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Solar Water Heater Key Facts

In Solarpower Knowledge by Georg Kogler

While solar PV systems are rightly recognized as a major driver for the solar energy movement and the growth of green energy overall, solar water heater systems also have a significant role to play. On average, hot water consumes 18% of a typical family energy bill, and solar water heaters are an excellent means of meeting this demand, via clean, proven technology.

Solar water heater systems are also highly efficient at converting sunlight into heat. As solar technology continues to improve, these systems will no doubt continue to play a strong role.

What Is A Solar Water Heater?

A solar water heater is a solar panel (called a collector) that instead of converting sunlight into electricity, converts sunlight into heat which is used to heat the water supply in the house or building where it is installed.

How Does A Solar Water Heater Work?

The way a solar water heater works depends on the type of system, taking in the collector type and the circulation system type. The types and their working methods are as follows:

Solar Water Heater Collector System Types

  1. Batch Collectors, also known as Integrated Collector-Storage (ICS) systems, operate by heating water contained in tanks or tubes within an insulated box. The water is stored in this tank until needed. The water can be retained in the collector for prolonged periods, thus keeping the water temperature high. At the tap, the temperature is moderated using what is known as a tempering valve, which adds cold water as necessary make the water temperature safe for household use. Batch collectors do not work with closed-loop circulation systems. Therefore, batch collectors are not normally deemed suitable for cold climates.
  2. Flat-Plate Collectors operate via a set of tubes fitted to the absorber plates. These tubes are typically copper and usually comprise rows of parallel tubes, with an inlet pipe on one end and an outlet pipe on the other end. The flat plate and pipes are housed within an insulated box, which has a tempered glass exterior. Flat plate collectors are typically sized to contain 40 gallons of water. Two collectors provide roughly half of the hot water needed to serve a family of four.
  3. Evacuated Tube Collectors are regarded as the most efficient solar water heater collectors, because they extend the heat efficiency of the flat-plate collector concept. Once again, a series of tubes are used, each of which contains water (or a heat transfer fluid) that is heated by the collector plate. However, in this case, the tube containing the fluid is encased in a larger glass tube. In addition, the space between the inner and surrounding tubes is a vacuum, similar to the principle for a thermos flask. As a result, very little heat is lost from the heated water (or transfer fluid). Because of this extra efficiency and insulation, evacuated tube collectors can work well in overcast and cooler conditions, even in temperatures as low as -40°F (-40C). Another valuable feature is that tubes can be replaced on an individual basis as needed. However, it should be noted that the added efficiency and flexibility of this system comes at a cost: evacuated tube collectors can be twice as costly per square foot as flat plate collectors.

Solar Water Heater Circulation System Types

  1. Direct systems operate by circulating water through the solar collectors, which heat the water by converting sunlight to heat. The heated water is then stored in a hot water tank. These systems are best suited to locations where freezing temperatures are a rare occurrence.
  2. Closed-loop, or indirect, systems use a heat transfer liquid to transfer heat from the collectors to water in a storage tank. The heat in the transfer fluid then heats the water in the storage tank, by means of a heat exchanger in the storage tank, which transfers the heat from the transfer fluid to the water. The transfer fluid then returns back through the collectors and so on. These systems are safe for use in freezing climates.
  3. Active, or forced-circulation, systems use a system of electric pumps, valves and controller units to pump water from the solar collectors to the hot water storage tank. This type of system is common in the U.S.
  4. Passive systems use natural convection (based on heavier cold water forcing hot water to rise) to transfer water from the collectors to the hot water storage tank. These systems do not require pumps.

Advantages and Limitations of a Solar Water Heater

Advantages

  • Zero cost to operate, using clean, natural energy, with no external electricity required
  • Zero maintenance required (apart from keeping free of obstructions, e.g. dirt)
  • Year-round operation (performance will depend on specifications)
  • Saves money on fuel bills (based on previous cost to supply hot water)
  • Less space required than for a solar PV system (for example, a typical house would require only 2-3 solar water heater collector panels to heat water for the home, versus a typical requirement of 10-16 solar PV panels to provide sufficient electricity to meet the needs of that home
  • Highly efficient, with roughly 80% of the sunlight received by the collector panels being converted into heat for hot water
  • May qualify for renewable energy incentives

Limitations

  • Solar collector panels can only heat water, whereas solar PV panels generate electricity, some of which could also be used to heat water
  • Annual maintenance recommended, to ensure that the pump and antifreeze (depending on the system) are functioning smoothly
  • Usually requires a new hot water cylinder to be installed, which requires space and incurs an additional upfront investment

Note: Solar water heater systems can be supplemented with a solar PV system if there is sufficient roof space and budget for this additional system. If so, a portion of the electricity generated by the solar PV system could be used to provide extra hot water heating to boost the water temperature if required. This approach is a very efficient way of meeting hot water needs.

Overall, solar water heater systems are a strong contributor to the solar energy story, and no doubt will continue to be so with the ongoing improvements in solar technology.

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