JOULE (J) is the unit of energy in SI units
A Watt (W)
is the SI unit for power i.e. the energy used (or supplied) per unit time.
1 J/s (Joule per second) = 1 W (Watt)
Commonly Used Energy Units Conversion 1kJ (kiloJoule) = 0.9485 BTU=0.0002778 kWhr
Commonly Used Power-Units Conversion 1kW (kiloWatt) = 3414 BTU/hr
often encounter power and energy in our daily lives. The concept of Energy (or more appropriately the exchange of Energy) is related to the latent ability to do work or transfer heat (and thus cause a change in the Temperature). Energy is a relative quantity and only the change is energy is measurable because the amount of useful (or usable) energy for any objective is always measured relative to some base state. Base states for example could be room temperature and room pressure or 0°K). The amount of energy-exchange however does not need to be referred to a base state. The energy exchange can be by transfer of Work, Heat or Mass between a system and its surroundings or vice-versa. Very often the system of interest is thought to be enveloped in what is called the control volume boundaries and the system itself is referred to as the control volume. This control volume is the object of study in most energy-problems. In the paragraphs below you will learn that that the efficiency of the first objective of the previous sentence is limited by what we will call the First Law of Thermodynamics and the second objective is limited by the Second Law of Thermodynamics. The Joule (J) is the unit of energy in SI units. Other common units are BTU's, kJ (kiloJoules), and kWhr (kiloWatt-hour) or units from combination of properties such as PV, TS or uM. Additions of these combinations will also have the units of energy. Here P is absolute pressure, V is Volume, T is absolute temperature, S is entropy (explained below in detail), u is chemical potential (per unit mass) and M is mass.
There is a conversion calculator table below to assist you with a feel for every unit and another that is a comprehensive units conversion calculator.
Power is oomph! Power is the amount of energy that is converted
in one unit of time (one second). Generally speaking, power is the rate at which you can do something, e.g. higher power can make an automobile accelerate faster, make a reaction go faster, produce more, and so on. A Watt (W)
is the SI unit for the energy used (or supplied) per unit time. 1 J/s (Joule per second) = 1 W (Watt). Other common units are BTU's/hr and kW (kilowatts). For historical reasons we use many different units for power and energy. The
calculator(s) below will help you to convert energy from
one type of units to another. If you know the time over which
the energy is delivered, it will allow you to calculate the
power (i.e. (energy-used)/time). You can do more with the calculator. Say you wanted to get a feel for the amount
of energy used in terms of equivalent amount of gasoline
burnt, or in terms of how much ice the same energy would melt; you
can do that also! Have fun!
Cars, refrigerators and
motors are often priced by the device horsepower (power); light bulbs
are often priced by their output watts (power); natural
gas is sold by its combustible energy content namely therms (energy) and
of course the supply-demand situation of the gas; electricity by
kilowatt-hours (energy); convective units are normally priced by power (examples are Convection Air Movers and Air Conditioners); Furnaces are priced by kilowatts
(power) and their temperature capability in Kelvin (K); and air
conditioners by tons or BTU's per hour (power). Power multiplied by the time of use of that power, gives you energy. The SI units for energy is Joules. Expect
to pay a little more when demanding higher power! or expect to earn more when delivering products or service with a high-power high-productivity device. When
you purchase a energy conversion device (say electrical
energy to heat and steam), the device will often be rated by its "power". This enables you to calculate
how much energy you will use (and pay for) by simply multiplying
the rated power by the hours used. When using electric power the commonly used units of energy are kilowatt-hours or kWhrs (which is an unit for energy, 1kWhr= 3600,000J). Each electric
unit that you pay for, is normally one kilowatt-hour (also sometimes called one unit).
When you pay for units, your US electric utility bill
contains a price the for the units used during normal usage, and
the price for units used in a "demand penalty" condition (peak hour use).
You should always carefully watch this penalty number,
and contact your electric utility provider to find ways
for reducing the "penalty" charge. This why energy efficiency is important to you. Devices like the Airtorch™ are highly power transfer efficient.
Energy and Power are the key concepts that are taught in a subject called Thermodynamics. The science of Thermodynamics also deals with issues like energy transfer and other variables that define the direction of any process. Einstein considered thermodynamics to be the most important discipline because it is a basis for understanding why time change occurs in only one direction. Thermodynamics also enables you to understand if something is at equilibrium (equilibrium generally indicates unchanging measurable properties). At the microscopic level (think atoms and smaller entities that make up atoms) however things are always moving. Thermodynamic considerations at the microscopic level are only possible by taking into account an aggregate of small things (like a mole of molecules ~ 6.02x 10^23 atoms). This branch of thermodynamics is called Statistical Thermodynamics. At the macroscopic level we can define measurable properties like pressure (P) and temperature (T) when an object or control volume (V) (of a shape that describes the object of study) is moving at a fixed velocity or is stationary. The study of thermodynamics at the macroscopic level level is called Classical Thermodynamics. The connection between the two branches was described by Boltzmann and Claussius who were able to describe and interrelate macroscopic properties by describing the microstates available to the system for distributing its energy. Thermodynamic is a subject that is used in an important manner across many different scales of understanding the world from large power generation plants to the shapes of the internal structure of matter. The key laws of Thermodynamics that are discussed below are applicable to small scale understanding where quantum mechanics is important and also to larger scale understanding where classical Newtonian mecahnics is valid. This is a good place to note that the cost of enegry production is very much related to energy innovation.
There are many measurable properties that define the thermodynamic state of any system like P, V and T. A system may be associated with other properties not directly measurable. Energy and Entropy are two of them. Energy, E (Joules) is always conserved (such a conservation
process is known as the first law of thermodynamics). Entropy, S (loosely defined as a property when changed, modifies of the quality of the energy in a given material) is not conserved. This is not a difficult concept when you realize that some properties like mass are conserved and some like density are not conserved. Of course we are only speaking about properties of materials that travel much slower than the speed of light and do not involve consideration of electromagnetic radiation of high frequencies where matter and antimatter can transform to energy and vice-versa.
Entropy is also not a difficult concept. It is a thermodynamic variable (property of a material at equilibrium) just like any other property. The overall entropy of the Universe is not conserved, so when a process occurs- the state of the system is changed by the process - and new entropy may be generated and distributed in the system or surrounding. You will learn about other properties called Enthalpy, Landau Potential, Helmholtz Free Energy and Gibbs Free Energy in MHI 102. Finally in MHI 103 you will learn about entropy generation, Sgen. Suffice it to know for now that these variables reflect or modify the amount of useful work that can be recovered from any system while holding other select thermodynamic properties especially Energy and Temperature, constant for a system that changes its state. Entropy is associated with the order prevalent in a system or object. More order - lesser entropy. More disorder - higher entropy, when compared at the same temperature or for the same material. An easy way to appreciate this is to think about a system containing a known material, say water, at the melting point at one-atmosphere pressure. If at the melting point the water is in the form of a solid it has lesser entropy than if the same amount of water when in a liquid form, at the same temperature (melting point). When you posses energy with the lowest entropy,
you posses the most useful form of energy for conversion
to work, i.e. should work be your objective and only when you are comparing for constant energy. However the inherent value of the energy (quality) to you depends on what you need it for. For example, if you are cold and you need heat, your objective is not for an ordered form of energy that takes you (allows you e.g. to drive) to another cold place, but more for an increase in temperature which is best gained by heat transfer to you. Similarly many chemical processes require heat for faster kinetics (i.e. for improved productivity). On the other hand when you have an automobile, it seems important that you be able to order the energy of the automobile motion in order to stay in your defined lane!
Chaos or Order are simply different types of objectives for a process. A process involves a starting state and the final state (the objective). The objective may be a higher temperature (more chaotically energy arrangement) or a lower temperature (less chaotic energy arrangement). Sometimes Chaos is better than Order and sometimes the other way- depends on the objective. When a change of state occurs there are possible implications on the work ability from the new state. The chances are high that you may have during a process one may have converted some of the mechanical energy to a more disordered form of energy (by processes such as friction) or rearranged the microscopic distribution of energy - these are called irreversiblities. Much more on this in the laws of thermodynamics section below. Sometimes the objective of changing a state involves the choice of a detrmined process path. In an ideal situation, when process work is the intended objective, one seeks a process that does not cause
any irreversibility, and consequently no entropy is generated (see bulleted points below). Electrochemical processes, and processes where energy is converted to other forms of energy without any generation of entropy (work to work) are close to such ideal (reversible) processes. But almost no processes is really reversible. Modern process designs are those where the process otimizationinvloves also what was previously thought of waste. If during an irreversible entropy generating process, some heat is produced, this heat can be a useful feature also. Such a concept is used to make electric co-generation power plants. In thermodynamic terms, any gradient (i.e. the difference of a thermodynamic property with distance) in potential, including gradients in temperature, composition, height or charge can cause energy to flow spontaneously with a corresponding generation of entropy. Whenever entropy is generated, the process is called irreversible. Note that the word irreversible also has the connotations of the direction of time and is neither inherently good or bad - remember it depends on your objective. Both time and energy flow "spontaneously" only in a certain direction. This is the direction that produces positive new entropy. In the module, MHI 103, you will learn about a clever way that natural processes choose when trying to spontaneously find a process-path. Nature's clever way for process-path and shape selection is called the MEPR principle. MEPR (sometimes simply called MEP) stands for 'maximum entropy production rate'. Entropy generation rate is fundamental to shape selection.
The inherent energy (E) of a system is useful to know because it can achieve an objective (e. g. heating or lighting or locomotion) when this energy is exchanged by the system with its surrounding (a system can also rearrange the way it partitions energy within itself but we will leave this to MHI 102). When the state of Energy of a system changes, the path taken for this change is important to know because it relates to the objective of the intended change.
A system may changes its energy content when it interacts with the surroundings. A process-path that defines the exchange process for energy exchange. This path represents a change in the thermodynamic-property(s) variable during which energy is exchanged by the system with its surroundings. The exchange of energy occurs by either heat transfer, work (both boundary and non boundary) or by mass transfer. Heat and Work are only recognized during the process path. They are not thermodynamic properties because we cannot associate them with a state of a system at equilibrium. Heat flow, on account of energy transfer occurs from a difference in temperature.
Heat flows from a higher to lower temperature only. Heat flow can be further classified as conduction, convection or radiation. Heat flow is designated by the symbol q (Watts/m2). For radiation q is proportions to T4. For convection q is proportional to T and for conduction heat flow is also propotional to the T, where T is some average absolute temperature between the high and low temperatures between which the heat is transferred spontaneously. When you convert energy from one form to another,
you may create new entropy (almost always) - and when you do that, the
process is called irreversible (because you generate new entropy and end-up degrading the quality of energy). Process-paths need not be continuous or reversible and not just because of quantum effects! (more in MHI 103). Let us define P and V as our symbols for pressure and volume respectively. We can now define a new useful property, Enthalpy (H) (defined as H= E+PV). Enthalpy is an important thermodynamic property which is particularly useful to know during phase change processes. When enthalpy is changed (say by combustion or heat transfer) of a control volume, the process for the change
is called irreversible, because entropy is generated by the process. Efficiencies depend on your objective. Using the best energy efficient device is important - you end up degrading or wasting the least amount of energy for your objective. Efficiency therefore generally depends on how much of energy gets converted to the objective. However, Energy is always conserved and there is a best efficiency when converting heat to work which is a limiting law of nature (described more below- see section on Laws of Thermodynamics). MHI devices are energy efficient. What is Green about Thermal Devices?
New: A Heat Transfer Mechanism that enhances convection and radiation? The fastest way for heat transfer is...?
Energy Efficiency (First Law and Second Law Efficiencies)
Rule: There is no process that can violate the First or Second Law of Thermodynamics. Each law provides a limitation on the objective during any process.
The first law states that energy is always conserved (but of course may be converted from one form to other form by heat flow, mass flow or work interactions). In this context, the efficiency indicates the efficiency of your conversion to the form of energy you require from the form of energy you start with. For example you may wish to tap the electric energy (also called electric work) from an electric power socket and wish to convert it to the thermal energy contained in a hot gas -for example with an with an Airtorch™. The time averaged efficiency of this process will be defined by the power delivered into or by by the hot gas when compared to the power input from the electric source. The First law efficiency depends on the objective of the energy conversion/use and can be as high as 100%.
The second law efficiency is more complicated. When examining a process for second law limited process conversion, the efficiency is the measure how close the process is when compared to a reversible process (see above for the meaning of reversible). It is important to know at this stage that the second law can be stated in many forms e.g., the Kelvin-Planck Statement or the Clausius Statement, and has to do with the impossibility of spontaneously converting disordered forms of energy totally into ordered forms of energy without doing extra work (the penalty). Simply stated one is limited from making all heat into work. This law sets the ultimate limits on your car efficiency (assuming that the car has a heat producing combustion engine) or solar cell efficiency.
The limitation imposed by the First Law is that energy cannot be created or destroyed and for all purposes energy is a conserved quantity.
The limitation imposed by the Second Law is that entropy can only be created i.e. new entropy can only be generated - never destroyed. This is why there is a direction to heat flow, i.e. only spontaneously from hot to cold (if it could flow from cold to hot then you will destroy entropy which is not allowed in our universe).
Entropy is not a conserved quantity (energy is conserved). Any exchange of energy of the system and surroundings can produce new entropy or keep it the same. You should not confuse entropy generation with entropy as a thermodynamic variable. The second law limitation only applies to entropy generation, not to the change of entropy between the states of a system (a system can have a higher or lower entropy depending on how it interacted with the surroundings during a process). Regardless of the process path is chosen, the entropy generation can only be zero (a reversible path) or greater than zero (a irreversible path). The higher the temperature of the sourcewhen the sink is maintained at room temperature, the higher will be the efficiency of the conversion of thermal energy into an ordered form of energy (say mechanical energy). There is no significant penalty going the other way i.e. from work to heat i.e an ordered form of energy can be converted to a disordered form- theoretically without penalty.
There is also a Third Law of Thermodynamics which is that for a perfect crystal at 0 K there is no translation, rotation or vibration of molecules.The Third law states that the entropy of a perfect pure crystal at 0 K is zero.
•Entropy will increase as we increase the temperature of the perfect crystal.
•Molecules gain vibrational motion and the degrees of freedom (related to entropy) increase with an increase in the temperature.
•As we heat a substance from absolute zero, the entropy must increase.
•The entropy changes dramatically at a phase change. When a solid melts, the molecules and atoms have a large increase in freedom of movement.
•Boiling portends a much greater change in entropy than melting.
are some typical efficiencies encountered in everyday
processes . Some limitations are from the first law limitations and some from the second law limitations. Which one matters to you is dependent on the specific objective that you seek.
fossil fuel to get useable heat - about 85%
(running a gas-fired water heater, or making
steam to power a turbine...some heat goes up
the smokestack). Here we are speaking about first law efficiency.
fossil fuel to get electricity - about 33% (active
world-wide research area to improve several
types of efficiencies). The big drop in efficiency comes from the second law limitation which states that not all heat (a disordered form of energy) can be converted to work (an ordered form of energy). Here we are speaking about the second law efficiency.
to electricity in a PV (photovoltaic) cell - about 10% (active
world-wide research area to improve conversion
efficiency, some polymer photovoltaic's are now reporting over 35-44% i.e. close to silicon technology). Can you tell if this process has a second law limitation? Hint - consider if any heat is produced. Also see answer to quiz problem 6 below.
electrical energy in a battery (charging it)
and pulling it back out -about 90%
electrical energy into mechanical energy with
an electric motor - about 85%
- Fuel Cell about 60% or lower. Theoretically this can be even as high as about 92%. Electrochemical processes are limited in efficiency by how much enthalpy of the electrochemical reactions can be used as free energy and produce useful work (called exergy). (Any entropy generation that occurs from the of mixing of constituents limits the electrochemical conversion efficiency).
- IC (internal Combustion) engine about ~ 30% (second law thermal efficiency). Higher temperatures are better for efficiency.
- Wind energy about 59.5%. Wind energy mills convert the kinetic energy of the wind to rotational shaft energy which can be used to create electrical energy. The limitation of wind energy conversion limitation comes from having to cater for residual kinetic energy in the downwind past the windmill blades. Here the first and second laws are both in play. Frictional heat causes entropy generation as do eddys.