Difference between revisions of "Solar thermal panels"

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== Resources ==
 
== Resources ==
* See [[Solar Thermal Collectors]], our design page for the panels.
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* See [[Solar Thermal Collectors]], our original design page for the panels.
  
 
[[Category:site index]]
 
[[Category:site index]]
  
 
[[Category:As Built]]
 
[[Category:As Built]]

Revision as of 18:36, 9 August 2012

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What are solar thermal panels?

Solar thermal panels or collectors capture the energy from the sun and transfers it to water as heat to provide radiant heating for the building. It does so with one or more solar collectors positioned so the sun shines directly on them. The collectors are often located on top of buildings or in open areas adjacent to the building. The sun's energy heats up the collector. Water flowing through copper tubes in the collector is heated, and returned to mass thermal storage tanks. The stored hot water is then used for radiant heating.

There are two typical solar thermal panel types, flat panel and evacuated tube. They both have up to an 80% efficiency in converting the sun's energy into hot water. There's much debate as to which collector is best. It does appear that the flat panel has a longer life expectancy due to fewer points of failure. The tube seals on evacuated tubes are the weak link. Also, when looking at the efficiency of the collectors, you need to look at how efficient they are when placed into a system.

There are also two basic configuration designs for cold climates, closed loop glycol and open loop drainback. The main reason for the glycol is to prevent freezing of the water in cold winters, but a drainback design can also be used in cold climates.

A typical solar thermal panel subsystem includes:

  • Solar collectors and framing to hold the collectors
  • Piping carrying the water to and from the collectors and the thermal storage
  • Thermal fluid...water and/or glycol

The size and number of collectors is determined by the amount of energy that is needed by the building, the location of the building, amount of sun energy available, and the efficiency of collectors.

Differentiation from PVC panels

Solar thermal collectors are different from solar photovoltaic collectors in that they translate the sun's energy into heated water instead of electricity. A big difference is that solar thermal collectors are much less costly, but they are 3 to 4 times as efficient... an 80% efficiency versus 15% for photovoltaic. The efficiency of the solar thermal collectors varies greatly with delta T (Temp of the water in the collector -(minus) Temp outside air)...the greater the difference, the lower the efficiency of the collector.

When to use them

A solar thermal panel subsystem is best suited for locations where:

  • The space heating demand has already been reduced through insulation of the the building envelope and domestic hot water needs have been reduced through use of low-flow shower heads and efficient appliances.
  • There is adequate access to direct sun light in a location on or near the building for locating the solar thermal panel subsystem.
  • There is an adequate demand for space heating and domestic hot water.
  • You'd be looking for quickest payback renewable energy systems, as domestic hot water has one of the quickest paybacks of any renewable energy application.
Solar thermal panels on the roof of the annex at the Green Garage.

How are solar thermal panels being used at the Green Garage?

  • We used the flat roof of the side building as a location for the panels. We didn't want to deal with any structural issue associated with placing the collectors on the barrel roof of the historic building.
  • We used thermal storage in the form of two 2,500 gallon insulated steel storage tanks to reduce the number and size of the solar thermal collectors. The tanks serve as a form of a battery, allowing us to store heat for a number of days.
  • We chose flat panels because of their slightly lower cost and higher durability (twice the life).
  • We have 10 collectors, each 4 ft x 10 ft (400 sq ft), positioned vertically. Each weighs about 185 lbs when you consider water, framing and hardware.
  • Vertical positioning is 60-70 degrees. Horizontal positioning is 25 degrees west of a line perpendicular to the south wall, or our "true south."
  • We used an open-loop drainback design because of its simplicity. It eliminates the use of glycol, which is toxic and is the main cause of the deterioration of the collectors. It also eliminates the need for costly heat exchangers as the water is circulated directly into the thermal storage.
  • The flow through the collectors is triggered by a thermostat positioned inside the top of the collector reaching 90F (and above the temperature of the water in the tank.) Then the pump begins pumping the water from the thermal storage tank up into and through the collectors. When the temperature drops below 90F the pumps stop.
  • The panels are mounted in a straight row and connected with high temp silicone heater hose for the purpose of simplicity.
  • The panels are connected to the thermal storage tanks which is connected to the pex tubing which runs under the floors of the main building and under the perimeter of the annex.
  • We bought the panels from Rushforth Solar in Philadelphia.
  • For more specifics on how the panels operate, please see the Key Design Criteria section of our Solar Thermal Collectors Design Page.

How are they working?

  • Freezing incident. In their first year of operation we had an incident in which some of the panels froze. After careful study, it was determined that the evacuation tubes had become submersed in the tank water so the panels could not evacuate the water adequately. The tubes were trimmed and we have not had this situation repeated. We were able to salvage the panels.
  • General operation. The panels have performed well, providing enough warm water to heat the building during an entire winter season.

Photos

Resources