ASK THE EXPERTS: Seasonal Heat Storage

A heat dissipater, such as this one by Apricus, can be used as a dump load to prevent overheating in a solar water heating system.

I am interested in applying some of the principles outlined in Chuck Marken’s article “Overcoming Overheating” (HP142). I am especially intrigued with the idea of routing the excess solar heat produced during the summer to a heat dump for melting winter snow. The question is how to manage the process to store the heat.

We have a second home in the San Bernardino Mountains at an elevation of 7,200 feet, near Green Valley Lake, California. Here, snow can accumulate to several feet—and it can remain for months.

In 2010, we installed nine 4-by-10-foot solar collectors in an antifreeze-based system for space and water heating. To prevent system overheating during warmer months, I cover several of the collectors, which are mounted on the roof. In April, I cover about 20% of the array; by the end of May, about 75% is covered. I reverse that process beginning in August.

While this strategy is effective, it is very labor-intensive and does not accommodate cloudy days well. Lately, we are experiencing more cloudy summer days. Since it is impractical to climb onto the roof to uncover collectors as the clouds come and go during the day, we now find ourselves relying on imported power for domestic water heating. Installing a heat-dump controlled with a thermostat is my next choice for dealing with overheating, since I could still make use of the excess heat.

Walter Farmer • via

Beyond heat storage in a bed of sand under a radiant floor, I don’t know of anyone who has had success with seasonal storage and retrieval systems. Even sand beds only provide a few days’ worth of heating. A successful seasonal thermal storage system is an idea waiting for innovation and, to my knowledge, has been waiting for 40 years.

A heat dump, however, is a pretty straightforward solution. A motorized and dedicated pump that diverts the glycol solution to a radiator or underground tubing is not my favorite solution since it uses up some parasitic energy to dump the heat. I prefer a passive inline radiator—a thermostatic valve diverts the glycol solution to the radiator when the system is running normally and when the fluid is warmer than 170°F. An example can be found at

You’ll need about 8 feet of 3/4-inch hot water fin tube element per 4-by-10 collector—maybe less.

Chuck Marken • Home Power solar thermal editor

We normally recommend one Apricus heat dissipater for each 4-by-10 flat-plate or 30-tube evacuated-tube solar collector. They must be located outside—with good airflow—to maximize heat transfer. They can be installed in parallel (not series) to increase energy dissipation.

For winter heating, we normally recommend that the collectors are tilted at an angle that’s 20° to 25° greater than the location’s latitude. This helps maximize winter, fall, and spring output, while reducing summer output. Another good option is to mount collectors on a south-facing wall at 60° to 70°. In the winter, the collectors will benefit from diffuse radiation from the snow-covered ground, which further helps increase output during the cold months.

Mick Humphreys

Comments (6)

andrejones's picture

I have a home-based business and I'm trying to cut some costs. I've thought about installing solar panels at home to save energy, but my concern is if it's really worth the price.
The installation is very expensive and during winter, the panels generate no energy at all. So, I'm not sure if the savings really justify all the trouble I have to go through in order to install the panels.
Could someone share a personal experience? Thanks!

Fred Golden's picture

Ove Schou,

The first time I ever heard of phase change heat storage was in 1977, I was in grade 7, and my class mate's dad worked on Solar 1, in Barsto CA. It used the sun and about 1,000 mirrors to heat a tower with salt in it, changing it to liquid, and turning a large steam turbine. It ran until about 1999 or 2000.

You might try wax, and find something in the 150F melting range. Then place this in the outside compartment of a thermos vacuum bottle. By pouring in hot water (212F) you would change the wax to melt it, then pour the water out after 10 minutes, and pour in the coffee or soup that you wanted to keep warm for hours. As the wax changes back from liquid to solid, it will give off all that heat back to the thermos bottle.

Good luck finding the correct material for phase change at whatever temperature you desire. It will take testing 500 materials, and then you will know that 490 of them will not work, but will discover something if you keep trying!

Ove Schou's picture

There are some phase change materials out there that change from a solid to a liquid when warmed up, and then back again. This isn effect "stores heat" for later use.
However, I have never used this technology, other than hand/glove warmers! The UK University I work for is doing research in this area. (Google Centre for Renewable Energy Systems technology, or CREST). As such, I expect to see fruits of that research in the next few years.

Jim Ballenthin 2's picture

The professionally designed/installed 300 evacuated tube system in northern MN with which I am familiar had serious overheat and freeze issues, all of which were solved during the summer/fall of 2014, The system includes a 1000 gallon DHW storage tank. In addition to plumbing changes, the overheat problem was solved 1) by using the cool down feature of the Resol controller, which circulates hot water from the DHW tank through the tubes at night to drop the tank temperature sufficiently to allow heating capacity during the day and 2) plumbing in a hot water unit heater ("heat dissipater" function), also controlled by the Resol, to dispel excess heat when top of DHW tank reaches the trigger temperature. The freeze issues also were solved to deal with the potential -40F winter cold snaps, which challenged the antifreeze protection in the system. These involved two levels of hot water circulation through the tubes, both controlled by the Resol, and a fail safe level of heat tape activated by a separate controller.

Prior to these changes, overheat was dealt with by covering collectors, simple but cumbersome and subject to human error in failing to cover timely, and freeze was dealt with by draining and shutting down the system.

Fred Golden's picture

Kalim Khan,

It would be much more simple to cover one collector during the warmer months. If you can install the collectors near the ground, say on a south facing wall, it will make it really easy to control the overheating by covering one panel in the summer. Also tilting to a steep angle, say 60* will work well to increase winter collection, and drop summer collection.

If the system is on the roof, then heat dissipation is easier, like described above. Even several feet of 3/4" copper tubing in free air, and have that on the line going from the storage tank to the collectors, and open a valve to warm the 20' of tubing, or close the valve when you want to bypass the cooler line.

You could also bury a copper or PEX line in the ground, to warm the ground when you want to disperse heat in the summer. This will melt the ice from a driveway in Big Bear area. Just cover the water loop with 4" concrete, and it will last a long time.

Kalim Khan's picture

I intend to use 2 flat plate collectors for hot water and floor heating in a small house to be built @ 35' x 70' lot in a foreign country. The lowest temperature in winter drop down to zero degree C and highest in summer climbs up to 40 and some time 45 degrees C. Can I do my plumbing in a way to shut one collector during the summer?

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