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Bristol Stickney Solar Solutions |
Bristol’s six principles for good solar hydronic design
By BRISTOL STICKNEY,
contributing writer
In this article, I continue our ongoing discussion of the broadest principles of good solar heating design. Over the past few decades I have found that the best solar heating installations do not depend upon any single idea or product, but rather the careful integration of all of these principles into every installation. The key ingredients for solar/hydronic design and installation can be divided into six categories, listed below, roughly in order of their importance:
The success of any solar hydronic home heating installation depends on the often-conflicting balance between any of these six principles. In past columns I described the conflict between many of these principles to illustrate the need for a balance in the overall design. This month I would like to focus on Effectiveness and Serviceability. These concepts do not always conflict; in fact, sometimes they reinforce each other nicely. Let’s take a closer look at these two principles. Also this month I would like to talk about how you know when the design job has been done well. An example of good integrated solar heating design comes from another industry -- the automotive industry. Hybrid car designers have a very similar challenge to solar heating designers.
Principle 2: Effectiveness -- think about user satisfaction
Don’t you hate it when they complain? Well, if we offer solar heating systems that are Effective, they will have little to complain about. This category should include everything that the owner needs to feel satisfied with the system. The initial cost must be reasonable and justifiable (cost-Effective). Part of the cost may be justifiable in terms of lower carbon emissions and other environmental benefits, solar tax incentives or other intangibles unrelated to heating system performance. But, it must be a good heating system as well, providing heating comfort (or hot water) upon demand, low fuel consumption, reasonable operating cost, low maintenance, no special attention and uncomplicated user controls.
In fact, an Effective solar heating system has all the same requirements as, say, a hybrid car. A well designed hybrid car will meet the driver’s expectations in every way when compared to their old car. A solar heating system in today’s market must do everything that the old heating system did, but with the added benefits of comparatively low operating cost and low emissions. Solar heating will enjoy the same acceptance as hybrid cars are gaining only if they are more Effective, reliable, compatible, elegant, serviceable, and efficient than the old home heating system was.
Solar heating -- hybrid energy for your home
Solar hydronic heating systems, whether used for domestic hot water, or your larger heating needs, apply the “hybrid car approach” to home heating. In the car, electrical energy is stored in batteries to be delivered to the drive wheels later, at times when the battery power can offset the use of gasoline. This allows the gasoline engine to run less often and at a rate that is more fuel efficient.
Active solar heating systems have developed over time to work the same way. The solar heat is stored in thermal mass, to allow it to be used later to offset conventional heating fuel. This reduces the use of conventional fuel and also can be designed to improve the efficiency of the fuel consumption. The thermal mass can be installed in several different forms such as domestic hot water tanks, other insulated water tanks, a swimming pool or hot tub, or the thermal mass of the masonry radiant heated floors themselves.
Just as in the hybrid car, the mechanical system and controls must be in careful balance. The car batteries must be regulated to prevent overcharging them. When in danger of overcharging, there must be a safe way to deal with the excess electrical energy. When the batteries are low, the control system must decide when to start the gasoline engine, and when to send energy-regeneration from the brakes to the batteries. When the battery or regulator malfunctions, you still want the gasoline engine to get you home reliably. The designers of the hybrid car had to anticipate every possible contingency and mode of failure. Then they had to build in enough intelligence in the control system to make it both fail-safe and reliable. If they didn’t do this, few people would want to own one.
This is the same approach that we are taking with solar heating these days. We don’t make the solar collectors too big, because that would require too much heat storage. The controls have enough intelligence to prevent solar overheating, while providing seamless operation of the conventional backup heat in a prudent yet efficient way. All types of possible malfunctions are anticipated and compensated for by the controls without sacrifice of reliability. The mechanical configuration and the control system go hand in hand to allow every practical heating need to be supplied by solar heat automatically, when ever possible.
Principle 5: Serviceability -- Make it easier to install, repair and adjust
Isolation valves can be a bargain. When things go wrong, a quick and economical repair is essential to maintain the confidence of the owner. When purchasing a solar heating system, a buyer will often apply pressure to get a lower price. If we give in to this pressure, we can do things that we will soon regret. It is false economy to eliminate critical ball valves, drains, air vents, check valves and other equipment that allows for rapid filling, draining, air elimination or component replacement. I hate to see a repair crew struggle for hours to do something that should only take a few minutes if it had only been designed properly.
Pre-fabricated plumbing modules are being offered by more manufacturers these days for both hydronic heating applications and active solar installations. These modules typically contain a pump, isolation valves, gauges, check valves, relief valves, air vents and other components already pre-assembled. The use of pre-fabricated plumbing modules promises to reduce the labor cost of an installation, and allow more installations to be completed each month. Furthermore, it is not possible to remove the critical components from a pre-fab module, and so that temptation is eliminated.
The following heat exchanger experience illustrates the questions regarding cost Effective versus Serviceability, and how they can reinforce each other:
The exchanger experience
Have you noticed the solar equipment ads that show how easy it is to hang an external heat exchanger on an existing hot water tank to transform it into a solar heated tank? The main advantage of doing this is you can save the cost of a new solar storage tank. It is presumably cost Effective. Some are modular with two pumps and a ?T control pre-assembled. Some have only one pump (for the solar fluid), and the water is supposed to thermo-siphon by gravity flow into the water tank. These modules conform to the laws of physics, and when first installed, usually work quite well.
But then something insidious can happen. It may take a few months, or a few years, but the water side of the heat exchanger can clog up with minerals and the water-side pump will then burn out. In my region of New Mexico this happens with such regularity that I no longer use external heat exchangers on open domestic water unless there is a proven source of soft water or a maintenance contract solution in place. Even if this happens to only a small number of solar equipment buyers, those people will not get their expected solar benefits, and will tell everyone they know about it. The last thing the solar heating industry needs is another black eye.
At Cedar Mountain Solar, we religiously use in-tank heat exchanger coils for solar domestic hot water. This is because even when the coil becomes coated with minerals, it still continues to make hot water and there is no water-side pump to burn out. The solar thermal efficiency may drop over time, but the solar hot water maker does not simply quit, causing the solar collectors to overheat -- a clear case of long term Reliability winning over short term cost Effective (false) economy.
A major trend in solar heating, or at least in solar water heating, is the modularization of components, reducing guesswork, fieldwork, warranty liability and labor. The new packaged Caleffi solar hot water systems are an example of a solar equivalent to the hybrid car. The package includes two heat sources -- a flat-plate solar collector array and an electric element internal to a tank. It also includes the tank itself (with internal heat exchanger -- good job, guys!), and a pump module consisting of pump, supply and return pipes, differential controller with sensors, change ports, and everything else you need for a successful installation. This company has also helped ensure reliable installation by providing unions on the collectors, flexible stainless pre-insulated piping, expansion tank, and even heat transfer fluid for a truly complete kit.
My company, Cedar Mountain Solar, has modularized design and controls for whole house heating applications for our own installations, but this process is still quite labor-intensive for design/build solar heating firms. The trend towards modularization will make these systems easier to install in the future. I’ll write more about our design, piping and control systems in upcoming articles.
Bristol Stickney, partner and technical director at Cedar Mountain Solar Systems in Santa Fe, N.M., has been designing, manufacturing, engineering, repairing and installing solar hydronic heating systems for more than 30 years.
