Electrical and Thermal Storage

Our calculations thus far have studiously avoided the largest component of BMR and building energy use generally - hot water and how to make it. We know in addition to the previously noted need for 30+ gallons of hot water for showers or other DHW uses per day, we will need hot water to circulate in the hydronic heating system. How much? This is a fairly complex question, and modeling software would be needed to answer precisely.

However, we can exploit the fact that hot water can be stored with minimal energy loss, at least for short periods. This suggests a load-shifting strategy where water is heated when power is more plentiful (during PV 'working hours'), and used later when it is not (overnight). Thermal storage in combination with battery (electric) storage can significantly help close the gap between energy production and consumption, and while the energy density (by weight or volume) of heated water is less than, say, Li-ion batteries, its cost is vastly less. And fortunately, current air-to-water heat-pump technology has become quite efficient (one data point: the Sanden SANCO2 boasts a COP of 5.0, and ability to operate at -20 degrees F), so the contribution to BMR is greatly reduced.

Interestingly, such a (short-term) thermal storage strategy can be adopted in cooling climates as well: products such as Ice Energy's 'Ice Bear' can freeze water during off-peak times, and use it for cooling at peak times.

And should it be necessary, we can reply on additional electrical storage: portable electric storage in electric vehicle batteries. Technologies such as EV 'V2G' or 'V2H' (vehicle to grid/home) could allow us during the most challenging energy production periods (for PV, late December to January) to supplement our local thermal and battery resources with substantial energy - conceivably as much as 20-30 kWh per day, if charging is nearby, and we are willing to endure a bit of inconvenience for the few occaisions when it would be necessary. V2G could also be an effective strategy at the other extreme - when excess energy is produced (long summer days), it could be exported to the grid.

While we have not quantitatively established it, we have outlined enough considerations to build a plausible case for believing that the storage demands of the reactive house BMR might not be completely out of reach, given current technology.