Lesson 2
Solar Hot Water Basics

Overview

In this lesson, you will learn about using the sun to provide heat. For this portion of the course, we will emphasize heating domestic hot water for a building.

In a solar water-heating system, heat collection is the main objective, along with moving the heat from the collecting surface, transferring it to storage, and ultimately using it to heat the domestic hot water.

solar panels
The shallow water of a lake is usually warmer than the deep water. That's because sunlight can heat the lake bottom in the shallow areas which, in turn, heats the water. It's nature's way of solar water heating. The sun can be used in basically the same way to heat water used in buildings and swimming pools.

Most solar water-heating systems for buildings have two main parts: a solar collector and a storage tank. The most common collector is called a flat-plate collector. Mounted on the roof, it consists of a thin, flat, rectangular box with a transparent cover that faces the sun. Small tubes run through the box and carry the fluid — either water or other fluid, such as an antifreeze solution — to be heated. The tubes are attached to an absorber plate, which is painted black to absorb the heat. As heat builds up in the collector, it heats the fluid passing through the tubes. Different types of solar collectors are described below.

The storage tank then holds the hot liquid. It can simply be a modified water heater, but it is usually larger and very well-insulated. Systems that use fluids other than water (usually a propylene-glycol mixture) heat the water by passing it through a heat exchanger, which transfers the heat from the glycol mixture to the water being heated.

Solar water-heating systems can be either active or passive. Most common are active systems, which rely on pumps to move the liquid between the collector and the storage tank. Passive systems, on the other hand, rely on gravity and the tendency for water to naturally circulate as it is heated.

a home in Nevada
This home in Nevada has an integral collector storage (ICS) system to provide hot water.
Solar collectors are the key component of active solar-heating systems. Solar collectors gather the sun's energy, transform its radiation into heat, and then transfer that heat to water, solar fluid, or air. The solar thermal energy can be used in solar water-heating systems, solar pool heaters, and solar space-heating systems. There are several types of solar collectors:

  • Flat-plate collectors
  • Evacuated-tube collectors
  • Integral collector-storage systems

Residential and commercial building applications that require temperatures below 200°F typically use flat-plate collectors, whereas those requiring temperatures higher than 200°F use evacuated-tube collectors.

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Solar Water-heating System Types


Active Solar Water-Heating Systems

Active solar water heaters rely on electric pumps, valves, and controllers to circulate water, or other heat-transfer fluids (usually a propylene-glycol mixture) through the collectors. There are the three types of active solar water-heating systems:

1. Direct-circulation systems (or open systems) use pumps to circulate water through the collectors. These systems are appropriate in areas that do not freeze for long periods and do not have hard or acidic water. These systems are not approved by the Solar Rating & Certification Corporation (SRCC) if they use recirculation freeze protection (circulating warm tank water during freeze conditions) because that requires electrical power for the protection to be effective.

2. Indirect-circulation systems (or closed systems) pump heat-transfer fluids, such as a mixture of glycol and water antifreeze, through collectors. Heat exchangers transfer the heat from the fluid to the potable water stored in the tanks. Some indirect systems have overheat protection, which protects the collector and the glycol fluid from becoming super-heated when the load is low and the intensity of incoming solar radiation is high.

3. Drainback systems, a type of indirect system, use pumps to circulate water through the collectors. The water in the collector loop drains into a reservoir tank when the pumps stop. This makes drainback systems a good choice in colder climates. Drainback systems must be carefully installed to assure that the piping always slopes downward, so that the water will completely drain from the piping. This can be difficult to achieve in some circumstances.

Drainback solar water-heating systems are a good choice for cold climates like Pennsylvania. Illustration: North Carolina Solar Center.

 

Passive Solar Water-Heating Systems

Passive solar water heating systems are typically less expensive than active systems, but they're usually not as efficient. Passive solar water heaters rely on gravity and the tendency for water to naturally circulate as it is heated. Because they contain no electrical components, passive systems are generally more reliable, easier to maintain, and possibly have a longer work life than active systems.

1. Integral-collector storage systems consist of one or more storage tanks placed in an insulated box with a glazed side facing the sun. During the winter, they must be drained or protected from freezing. These solar collectors may be best suited for areas where temperatures rarely go below freezing. They are also good in households with significant daytime and evening hot-water needs; but they do not work well in households with predominantly morning draws because they lose most of the collected energy overnight.

2. Thermosyphon systems are an economical and reliable choice, especially in new homes. These systems rely on the natural convection of warm water rising to circulate water through the collectors and to the tank (located above the collector). As water in the solar collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, the cooler water flows down the pipes to the bottom of the collector, enhancing the circulation. Some manufacturers place the storage tank in the house's attic, concealing it from view. Indirect thermosyphons (that use a glycol fluid in the collector loop) can be installed in freeze-prone climates if the piping in the unconditioned space is adequately protected.

Solar water-heating systems almost always require a backup system for cloudy days and times of increased demand. Conventional storage water heaters usually provide backup and may already be part of the solar system package. A backup system may also be part of the solar collector, such as rooftop tanks with thermosyphon systems. Since an integral-collector storage system already stores hot water in addition to collecting solar heat, it may be packaged with a demand (tankless or instantaneous) water heater for backup

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Solar Water-Heating System Components

Components: Collectors

1. Flat-plate collectors

Flat-plate collectors are the most common solar collector for solar water-heating systems in homes and solar space heating. A typical flat-plate collector is an insulated metal box with a glass or plastic cover (called glazing) and a dark-colored absorber plate. These collectors heat liquid or air at temperatures less than 180°F. (see Figure 1) Liquid flat-plate collectors heat liquid as it flows through tubes in or adjacent to the absorber plate. The simplest liquid systems use potable household water, which is heated as it passes directly through the collector and then flows to the house. Solar pool heating also uses liquid flat-plate collector technology.

a flat plate collector
  Fig 1

unglazed filter
Unglazed solar collectors are typically used for swimming pool heating.

a solar air collector
Air flat-plate collectors are used primarily for solar space heating. The absorber plates in air collectors can be metal sheets, layers of screen, or non-metallic materials. The air flows past the absorber by using natural convection or a fan. Because air conducts heat much less readily than liquid does, less heat is transferred from an air collector's absorber than from a liquid collector's absorber. Air flat-plate collectors are used for space heating.

 

 

 

 

 

 

 


2. Evacuated-tube collectors

an evacuated-tube collector
Fig 2 | Evacuated-tube collectors are efficient at high temperatures.

Evacuated-tube collectors can achieve extremely high temperatures (170°F to 350°F), making them more appropriate for commercial and industrial application. However, evacuated-tube collectors are more expensive than flat-plate collectors, with unit area costs about twice that of flat-plate collectors. (see Figure 2)

The collectors are usually made of parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin is covered with a coating that absorbs solar energy well, but which inhibits radiative heat loss. Air is removed, or evacuated, from the space between the glass tubes and the metal tubes to form a vacuum, which eliminates conductive and convective heat loss.

A new evacuated-tube design is available from the Chinese manufacturers, Beijing Sunda Solar Energy Technology Co. Ltd. The "dewar" design features a vacuum contained between two concentric glass tubes, with the absorber selective coating on the inside tube. Water is typically allowed to thermosyphon down and back out the inner cavity to transfer the heat to the storage tank. There are no glass-to-metal seals. This type of evacuated tube has the potential to become cost-competitive with flat plates.


3. Integral collector-storage systems

Integral collector-storage (ICS) systems, also known as batch systems, are made of one or more blank tanks or tubes in an insulated glazed box. Cold water first passes through the solar collector, which preheats the water, and then continues to the conventional backup water heater.

ICS systems are simple, reliable solar water heaters. However, they should be installed only in climates with mild freezing because the collector itself or the outdoor pipes could freeze in severely cold weather. Some recent work indicates that the problem with freezing pipes can be overcome in some cases by using freeze-tolerant piping in conjunction with a freeze-protection method.


Components: Heat Exchanger
Solar water-heating systems use heat exchangers to transfer solar energy absorbed in solar collectors to the liquid or air used to heat water or a space.

Heat exchangers can be made of steel, copper, bronze, stainless steel, aluminum, or cast iron. Solar heating systems usually use copper, because it is a good thermal conductor and has greater resistance to corrosion.

Solar water-heating systems use two types of heat exchangers:

1. Liquid-to-liquid heat exchangers

Liquid-to-liquid heat exchangers use a heat-transfer fluid that circulates through the solar collector, absorbs heat, and then flows through a heat exchanger to transfer its heat to water in a storage tank. Heat-transfer fluids, such as antifreeze, protect the solar collector from freezing in cold weather. Liquid-to-liquid heat exchangers have either one or two barriers (single wall or double wall) between the heat-transfer fluid and the domestic water supply.

A single-wall heat exchanger is a pipe or tube surrounded by a fluid. Either the fluid passing through the tubing or the fluid surrounding the tubing can be the heat-transfer fluid, while the other fluid is the potable water. Double-wall heat exchangers have two walls between the two fluids. Two walls are often used when the heat-transfer fluid is toxic, such as ethylene glycol. Double walls often are required as a safety measure in case of leaks, helping ensure that the antifreeze does not mix with the potable water supply. An example of a double-wall, liquid-to-liquid heat exchanger is the "wrap-around heat exchanger," in which a tube is wrapped around and bonded to the outside of a hot water tank. The tube must be adequately insulated to reduce heat losses. Some local codes require double-wall heat exchangers on solar water-heating systems.

While double-wall heat exchangers increase safety, they are less efficient because heat must transfer through two surfaces rather than one. To transfer the same amount of heat, a double-wall heat exchanger must be larger than a single-wall exchanger.

2. Air-to-liquid heat exchangers

Solar heating systems with air-heater collectors usually do not need a heat exchanger between the solar collector and the air distribution system. Some solar air-heating systems are designed to heat water if the space heating requirements are satisfied. These systems use air-to-liquid heat exchangers, which are similar to liquid-to-air heat exchangers.


Heat Exchanger Designs
There are many heat exchanger designs. Here are some common ones:

1. Coil-in-tank heat exchanger

The heat exchanger is a coil of tubing in the storage tank. It can be a single tube (single-wall heat exchanger) or the thickness of two tubes (double-wall heat exchanger). A less efficient alternative is to place the coil on the outside of the collector tank with a cover of insulation.

2. Shell-and-tube heat exchanger

The heat exchanger is separate from (external to) the storage tank. It has two separate fluid loops inside a case or shell. The fluids flow in opposite directions to each other through the heat exchanger, maximizing heat transfer. In one loop, the fluid to be heated (such as potable water) circulates through the inner tubes. In the second loop, the heat-transfer fluid flows between the shell and the tubes of water. The tubes and shell should be made of the same material. When the collector or heat-transfer fluid is toxic, double-wall tubes are used, and a non-toxic intermediary transfer fluid is placed between the outer and inner walls of the tubes.

straight tube heat exchanger

3. Tube-in-tube heat exchanger

In this very efficient design, the tubes of water and the heat-transfer fluid are in direct thermal contact with each other. The water and the heat-transfer fluid flow in opposite directions to each other. This type of heat exchanger has two loops similar to those described in the shell-and-tube heat exchanger.

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Sizing

A heat exchanger must be sized correctly to be effective. There are many factors to consider for proper sizing, including the following:

  • Type of heat exchanger
  • Characteristics of the heat-transfer fluid (specific heat, viscosity, and density)
  • Flow rate
  • Inlet and outlet temperatures for each fluid.

Usually, manufacturers will supply heat transfer ratings for their heat exchangers (in Btu/hour) for various fluid temperatures and flow rates. Also, the size of a heat exchanger's surface area affects its speed and efficiency: a large surface area transfers heat faster and more efficiently.


Installation

For the best performance, always follow the manufacturer's installation recommendations for the heat exchanger. Be sure to choose a heat-transfer fluid that is compatible with the type of heat exchanger you will be using. If you want to build your own heat exchanger, be aware that using different metals in heat exchanger construction may cause corrosion. Also, because dissimilar metals have different thermal expansion and contraction characteristics, leaks or cracks may develop. Either of these conditions may reduce the life span of the heat exchanger.


Components: Heat Transfer Fluids

Heat-transfer fluids carry heat through solar collectors and a heat exchanger to the heat storage tanks in solar water heating systems. When selecting a heat-transfer fluid, you should consider the following criteria:

  • Coefficient of expansion – the fractional change in length (or sometimes in volume, when specified) of a material for a unit change in temperature
  • Viscosity – resistance of a liquid to sheer forces (and hence to flow)
  • Thermal capacity – the ability of matter to store heat
  • Freezing point – the temperature below which a liquid turns into a solid
  • Boiling point – the temperature at which a liquid boils
  • Flash point – the lowest temperature at which the vapor above a liquid can be ignited in air.

For example, in a cold climate, solar water heating systems require fluids with low freezing points. Fluids exposed to high temperatures, as in a desert climate, should have a high boiling point. Viscosity and thermal capacity determine the amount of pumping energy required. A fluid with low viscosity and high specific heat is easier to pump, because it is less resistant to flow and transfers more heat. Other properties that help determine the effectiveness of a fluid are its corrosiveness and stability


Types of Heat-Transfer Fluids
The following are some of the most commonly used heat-transfer fluids and their properties:

Air
Air will not freeze or boil, and is non-corrosive. However, it has a very low heat capacity, and tends to leak out of collectors, ducts, and dampers.

Water
Water is nontoxic and inexpensive. With a high specific heat, and a very low viscosity, it's easy to pump. Unfortunately, water has a relatively low boiling point and a high freezing point. It can also be corrosive if the pH (acidity/alkalinity level) is not maintained at a neutral level. Water with a high mineral content (i.e., "hard" water) can cause mineral deposits to form in collector tubing and system plumbing.

Glycol/water mixtures
The most common fluid used in closed solar water heating systems is propylene glycol. Glycol/water mixtures have a 50/50 or 60/40 glycol-to-water ratio. Ethylene and propylene glycol are "antifreezes." Ethylene glycol is extremely toxic and should only be used in a double-walled, closed-loop system. You can use food-grade propylene glycol/water mixtures in a single-walled heat exchanger, as long as the mixture has been certified as nontoxic. Make sure that no toxic dyes or inhibitors have been added to it. Most glycols deteriorate at very high temperatures. The pH value, freezing point, and concentration of inhibitors should be checked annually to determine whether the mixture needs any adjustments or replacements to maintain its stability and effectiveness.

Refrigerants/phase change fluids
These are commonly used as the heat transfer fluid in refrigerators, air conditioners, and heat pumps. They generally have a low boiling point and a high heat capacity. This enables a small amount of the refrigerant to transfer a large amount of heat very efficiently. Refrigerants respond quickly to solar heat, making them more effective on cloudy days than other transfer fluids. Heat absorption occurs when the refrigerant boils (changes phase from liquid to gas) in the solar collector. Release of the collected heat takes place when the now-gaseous refrigerant condenses to a liquid again in a heat exchanger or condenser. Evacuated tube heat pipe solar collectors use this kind of fluid.

For years chlorofluorocarbon (CFC) refrigerants, such as Freon, were the primary fluids used by refrigerator, air-conditioner, and heat pump manufacturers because they are nonflammable, low in toxicity, stable, non-corrosive, and do not freeze. However, due the negative effect that CFCs have on the earth's ozone layer, CFC production is being phased out, as is the production of hydrochlorofluorocarbons (HCFC). The few companies that produced refrigerant-charged solar systems have either stopped manufacturing the systems entirely, or are currently seeking alternative refrigerants. Some companies have investigated methyl alcohol as a replacement for refrigerants.

If a refrigerant-charged solar system and it needs servicing, a local solar or refrigeration service professional should be contacted. Since July 1, 1992, intentional venting of CFCs and HCFCs during service and maintenance or disposal of the equipment containing these compounds is illegal and punishable by stiff fines. Although production of CFCs ceased in the U.S. 1996, a licensed refrigeration technician can still service your system.

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Components: Circulating Pumps

a circulating pump
Centrifugal-type circulating pumps are most commonly used in solar water-heating systems. Centrifugal pumps generally have low power consumption and low maintenance and are highly reliable. The bodies are typically made with cast iron, bronze, or stainless steel. For closed loop systems lower cost, cast iron circulating pumps are adequate. For open-loop systems, circulating a replenishing supply of water, a bronze circulating pump is necessary. Stainless steel pumps are used in swimming pools and other applications where chemicals are present.

Once it is determined that the pump is to operate in a closed loop, open loop, or other particular environment, the solar system head and flow requirements are used to select the appropriate pump. Head is the pressure the pump must develop in order to create desired flow through the system. The overall pressure a pump must create is determined by the height the water must be lifted and the frictional resistance of the pipe.

Static head is pressure resulting from the vertical height and corresponding weight of the column of fluid in a system. The higher a pump must lift the fluid against gravity, the greater the static head it must develop. Dynamic head includes the frictional resistance of the fluid flowing through the pipe and fittings in the system. The pressure a pump must develop to overcome dynamic head varies with the size and length of the pipe, number of fittings and bends, and the flow rate and viscosity of the fluid.

Circulating pumps are typically categorized for low, medium, or high head applications. Low head applications have 3 to 10 feet (0.9-3 m) of head; medium head applications, 10 to 20 feet (3-6 m) of head; and high head applications, over 20 feet of head.


Components: Sensors and Controls

a differential controller The differential controller tells the pump when to turn on and off. The controller, via sensors connected to the collector and the storage tank, determines whether the collector outlet is sufficiently warmer than the bottom of the tank to turn the circulating pump on.
a connector The sensors are located at the collector outlet, and at the bottom of the solar storage tank. These sensors are thermistors that change their resistance with temperature. The differential control compares the resistances of the two sensors. It turns the pump on when the collectors are warmer (usually 20°F) than the bottom of the solar storage tank to collect useful heat. The controller usually shuts the pump down when the temperature difference is 3 to 50F.


Components: Storage Tank

A solar water-heating system is generally installed between the cold water coming into the home and the conventional water heater, and is used to pre-heat the water entering the conventional water heater. A storage tank is necessary to hold water heated by the solar water heating system. Adding another storage tank to hold solar heated water is not only more efficient than have just the conventional water heater, but the solar water storage tank acts as a means to keep the solar panels from over heating. This picture shows the 80-gallon storage tank on the left and the natural-gas fired conventional water heater with the add-on insulation blanket on the right.

a storage tank
For the summer months that can be satisfied with solar hot water alone, you can install a "bypass valve assembly" between the solar storage tank and the backup water heater. The solar bypass consists of three valves (or two 3-way valves), which will allow the house to be supplied with solar heated water directly. A tempering valve should be added when solar heated water is hotter than normally produced by thermostatically controlled conventional tank. The tempering valve is installed on the hot water line feeding the home. The desired maximum temperature of the water delivered to the tap can be adjusted. Hot water enters one side, cold water, if necessary, enters from the bottom and mixed water goes out to the home’s hot water plumbing.






a check valve
Components: Check Valve

A check valve permits fluid to flow in one direction only. It prevents heat loss at night by convective flow from the warm storage tank to the cool collectors. Check valves are either the "swing" type or the "spring" type. Swing-type check valves should be properly installed (i.e. not vertically upside-down which allows them to hang open). A swing-type check valve should be used with pump powered directly from a PV module. Low sun conditions produce lower flow rates, which may not be strong enough to overcome a spring-type check valve. For systems using AC circulating pumps, spring-type check valves should be installed. The spring resists thermosyphon flow in either direction.


a check valve
Components: Expansion Tank

An expansion tank allows the fluid in a closed-loop system to expand and contract depending on the temperature of the fluid. Without the expansion tank, the plumbing would easily burst when the fluid is heated. Diaphragm-type expansion tanks are constructed with an internal bladder and a pressurized air chamber. Heated fluid expands in the closed loop against the bladder and pressurized air chamber. As the fluid contracts while cooling, the air chamber maintains pressure in the closed loop. The size of the expansion tank must be able to handle the expansion based on the volume, coefficient of expansion, and range of temperature fluctuation. The size and number of collectors, and the size and length of piping and fittings determine fluid volume. Diaphragm-type expansion tanks are readily found in most plumbing supply houses.




Components: Pressure Relief Valve

Every hydronic heating system must have protection against high pressures due to high temperatures. A pressure relief valve of 50 psi is usually adequate to protect closed-loop plumbing systems from excessive pressures. Temperature/pressure relief valves are not commonly used in the closed loop because high temperatures are common. Pressure-only relief valves are most commonly used. Pressure relief valves should be have a vent tube that directs escaping fluid to a bucket or floor drain. Once one of these valves opens, it is wise to replace it, since they often do not fully reseat, scale or dirt particles may allow a slow leak.


a pressure gauge
A pressure gauge
Components: Pressure and Temperature Gauges

A pressure gauge shows if the closed loop system is within an acceptable range of pressure. A typical system pressure is on the order of 10 to 15 psi. A pressure gauge is used as a diagnostic tool to monitor the state of the glycol charge. A loss of pressure indicates a leak in the system that needs to be located and repaired.



a temperature gauge
A temperature gauge
Two temperature gauges in the closed loop and one in the water loop are optional, but valuable indicators of the system’s function. One gauge on each side of the heat exchanger in the collector loop shows the temperature rise across the collectors and the temperature change across the heat exchanger. A temperature difference of 15 to 20°F indicates effective system operation. One temperature gauge in the water loop between the exit of the heat exchanger and the entry to the storage tank will display the current temperature of solar heated water entering the storage tank. The temperature gauges should have a range of 0 to 240 or 300°F. A hot summer day may produce water temperatures in the solar loop around 200°F, although normally 180°F is the maximum temperature attained.

Lesson 2 Questions

  1. Briefly describe the two main types of active solar water-heating systems.


  2. In passive solar water heating systems, what drives the fluid’s circulation from the collector(s) to the storage tank?


  3. What is the most common solar collector type?


  4. When is a double-walled heat exchanger required in a solar water-heating system?


  5. Why are refrigerant heat transfer fluids, such as chlorofluorocarbon, being phased out of solar heating systems?


  6. In a typical residential, closed-loop solar water-heating system what type of pump is commonly used?


  7. What is the difference between static head and dynamic head?


  8. Where should the controller’s sensors be placed in a solar water-heating system?


  9. Why should a check valve be installed on a solar water heating system? Where should a spring-type check valve be installed?


  10. What is the main function of the expansion tank in a closed-loop system?


  11. Where should temperature gauges be installed to indicate how the system is functioning in an open-loop and in a closed-loop solar water-heating system?

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Answers