Energy Storage

Solar PV Energy Storage – Batteries

If you are not connected to Mains (Grid) power, or the Mains power is unreliable, you may need to store power from your solar panels Solar PV system to use when the Mains is not available, or at night.

My Solar Power system has 2 separate systems. We have a Grid Connect Inverter for a 1KWp solar PV array and we have a Stand Alone power system, which comprises a 3.6KWp solar array, a MPPT battery charger, a large battery bank and an inverter to power our 240V appliances. I will include photographs of our system at a later date.

We have a power source selection switch to allow us to control the source of our 240V Power. We can either select Mains (Grid) power or we can select Inverter (Solar) power.

The switch also has an OFF position between these two sources.

Most of the time we use Solar PV power from the Inverter, but when we do not have adequate sunshine, or our usage is high, we select the Mains power source.

This article concentrates on the different batteries available for Solar Power storage.

Batteries:

There are 3 main types of batteries used for Solar PV Power storage;

  • Lead – Acid
  • Nickel Cadmium  (NiCd)
  • Nickel – Iron (NiFe)

Lead – Acid

These are the most commonly used storage batteries for Solar PV Storage, due mainly to their relatively low price.

Lead – Acid batteries can have a long service life of between 15 and 25 years in Float applications and between 5 and 10 years in solar applications. Their “self-discharge is low at about 40% per annum. The lead in these batteries is recyclable, but lead is a toxic metal, so end of life recycling is important.

There are 3 technologies available:

  • Vented (Flooded) Cells – These have liquid electrolyte in them. This then has the risk of leakage or spills from the battery. Vented cells are suitable for use in high temperature locations, and in Solar applications have additional electrolyte capacity to cater for regular (automatic) equalisation charges. They also require regular maintenance (checking electrolyte levels, cleaning terminals, recording cell voltage and Specific Gravity.
  • Gel Cells (Sealed Lead Acid – SLA) – These overcome the risks of electrolyte leakage by having the electrolyte in a gel form.  These batteries have a relatively long life and most manufacturers have overcome terminal corrosion issues. They are lighter than equivalent capacity vented cells. They batteries are suitable for moderately high temperature locations as there is adequate electrolyte.  Some manufacturers produce GEL cells specifically for Solar power applications. These batteries will have a maximum cell voltage specified by the manufacturer.  Operation above this voltage will cause excessive “gassing” and this may cause the pressure valve to vent pressure (gasses from the electrolyte) which will reduce the available electrolyte.  Regular maintenance should include measuring cell voltages and temperature.
  • AGM Cells (Sealed Lead Acid – SLA)  – These cells have an absorbent glass mat to hold the electrolyte. These are usually the cheapest type of Lead – Acid battery.  They are very light compared with Vented or Gel cells, as they do not have the large volume of electrolyte used in Vented or Gel cells. As there is little excess electrolyte operation in high temperature areas is not recommended.  These also have a maximum cell voltage specified and operation above this voltage will shorten battery life significantly.  These are not recommended for use in “charge / discharge” applications.

 

Nickel Cadmium

These have a long life but have a high initial cost and are not as efficient in storing power than Lead–Acid. Nickel – Cadmium (NiCd) batteries are available as vented or sealed varieties. They have a higher energy density than Lead Acid batteries, are smaller, longer lived and have lower maintenance needs but they are less efficient (losing up to 40% of power in a charge / discharge cycle) and have a high self discharge rate (up to 120% per annum). At “end of life” these batteries must be disposed of in an environmentally sound manner to prevent contamination of the environment with the Cadmium they contain.  NiCd battery disposal costs can be high. The high purchase price and high disposal cost are real disincentives to the use of NiCd cells in solar Systems.

 

Nickel Iron

Nickel Iron batteries have the longest life of the 3 technologies discussed. They contain no environmentally damaging metals and perform well in deep cycle operation. They have a relatively high charge – discharge efficiency (approaching 100% at 50% Charged), but relatively low charge retention, losing 20% – 30% of capacity per month. The main disincentive to NiFe cell use in solar Systems, is the high purchase price.

Here is a Table Comparing these 3 battery types (Based on information from Wikipedia)

Parameter

Lead-Acid

NiFe

NiCd

specific energy

30–40 Wh/kg

30−50Wh/kg

40–60 W·h/kg

energy density

60–75 Wh/l

30 Wh/l

50–150 W·h/L

specific power

180 W/kg

100  W/kg

150 W/kg

Charge/discharge efficiency

50%–92%

65%–80%

70–90%

Energy/consumer-price

7(sealed)-18(flooded) Wh/US$]

1.5– 6.6Wh/US$

Self-discharge rate

3–20%/month

20%– 30% /month

10%/month

Time durability

30 – 50 years

Cycle durability

500–800 cycles

Repeated deep discharge does not reduce life significantly.

2,000 cycles

Nominal cell voltage

2.105 V

1.2 V

1.2 V

Charge temperature interval

min. -40 °C to max.46 °C

 

Battery Lifespans (from Windsun.com):

  • Starting (Automotive): 3-12 months
  • Marine: 1-6 years
  • Golf cart: 2-7 years
  • AGM deep cycle: 4-8 years
  • Gelled deep cycle: 2-5 years
  • Industrial deep cycle (Crown and Rolls 4KS series): 10-20+ years.
  • Telephone (float): 2-20 years. These are usually special purpose “float service”, but often appear on the surplus market as “deep cycle”. They can vary considerably, depending on age, usage, care, and type.
  • NiFe (alkaline): 5-35 years
  • NiCad: 1-20 years

Here are photographs of these types of batteries:

This image shows Flooded Lead – Acid Cells designed specifically for Solar applications.

This image shows Flooded Lead – Acid Cells designed specifically for Solar applications.

This image shows examples of Lead – Acid Gel cells designed specifically for Solar applications.

This image shows examples of Lead – Acid Gel cells designed specifically for Solar applications.

This image shows AGM batteries, configured as 12V batteries.

This image shows AGM batteries, configured as 12V batteries.

You may note that on this page  only the Enerlyte, Suncycle and SunGel batteries have a Deep discharge Cycle life tables available for download.  This indicates that the manufacturer does not recommend the other products for Deep Discharge operation.

In this image, a 2 cell flooded NiCd block is shown (if wired in Series, this provides 2.4V)

In this image, a 2 cell flooded NiCd block is shown (if wired in Series, this provides 2.4V)

In this image, a 1.2V NiFe cell is shown.

In this image, a 1.2V NiFe cell is shown.

References:

http://pvcdrom.pveducation.org/BATTERY/charlead.htm
http://www.altestore.com/howto/Solar-Electric-Power/Design-Components/How-to-Size-a-Deep-Cycle-Battery-Bank/a94/
http://corrosion-doctors.org/Batteries/self-compare.htm
http://www.siomar.com/GELvsAGM.pdf
http://ironedison.com/technical-charts
http://www.thebatterybank.com.au/index.html
http://www.batteryenergy.com.au/

 

About

You may also like...

Comments are closed.