Storing solar energy in the form of hot water is an effective and low-cost way of maximising the use of generated excess PV electricity.
Australia’s net-metering arrangements and despicably low feed-in-tariffs typically mean that the large amounts of excess PV electricity generated during the middle of the day – when residential demand is close to its lowest levels – effectively goes to waste, at least from the PV owner’s end. We pay in the range of $0.30–0.60 per kWh to retailers at peak times, either as blended or as time-of-use rates, while only getting back on average something on the lower end of $0.05–0.10 per kWh of PV electricity we export at any time of the day.
This means that if you manage to export the same amount of electricity back to the grid as what you consume within a metered time period, as a PV consumer on this arrangement, you will still pay the difference between the retail rate and the $0.05–0.10 feed-in-tariff per kWh consumed.
Perhaps this is not a fair ‘apples-for-apples’ or ‘electron-for-electron’ comparison but, to me, it still looks like a dodgy, one-sided deal. There’s very little consumers can do to enforce a swift change in this unfair pricing structure – I don’t expect current policy-makers to show any initiative on this front – so it’s no wonder retailers and energy providers are paying PV consumers as little as possible for the services they could potentially be providing to the grid.
There are, however, two simple things PV owners can do to work around this pricing imbalance: (i) shift loads to peak PV generation periods of the day; and (ii) store that excess energy created in peak generating periods for use later (e.g. in batteries). This article will describe ways in which excess PV electricity can be stored in the form of hot water using both of the above-mentioned strategies.
Make hot water while the sun shines
The basic idea is to transfer excess generated solar energy to an electric* water heater when onsite generation is higher than the load. This effectively displaces the need to heat water using higher-priced electricity from the grid and ensures that all the generated solar electricity is used.
A simple timer, programmed to turn on a water heater in the middle of the day when the sun shines at its brightest, can perform this function quite well. It’s simple, relatively easy to use but has the disadvantage of not being able to respond accordingly to changes in onsite electricity use. Some estimates have this set-up costing between $200–250 installed and paying itself back within 1-2 years for average Aussie homes.
There are also much more sophisticated programmable solar power diverters, such as the SolarImmersion and SunMate units, currently available in Australia. They are packed with control gear which senses the flow of electricity into and out of the switchboard and diverts any excess electricity to a water heater, with grid export as a last resort. These units are responsive to changes in the net load but are more expensive, coming in at a figure of $1,000–1,100 installed and likely paying back in approximately 3–5 years at current rates. This option tends to make sense only for PV households with quite high hot water needs or a well-integrated load monitoring and control system.
How much energy can a hot water tank hold?
The amount of heat that can be stored in a hot water tank is related to the heat capacity of water. The equation below outlines how to calculate the amount of heat stored in water at a specific temperature:
– Q is the amount of heat energy added/stored in the tank (Wh);
– c is the specific heat of water (1.163 × 10-3 Wh/°C.g);
– m is the mass of water (g); and
– ΔT is the change in temperature of the water (~ 20°C to 60°C).
A large 400L tank can effectively store ~ 19kWh of USABLE energy, which is more than double the NOMINAL capacity of Tesla’s new daily-cycle 7kWh Powerwall unit. And if the hot water storage system is fitted with a tempering valve, the USABLE storage capacity of the 400L tank increases to ~ 33kWh with water stored at 90°C.
I’d say this is something also worth getting excited about. The bottom line is that most PV owners don’t realise that they already have a multi-kWh energy storage sitting at home right now! Waste not, want not.
* For consumers with gas water heating, an upgrade to a heat pump system is a high-efficiency option which may be worth considering.
Top Image Credit: Pipe Safe Plumbing
© 2015 Solar Choice Pty Ltd