Solar heat exchanger
Solar hot water heaters and heat exchangers
A solar heat exchanger can be a required component of a solar hot water heater. Heat exchangers are used to transfer heat from the system's heat transfer fluid to the domestic water. Without the assistance of a heat exchanger, solar hot water heating systems in harsh climates would be highly susceptible to freeze.There are several design and performance considerations to make when selecting a heat exchanger. In some instances, the solar water heating design and the local building codes can determine application.
Heat exchangers are required in all closed loop solar water heater designs, also called indirect circulation systems. Whether your considering purchasing the major components and tailoring a design, or your going with a reputable solar supplier; understanding how heat exchangers function within whole systems is important.
In all external applications, the solar heat exchanger is located in the plumbing loop between the hot water panel and the storage tank. An external application can allow for the additional flexibility of incorporating a used (and slightly modified) conventional storage tank to serve as the system's solar storage tank.
Solar hot water heaters that have external heat exchangers generally require two solar hot water pumps, one to circulate the domestic water from the storage tank and another to distribute the heat transfer fluid to the panel (or collector) and the solar heat exchanger.
Internal or immersed application
In all internal applications, the solar heat exchanger is immersed in the storage tank. Internal applications place the stored domestic water in direct contact with the outer wall of the heat exchanger.
In fact many manufacturers have introduced integrated solar hot water tanks that already come equipped with heat exchangers. Many industry experts consider an internal heat exchanger as the more energy efficient application.
Single wall and double wall
With both applications (external applications and internal applications), solar heat exchangers come both single walled and double walled.
Single Wall Heat Exchangers
Single wall models have one layer of material between the domestic water and the heat transfer fluid. These models can be more efficient, simply because there is less material between the fluids.
Double Wall Heat Exchangers
Double wall models have two layers of material between the two fluids. These models also provide an extra air space between the two fluids. In the event one of the inner walls is compromised, this extra air space will ensure that the two fluids will never mix.
Many double wall models allow for easy leak detection as well. In these models small exit holes are pre manufactured in the vented air spaces, so if a leak were to occur, the fluid will noticeably leak outward from the holes in the air space.
It's important to note the International Association of Plumbing and Mechanical Officials, or IAPMO, has developed the 2006 Uniform Solar Energy Code. This uniform or code recommends that only double wall solar heat exchangers be used in residential water heating systems.
Some U.S states, and local municipalities, have since adopted this code while other states have approved the use of single wall heat exchangers as long as non toxic heat transfer fluids are used.
It would be strongly advised to check with a local solar installer, or with your local or state government about such potential building codes.
Solar heat exchanger designs
All internal or external and single wall or double wall solar heat exchangers come in one, or more, of three different design types; coil (which is sometimes referred to as wraparound), tube in shell (or shell in tube) and plate design.
Coil or Wraparound
Coil and wraparound heat exchangers often come both single walled and double walled. Coil and wraparound models are either internally immersed or wrapped around the solar storage tank.
This design type is essentially a twirled pipe that's wrapped around the the tank between the outer lining and the internal insulation or immersed deep inside the tank in direct contact with the domestic water.
Tube in Tube, or Shell in Tube
This design is essentially made up of a layer of pipe (or tube) within a separate layer of pipe (or tube). The heat transfer fluid circulates through one tube (or pipe) and the indoor domestic water simultaneously circulates in the opposite direction through the other tube (or pipe).
Tube in tube, or shell in tube, solar heat exchangers are generally used in external applications, and more times than not come in double wall designs.
The plate design is essentially a number of plates spaced apart and enclosed, or capped, around the ends and/or sides. This design provides separate directions and passage ways for the two fluids to simultaneously circulate through.
More times than not plate solar heat exchangers offer a relatively large amount of heat transfer surface area, and the heat transfer surface area is effectively and efficiently packed into a relatively small space.
Plate heat exchangers are used in external applications, and more times than not, come in single wall designs.
Important design factors
The material used to construct the heat exchanger, the heat transfer surface area and the flow rates of the fluids circulating through the heat exchanger will all have a large impact on the efficiency and performance of the solar hot water heater.
The material used to construct both the inner and the outer surface of the solar heat exchanger is important. The thermal conductivity, or simply put the amount of heat that the material absorbs and transfers, will be key.
Most quality heat exchangers in solar hot water heaters are made up of copper, because copper has a high thermal conductivity as well as a high resistance to rust or oxidization.
Heat Transfer Surface Area
The heat transfer surface area is the where the all important heat transfer takes place. More specifically, it's the area where the outside portion of the tube, coil or plate comes in direct contact with the circulating or stored domestic water.
The heat transfer surface area must be large enough to enable the solar fluid to transfer enough heat to all of the circulating or stored domestic water.
Fluid Flow Rate and Volume
The volume or flow rate of the fluids circulating through the solar heat exchanger will be important as well. Simply put, the higher the volume or flow rate of fluids generally the better and more efficient the transfer of heat.
It's important to note, a properly sized and installed solar hot water pump will ensure an acceptable flow rate of fluids throughout the solar hot water heater.
Keep in mind
At the end of the day a high surface area to flow rate ratio is what's most ideal. But the heat transfer surface area is generally the single most important design factor, because more heat transfer area for a sizable amount of volume or flow rate will always produces a quality heat transfer.
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