First, we should distinguish between thermal and electrical energy storage. The latter is convenient because electricity is versatile and therefore valuable. However, systems that store electricity are typically quite expensive. Whereas thermal energy storage is relatively inexpensive, but its potential uses are far fewer. Both types have their place and can be used to buffer some of the peaks and troughs in energy production and consumption.
When we’re talking about thermal energy storage, anything that has a high thermal mass that you can easily input and extract energy from can be used. Large tanks of hot water are a great option because many conventional heating technologies are already adapted to using hot water and there are many choices for heating the water. Space heating systems that employ the use of thermal storage have a broad range of options for how that stored thermal energy is used.
For example, lets imagine a district energy system with a thermal storage lagoon that is tied to a ground source heat pump system. The buildings tap into the thermal storage system with individual heat pumps that lift the temperature to domestic needs. The community employs solar PV, wind turbines, as well as tying into the grid for electrical energy. So, the factors we need to consider are grid energy price, solar insolation, wind, air and ground temperature, as well as the behaviour of those occupying the buildings. We want to be able to charge the lagoon with the ground source heat pumps when we have solar or wind energy or when the grid electricity is relatively cheap. However, we should also be mindful of when the energy will be needed by looking at community usage behaviours and weather forecasting.
You also need to decide what sort of temperature range you want the lagoon to achieve. A warm lagoon with temperatures between 20 and 50 degrees Celsius will require precautions against legionella, a separation between the system and domestic water would likely be needed. If the temperature range is higher, above 60 degrees, it should be safe from bacteria, but will be difficult to insulate adequately and will likely result in higher losses. You may also strive for a shifting temperature range throughout the year to account for shifting community demand. If the thermal storage’s main use is space heating, then it would make sense to lower the temperature range moving into the summer to account for increased cooling demand. Doing so would also help recharge the geo-field with heat.
As you can see, there are many factors to consider when deciding how to employ thermal energy storage. Your control algorithm might have a hierarchy that prioritizes maintaining the desired temperature band and then charging when surplus wind or solar is available. When grid electricity is cheap, it can be used to charge the lagoon if bad weather is forecasted. However, when the electricity on the grid is expensive and the forecasted weather is temperate, you might choose to sell surplus solar and wind energy to the grid rather than charging the lagoon. Depending on how complex you want to get, there are some interesting control schemes that integrate the different system potentials being employed.
The case is almost completely different when it comes to electrical storage. For one, you aren’t dealing with temperature, at least not directly, and so all this concern about temperature bands is completely irrelevant. Here, all we really need to worry about is how electricity is coming in, how the storage system is going to keep it there, and where it’s going once it’s gone. Obviously, when we’re talking about electricity the source is going to be electrical, but what’s the voltage and current? Is it alternating current or direct current? Is the signal regular? Depending on the generator and how it’s being transported, an electrical signal can behave in a wide variety of ways, some of which can’t be used by energy storage devices. For the sake of simplicity, lets assume our signal is like that we see on the grid. This just means that any renewables we’re talking about are linked to some sort of inverter technology that produces the same sine wave it sees on the grid.
Once you have a regular electrical signal coming into the energy storage device, it can be designed to work with that signal. Different energy storage technologies work better in different usage conditions. Energy storage devices vary in their desired charge and discharge rates, the amount of energy they can store, how far they like to be charged or discharged, and how efficiently they store energy, to name a few. Something like an ultracapacitor can’t hold as much energy as a battery or pumped hydro, but it can dispatch its energy very quickly. Furthermore, they can remain charged without constantly losing a lot of energy. This makes a charged bank of capacitors good at responding to momentary dips in the grid that only last a few seconds but require near instantaneous responses. Something like pumped hydro is quite different. The efficiencies of energy input and extraction (pumping water and running a turbine) are significantly lower than charging a capacitor. However, you can keep a reservoir of water elevated for extended periods of time virtually without storage losses. This makes pumped hydro storage better at taking surplus renewable energy generation and storing it for days, weeks, or even months until it’s needed.
Depending on the desired use for the electricity being stored and how that electricity is coming in, the choice of electrical energy storage will vary. If you’re in Canada, trying to take the surplus of solar energy in the Summer and use it in the Winter, obviously you want something with low storage losses. However, if you’re main renewable energy is coming from wind in an area with relatively consistent winds throughout the year, your storage system might be doing more hour to hour or day to day work and so a high transfer efficiency may be more important. Perhaps all the system is doing is buying grid energy when it’s cheap and selling it when it’s expensive. Then you’re going to want a high rate of charge and discharge so you can buy and sell plenty of energy during the troughs and peaks. Energy storage devices are highly site specific, like many renewables. However, their site constraints are generally the nature of the generators and loads that are tied to them. That said, pumped hydro usually only makes economic sense when the landscape lends itself to the creation of an elevated reservoir.
As the grid changes from central and dispatchable generators to distributed and renewable generators, the energy storage requirements will change in kind. Right now, operators don’t really have much need for energy storage. What capacity exists is typically used to ride out times when generated energy is relatively expensive. However, once the grid is populated with more renewables, it will likely be necessary for grid operators to set up strategically located energy storage banks to buffer the intermittent generation of solar and wind.
If this hasn’t been confusing enough, let me conclude by saying that thermal and electrical energy storage are far from our only options. Finding ways of preserving the summer harvest to be eaten throughout the winter was one of the first ways we started storing energy. The plants act as our energy input devices, taking in solar energy and storing it in fruits, veggies, roots, stalks, and the like. Our methods of preserving the food increase the storage efficiency and our bodies act as tools for extracting the chemical energy.