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Solar H2O
Solar water heating systems use “solar collectors” to capture solar energy in the form of heat. The heat collected 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.
Solar Water Heating System Types
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’s preferred design: top
Cedar Mountain Solar offers closed loop water heating systems that use a specific design approach. This approach has been refined by over 25 years of system design experience. It is geared towards optimizing performance, simplicity, reliability, and both electrical and heat-transfer efficiency.
Our systems usually use a solar-electric driven pump to circulate fluid through the solar collectors. There are two reasons for this design:
1. In the event of a power failure, the solar-electric
pump continues to operate. If the power failure occurs during a very sunny
period, there is 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. As long as fluid is circulated through
the collectors, overheating can be prevented.
2. 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 turns
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. This function is performed by a valve, which is in turn actualized by a differential controller. 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 OK 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.
1. 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.
2. 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.
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 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.
Selective Surface Flat Plate Collectors:
These collectors absorb more energy per surface area than other flat plate collectors. We tend to use these because they perform very well in cold conditions and fairly well in low-light conditions while remaining cost-effective compared with more efficient technologies. They typically operate at temperatures between 110 and 160 degrees. This range is suitable for domestic water heating and for several types of space heating systems including radiant floors, hydronic air-handling units, and some baseboards.
These collectors are usually very well insulated on the back and sides, and glazed with low-iron tempered glass. This type of glass allows as much sunlight through as possible, similar to greenhouse windows, while withstanding hail and other abuses. Underneath the glass a network of pipes exists, consisting of a header across the top and bottom and several risers connecting the headers to one another. These pipes are typically made of copper, with the headers having a 3/4” or 1” diameter and the risers much smaller, almost like capillaries. Fluid is pumped into the lower header and as it heats up it rises by convection through the risers into the top header. Different methods ranging from brazing to gluing are used to fuse the risers to the headers. Beneath this network of pipes is the absorber, to which the risers and header are fused. In selective surface collectors, the absorbers are usually chrome or copper plated to maximize the energy absorption of the collector. As the absorber heats up, that heat is transferred to the risers and thereby to the fluid in the collectors.
Painted Flat Plate Collectors:
These collectors use methodology very similar to that of the selective surface collector described above. The main difference is the quality and cost of the absorber. Instead of chrome or copper plating, these collectors are painted black. This practice limits cost and also reduces efficiency and performance. These collectors are appropriate in some applications, as in high altitude installations with very intense sunlight, or for pool heating where lower fluid temperatures are acceptable.
In this type of collector the absorber is enclosed in an air-evacuated tube. This allows for better insulation as well as more efficient energy absorption compared to flat-plate collectors. Temperatures over 300 degrees F can be reached, which can be appropriate for steam and other high-temperature heating applications. These collectors also produce a useful amount of heat in partly cloudy and very cold conditions, making them a suitable choice in some climates. They are also used in industrial applications such as steam process heating and district hydronic heating.
The manufacturing process of evacuated tube collectors is more expensive than that of flat plate collectors, and in many climates the higher efficiency does not justify the additional cost. The installation of these collectors can also be more expensive than that of flat plate collectors, as all components in the collector loop must be selected for their ability to withstand very high temperatures.
Plastic collectors are basically arrangements of black plastic pipe, treated to withstand UV degradation and other environmental hardships. They are popular for heating pools in seasonal applications or year-round in warm climates.
Plastic collectors are impacted by ambient temperature to a much greater degree than the other collector technologies discussed here. When the outside temperature is cold even on a clear day, these collectors will deliver little or no heat. This is because they are neither glazed nor insulated. Since pool heating, especially of outdoor pools, is often a seasonal concern, plastic collectors can be very effective. However, for domestic hot water and space heat, or for year-round pool heating, higher temperatures are required, and plastic collectors can’t keep up.
A developing application for plastic collectors is nighttime heat radiation for cooling purposes. Since they are not insulated or glazed, they release heat as readily as they absorb it. Fluid can therefore be pumped from radiant floor tubing in a home through these collectors, and will return to the floor cooled.
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The website for the US Department of Energy contains additional information on the different types of collectors and systems available for an array of applications.
http://www.eere.energy.gov/consumerinfo/heatcool/hc_water_type.html
http://www.eere.energy.gov/consumerinfo/factsheets/solrwatr.html