Heat batteries were tried in furnaces a couple of decades ago, but they used some sort of high heat capacity bricks and forced the house air through them. As GP asks, the matter of maintenance was forgotten.
Recently I heard someone is trying this again with water heaters. The working fluid has roughly four times the heat capacity of water, so your “water” heater is substantially a fluid heater with either no water tank or a very small one. Incoming water is heated with the fluid, and the fluid is heated on demand. Only the heat exchanger has maintenance concerns.
Driving the whole thing with a heat pump makes the math a little more fiddly, since peak pump efficiency and peak power prices overlap some of the same parts of the day. You’re calculating BTUs per dollar, not watt hours per dollar.
Recently I heard someone is trying this again with water heaters. The working fluid has roughly four times the heat capacity of water, so your “water” heater is substantially a fluid heater with either no water tank or a very small one. Incoming water is heated with the fluid, and the fluid is heated on demand. Only the heat exchanger has maintenance concerns.
Driving the whole thing with a heat pump makes the math a little more fiddly, since peak pump efficiency and peak power prices overlap some of the same parts of the day. You’re calculating BTUs per dollar, not watt hours per dollar.