Solar Thermal Simplified
Solar Thermal uses radiation from the sun to produce heat energy. Solar Hot Water (SHW) systems use basic principles and components to capture incoming solar radiant energy and convert it to thermal energy by the collector. This thermal energy is then transferred to water (heating it) in a storage tank. SHW systems can operate in any climate, require little maintenance, use the free energy of the sun, and are cost effective. Performance factors include the solar energy input from the site, temperature, orientation, and tilt.
The major applications of solar thermal energy at present are heating water for domestic use (DHW), Radiant Floor Heating (RFH), space heating, and swimming pools. For these purposes, (with the exception of the pool), the general practice is to use flat-plate solar energy or evacuated tube collectors with a fixed southern orientation.
There are two main types of solar water heating systems:
Passive systems – Operate without the use of pumps or controls. Typical passive systems are Thermosiphon and Integrated Collector Storage (ICS). Some people favor passive solar systems because they offer a simple, low-cost system with no maintenance, no moving parts, and zero operational costs. The following should be considered for the installation of a passive thermal system: hot water load, hot water usage habits, the weight of the Thermosiphon tank, and the weight of the batch for the ICS system.
Active systems – Contain pumps and controls to regulate the flow and hot water production. Types of active systems include closed loop glycol, drainback, and open loop, all of which require freeze protection in areas that have any freezing temperatures.
How Solar Hot Water Works
In the simplest solar thermal application, a discrete solar collector gathers solar radiation to heat air or water for domestic, commercial or industrial use. The solar domestic hot water system is designed to efficiently take full advantage of the sun’s radiant energy to heat water. The system is comprised of solar collectors and an automatic control unit that work together to create a reliable and carefree solar hot water heating system. The panel is usually a flat plate collector that consists of a metal box with a glass or plastic cover and a black absorber plate at the bottom and is usually mounted on a south-facing roof with a pitch.
Absorber plates are usually painted with selective coatings that absorb and retain heat better than ordinary black paint. They are normally made of metal, typically copper or aluminum, because they are good conductors of heat. Although copper is more expensive, it is a better conductor and is less prone to corrosion than aluminum. The sides and bottom of the collector are usually insulated to minimize heat loss.
There are different types of domestic solar hot water systems: active open loop, active closed loop, and unpressurized drainback systems. Closed loop hot water systems come in several variations and are slightly different than the open loop active systems as they utilize heat exchange solution. Used commonly in colder climates where freezing weather occurs, this FDA approved heat exchange solution, or food grade propylene glycol, works well in places where there are greater temperature differences. The closed loop hot water heater has the storage tank built in and directly connected to the solar collector panels, so that the solar energy collected is transferred directly to heat the water in the tank, and the tank itself uses no natural gas. A fully automated thermostat turns the booster switch on or off as needed to maintain the pre-set temperature. If the switch is on, it doesn’t mean you’re paying for supplementary electric or gas energy. Easy to operate, set the temperature desired, and the system does the rest.
Basic System Components of a Typical Active Pressurized or Closed Loop Glycol System:
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