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Comment by 3eb7988a1663 | Hacker News Reader

A suggestion I liked was to time-shift water heaters as a thermal battery. Water heaters are a significant sink (~20%) of household energy. They are already installed and well insulated, you just want to over-heat them during cheap noon solar to off-set the heat losses over the day.

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This is basically how solar hot water heaters work today (I have one). They are superheated during the day (75C/170F) with a mixing valve to produce warm, but not scalding, water to the tap. The tanks are also oversized with a backup electric heater for the top half of the tank.

It works pretty well for my climate. I wonder how it compares to PV panels driving an electric heater though. Neither is going to work very well during the winter but the hydronic based system probably needs to get above some base level of solar input to meaningfully heat the water. Whereas PV will always produce some electricity in the winter which can always be converted to heat.

this seems like it makes sense. how do you keep the dispensed water from being too hot? is there such a mechanism?

There are mixing valves which combine the too-hot water with cold tap water to produce hot water at a safe temperature.

> It is increasingly common practice around the world to regulate the storage water temperature to above 60 °C (140 °F), and to circulate or distribute water at a temperature less than 50 °C (122 °F). Water above these temperatures can cause scald injuries. Many countries, states, or municipalities now require that the temperature of all bath water in newly built and extensively refurbished domestic properties be controlled to a maximum of 48 °C (118 °F). Installing thermostatic mixing valves can ensure that water is delivered at the required temperature, thereby reducing the risk of scalding accidents; it also reduces hot water consumption from a supply that is maintained at a higher temperature.

https://en.wikipedia.org/wiki/Thermostatic_mixing_valve

I see these a lot in commercial domestic hot water systems, a lot of them use steam or boiler loop water to heat the domestic hot water.

When we bought our house we had a heat pump water heater installed. The nominal tank water temperature is 80°C, and it mixes it to the desired temperature on output. It can also circulate water in a heat exchanger to keep your bath water from going luke-warm while you're in the bath.

We don't have solar but we do have a solar energy plan from our power company, so power during solar hours is charged at 0.5x rate in trade for the solar during the duck curve hours being 2x. So we have the hot water heater set to only heat during the solar hours and A/C set to shut off during duck curve hours.

This has resulted in our electricity costs for our house being lower than the combined electricity+gas costs for the apartment we lived in before (which was much smaller).

Mixing valve. Lets me use my water heater like it's 20 gallons larger than it actually is.

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