As the sun begins to sink lower in the sky, solar BTUs available each day actually increase. This is because the rays begin hitting the collector panels more squarely – less sun heat glances off the glass panels.

On September 17th (or maybe Oct 1, or Labor Day, or…, experience will help pick this “start” day in the future) we flip a switch to activate the large collector bank and turn a valve to shut off the earth heat dumping loop. Now we will start collecting heat from both collector sets.

When heated collector fluid (water and a low percent of propylene glycol) returns hot from the collectors, it first heats the water in the 300 gallon storage tank, as it did all summer. Until the tank’s stored water reaches 130 degrees, the collector fluid is sent directly back to the collectors for more heat. The reason for this strategy is to maximize the availability of useable heat for domestic hot water and targeted space heating. Tepid, 80 degree water is not very useful.

When domestic hot water is called for, potable water passes through a heat transfer coil located toward the top of the 300 gallon storage tank. Heat stratification in the tank assures that this coil will run through the hottest water. Water exiting this exchanger will most always be hot enough for showers – even if it isn’t always as hot as we’d like!

An option is to have a small, 25 gallon, “top-off” electric water heater directly after the storage tank. This would assure 10-20 gallons of always hot water is available for showers. Because this small tank is electric, it can be very well insulated to minimize stand-by losses. It will only add heat when the entering water is less than 115 degrees. And, very importantly, it can be turned on and off, depending on the situation. While we want to live within our solar means, I expect, if we have the option, we will run short term deficits. A big advantage of being “grid connected” is that you can balance your books – solar heat and electric – annually, rather than for every sunny/cloudy cycle.

When the storage tank reaches 130 degrees, a valve automatically opens, directing the collector fluid through the PEX tubing at the bottom of the sand and concrete Solar-Battery beneath the basement floor before returning to the collectors to gather more heat. This is longer-term heat storage. The mass is relatively cheap compared to water mass and it will provide general heat throughout the house, initially heating much of the space and later providing higher minimum temperatures, especially in the basement.

There are several reasons for the combination of wet and dry thermal storage – see Solar Concepts, Solar Thermal Storage. The main reason is to maximize our harvest of solar energy. After the water storage tank reaches 130 degrees, collector fluid exiting the storage tank’s heat exchanger temperature will exceed 140 degrees. The laws of thermal dynamics dictate that this fluid will collect less heat from the sun than if its temperature was significantly lower, which it will be after giving off much of its heat to the concrete and sand Solar Battery.

During this stage the primary heat demand will be the bathrooms in the morning. Though not much heat is needed, it is nice to have a warm floor for morning showers.

In the beginning of the annual heating cycle, the Solar Battery average temperature will be a fairly uniform 60 degrees – a long-term balance between the earth’s temperature (50 degrees) and the house’s internal core temperature (70 degrees). When we start charging the battery in September, after taking a long slow path through the bottom of the Solar Battery, the collector fluid will return to the collectors at about 75 degrees. At the end of October, when the Solar Battery is fully charged, the water will return to the collectors at over 90 degrees – still low enough to reap high efficiency heat collection.

It will take a couple of weeks before we start sensing any heat in the basement. At first the extra heat will be minimal, and because the room itself will be fairly warm, about 68 degrees, the heat will flow very slowly through the sand/concrete into the space. Thus the battery will be hottest toward the bottom and slowly warm toward the top. During the later days of October, we will keep basement windows open to prevent the space from becoming too warm. The Solar Battery temperature should peak sometime between November 1st and the 15th. This is when we will start drawing more heat, on a daily basis, from the Solar Battery than we are putting back in, on average.

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