Wednesday, 31 December 2014

Plastic Photovoltaic Cell

While I was in the process of moving from Tasmania to Victoria, I payed a visit to one of my friends in Tassie. We were having a relaxing time discussing what we had been doing for the last ten years or so, while I was in Tasmania. My friend suddenly got excited and ran off to the other end of his house and came back with a smallish rectangle of plastic (140mm x 100mm).

I have had to wait a while to play with the cell, but now that I have, I am super impressed.
The cell came from Their website says that the Department of Energy Conversion and Storage at the Technical University of Denmark is behind this little gem.
I have tested the cell both indoors (0.67V) and outside in full sunlight (7.56V) and I must say that I am very impressed with the freeOPV. By the way, mine is version 2014 – V2.0 – A1 25.3. The website indicates that I should be able to get around 8V from the 16 strips. I’m getting close to that, so … woot!
Okay, so … what am I going to do with my freeOPV? Well, obviously it has to be something that is in keeping with the renewable energy/green principles of an organic solar cell.
I have a small vegetable patch that is about 3m x 3m sitting next to a 8m x 3m shed. The guttering on the shed feeds in to a 10,000 ltr water tank.. What I would like to do is to make the water usage of the vegetable patch as efficient as I can.
The plan is to make a watering system that responds to the moisture level detected in the soil, turning the water on for a pre-set amount of time when the moisture level falls below a set value.

I will need to embed a hygrometer in the garden that will periodically take a moisture level reading. The Arduino will read the moisture level and compare it to a stored moisture level value. If the moisture level is below the stored value, the pump is turned on for a set amount of time (say 5 minutes). If the moisture level is equal to or above the stored value, the pump remains turned off.
I’ve calculated that the roof of the shed is approximately 24m2. I’ve also calculated the average rainfall (mm) in my area and factored in a water collection efficiency value (0.9).
Area x Average Annual Rainfall x collection co-efficient = Litres Collected Per Annum.
For my system, this means an average annual rainfall of something over 14,000 litres per annum. This gives me a daily water consumption rate of around 38 litres.
Now for some more calculations. Some horticulturalists give an average of 1 inch of water per week or 630 gallons per 1,000 square feet). Damn … conversion time. That’s equivalent to 2384.80942 litres per 304 square meters … roughly 8 litres per square meter, per week.

My garden is 9 square meters, so that would mean that I need to use 70.5 litres per week … or 10.08 litres per day. Right … so I have almost 4 times as much water as my vegetable patch will use.
There are other factors to consider, different vegetable crops require different water levels at different times in their development … plus! If I use water efficient planting (using mulch and increasing the humus in the soil, I need even less water input.
Well, for the sake of this experiment, let’s keep things simple.
If the water level drops down below 10% moisture content, turn the pump on for 5 minutes.
I’m expecting that the pump will not come on all that often and it shouldn’t come on at all on rainy days..
Of course, I will want to be able to log in to the vegetable patch, night or day, from ANYWHERE on the globe and get mission critical information pumped straight to my smart-phone. I want to be interrupted while having a highly charged and pressure lunch with senior business partners with information on the drinking habits of my legumes. Clearly, I will need to embed some Wi-Fi capability into my circuit so that I can interrupt Mr CEO while I water my dinner (more likely I will get a beep while in a job interview).
An Arduino Hygrometer module comes in at around $2.00 on eBay. I’m not sure about the pump control yet, so I can’t comment on cost, but lets say about $20

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