Solar Hot Water Systems - How it works.

Heating hot water with the sun can be accomplished in several ways, some simple, some a little more complex to build --but easy to use.

Passive Solar Systems

The bucket example: The simplest way to heat water passively with the sun, is to put water into a container or bucket, paint the bucket black, stick it out in the sun, wait a few hours, and presto, free hot water, compliments of Ra , the ancient Egyptian Sun God. The bucket system is "passive" because it does not use any moving parts or electricity.

Scientific explanation: Billions of photons beaming down from the sun excite the bucket's atoms--making them move and "spin" faster. We observe the faster moving atoms as an increase in temperature. The bucket's more excited atoms in turn excites the H₂O water molecules. The transfer of heat is called thermal conduction and obeys the laws of thermodynamics --and the scientific law of conservation of energy --not to be confused with human-created conservation law!

We enjoy hot water in the form of hot showers and also the important side benefits of loosening grit and grime from our skin, clothes, floors and pet dog Spot.

"Batch" collectors are a more sophisticated form of the bucket example. The collectors are black coated containers or tanks that are housed in an insulated metal box and covered with a solar glass or glazing material, and are larger than buckets. Usually batch collectors are filled with pressurized water.

Strengths: Batch collectors operate without the need of "active" pumps or controls, so they don't need much maintenance. Also, because they don't have many parts, they can be the cheapest system to purchase or build.

Weaknesses: Batch collectors are not as efficient as the following "active" collector systems. Also, batch collectors should be installed only where freezing is not a problem, because the water feed pipes and storage tanks are prone to freeze damage. Owner participation is usually required to prevent freeze damage, by manually draining down the batch collector during near freezing weather conditions. Also see *Thermosiphon Systems

Active Solar Systems

From this point, solar heating gets a little more complex to explain, but increasingly easier to operate. Since no one but your basic camper would really appreciate the simplicity of the black painted bucket method, engineers, long ago, developed highly efficient "flat plate" solar collectors.

Flat plate solar collectors are used in active solar systems. They are usually made out of a set of parallel copper pipes ("riser tubes" or "risers") that are embedded or soldered onto a thin copper "fin" that runs the length of the riser tubes. The "fins" increase the absorption of the solar rays, transferring more heat into the water or various kinds of solar fluids that can be circulated through the riser tubes. The riser tubes and fin assembly are brazed into 2 horizontal "header" pipes, at the bottom and the top of the riser tubes. The solar absorber plate is then installed in an aluminum framed box surrounded on the bottom and sides with insulation and covered with tempered glass. Flat plate solar collectors ("solar panels") are a key component of active solar systems because the require constant flow of fluid through the panels.

Open loop

Open loop systems directly heat the water. Circulation of the fluid through the solar collector is accomplished via a small pump mounted on a solar storage tank. The solar pump is activated by a differential thermostat controller that senses when heat is available in the solar collectors. The solar storage tank connects to the existing hot water heater and feeds the preheated solar water into the gas or electric hot water heater as hot water is used. The solar collectors and feed lines are protected from freezing by automatic drain down controls which allows the water in the pipes and panels to fall safely back out of the solar collectors and feed pipes. These types of systems are called "open loop" since the energy collection loop is not separate from the rest of the the hot water system --i.e. it is "open" to using the same water.

Strengths: Regular maintenance costs are associated less with open-loop systems because they don't use any special fluids or heat exchangers used with closed-loop systems. They are usually cheaper to install than closed-loop systems. Open-loop systems are more efficient at heating water than closed-loop systems, because water is directly heated from the solar panels.

Weaknesses: Open-loop systems' "drain down" freeze protection can fail, subjecting system owners to risk of substantial repair costs from freeze damage and property damage such as flooding from broken pipes. These costs can significantly outweigh the benefits of using this kind of system. A system drained-down for protection purposes can miss opportunities for providing high-performance benefits of solar hot water heating.

Closed loop

Active solar hot water heating systems can also employ the use of heat exchangers that circulate heat exchange fluids through the panels and feed pipes. This type of system is called a closed-loop system, because the solar exchange fluid is closed-off from the external atmosphere or isolated from the potable water through utilization of a heat exchanger. In a closed-loop system the heated solar fluid is pumped through the solar collectors. The heated solar fluid flows through a copper or stainless steel heat exchanger located near the solar storage tank. The heat from the solar fluid transfers to the potable water within the solar storage tank. Another small circulator pump may be used to circulate the water through the potable side of the heat exchanger.

Strengths: The anti-freeze heat exchange fluids can withstand freezing temperatures, allowing the system to operate during periods when there is the greatest temperature difference between cold incoming water, and temperatures reached in the solar collectors. The system can have the greatest performance benefits at this time. Also, if maintained properly, these systems will not corrode or scale the passage ways in the solar collectors and pipes. Closed-loop systems tend to have the lowest overall operating costs --other than passive systems.

Weaknesses: Closed-loop systems tend to have the highest installation cost associated with them. They heat water slightly less efficiently than direct open-loop systems, (but can work more and longer when it's risky to operate open-loop systems.)

The above are the basics of solar water heating. There are various modifications and hybrid systems that utilize parabolic troughs, concentrating mirrors, freon, and vacuum tubes to accomplish the same result. More information on these hybrid systems is available, but the majority of solar hot water heating systems installed in the United States are either batch, open loop, or closed loop systems due to their cost efficiency and proven performance over decades of use.

*Thermosiphon Systems

Thermosiphon systems are a kind of "passive" solar hot water heating that employ the use of flat plate solar collectors. The solar panels usually are mounted at a lower elevation than the storage water to be heated. Thermosiphon systems can circulate potable water or utilize a heat exchanger and heat exchange fluid.

For potable water systems, the cooler water at the bottom of the storage tank is thermally siphoned to the hotter water near the solar collector by the raising temperature and volume of the warmer water, initiating a circulation of the storage water through the collectors fluid passage ways back into the top of the storage tank. The circulation continues until the temperature at the bottom of the storage tank is about the same as the temperature of the outlet pipe at the top the solar collector.

Thermosiphon systems using a heat exchanger work on the same principle by circulating a nonfreezing solar exchange fluid through an internal heat exchanger installed inside the solar storage tank, instead of circulating potable water. This type of system is usually installed in climates that experience freezing temperatures to prevent freeze damage of the solar collectors and siphon pipes.

Strengths: Thermosiphon systems, like other passive solar systems, usually cost less than active solar systems to purchase and maintain.

Weaknesses: Regardless of the thermosiphon system used, care needs to be taken that the water feed pipes connecting the solar collectors to the water heater inside the house or building, do not freeze. Insulation of the feed pipes does not guarantee against freezing. We recommend a manual drain down of the pipes and tank preceding freezing conditions. Also, thermosiphon systems need special installation conditions not associated with the active solar systems.