Energy FAQ

Question:

After 20 years my thermo is a little rusty. I’m trying to relearn it from my college textbook (Thermodynamics, JP Holman, 1974) to understand this solar/steam stuff a little better. Can someone tell me if I understand the following stuff correctly? 1. Internal energy, u. – basically a measure of the thermal energy – the energy stored in the random kinetic motion of the molecules. 2. enthalpy, h. –  the internal energy plus the work available because the gas is compressed. To determine the work available between two states, compare the enthalpies. (I’m thinking in terms of moving a piston here.) If I’m wrong, correct me in public so no one follows me down a dark road. Please e-mail me if you’d be willing to answer an occasional question. I won’t ask till I’ve thought. Thanks in advance, Steve S

Response:

>1. Internal energy, u. – basically a measure of the thermal energy – the >energy stored in the random kinetic motion of the molecules.

Close enough for government work. >2. enthalpy, h. –  the internal energy plus the work available because the >gas is compressed.

Enthalpy = internal energy plus the product of pressure and specific volume. (Multiply pressure in N/m^2 by volume in m^3/kg, you get N-m/kg, or specific energy.) >To determine the work available between two states, compare the >enthalpies. (I’m thinking in terms of moving a piston here.)

Not quite.  Enthalpy is used when the fluid crosses a boundary, e.g. steam flowing out of a boiler.  The pressure-volume product is a transfer of energy which has to be accounted for.  In hydraulic systems, nearly all of the energy transfer is from the pressure-volume product, a negligible amount is from internal energy changes. If you are heating, expanding, cooling, and compressing a fluid inside a closed cylinder, you have nothing crossing any boundaries such as nozzle apertures and you’d use the internal energy throughout. >If I’m wrong, correct me in public so no one follows me down a dark road.

I’m always happy to help put the public discussions on a firmer foundation. I post publicly when I have the ability and time.

Response:

After 20 years my thermo is a little rusty. I’m trying to relearn it from my college textbook (Thermodynamics, JP Holman, 1974) to understand this solar/steam stuff a little better. Can someone tell me if I understand the following stuff correctly? 1. Internal energy, u. – basically a measure of the thermal energy – the energy stored in the random kinetic motion of the molecules. 2. enthalpy, h. –  the internal energy plus the work available because the gas is compressed. To determine the work available between two states, compare the enthalpies. (I’m thinking in terms of moving a piston here.) If I’m wrong, correct me in public so no one follows me down a dark road. Please e-mail me if you’d be willing to answer an occasional question. I won’t ask till I’ve thought. Thanks in advance, Steve S

Response:

> After 20 years my thermo is a little rusty. I’m trying to relearn it from > my college textbook (Thermodynamics, JP Holman, 1974) to understand this > solar/steam stuff a little better. Can someone tell me if I understand the > following stuff correctly? > 1. Internal energy, u. – basically a measure of the thermal energy – the > energy stored in the random kinetic motion of the molecules.This is my first try to get something in a newsgroup, so sorry if it is

not in the right place or so.. Together with Kees Hulsbergen I have found a new concept to extract huge amounts of tidal energy from shallow seas where the tidal wave propagation is typically parallel to the shore. Typically 1000 to 2000 MW plants are possible. This concept is new, but most promising for the future. At this moment we would like to know if anyone ever dealt with tidal energy plants. In addition we are looking for finances for a feasibility study, so if anyone knows the right place, pls let me know Robbert

Response:

We (two persons working on a private basis) have developed a new concept for winning tidal energy (1000 – 3000 MW-plants). Patent pending. This concept works for areas where tidal wave propagation is typically parallel to the shore and where the foreshore is shallow. Who has ever dealt with tidal wave energy ? We would also like to be informed on possible financing agencies for a further feasibility study. Please reply sept. 1996). Robbert

Response:

>1. Internal energy, u. – basically a measure of the thermal energy – the >energy stored in the random kinetic motion of the molecules.

Close enough for government work. >2. enthalpy, h. –  the internal energy plus the work available because the >gas is compressed.

Enthalpy = internal energy plus the product of pressure and specific volume. (Multiply pressure in N/m^2 by volume in m^3/kg, you get N-m/kg, or specific energy.) >To determine the work available between two states, compare the >enthalpies. (I’m thinking in terms of moving a piston here.)

Not quite.  Enthalpy is used when the fluid crosses a boundary, e.g. steam flowing out of a boiler.  The pressure-volume product is a transfer of energy which has to be accounted for.  In hydraulic systems, nearly all of the energy transfer is from the pressure-volume product, a negligible amount is from internal energy changes. If you are heating, expanding, cooling, and compressing a fluid inside a closed cylinder, you have nothing crossing any boundaries such as nozzle apertures and you’d use the internal energy throughout. >If I’m wrong, correct me in public so no one follows me down a dark road.

I’m always happy to help put the public discussions on a firmer foundation. I post publicly when I have the ability and time.

Response: