Specific energy per unit mass

The most important performance indicators for power supplies designed for portable equipment are the specific energy per unit mass (weight), Ym (in J/kg) and/or per unit volume, Yi> (in J/L). Often, miniplants with fuel cells are used in portable equipment as a replacement for lithium-ion batteries, which have specific performance indicators of 150 Wh/kg and 350 Wh/L. 

The first step in an application of the first law is to identify the system. This is conveniently done by drawing a line around the system, which is called the control volume. The quantities Q, W, and AE are then introduced into Eq. (11.5). The change in thermal energy AE) in thermal problems usually involves changes in temperature T) and pressure P). Tables of relative values of specific energy per unit mass (m) in kJ/Kg for different materials such as air, water, steam, refrigerant, etc. are available. These tables also give values of other properties at different combinations ofP and T, such as volume per unit mass. The Bernoulli equation [Eq. (5.15)] is a special application of the first law for applications where there are no losses, no heat transfer, and no work done. 

In (2.19), F has been replaced by P because force and pressure are identical for a one-dimensional system. In (2.20), S/m has been replaced by E, the specific internal energy (energy per unit mass). Note that all of these relations are independent of the physical nature of the system of beads and depend only on mechanical properties of the system. These equations are equivalent to (2.1)-(2.3) for the case where Pg = 0. As we saw in the previous section, they are quite general and play a fundamental role in shock-compression studies. 

The time rate of change of total specific energy of the element due to work is thus d(uP)ldh)lpo. If heat is added to the element from the surroundings at the rate (dQ/dt) per unit mass, then these contributions from work and heat can be equated to the change in internal and kinetic energy per unit mass... 

Here Q(t) denotes the heat input per unit volume accumulated up to time t, Cp is the specific heat per unit mass at constant pressure, Cv the specific heat per unit mass at constant volume, c is the sound velocity, oCp the coefficient of isobaric thermal expansion, and pg the equilibrium density. (4) The heat input Q(t) is the laser energy released by the absorbing molecule per unit volume. If the excitation is in the visible spectral range, the evolution of Q(t) follows the rhythm of the different chemically driven relaxation processes through which energy is... 

Specific Energy—The actual energy per unit mass deposited per unit volume in a small target, such as the cell or cell nucleus, as the result of one or more energy-depositing events. This is a stochastic quantity as opposed to the average value over a large number of instance (i.e., the absorbed dose). 

Notice that the dose has a strict definition of energy per unit mass of the absorber and, in principle, can be measured for a given radiation at a certain energy in a specific material. The equivalent dose is a relative unit in that a radiation weighting factor is applied to a measured quantity. The dose can be measured from ionization in an electronic radiation detector the equivalent dose must take into account the type of radiation causing the ionization. 

For latent heat, we look up the corresponding entry in the tables for either the latent heat of vapourisation (or simply the heat of vapourisation) or the heat of fusion, depending on the type of phase change encountered (liquid to vapour and solid to liquid, respectively). These quantities are in units of energy per unit mass and are given for a specific reference state (often the 1 atm boiling point or melting point of the substance). 

Units specific energy, energy per unit mass of fluid flowing [J kg-1, m2 s 2], To get rates [J s 1. W] simply multiply each term by mass flow rate, G. ... 

The energy of the fluid volume element comprises the specific internal energy e per unit mass and the kinetic energy per unit mass which is 0.5(u2+v2). The internal energy of the volume element AxAyAz is... 

G Vapor enthalpy vector g is specific enthalpy of vapor in stage i, energy per unit mass... 

Specific Internal Energy (u) is the internal energy per unit mass of the... 

Reference temperature for internal energy of enthalpy (K) Internal energy per mole, or molar internal energy (J/mol) Internal energy per unit mass, or specific internal energy (J/g) Volume per mole, or molar volume (m /mol)... 

Stored energy per unit mass, expressed in MJ kg Wh kg or kWh kg Specific power... 

Example 4.10. The vessel in Example 4.9 had a final pressure of 20psia and a temperature of 161°F. The internal energy per unit mass in the container was 92.0Btu/lbm the specific volume, 2.8fi/Ibm, Now we connect the container to another Freon 12 line, in which the pressure is 40 psia and the temperature is 200 F. We open the valve and let gas flow in until the pressure in the vessel is 40 psia. What is the final temperature in the vessel j... 

Now, for a check of whether we have made the correct assumption, we see that three properties of the final state are known, i.e., the pressure, the internal energy per unit mass, and the specific volume. If we have made the correct assumption, these must be consistent. In App. A.2 for 40 psia and 1.8ftVlbm, we read an enthalpy of 134 Btu/lbm. The corresponding internal energy is... 

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