The Main Components of a Solar Hot Water System

Solar Collector Panel Mounted on your roof, the collector captures the heat from the sun and transfers it to the liquid circulating through the panel. Sometimes this liquid is water, but it can also be a special type of fluid that eventually transfers the heat to your home’s storage tank through a heat exchanger.

Storage Tank The heater storage tank is your home’s current boiler/hot water heater. The heated water captured by the solar collector panel is stored in the tank for later use.

Heat Exchanger The heat exchanger transfers the heat energy captured by the solar collector panel to the potable water that is stored in the heater tank. In our system, the heat exchanger is external to the solar storage tank.

Expansion Tank The expansion tank ensures that the system’s pressure does not exceed the pressure limits set by the system designer.

Control System The control system consists of a controller and the circulating pump. The controller compares the temperature difference between the heat exchanger exit point and the solar collector’s exit point. When the collector panel’s temperature is higher than the heat exchanger’s temperature, the controller turns on the pump and circulates the liquid through the system until the temperature is equalized, and then turns the pump off.

Flat Plate Collectors

Solar Collector Panel

The principal component of a flat plat solar collector is the absorber plate, which consists of an assembly of a copper sheet and copper tubing.

The top surface of the absorber plate is coated with either a dark colored material or with a selective absorbent coating that is designed to extract as much as 15% more heat for the same active area. The solar radiation that strikes this surface is converted to thermal energy that’s used to heat the fluid flowing through the tubes.

To ensure extended service life, Free Hot Water collector parts are housed inside an anodized Aluminum case that’s capable of withstanding many years of exposure to the elements. The components inside and the back of this enclosure are insulated in order to minimize potential heat loss.

The upper surface of the collector is covered with tempered glass with low iron oxide content. The glass is designed to withstand high thermal stress (heat from the sun!) as well as impact from hailstones.

Free Hot Water’s Flat Plate Solar Collectors are high performance thermal collectors certified by the Solar Rating and Certification Corporation (SRCC) as OG-100, appropriate for large installations such as apartment buildings, hospitals, restaurants, retirement homes, and other businesses.

Evacuated Tube Collectors

Evacuated Tube Collectors consists of several glass tubes, each of which has concentric inner and outer walls. The inner space is evacuated and the vacuum helps keep the inner tube isolated.

The system works on the same principal as a Thermos® bottle. Just like a thermos, the solar evacuated tube allows most of the heat loss to be eliminated. That makes evacuated tube design a preferred system in cold climate areas. However, you always need to pay attention to the header design. This is the weakest area of the Evacuated Tube collector design.

How it Works

The sun’s energy is trapped by the absorbing strips and transferred to a specialized fluid that is sealed within the internal copper tubing. When heated, this fluid changes from liquid to vapor and rises toward the top of the tube, and into a manifold assembly located at the top of the collector.

Heat conducts though this copper capsule into a fluid circulating along the manifold where it heats the water that flows into the manifold. Once the fluid cooled, it condenses back to a liquid and flows back to the bottom of the tube, ready to repeat the cycle.

Collector Efficiency

Now that we’re more familiar with the two main heat collection methods available, one may ask how we decide when to use one over the other? The answer is collector efficiency.

The first step in designing active solar energy systems is to pick a system that will maximize energy extraction over a wide range of operating conditions.

One method is to compute the thermal efficiency of a collector, which is the ratio of the average heat output from the collector divided by the rate that solar radiation strikes the panel.

The thermal efficiency of a collector is calculated by using the formula bellow:

P= [(Ti – Ta) / I]
P= Inlet Fluid Parameter
Ti= Inlet fluid temperature to the collector (ºF)
Ta = Ambient air temperature surrounding the collector (ºF)
I = solar radiation intensity striking the collector (Btu/hr/ft2)

For the value of I (isolation) factor, look up the isolation table (Nasa Surface meteorology and Solar Energy Data Set) The greater the delta value in fluid temperature of the inlet fluid Vs ambient, the harder the collector needs to “work.”

For example, a flat collector that has the efficiency characteristics of above line that receives water that has temperature of 55° F and the ambient temperature is 75° F with a radiation intensity of 110 Btu/Hr/Sqft (see above NASA link for your respective area) would compute as follow: P=[(55-75)/110] = 0.18

Looking up 0.18 at the above graph would show that a flat collector would work best for that kind of environment.

SRCC Collector Ratings

Any quality collector sold in the United States is examined and approved by what’s commonly known as the SRCC, which stands for the “Solar Rating and Certification Corporation.” States such as California will only issue rebates to collectors and complete systems that have been rated by the SRCC.

And who is the SRCC? The SRCC is an independent agency that administers a certification, rating, and labeling program for solar hot water collectors. They also have a similar program for complete residential solar water heating systems.

Solar thermal collectors have what’s known as an "OG-100" certification. These can be used on any roof with a custom installation. Apartment buildings, hotels, laundry mats, car washes, and hospitals all require custom installations that include OG-100 panels.

Complete solar water heating systems that include OG-100 solar collectors, storage tank, and other components have an "OG-300" certification. Residential systems don’t typically need custom engineering, which is why most residential systems are sold as complete SRCC rated kits.

All equipment that the SRCC certifies and rates is required to bear a certification label, which shows the performance rating for that product. In addition, each certified product is published by SRCC in a directory.

Each product’s directory listing contains information on the product’s material and specifications as well as the certified thermal performance rating. For example, our Free Hot Water 7000 and 5000 and 4000 series are all OG-100 certified.

See the label for our 7000 series specifications and how the collector performed on sunny days, cloudy days, and other specifications. You can then compare Free Hot Water solar collectors to other certificates from other brands and make apples to apples comparison of performance and the materials used to make the panels.

Sizing Your Solar Water Heater Tank

The simple rule of thumb is that on average, you should consider 1.5 gal of hot water storage for every square foot of solar panel installed.

For example every FHW-FC7275 collector has 25.25 sq ft of active area (the “aperture area”). An average system with two FHW-FC7275 should have a 75 gal storage tank capacity.

Please note that this type of calculation is a rule of thumb and is only intended to give you a good “ball park” number. In reality, storage capacity should be calculated based on:

If you’re not sure of the storage capacity you need for solar hot water home or business, please contact us and one of our experts will walk you through the process. Contact Us.