“Solar water heating is only practical in southern climates…”

We often hear that solar water heating doesn’t make sense in northern states, like Minnesota and Wisconsin. In fact, Holladay makes the case that with plummeting PV prices, it may actually be cheaper to heat water with PV now than it is to us a solar thermal system. With PV panel prices dropping below $1/watt, this may be even more true than when Holladay’s article was initially published. Still, thanks to incentive programs, affordable solar domestic hot water (sdhw) systems are still going up in northern states. According to the Daily Northwestern, The city of Evanston Illinois has had 85 new DSHW systems installed this year. In addition, niche markets for solar water heating are popping up, in the hotel industry, greenhouses and residential and public pool heating.

The solar heated greenhouse at Dickinson college photo: dickinson.edu

Meanwhile, SDHW continues to see modest growth and continued popularity in southern states like Arizona and Florida. Can solar water heating make a comeback in the US? As with PV, the key will be seeing the installed price come down, and sadly, right now, that isn’t happening.

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The water tank is installed above the collector, directly on the roof. As hot water rises in the tank, cold water flows to the bottom and into the collector. The cold water directs the heated water through the collector outlet and into the top of the tank. This heated water flows from the rooftop tank to the ground-level storage tank.

Photo Credit: ClimateChange.gov.au

Thermosiphon systems are very efficient as they are simple and rely only on the laws of thermodynamics, allowing them to function even when mains power is down. However, these systems’ efficiency declines in colder climates.  Plus, other factors make such systems unsuitable for some households: the roof may not be able to support the weight of the storage tank, and the appearance of a fully roof-mounted system does not appeal to all property owners.

Nowadays, with the development of new technologies, the use of thermosiphon systems has decreased. Contact your installer to find out whether such a system is appropriate for your home.

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• solar resource: the amount of sunlight reaching your solar collectors impacts the efficiency of your solar thermal system. Learn about solar radiation in your region.
• climate: a cooler climate may require an active heating system, whereas a passive system could suffice in warmer climates.
• local building codes and regulations

What direction should the solar heating system face?

Ideally, solar thermal collectors should face south to maximize the amount of solar radiation reaching the collectors. If your roof faces another direction, an extra frame can be installed to angle the collector toward the south.

Photo Credit: Arizona Solar Hot Water

Can water heaters be placed in the shade?

Avoid mounting solar collectors in areas that are shaded by trees, electrical cables, buildings, chimneys, vents or any other source. Shading in an area can change with the sun’s path, from summer to winter. An installation professional can estimate the shading your system will receive.

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A rooftop solar thermal collector.

The cost of solar thermal energy depends on:

• price of fuel from local utility, used to heat water
• federal, state and local government incentives
• the amount of hot water you use
• your system’s performance
• the type of back-up heating you use
• the solar radiation resource in your area

Reinforcing your roof to hold the weight of a solar collector can add to the solar water heating cost. Another factor to keep in mind is that some systems require more frequent and costly maintenance than others.

You can estimate the annual operating cost of a solar water heating system once you know your system’s solar energy factor (the measurement of efficiency for solar thermal systems) and the cost of your backup heating. Check out this great U.S. Department of Energy guide to calculating the cost of the system you have in mind.

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Solar Collector Size

The conventional estimates for surface area of solar collectors are as follows:

• 20 square feet of collector area for two family members
• 8 square feet for each additional household member in the U.S. Sun Belt region or
• 12-14 square feet for each additional household member in the northern U.S.

Storage Tank Size

In general, an active water heating system requires 1.5 gallons of storage per square foot of collector. Some specialists suggest installing an even larger storage tank – about 2 gallons per 1 square foot of collector – in very warm, sunny climates. The more tank space, the less likely the system is to overheat when demand for hot water is low.

This water heater size chart is an approximate guide to choosing the right collector area and tank capacity. Households using a lot of hot water – for example, for spas or dishwashers – should consider the next largest system size.

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Photo Credit: U.S. Department of Energy

• Solar collectors convert sunlight to heat energy. Learn more about the different types of solar collectors.
• Heat transfer fluids carry the heat from solar collectors to water storage tanks. In warm climates, the heat transfer fluid may be potable water; in cold climates, a non-toxic anti-freeze. Learn about the importance of heat transfer fluids.
• Heat exchangers transfer solar heat from the transfer fluid to the home water supply. Learn more about the types of heat exchangers.
• Storage tanks store hot water when it is not in use. Find out what type of storage tank is best for your home.
• Pumps control the flow of the transfer fluid through the collector and storage tank.
• Pump station/system controller. These are optional.

In addition to the various parts needed for a solar hot water heating system, you will need to select the correct system size for your household.

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Photo Credit: Tallahassee Energy & Construction

• single-wall: fluid in the pipe and outside of the pipe are different.
• double-wall: used as a safety measure when the transfer fluid is toxic. These are less efficient because the heat must go through two surfaces instead of one.

The most common designs of heat exchangers are:

• coil-in-tank where the exchanger is a tube in the storage tank
• shell-and-tube, where two tubes with water and heat-transfer fluid circulating in opposite directions are encased in a metal shell outside the storage tank.
• tube-in-tube, with the two tubes of water and heat-transfer fluid  in direct thermal contact with each other, is the most efficient type.

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There are many potential heat transfer fluids. The right fluid depends on the type of solar hot water heating system and the climate. Factors to consider are: freezing point, boiling point, and viscosity (which determines how much energy is needed to pump the fluid). at transfer fluids used in solar hot water systems are non-toxic. The most common transfer fluids are:

Different heat transfer fluids are suitable for different climates.

Water

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Performance

The vacuum prevents heat loss, making these the most efficient collectors. An evacuated tube collector can work well in cloudy conditions and in temperatures as low as -40°F. This characteristic is most advantageous in colder climates, or during the winter, where other solar collectors may not produce as much energy.

Cost and Durability

Photo Credit: Solar Colorado

Depending on the brand, evacuated tube collectors can cost more than flat plate collectors – though the additional up-front cost may be made up for in utility bill savings. Flat plate collectors are usually more cost-effective for residential solar water heating systems. Thus evacuated tube collectors are more commonly used for commercial applications. The added winter and cold climate efficiency makes them a good choice for solar space heating.

Evacuated tube technology has been around since the 1980’s and most evacuated tube collectors have a similar warranty to flat plate collectors. A major benefit is that individual tubes can be replaced as needed, rather than replacing the whole collector.

If you’re considering a solar water heating system, we highly recommend you purchase this comprehensive guide to solar water heating systems by Bob Ramlow.

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Photo Credit: Physics World

Performance

Flat plate solar collectors suffer more heat loss than other types of solar collectors, and are thus not the most efficient type. However, flat plate collector are suited to a wider range of climates. Because these collectors can use a heat transfer fluid such as antifreeze, rather than directly heating the domestic water supply, this type of solar thermal panel can be used in colder climates.

Cost and Durability

While they are not the most efficient solar collectors, flat plates last for over 25 years and are one of the most cost-effective options for a residential solar water heating system. Typically, one flat plate collector can supply adequate energy to heat about 40 gallons of water for domestic hot water use. According to EnergyStar.gov, a family of four can obtain about half its hot water from two collectors.

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Performance

Photo Credit: Florida Solar Energy Center

Batch systems are only suitable for warmer climates. The collector itself and any outdoor pipes are susceptible to freezing in cold weather, and the batch collector can become very hot if the water is not drawn during the day. Batch systems lose heat during the night time. Nevertheless, batch systems are very good for heating water during the day. They are best for households with most hot water demand during the day and evening.

Cost and Durability

ICS systems can be quite heavy when filled with water and may require extra roof support. But aside from that, batch solar water heaters are simpler than other types of solar collectors, and are correspondingly less expensive. These can be one of the most cost-effective solar water heating systems for warm, sunny climates. Colder climates require additional components to the system, in which case another type of solar collector is likely more economical.

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An absorber coating inside the tube absorbs the solar radiation. Evacuated tube solar collectors heat the transfer fluid (often simply water) moving through the collector and then to the hot water storage tank. In cooler climates a heat exchanger is used to separate the potable water from the non-toxic anti-freeze in the collector.

Photo Credit: Solar Colorado

Generally, evacuated tube collectors are used to heat greater volumes of water, or to heat water to higher temperatures, as is often the case in commercial applications. These types of collectors can also be used for steam production and solar space heating.

Efficiency of Evacuated Tube Collectors

The vacuum is a very effective insulator that keeps the water inside the collector hot while the actual tubes are only a few degrees warmer than ambient air temperature. This low heat loss makes these the most efficient of all solar thermal collectors.

An evacuated tube collector can work well in cloudy conditions and in temperatures as low as -40°F. This characteristic is most advantageous in colder climates, or during the winter, where other solar collectors may not produce as much energy. The cylindrical shape also allows the collector to catch ground reflected radiation, known as the “passive tracking effect,” when facing true south.

Nevertheless, a less efficient collector (such as a flat plate collector) is often sufficient for a residential hot water heating system in warmer climates, as the added solar radiation makes up for any efficiency loss. But the higher efficiency makes an evacuated tube system preferable when collector space is limited.

Cost and Durability of Evacuated Tube Collectors

The cost of evacuated tube collectors can be significantly more than flat plate collectors – though the additional up-front cost may be made up for in utility bill savings. Because flat plate collectors are usually more cost-effective for residential solar water heating systems, solar hot water evacuated tube collectors are more commonly used for commercial applications.

Photo Credit: metaefficient.com

Evacuated tube solar collectors have been around since the 1980’s and most have a similar warranty to flat plate collectors at around 20 years. But these types of collectors tend to need more repairs for the following reasons:

• the thin glass (less than 2 millimeters thick) is more susceptible to breaking, and
• the reliance on a vacuum seal means that over time, the seal can be lost

Despite these issues, the tubes can be replaced individually without having to replace the entire collector.

Maintenance of evacuated tube collectors is slightly more complicated than for other types of collectors. It is true that an evacuated tube solar water heater can be easily cleaned by falling rain water. But snow tends to melt more slowly off these collectors, and due to the fragility of the glass, it cannot be removed with a snow rake. Thus the practicality of cleaning the solar collector can vary depending on climate.

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Photo Credit: Physics World

A glazed flat plate solar collector is an insulated box covered by glass or plastic with a metal absorber plate on the bottom to absorb the sun’s radiation. The weatherproofed collectors are usually glazed with a coating to better retain heat. Heat transfer fluid flows through metal tubes lying below the absorber plate. The fluid then flows through a heat exchanger before entering the storage tank.

Unglazed flat-plate collectors (without insulation or absorber coatings) do not operate in cool or windy climates but are excellent for heating water in a pool.

Efficiency of Flat Plate Solar Collectors

A number of factors influence how well the collector is able to heat water, including the ambient temperature and available sunlight.

Flat plate solar thermal collectors suffer more heat loss than other types of solar collectors, and are thus not the most efficient type. However, flat plate solar water heaters are suited to a wider range of climates. Because these collectors can use a heat transfer fluidsuch as antifreeze, rather than directly heating the domestic water supply, this type of solar thermal panel can be used in colder climates.

A flat plate collector installation. Photo Credit: Viessman

Evacuated tube collectors are often more efficient than flat plate collectors as they are able to reach higher temperatures. However, higher temperature capability should not be the sole characteristic to focus on when choosing a solar water heating system.

Evacuated tube systems are more suited for commercial purposes, when greater amounts of water at higher temperatures are needed, such as at a laundromat.  In most instances, a flat plate system is suitable for heating water for your home.

Cost of Flat Plate Solar Collectors

While they are not the most efficient solar collectors, flat plates last for over 25 years and are one of the most cost-effective options for a residential solar water heating system. Flat plate collectors are the most durable type of collector, and can be cheaper than evacuated tube systems.

Typically, one flat plate collector can supply adequate energy to heat about 40 gallons of water for domestic hot water use. According to EnergyStar.gov, a family of four can obtain about half its hot water from two collectors.

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Photo Credit: U.S. Department of Energy

Integral Collector-Storage Systems

Also called batch systems, ICS systems have storage tanks or tubes inside an insulated box, with the glazed side facing the sun. Cold water passes through the solar collector, which warms the water. The water then continues to the conventional backup water heater. This type is the least expensive and least efficient solar water heating system.

Because the outdoor pipes could freeze in very cold weather, solar batch heaters work best in milder climates. Since most heat loss occurs at night, batch systems work better in households that use most water during the day and evening, rather than the morning.

Thermosyphon Systems

A thermosyphon system takes advantage of the fact that warm water rises. The tank is placed above the collector, and as water in the collector heats up, it naturally rises to the tank. The colder water flows downward into the pipes at the bottom of the collector. Learn more about thermosyphon systems.

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Direct Circulation Systems

A direct circulation system pumps water through the collectors directly into a storage tank. A direct circulation system is more efficient, and is cheaper than to install, than an indirect system. However, as the collector and water in the pipes are outside, this type of system is susceptible to freezing. These are best in warmer areas that do not have hard or acidic water.

Indirect Circulation Systems

An indirect circulation system is better for colder climates that experience freezing conditions. A non-freezing heat-transfer fluid, rather than potable water, is pumped through the collectors. Then the fluid flows through a heat exchanger to heat the residential water supply. The following components are required:

Photo Credit: U.S. Department of Energy

Heat-Transfer Fluids carry the heat from solar collectors to water storage tanks. These heat transfer fluids are non-toxic and protect the collector and piping from freezing. The right fluid depends on the type of solar thermal system and the climate. The primary factors are freezing point, boiling point, and viscosity (which determines how much energy is needed to pump the fluid).

Heat Exchangers transfer solar heat from the transfer fluid to the home water supply. Heat exchangers are often made from metals such as copper and stainless steel. The main type of heat exchanger is liquid-to-liquid, which uses transfer fluid, with one or two barriers between the transfer fluid and the water supply. These are described below:

• single-wall: the heat transfer fluid and the potable water are separated by the heat exchanger material.
• double-wall: the heat transfer fluid and the potable water are separated by two layers of heat exchanger material with air in between. If the heat exchanger has a problem the two fluids will not touch each other.

The most common designs of heat exchangers are:

• coil (shown above) where the exchanger is a tube in the storage tank. If you have an indirect oil or gas water heater, you are already are using a coil heat exchanger!
• shell-and-tube, where two tubes with water and heat-transfer fluid circulating in opposite directions are encased in a metal shell outside the storage tank.
• tube-in-tube, with the two tubes of water and heat-transfer fluid  in direct thermal contact with each other, is the most efficient type.

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Active Solar Water Heating

These systems have pumps to circulate heat transfer fluid through the solar hot water heater. Learn about active solar water heating for your home.

Photo Credit: metaefficient.com

Passive Water Heating

Cheaper but less efficient than active systems, passive water heating systems rely on gravity and to circulate the water or heat transfer fluid. Learn about passive solar hot water heating for your home.

A solar water heating system generally requires a backup system for days without much sun and periods of increased demand. Many solar heating systems for homes have a built-in backup. Often, your existing hot water storage system can be integrated into the solar thermal system. Sometimes, however, the addition of an on-demand backup water heater makes the most sense.

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