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Green Collar Careers Solar Heat Overview

Component Design Review

• System Flow - Open Loop, Drainback & Closed Loop
• Solar Collectors
• Loop Pump
• Controller
• Storage Tank
• Supplemental Electric Water Heat
• Installer Related Materials
• System Diagram

System Flow - Open Loop, Drainback, Closed Loop

Open Loop feeds the potable water directly to the to the solar collector for heating.  The advantage is its simple and slightly more efficient then a Closed Loop system.  Of course if the ambient temperature drops below about 28° the collector will almost certainly be damaged.  When the sun is not heating the collector the water flow is bypassed from the collector to avoid loosing stored heat. Drainback is an Open Loop system with a series of valves that allow the collectors to be drained rather then just bypassed.  Over 95% of America has some risk of freeze temperatures below 28° at least once every 3 years and if the system poses enough risk in the event of failure that they are not preferred. Closed Loop uses an antifreeze solution to circulate between the collector and storage tank which has to be pumped.  Simple circulation pumps are relatively low cost, use very little power and have a very long life.  For that reason we strongly recommend a Closed Loop system.

Solar Collectors
 

Flat Plate Solar Collectors are a simple low cost design that has good efficiency in moderate climates.  Evacuated Tube Solar Collectors are about 50% higher cost then Flat Plate and in moderate climates are less efficient. Other designs including Parabolic Dish and Trough Solar Collectors are currently only gaining popularity in commercial applications.

Flat Plate Solar Collector

Evacuated Tube Solar Collector

Both types of collectors work well however Flat Solar Panels tend to provide higher efficiency in warm-moderate climates while Evacuated Solar Tube is more efficient in moderate to cold climates. Hail damage is another consideration. Evacuated Solar Tubes are more susceptible to damage from direct hits of hail (usually 1" or larger) if the hail strikes it just about dead center - otherwise the hail is deflected. When they do break, its next to impossible to replace a single element. Flat Panels have more flat area, and are generally resistant up to about 1-1/4" hail.

One advantage the Evacuated Solar Tubes do offer is that they do tend to be more efficient when generating higher temperature loop fluid which can increase a Closed Loop efficiency.  Closed Loop Antifreeze will be heated 30° - 40° higher then the water in the storage tank; if the hot water is set to 135° that means the solar collectors will be heating up to 175°. 

The Collector Efficiency chart below shows the efficiency of both solar collectors based on the difference (or delta) between the air and fluid temperatures.  When the delta is greater then about 107° the efficiency of the Flat Plate drops below the efficiency of the Evacuated Tube.

Delta Temp °F	Flat Plate Efficiency	Evac Tube Efficiency
25°	66%	50%
50°	58%	48%
75°	50%	44%
100°	42%	41%
125°	33%	38%
150°	27%	36%

If the antifreeze has to reach 175°, then in ambient temperatures above (175°-107° =) 68° the Flat Plate is more efficient, and below 68° the Evacuated Tube is more efficient.

Loop Pump
To circulate antifreeze through the solar collectors and back to the storage tank heat exchanger requires a pump.  Pumps are available to run on 12/24 volts powered by a Solar Cell, and there are conventional 120/240 VAC pumps.  The pump doesn't take much power to run, generally about 10-20 watts.

Controller
To switch the pump on/off the Controller monitors temperatures of the high side and low side of the antifreeze in the loop.  Usually there are two thermocouple inputs to the Controller and a relay output that switches the pump circuit.  In some systems (usually an Evacuated Tube in hotter climates) if no hot water is used for a period of time (often referred to as "vacation mode") the temperature in the storage tank can reach close to boiling.  To overcome that risk, some Controllers have a bypass switch that controls added valves to bypass the storage tank.  This mode may not be needed with a Flat Plate as the temperatures of the antifreeze rarely gets close to boiling.  Consult the supplier for your application as they will have specific information on this.

Storage Tank
The general rule of thumb is each member of the household will use 20 gallons of hot water a day.  That may be a bit excessive depending on the family but its a safe number to use.  Storage tanks are similar to a hot water heater except they don't have a heater element and the have an internal heat exchanger the antifreeze passes through.

While a non-toxic antifreeze is used it there were to be an internal leak the possibility of the antifreeze mixing with potable water is eliminated by using a double wall of protection.  Early systems mounted the heat exchanger coils inside the storage tank.  Current storage tanks have the heat exchange tubing coiled around the outside of the tank.

Sizing the system is generally based on a daily 24 hour cycle.  Trying to store some extra hot water for cloudy days isn't going to hurt except that to return to temperature means it will either take extra time or more solar collectors.  Plan on a 24 hour cycle for an every day dwelling, although for weekend retreats etc, it may be more cost effective to plan on a ratio 7 days of heating for every 2 days of storage.

Tanks are generally sized to 80 gallons and 120 gallons.  A family of 4 would need an 80 gallon tank.

Size and location of the storage tank may require squeezing an extra tank in the mechanical room where the water heater was.  It would make sense that someone should make an electric water heater with a heat exchanger right?  Well someone has.  Rheem Solaraide HE.

Supplemental Electric Water Heat
More about the Rheem Solaraide HE - its the first dual-purpose system built to accommodate both a closed loop solar water heater that includes a heating element.  Available in 80 and 120 gallon sizes this system is lower in cost then buying both a storage tank and a good quality water heater.   If you choose the Rheem Solarade HE option we would recommend the 120 gallon system for 4 people as this will keep a more uniform temperature and provide a longer cushion for depleted hot water before the heater switches on.

Unless your customer can go without hot water during extended periods of inclement weather they still will need a conventional hot water heater.

Installer Related Materials (IRM)
Mounting provisions will need to be included no matter where you plan on placing the solar collectors.  Mounting provisions will include stilts that angle the solar collectors for optimal alignment with solar altitude.

Not all solar collectors are roof mounted as not all homes have the right geometry or conditions for that. Some solar collectors are ground mounted which actually has a lot of advantages if the space is available.  Some solar collectors have been built into cleverly designed awnings to provide shade over patios or south facing windows.

Different roof styles take different mounting hardware. Tile roof construction can be compatible with a roof mounted solar collector.

Plumbing will need to be tallied up in lengths and sizes of piping, along with any elbows, couplings, reducers, caps, and hand valves required.  During the site survey you'll get an idea of how long the runs are of what sizes and how may turns it takes.  All of the loop plumbing will need insulation.  Also hanger iron fasteners and more little parts.

We've put together an "IRM Kit" of suggested materials that should have more then enough materials to do the job, and when the job is done, merely replenish the kit.

Section 402 will cover System Configuration.