Water heating is one of the most efficient uses of solar energy and one that has matured over more than 100 years. For home use, a solar hot water heater is a way to reduce energy bills significantly with a proven technology.
In general, the payback period for installing a hot water heating system is less than that of a PV electric system. In this section, collectors for hot water systems and representative systems are discussed.
Solar water heating can be divided into passive and active systems.
Passive systems are simple systems that do not use auxiliary power such as pumps to operate, whereas active systems require electrical power for external pumps or fans.
In some passive systems, the heat collection and storage are separate; others combine the two functions. One of the first solar water heaters used black cans to warm water, an example of a totally passive system but one suited to locations that do not freeze.
Systems are still in place in many parts of the world that are essentially no more than an insulated box with a glass cover and an enclosed black tank.
This type of hot water system can be quite heavy, depending on the size of the tank, so the structure supporting it needs to be strong enough to carry the weight.
Considerations for choosing a system for heating water include climate, hot water requirements (quantity, time of day, temperature), physical size (storage tanks, etc.), available space for collectors, and cost.
The most important factor is climate, which includes the effects of temperature and snow and wind loading. It is important that the system is designed for worst-case conditions, keeping in mind that weather records can be broken!
Solar Thermal Collector Types
Several different types of collectors are used in hot water heating systems. Choosing the best option for a particular application depends on the climate where the collector is installed and the temperature requirement for the water.
In warm climates where there is no chance of freezing, potable water can be circulated through the collector, and a simple flat-plate collector is the most economical solution.
Figure 1 shows two flat-plate collectors ganged together. They are essentially made from a strong aluminum frame with a copper pipe manifold in a tightly enclosed, tempered glass enclosure.
The frame is insulated on the sides and back, and an absorber plate is placed in direct contact with the copper manifold.
The absorber plate is made from a good heat conductor that can transfer heat to the pipes; it is coated with a material that has high absorptance and low emittance.
Absorptance is a dimensionless number that is the ratio of absorbed radiation to incident radiation.
Emittance is the total flux (radiant energy) emitted per unit area from a material; it is related to the ability of the material to give off radiant heat.
In an insulated flat-plate collector with a good absorber plate, the inside temperature can reach 180° F. The water in this system is circulated to an inside storage tank using a small pump.
Figure 1 Two Flat-Plate Collectors Ganged Together
Another type of flat-plate collector is the manifold swimming pool heater constructed from an unglazed ultraviolet-resistant polymer material.
Swimming pool heaters are used in any climate because they are used only when there is no danger of freezing; in the winter, water is diverted from the collectors and the collectors are drained.
The basic pool heater is mounted on a south-facing roof; cool water is run into the bottom and warm water emerges from the top.
Usually, the water to the pool heater is moved through the system using the pool’s pump; an automatic controller and sensors determine when solar heat is available and the pool needs heat. In this case, an automatic diverter valve sends water from the pool to the collectors.
Solar Heat Pipes
The second type of collector consists of an array of evacuated solar heat pipes, which function on the principle of an evaporation and condensation cycle. The cross-section of the heat pipe is shown in Figure 2.
The basic pipe is a coaxial arrangement with a glass outer tube and a closed copper inner tube that holds a nontoxic fluid. The inner tube has low pressure in it so that the small amount of fluid vaporizes at a lower than normal temperature.
Solar insolation striking the assembly causes the fluid to evaporate easily, and the hot gas moves to the top of the inner copper tube, where the heat is transferred to a heat transfer fluid and eventually to the potable water.
The heat pipe is very efficient at moving heat because, when compared to liquid, the gas carries energy called the latent heat of vaporization; this latent (or hidden) heat is released at the top of the pipe, where it makes contact with a transfer material.
The heat of vaporization is the heat absorbed or released during a change of state from a liquid to a gas and is very large compared to the heat absorbed to cause a temperature change in a substance.
As the gas moves to the top of the pipe, it cools and releases the heat of vaporization as it condenses back to a liquid. The liquid runs down the tube, completing the cycle.
The outer tube contains a hard vacuum, so it eliminates conduction or convection loss from the gas.
Figure 2 Solar Heat Pipe Construction
For the evaporation and condensation cycle to function properly, heat pipes normally need to be mounted so that they are raised a minimum of 25° from the horizontal (although there are heat pipes designed to be laid flat). Water temperatures are typically between 120° F and 190° F.
In freezing locations, the transfer fluid is usually food-grade propylene glycol—not to be confused with poisonous ethylene glycol, which is antifreeze used in radiators but is hazardous for solar water heaters, where it should never be used.
Solar hot water heating panels composed of heat tubes are more efficient in cold climates that are flat panels, and they are not affected by outside air temperature or wind because the evacuated glass prevents heat loss by conduction and convection.
Figure 3 shows heat pipe collectors for a hot water system in Switzerland, where freezing is common.
Figure 3 Hot Water Heating System in a Cold Climate (Switzerland) Using Enclosed Heat Tubes
Another type of collector that can be used in any climate is the concentrating collector, which is useful for producing very hot water and process heat.
The concentrating collector can also be used for electricity production by installing PV cells at the focus. Process heat is useful in a number of industries, including food, chemical, and textile.
Concentrating collectors need to have tracking to optimize output, but they can use simple one-axis tracking. For this reason, this type of system is more suited to larger installations.
Figure 4 shows an Absolicon ×10 parabolic trough concentrator with built-in tracking for producing hot water. The trough is covered with glass to help retain heat.
In cold climates, non-toxic antifreeze is used to circulate to the collector, and the heated fluid is carried to a separate heat exchanger.
Figure 4 Solar Concentrators Used for Heating Water in a Cold Climate (Sweden)
In an open-loop or direct system, potable water is circulated through the collectors. The simple black-can water heater that was mentioned previously, in which the collector and water storage are integrated into the same unit, can be considered a passive, open-loop type system.
These systems can be used in warm climates where freezing is not a problem or in seasonal applications (like campgrounds or summer cabins) where they are drained for the winter months.
A more sophisticated open-loop passive system is a thermosiphon, in which hot water is stored in an insulated solar storage tank mounted above the collector.
Figure 5 shows a basic thermosiphon system that uses a heat pipe collector, an arrangement that works in cold climates.
The heat pipes move heat to the solar storage tank, and an internal heat exchanger warms the water in the tank. Cold water is routed directly to the solar storage tank, where it is warmed because of passing through a heat exchanger.
When hot water is drawn from the system, it is taken from the backup tank, and preheated water from the solar tank goes to the inlet of the backup water heater. Exposed water pipes in this system should have minimal exposure to the cold and must be insulated to prevent freezing.
The insulation for outdoor piping is limited to certain types of insulation that can withstand temperature extremes of the hot water in summer to freezing conditions in winter.
In exterior applications, the insulation should have a jacket to protect it from UV radiation, rain, and snow, as well as squirrels, insects, and birds, which find insulation handy to use in nests.
Figure 5 Thermosiphon Hot Water Heater That Uses Heat Pipes for the Collector
In a closed-loop water heating system, potable water is never exposed to the outside environment: A separate loop is used with a fluid that is heated. Generally, this fluid is a propylene-glycol mixture that is heated and sent to a heat exchanger, where the heat is transferred to the potable water.
In addition to protection from freezing, a separate loop has the advantage of protecting the collectors from corrosion and deposits caused by hard water. There are two basic types of closed-loop systems: pressurized systems and drain back systems.
Closed-Loop Pressurized System
A closed-loop pressurized system uses a propylene-glycol-water mixture that is circulated to the collector using a recirculating pump.
Typically, a flat-plate collector is used, but any type of collector will work. At the collector, the propylene-glycol-water mixture is heated and returned to a solar storage tank that contains a heat exchanger. (In some systems, the heat exchanger is an external component.)
A common mixture of propylene glycol and water is a 50-50 mixture, but the particular ratio depends on the climate and the type of system.
A basic closed-loop pressurized system is shown in Figure 6. The system in the figure has a separate backup water heater that provides hot water if the solar system is unable to do so, but some systems are constructed without the separate tank.
Figure 6 Closed-Loop Pressurized Hot Water System
The system is monitored and controlled by a controller, which monitors temperatures to and from the collector and determines when heat is needed and available. In this case, the controller turns on the recirculating pump to move heat from the collector to the solar storage tank.
The backup hot water heater is connected so that pre-warmed water from the solar storage tank is used in place of cold water.
Because temperatures may be hotter than desired, a tempering valve automatically adds cold water as needed to set the final output temperature.
In the propylene-glycol-water loop, an expansion tank is necessary to prevent a system failure that could occur when the fluid expands. As you know, liquids expand when they are heated, and they are not compressible; the expansion tank provides room for the hot fluid.
The expansion tank has an enclosed air chamber that is separated from the circulating fluid with a bladder that expands and contracts as the fluid temperature changes.
Drain Back Systems
A drain back system is one in which the fluid is heated in collectors only during times when there is available heat.
A drain back system is a popular method of heating hot water and avoiding freezing problems. The circulating fluid can be pure water or a propylene-glycol-water mix in extremely cold climates or if extra protection from freezing is desired.
A basic system is illustrated in Figure 7; other configurations are possible, but this system shows the basic components. The system pump is on only when heat is available at the collectors and needed at the storage tank; otherwise, it is off.
When the pump is off, the collectors drain by gravity back to the drain back tank. While this action should prevent freezing problems, it is possible to see some damage if complete drainage of the system does not occur, so it is important that the system is designed to drain completely and rapidly.
The collectors must be installed at an angle and the drain must be set up at the lowest point to ensure drainage.
Pipe runs cannot be allowed to sag or collect water when they drain (avoid horizontal pipe runs), and the pump needs to be adequate to ensure reasonable head pressure at the top of the collector.
Some system designers prefer to use a propylene-glycol-water mixture no matter what the conditions, but particularly where conditions can be severe.
Figure 7 Drain back Hot Water System
Instead of the expansion tank used in a pressurized system, a drain back system requires a reservoir or drain back tank, which is an unvented tank designed to hold all of the recirculating fluid when it is not in the collectors.
The tank is located in a protected environment, where it has no chance of freezing and includes a means of checking the level within the tank. It must also be located below the level of the collectors and within an area safe from freezing so that gravity allows the fluid to drain completely from the collectors and outside plumbing when the collectors are not in use.
The drain back tank should also be located as high as possible within the warmer space to reduce the load on the pump.
As in the case of the pressurized closed-loop system, a controller is used. The controller monitors the temperature of the water in the tank and the collector temperature and turns on the pump if there is enough temperature difference to make it worthwhile to turn it on. One option is to use a DC pump powered by a dedicated PV module.
Hybrid PV Systems
A new trend in solar systems is to combine the generation of electricity with hot water heating. Hybrid PV/thermal systems can represent a cost-effective system for some locations.
Flat-plate PV collectors naturally get hot in the sun, so they are a ready source of heat for a thermal water system.
One hybrid PV system called the Echo system, draws outside air under the PV modules, thus warming the air. The hot air is drawn into the attic of a structure through a roof vent and then passed into a heat exchanger.
During the heating season, it can help warm the house (or any structure) as needed; otherwise, it can serve as a preheater for the hot water system.
Failure can occur in any system. Solar hot water systems can be designed to be very reliable, but a leak can occur, or the pump can even become stuck on, in which case, water may be released or water could be exposed to freezing conditions.
It is important that the system is checked regularly and maintained as needed. For example, the recirculating fluid can evaporate over time, requiring new fluid to be added.
In cases where propylene-glycol is used, the propylene-glycol can become acidic over time and need replacement.
A visual check of the system for problems such as rodent damage, leaks, or corrosion is a useful exercise in keeping a system active.
- What is the difference between an open-loop and a closed-loop solar hot water system?
- What are the characteristics that make a certain material a good absorber plate in a flat-plate conductor?
- How does the latent heat of vaporization make a heat pipe more effective at moving heat to the top of the pipe?
- Why is it necessary to include an expansion tank in a pressurized hot water system?
- What steps are important to prevent freezing in a drain back hot water system?
- In an open-loop system, potable water is circulated through the collectors; in a closed-loop system, a separate fluid, usually a propylene-glycol-water mixture, is sent to the collectors and heat is exchanged with potable water.
- The plate is constructed from a good heat conductor and it is coated with a material that has high absorptance and low emittance.
- The heat of vaporization is very large. Because of the change of state, more heat is transferred to the liquid vaporizes.
- The expansion tank prevents a system failure that could occur when the fluid expands as it is heated in the collectors.
- The circulating fluid must drain completely when heat is not available at the collector, so the collectors and all plumbing should be checked for proper slope and any sags, and the pump needs to be adequately sized to bring water to the top of the collector with reasonable pressure.