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Efficiency figures

arild jensen


Joined: Mar 02, 2012
Posts: 37
Location: New Hazelton BC zone 3 lat 56 north
I often see values for efficiencies quoted in various comments dealing with technology. Sometimes I recognize them as being valid but sometimes I see them as being misleading.
Few people realize solar panels are only around 22% efficient. To them solar panels cost nothing to operate so their thinking is they must be 100'% efficient.
Scientists who specialize in such things have determined just how much sunlight energy reaches the Earth's atmosphere.
They have further determined that after losses due to atmospheric absorption only 1000 watt per square meter reach the earth's surface with the sun directly overhead.
At higher latitudes sunlight comes through the atmosphere at an angle thus resulting in more losses. Clouds or thin films of dust on the panel also result in losses. Most users rarely allow for this.
Somewhere in one of the posts on these forums I saw mention that an alternator is less than 50% efficient. Not exactly true but it is true that a typical generator ( which uses an alternator) is about 37% efficient. However, than same alternator driven by water flow can be around 80% efficient sometimes better.
It all depends on where you measure your values and under what operating conditions the measured device is run.
A solar panel has zero % efficiency at midnight. Do not let misleading sales brochures mislead you.
Marcos Buenijo
pollinator

Joined: Dec 18, 2011
Posts: 583
Location: Southwest U.S.
    
  12
Excellent topic. I agree that efficiency is often not properly understood, nor is it properly considered. One example that is highly relevant is the dramatic variation seen in the efficiency of a gas generator over its power range. To bring this message home, consider the figures listed for an 8.5 KW Kohler natural gas standby generator:

100% load: 3.7 m³/hr (132 cfh)
75% load: 3.2 m³/hr (113 cfh)
50% load: 2.6 m³/hr (93 cfh)
25% load: 2.2 m³/hr (77 cfh)

See www.kohlerpower.com

When the load drops from 100% down to 25% the fuel consumption rate does not drop by a factor of 4, but by only 40%. The engine is about 25% efficient at full load, but drops to about 11% efficient at 25% load! It only gets worse as the load continues to fall. This is particularly important to understand because many generators operate at a small percentage of their rated load the vast majority of the time. I've seen some highly inefficient power plant configurations devised by not considering dynamics such as this. For example, people often automatically assume a battery system will reduce overall efficiency. However, this is not necessarily the case when the charging system is designed to operate at a constant charging rate where the efficiency is optimal. Also, running large loads off the generator while the battery is charging will avoid many losses altogether.

About alternators and some other devices: I understand that most conventional automotive alternators are about 50-60% efficient. A good permanent magnet alternator is about 70-80% efficient when operated at an optimal rate. Good generator heads for generators on the order of 5-10 KW can be 80% efficient or even a bit more. Small Diesel engines (less than 10 KW) are normally 25-30% efficient. The efficiency of Diesel engines do vary like in gas engines, but it's not nearly so bad.

Some other devices relevant for the off grid setting include wind turbines, inverters, and batteries. My research suggests that a good small wind turbine will deliver convert about 20-25% of the wind energy available in its cross section to mechanical energy. The efficiency is highly dependent on having blades and a turbine speed optimized for the wind conditions present. Inverters are typically 85-90% efficient. Lead acid batteries are about 80% efficient.

My research suggests most PV panels on the market today are about 15% efficient at converting incident solar radiation to DC electricity. Of course, there are all sorts of losses as discussed in the previous post. One significant loss is seen by not matching panel voltage to battery charging voltage, and Multi Power Point Tracking (MPPT) charge controllers do this automatically.

arild jensen


Joined: Mar 02, 2012
Posts: 37
Location: New Hazelton BC zone 3 lat 56 north
Marcos Curiously my own information differs from yours. Because I worked in a naval design office I often had to extract fuel flow data for on board generators. When I bemoaned the lack of full BSFC curves and only got point data for 50% and 100% load data for various gensets the engineer informed me that most of their gensets had essentially linear curves between the two points. And if I was any sort of decent designer I would avoid selecting a genset that operated at less than 50% because of known service issues associated with this. And this also explains why we normally design in two gensets per yacht. Sometimes three.
Marcos Buenijo
pollinator

Joined: Dec 18, 2011
Posts: 583
Location: Southwest U.S.
    
  12
arild jensen wrote:Marcos Curiously my own information differs from yours. Because I worked in a naval design office I often had to extract fuel flow data for on board generators. When I bemoaned the lack of full BSFC curves and only got point data for 50% and 100% load data for various gensets the engineer informed me that most of their gensets had essentially linear curves between the two points. And if I was any sort of decent designer I would avoid selecting a genset that operated at less than 50% because of known service issues associated with this. And this also explains why we normally design in two gensets per yacht. Sometimes three.


I don't doubt it. Large generators are usually a lot more efficient, especially large Diesel generators. This is so for many reasons, particularly the lower cylinder surface area/volume ratios in larger engines that lessen thermal losses, the often higher compression ratios in large industrial gensets, and the attention to minimizing friction in industrial engines designed for longer life. I expect a large slower moving Diesel to see a net thermal efficiency on the order of 35%. Extremely large slow moving Diesels can reach 50% net thermal efficiency. The efficiency of Diesel engines also do not vary nearly so much over their power range like gas engine generators operated at a constant rpm (especially 3600 rpm where the friction losses are higher).
arild jensen


Joined: Mar 02, 2012
Posts: 37
Location: New Hazelton BC zone 3 lat 56 north
On the subject of generator use either permanently or as occasional back-up. By now most people have heard of the HONDAi generators.
Yamaha and a couple of other makers have by now copied the idea.
The generator adjust its speed dependent on load. Light load = slow speed, high load = fast speed. Not only does this save fuelbut also wear and tear on the engine. Running a conventionalconstantm speed genset at light loads is not only fuelinefficient but causes ssomething called "wet stacking" resulting in premature wear and glazed cylinder walls.
If your primary use is charging a battery, an even better approach is to get a DC genset. I built my first such generator back in 1965 before I even knew a name for it. My latest version has a diesel engine and can deliver 100 A @ 12V. Some clever bargain hunting meant it cost me $200 but realistically expect to pay closer to $1000 unless you are really good at bargain hunting like me. This is still less than what you would pay for a normal AC powered charger of equivalent capacity. 100A chargers typically cost $1500 new.
If you are willing to settle for a gasoline or propane fuelled genset and a taper charger instead of a 3 stage smart charger you can build one from scrap $200
R Scott


Joined: Apr 13, 2012
Posts: 2508
Location: Kansas Zone 6a
    
  22
If you are just charging batteries, you can build a genny from car alternators, an engine, a few pulleys and a belt.
Marcos Buenijo
pollinator

Joined: Dec 18, 2011
Posts: 583
Location: Southwest U.S.
    
  12
Wow, it's scary how much I'm agreeing with you here. You're describing the battery charging system that I've considered for my (future) off grid cabin. I intend to belt drive a good permanent magnet alternator from a small Diesel engine (or gas engine if I can't find the right Diesel engine) taking care to find the optimal speed for both engine and alternator to optimize efficiency. I plan to send charge through a 3 phase bridge rectifier directly into a "small" forklift battery. I intend to optimize the system for a constant charging rate while dual-fueling the engine with a small wood gasifier. I expect the optimal engine speed to be on the order of 1500-2000 rpm. That should make for a long-lasting engine. Then there's the auxiliary oil filter system, but that's another story. If you have any suggestions, then please throw out some ideas. In particular, I'm wondering about the viablity of charging directly through a rectifier. Although, I did plan on using a diversion load charge regulator, and using a small solar array to float charge the battery. I'm weak on the electrical side of things.

 
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