Water heater, refrigerator, HVAC as energy storage
There was a YouTube video on using equipment as energy storage through programmable thermostats for HVAC, the water heater, and the refrigerator. Since these devices work on a temperature range, the set temperature can vary based on available renewable energy. It could also respond to times with low energy cost from the grid. Some examples of temperature settings:
Refrigerator: 34-38 degrees F
AC: 70-76 degrees F
Heat: 68-74 degrees F
Hot Water: 120-140 degrees F
The water tank level would be heated based on the history of demand:
Low cost: max tank, max temp
Med cost: medium tank, medium temperature
High cost: min tank, min temperature
PV available: full tank, max temperature
The HVAC and refrigerator would also respond to the type of electricity available by varying the set temperature. This would reduce energy use and serve as a “battery” for renewables. Do such devices exist?
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Yes, it's easy to set things like this up, although making them fully "smart" is more complex.
I would not bother with the refrigerator. The enclosed mass is so small that the energy "stored" over the very small temperature swing that is allowable (remember that the primary concern in your fridge is keeping your food from going bad), is too small to be worth bothering with.
The hot water tank is an easy one, and you can stuff a fair amount of energy into it. The simplest way to make this work would be to tie into your solar system so that the water heater runs two stages (assuming it has two elements) when on solar, and only one stage the rest of the time. Fancier control would require a fancier controller. It wouldn't be difficult to build, but I'm not aware of any that are commerically available.
Heat and A/C are also pretty good for this, especially if you can tolerate larger temperature swings in your home. I have, for a long time now, used a simple set back thermostat (now a fancier EcoBee "smart" thermostat) to precool my house in the early morning when power is cheaper (I'm on a "time of day" electric rate that is more expensive between 11am and 7pm on weekdays during peak time. I save money with this rate). The precooling means the A/C doesn't have to run as much during the more expensive on-peak power time. I could do the same thing with heat, but haven't bothered since I heat with natural gas. The EcoBee thermostat recently added a function to be smart about the utility's time-of-day rates too, so it can automatically optimize around that instead of needing a user program the way I used to do it.
BTW, this isn't really acting as "storage", it's closer to what is known in the industry as "peak shave", which is a way to reduce peak demand. Sometimes this type of thing is implemented with additional generation (which could be solar), but other times it's implemented by shifting demand patterns to "spread out" the peak. In the commercial world, so-called "ice chillers" were used to do this, which allowed large commercial properties to make ice at night with cheaper off-peak power, and then thaw that ice during the day to air condition more economically when the power was more expensive. Since the ice chillers also involved phase change (water into ice and back), they could store significantly larger amounts of energy compared to what we're talking about with precooling our houses.