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FWIW, I spent some time studying and experimenting with thermal storage systems in the late '70's/'early '80's.
One thing I learned several times was the concept of a thermal time constant, somewhat similar in concept to charging (discharging) a capacitor through a resistance in an electrical circuit with the voltage change as f(time) analogous to the temp. change, inside to outside.
Cut to the chase: A dwelling with a large thermal mass will change internal temp. slower than one with a smaller thermal mass. Also, a dwelling with a small total heat loss coeff. (i.e.,well insulated) will change internal temp. slower than one with a larger total heat loss coeff. The building with a large thermal mass and a low total heat loss coeff. will change temp. much slower than either of the others somewhat proportional to: (Effective Building Thermal Mass/Total Building Heat Loss).
Using the analogy of the thermal mass to a capacitance and the (1/total heat loss) to a resistance, most stick/tract type dwellings have building time constants of the order of very approx. ~ 12-24 hrs. or less. High mass passive solar designs can have time constants of the order of several days or more. Seasonal storage dwellings have effective time constants on the order of 6 months - Store in the summer- use in winter - possible but usually impractical and not cost effective.
A common misconception is that high mass alone equals low heat loss. Not true - in act, usually the opposite because most brick/stone/masonry is a lousy insulator, pretty much regardless of thickness, and so a high heat loss --->>> high heat loads --->>> high bills.
Water is the most compact storage medium and is (was) the most common. Rock bins/ pebble beds are OK, but may have long term problems with mold and critters after a while. Think access to the middle of a 20 X 20 rock bin that's 8 ft. deep. Heat distribution in pebble beds can be a tricky issue as can flow rates, and as f(partical size/shape/void fraction, etc.). Also, it's difficult/impossible to add and withdraw heat from a bed simultaneously.
On a retrofit, the most practical, common and cost effective methods to reduce a dwellings HVAC demand is to insulate the living crap out of it, and tighten it to the safest level possible from an air quality standpoint. BTW, air/air HX are a nice idea, that doesn't usually produce effective results or trouble free operation (read, lousy performance and real maint. issues, including health problems). Adding thermal mass in an effective way, in the quantity necessary to be effective beyond a feel good placebo sort of way is pretty difficult. One exception to that might be effectively insulating a brick, stone or masonry dwelling on the outside. It's a tough job with lots of details, and may involve what's called environmental trashing, but again, it's possible.
One thing I learned several times was the concept of a thermal time constant, somewhat similar in concept to charging (discharging) a capacitor through a resistance in an electrical circuit with the voltage change as f(time) analogous to the temp. change, inside to outside.
Cut to the chase: A dwelling with a large thermal mass will change internal temp. slower than one with a smaller thermal mass. Also, a dwelling with a small total heat loss coeff. (i.e.,well insulated) will change internal temp. slower than one with a larger total heat loss coeff. The building with a large thermal mass and a low total heat loss coeff. will change temp. much slower than either of the others somewhat proportional to: (Effective Building Thermal Mass/Total Building Heat Loss).
Using the analogy of the thermal mass to a capacitance and the (1/total heat loss) to a resistance, most stick/tract type dwellings have building time constants of the order of very approx. ~ 12-24 hrs. or less. High mass passive solar designs can have time constants of the order of several days or more. Seasonal storage dwellings have effective time constants on the order of 6 months - Store in the summer- use in winter - possible but usually impractical and not cost effective.
A common misconception is that high mass alone equals low heat loss. Not true - in act, usually the opposite because most brick/stone/masonry is a lousy insulator, pretty much regardless of thickness, and so a high heat loss --->>> high heat loads --->>> high bills.
Water is the most compact storage medium and is (was) the most common. Rock bins/ pebble beds are OK, but may have long term problems with mold and critters after a while. Think access to the middle of a 20 X 20 rock bin that's 8 ft. deep. Heat distribution in pebble beds can be a tricky issue as can flow rates, and as f(partical size/shape/void fraction, etc.). Also, it's difficult/impossible to add and withdraw heat from a bed simultaneously.
On a retrofit, the most practical, common and cost effective methods to reduce a dwellings HVAC demand is to insulate the living crap out of it, and tighten it to the safest level possible from an air quality standpoint. BTW, air/air HX are a nice idea, that doesn't usually produce effective results or trouble free operation (read, lousy performance and real maint. issues, including health problems). Adding thermal mass in an effective way, in the quantity necessary to be effective beyond a feel good placebo sort of way is pretty difficult. One exception to that might be effectively insulating a brick, stone or masonry dwelling on the outside. It's a tough job with lots of details, and may involve what's called environmental trashing, but again, it's possible.
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