[Greenbuilding] Energy and Power...

[Haudy Kazemi]

That's a good presentation/PDF.  I suggest anyone who isn't completely 
clear about energy vs. power to take a look at it.

I also suggest that part of the kWh /  kW per hour confusion is based on 
how we see speed measured.  Our speedometers says MPH not M/H, even 
though 'per' means divided by.  We are familiar with speed, but less so 
with invisible electricity flowing in wires.  When people see KWH, 
perhaps the more familar MPH comes to mind eventually leading to other 
confusion as the meaning of P as 'per' evades them.


A few notes related to the presentation:

1.) an analogy of energy vs power is the water vs water flow (you did 
touch on this in the presentation).  More specifically, water used 
(gallons or cubic feet) vs how quickly it is flowing (gallons per minute 
or cubic feet per second).  Filling a swimming pool takes a certain 
number of gallons of water, this is analogous to the number of kWh or 
Joules used.  The water company charges for how much water is used and 
likewise the electric company charges for how much electricity (usually 
measured in kWh) is used.  Filling the swimming pool slowly with a 
garden hose (fewer gallons per minute) instead of a fire hose will not 
change the gallons of water it takes to fill the pool.  Using 
electricity at a faster (higher watts( or slower rate (lower watts) will 
not change the total amount of electricity used to complete a specific 
task, all else being equal.  (A 1500 watt electric electric tea kettle 
will use the same amount of electrical energy to heat 1/2 gallon of 
water to 190 degrees as a 750 watt tea kettle will use to heat 1/2 
gallon of water to 190 degrees.  The 1500 watt kettle will do the job in 
1/2 the time, but the energy used is the same*.)


2.) The following caveat mights confuse some people.  Be sure to 
understand the more basic concepts in the presentation and above before 
trying to make sense of the caveats.

2a.) unlike most residential electric customers, some commercial and 
industrial electric customers (mostly those with large capacity 3 phase 
service) pay both an energy charge and a power demand charge.  The power 
demand charge is intended to account for the additional infrastructure 
(bigger wires and transformers) needed to support uneven, bursty loads 
compared to even loads.  In the previous analogy, the water company 
could charge extra for filling the pool with a fire hose rather than a 
garden hose because bigger pipes need to be installed in the 
distribution system to support a fire hose than to support a garden 
hose.  (Water companies do this indirectly by charging different meter 
fees based on the size of your water meter (3/4" vs 1" vs 1.5" etc).)

2b.) electric company power demand fees are a way to manage customer 
power usage.  Power demand fees can make big on-demand appliances (like 
instant electric water heaters) unattractive to use from a cost 
perspective.  (An analogy is being charged extra to use fire hose 
instead of garden hose to fill a swimming pool, because the fire hose 
can deliver the same amount of water in less time.)

2c.) electric company time-of-use (TOU) rates are a way to manage 
customer energy consumption at different times of day.  TOU rates make 
use of devices during expensive times of day unattractive from a cost 
perspective.  TOU energy rates are like variable tolls on toll 
roads...with extra tolls for the express lanes during rush hour and deep 
discounts during the off peak hours.

2d.) *second order effects can result in differences between how much 
energy is used even though mathematically one might expect it to be the 
same.  One of these are the I^2R (current squared x resistance) losses 
in the wires supplying the load.  Doubling the current will quadruple 
the losses in a wire.  These losses are used chose proper minimum wire 
sizes for various electric loads.  I^2R losses become dramatically more 
significant during peak power demand and peak energy times of day, which 
is one reason peak shaving, load shifting, and demand control switches 
on air conditioners and water heaters is important.  I^2R losses also 
factor into energy prices.

---

Another set of items that are often confused are data storage capacity 
(bytes) and communications bandwidth (bits per second) and latency 
(delay, lag).  Data storage capacity is measured in 
bytes/kilobytes(kB)/megabytes(MB)/Gigabytes (GB) etc.  Bandwidth is 
usually measured in bits per second.  8 bits = 1 byte, but there is 
overhead/retransmissions that makes 10 bits = 1 byte a better 
approximation for communication bandwidth calculations.  Latency is a 
measure of delay, lag.

In an analogy to energy and power, here is how data and power networks 
line up:
Data storage capacity (bytes used or moved) (5 gigabytes = 5000 
megabytes) ~= energy (kWh used)
Max communications bandwidth (50 Mbps = 5 megabytes/second) ~= maximum 
power available (service drop size ... 240 volts x 200 amps = 48 kW service)
Latency (36 milliseconds to transfer a piece of data from Google's 
homepage) ~= seconds to minutes to hours delay in bringing up standby 
power plants; years delay in constructing new power plants.

Low latency in responding to demand allows power networks to more 
accurately compensate for source variability.  Buffering (temporary 
short term storage) in the form of batteries, flywheels, and 
hydroelectric/pumped storage hydroelectric can bridge the gaps between 
sources with high variability and backup sources with high 
latency/delays in starting up to replace the high variability sources.

bandwidth vs latency (which latency matters a lot in how well 
communications systems work)
http://www.stuartcheshire.org/rants/Latency.html




On 8/6/2011 3:25 PM, Gordon Howell -- Howell Mayhew Engineering wrote:
> Greenbuilding List:
>
> Further to the interchange below highlighting people's incorrect use 
> units for energy and power,
> though it may seem reasonable on the surface to say "almost every 
> Canadian on this list (including myself) and, I suspect, most others 
> in North America and around the world either know or have the capacity 
> to figure out what your original message meant":
>
> it is the same as mixing up speed (km/h) and distance (km), or volume 
> (litres) and flow (litres per second) (which we would never do and 
> would be disparaged by society if we did)
> and
> then we get these same people who "apparently" understand how to 
> "figure it out" then making huge calculating errors when the look at 
> solar PV economics or sizing (as I've seen many times) because they 
> don't fundamentally know the difference between energy and power.
>
> *_I would suggest that it is extremely important to not mix up these 
> units.
> _*A watt is a rate of energy flow -- it is like speed, which is a rate 
> of distance "flowing".  A watt means a joule per second.  Whenever you 
> use "W" think of "joule per second"... so a 230 W solar PV module 
> generates 230 joules of electrical energy per second (at rated solar 
> conditions of 1000 joules per second per m2).
>
> A kWh (or preferably joule) is an amount of energy -- it is like distance.
>
> kWh means "thousand times watts times hours", which is a correct 
> energy unit.
>
> kW/day means "thousand times watts per day", means "thousands x joules 
> per second per day" -- it is a unit of energy-production 
> acceleration.  It is never used on a small scale as in kW or days.  
> The electric utilities and regulators use MW/hour units, because it 
> speaks of the rate at which generators can ramp up or down (which is 
> an acceleration) the rate (MW or millions of joules per second) at 
> which they can produce energy.  We also use MW/year to describe the 
> capacity of a PV factory, because it also implies the acceleration of 
> PV energy generation in the world due to that factory.
>
> So I made the attached PowerPoint presentation to help (I hope) people 
> understand the difference between the two and the importance for not 
> mixing them up.
>
> I have to say that when I see people mixing up energy and power units, 
> then immediately my mind starts to discredit them and their 
> professional expertise... and I've even seen engineers, electric 
> regulators and electric utilities mix them up!
>
> I am quite fascinated that peoples' mixup in all this fundamentally 
> arises because we have non-metric time, and this is due to the 
> Sumerians of some 4000 years ago who gave us the sexagecimal system, 
> which we use for hours, degrees, minutes and seconds.
>
> I would value anyone's comments on this.
>
> +Gordon Howell
> Edmonton
>
>
>
>> Please see my replies to your questions, below. You may not comprehend
>> all of my answers, but that is all right.
>>
>> > We've mounted 13,000 watts DC panels, in agreement with the Ontario
>> > Power Authority to sell, at max., 10 Kw per hour.
>>
>> I mean that we have 52 Solar Worldphotovoltaic modules, rated at 250
>> watts Direct Current output each, mounted in four strings, feeding two
>> SMA 5,000 watt output inverters, and that the Ontario Power Authority
>> will buy 10 Kilowatts of electricity, maximum, per hour, produced from
>> these modules.
>>
>> > These sunny summer days, we vary from 75 to 92 Kw per day...
>>
>> Each day varies, as does the weather, and we typically sell between 75
>> kilowatt hours to 92 kilowatt hours per day, to the Ontario Power
>> Authority.
>>
>> > They pay 81 cents per Kw produced, up to a maximum of 10 Kw per hour...
>>
>> Each day, the Ontario Power Authority pays $0.81 Canadian Dollars for
>> each kilowatt that they buy from us. However, they will only pay for 10
>> kilowatts per hour, even if we produce more than that. The limit of 10
>> kilowatts per hour is a part of the definition of the Ontario microFIT
>> distributed generation program.
>>
>> > This means we produce more full 10Kw each hour than the standard
>> > 11,000 watts arrays
>>
>> I am impressed by your courteous (and slightly humorous) response to 
>> what I considered a rather inappropriate email.
>> Almost every Canadian on this list (including myself) and, I suspect, 
>> most others in North America and around the world either know or have 
>> the capacity to figure out what your original message meant.
>
>
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