Storing and using energy

Caution

• Most forms of easily obtainable renewable energy sources and storage options are sufficiently powerful to cause injury or property damage if not installed and operated correctly. Obtain supervision if you're even slightly uncertain.
• Do not attempt to connect any generation or storage system to the grid without consulting a licensed electrician first. Not only is there significant risk of injury to yourself, you may injure others.
• Read and follow the safety and handling instructions accompanying any devices you obtain (even simple car batteries).
• An unconnected solar panel in the sun, or a battery with bare terminals is capable of injuring you. Be very careful around them and assume wiring is live.

Overview

Small scale solar, hydro and wind power systems can generate AC or DC at a variety of voltages. On the other hand, most common loads require household style single phase power (e.g. 230V/50Hz AC in New Zealand), or, perhaps 12V or 5V DC. The key requirements to understand in designing a storage system is therefore how to how to convert the input energy into a form that a battery charging and management system can use, and then how to best to extract that energy from the batteries for use. In general it is more efficient to convert the energy output from the batteries as few times as possible. That means if you have loads than can accept for example 12VC, that will be much easier to extract that needing to go via an inverter back up to AC.

Battery management systems (BMS)

A battery management system (often called a charge controller) takes care of both charging and extracting energy from a battery (or potential several batteries). While it is possible to build and integrate your own, it is far easier and less complex to obtain an off the shelf BMS. Most BMSes only accept solar power, can charge only one battery, and output 12 or 24V at a few hundred Watts. However such systems are very common and very reliable, simple to operate, and can manage different kinds of batteries (different batteries need to be charged at different rates and voltages even though they may provide similar output voltages - it's important to get a BMS that is capable of charging your particular batter).

A good option to start with is a 12V system, which means you might have one more 12V batteries (which must be of the same type and capacity if wired together). Such a system is simple to build with the BMS' instructions. However it will not be efficient past a few hundred Watts due to higher losses (higher currents/head lower voltage). 24V is a worth while step up, 48V is even better for larger systems. A BMS can accept a charge from most any DC source that is higher than the battery voltage and also below the BMS' limit (for example, you can charge a solar BMS from another 12V source that is not a solar panel).

Batteries

There are a wide range of batteries available for storage. Regular car batteries of various sizes are very cheap and easy to obtain. However they are very big and heavy, produce potentially dangerous gases while charging, tend to self-discharge when cold and are not capable of being routinely discharged below 80% capacity without damaging them. So called "deep cycle" batteries are available that can sustain routine discharges below 80%, but they are even heavier and more expensive.

There are also different kinds of Lithium-chemistry batteries (the phrase "Lithium battery" covers several different systems which have quite different designs and features and charging requirements). These batteries generally are much lighter and smaller and do not produce gases when charged, but are significantly more expensive, and should not be discharged below approximately 20% repeatedly without damage.

Most BMS cannot handle mixed battery types, so choose a battery type you can easily obtain and replace and operate safely (e.g. with adequate ventilation), and that your BMS supports, and that can accommodate your power needs (calculated in Amp Hours - the amount of energy you need to extract from them over time without damaging the battery).

Inverters

You will need an inverter, which converts DC power to AC for loads that cannot accept DC. Very sensitive loads (some computers) require a "pure sine wave" inverter, whereas less sensitive loads (lights) do not. Some loads involving motors or heating will require powerful inverters. A waterpump might use only a few hundred Watts when running, but to start it may require a kilowatt or more. Make sure your inverter can meet the requirements of the loads you have.

Wind and hydro BMS

BMSes for these energy sources have special requirements - ways to divert excess generation capacity. With solar cells a BMS can simply reduce intake. However motor-based generation sources require a way to dump any excess generation capacity to avoid burning out. A wind or hydro BMS generally has a resistor/heating element to accomplish this. Do not use a solar BMS for wind or hydro power. Some BMS can accept all kinds, though these are rare.