Storing energy - Comparison of battery technogies

Be it to deal with solar energy storage or to deal with the load-shedding solutions, batteries can be an important part of a an energy solution.

New developments by manufactures have developed energy storage systems with a capacity of 10kilowatt hours capacity made from lithium batteries costing about US$3000.

These are very poor offers compared to older competitive solutions available to the home owner.

Battery systems are the most expensive part of any solar installation as they are effectively a consummable having a limited life and need periodic replacement.

The lowest energy cost storage systems for the home owner are;

  1. To consume the energy immediately as available by scheduling your power hungry devices to operate when solar energy is being converted - for example to top up heat in geysers by using time switches to switch on during the middle of the day.
  2. Grid tie inverters to pump energy back into the municipal grid and then draw it back during the night.

However if you do need to store the energy in an electrical form to use when no solar power is available, then you are going to use some batteries.

Battery systems

There are three different chemistries available to the home owner that can store energy. Although we are giving as an example 10KwH systems, for the home owner with load shedding limitations 1.2kwH will often surfice.

Lithium batteries
Life usage 5-8 years
Range of working temperature -20C to +65C
No of life cycles expected for discharge cycle at 80% = 1 000
Discharge ability at high current Normal
Work ability as a battery pack Normal
Cost value for money Much more than gel (US$3000/R54K for 10kwH)

Gel lead acid batteries
Life usage 3-4 years
Range of working temperature -18C to +50C
No of life cycles expected for discharge cycle at 80% = 500
Discharge ability at high current No good
Work ability as a battery pack OK
Cost value for money Low (US$1300/R24K for 10kwH)

Wet cell lead acid batteries.
Life usage 2-3 years
Range of working temperature -18C to +45C
No of life cycles expected for discharge cycle at 80% = 450
Discharge ability at high current No good
Work ability as a battery pack OK
Cost value for money Lower (US$1000/R18K for 10kwH)

There are other chemistries like NiCd and Nimh which are not suitable for high energy storage such as needed to operate a home. .

Battery densities
Lithium-ion 0.6Mj/kg
Nimh 0.28Mj/kg
Lead acid battery 0.17Mj/kg

Battery choice

  • If you need a light weight solution, then the wisest choice is Lithium chemistry. This is necessary for mobile applications with sophisticated charging controllers .

  • If you are not worried about the physical weight of the batteries but want batteries that will have a very long life and are very tolerant for charging/discharge abuse, then you need lead crystal batteries. These batteries are very heavy - almost twice the weight of lead acid batteries.

  • In articles we have read, users recommend using wet cell lead acid batteries compared to gel lead acid batteries for home storage if the lead crystal batteries are not available. The wet cell lead acid batteries do need periodic checking of electrolyte levels.

  • How much energy do you need to store

    The advantage of battery and solar systems is that they are modular and you can increase the capacity in modules when your budget can afford the expansion.

    But how big do you want to get?

    The key formula is

    Kilowatt hours storage(kwH) = Battery voltage (V)* Amp hour capacity (AH)/1000

    So if you have a 100 amp hour 12 volt battery - that can store 1.2kw hours of energy.

    In South Africa with rapid expansion of industries, we are short of municipal electricity at peak times, and there is a program of load shedding in operation by the utilities, where zones of customers are cut-off from supply for up to four hours in rotation. This means that we have a need for solutions for periods of four hours when our turn to be cut-off occurs. Luckily most of the country has lots of sun shine and a warm climate so we are ideally placed for a solar/storage solution. More power stations are being built but they take many years to come on-line.

    Typicfal needs and solutions are:

    There have been massive developments in portable lighting where lithium battery packs are built inside LED lighting systems allowing the batteries to be charged from solar or mains when not in use.
    Heating water
    A 200 liter 4 kilowatt heater coil will take about 2 hours to reheat a geyser using 8kwH of electricity. This can be replaced by a solar geyser using sun power to heat the water.
    Gas appliances can be used for cooking when a meal occurs during load shedding.
    Space heating
    A single bar heater would use 1 kilowatt to operate. Gas heaters can be used to supply heat during load shedding.
    Electricity - Some devices need electricity to operate.
    A freezer uses about 250 watts to keep operating.
    A fridge uses about 250 watts with frequent cycling
    Desktop computer uses 200 watts
    DSTV satelite decoders use 25 watts to operate
    A 40 inch smart LCD TV uses 70 watts to operate

    Getting help and advice
    This description is provided to show how easy it is to get going. I am not selling anything, just encouraging users to take the first step. You can contact me for help and advice from Mike at Tel +27 72 992 6040

    Also see an article on Cost effective solar systems for South African conditions
    Also see an article on Smart Home using solar systems
    Also see an article on battery storage systems
    Also see an article on Solar water heating - solar geysers
    Also see an article on Measuring systems for solar systems
    Also see an article on Gridtie inverters -Converting solar energy
    Also see an article on Stand alone inverters -Converting battery energy
    Also see an article on Energy equivalents

    12 April 2023