Storing solar energy - Comparison of battery technogies

Also see an article on Grid-tie inverters and solar geysers
Also see an article on Energy equivalents

The recent announcement by Tesla (USA) and Mercedes (Germany) about plans to enter the home energy storage market was greeted with excitment by journalists. Figures quoted were that a solar energy storage system with a capacity of 10kilowatt hours capacity made from lithium batteries would be available for about US$3000.

These are very poor offers compared to competitive solutions available to the home owner and also ignore the fact that you have to add to these costs the cost of the solar panel installation to convert the sun power to the electrical energy stored in the batteries.

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 availalble, then you are going to use some batteries.

Battery systems

There are four different chemistries available to the home owner that can store energy.

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 for 10kwH)
Lead crystal lead acid batteries
Life usage 7-10 years
Range of working temperature -40C to +65C
No of life cycles expected for discharge cycle at 80% = 3 100
Discharge ability at high current Good
Work ability as a battery pack Good
Cost value for money Slightly more than gel midrange (US$1800 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 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 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 .6Mj/kg
Nimh .288Mj/kg
Lead acid battery .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.


    The lead crystal battery technology consists of lead plates, and an acidic solution of SiO2 as electrolyte. During the initial charge and discharge cycles the electrolyte solidifies and forms a non toxic white crystalline substance. This eventually results in a safe, fluid-less high performance, environmentaly friendly battery.

    The lead crystal batterys performance is superior to conventional lead acid batteries. With the following functional advantages:

    Battery life
    Lead crystal batteries have a float life of 18-20 years at 20C. Having a battery cycle life of 3 100 (charge discharge) cycles depending on the Depth of Discharge (DOD).
    Shelf life
    Lead crystal batteries can be stored for 2 years without additional charging. This impacts significantly on rotational charge and logistics.
    High-rate discharge
    Lead crystal batteries have a discharge rate of up to 10V, without significantly impacting on the battery life.
    Excellent charge performance
    The lead crystal battery has a charge time, 3-5 times faster than conventional batteries, when charged by conventional means, a generator or solar.
    Depth of discharge
    The lead crystal battery can be discharged to 0 Volt (100% DOD) and then restored to full rated capacity.
    Temperature resistance
    Lead crystal batteries operating temperature ranges from -40C to +65C. Due to the low internal resistance, the internal temperature remains low when charged and discharged. Tested and cycled at +41C ambient temperature resulted in only 23% loss of battery life, which is rated between 7 to 10 years. The battery delivers more than 85% of its rated capacity at -40C.
    A greener option
    Lead crystal batteries emit no mist or harmful, gaseous emission, as the basic electrolyte is neutral and non-corrosive. Lead crystal batteries will not cause pollution in line with ever increasing environmental protection requirements (ISO 14001 certification).
    Safe to transport
    Lead crystal batteries are classified as non-hazardous devices and are safe to transport by land, sea or air.
    Product range
    2 Volt, from 100 Ahr to 3 000 Amphour
    6 Volt, from 4 Ahr to 12 Amphour
    12 Volt, from 2.3 Ahr to 200 Amphour

    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.

    Our needs 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 useed 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 50 watts to operate
    A 40 inch LCD TV uses 156 watts to operate

    Also see an article on Grid-tie inverters and solar geysers
    Also see an article on Energy equivalents

    26 July 2015

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