catching a bit of sun

In some of the posts about sports I've noted the efficiency at converting the energy stored in food into moving our muscles is in the 20 to 25% range.  Much of the remaining energy is converted into thermal energy and we have to get rid of quite a bit of heat in the process.  In theory it is possible to convert energy from one form to another, but much of our world is less than perfect.  While energy is never created or destroyed, often its transfer or conversion results in at least some of it being difficult or impossible for us to easily use and we can talk about the efficiency of a process.¹  

 

Here are a few examples:

conversion process	type of conversion	efficiency %
large electric generator	m -> e	98 - 99
bicycle chain drive	m -> m	95 - 99
large power plant boiler	c- > t	90 - 98
natural gas furnace	c -> t	95 - 97
water turbine	m -> m	90
human lactation	c -> c	75 - 80
small electric motor	e -> m	65 - 75
best bacterial growth	c -> c	50 - 60
wind turbine	m -> m	50 - 55
steam turbine	t -> m	40 - 45
supertanker diesel engine	c -> m	50 - 55
turbofan under ideal conditions	c -> m	50
electrolysis of water	e -> c	40 - 50
large gas turbine	c -> m	40
small electric motor under 200w	e -> m	30 - 50
automotive diesel engine	c -> m	25 - 35
automotive gasoline engine	c -> m	15 - 25
photovoltaic solar cell	r -> e	5 - 30, 15 common
home refrigerator	e -> t	10 - 20
human muscles	c -> m	15 - 25
pregnancy	c -> c	10 - 15
modern chicken growth	c -> c	10 - 15
gas stove	c -> t	10 - 15
fluorescent lamp	e -> r	8 -15
beef production	c -> c	3 - 10
peak photosynthesis	r -> c	4
watt steam engine	c -> m	2.5
incandescent lamp	e -> r	0.7 - 3
beeswax candle	c -> r	1
neucomen steam engine	c -> m	0.5
global photosynthetic mean	r -> e	0.3

            Here c is chemical energy, t is thermal, r is radiant, m is mechanical, e is electricity

You'll notice that some conversion devices like large generators and bicycle chains are fantastically efficient while those that involve burning fuels, metabolism, most forms of producing visible light and photosynthesis are fairly inefficient.²

 

When dealing with systems that cascade these processes, you can often find short cuts and extract a lot of unmined efficiency.  Co-generation is an example - a large power plant with a steam engine can create a lot of thermal energy that won't run a conventional steam turbine, but can heat a massive greenhouse or office building.  Putting solar cells on the roof of a building to power electric lights may only be 10% times 10% - or just 1% efficient.  Rethinking the building and using daylight through the windows most of the time offers an enormous savings.  It can be like finding diamonds on the beach.

 

Most of our energy comes from the Sun.  The dominant source of its energy comes from a set of light element fusion reactions that are "fueled" by the crush from the Sun's internal gravity.  Most of this takes place in the densest part - well within the first quarter of its radius.  The power density is surprisingly low - about the metabolism of a lizard and about a quarter of our metabolism - but there is a boatload of star out there.  This energy is mostly radiated out into space as light - about half of it visible light.  Most of it misses the Earth - we only catch about a billionth of its output.  Some of the light is reflected out into space, some is absorbed heating our atmosphere, land and ocean and some of it is converted into chemical energy by photosynthesis.

 

We should have a hooray for photosynthesis! day.  Radiant energy is ephemeral.  It takes about 500 seconds to make the trip from the surface of the Sun to us and then needs to be captured.  It would be nasty to not be able to make it through the night or even cloud cover ...  fortunately we have this wonderful chemical fuel and our cells are filled with little chemical fuel cells.  

 

But photosynthesis is fundamental to life on Earth and its efficiency is terrible - the world average for the biosphere is about 0.3%.  This creates an upper limit on how much life can be sustained.  You can work out how much energy can be fixed by certain types of agricultural crops.  Our main food crops are highly optimized for our metabolism needs, but the overall efficiency of a field is better than the global average, but is generally less than one percent.³

 

It is fun to work out the efficiency of modern post green revolution mechanized agriculture.  The world's cultivated area has roughly doubled since 1900 (we are close to the limit now), but the energy available in the form of edible crops has increased sixfold.  This has been made possible by an increase of the energy used by agriculture on the order of 8500% - we are using about ten times as much energy per kilogram of harvested crop, but we are getting much more.  We have increased the number of people who can be sustained on earth from perhaps a billion and a half to about seven or eight billion.  

 

This is enormous progress.  In 1900 a hectare of good farmland could produce about 15 megajoules of food energy per day - about 3,600 nutritional Calories. The world supply worked out to about 2,400 Calories per capita per day - not much of a buffer if there was a bad harvest.  Now we can supply about the same for all of the world's people.4  A huge challenge for science and policy is where to go as there are no clear paths to guaranteeing a good supply of food to 9 billion people - the projected population for 2050 - given current technology and consumption trends.  A second green revolution isn't immediately apparent.

 

 We need to revisit efficiency.  About ten years ago I wrote a paper noting:

 

let light be light

let heat be heat

let food be food

The notion was that it is generally inefficient to have serial energy conversions.  It is much more efficient to eat a vegetarian diet than one that uses meat.  At this point most of the undeveloped world is largely vegetarian, but as the population becomes more wealthy, people tend towards eating more meat.  It takes about ten Calories of the energy in grain to make one Calorie of beef - and that assumes the animal is completely consumed. Poultry is somewhat more efficient than beef, but a largely vegetarian diet may be necessary to share meat with others who are newly wealthy as well as to ensure everyone has something to eat.⁵  A potentially larger problem is growing food as a fuel to displace petroleum.

 

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¹ Some people like to talk about exergy which is loosely the maximum amount of useful work in a process.  Some chemists and engineers like that way of looking at the world, I prefer using efficiency.

² It turns out there are strict theoretical limits on many of these.  One of the triumphs of 19^th century physics was sorting out how to calculate the efficiency of heat engines.  That led to a basic understanding of thermodynamics and one of the greatest mechanisms for dealing with "big data" on a scale that is unimaginable to today's big data computations.

³ Getting a handle on these numbers can be very difficult as you have to consider the question of temperature, season, water availability, pests, and energy associated with running the farm, pumping water and making fertilizer.

The theoretical efficiency for photosynthesis is about 11%, but a variety of processes make that very impractical.  Ping me if you want a detailed discussion on how many photons of a certain wavelength are necessary to fix an molecule of carbon dioxide (about ten it happens)

⁴ There is a distribution problem and a huge waste problem.  About a billion people on Earth are malnourished.

⁵ It turns out I'm a vegetarian, but I tend not to care about what others eat as that is a personal choice.  There is a huge energy advantage to that way of life and that may be important as time goes on, but currently it isn't a huge issue locally in North America.

 

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The beautiful image credit:

On August 31, 2012 a long filament of solar material that had been hovering in the sun's atmosphere, the corona, 

erupted out into space at 4:36 p.m. EDT. The coronal mass ejection, or CME, traveled at over 900 miles per second. T

he CME did not travel directly toward Earth, but did connect with Earth's magnetic environment, or magnetosphere, 

causing aurora to appear on the night of Monday, September 3. 

Picuted here is a lighten blended version of the 304 and 171 angstrom wavelengths. 

Credit: NASA/GSFC/SDO

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Recipe Corner

 

Before fresh sweet corn goes away.  It was fantastic.  My vegetable broth was incredible with all of the fresh produce this time of year.

 

Cashew Corn Chowder 

 

Ingredients

 

° 1 tbsp. coconut oil 

° 2 medium yellow onions

° 6 cloves garlic chopped

° 2 tsp. sea salt

° 2 tsp. turmeric

° 1 tsp. cumin

° cayenne to taste

° kernels from 3 ears corn

° 1 kg vegetable broth

° 1 tbsp. freshly-squeezed lime juice

° 60g raw cashews, soaked overnight 

 

Technique

 

° Heat oil in a large pot over medium heat. When hot, add turmeric, cumin and cayenne and cook for about a minute, until fragrant.

° Add onions and salt, and cook for 5 minutes until softened, then add garlic. 

°  Add corn kernels to the pot and stir to coat with spices. Cook for 5 minutes then add the remaining broth and bring to a boil

° lower to a simmer and cook until the corn is bright yellow and sweet (about 5 minutes) 

°  Remove the pot, add the cashews and blend with an immersion blender.  You can reserve a few tablespoons of the whole corn kernels for garnish if you're into presentable food.

° Season to taste.

° Return soup to the pot to keep warm. Serve with cilantro cream, a drizzle of olive oil, a few kernels of corn and plenty of cracked black pepper.

 

Cilantro Cream

 
° 60g  raw cashews, soaked overnight

° 1 tbsp. freshly-squeezed lime juice

° 1 cup cilantro (leaves only, loosely packed)

° 120g water

° 1 tbsp honey

° 1 tsp. sea salt

° pinch cayenne 

° ½ small clove garlic 

Technique 

° Throw in a blender and blend until completely smooth and season to taste