Solar Hot Water
Solar water heating systems use “solar collectors” to capture solar energy in the form of heat. The solar heat is then transferred into a water tank, thereby heating it. There are several types of solar collectors, and several categories of solar water heating systems that utilize different approaches in heat transfer.
Types of Solar Hot Water System
There are three basic types of solar hot water systems considered appropriate for freezing climates:
• Closed-loop
• Drain-back
• Batch
Closed-loop Water Heating Systems
In a closed loop water heating system, there are two separate, pressurized vessels that interact with one another. One is the water tank itself. The other is the solar collector loop. Heat is transferred from collectors to tank by use of a heat exchanger. The heat exchanger can be internal or external to the tank.
Depending on climate conditions, the solar collector loop may be filled with either water or propylene glycol. The glycol serves as a non-toxic antifreeze. As it remains in the pressurized collector loop even at night, it protects the collectors from freezing and subsequent damage. High quality glycol also is “high temperature inhibited” meaning that it doesn’t break down chemically and become acidic until it reaches very high temperatures.
Cedar Mountain Solar offers closed loop water heating systems that use a design approach refined by over 25 years of experience. It is geared towards optimizing performance, simplicity, reliability, and both electrical and heat-transfer efficiency.
Our systems use a solar-electric (photovoltaic) driven pump to circulate fluid through the solar collectors. There are two reasons for this design:
• In the event of a power failure, the solar-electric pump continues to operate, postponing or preventing collector overheating. With other systems, if the power failure occurs during a very sunny period, there is immediate danger of the collectors overheating, which can cause damage to both the fluid in them and to the collectors themselves. After overheating, typically a service call is required.
• Solar-electric pumps operate at variable speeds, and therefore varying flow rates, depending on the amount of available sunlight. The solar heating collectors also absorb and deliver varying amounts of heat depending on sunlight intensity. In other types of systems, pump operation needs to be controlled with sensor-based controllers that determine collector temperature and turn pumps on and off accordingly. In our systems, the controller is not needed because the solar collectors and solar-electric pump operate synchronously with one another, both controlled by the sun. When more sunlight is available, more heat is produced, and the pump speed increases to deliver more heat to the tank.
Our systems generally also incorporate storage tanks with internal heat exchangers. This approach eliminates the need for a pump to circulate water through the tank side of the heat exchanger. Since the heat exchanger is submerged in the storage tank, it delivers heat to the tank extremely efficiently. External heat exchangers, since water is circulated through them, also have the disadvantage of calcifying and becoming gummed up over time, especially if water softeners are not used.
Based on our experience, we also incorporate many less obvious features into our installations, including insulation types and our choices of valves and other small components that ensure reliability and efficient performance.
Drain-back Solar Water Heating Systems
In drain-back solar water heaters, fluid is drained out of the collectors to prevent freezing in cold conditions. A vacuum relief valve allows air to fill the collectors as the fluid is removed. When the tank is cooler than the solar collectors, the controller turns on a pump that fills the collectors with fluid, which is then pumped through the collectors. The heat absorbed by this fluid from the collectors is then delivered to the storage tank. When outside temperatures fall or when the tank does not call for heat, the collectors are drained into a drain-back tank.
In our opinion, these systems are acceptable for areas that rarely freeze such as Florida or southern California. However, there are a couple of reasons that Cedar Mountain Solar does not use drain-back systems. Plumbing for the collectors must be installed so that no fluid can possibly remain anywhere in the collector loop in case of a freeze. If fluid does freeze in the collectors, they can be damaged and require repair or replacement. A small error in plumbing can cause major damage to the system.
The electrical requirements for such a system are higher than in closed loop systems because the collector loop is not pressurized. Pumping fluid from a basement mechanical room to a second-story roof takes much more power than pumping around a sealed and pressurized loop. In the latter circumstance, there is no lift for the pump to overcome; as it pushes fluid from its outlet, fluid is automatically pushed to its inlet.
Batch Solar Hot Water Heaters
This type of solar water heater incorporates the solar collector right on the water storage tank, which is then typically mounted on the building roof. Such a system has the advantage of being completely passive. No pumps are required as hot fluid thermo-siphons naturally from collector to tank. A batch solar hot water heater relies on the insulated thermal mass of the water, heated during the day, to hold heat throughout the night thereby preventing freezing.
In our opinion, batch heaters are a great low-tech solution to making hot water. However, issues of large weight loads on the roof and the potential of freezing in a longer cold spell must be addressed in the installation.
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