by Paul Shippee
Using “today’s sun today” for home heating is an alternative to using up the last hours of ancient sunlight—also known as fossil fuels. These fuels—coal, oil, and gas—are reaching their end-times due to pollution, climate change, peak oil, security risks, and rising prices. Cheap fuel will become a relic of our high growth industrial age.
Meanwhile, the solar industry is heating up, so to speak, with slogans like “Stop Global Warming—One Rooftop At A Time.” I’ll cover some popular active solar system types here, as well as conservation, efficiency, solar financing, tax credits, and Colorado rebates.
Heating your home, sometimes called space heating, with solar energy from daily sunshine can be as simple as south-facing windows that let the sun’s warmth indoors—passive solar heating. When this is not possible, only partly feasible, or not enough, then flat-plate solar thermal collectors—thin, glass-covered black boxes—can be mounted on the roof or on the ground to capture the daily renewable sun’s energy.
Sun shining into these glass-covered boxes is converted to heat and absorbed by the black interior. This heat is transported indoors by a pump that moves liquid through copper pipes, or a blower fan that pushes hot air through ducts, where it is then stored in a tank or thick floor mass or water containers, etc. for use on winter nights. The fact that the solar heat from the black boxes is transported into the home by pumps or fans that use electricity is what makes these systems “active” solar thermal systems, as opposed to passive systems (windows).
Leaving solar air collectors aside for now, let’s look at active solar collection systems that use liquid to transport the sun’s heat into the domestic hot water, or into a “whole house” system for space heating and domestic hot water together. Liquid systems are versatile, popular and widely used. Active solar systems can be complex with many options, many designs, and many variables. I’d like to narrow the field of discussion here by offering to clarify only one aspect—the difference between the two most popular liquid system types. These two are known as “closed loop” systems and “drain-back” systems. Their primary difference stems from the way they protect the liquid against freezing on the roof in winter.
First, the closed loop system. If you have water in pipes on your roof in a cold climate there is a freeze hazard that can burst copper pipes—an expensive problem. One way to protect against this freezing is to employ a mixture of anti-freeze chemicals and water, like in your car. With a water/antifreeze mixture in the collectors and pipes, the risk of burst pipes is eliminated. This is called a closed loop system because it is full of liquid, sealed into pipes under pressure and circulated to heat storage media in the house by a pump.
This closed loop antifreeze technology is borrowed from the tried and true hydronic baseboard fossil fuel home heating practice. This method of freeze-proofing is becoming an “industry standard” for active solar, a sort of top-down technology transfer. It is something that plumbing and heating supply corporations, contractors, and tradesmen are used to. However, anti-freeze chemicals inside the home tend to be messy, can become toxic and corrosive, are prone to leaking, require expensive replacement maintenance, and have lower heat transfer capacity than water.
Closed loop systems are also subject to overheating in the summer, and perhaps boiling under the sun when stagnant (i.e., not circulating) causing dangerous high pressures throughout the system, resulting in corrosion and damage to system components.
Ways to dump this excess heat in summer are then considered, like into pipes in the ground under the house or into a couple of hot tubs. It is necessary to keep the solar pump running under the summer sun to avoid boiling and explosive pressures during any stagnation. However, a stall or stagnation condition will occur during a power grid failure or a solar system component failure. Various technical means are available to mitigate against such potentially destructive failure events, but are not fool proof.
Solar heating is simple but not easy!
An alternative to the closed loop glycol anti-freeze system is the drain-back solar system. This is another way to prevent freeze-ups, and it eliminates the need for anti-freeze chemicals. This system was developed from the bottom up by grass roots hippy inventors and pioneer do-it-yourself solar engineers, who reasoned that using anti-freeze in the home around drinking water might not be the best idea after all. They asked, “Can we just use plain water?” In drain-back solar heating systems, yes, plain water is used to transfer the solar heat from the roof collectors to the home interior.
But what about freezing? One type of drain-back system employs a reservoir of air inside a closed piping system. When the solar pump turns off, the system piping, which must be pitched slightly downhill, allows the plain water to drain back into the warm areas of the home. After draining back in this way, the collectors are now filled with air, which does not freeze.
This drain-back system type can be more efficient and require less maintenance than closed loops when installed properly. Surveys done in Denver during the 80’s and 90’s appear to verify this.
The drain-back system also avoids the summer boiling and overheating problems associated with closed loop systems. This is accomplished by allowing the solar pump to turn off whenever the heating requirements of the home are satisfied. The hot water simply drains back into the heated areas of the home. The black box collectors are made to withstand hot temperatures of up to 400F when filled only with air, sitting empty in the summer sun.
Simply by installing the piping and collectors with a suitable slope, drain-back systems are made fail-safe for winter freezing and summer overheating and boiling. That is to say, they can be made to drain back under any power failure or system failure when designed and installed properly.
There are many other aspects of active solar systems that are beyond the scope of this article. For example, there is often the challenge of integrating solar domestic hot water into a variety of existing space heating delivery systems: What about retrofitting baseboard heating systems or a warm air furnace? Why
are radiant floor solar systems more efficient? What about running the solar pump with a PV panel? When should I use fan coil units to heat rooms? What if the pump runs at night? The list goes on and on.
At the Crestone Solar School http://www.crestonesolarschool.com we address all of these questions and many more.
To conclude, I’d like to say something about efficiency and conservation. The most efficient solar home heating delivery system, whether drain-back or closed loop, is the radiant slab floor. The reason for this is simple: a large floor area can heat a home effectively utilizing low temperatures (80-100F). When the black box solar thermal collectors on the roof operate at these low temperatures, they are very efficient because they do not lose much heat out of the box to surrounding cold winter air.
Conservation is also effective. It is not the same as efficiency. Conservation means you can significantly reduce the size and the expense of an active solar heating system for your home by investing in smart insulation: upgraded attic insulation, systematic caulking of cracks, and insulating windows at night. These are cheap conservation improvements compared to investing in extra solar panels. Up to a point, money is better spent on limiting heat loss than capturing more heat with solar equipment.
Both solar improvements, efficiency and conservation, should be considered smart investments (not expenses) because they aim to avoid the future use and expense of fossil fuels. It’s like buying all your fuel at once for the next 25 years. What fun! And this promises to make you and the world more secure. Therefore, financing—how to best arrange and manage your investment—is the next creative frontier for deploying renewables. Stay tuned.
Nowadays, the Colorado Governor’s Energy Office (GEO) is pushing to make our state outstanding in saving energy and expanding the renewable energy market. One way they are doing this is by partnering with local utilities to award substantial rebates for residential solar hot water, wind power, and solar electric installations. Current indications are that these rebates are not going away. This coupled with the 30% Federal income tax credit can save you more than half of the installed cost of solar! Look at the Crestone Solar School website above for a list of resource links to help you track the developing progress of these market incentives. Let’s get those rooftops to work!