Mother Earth Living

Bathe in the Sun's Rays: Solar Hot Water Heaters

Save money and energy with solar water-heating systems
By Bob Ramlow
November/December 2007


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Imagine filling your bathtub full of steamy water, knowing you did it without creating pollution or wasting money. Solar water heaters can supply hot water to your tap for less money while helping you reduce your contribution to global warming by decreasing the amount of nonrenewable energy you use.

Solar water-heating systems use clean, free energy from the sun to heat water for daily use or for space heating. About 15 to 20 percent of a household’s energy outlay goes to heating water for bathing and washing clothes and dishes. A solar water-heating system is the most cost-effective renewable-energy investment a homeowner can make.

The upfront cost for a system and installation is steep, typically ranging from $4,000 for a small system to $20,000 or more. However, you’ll enjoy an instant increase in home equity, and you’ll immediately reduce utility bills while also protecting yourself from energy rate increases. Because the energy is free, there are no monthly bills. Once the system has paid for itself in savings, the hot water your solar system provides costs you nothing. On average, a solar hot water system pays for itself in five to 15 years, depending on the system type and financial incentives in your area.

Many people finance solar heating systems, making monthly payments. The energy savings are usually large enough to offset the loan payment, so your monthly expenses don’t increase.

How do solar water heaters work?

Solar collectors need direct sun to function. They’re mounted on a rack in a sunny spot, either on your roof or in the yard. Insulated piping connects collectors to a storage tank near your existing water heater. A pump circulates antifreeze or water through the collectors whenever the sun is shining. This hot solar fluid then goes through a heat exchanger to preheat your water and store it in a tank for later use.

A solar professional can assess your home to determine the best location for the system, give you a cost estimate and determine what services a solar system could provide at your house. Solar collectors should be mounted within 30 degrees of true south and tilted at an angle equal to your latitude.

Solar water-heating systems aren’t designed to provide 100 percent of a home’s water. A typical system provides 50 to 75 percent of the annual load. The amount of energy your solar system can provide depends on your climate and the time of year. In warmer climates or during the summer, you can expect your solar water heater to provide nearly all of your needs; during overcast periods, output can decrease to 50 percent. You’ll need a backup water heater for use during cloudy weather.

A solar water-heating system can also contribute to space heating through your forced-air furnace, radiant floor or other type of heating system. It won’t replace your current heating system, but it will preheat the fluid or air that’s warming your home. This means your current system does much less work, saving both money and fossil fuel.

Selecting a system

Your climate is the most important consideration when selecting a solar water-heating system. In climates that stay above freezing year-round, the system’s solar fluid can be water. Where temperatures plunge, you’ll have to drain the system during cold months or use an antifreeze mixture. The many system types make it economically viable to use a solar water-heating system anywhere in North America, provided it’s sized appropriately and installed properly.

An experienced professional in your area will know which solar heating systems work best in your climate and can inform you of potential state or local incentives for renewable energy installations. Incentives are often offered through state programs or utilities. To find out what’s available in your area, visit the Database of State Incentives for Renewables and Efficiency website, a source of information on state, local, utility and federal incentives for renewable energy and energy efficiency.

The current federal tax credit is applicable until December 31, 2008; it pays for 30 percent of the cost (up to $2,000) of a residential solar hot-water system, provided it meets 50 percent of the home’s water-heating needs and uses certified collectors.

Solar water heaters can last 40 years or more if the design is climate-appropriate and incorporates high-quality materials and workmanship. A solar investment keeps energy dollars at home and reduces our dependence on foreign oil. With solar water-heating system, your system pays for itself and you help the earth.  

Collectively Speaking

There are two popular kinds of solar collectors:

Flate-plate collectors:

• Are shallow, rectangular boxes (typically 4 by 8 feet) with tempered glass tops and insulated backs and sides. An absorber plate inside gathers solar heat and transfers it to a network of copper tubing, through which the solar fluid flows.

• Work well in all climates.

• Have been used the longest.

• Are efficient and competitively priced.

• Shed snow and frost well.

• Are very durable, but they need secure mounting against high winds.

Evacuated-tube collectors:

• Are glass tubes (3 to 4 inches in diameter, 4 to 6 feet long) with a vacuum inside that avoids some heat loss. An absorber plate inside the tube gathers heat and transfers it to the solar fluid that runs across the top of the collector.

• Can be fragile because the glass is untempered. They can lose their vacuum over time.

• Need a generously sized storage tank to avoid overheating.

• Work best when there's a consistent, year-round, daily load.

• Are safer for windy areas because of spaces between the tubes in a collector.

Which System Is Right for You?

Integral Collector Storage

Best for: hot climates or seasonal use

Pros:
• Proven design
• Low maintenance
• High efficiency
• Simplicity
• Collectors mounted on roof or ground

Cons:
• Collectors also store the water, so they're very heavy.
• Use only where freezing never occurs.
• Loses heat because storage is outside

Pressurized Closed-Loop Antifreeze

Best for: cold to hot climates

Pros:
• Proven design
• Low maintenance
• High efficiency
• Usable in any climate, including extreme cold
• Most versatile collector mounting options
• No limit to pipe runs or piping configuration
• Can be used for space heating

Cons:
• Antifreeze solution degrades when system is idle for extended periods.
• Antifreeze must be replaced every 10 to 20 years.

Drainback

Best for: moderate to hot climates

Pros:
• Proven design
• Low maintenance
• High efficiency
• Freeze protection in nearly all conditions
• Fluid drains when idle, preventing degradation of solar fluid if antifreeze is used
• Can be used for space heating

Cons:
• Collectors must be mounted above drainback tank.
• Installation requires care to ensure quick, complete drainage.
• Vulnerable to freezing in extreme cold


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