Solar Hot Water Panels

Solar hot water panels, or collectors, are essential components of any solar water heater. These panels function as the engines that harness the sun's radiant heat.

Panels are responsible for absorbing solar heat and for insulating as well. Any reputable industry professional will confirm the incredible importance of selecting the right collector for a solar hot water heater.

Doing your homework in advance will ensure that your solar hot water panels will meet the needs of the particular heating design. There are three types of solar hot water panels; integrated collector storage (or ICS), flat plate collectors and evacuated tube collectors.

Integrated collector storage

ICS collectors have been used worldwide for hundreds of years. ICS collectors are classified as such simply because the collector and the storage component of the solar hot water heater are integrated into one unit. These collectors are used in passive solar water heating designs.

It's important to note that there are different design methods that integrate the system's storage volume and collector.

The batch, or collector box, is the oldest and most time tested integrated collector storage design. The batch or box design is simply an unpressurized storage tank with painted absorber coatings. The tank is enclosed in a box that's covered by glass, or glazed, and in the box is a layer of foam insulation.

Pre manufactured batch collectors, more times than not, use an aluminum or steel skin box. The collector box is also weatherproof. Collector box ICS units are solely used in batch solar water heating systems.

Progressive tube batch collectors are more modern versions of box ICS units. They use an enclosed looped tube to circulate fluid. The looped tube is enclosed in a panel or flat plate, instead of an enclosed storage tank. Domestic water enters from the bottom of the tube and exits from the top of the tube when heated.

With progressive tube batch collectors, there's less mixing of cold and hot water. This design allows for a more efficient circulation of domestic water than a conventional batch or box ICS. This design also provides more thermal surface area, or net aperture area, because generally there's more glazing surface area.

Other ICS Solar Hot Water Panels

Grid style flat plate collectors, because of the solar water panels' design, are sometimes used as integrated storage collector units in some heating systems.

Thermosyphon evacuated tube collectors are joined with unpressurized solar storage tanks, making this design an ICS unit as well.

These types of solar hot water panels are applied, as described, in most passive thermosyphon solar water heater designs.

Flat plate collectors

Flat plate collectors are time tested also. These solar hot water panels have been used for several decades throughout the U.S and the world. There primarily used in active solar water heating systems and pool solar water heaters as well as space heating systems.

These solar hot water panels have earned they're name from they're appearance. There essentially made up of a flat absorber plate that's enclosed in a very light weight metallic frame that's usually made up of aluminum.

Absorber coatings are placed on the absorber plate, to assist with the collection of solar radiation. The coatings used will vary from model to model. It's important to note that some coatings perform better than others.

Bonded to the metallic absorber plate are pipes, which are commonly referred to as risers. These risers are designed to contain the circulated fluid throughout the solar hot water heater. Depending on the systems' design, the fluid can be either domestic water or a heat transfer fluid.

Inside the solar hot water panels' enclosed frame is a layer of panel insulation. The panels' frame is covered, or glazed, with a single layer of glass or special plastic. The panel glazing is quite important because it must allow as much solar radiation in, while insulating as much of the heat gathered as possible.

Flat plate riser configuration

The configuration of the riser, or the pipes, determines how the fluid is circulated throughout the panel. With flat plate solar hot water panels, the risers bonded to the absorber plate can be designed generally in two different ways.

Flat plate solar hot water panels with grid style risers have several separate risers that are evenly spaced out and are aligned parallel. The risers are each joined to the panel or collector header separately.

This configuration allows for an effortless and even fluid distribution. Making these solar hot water panels ideal for thermosyphon water heating designs as well as drainback solar water heater plans.

Flat plate solar water panels with serpentine style risers have a single looped riser. The single riser is joined to the panel header at two separate points. This configuration allows for less absorber plate bonding but there are note worthy downsides.

The looped or snaked pipe can leave small amounts of fluid in the solar hot water panel when the water heater is idle. Making this riser configuration less than ideal for drainback and thermosyphon solar powered systems. Serpentine style flat plate collectors are widely used in antifreeze solar domestic water heaters.

Evacuated tube collectors

Evacuated tube collectors are overall more modern, but they have been widely used in certain parts of the world since the 1970's. Evacuated tube collectors are primarily used in active solar water heating systems and space heating systems.

These solar hot water panels have also earned they're name because of they're appearance. There made up of several absorber plates, each one is coupled with absorber coatings, and each plate and coating is covered by a glass tube.

This design provides each heat gathering tube in the panel with an insulating vacuum. The vacuums between the absorber plates and the glass tubes reduce heat loss significantly more than the internal insulating that flat plate collectors have to offer.

With the assistance of each of the vacuums, evacuated tube collectors generally produce higher fluid temperatures than they're flat plate counterparts as well.

Single Glass Tubes and Double Glass Tubes

Evacuated tube solar hot water panels come with both single and double glass tubes. The first models that were introduced decades ago were primarily single glass, and these tubes would eventually crack.

The tubes today use a more durable glass, usually borosilicate or soda lime glass. So with that said, the only real difference between double glass and single glass evacuated tube collectors today is the location of the vacuum.

Evacuated tube configuration

With these solar hot water panels, the configuration of the tube is what's really important. There are a few different tube configurations, and these differences can determine how the fluid is distributed throughout the solar water panel. The configuration is also likely to determine which application would be more ideal.

In heat pipe evacuated tube collectors a heat pipe (usually made of copper) is attached to each absorber plate within each tube. The pipe serves as a heat exchanger and it contains a small quantity of fluid, usually alcohol or purified water.

When solar heat is introduced to the tubes, the liquid in the heat pipes rises and vaporizes into the top portion of solar hot water panel. This top portion is often referred to as the manifold. When the vapors enter the manifold, heat is transfer to the fluid circulating through the manifold.

This tube configuration is gaining popularity world wide. The one advantage heat pipe evacuated tube collectors have is flexibility. In the event a tube cracks or breaks, the the fluid in the loop won't be pumped out and the entire function of the solar hot water heater won't be compromised.

This flexibility makes heat pipe evacuated tube solar hot water panels ideal for closed loop solar designs.

With direct flow evacuated tube collectors the heated fluid is circulated in the outer tube, and the cold inlet fluid is circulated through the center of the inner tube. This particular tube configuration in many ways is similar to flat plate panels, with the exception of the vacuum provided by the outer tube.

Many solar industry professionals believe direct flow designs are more energy efficient than heat pipe designs, because with direct flow, there isn't a heat exchange between fluids.

The one setback with direct flow evacuated tubes is the lack of flexibility. If a tube breaks, the fluid will likely require replacing. And if the fluid leak were significant enough, the entire function of the hot water heater would be interrupted. This tube configuration is less than ideal for antifreeze solar designs.

Evacuated tube solar hot water panels with this type configuration are combined with unpressurized storage tanks. These panels use the principle that heated water will naturally rise.

The heated domestic water rises in the tubes and into the storage tank installed above the solar water panel. Solar hot water panels with this tube configuration are technically integrated collector storage, or ICS, panels.

Flat Plate Collectors vs. Evacuated Tube Collectors

Solar Hot Water Panel Characteristics	Flat Plates	Evacuated Tubes
More Readily Available Technology
More Durable Surface Glazing Area
Less Expensive Per Square Foot
Compatible With Most Systems
Better For Cold and Windy Climates
Sites With Minimal Solar Resource
Panel Mount and Tilt Is Less Important
Easier To Install and Repair

ICS, flat plate collectors and evacuated tube collectors

Integrated collector storage

Flat plate collectors

Flat plate riser configuration

Evacuated tube collectors

Evacuated tube configuration

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