Introduction to Energy Density (By Weight and Volume)
The Energy density of a battery is generally expressed in two ways, in the form of gravimetric energy density and as volumetric energy density . The gravimetric energy density or the specific energy of a battery is a measure of how much energy a battery contains in comparison to its weight, and is typically expressed in Watt-hours/kilogram (W-hr/kg). The volumetric energy density or the energy density of a battery is a measure of how much energy a battery contains in comparison to its volume, and is typically expressed in Watt-hours/liter (W-hr/l). Fig.1 shows gravimetric density (specific energy) and volumetric density (energy density) of three batteries.
|GRAVIMETRIC DENSITY (W-HR/KG) specific energy||55||50||90|
|VOLUMETRIC DENSITY (W-HR/L) Energy density||180||140||210|
FIG 1. ENERGY DENSITY COMPARISON (source: www.ti.com)
As we can see in Fig 1, that the Li-Ion has a better gravimetric density compared to the Ni-Cd and Ni-MH values. The implication of this is that appliances powered by Li-Ion battery can be made much lighter without sacrificing the run time. Also, keeping the battery weight same, the run time will double if Li-Ion batteries are used. Because of this advantage Li-Ion chemistry is quickly displacing Ni-MH in cell phones and laptops.
Specific Energy And Volumetric Energy Density- A critical Parameter in PV Systems
Energy density is one of the many battery characteristics used chiefly to compare one type of battery system to another. The energy density is a function of the weight of the battery, and the volumetric energy density (in Whr/litre3) is a function of volume of battery. A battery with a higher energy density will be lighter than a similar capacity battery with a lower energy density. Talking about portable systems, the energy density is a critical parameter but in conventional PV systems, which provide power for a stationary object, the energy density may be less important. However, since it is costly to transport batteries to remote locations, so a high energy density battery is typically an advantage.
Energy Density Not Same As Power Density
Power density is normally mentioned in the units of Watts/L or Watts/Kg and is not to be confused with Energy density, which is expressed as Wh/L or Wh/Kg. To explain this better, Energy density implies how much energy a battery can hold. The higher the energy density, the lower the run time of battery will be. Example is Li ion chemistries. On the other hand power density implies how much power a battery can deliver on demand. The focus is on power bursts like drilling rather than the run time. Example is Ni based chemistries.
Water bottle Analogy to explain Energy density and Power density
The size of the bottle is analogous to Energy density and the spout analogizes Power density. A large bottle can carry lot of water (energy density) while a large opening (power density) can empty it quickly. Large bottle with large opening is an ideal and best combination.
Battery Costs Corresponding To Energy Density Values
Cost is an important factor in choosing a battery . In the table below is presented the cost per Watt-hour, Specific Energy i.e. Watt-hours per kg and the Energy Density i.e. Watt-hours/liter for various types of batteries.
$ per Wh
We can see that the Lead-acid storage batteries head the list in low cost. Li ion tops in energy density and is priced at $0.47 per Wh.
Energy Density Comparison of Size & Weight
The below battery comparison chart illustrates the volumetric (i.e. energy density) and specific energy densities showing smaller sizes and lighter weight cells.
Figure 2:Battery Energy densities (source: www.epectc.com)
Energy Density And Specific Energy Advancements in 2019
Licerion battery by Sion power:
Sion Power has created technology that has vastly improved the energy and cycle life of rechargeable lithium metal batteries. Lighter, higher-energy batteries have always been in demand and have led to the research and development by the team at Sion Power to create the Licerion® rechargeable battery. Licerion batteries are setting a new standard for lithium batteries by offering the highest combination of energy density and specific energy available. This ultra thin li metal battery has energy density and specific energy as 500 Wh/kg and 1000 Wh/L