No space for a traditional two-tank solar hot water system? No problem. Single-tank solar water heating systems offer great performance and high efficiency, all in one small footprint.
A single-tank solar water heating system combines solar preheated water and backup heating into one tank, in contrast to a typical two-tank system, which separates these functions. Both single-tank and two-tank solar hot water heating systems can save 55% to 85% on water heating costs, depending on your local climate. Considering that water heating is the second-largest energy guzzler in most homes, this translates into serious savings over the long haul, since solar hot water systems have an expected life between 15 and 30 years. But does one system have an advantage over the other?
Yes, says University of Oregon researcher Steve Baker. In the 1980s, Baker monitored 37 solar water heating systems, both single tank and two tank, for the Oregon Department of Energy (ODOE), using common electric water heaters as a standard of comparison. Oregon enacted a solar energy tax credit in the late 1970s, and ODOE wanted to find out how the systems were performing.
The monitoring study found that the typical total energy use for household water heating was about 5,000 kilowatt-hours per year. Several SHW systems in the sunnier parts of the state were able to meet more than 80% of the yearly hot water loads. The study also found that single-tank solar water heaters often saved more energy than a comparable two-tank system.
Single-tank systems performed better for two reasons related to the two loads of water heating. First is the amount of energy needed to raise the temperature of cold water coming into the home (about 50°F) to the temperature setting of most water heaters (120°F). Second is the energy needed to maintain the water heater at the desired temperature throughout the day and night—even when no hot water is being used. This is known as the standby load or standby loss.
In a two-tank SHW system, heated water from the solar collectors is stored in a solar preheat tank located near a backup water heater. Household water pressure pushes the preheated water from the storage tank into the backup water heater when hot water taps are opened. During the sunny, warm season, the solar preheated water usually remains above 120°F and the backup water heater doesn’t need to come on at all. But in the winter, when the temperature of the solar preheated water typically ranges from 60°F to 100°F, the backup heater works to boost the water temperature to 120°F. In a two-tank system, when hot water isn’t being used, the water in the backup heater cools and the heater comes on—even if the solar tank is full of 120°F water. Maintaining this standby load can account for 15% to 20% of the total energy used for water heating.
In contrast, most single-tank solar water heaters feature a 240-volt electric heating element in the top half of the tank. The element heats the water in the upper half of the tank, while the water in the bottom half is heated by the solar collectors. Water in the tank naturally stratifies, with warmer water rising to the top and cooler water sinking to the bottom. In a single-tank system, the solar collectors can heat water well above the electric element’s thermostat setting of 120°F (130°F to 150°F is common). Because of this, the tank water may remain above 120°F even if it cools by 5°F to 10°F overnight. The result? The element in the single-tank heater doesn’t need to kick on at all, yielding a 10% to 20% savings advantage in standby losses compared to two-tank systems.
Because of the inherent inefficiency of gas water heaters, the difference is even greater if the backup water heater in the two-tank system is fueled by natural gas. Until recently, natural gas was less expensive than electricity, so gas water heaters got away with having lower efficiencies. But cheap gas is no longer available in most of the country. When the burner is firing, or even between uses, unused heat escapes up the flue. This, coupled with standby loads, can amount to 20% to 30% of the entire water-heating load. A single-tank system with backup electric heating offers at least an additional 10% energy savings compared to a two-tank system with natural gas backup. When a single-tank system replaces a gas water heater with a continuously burning pilot light, the savings can be 20% or more.
In any two-tank system, whether electric or gas backup, heat loss through the insulation of the backup tank and between the pipes going in and out of the tanks can also contribute to standby losses. Of course, this is true for a single-tank system too, but to a lesser degree, since there’s only one tank wall and less piping. When the two tanks are combined into one, energy use that typically goes to standby losses can be reduced.
The efficiency of single-tank systems can be further improved when a timer is included. Using a timer accommodates the typical hot water habits of most households, where usage normally occurs first thing in the morning and again in the evening. During midday, when the house is unoccupied, the timer shuts off the element, giving the solar collectors the opportunity to heat the entire tank—even the top portion. The timer can be set to activate the heater an hour or so before wake-up to ensure ample hot water in the morning, and also programmed to allow the element to come on again for evening hot water use. Committing to a timer may mean altering your lifestyle so that you preplan your hot water usage. If you are typically at home during the day, having a timer turn off the tank’s heating element will leave you without hot water on cloudy days. However, in many cases, having a timer turn off the electric water heating element at night is convenient and well worth it. And if you plan ahead—showering and doing laundry in the morning, for instance—a timer can save you money without cramping your lifestyle.
Allowing the single-tank system to utilize as much solar energy as possible will cut energy costs. During the middle of the day, the sun’s energy can take over, heating water in the tank to 130°F or higher. When this happens, the tank can incur standby losses of 8°F to 10°F and still keep the water temperature above the electric element’s 120°F setpoint, effectively keeping the element off. If this occurs half of the year, overall standby losses are cut in half. And it does no harm to store hotter water in the tank—a mixing valve adds enough cold water in the pipes to keep the temperature at 120°F when it comes out of the hot water tap.
At night, having the timer turn off the element can save on standby losses too. Although it seems like this strategy would require more energy the next morning to raise the temperature again, heat loss occurs at a rate directly proportional to the difference in temperatures. A 120°F tank of water in a 50°F garage has a 70°F difference in temperature, or DT. When the tank’s temperature drops to 110°F, the DT is reduced to 60°F, with a corresponding drop in the rate of heat loss. As the temperature drops, the percentage of heat loss drops as well. Even though the backup heater will be on longer first thing in the morning, it will not be on as long as it would have been if it had to maintain the temperature at 120°F through the night. Over the entire year, this amounts to many KWH of energy savings.
For most single-tank solar water heater systems, a 120-gallon tank will provide the best service. The single 4,500-watt element is strategically located a few inches above the middle of the tank and will effectively heat about 50 gallons of water electrically, with the remaining 70 gallons in the bottom portion of the tank heated by the sun. Although 80-gallon tanks are available, you’ll run the risk of not having enough hot water on rainy days because the element will only heat the top 30 gallons.
For $4,000 to $5,000, you can purchase a single-tank drainback system kit that includes 40 square feet of solar thermal collectors, a 120-gallon tank heater, and all the other needed components. If you want to purchase the components separately, calculate your collector area based on the water in the tank that will be heated by the sun. In a 120-gallon tank, that would be the bottom 70 gallons. Divide this number by 1.5 to 2 gallons to get the recommended collector square footage. In general, most single-tank systems work well with a 120-gallon tank and a collector area between about 40 and 60 square feet.
Federal tax incentives can help offset costs of a SHW system, and many states, cities, and local utility districts have incentives as well. The current federal tax credit pays 30% of a solar hot water system’s cost, up to $2,000. However, qualified systems must be installed and operational on or before December 31, 2008, to receive the federal tax credit. And at least a dozen states offer incentives based on cost or performance, usually topping out at about $1,500. Utility incentives are also available in many regions of the country. To check incentives in your area, visit the Database of State Incentives for Renewables & Efficiency at www.dsireusa.org. You can also contact your local utility or solar contractor to find out what’s available.
Keen energy awareness and dedicated conservation tactics can really make a single-tank system pay off, especially when coupled with a timer. There are always ways to conserve more, such as using low-flow showerheads and using hot water sparingly during cloudy weather. As your usage shrinks, the ratio of water heating by the sun increases. A retired couple from Portland, Oregon, once told me that I misrepresented their solar savings from the system I’d installed a year before. I was perplexed, since I had conservatively told them that they could expect to cut their water heating costs by 55% to 60%. But then they told me that their actual energy savings was 70%. I was amazed. After all, we’re talking Portland, a place known for its soggy skies.
The key to the couple’s exceptional savings was their enthusiasm. They were excited about their system and made the most of it, spreading their hot water use through the day to optimize solar efficiency and waiting for sunny days to wash clothes. Some of us don’t want to change our lifestyles to accommodate the sun, but if you’re so inclined, you’ll reap extra dividends.
Allen and Laura Bernstein installed a single-tank water heating system in August 2007, when their old water heater needed to be replaced. “We went with a single-tank system because we only had space for one tank in the laundry room,” says Allen. The tank wedges between the wall and the laundry sink, with only a scant half inch to spare.
For their two-person household, the setup is ideal. They are energy-conscious and conservation-minded, and their routines allow them to set the timer to garner the most benefit from solar-heating hours. “The system is performing really well,” says Allen. “We’re getting plenty of hot water.”
Allen is so impressed that he’s planning to put similar systems on rental property the couple owns. “I think this makes sense for my business, especially with the tax credits available. I have a bunch of water heaters that are at the end of their useful lives and need to be replaced. I’m in the business of providing a comfortable place to live, and I believe my apartments will be more rentable as power rates go up.”
John Patterson is president of Mr. Sun Solar and inventor of the Sol-Reliant solar water heating system. He has installed more than 1,000 solar water heaters over the past 28 years.
Suzanne Olsen is a writer and photographer specializing in renewable energy and the environment.