By Anton Lang ~
These two similar sounding terms are perhaps the most misunderstood things in the whole electrical power generation debate, and while there are some important things in this debate, these two terms are in that small group of the most important of them all.
Firstly, the simple explanation for both terms.
Megawatts means the design specification maximum power that the generator can actually deliver. This is what I refer to as the Nameplate for the generator. The acronym for Megawatts is MW.
MegaWattHours is what that generator, while it is actually working, delivers in power to the grid over a period of time, here hours, and that period of time can be an hour, a day, or a year. The acronym for MegaWattHours is MWH.
I will explain it in a little more depth below, and show you, with the use of some graphs what the difference actually is.
What made me think of it is that this week, here in Australia, there was a proposal put forward to construct two new technology coal fired power plants in New South Wales. That proposal was for a total of 2000MW of coal fired power. I did the math for the yearly output for these two coal fired plants, and found that they will deliver power to the grid in that State, a total power (in MWH) a little more than the total delivered power from EVERY wind plant in Australia combined, FIFTY ONE of them in all, and that wind power has a total Nameplate (Megawatts) of 5661MW. So then, how can a coal fired plant deliver more MWH than ALL those wind plants when the wind plants have a Nameplate that is 2.83 times larger than those coal fired plants? I will also explain that below as well.
I might think that this proposal is more along the lines of a thought bubble to gauge public opinion, and that they most probably will not be constructed, but what it does do is to get people talking about it at last, something sorely needed in all of this latest and most current electrical power debate. The proposal did have the desired effect though, as Greens politicians went ballistic into apoplectic fits of rage, saying that this has zero chance of coming to fruition.
So then, Megawatts and MegaWattHours.
It’s an electrical principle, so it’s a little difficult to explain for the person not trained in electrical theory, hence it is so often misunderstood.
I have two graphs below which I will use to explain the difference, and as with all the images I use in my Posts, if you click on the image, it will open in a new and larger window, so you can better see the detail.
Now, for the sake of comparing ‘apples with apples’, I have used a wind plant and a single coal fired Unit both of the same Nameplate (MW) for my examples.
The wind plant is the Macarthur wind plant in the State of Victoria, and this is the largest wind plant in the Country. This plant was opened in 2013, so it is still only 6 years old. It has 140 individual wind towers at the site. Each tower has a single generator at the top of the tower, and each generator has a Nameplate of 3MW, so the total design specification, (MW) the Nameplate for this plant, is 420MW. The wind turns the three large blades out the front of the generator, and that is what drives the generator.
The single coal fired Unit I am using as my example is Unit 1 at the two Unit Millmerran power plant in the State of Queensland. This Unit is the closest I can find to the same Nameplate as for the Macarthur Wind plant, and it has a Nameplate of 425MW. This plant was first opened in 2002, so it is now 17 years old. It is a Supercritical plant, so one level of technology lower than these proposed new coal fired plants. This plant is only one of four of these Supercritical plants in the whole of Australia, and all of them are located in that State of Queensland, the others at Callide, Kogan Creek, and Tarong North. The remaining coal fired plants still operational in Australia are all either one level lower than this one, (Critical) or two levels lower, (SubCritical) the older of the currently operational coal fired plants here in Australia.
For further equal comparison, I have selected the same day for these graphs, March 6th 2019, barely three days ago now, and that date is shown at the top of each graph.
The graph at left is for the Macarthur wind plant, and as you can see from the list under that graph, all of those 51 wind plants have their boxes unticked, so the only one shown is that Macarthur wind plant, indicated on that list as Macarth1, shown near the bottom right of that image.
The graph at right is for the Millmerran coal fired plant, Unit 1, and that box is the only one ticked in that list under the graph, shown here as MPP_1, also shown near the bottom right of the image.
What both images show is the total power delivered across the whole 24 hour period of that day, 6th March 2019. I have also added the time indicator for Midday, so it shows the power being generated at that single point in time, and that is shown at the left of the main body of the graph, 374.3MW for the Macarthur wind plant, and 426.8MW for the Millmerran Unit 1. Note the scale change with each graph indicating MW, (the left vertical scale) and for the wind plant it shows a maximum of 400MW, and for the coal fired Unit, it shows a maximum of 2000MW on that left vertical axis on both graphs.
Now, (nearly) everybody knows that wind power is variable with the speed of the wind blowing at any one point in time, and the graph for Macarthur wind plant shows that variability, with the output varying up and down across the day.
With the coal fired plant, whilst ever they feed in coal at the front end, the total design specification power (MW) is delivered from the generator, so that is why that graph is a virtual straight line across the page at the maximum design specification for that Unit.
To calculate the total power delivered across the day, I took separate readings at every hour mark across the day for both the wind and coal fired graphs, and added each of them individually for a total for each graph. I then divided that total by the number of readings to give me an average. I then multiplied that average by 24 (the hours in a day) to give me a total generated, and delivered power for that day in MWH.
The total power delivered across the day by Macarthur wind plant came in at 4152MWH.
The total power delivered by Millmerran Unit one across this same day was 10200MWH.
See the difference there? The coal fired Unit with the same Nameplate (MW) generated and delivered 2.46 times the power. (MWH)
There is a relationship between the Nameplate (MW) and the generated power (MWH) and that relationship is referred to as the Capacity Factor, and that is usually calculated across the whole 12 Month period, and this is the Industry Standard. The most current average Capacity Factor for all the wind power plants in Australia across the whole year is 30%, meaning that across that year, all of those wind plants will deliver 30% of their Nameplate in total power that they all generate.
Now, for this example above, I have actually used a good day for Macarthur wind plant, because on this day, that Capacity Factor for Macarthur Wind was 41.2%, well above that year round average of 30%. That can be better shown by what happened on the following day, as these two graphs below show the output for both of these same plants for that following day, 7th March 2019.
Now, using the same maths, the total power delivered from Macarthur wind plant was 345MWH, (so, the Capacity Factor was only 3.4% for that day) while for the Milmerran coal fired Unit, it was the same total as it was the day before, 10200MW, so on this day, that coal fired Unit delivered 29.6 times the power in MWH.
So, while there is a difference between MW and MWH, that changes on a daily basis, as you can clearly see with just these two sets of graphs.
So then, that brings us to what I mentioned above, how these two proposed new tech coal fired plants will deliver more power across a year (MWH) than every wind plant in the Country, even though the Nameplate (MW) for wind power is 2.83 time larger.
This proposal for those two new coal fired plants is for USC coal fired plants, and here USC stands for UltraSuperCritical, and that is one level of technology higher than the existing most modern plants in Australia, those four SuperCritical plants in Queensland. Those new USC plants are now being referred to as HELE power plants, and that acronym stands for High Efficiency Low Emissions.
Now, unlike older technology coal fired plants, these new tech ones will operate at close to their maximum all the time that they are delivering their power. The only time they are off line is when they are scheduled for maintenance when their power output goes back to zero. Because they are so efficient, they will operate at (or close to) their total Nameplate (MW) for all the time they are on, and you can see that in both of those graphs for the Millmerran plant above, and that will be close to, or above 90% across each year for at least the first few years, and all those existing new tech plants in Queensland are still operating at or above 90% after many years of operation, and, as you can see from this Unit at the Millmerran plant, it is still delivering its maximum power after 17 years of operation.
So, with a Nameplate of 2000MW, then across the whole year these plants will deliver 15.8TWH, and here TWH stands for TeraWattHours, and a TWH is one million MWH.
Now, all of those 51 wind plants currently in Australia have a total Nameplate ofs 5661MW, so that’s 2.83 times the Nameplate for these proposed new coal fired plants. When you take that relationship between Nameplate and generated power, (the Capacity Factor) then for all these 51 wind plants, then the total generated power across the whole year is 14.9TWH.
So, these proposed new tech coal fired plants will deliver 6% more power (MWH) over each year than all the currently existing wind plants in the Country.
THAT is why the difference between Megawatts and MegaWattHours is so important.
Anton Lang uses the screen name of TonyfromOz, and he writes at this site, PA Pundits International on topics related to electrical power generation, from all sources, concentrating mainly on Renewable Power, and how the two most favoured methods of renewable power generation, Wind Power and all versions of Solar Power, fail comprehensively to deliver levels of power required to replace traditional power generation. His Bio is at this link.
Robber
Sun 03/10/2019
Another way of expressing the reliability of coal versus the variability of wind:
Across the AEMO grid, coal delivers average power of 16,800 MW, ranging from a minimum of 14-16,000 MW to a maximum of 18-20,000 MW in response to demand.The minimum occurs overnight while peak demand is early evening.
In contrast, wind delivers average power of 1,700 MW, ranging from 150 MW to 4,100 MW, not in response to demand but to the vagaries of the wind.
So when the wind doesn’t blow there must be 100% reliable alternative generation available. Same applies to solar that varies from 0-6,000 MW, average 1,400 MW.
Other generators include gas and other fossil fuels 2,300 MW (range 800-9,000 MW), and hydro 1,900 MW (range 400-5,000 MW, both varying in response to demand.
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Child Of God
Sun 03/10/2019
Forgive me for NOT understanding. My mind is not the way it used to be and I need a real Class to begin to understand, I am speaking as a beginner. I appreciate your work that I will on occasion share with my other followers that might have a better understanding and true appreciation. God Bless you.
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TonyfromOz
Sun 03/10/2019
Child Of God,
you are in fact a big help to me here.
Right from the start when I began doing these Posts on Electrical Power Generation, I thought it would be really boring. Even though I had a background of 25 years in that electrical trade, I knew it had concepts that would be difficult to understand. I was fully trained in it, and even I had times when there were things I could not understand, and that also went for others in my trade as well. The problem I had right from the start was then to attempt to explain it for those people with no training in it at all. I had to find ways to make it so that people could at least get the basic idea of what it was all about. Having actually spent five years teaching the electrical trade made that task a little easier, but not that much.
So, even now, after doing this for so long, there are still some refinements I can make to make it even simpler to understand, and oddly, that sometimes gets me into trouble. When people who actually ARE trained in those aspects read some of the things I write about, they comment that I’m not including the intricacies of what it is about, so it seems like I’m leaving things out, so I have to explain that the target audience is not people who are knowledgeable in the subject, but those who are NOT knowledgeable in the subject.
So, every comment is good. There’s no need for you to apologise for not understanding it either, so I hope this helps.
Tony.
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nickreality65
Sat 03/09/2019
“…delivers in power to the grid over a period of time, here hours,”
A MW is a power unit, energy over time, 3.4 Btu/h or 3.6 kJ/h. Like the 300 Hp engine in your car.
A MWh is an energy unit same as Btu or kJ. Say your commute averages 200 hp for an hour, or 200 Hph and will use 10 gals at 20 mpg of xxx Btu/gal and $3/gal.
Your utility bill might show a demand change. This is for the MWs and other physical assets, lines, transformers, etc. Think of it as your monthly car payment, insurance, parking, etc. You pay this whether you drive 1 mile or 1,000 miles. The more miles you drive the lower the incremental cost. Say your fixed monthly payments are $200. 1 mi = $200/mi. 1,000 mi is $0.2/mi
Your utility bill will show an energy charge for kWh actually used. This is like the gasoline you burned by your car for the month. Might include wear and tear on the tires and mileage based ,maintenance, oil changes, etc. For instance, gal / mi * $ / gal = $ / mi. So this variable expense depends on the car’s efficiency.
Of more concern to the utility’s MW capacity is the load. Charging an electric vehicle in 15 minutes takes 4 times as much capacity, i.e. MWs, then charging it in over 4 hours.
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NEO
Sat 03/09/2019
Close but not quite. Anton is spot on, MW has no time involved, it is the maximum a device can deliver, or as he says the nameplate rating. MWH is the amount delivered over the period of an hour, not the maximum, but the average.
A demand charge (in the US, usually only on 3 phase, fairly large services) is figured on the power factor, whether leading or lagging combined with the maximum current drawn. If you do not correct for power factor on inductive loads and/or start many motors simultaneously, you place excessive demand on the distribution system and this charge usually covers it. If you correct power factor (usually not very expensive) and stage starting the loads, you can reduce the demand on the supplier, and in recognition of that, your bill will go down.
Correcting for it is an engineering task, and pretty simple. We’ve been doing it for at least a hundred years. But the necessity of the charge is really more about the rather high fixed cost of central station electric power, and actually more to do with accounting than technical subjects. In my experience, most utilities require correction on 5hp to 10 hp motors, and on large banks of inductive lighting (Fluorescent, HID, and so forth).
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nickreality65
Sat 03/09/2019
My monthly residential utility bill has a demand component. It’s what I pay for a meter wired to the grid whether I use 0 kWh or 700 kWh
MW is power, energy over time, MWh is energy.
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TonyfromOz
Sat 03/09/2019
nickreality65,
that’s a supply charge, as they call it here in Australia. It has nothing at all to do with the power you actually consume.
P=EI and is expressed in Watts. (or KW, MW, GW etc) Power that is generated (or consumed) over a period of time is MWH. (or KWH, GWH or TWH)
Tony.
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nickreality65
Sun 03/10/2019
“…with the power you actually consume.”
I don’t “consume” power I “consume” energy.
My electrical panel is sized for 200 amps. If I use all of that the utility has to deliver that capacity. 200 * 120 = 24 kW. The planners know I won’t do that so maybe they figure half that or 12 kW.
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NEO
Sun 03/10/2019
That’s a fee for service, not a demand charge.
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nickreality65
Sun 03/10/2019
Pot-ay-to Pot-ah-to
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TonyfromOz
Sun 03/10/2019
nickreality65,
they are two separate things.
One is the actual connection to your power box which enables you to be supplied with grid electricity. It is a fixed charge, usually a fixed amount per day.
The second is the actual power you draw for your home from the grid, and that is a charge per KWH, not KW, but KWH, the power your home actually consumes.
If you look at your most recent electrical power bill, you’ll see that while that fixed amount stays the same, and is a fixed amount per day for the entire billing period, while the other and higher charge is a per KWH cost for the amount of electricity your home has actually consumed across the entire billing period.
Tony.
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nickreality65
Tue 03/12/2019
You know, we are pretty much saying the same thing.
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Hifast
Sat 03/09/2019
Reblogged this on Climate Collections.
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