Solar thermal panels

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What are solar thermal panels?

Solar thermal panels or collectors capture the energy from the sun and transfer 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 used in a variety of ways.

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 size of the building, the location of the building, available space for the panels, 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.

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're 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.
Schematic drawing of the solar panel piping.

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 insulated steel storage tanks totaling 5,000 gallons 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 solar 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. In our design, it 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. If the air temperature in the panel is a minimum of 90 degrees and is a minimum of 30 degrees warmer than the tank temp, the pump starts and circulates water from the storage tanks through the panels back to the storage tanks. When the water temperature leaving the panels drops below the water temp in the tank the pump stops and the water drains out of the panels back into the tanks.
  • The panels are mounted in a straight row and connected with high temp silicone heater hose for the purpose of simplicity.
  • Hot water from the storage tanks is pumped through a heat exchanger which transfers the heat to the water being pumped through the radiant loops which run under the floors of the main building and around the perimeter of the annex.
  • We bought the panels from UMA Solar.

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 end of the return water tube was submersed in the water in the tank. Because of this, the panels did not fully drain when the pump shut off. The tube was shortened and we have not had this situation reoccur. We went through the remainder of the winter incident-free.
  • General operation. The panels have performed well, providing enough warm water to keep the building warm during an entire winter season.