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Training Video for Inspection of Solar Water Heating Systems
Overview
This segment will give you a background knowledge of the types of solar water heating systems you will find in the field.
Solar domestic hot water systems use the sun’s energy to heat water and store it for later use. Therefore, solar systems need solar collectors located outside and facing the sun to collect sunlight. They need a tank to store the energy, and they need piping to connect the two together. Piping is also needed to connect the solar system to the home’s hot water distribution system. Just like conventional water heating systems, various other components are also needed for safe and proper operation such as valves, pumps, sensors and controllers.
System designs usually depend on climate or water quality, with more complex designs being required where freezing conditions are more common, or where poor water quality requires protection of the components by physical separation of the collector fluid loop.
There are many ways to describe solar domestic hot water systems. Here are some common basic designs.
The thermosyphon design is perhaps one of the most common found in the world, but is not often seen in the US.
As the name implies, solar energy heats the fluid in the collector. Buoyancy forces, or natural convection, make the heated fluid rise to the tank located above the collector.
The Integral Collector Storage, or ICS uses solar collectors that have the tank also functioning as the solar collector.
Many early designs had a single large tank/collector in an enclosure.
In recent years, ICS collectors using multiple tube tanks have dominated the US market.
However, single tank plastic units are currently under certification test and may soon be seen on the US market.
PV Pumped systems use a DC circulator to pump water around the collector loop. The pump is powered by a photovoltaic, or PV, panel.
In a certified system, the PV panel and pump have been sized such that when there is sufficient sunlight to power the pump, there is sufficient sunlight to heat the water in the collector.
Two common designs are seen in freeze-prone regions of the country. The exposed components in an antifreeze system are protected from freezing by a glycol or other freeze-tolerant fluid in the collector loop.
All antifreeze systems are indirect. That is, the collector loop fluid is separated from the potable water by a heat exchanger. Antifreeze systems always have fluid in the collector.
The other common system design seen in freeze-prone regions is the drain-back design.
As its name implies, the drain back design drains all of the liquid out of the collector when it is not collecting solar heat. The fluid drains into a drainback tank located in conditioned space. Thus, when freezing conditions occur, there is no liquid in the exposed components that can freeze. Water can be used in the collector loop. A circulator is usually insufficient to raise the column of water up to the collector, so a larger pump usually must be specified. Properly installed, this design provides positive freeze protection. However, care must be taken during installation that all exposed plumbing is sloped to allow drainage.
In fact, there are many different configurations seen in solar water heating. Several descriptors are commonly used in the industry to categorize basic SDHW system configurations.
An active system requires a pump to move the heat transfer fluid between the collector and the storage tank.
Pumps may be DC and powered by a photovoltaic panel located on the roof next to the collector, as seen in the PV-Pumped design.
Pumps may require AC line power and be controlled by a differential controller. The AC pump in this configuration is a circulator sized to move water around the collector loop. A circulator could be found in a direct system or an indirect antifreeze design. AC pumps in other systems might be sized larger if they have to lift a column of water up to the collector, such as in a drain-back design.
A passive system has no mechanical pumps. The thermosyphon and ICS examples we saw earlier are passive systems.
Solar generated heat is transferred to the potable water by means such as thermosyphon effect, or heat pipe, or by mains pressure moving the water through the collector and tank.
In a direct system, potable water moves through the collector loop. The collector is exposed to mains water pressure and the collector waterways are always wetted.
A direct system could be active or it could be passive. The direct configuration is simple and efficient, hence a good choice in tropical climates or even very mildly cold climates. Collector wetted materials must be approved for contact with potable water. Direct systems are not recommended in areas with aggressive or hard water because the small collector passage ways have a high tendency for scale or corrosion.
Also, it is not a good choice for heavy freeze areas because when freeze protection is active the heat lost may exceed the heat collected from the sun. This would mean a net energy loss for the consumer.
Here we see an active direct system. Note the location of the freeze valve near the outlet of the collector, and the pump. Note also that this system is a one- tank system.
Most ICS units would be passive direct systems since mains water passes through the solar system before heading to the conventional tank.
In an Indirect system, the collector loop is isolated from the potable water supply and mains pressure by a heat exchanger. Isolation of the collector loop may be necessary in some areas that have corrosive or hard water, or otherwise poor water quality. Isolation protects the small passages in the collector from corrosion or scale. It also may be necessary to protect the collector from mains pressure, or if a freeze protection mechanism such as anti-freeze fluid or drain back is used.
As mentioned before, thermosyphon units are passive, using no pumps, and may be direct or indirect.
It is difficult to visually tell the difference between direct and indirect thermosyphon from the outside,
but note in this cutaway view of an indirect thermosyphon unit that the collector fluid does not come in contact with the potable water.
This solar system is also a passive indirect.
It uses a heat pipe to transfer the solar energy from the evacuated tube collector to the potable water passing through the insulated header at the top.
Here we see an active indirect system with one tank. Note the location of the pumps. Note also that there is a heat exchanger isolating the collector loop from the potable water.
Now a word about describing the tanks in a solar water heating system.
Many solar systems are designed as pre-heat to electric or gas tanks. That is, mains water is first heated by solar energy then is passed to the ‘conventional’ water heater. If necessary, water temperature is boosted to the set point using gas, electricity or heating oil. The conventional heater is considered a ‘backup’ to the solar system, assuring hot water on very cloudy days. While a solar system that relies exclusively on solar is acceptable in the Code, it is not commonly found in the US and is not accepted for SRCC-certified systems. Domestic solar systems are typically sized to provide 50 to 70 per cent of a household annual hot water load.
You may find a one- tank system where storage of solar heat is in the bottom two-thirds, and electricity heats the upper third.
Any lower electric element must be disconnected to allow the solar system to work.
Or, you may find a two-tank system, where solar heat has its own tank that feeds the conventional backup water heater. Multiple tank systems should be connected in series, with the solar storage pre-heating the feed line for the conventional fossil fuel backup storage. Two-tank systems are necessary when gas is used as a back-up, but might also be used with electric back-up. In addition to providing an extra tank of hot water storage, a two-tank system may be more efficient if the second tank is properly insulated and properly sized. However, a single tank unit takes up less floor space and requires less equipment… about one tank’s worth.
Further, in some two-tank systems, losses from the second tank may actually hurt thermal performance if it is not properly sized or insulated.
With the improvement in instantaneous water heaters, you will probably soon see more solar water heaters using instantaneous heaters as backup units.
Solar storage tanks may be found indoors in the utility room or outside the house.
Outdoor tanks may be found either on the roof, or on the ground. In the US, a very common outdoor tank design is the Integral Collector Storage system, or ICS. The ICS uses solar collectors that have the tank functioning also as the solar collector.
Unless the system has no back up and is solar-only, ICS systems are exclusively two-tank systems, where the ICS storage tank feeds solar heated water to a conventional tank in the utility room.
Another outdoor tank design is the thermosyphon collector.
The thermosyphon configuration is quite possibly the most common outside the US. It is less common in the US, possibly due to aesthetics concerns.
At least two manufacturers have taken the aesthetics issue into consideration in their designs. One hides the tank behind the collector, and in another the collector is flush-mounted to the roof and the tank is hidden inside the attic. Both designs use buoyancy flow to transfer solar heat from the collector to the storage tank.
Thermosyphon systems can be single-tank with an electric element to be used as backup on cloudy days, or they might be two-tank pre-heat configuration with the conventional tank located in the utility room.
System controls are another way to differentiate between solar heating systems. Various levels of control could be used to assure that solar heat is collected when it is available. In a thermosyphon system, natural buoyancy controls how much heat is collected in the tank above the collector. In a PV-Pump system, the pump will only turn on where there is sufficient sunlight available to the PV panel to energize the pump. If the system is properly designed, this will coincide with sufficient sunlight to heat the collector. When the sun goes down, the pump loses power at about the same time that the collector cools down.
Another means of control is to place a timer on the solar collector pump. This has been seen in the simpler direct systems. The timer turns on and off at specified times, presumably in the morning and evening when the sun is expected to shine.
A timer control is simple and inexpensive. However, it may cause the pump to run during cloudy days when there is no solar heat to collect. This would cool the tank, rather than heat it. Also, power outages will require the timer to be readjusted.
Even timers with battery backup have been found neglected after a couple years of use. Timer controls are not recommended unless the homeowner is very diligent about home maintenance.
Most solar controllers are of the differential temperature type.
That is, the controller monitors the temperature difference between the coldest part of the system at the bottom of the storage tank … and the hottest part of the system at the outlet, or top, of the collector. When the temperature difference exceeds a predetermined setting, usually between 8 and 20 degrees F, the controller turns the pump on. When the temperature difference drops to a lower predetermined setting, usually 2-4 degrees F, the controller turns the pump off. There may also be other control settings, such as a maximum temperature shut-off, a freeze protection recirculation setting, and a manual operation override.
It is important that the temperature sensors be properly placed and the correct extension wire be used. Otherwise, the system will not operate correctly.
In almost all parts of the US, some form of freeze protection is necessary to protect the collector and external piping from damage when the weather gets cold. Freeze protection mechanisms might include freeze valves, recirculation, or even thermal mass. The thermal mass of the water and metals in this collector allow it to resist freezing for short periods in very mildly freezing climates. Care must be taken that the external pipes are also sufficiently insulated or they will freeze and split.
Recirculation used to be a common freeze protection means in areas with light freeze, but it is rarely seen anymore. The controller senses freeze danger conditions using a snap switch.
When the switch opens, the controller turns the pump on to circulate heated water from the tank through the collector. Some heat is lost from the tank but the collector is protected from freezing. Obviously, this would not be a good choice in climates where it freezes more than a few hours per year. Also, the controller and pump must have power to function, making this freeze protection option prone to failure during power outages.
Freeze valves operate automatically and do not depend on the controller. Because they require mains pressure to operate, they should only be found on direct systems.
When ambient temperature approaches freezing, the valve opens, allowing relatively warm water from the tank or mains to slowly flow through the collector and out onto the roof, to keep the collector temperature above freezing.
To be sure that the warm water will follow a path through the collector, the valve must be located on an uninsulated tee at the collector outlet and a check valve should be located on the hot return leg from the collector, downstream of the valve.
The valve must be located on an uninsulated tee partly because the valve must not be unduly influenced by the thermal mass of the collector at night, and partly because the control element in the valve is susceptible to high temperatures over time during normal daytime operation.”
This valve has had the tip cut off in hopes of discouraging nest building by wasps.
This valve has failed prematurely. When the valve functions normally, it should offer only a fast drip at full open.
Freeze valves are sized to protect a collector of a certain size defined in the manufacturer’s literature accompanying the valve. If the collectors are oversized or if there are multiple collectors, then multiple freeze valves may be required.
Thermal mass, Recirculation, and freeze valves are common freeze protection where ambient temperatures rarely approach freezing. As one moves north, of course cold nights and freezing become more common. If too many cold nights are experienced and the air temperature is low for too many hours at night, then the systems will tend to lose overnight most or all of the heat they collected during the day. More detailed freeze protection mechanisms are necessary in these colder climates.
In climates with hard freeze potential, two protection mechanisms are commonly used. As their name suggests, antifreeze systems use a fluid in the collector loop that will not freeze such as a glycol solution.
The glycol should be food grade propylene glycol or listed in CFR Title 21, Part 182 or Part 184. If a single walled heat exchanger is used in the system, the system should operate at a pressure less than the normal operating pressure of the potable water system.
Systems should use a double walled heat exchanger with positive leak detection to avoid contamination of the domestic water supply. This wrap-around heat exchanger wrapped around the outside of the water tank constitutes a double-wall design. SRCC-certified systems will have been reviewed for compliance to this issue.
The drain-back configuration is another good design in climates with hard freeze potential. A drainback system will have a small tank to store the collector fluid when not in use.
The collector pump must be sufficiently sized to lift the collector fluid to the highest point in the collector loop. When the pump is turned on, water is pumped through the heat exchanger and up to the collector where it is heated and it falls back down to the tank.
When the pump is turned off, all of the fluid in the collector loop will drain back into the tank. The drain back tank must be in conditioned or heated space, so that the collector fluid does not freeze.
Piping must be properly sized and sloped so that all fluid drains out of unconditioned pipe and into the drain back tank in conditioned space.
Here the plumber chose to make a single piping penetration. Although the piping isn’t pretty, it should be able to drain adequately.
Here the pipe rises to enter the penetration. Water will puddle in the pipe on the roof and freeze causing a rupture. This is a poor installation.
A heat exchanger separates the collector loop from the potable water. Sometimes the heat exchanger might not be visually apparent, and may be located in the small drain-back tank or even in the potable water tank, or outside both tanks, either as a wrap-around heat exchanger inside the back up tank’s insulation, or completely external to any tank. Sidearms often use natural convection on one side to transfer heat.
This side arm heat exchanger is poorly insulated.
This U-shaped heat exchanger uses a pump on either side of the heat exchanger to transfer the heat from the collector loop to the potable hot water tank.
One final comment on collector arrays and proper collector plumbing. Regardless of the design, multiple collectors should be plumbed in parallel for greatest solar collection efficiency.
With both single collector and multi-collector systems, the collectors should be plumbed in reverse-return configuration to balance the flow through the collectors. ‘Reverse-return’ means that the inlet and outlets are on opposite far corners. Otherwise, expensive balancing valves may be necessary to balance the flow through the collectors.
We hope this segment has been informative and has given you a background knowledge of the types of solar water heating systems you will find in the field. Please view the other segments to see how the ICC Codes address proper design and installation.