector panel on early designs, however this has changed as the technology has advanced to maximize the absorption area. In extremely hot climates, flat-plate collectors will generally be a more cost-effective solution than evacuated tubes. When employed in arrays of 20 to 30 or more, the efficient but costly evacuated tube collectors have net benefit in winter and also give real advantage in the summer months. They are well suited to extremely cold ambient temperatures and work well in situations of consistently low-light.
What About Maintenance
There are no moving parts to wear out. All components are made from high-grade stainless steel, copper or non-corrosive materials.
Usage
Hot water heated by the sun can be used to:
Heat water (e.g. for sanitary purposes such as showering, washing, ...)
Generate electricity
Designs suitable for hot climates can be much simpler and cheaper, and can be considered an appropriate technology for these places. The global solar thermal market is dominated by China, Europe, Japan and India.
The typical 50 gallon electric water heater uses 11.1 barrels of oil a year, which translates into the same amount oil used by a typical 4 door sedan driven by the average consumer.
Electric utility companies often provide electricity by burning and releasing energy from fuels such as oil, coal and nuclear energy. An electrical home hot water heater sits on an electrical grid and may be driving the use of unclean fuels on the other end of the grid.
SOLAR WATER HEATER
We are blessed with Solar Energy in abundance at no cost. The solar radiation
incident on the surface of the earth can be conveniently utilized for the benefit of human
society. One of the popular devices that harness the solar energy is solar hot water
system.
Solar water heating or solar hot water is water heated by the use of solar energy. 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, under floor 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. This can include domestic non-electric concentrating solar thermal systems
Residential solar thermal installations can be subdivided into two kinds of systems: compact and pumped systems. Both typically include an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) that is activated when the water in the tank falls below a minimum temperature setting such as 50 °C. Hence, hot water is always available. The combination of solar water heating and using the back-up heat from a wood stove chimney to heat water can enable a hot water system to work all year round in cooler climates, without the supplemental heat requirement of a solar water heating system being met with fossil fuels or electricity.
Among pumped options, there is an important distinction to be made regarding the sustainability of the design of the system. This relates to what source of energy powers the pump and its controls. The type of pumped solar thermal systems which use mains electricity to pump the fluid through the panels.
History
Flat-plate collectors for solar water heating were popular in Florida and Southern California in the 1920s. Levi Yissar built the first prototype Israeli solar water heater and in 1953 he started NerYah Company, Israel's first commercial manufacturer of solar water heaters. Despite the abundance of sunlight in Israel, solar water heaters were used by only 20% of the population by 1967. Following the energy crisis in the 1970s, in 1980 the Israeli Knesset passed a law requiring the installation of solar water heaters in all new homes (except high towers with insufficient roof area). As a result, Israel is now the world leader in the use of solar energy per capita with 85% of the households today using solar thermal systems (3% of the primary national energy consumption).
During this time, there was some resurgence of interest in solar heating in North America. Technical innovation has improved performance, life expectancy and ease of use of these systems. Installation of solar water heating has become the norm in countries with an abundance of solar radiation, like Cyprus, Israel and Greece, as well as in Japan and Austria, where there is less.
In 2005, Spain became the first country in the world to require the installation of photovoltaic electricity generation in new buildings, and the second (after Israel) to require the installation of solar water heating systems in 2006. Australia adopted the mandatory regulation for solar thermal for new construction in 2006 as well.
Types
Solar water heating systems can be classified in different ways:
The type of collector used (see below)
The location of the collector - roof mount, ground mount, wall mount[29]
The location of the storage tank in relation to the collector
The requirement for a pump - active vs. passive
The method of heat transfer - open-loop or closed-loop (via heat exchanger)
Compact systems (passive systems)
A compact system also known as a monobloc has a tank for the heated water and a solar collector mounted on the same chasis. Typically these systems will function by natural convection (thermosiphon) or heatpipes to transfer the heat energy from the collector to the tank.
A integrated collector storage system
A special type of compact system is the Integrated Collector Storage (ICS or Batch Heater) where the tank acts as both storage and solar collector. Batch heaters are basically thin rectilinear tanks with glass in front of it generally in or on house wall or roof. They are seldom pressurised and usually depend on gravity flow to deliver their water. They are simple, efficient and less costly than intense plate and tube collectors but only suitable in moderate climates with good sunshine. [30][31] A step up from the ICS is the Convection Heat Storage Unit. These are plate type intense collectors with built-in insulated tanks. The unit uses convection (movement of hot water upward) to move the water from heater to tank. Neither pumps or electricity are used. It is more efficient than an ICS as the intense collector heats a small(er) amount of water that is constantly rising to the tank. It can be used in areas with less sunshine than the ICS.
A passive open loop system
Direct ('open loop') compact systems, if made of metals are not suitable for cold climates. At night the remaining water can freeze and damage the panels, and the storage tank is exposed to the outdoor temperatures that will cause excessive heat losses on cold days. Some compact systems have a primary circuit. The primary circuit includes the collectors and the external part of the tank. Instead of water, a non-toxic antifreeze is used. When this liquid is heated up, it flows to the external part of the tank and transfers the heat to the water placed inside. ('closed loop'). Open loop (direct) systems have the disadvantage that during the night-time, where the temperature of the solar panel starts to drop below that of the water tank, the system starts working in reverse heating the water in the panel and cooling the water inside the tank. This problem is least noticeable in closed loop system using a heat exchanger as only the water in the heat exchanger and not the whole tank is affected by it. Zhuhai Tianke Energy Saving Equipment Manufacture Co., Ltd. managed to solve this problem using a patented design in their solar water heating systems heat exchanger which forces the flow of the water in the in the heat exchangers inlet pipe in an upward flow, thus restricting cold water flowing down to the panel.The force of the flow is made through a driver inside the heat exchanger (jacket). The jacket and the driver part is made of the same heat conducting material therefore sharing the same temperature on same levels of the heat exchanger.
A compact system can save up to 4.5 tonnes annually of greenhouse gas emissions. In order to achieve the aims of the Kyoto Protocol, several countries are offering subsidies to the end user. Some systems can work for up to 25 years with minimum maintenance. These kinds of systems can be redeemed in six years, and achieve a positive balance of energy (energy they save minus energy used to build them) of 1.5 years. Most part of the year, when the electric heating element is not working, these systems do not use any external source for power (as water flows due to thermosiphon principle).
Flat solar thermal collectors are usually used, but compact systems using vacuum tube collectors are available on the market. These generally give a higher heat yield per square meter in colder climates but cost more than flat plate collector systems.
Pumped systems (active systems)
Schematic of an pasive solar heating system
Schematic of an active solar heating system
An active solar heating system uses a pump for the circulation of water
How the solar water heating system is pumped and controlled determines whether it is a zero carbon or a low carbon system. Low carbon systems principally use electricity to circulate the fluid through the collector. The use of electricity typically reduces the carbon savings of a system by 10% to 20%.
Conventional low carbon system designs use a mains powered circulation pump whenever the hot water tank is positioned below the solar panels. The storage tank is placed inside the building, and thus requires a controller that measures when the water is hotter in the panels than in the tank. The system also requires a pump for transferring the fluid between the parts.
The electronic controllers used by some systems permit a wide range of functionality such as measurement of the energy produced; more sophisticated safety functions; thermostatic and time-clock control of auxiliary heat, hot water circulation loops, or others; display or transfer of error messages or alarms; remote display panels; and remote or local datalogging.
The direct pumped system, has one or more solar energy collectors installed on the roof and a storage tank somewhere below, usually in a garage or utility room. A pump circulates the water from the tank up to the collector and back again. This is called a direct (or open loop) system because the sun's heat is transferred directly to the potable water circulating through the collector tubing and storage tank; no anti-freeze solution or heat exchanger is involved.
This system has a differential controller that senses temperature differences between water leaving the solar collector and the coldest water in the storage tank. When the water in the collector is about 15-20° F warmer than the water in the tank, the pump is turned on by the controller. When the temperature difference drops to about 3-5° F, the pump is turned off.
In this way, the water always gains heat from the collector when the pump operates.
A flush-type freeze protection valve installed near the collector provides freeze protection. Whenever temperatures approach freezing, the valve opens to let warm water flow through the collector.
The collector should also allow for manual draining by closing the isolation valves (located above the storage tank) and opening the drain valves.
Automatic recirculation is another means of freeze protection. When the water in the collector reaches a temperature near freezing, the controller turns the pump on for a few minutes to warm the collector with water from the tank.
The dc pump and PV panel must be suitably matched to ensure proper performance. The pump starts when there is sufficient solar radiation available to heat the solar collector. It shuts off later in the day when the available solar energy diminishes. As in the previous systems, a thermally operated valve provides freeze protection.
Types of thermal collector
There are three main kinds of solar thermal collectors in common use. In order of increasing cost they are: Formed Plastic Collectors, Flat Collectors, and Evacuated Tube Collectors. The efficiency of the system is directly related to heat losses from the collector surface (efficiency being defined as the proportion of heating energy that can be usefully obtained from insulation). Heat losses are predominantly governed by the thermal gradient between the temperature of the collector surface and the ambient temperature. Efficiency decreases when either the ambient temperature falls or as the collector temperature increases. This decrease in efficiency can be mitigated by increasing the insulation of the unit by sealing the unit in glass e.g. flat collectors or providing a vacuum seal e.g. evacuated tube collector. The choice of collector is determined by the heating requirements and environmental conditions in which it is employed.
Formed plastic collector
Formed plastic collectors (such as polypropylene, EPDM or PET plastics) consist of tubes or formed panels through which water is circulated and heated by the sun's radiation. These are often used for extending the swimming season in swimming pools. In some countries, heating an open-air swimming pool with non-renewable energy sources is not allowed, and then these inexpensive systems offer a good solution. This panel is not suitable for year-round uses like providing hot water for home use, primarily due to its lack of insulation which reduces its effectiveness greatly when the ambient air temperature is lower than the temperature of the fluid being heated.
Flat plate collector
A flat plate collector
A flat plate collector consists of a thin absorber sheet (of thermally stable polymers, aluminum, steel or copper, to which a black or selective coating is applied) backed by a grid or coil of fluid tubing and placed in an insulated casing with a glass or polycarbonate cover.
Fluid is circulated, using either mains or solar electricity, through the tubing to remove the heat from the absorber and to transport it to an insulated water tank, sometimes directly or otherwise to a heat exchanger or to some other device for using the heated fluid. Some fabricants have a completely flooded absorber consisting of 2 sheets of metal stamped to produce a circulation zone. Because the heat exchange area is greater they may be marginally more efficient than traditional absorbers.
As an alternative to metal collectors, new polymer flat plate collectors are now being produced in Europe. These may be wholly polymer, or they may be metal plates behind which are freeze-tolerant water channels made of silicone rubber instead of metal. Polymers, being flexible and therefore freeze-tolerant, are able to contain plain water instead of antifreeze, so that in some cases they are able to plumb directly into existing water tanks instead of needing the tank to be replaced with one using heat exchangers. By dispensing with a heat exchanger in these flat plate panel, temperatures need not be quite so high for the circulation system to be switched on, so such direct circulation panels, whether polymer or otherwise, can be somewhat more efficient, particularly at low light levels.
As with evacuated tubes, most flat plate collectors have a life expectancy of over 25 years.
Evacuated tube collector
Evacuated (or vacuum) tubes panel.
Evacuated tube collectors are made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change. This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flat-plate collector). In some cases, the tubes are covered with a special light-modulating coating. In an evacuated tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it. The absorber can either consist of copper (glass-metal) or specially-coated glass tubing (glass-glass). The glass-metal evacuated tubes are typically sealed at the manifold end, and the absorber is actually sealed in the vacuum, thus the fact that the absorber and heat pipe are dissimilar metals creates no corrosion problems. Some systems use foam insulation in the manifold. Soda-lime glass is used in the higher quality evacuated tubes manufacture.
Later technology evacuated tube systems use the glass coated absorber. The glass is a boron silicate material and the aluminum absorber and copper heat pipe are slid down inside the open top end of the tube. In lower quality systems moisture can enter the manifold around the sheet metal casing is eventually absorbed by the glass fibre insulation and then finds its way down into the tubes. This leads to corrosion at the absorber/heat pipe interface area, also freeze ruptures of the tube itself if the tube fills sufficiently with water.
Two types of tube collectors are distinguished by their heat transfer method: the simplest pumps a heat transfer fluid (water or antifreeze) through a U-shaped copper tube placed in each of the glass collector tubes. The second type uses a sealed heat pipe that contains a liquid that vapourises as it is heated. The vapour rises to a heat-transfer bulb that is positioned outside the collector tube in a pipe through which a second heat transfer liquid (the water or antifreeze) is pumped. For both types, the heated liquid then circulates through a heat exchanger and gives off its heat to water that is stored in a storage tank (which itself may be kept warm partially by sunlight). Evacuated tube collectors heat to higher temperatures, with some models providing considerably more solar yield per square meter than flat panels. However, they are more expensive than flat panels, but generally of a less cost to repair in the event of damage. Evacuated heat tubes perform better than flat plate collectors in cold climates because they only rely on the light they receive and not the outside temperature. The high stagnation temperatures can cause antifreeze to break down, so careful consideration must be used if selecting this type of system in temperate climates. Tubes come in different levels of quality so the different kinds have to be examined as well. High quality units can efficiently absorb diffuse solar radiation present in cloudy conditions and are unaffected by wind. They also have the same performance in similar light conditions summer and winter.
For a given absorber area, evacuated tubes can maintain their efficiency over a wide range of ambient temperatures and heating requirements. The absorber area only occupied about 50% of the collector panel on early designs, however this has changed as the technology has advanced to maximize the absorption area. In extremely hot climates, flat-plate collectors will generally be a more cost-effective solution than evacuated tubes. When employed in arrays of 20 to 30 or more, the efficient but costly evacuated tube collectors have net benefit in winter and also give real advantage in the summer months. They are well suited to extremely cold ambient temperatures and work well in situations of consistently low-light.
What About Maintenance
There are no moving parts to wear out. All components are made from high-grade stainless steel, copper or non-corrosive materials.
Usage
Hot water heated by the sun can be used to:
Heat water (e.g. for sanitary purposes such as showering, washing, ...)
Generate electricity
Designs suitable for hot climates can be much simpler and cheaper, and can be considered an appropriate technology for these places. The global solar thermal market is dominated by China, Europe, Japan and India.
The typical 50 gallon electric water heater uses 11.1 barrels of oil a year, which translates into the same amount oil used by a typical 4 door sedan driven by the average consumer.
Electric utility companies often provide electricity by burning and releasing energy from fuels such as oil, coal and nuclear energy. An electrical home hot water heater sits on an electrical grid and may be driving the use of unclean fuels on the other end of the grid.
