Storing Renewable Energy

Rob Bell

Issue 1, July 2017

We’re all well aware of the limitations of renewable energy, especially at a home-installation level. Government initiatives and persuasive marketing has seen Australia claim one of the highest per household uptakes of solar panel installation. This means we’re generating massive amounts of solar electricity through the day, but relying on a traditional power grid at night.

The Unseen Buffer

The one thing we don’t really talk about is the way many installations have been setup. Essentially, they feed excess power into the electricity grid during the day (at a continuously reducing rate) and draw energy back from the grid at night (at standard retail rates). This is terribly inefficient and has a major Achilles heel because if the main electricity grid fails at night (or in extremely poor weather), your renewable energy isn’t likely to help you.

With grid power prices ever-increasing and the feed-in tariffs falling closer and closer to zero, it makes financial sense to look for alternatives. Beyond the issue of dollars though, if we can create a localised power storage method for our own solar power, we could remain totally self-sufficient even if the main grid goes offline. Of course, this also means you could operate from your renewable energy anytime – even when the sun isn’t shining and the wind isn’t blowing.


Solutions such as a 14kWh Tesla Powerwall 2 are making vast inroads to allowing us to store and use our captured solar power. Storing electricity – in what is effectively a giant battery – makes a lot of logical sense. But what else is there?

Electrical energy doesn’t have to remain as electricity. It’s all energy and we can convert energy from one type to another. In any energy conversion there is always an overhead; that is, energy is lost during the conversion. Even when electricity is stored in a battery, energy is lost in the conversion to chemical energy and back to electric energy.

There is a growing number of experiments (and indeed, viable commercial installations) testing different ways to store this energy.

GRAVITATIONAL Potential as Storage

One idea that seems interesting at a DIY level is converting excess energy into some sort of gravitational potential energy, energy contained in a mass at an altitude. By lifting an object we add height and therefore GPE. When the object is allowed to fall, gravity potential is converted into electricity. By using electrical energy to raise the mass, and then generating electricity using the change in height of the mass, electricity can be stored for use when solar/wind generation is offline. This provokes a very interesting idea. What if we could create a mechanism to utilise gravitational potential for our energy storage?

Water Storage

Pumped hydro storage has been used in the power industry around the world (e.g. Wivenhoe Dam, QLD), but what if the householder could use renewable energy to pump water into a raised tank? You have a large water tank down low and another up quite high. During the day, the water is pumped from the lower tank to the higher tank. At night, gravity forces the water in the high tank to flow back to the low tank. We can utilise that energy to drive a turbine (depending on the pump, it could be used in reverse as a generator too).

The result? We’re using gravity the same way a hydroelectric dam does. The biggest difference here, however, is that we’re not using weather to send the water to the top tank – we’re using a pump; of course, the volume of water being moved is quite a bit less too! The height between the two tanks and the volume of water being stored would determine generation capacity. To store 1kWh of electricity, the pump would need to raise 1000L of water 367m, or 100 tonnes of water 3.67m... assuming perfect efficiency. With those numbers in mind, we have to consider if it's even a real-world solution.


We're not about to go installing these on a flat quarter acre block. But what about apartment buildings? A 10-floor apartment block has, say 30m of height from top to bottom - 40m if we include a few levels of basement parking. A 100kL water tank (not difficult to include in building design, but i suspect wouldn't be possible to retrofit) could provide around 10kWh of energy. In an inner-city area, perhaps this is just a ridiculous notion. That's a whole lot of water, and 10kWh will not go far in an apartment building. But not everything happens in the city.

What about a small farm in a hilly part of the countryside, where grid power can have reliability issues due to distance, and high tech solutions might lack the support. In a rural area, it's not as difficult to find somewhere suitable for this type of installation. A 50kL tank (basically the size of a suburban swimming pool) installed with 100m of elevation from the lower tank, can provide that same 10kWh of energy, which is plenty for even a large house. Indeed many farms already have these tanks in the form of water dams, often with capacity far greater than 50kL.

The renewable solar or wind capacity required to provide this power for "recharging" (plus required daily use) is easily installed too, being no larger than many existing rooftop solar installations.

Sound ridiculous? Sure, but some of the most creative solutions come from the most ridiculous ideas.