Good points!

It should be noted that higher efficiencies can be realized using compressors/expanders to increase/decrease the temp of the heat sink and cold cold sink. This is done by concentrating heat using the compressor and extract much more energy than the original Carnot efficiency would allow. This compression costs energy, but you expend more to get more.

Fridges also work using the house as a cold source, and the fridge inside as a heat source. To accomplish this the heat flux storing material must be expanded such that its temperature is below that of the fridge inside, the hotter fridge then transferrs heat to the cold flux. The cold flux is then moved to a compressor, and is compressed to hotter than the house temperature, the cold flux not hot, contributes heat to the house!

It really is best to take a course on thermo, preferably an engineering one which actually deals with enthalphy and entropy of different mass streams. You will learn a great deal of practical knowledge which will save you money. (combined with engineering finance and you can do ROI/ROE for housing equipment you purchase and when is the best time to replace!)

Replacing an aging refrigerator with an Energy Star model decreases electrical consumption substantially. Maybe as much as 80 to 100 kwh per month according to my records. I also replaced the washer/dryer and dishwasher with very efficient models at the same time as the fridge replacement.

Dr. Schmitz presents a good description of the entropy problem. The beginning of his article is almost the same as that of David Goodstein in his little book, "Out of Gas" (Norton, 2004), which I happen to be reading just now. Dr. Goodstein is a physics professor at Cal Tech. Schmitz misses a minor point in not mentioning that the temperature to be used is absolute temperature, that is, degrees Kelvin in SI units, although he uses Kelvin in his calculations.

Unfortunately, he completely screws up the energy efficiency calculation for a steam engine, using the boiling point of water at atmospheric pressure as his high temperature. Steam boilers operate at a much higher temperature and pressure than this and the efficiency of a steam power plant is much above his calculated 25% Carnot value as a result. The same is also true for an automobile engine, when operated at peak efficiency, which is seldom the case for a gasoline powered vehicle.

Worse yet, his description of a refrigerator is also completely incorrect. Refrigerators (and air conditioners) operate by reversing the Carnot cycle, in a sense. They use energy from some external source to pump heat to a lower enthropic state at a higher temperature. In so doing, they "create" more enthropy than the reduction in enthropy that is produced as high temperature fluid or exhaust air. The temperature of the "exhaust" heat exchanger is above ambient temperature. As Goodstein pointed out, the thermal energy made available at the exhaust temperature compared to the temperature of the inside of the refrigerator can not then be used in another heat engine to recover the initial input of energy by the electric motor. That would be a perpetual motion device. As for the efficiency of the appliance, that could be defined as the difference between an ideal heat pump with Carnot efficiency and an actual system using some real working fluid, real pumps and real motors. The SEER rating of an air conditioner is another thing entirely.

I guess Dr. Schmitz's confusion to be expected from someone without a background in physics or mechanical engineering. Perhaps his experiences working with electrical engineers has clouded his vision. That he gets it wrong just shows how difficult it is for people to understand the energy problem, especially as so few of us have engineering or science degrees to begin with.

E. Swanson

The performance of a refrigeration cycle actually increases in situations where you are trying to move heat against a relatively modest temperature gradient. So a refrigerator can in theory be more effective than a freezer, and so on. In fact, for modest temperature differentials, you can actually move significantly more heat than the amount of work you put in.

The important parameter is called Coefficient of Performance (COP), and it's defined as:

COP = Ql/W = (qty of heat removed from cold side) divided by (qty of work done by the motor)

The best possible theoretical performance occurs in the reversible case, where

COP = 1/[(Th/Tl) - 1]

In this case, we have 1/[(293/283) - 1] = 28.3

(all this is from Engineering Thermodynamics, by Cravalho and Smith)

In reality, you will never get performance this good, but you can certainly get COPs well over 1 from common appliances. That's why it can make sense to use a heat pump.

Please to examine constructions English posting before to on Oil drum being.

Sentences constructed in the badly sense are to the hard side of the reading spectrum unfortunately.

I'd suggest however, that some of the biggest "wastes" of energy are little to do with the 2nd law of thermodynamics, but rather, using energy to do "work" that has little benefit: e.g. the energy used to move a 2 tonne vehicle, when all you really need is the ability to move a 80kg person. Or the energy used to light and keep the temperature of large buildings within a tiny range, when we could work just as effectively with small amounts of light just where we really need it, and minimal amounts of heating and cooling in specific areas.

Thanks for the reactions to my contribution Why We Waste Energy.
Allow me to make a few comments.
First about the efficiencies of a power plant: sure you can increase that by working at above atmospheric steam pressures that take the temperature of the heat source up. Then the Carnot cycle will give you higher efficiencies, and indeed that is what happens in practice. But fact of the matter is that also in that case the upper efficiency will still remain limited by the Carnot cycle and runs at about 50% best case.
Second my remark about refrigerators: they are big users of electricity in a normal household (see for instance http://www.nypa.gov/press/2003/030211a.htm ), that was my main point I tried to make. Certainly you can make improvements in the efficiency by compression but for safety reasons you do not want to take the exhaust temperature up too high. Thus the fact remains that there will be a relatively small temperature difference between the cold and hold spot of the refridgerator keeping the overall efficiency low of this type of devices and that leads then of course to large electricity bills…
Therefore I still think that Carnot's efficiency rule is the root cause of our "waste" of energy.

John Schmitz

Another word for the "concentration of usable energy" in a carrier is its EXERGY. ( Google link )

From Wikipedia:

In thermodynamics, the exergy B of a system with respect to a reservoir is the maximum work done by the system during a transformation which brings it into equilibrium with the reservoir. ("Reservoir" in practice is the surrounding with high capacity for receiving heat). Energy that has a high convertibility potential is said to contain a high share of exergy. Electricity and mechanical work are perfectly convertible and for these forms exergy contents equals the energy content. For nuclear and fossil fuels theoretical conversion potential is close to perfect, but severely limited by available technical processes. Reversely, heat at temperature close to the reservoir has low convertibility potential, the exergy content of such heat is much lower than its energy content. Exergy analysis is used in the field of industrial ecology as a tool to both decrease the amount of exergy required for a process, and use available exergy more efficiently. The term was coined by Zoran Rant in 1956, but the concept was developed by J. Willard Gibbs in 1873. Z. Rant introduced also the concept of anergy, which is the complementary part of the (heat) energy that can not be converted into work.

Steve

One way of cheating this is if the condensation temperature gives heat energy that is usefull for some industrial process like drying or heating homes or greenhouses or running absorbtion coolers. This often means giving up some of the mechanical energy extracted in a turbine to make the waste heat hotter and more usefull.

John: with regard to your fridge example I think you are using the wrong equations, the ones for heat engines, not fridges.

The first step, I think, should be to calculate the Coefficient of performance for an ideal or "Carnot limit" fridge at the temp. specified.

Using the constants you did in your example:

COP "ideal" = Tl/(Th-Tl) = 283 / 10 = 28.3
where
Tl = temperature low and Th = temp high in deg. Kelvin

so the best "theoretical" fridge could move 28.3 units of energy from the cold to hot sides of the unit for each unit of energy supplied from the electrical outlet at these temperatures.

Given that we need to move 40,000 joules in your example we would need to add 40,000/28.3 = 1,413 joules of energy to do the move.

1 watt hour = 3600 joules so that's: 1413 / 3600 = 0.39 watt hours.

You have specified a 350 watt cooling plant so: 0.39 / 350 * 3600 seconds in an hour = 4 seconds of run time on the motor to cool the water not one hour.

Now no real fridge can achive the Carnot limit. To qualify for the "Energy star" rating a real fridge needs to meet a COP of 3.6 or better

Such a fridge would need to run 7.8 time longer than an "ideal" fridge to cool the liter of water, or about 30 seconds, if it had a 350 watt power draw as per your example.

This does not include other losses such as through the walls of the fridge, rechilling the air inside the fridge when you opened the door etc. but these were not in your example, so I didn't worry about them

Why do we waste energy?

Because we can.

All

Thanks for pointing out your concerns. In my haste I did indeed mix up the formulas of a heat engine like a steam engine and a heat pump like a fridge. Appology for that.
But for both the Carnot cycle is the starting point (in different directions though) and poses an upperlimit to the efficiency that we can get, right?
We simply cannot escape Carnot principle and will always get less favorable results than the ideal reversible case.

Interesting dialog

As soon as I read this:

an efficiency as poor as 50% or often even lower

I was concerned about the logical integrity of the post, but then reading on, I gave some ground, spelling and grammar making me make allowances for use-of-language.

What I'd like to know from the critics, who understand the physics, is whether the statement

Suppose you want to cool one liter of water from 20°C to 10°C ... you have to extract ... about 40kJ. ... it will "cost" us about 1300kJ.

is fundamentally correct or not.
--
Jaymax (cornucomer-doomopian)

Folks, my apologies.

Various miscommunications occurred prior to the posting of this piece, the first being that I thought it had been copyedited, so I didn't read it over as I have been in and out of meetings today and haven't had time to read it over until now.

Second, as for the substance of the post, well that's already been well documented in the thread. Sorry gang.

My comment is that a technique used in Combined Heat and Power (CHP) projects is to max-fire a Heat Recovery Steam Generator, HRSG to use all of the Oxygen available in the Gas Turbine. This max-fired CHP type of power plant is 92% or 93% thermal efficiency. One example might be a 15mw gas turbine max duct fired to make 250,000 pph of steam for an industrial plant or for a University or for a large hospital etc. Between the steam and the gas turbine generator kw, total thermal efficiency is about 92%. It is possible to get slightly higher to 93% if a back pressure steam turbine is added to the process.

So many more CHP plants would greatly help . Also they avoid the very large penalty in electric transmission losses. They save perhaps the most emissions and CO2 simply be being far more efficient.

So my point is that with Peak Gas and Peak Oil, more than ever - we need to maximize efficiency first and that CHP is one excellent way to help continue to grow the economy with a fixed amount or even a decreasing amount of fuel. It is not a panasea but simply (I think) the first step to take. First use efficiency improvements starting with maximizing worldwide CHP to help manage the problem. We still must do much more and this only buys time but at least has an ROI even if 10yrs.

We have many barriers to CHP but it has really developed rapidly in Europe (especially Denmark) and amazingly in third world countries putting in new process plants, food and drug plants, paper mills, etc. This happens while USA CHP investment suffers.

>electric cars with regenerative breaking offer some nice things such as recapturing 80% of the energy used to accelerate that 2080kg mass and using it later, thus making it as if you are accelerating a 416kg mass.

There's a fallacy here. Braking has to be done at high deccelerations, at least in an emergency. The generator has a finite power rating as does the battery the energy is stored on. Now figure out the mass of the generator and the mass of the battery that stores the regenerative braking energy. How much mileage does it cost pushing that extra weight up and down hills?

I'm sure you can get 80% efficiency on the test track. I don't think you can get it in a supermarket parking lot.

The worst case seems to be pumping heat out of the fridge into the house then cooling the house with a compressor air conditioner.

OK, we see the problem. Now who can supply a refrigerator that can:
1 In winter use ambient/outside air temp below 4C to cool.
2 Not heat the indoors during air conditioning conditions.

I assume a 'split system' like groceries stores use would fit the bill, but I have never seen one in a house.

I also know there are 'meat safes' and I've lived in places where I can leave the outside freezer unplugged for 5 months. But I mean something that most of civilization would like to have in their house.

A refrigerator is a type of heat pump. A heat pump may have an efficiency of 300% - for a small temperature gradient.

Need I say more?