Information about Solar Hot Water Heaters

Things you need to know about Solar

Sunshine is a free source of energy which makes solar water heaters an extremely viable home appliance.

A solar water heating system works by capturing the energy from the sun and converting it to energy which heats the water, there are two basic types of solar heating systems for household water.  The first is an active solar water heating system which circulates the hot water through the household water system using a water pump, the second type of solar water system is passive which works much like your electric hot water system in that it holds the solar heated water in a central tank and then feeds it to the destination using household water pressure.

The setup of a home solar water heating system is usually configured as a collector (see below) which captures the energy from the sun and transfers that energy to the water in the form of heat, the heated water then flows down to a heat exchangers which, in a solar water preheating system is a tank that holds water or other fluids. This solar preheater is feed directly from the solar collector and usually contains a set of copper pipes coiled and submersed into the fluid, these copper coils allow the primary water heater to circulate it's water source through the solar water preheating system without contaminating the household water source. As the water from the primary water heater flows through the coil in the preheater, the water is heated through natural convection and then returned to the primary water heater's storage tank. This cycle takes place 365 days a year and significantly cuts down on the average household electric bill.

There are generally three types of solar collectors used for residential solar water heating applications:

Flat-plate collector
Glazed flat-plate collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors—typically used for solar pool heating—have a dark absorber plate, made of metal or polymer, without a cover or enclosure.

Integral collector-storage systems
Also known as ICS or batch systems, they feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. They should be installed only in mild-freeze climates because the outdoor pipes could freeze in severe, cold weather.

Evacuated-tube solar collectors
They feature parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin's coating absorbs solar energy but inhibits radiative heat loss. These collectors are used more frequently for U.S. commercial applications.

Active solar water heating systems come in two types:

The direct circulation systems pumps water through the collector directly into the home and does not use a preheater tank or a primary electric water heater, this type of system may work well in southern Nevada and other places where it rarely freezes but in colder climates they generally perform poorly.

The indirect solar water heating system pumps water or heat-transfer fluid through the collector into a heat exchanger, much like Trendsetter's Piggy Back™ solar water heating system which can be configured as passive or active. The indirect system of solar heating is preferred in almost all climates from desert heat to climates prone to freezing temperatures.

When cost is an issue, the passive solar water heating system is a good option over an active system. Although a passive solar heater is less efficient than an active solar water heater, the passive system can be more reliable and outlast an active system. The two basic types of passive solar water systems are:

Integral collector-storage passive systems
These work best in areas where temperatures rarely fall below freezing. They also work well in households with significant daytime and evening hot-water needs.

Thermosyphon systems
Water flows through the system when warm water rises as cooler water sinks. The collector must be installed below the storage tank so that warm water will rise into the tank. These systems are reliable, but contractors must pay careful attention to the roof design because of the heavy storage tank. They are usually more expensive than integral collector-storage passive systems.

With almost all solar water heating systems a primary or backup water heater is required when there are periods of low solar activity such as night or long lasting cloudy days or during increased demand. The backup water heater is usually an electric water heater which must run when solar heated water is not available.

One of the better solar water systems systems use a collector which feeds a heat exchanger which preheats the actual electric water heater. What this does is to allow for the backup system to be directly heated by the solar hot water heating system and still be able to take over when the water temperature falls below proper levels.

In this diagram you can see the Trendsetter piggy back system™, This system addresses many issues in solar water heating as it saves space by having the heat exchanger and primary water heater together, it transfers solar heat 24 hours a day by natural convection, it uses no electrical power from the grid, it passively optimizes solar storage capacity and it provides damping of temperature swings much like a surge protector does for your home computer.

The generic description of the system is classified as a drain-back system. The non pressurized solar storage tank (1) contains 40 gallons of tap water initially filled from a garden hose or the fill line. The level gage (12) shows when the tank is filled to the proper level. The tank water does not mix with the potable hot water discharged at the fixtures. When the sun comes up, the PV panel (2) receives sunlight. The controller (3) conditions the voltage and provides power to the dc pump (4) for a soft start and maximizes the power output of the PV panel (2). The water in the tank gradually heats up as water is circulated from the tank through the 5 micron filter (13) to the solar collector (6) and back to the tank. The phase change material (7) will melt as the water in the tank begins to exceed 115oF. This process will significantly improve the thermal performance of the system.

Transfer of Solar Heat to the Water Heater
Cold water normally enters the water heater (8) through the cold inlet at the top of the tank. The ball valve (11) blocks off this path and diverts the cold water through the heat exchanger (9) to “solar heat” the water before it enters the tank at the P&T valve (12) location near the top of the tank. The check valve (5) prevents the cold water from entering the tank at the discharge port at the bottom of the tank.

Natural Thermal Convection
The water heater (8) is also passively thermally coupled through the heat exchanger connections at locations P&T valve (12) and check valve (5). The normal temperature of water heater (8) is 120oF. Normally the temperature at the bottom of a tank type water heater would be approximately 90oF when the top of the tank is 120oF. Hot water wants to rise to the top because it is less dense than cold water. We use this principle to transfer the solar energy from the solar storage tank (1) to the water heater (8) once the temperature in the solar tank exceeds the water temperature at the bottom of the water heater tank

Regular maintenance on simple systems can be as infrequent as every 3 to 5 years, preferably by a solar contractor. Systems with electrical components usually require a replacement part after or two after 10 years.

Benefits of Solar Water Heating
  • Reduce water heating portion of utility bill by as much as 90%
  • No water heater storage tanks to replace ever
  • $2,300 Federal Tax Credit
  • 100% property tax exemption
  • Saves tons of polluting CO2
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