Sunday, 20 July 2014

Light, Energy and Plants

Light, Energy and Plants

While thinking about weather, particularly rain, in providing what plants need to grow well my attention has turned to light and temperature.

Permaculture keeps telling us the reduce energy, grow diverse food plants and generally observe nature to better interact but I keep coming back to the same old issue. Permaculture ideals,or ideas, are all well and good but very few Permaculture people appear to have an good understanding of some of some the issues and therefore can't act, interact, reduce or create better alternatives to achieve some of the goals.

I'm in the same position, how do you grow a wider range of food successfully if you don't know what the plants needs are other than to repeat what commercial people (farmers, seed companies and conventional books) tell you. How can you reduce energy effectively if you don't know where or how you are wasting it. How can you capture enough rain water if you don't know how much water you need.

Observation is the key word, but understanding what you are observing requires learning. The sun's energy is key to everything but apart from saying that what do we actually know about it? The plants use the sun to photosynthesise, great but how and how much? How much energy does the sun produce? I've been reading up and trying to answer some questions.

First of all how much energy does the sun give the planet?

The energy from the sun arrives at the planet in the form of different frequencies. Some of these frequencies our eyes can pick up in the form of colours of light while others are invisible. These frequencies are put into different ranges. From Ultra Violet, through Visible light to Infra Red. Ultra Violet light we can not see but this range is split further down into UV a, UV b and UV c light. We've heard about that because UV can cause skin cancer. Visible light is split into smaller ranges, Red, Orange, Yellow, Green, Blue, Indigo and Violet. After that we have Infra Red.

How much energy in these frequencies has been calculated and put in a form we can understand and compare to other forms of power. The combined power in the Sun's light that hits the earth's atmosphere is 1,366 Watts per Square Metre. There is about the same energy hitting every square metre of the earth's atmosphere to power your electric kettle.

Of that energy, plants take some and use it to make other things, some of which gets stored in the ground in the form of carbon (coal, gas and oil being the most notable) but some energy gets absorbed and heats things up which ultimately causes warm or hot spots which creates differences in temperature which then helps to drive wind and the weather, and of course we can capture some and convert into electricity.

Converting the sun's energy into electricity is something that interests me. Solar panels. The Silicon crystals are made in such a way that they respond, as well as can be made, to as many different frequencies of light as possible. Currently they can convert approximately 15% of light into electricity.

Knowing how much light reaches the atmosphere, and knowing that a solar panel can convert 15% of that into electricity suggests that I can take a solar panel and work out how much light there is at any given time on any given day. From that I could compare how well my plants grow when given certain amounts of light.

Knowing how much light there is should enable me to work out what foreign food plants will grow in my garden.

Things got complicated when I realised that some of the 1366 watts per Meter doesn't reach the ground because of haze, clouds etc but on a clear day at noon approx 1000 watts per meter reaches the ground. While doing some calculations with a solar panel I realised that things got even more complicated because of temperature. All the quoted ratings of a solar panel are based upon a solar panel running at 25 degrees Celsius. When the sun hits the solar panel it warms the panel, often to between 40 and 75 degrees. For each degree above 25 degrees C the solar panel becomes less efficient and loses approx 0.4% of its power for each degree. On a very hot day the solar panel, although supposedly 15% efficient will loose 20% of this.

The long and short of this is that although complicated I should be able to measure how much light and how much energy hits plants.

How efficient are plants?

This then led me to wonder how much energy plants take to photosynthesise. It turns out that they only need between 1 and 4% of the suns energy depending upon the plant. Not only that but plants don't use the full spectrum of light, which once I read about it makes sense. The fact that leaves are green shows that the plants don't use that part of the spectrum as they reflect those frequencies. Plants absorb light in the red and blue parts of the spectrum. This has been demonstrated by looking at how much oxygen a plant produces when different frequencies of light are applied. Plants have evolved as very inefficient energy converters because there is no pressure for them to be efficient, ie, there is plenty of light, plenty of energy in the sun.


If plants only absorb and use red and blue light (broadly speaking) then why do commercial greenhouse people waste a lot of energy producing light in the full spectrum only for the plants to reject much of it. Perhaps the lighting energy bill could be halved if you only produce certain coloured light. Better testing of how well plants do could also mean that not only the colour of light can be tailored but also the intensity.

Looking deeper into observations and having a deeper understanding of the issues can and will open up many more opportunities for reduction in energy, but also a chance to grow better plants and a wider range of plants.

If I know how much energy (light) falls on my plants, and I know the temperature and rainfall I can choose the plants I grow and be more certain as to their success. Also knowing that the season has started badly (not enough light because of clouds and lower temperatures) should enable me to give up on some plants at an early stage knowing they won't ripen and still have time to plant a second crop that will mature and ripen with the time left.

The biggest problem that I see is that there seems to be little or no information on how much light a plant needs at certain temperatures but the data from solar panels around the world gives enough information about the conditions and this information can be compared against plants.

Uses of light upon disease
It occurs to me that if plants only need certain wave lengths of light to photosynthesise then perhaps there is a chance to beat some plant diseases, such as blight, by not giving the disease the wave lengths it needs. It is perfectly possible that by filtering out some frequencies you can stop fungus, such as potato and tomato blight from starting. UV is known to stop algae in ponds and hence the use of UV lamps to keep the water clear. Something similar could be used on plants.

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