Solar Water Heating Systems
Solar water heating systems, probably the most common application of solar thermal technology, are the best first investment for solar in most homes. It is accepted within the solar industry that a house should be fitted first with a solar water heater, either before, or, during the addition of a photovoltaic (PV) solar electrical system.
DHS installs Solene brand solar water heating systems. For more information please visit the Solene website.
While there are many solar water heating (SWH) system types available it is important to only use system types that are appropriate for the climate you are in. In Oregon we have to consider both of our climate extremes, below freezing and above 100 degrees. Freezing conditions are addressed by using closed loop systems exclusively.
Closed loop systems can be anti-freeze based, or, water in a drainback configuration. At Dawson’s Home Specialties, Inc. (DHS) I install both system types, but prefer the drainback system because of the superior resistance to overheating during the summer. Both system types have positive and negative characteristics. I will list below the advantages and disadvantages to both system designs.
Anti-freeze Systems
As implied by the term anti-freeze, the most dominant feature is that this system uses anti-freeze to achieve freeze protection. It is, by nature, a closed loop system which means there is a heat exchange required between the collector fluid and the potable water used in the home. At DHS I prefer using a solar storage tank that has an integrated heat exchanger in order to simplify the exchange process. The same tank can be used in either the anti-freeze system and the drainback system. The list below reflects my opinions on the pros and cons of anti-freeze based systems.
Pros:
- Very versatile installation format, anti-freeze systems allow panels to be mounted almost anywhere in relation to the solar storage tank (drainback doesn’t).
- Plumbing can be routed almost anywhere and can have multiple traps without regard to maintaining slope.
- Allows for use of bundled pre-insulated flexable stainless steel tubing to supply collectors.
- Panels can be mounted square and level to the roof plane (they don’t have to be sloped to drain).
- Panels can be oriented in either horizontal or vertical positions. This can be helpful when trying to fit panels in a very specific place. In other words a 4’x 10’ or 4’x 12’ panel can be placed in any orientation.
- Anti-freeze systems may allow for smaller pumping requirements, especially on multi-story houses.
- Anti-freeze systems can use small PV panels and a DC pump to allow for autonomous operation electrically.
- Anti-freeze systems install in less time because of some of the above characteristics.
- Because the system operates under pressure it runs very quietly.
Cons:
- Anti-freeze solutions are problematic for several reasons, anti-freeze is messy and can stain if spilled, it breaks down with time and temperature.
- Anti-freeze systems require more maintainance because of the need to maintain the fluid, fluid losses because of high temperature overheating, and the nature that it will leak past plumbing connections that water wouldn’t.
- Anti-freeze doesn’t transfer heat as efficiently as water.
- It is very difficult for a homeowner to maintain the fluid in their own system (even very mechanically inclined people).
- Overheating can cause chronic problems if the residents don’t use much hot water during the hot months. There are techniques used to minimize the overheating effects, but they always add complexity and aren’t universally successful.
Drainback Systems
Pros:
- Uses water for a heat transfer medium, and because water doesn’t break down with age or temperature it can virtually last for the life of the system.
- Eliminates freeze danger and overheating because the panels drain out whenever the system is off. So when the storage tank gets to maximum temperature the system shuts off and the water drains back to the drainback tank located in a freeze proof location.
- Homeowners are able to maintain the fluid level in the system quite easily if they desire to.
Cons:
- Drainback systems are not under pressure so they make some noise and may not be suitable for extremely sound sensitive situations.
- More space is required because the 12 gallon drainback tank needs to be located near the 80 gallon solar storage tank. It is most commonly located above the solar storage tank.
- Panels must be located above the storage tank to allow the panels to drain back to the tank.
- Panels and piping must be slightly sloped to drain all water when the system is off. This takes a bit more time and skill to achieve.
Both closed loop anti-freeze and drainback systems have positive reasons to recommend them as a system of choice, but I tend to favor drainback as the system of choice.