Thermal mass of drums of water on top of a stack of concrete blocks

David Delaney
November  23, 2003

In a comment on A two-part heat store with large heat-transfer surface area, large thermal mass, and low resistance to air flow, Nick Pine suggested placing the 55 gallon drums of water on top of the concrete blocks. (See the thread, "Thermal mass for large thermosyphon solar air heater",  in starting on November 14, 2003.)

I found this suggestion tantalizingly attractive from the first, because it could lead to a higher average temperature in the water and a lower average temperature in the concrete,  potentially increasing both the efficiency of  the air heater and the amount of energy held in the store, but two negative effects of placing the drums on top of the concrete blocks bothered me. This article shows how to avoid the negative effects. I believe this is the best realization of the idea ot the combined concrete block and water drum thermal mass.

The first effect: When the air heater is hot, the much greater surface area of the concrete causes the concrete to cool the air in the heat store much faster than the drums can cool it. The hot air of the lower part of the heat store is quickly cooled by the concrete and replenished by the air heater, while the hot air of the upper part of the heat store is cooled much more slowly by the water drums and is replenished equally slowly by hot air risng from the lower part of the heat store. Nonetheless, the relatively cool drum bottoms are immediately above the concrete blocks.   If the top of the block stack is permeable to vertical air movement, cool air descending close around the drums will fall among the upper concrete blocks. The presence of the cool air prevents contact between the concrete blocks and warmer air at the same level of the heat store, wasting the capacity of the concrete blocks to absorb the higher temperature energy.

The second effect: unnecessary entropy creation. During a good sun day, especially after a sequence of poor sun days that have allowed the water drums to cool, the temperature of the concrete rises above the temperature of the water, even though the concrete block stack is below the water drums. At night after such a day, when the air heater is cool and the concrete is hotter than the drums, there are three sources of a high rate of entropy creation.  1) Air cooled by the relatively cool lower sides and bottoms of the drums falls among the hot upper blocks of the concrete block stack. 2) Air heated by the hot upper blocks of the concrete block stack rises to contact the cool lower sides and bottom of the drums. 3) Thermal radiation from the top of the hot concrete block stack impinges on the cool bottoms and sides of the water drums. It is both desirable and unavoidable that the concrete blocks transfer much of their heat energy to the drums at night, with some unavoidable entropy generation, but these three mechanisms generate much more entropy than necessary (they reduce the temperature of the energy being transferred unnecessarily).

These effects reduce temperature stratification in the concrete and in the water, making the energy stored in them less available. They might negate much of the benefit of placing the drums on top of the concrete.  There is a simple and inexpensive way to almost completely eliminate them.

First we place a thin horizontal partition between the concrete block stack and the drums to block vertical air movement and thermal radiation between the drums and the concrete.  The partition prevents cool air from falling from the drums directly into the hot upper concrete blocks. It also prevents hot air and thermal radiation from the hot upper concrete blocks from rising to impinge on the cool lower sides and bottoms of the drums. A single thickness of thin plywood might do for the partition, or a sheet of a insulating material,  polyisocyanurate foam, say. A single sheet of polyisocyanurate foam foil-faced on both sides would provide a very good radiation block.The drums should be raised 1-1/2" or 3" from the partition by supporting them on sections of 2"x4" or doubled 2"x4" running north-south on top of the partition.   The partition itself should also be raised on doubled 2"x4" to allow air to enter the top of the concrete block stack.

Next, we arrange a short vertical fence rising from the south edge of the top of the concrete block stack and extending for the full east-west extent of the concrete block stack.  (Drawing below.) This fence prevents air cooled by the drums from falling on the south side of the concrete block stack.  The cool air falls instead on the north side, or to the east and west.  The result is an organized flow of air in which hot air rising from the concrete block stack does not mix with cool air falling from the drums of water,  or rub against the cool lower parts of the drums. Instead the hot air from the concrete rises beside the drum stack to a level of neutral buoyancy before contacting the drums at a level where the temperature of the air is only slightly greater than the temperature of the water. See the air flow patterns below.

These arrangements allow a greater temperature difference between the top and bottom of the concrete by day, and reduce the average temperature difference of energy transfer from the concrete to the water at night,  maintaining and increasing the stratification of the water temperature. Entropy generation is reduced. The availability of the stored energy is increased.

Since the water drums absorb heat only slowly from the air, and air to warm the house is drawn from the top of the heat store, the warmest air from air heater, the concrete block stack, or the water drums will always be available for use to warm the house or preheat domestic hot water.

The details of the stack of concrete blocks may be found in "A concrete-block thermal mass with large air-concrete surface area".

For the context of this work, see Solar thermal energy for housing, and A two-part heat store with large heat-transfer surface area, large thermal mass, and low resistance to air flow

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