Flat Plate Collectors
Parallel-flow, flat-plate-collectors are common. They were developed by Hottel and Whillier in the 1950's and they consist of a dark absorber plate to change light into heat, a transparent cover that traps IR radiation, a heat-transport fluid like water, and an insulated backing. Commercial flat-plate collectors use copper absorber plates soldered to copper flow tubes. A selective coating such as CrO2 is applied to absorber to converts incoming radiation into heat. Commercial collectors are often built with a stainless steel or aluminum frames. Glazings materials made from low iron content plate glass materials BUT some manufacturers are starting to use Twin Wall Polycarbonate for glazing. Heat is normally transferred to flow tubes bonded to the absorber plate but today stainless steel plates with flow channels are made directly into heat exchangers to improve collector efficiency and lower manufacturing costs. As an alternative to metal absorber plates is the non-metallic heat exchanger with freeze-tolerant water channels are made of silicone rubber. Since polymers are flexible they will not have a freeze problem with plain water, so that they may be plumbed directly into existing water tanks.
Conventional metal collectors are vulnerable to damage
from freezing so when filled with water so they must be carefully plumbed to completely drain
when the pump turns off. A common type of open loop
system like this is called the drainback system. The same kind of collector,
used in a closed loop system, may also be protected from freezing by mixing propylene glycol antifreeze with
plain water. Unfortunately when propylene glycol is added the plumbing becomes more
involved and special precautions must be taken to isolate potable water from the
antifreeze. Closed loop pumps require less power to circulate water than open
loop systems, but plumbing and maintenance considerations are more involved.
This
flat plate collector is using thermo pane glazing to reduce heat loss... However it has been found that although thermo pane
reduces heat loss it also reduces the heat gain. For this reason the net
heat gain of a collector that uses thermo pane is reduced. Single sheets of
tempered glass are now used by most flat plat plate collector manufacturers.
Notice the flow tubes imbedded in the metallic absorber plate. This is where heat energy from sunlight is collected and transferred to the collector fluid. In a closed loop system this same collector fluid is circulated through heat transfer coils inside a heat storage tank.
Home made heat storage tanks sometimes use external heat exchange coils... but we'll save heat storage systems for another page.
Standard parallel-flow flat plate collectors are mounted so that the parallel flow tubes are arranged in a vertical orientation, but there will be times when special considerations will make vertical mounting difficult if not impossible. This may be done by tipping the collector on it's side. As a matter of fact an array may be assembled from a collection of standard parallel flow collectors but the plumbing is so awkward that it may be more practical to build a wide collector from scratch. This is how Bob Allen built a 96 sq. ft. collector under his patio.
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