Energy storage on the cheap, your hot water tank

Here in Toronto, most homes have a hot water tank. Its mainly used to supply domestic hot water for laundry, dish-washing, showers and the like. But could it also be a source for energy storage? Lets do some math and find out. Indeed a 50 Gallon (200L) tank has the potential to hold about 5 kWh of thermal energy. Converting excess heat back into electricity is not easy, but some simple lifestyle tweaks can quite easily shift your energy usage to the wee hours.

Lets start with some physics, how much energy can a hot water tank store? Well, a quick Home Depot search suggests hot water tank heaters come in sizes of approximately 150, 200 and 300 l. To be useful, the coldest usable hot-water temperature is around 40 C, and the hottest should be no more than 60 C. It takes 4280 Joules to heat 1L of water by 1 degree (source). That is about 1.19 Wh (source). So for a 200L tank, going from 40C to 60C, why that’s 4.75 kWh, about a third of the 13.5 kWh of the Tesla Powerwall (source).

There is the thought of bumping the temperature way north of 60C. After all, if I could use 80, 90 or even 99 C, why that’s 4.75 or even 9.5 kWh more storage without any extra hardware. Well, hot water can burn you if it gets too hot. The table below outlines how long it takes to cause some serious skin damage. Anything more than 60C seems dicey to me. While it is possible to use a “scalding-switch”, which will mix the hot water in your tank, with cold water, there is always the possibility of failure, with the scalding-switch sending water that’s too hot into your hot water system (source).

Temperature (C)	Time to 1st degree burn	Time to 2/3rd degree burns	Energy storage in 200 l tank [kWh]
43	Warm shower		0.71
46	35 min	45 min	1.4
50	1 min	5 minutes	2.38
60	2 seconds	5 seconds	4.76
67	Instant	1 seconds	6.426

How can I tap into that energy? The best way would be to use it for heat, Harvest-thermal aims to do just that. Store heat energy in the hot water tank, which then is used to heat your home during peak-hours. This concept might make some sense here, in particular if you do not need a huge amount of heat, keep in mind that the 4.76 kWh of heat storage, only allows us to run a 1.5kW space heater for 3 hours, which for a small house might be enough to heat through that pricey Ontario-ultra-low-overnight 25 c/kWh 4-9 pm window. Further, you could opt for a 300 L tank, or say more advanced tech (see the Tepeo boiler source). The simplest approach is just to be strategic about how you use your hot water, perhaps a shower in the evening makes more sense than a morning shower?

Turning heat into electrical energy is sadly fairly in-efficient. Several techniques are available: Thermo-electric, steam-electric, or turbines. Sadly efficiency is poor, usually around 33%. Power-plants using various tricks, such as combined cycle, so lets say 50% as an absolute max utilizing a combination of techniques (source, source, source). This would render our “hot-water-tank” battery fairly impractical for storing electricity, as the capacity is cut in half, plus we loose half of the energy we put in.

Its worth noting that storing heat, can be a lossy process. Most hot-water tanks loose between 1.5 kWh and 2 kWh per day (source), that might be OK for a limited time, just to get you through that 4-9 pm window, but it certainly will not work for multi-day/multi-month energy storage.

A hot water tank can operate as a heat storage device, which might allow you to shift some of your heat use to take advantage of cheaper electricity, be it the ultra-low overnight rate here in Ontario, or solar power. The simplest might be some life-style tweaks such as trying laundry and showering in the evening instead of the morning.