Active Solar (Solar Thermal)

In an active solar system, the collector is located someplace other than where windows are: on the roof, or possibly even not on the house itself.  Active system may heat air, water or some other fluid.   Heat from the collector is moved via pumps or fans into the house, or into a storage medium (traditionally, rocks or water).  Because air holds very little heat, air collection systems use more electric to collect heat than fluid systems, and so have fallen out of favor.  The most common active solar systems heat water (or other fluid), and are used to pre-heat hot water.

Due to the cost and complexity of active solar, it has less cost effective than passive, although it is still the only method of choice for water heating.  Compared to passive methods, active solar allows much more architectural flexibility due to only needing to find a place for the collection panels.   Often the solar collectors are on the roof, but they can also be built into a south facing wall or even detached somewhat from the house (although distance will create extra avenues for heat loss).

The collector consists of a sheet of glass covering an airspace in which a specially coated black metal surface collects upward of 90% of the solar energy hitting it.  Inside the collector, the black metal either heats the air itself, or a fluid (typically including anti-freeze for cold nights). In the more efficient versions, the metal is water running through it and the air is removed, significantly reducing the heat loss back outside.


((picture: active solar collector & pumps)

Solar Hot water

Collectors come in a number of varieties, and the best choice will depend on climate.  In order of cost, they are;

  • Batch collectors:  For non, or barely freezing climates only
  • Flat plate, drain-back collectors1: For mild freezing climates
  • Flat plate anti-freeze: For hard freezing, but sunnier climates
  • Evacuated tubes: For hard freezing, cloudier climates

Batch collectors: essentially these are insulated, glass covered tubs of water sitting in the sun that preheat the cold water line going to the water tank.  With enough insulation on the connecting pipes and enough water in the tub, they can withstand some freezing temperatures, but an entire night below freezing will likely destroy it.  These are typically not truly active systems, since water in them is pressurized to the same pressure as house water.

Flat plate collectors, in general: these collectors are made from an insulated box, with a glass cover.  Inside, blacked metal plates coated in a special high absorption/low emission material soak up the heat and transfer it to thin pipes embedded in the metal collection surface that transfer the heat to the collector fluid.

Drain-back collectors: in these system, water is generally the collection fluid, and the only pressure in the collector is due to the pump.  The storage tank must have a heat exchanger coil inside them to separate house pressure water from collector water. The key to these systems is that there is no water in the collector unless the collector is warmer than the storage tank, because that is the only condition the controller will turn the pump on.  When the pump is not running, water drains out of the collector into a reservoir.  Because the water drains back, a larger pump must be used than in anti-freeze collectors.  If the collector is 30 feet above the storage tank, a pump that can overcome a 30 foot head it needed.

Anti-freeze collectors: in these system, the collection fluid is generally water with food grade anti-freeze in it (propylene glycol: better than the highly toxic ethylene glycol used in cars, but you probably still don't want to ingest it). Since the water doesn't freeze, it doesn't have to drain-back, and as a result you can use a smaller pump than in drain-back collectors.  If the collector is 30 feet above the storage take, an anti-freeze collector can use the weight of the water returning to the pump to help, and so only must overcome the friction in the system, not gravity.

Evacuated tube collectors: these collectors are similar to flat plate collectors, but are insulated by removing almost all the air inside.  They are made as glass tubes, typically about 6" in diameter, and about 5 feet long. In many cases, rather than running the pumped anti freeze solution thru the tubes, a heat exchanger manifold is used: the tubes themselves contain their own heat transfer fluid that circulates in the tube by convection.  In sunny climates, evacuated tubes would be overkill, resulting in over collection during the summer, and so having to add a system to dump excess heat.  In cloudier climates, the main disadvantage is cost: they are as much as twice as expensive.  However, since they are more efficient, they collect the same amount of heat using less roof area.

Open Loop versus closed loop: batch collectors and some versions of drain-back collectors are open loop, meaning that the water in the collector comes straight from the tank.  The other collectors, especially ones with anti-freeze, are closed loop meaning the fluid in the collector never mixes with that in the tank.

The major disadvantage of open loop systems is that dissolved oxygen in the water will cause oxidation (rust) of collector parts, and hard water will also gum up the collector over time.  If any of these are issues, closed loop systems will last longer.

Orientation & Controllers

There are two parts of orientation: tilt angle, and compass direction.  Like every other solar collector, straight south is generally best, although slightly west of south is better in climates that tend to have morning haze or clouds.  Tilt is more arbitrary: ideally the collector would track the sun thru the entire seasonal 46° change in sun angle, but since that isn't simple, tilt is usually adjusted so that it more directly faces the winter sun, since there is less of the winter sun.  In cloudy climates, sunlight is dispersed and so orientation is not as critical.

Solar hot water collectors are operated by an electronic controller utilizing two temperature sensors: one in the collector and one in the tank.  The controller turns on when the collector is some set point warmer than the tank: in many controllers you can program the set point.  A good set point is around 10°F: warmer enough to justify using the pump energy.  The fancier controllers will give you more data, but otherwise they all do the same thing.

Hot Air/Rock Bin Systems

The general idea behind these systems is to use a large collector somewhere (roof, down a S facing slope, etc), and pump the air into a rock bin.  Smaller systems were designed just to hold heat for a few days, but larger ones were intended to collect summer heat and use it in the winter.  In order to be effective, a very large volume of rock is needed, and it needs to have a very large surface area to transfer the heat fast enough.  Because air hold very little heat, much air must be moved in order to transfer much heat to the rock bin, and as a result the heat stored per amount of energy used is not as good as with fluid transfer.  Indirect solar gain, thermo-siphoning systems are effectively active systems that use convection instead of pumps, so have a better collection efficiency, but are limited to the pressure available from convection.   As the temperature of the rock bin (or other mass) goes up, its heat leakage will also go up, so there is some practical limit to how much heat can be stored by increasing temperature.  In any case, the mass must be effectively insulated, especially if it is to hold heat from summer all the way into winter.

The advantage of air collector systems is that air leakage (compared to water leakage) doesn't damage the building.

Resources - Arizona Solar center website, covered much of this same material. - similar material to here from Home Power magazine.


1: as opposed to drain-down collectors, drain-back collectors are not under water system pressure.  Drain-down collectors have a