Question about grid efficiency

[Poindexter]

I am in Solar Power Your Home for Dummies, c2010, second edition, page 352:

"When you install a large PV solar system, you cut out a tremendous amount of pollution because our electrical power grids are extremely inefficient. For each kWh of energy you create with a PV system, you can save 3 or 4 times that much utility generated power."

True or false?

 

[begreen]

Our grid systems are lossy for sure. This blog says 8-15%.

https://blog.se.com/energy-management-energy-efficiency/2013/03/25/how-big-are-power-line-losses/

 

[Poindexter]

Thanks @begreen , the same page you linked does state that 100 units of energy saved in the home can save 300 units at the power plant - accounting for both transmission losses _and_ thermal inefficiency at the generating plant. Thanks.

 

[peakbagger]

I picked up a thermal camera and its interesting to take shots of electrical equipment and wiring around the house. I can see exactly what circuit is pulling amps. Every transformer loses 2 to 3% of its power. Typically a power plant is going to go through a step up transformer to get up to transmission voltage it they are sited well ( frequently two step ups), then 10s if not 100s of miles of transmission cable then usually has to be stepped down at least 2 more (likely 3 or 4 times) to get back to your house voltage. Utilities actually need to factor in how much the power lines will stretch and droop closer to the ground when heated up by resistance in the cables.

This is great graphic of the power system https://flowcharts.llnl.gov/

Power plants rarely if ever have a demand for the waste heat that is generated as a byproduct of the power generation process. If its a thermal heat plant, it has to reject heat and lots of it through cooling towers, air cooled condensers or direct cooling with some sort of natural source. That is why many densely populated areas in Europe have district heating systems and many are requiring that users of those systems are required to install low temperature radiant heaters. By having a use for that waste heat in local power plant, the overall cycle efficiency is bumped up. I built a combined heat and power plant in Mass for a papermill and we squeezed every bit of heat we could out of the power plant to heat and preheat water (papermills use incredible volumes of water) and we won an award as the most efficient project in its state and I think it was 73% of fuel input. A regular modern thermal power plant is in the low 40% and modern combined cycles are just breaking 50%. Heck nuclear plants only use 20% of the energy in their fuel. The rest ends up as high grade nuclear waste. The nuclear plant design used in the US and much of the world was picked to support the generation of feed products to make weapons grade plutonium. There were more efficient cycles developed and tested by US like the thorium cycle (that self extinguishes) but they didnt produce the plutonium the US needed.

I was involved with a power plant upgrade concept in Alaska. It was a rare natural gas plant with a line up of several reciprocating engines. We were going to put in organic rankine cycle equipment (a backwards heat pump cycle) on the exhaust stacks to add another 10% to the plants electrical output and a good bunch of low grade waste heat (probably good for greenhouses) It was a good project, (still is) but the group proposing it didn't have the political pull in the Alaskan government.

 

[woodgeek]

I agree with the above, but would suggest that the original quote is (confusingly) discussing the difference between primary and electrical energy/power. Many older thermal plants are very inefficient converting heat to power, so an efficiency computed from input thermal power could be 30-50% in many cases.

This table lists BTU/kWh figures for different US plant types. https://www.eia.gov/electricity/annual/html/epa_08_01.html

Since we all know that 1 kWh = 3414 BTUs, dividing through gets that Coal and Nuke plants are about 35% efficient (or closer to 30% after grid losses) while gas plants are closer to 50% (or 45% after grid losses).

Just another example of the confusion around primary energy statements.

 

[spuldup]

Believable. They burn *something* to boil water to turn turbines to move electrons. Then step the voltage up for transmission, then back to 240 for your house. Every part of the cycle generates heat, even the transformer on your pole. How can that be reasonably efficient? It is only profitable because the fuel/source is *relatively* cheap.

Solar gets electrons excited in the PV, then it gets inverted to AC. 2 steps vs 10 from the power company.

 

[EbS-P]

Is your end goal cost saving or carbon savings? Point of use PV is probably competitive on the carbon side. But probably not as competitive on the cost side vs a solar farm. What sort of carbon saving could the cost savings be invested in?

 

[incinerator]

I am in Solar Power Your Home for Dummies, c2010, second edition, page 352:

"When you install a large PV solar system, you cut out a tremendous amount of pollution because our electrical power grids are extremely inefficient. For each kWh of energy you create with a PV system, you can save 3 or 4 times that much utility generated power."

True or false?

Engineering Explained on Youtube had a video that covered some of this when he was comparing an EV with an ICE vehicle. I don't remember the specifics but i recall that transmission efficiency is just over 90%. Natural gas power plants are pushing 60% efficiency and nuclear reactors are around 33%.

 

[DBoon]

There is a lot of misinformation around this topic and bad assumptions that go into Energy Information Administration (EIA) data about this. Average transmission/distribution losses are about 7% for the transmission and distribution system, but this would certainly be higher for my very rural county compared to a small to medium sized city. This value for losses seems pretty reasonable on average. The EIA reports average electrical generation efficiency (BTUs in to BTUs as kWhs out) to be about 37%. This percentage never seems to change, despite increasing amounts of solar and wind (which use no BTU inputs) electricity generation. I did some research on this a while back and there are footnotes about assumptions made by the EIA in calculation of this data that basically assumes some BTU inputs for solar, wind, hydro, etc. that are really fairly ridiculous, and given how many more efficient newer natural gas plants there are compared to the many recently retired coal plants, and it gets even more ridiculous. We could have a 100% renewable powered grid and in 30 years the EIA will still be reporting the GT&D efficiency as 37% overall.

 

[peakbagger]

I used to work with a testing firm that established plant efficiencies usually to see if they met the guarantees. We used ASME Performance Test Codes (PTCs) that were very precise and generally accepted in court. The thing with plant output and efficiencies (also referred to as Heat Rate) is its one of the more closely guarded values in power plant. It is not something that is generally published or available for a plant. The reason is selling power to the grid is very competitive and someone selling into the markets would just love to get their competitors heat rate so that they can make an economic model of the regional grid so they know how much to bid into the grid for the various markets. Therefore, unless some government agency forces a power station to disclose the numbers, they are not giving it out. I used to develop detailed performance models of power plants on occasion and if I knew the basic configuration (usually available) I could plug in values for various major components and come up with functional model of a plant that would be fairly valuable to someone who was bidding into the market, If I had the heat rate, I could refine the model. Gas turbine power plants have an Achilles Heel and that is inlet air temperature, the warmer it is, the lower the performance. Inlet air cooling can be used but that takes power so the gain from cooling is complex. So a database for plant efficiencies is not something easy to get to develop an overall grid efficiency.

Plant efficiency also depends on what type of power is being dispatched onto the grid. The efficiency is going to change minute to minute day to day. I have no doubt that grid efficiency is increasing but from an illustration basis the number used by EIA is good enough.

I have done far more detailed research in the past on plant efficiency in the US but its covered by NDA and not worth the hassle.