Gravimetric and volumetric power densities

Although this notion is more difficult to define, we can speak of a gravimetric power density, specifying the conditions of discharge in which that value was obtained. To date, no standard or specification prescribes 

The same is true of the volumetric power density. The values given below are either calculated or read from the technical specifications where given. Once again, these values are difficult to compare because the conditions under which nominal power should be measured are not standardized. 

28 HPPC Hybrid Pulse Power Characterization. The HPPC profile was designed to measure the aptitude for dynamic power during charge and discharge pulsed regimes. 

However, the faradaic efficiency may decrease during the course of the life of a cell, because of the emergence of degradation phenomena, particularly relating to the passivation layer on the graphite electrode (such phenomena are described in section 6.2.21). This damage oeeurs by way of electrochemical reactions said to be irreversible, whereby electrons (and lithium ions) are consumed with no possibility of the reverse reaction occurring. 

31 Electrochemists use the term polarization to refer to this hysteresis, which is the difference in voltage between charge and discharge to a given SOC. 

---

For most mobile applications size and weight of the fuel cell is very important. Automotive fuel cell stacks have gravimetric and volumetric power density >1 kW/kg and >1 kW/1, respectively. For smaller stacks <10 kW, power density is significantly lower. 

Energy density. Both the weight and size of the battery are critical, particularly in aerospace, military, and spacecraft applications when multiple battery modules are required to meet the output power level for the desired service life. For both the gravimetric and volumetric energy densities, LiCF batteries enjoy a distinct advantage. 

An integrated heat exchanger-reactor for methanol steam reforming was developed by Hermann et al. [65] at GM/OPEL for a 50 kW methanol fuel processor. The system specifications included a volumetric power density of more than 5 kW dm, a gravimetric power density of more than 2.5 kWkg and a transient response to load changes from 10 to 90% in milliseconds. 

Power density. The amount of power a battery can deliver per unit volume at a specified state of charge, which is typically is 20%. This quantity is also known as volumetric power density and is expressed in watts per liter. Specific energy. The amount of electrical energy a battery is able to store per unit mass at a specified state of charge. This is also known as the gravimetric energy density and is expressed in watt-hour per kilogram. 

After more than two decades of persistent research and development on lithium-ion batteries, the specific capacities of the electrode materials and the energy densities (both gravimetric and volumetric) of the batteries at cell level are approaching their intrinsic theoretical values [9, 12]. On the other hand, due to the limited sohd-state diffusion coefficient, the intercalation/de-intercalation mechanism for most of the present inorganic cathode materials limits their high-rate performance for high-power applications [13]. Another concern on inorganic cathode materials comes from the... 

Direct Methanol Fuel Cell The liquid-fed direct methanol fuel cell (DMFC) is generally seen as the most viable alternative to lithium ion batteries in portable applications because DMFC systems require less ancillary equipment and can therefore potentially be more simphfied compared to an H2 PEFC. Additionally, the use of a liquid fuel simphfies storage. The DMFCs can potentially compete favorably with advanced Li ion batteries (which currently power many wireless portable apphcations) in terms of gravimetric energy density of 120-160 Wh/kg and volumetric energy density of 230-270 Wh/L. While both H2 PEFCs and DMFCs are strictly PEFCs (both use the same flexible polymer electrolyte), the DMFC feeds a liquid solution of methanol and water to the anode as fuel. The additional complexities of the low-temperature methanol oxidation reaction prevent the DMFC from... 

Arulepp et al. [81] reported an in situ technique of CDC activation whereby Ti02 was mixed with TiC and chlorinated. During synthesis, titanium in the Ti02 was etched, leaving controlled amounts of oxygen to etch the carbon. It was shown that the oxidation treatment improved the volumetric capacitance but not the gravimetric capacitance. This was perhaps because of unreacted Ti02 in the sample. The carbon was ideally polarizable at E < 2.8 V and showed a power density of 2.5kW/kg. 

Serenus H3-350W from Serenergy AS (Fig. 4b) with triple volumetric and double gravimetric power density, respectively, as compared to state-of-the-art direct methanol fuel-cell systems in the same power class, to be used either standalone or in 24 V hybrid battery—fuel-cell systems, (c) Aerospace Industry... 

In the past, several reports for the practical-sized Li-S battery were found. For example, high energy cell over 300 Wh kg was manufactured by Sion Power, and it was proved not to reach the thermal runaway [20]. But at this stage, there has not been a commercially available Li-S cells. It still will need time to put this system into the market. One of the main drawbacks is the lower density of the material and electrolyte in addition to the problem already stated in this article. Volumetric energy density cannot be so advantageous as gravimetric energy density because of the lower density. 

---