Solar Water Heating Off the Grid
By Dan Fink, Colorado
Solar electric systems are getting a lot of exposure these days, capturing media and public attention with massive utility-scale photovoltaic arrays covering acres, entire commercial rooftops covered with solar modules and home-scale systems popping up everywhere. But another solar energy option has been quietly lurking under the radar for decades: solar thermal, for directly heating water and air.
Considering that the average American family spends 18 percent of their energy budget on water heating and 53 percent on space heating, solar thermal can be a big cost saver. And it has one huge advantage over solar electric—anyone with basic fabrication skills and tools can build an effective solar hot water system from mostly scrap parts, at very low cost! Photovoltaic modules, on the other hand, take a high-tech factory to fabricate.
Solar thermal systems also have the advantage of collecting more thermal energy per area of roof or ground collector space than solar electric systems, as there are fewer energy conversions from sunlight to heat. For example, on average here in Northern Colorado about 13,000 BTU of solar energy per day hit each square meter (m²) of ground. Set out one m² of solar electric collectors to convert that energy to electricity, then run an electric space heater with it, and you’ll get only about 2,000 BTU per day. On the other hand, put out a one m² solar thermal collector at the same spot and you can expect more than 7,000 BTU per day. Don’t overlook heat gain from high-efficiency windows either, they are also more efficient heaters by area than photovoltaic, though storing the heat is more problematic. Hot water is an excellent thermal mass, and it can also be circulated within floors for an efficient space heating system.
Parts of a Solar Thermal System
The components in a solar thermal system are also a bit easier to understand than solar electric, as is their operation. Have you ever quickly pulled your hand back after touching a piece of black-painted metal that was heated by the sun? That’s stored thermal energy. The rest of a typical system is simply pumps, tanks, valves and plumbing, plus a thermostat. Very basic stuff, though it pays to learn from other people’s mistakes—especially on the DIY side—before diving in. I recommend the website www.builditsolar.com for information about a huge variety of successful home-built solar thermal systems.
It’s easy to collect solar thermal energy, the trick is to store it instead of immediately radiating it back into the surrounding air. That’s where important details in the design of solar thermal systems come into play.
Batch systems (some varieties are also called Integrated Collector Storage or ICS) are the simplest, both in operation and construction. These have been around since the invention of steel tanks and glass. The concept is simple: A black-painted steel tank full of water sits out in the sun and heats up, but it’s inside a glass-covered enclosure to reduce how much heat is released back into the air around it. Cold water is piped into the bottom of the tank, and hot water is removed from the top as needed.
Batch water heating systems are best suited for warm climates because they are prone to freezing, but they are also easy to drain for the winter for summer-only use. They are grouped under the term “passive systems” as they do not need pumps to circulate the water. These systems are not particularly convenient or efficient, but can be just great to meet certain needs, for example hand-washing out in the barn after chores or hot water at a remote hunting cabin. In Countryside May/June 2008 issue Rex Ewing explains how easy it is to build one of these.
Thermosiphon systems are another type of passive design, and use the effect of hot water rising above cold to circulate the hot water to a storage tank, which can even be located inside a house so it loses less heat to the ambient temperature. These systems were extremely popular in the USA and worldwide in the early 1900s, with hundreds of thousands of systems sold.
The trick is that the storage tank must be located above the collector for the thermosiphon effect to work, and any air bubbles in the piping must be bled out or the circulation will stop. These systems are also best suited for warm climates, as freezing can be a problem. Besides not needing a pump for circulation, another advantage of these designs is that home fabrication is not all that difficult, though there may be a learning curve getting the system working properly at first.
Active systems differ from the passive systems shown above in that they use one or more pumps to circulate fluid. They have the disadvantage of requiring electricity to run a pump, but the advantage of much better control of temperature using thermostats.
In an active direct system, the water being pumped through the solar collector is the same water that will be used for domestic hot water or radiant space heating, while in an active indirect system the fluid circulating through the collector never comes into contact with the end use water. In the simplest of direct systems—for example to pre-heat water for a hot tub—the pump can be powered directly by a small photovoltaic module. When the sun is up, it starts the pump, and when the sun sets the pump stops. A simple thermostat can be added to keep the water from getting too hot for comfort. The disadvantage is that outdoor piping will freeze and burst in cold climates if filled with water at night.
Drainback systems solve that freezing problem, even in cold climates. They are most commonly designed for indirect use, and include a “drainback tank” holding only enough water to fill the plumbing from the tank to the roof. The collector itself, the plumbing and the drainback tank usually hold only about 10 gallons of water. Inside the tank is a “heat exchanger” made from coiled copper tubing, through which the indirect end use water is pumped from the much larger end use storage tank.
A “differential temperature controller (DTC)”—basically a dual thermostat with some computer logic included—senses the temperature at both the collector and the drain-back tank. When the sun is heating the collector and the temperature difference (called ΔT, or delta T) between it and the drainback tank reaches about 10°F, it turns on the pump and starts circulating water through the collector. When the sun sets and that differential falls, the DTC shuts off the pump…and all the water in that outdoor collector and piping drains back into the tank, provided the installer correctly sloped all the plumbing so gravity can take its course. A “vacuum breaker” at the top of the collector lets in air so the water can properly drain. It’s an elegantly simple, freeze-proof solution that’s easily in the realm of a more advanced DIY project.
Active indirect, fully-filled systems are another popular type, and are especially common in the coldest of climates. The plumbing loop through the collector and into the heat exchanger is filled with a mixture of water and propylene glycol (non-toxic antifreeze), so nothing drains back at night and the outdoor line can remain fully filled. Advantages include no risk of freezing the collector or plumbing, excellent control of the system efficiency by the DTC, and a smaller pump that uses less energy, as it doesn’t have to lift fluid all the way up to the collector each morning.
The main disadvantage of these systems is the glycol itself; it’s a less efficient heat transfer fluid than plain water, is expensive, has to be changed out every few years, and expired fluid must be disposed of properly. Even though it’s non-toxic, you can’t just pour it on the ground or into a storm drain.
The other problem with glycol is called “stagnation,” where in a system that is not constantly circulating fluid during daylight hours, heat inside the collector can reach 400 to 600°F which can degrade the glycol mixture over time. If the end use water has reached maximum safe temperature, usually 140°F, the fluid circulation system must shut off, and heat transfer fluid (water mixed with glycol) is left in the collector.
This is usually caused by the homeowner who does not use enough hot water. For example, an extended vacation with nobody home, not enough hot water storage compared to collector area, or a system that overproduces thermal energy in the summer because it is designed to try and produce a high portion of heating needs in the winter—the “solar fraction.”
With drainback systems you don’t have to worry about stagnation, since once the end use water storage tanks reach 140°F, the pump simply shuts off, the collector empties and there is no fluid up there to stagnate.
The percentage of a home’s hot water needs—no matter what the end use—that is fulfilled by a solar thermal system is called the “solar fraction,” and it is critical in the design of any system.
In warm climates where there is little risk of extended freezing temperatures, it’s reasonable to design for a solar fraction of 75 to 100 percent, with 100 percent meaning that all of the home’s water heating needs are provided by solar. In these climates incoming sunlight is more consistent every month of the year and water can be used as the heat transfer fluid.
But in temperate and cold climates, a more realistic solar fraction to shoot for is 35 to 65 percent. It’s very similar to sizing an off-grid solar electric system in the same location—if you design it to provide 100 percent of your electricity even in the dead of winter, you will have spent a lot of money on extra PV modules that won’t even be turned on by the system controls in summer. Much better to use a backup electricity source for a few hours a week during those few weeks of snow and clouds.
Solar thermal works the same way. If you design the system to provide 100 percent of your hot water needs during winter, you’ll be overproducing energy during summer with no way to store it. The most cost-effective solution is to keep each collector working as hard as it can, most of the time, and use electric or gas backup water heating for periods with little incoming sunlight. At the end of the day, dollars per kilowatt-hour is the bottom line in both solar electric and solar thermal systems.
Hot Water Storage
Sizing the hot water storage tank(s) in a solar thermal system is very similar to sizing the battery bank in an off-grid solar electric system: too little storage, and your backup energy source has to run more often. Fortunately solar thermal storage is both less expensive and more long-lasting than a battery bank—it’s a very common practice to simply re-purpose old hot water heaters into storage tanks. The end use tank can simply be your existing hot water tank, with the heating system remaining in place. If it’s been sunny, the heater will need to run very little, and in periods of high hot water use or low sunlight with the heating element running, the water inside has at least been pre-heated to save energy.
Some general “rules of thumb” for sizing solar thermal systems are:
- Plan on 16 to 25 gallons of hot water use per person per day in your household. Your use may vary…usually on the high side.
- About 1.5 m² of collector area per person is a good place to start for sizing a system.
The recommended ratio of collector area to storage volume depends on your local climate:
- In the Sunbelt: 1 square foot of collector per 2 gallons of tank capacity (daily hot water demand).
- In the Southeast and Mountain States: 1 square foot of collector per 1.5 gallons of tank capacity.
- In the Midwest and Atlantic states: 1 square foot of collector per 1 gallon of tank capacity.
- In New England and the Northwest: 1 square foot of collector per 0.75 gallon of tank capacity.
Sound complicated? It is a bit, but it’s not rocket science either. And one of the things that intrigues me so much about solar thermal systems is the huge variety of ways to design and build them, combined with how easy it is to build the system yourself. Don’t forget that you may be eligible for Federal, State and local tax credits for a solar thermal system—though they may not apply if you build a system from scratch.
But with DIY fabrication and very low cost possible, why not just give solar thermal a try? Even a simple experiment on a kid’s science fair scale will show positive results, and might inspire you to expand the scope and build something bigger to really help reduce your water heating costs.