advise: First-build roof-shed water catchment @feelfree

We're planning on building a two floor 5x5m‘ workshop/temporary living space at our lot in Capurgana, which is within a tropical monsoon climate.

Our farm will start as soon as we have this liveable space ready on the lot. We'll be starting a nursery to the main structures side, too. The workshop will serve as a shopfront for the farm and storage and temporary living space until we can start the kitchen project. Rainwater falling on the 25m‘ roof will flood a catchment pipe which fills a (second floor) 500l water tank directly beneath the rain gutter point. The water will be used for washing (one faucet) below the tank (max pressure) and another leading outside for a simple jungle shower. The two systems will lead into a grey water treatment bed of plantain and beneficial plants.  
A messy scribble describes the idea for the more extreme visual learner here.
http://imgur.com/s6ujZQE

Q1. We're at 8˚ latitude here, and plan on mounting a solar system on the roof. I'm told that the optimum angle for panels relates to Latitudinal zone. Any idea how to best optimise solar panel light exposure, including magnetic direction (I assume the roof will slope slightly to the north)?
We will rarely struggle for water, but catching it seems the most logical way of supplying limited needs, and it would be great to harvest the abundance of rain, So:
Q2. Has anyone ever created a simple system like this one? I worry about the pressure being too low, the catchment too big for the tank «
http://imgur.com/JJpvHHI
Q3. That a tank of 500 litres will be too weighty for 8x 6"x6"x8' Wild Cashew timber building to hold on the first (raised, boarded floor)?
http://imgur.com/nZOTsoG
Please leave any feedback on the idea here as we're ready to build in June!

Contact us and follow our progress on http://www.facebook.com/feelfreehostel ;) Likers win free t-shirts (from the workshop!!!)

For the solar panels, I know they sell electronic trackers that optimize your panels to the best possible efficiency in that area.  I do not know if they make them for roof installation though. The ones I have seen were installed on stands on the ground.

Hey Matt

Thanks, I'll look into it, though I think it might break the budget! I was just thinking N-S Directions near the equator.

Regarding the panels, I don't have practical experience but I understand the geometry and some of the principles so take what I say as food for thought rather than definite advice.

Your Sun goes more or less overhead throughout the year so I would suggest a panel more or less horizontal. If it slanted 8 degrees towards the south (if you are northern latitude) or 8 degrees towards the north (if you are southern latitude) then you will be maximizing reception at midday around the equinoxes (March, September) and getting a good reception throughout the year around the middle of the day. I'm not sure if panels are subject to pooling water or if there is a recommended minimum slope angle to encourage water droplets to trickle off.

If you are storing up your energy for night usage this is probably best. Regular daily weather patterns may effect you choice of angle, so, for example, if you mainly get clear mornings and clouds build in the afternoon, then consider angling them a little to the east to catch more of the morning Sun.

If by any chance you mainly need most of your energy in the morning and middle of the day and you want to minimize battery capacity, you might consider positioning them more towards the east.

Dan, it looks like the larger tank is inside your structure. Is that the 500 liter tank? That sounds small for the amount of rain you will get. Can you fit a 1,000 liter tank?

If you are storing up your energy for night usage this is probably best. Regular daily weather patterns may effect you choice of angle, so, for example, if you mainly get clear mornings and clouds build in the afternoon, then consider angling them a little to the east to catch more of the morning Sun.

Hey Tom thanks for useful advice we certainly will be generating during the day for night time use, as well as a chest freezer and other, mostly kitchen, appliances. Pooling water wont be problematic as the panels will have to be transported inside at times anyway (to keep the local coyotes from half-inching the panels). Any idea what size panels and batteries I might need for a roundabout 1400kw p/month average?

Dan, it looks like the larger tank is inside your structure. Is that the 500 liter tank? That sounds small for the amount of rain you will get. Can you fit a 1,000 liter tank?

I thought so too until I looked at the plans, I think the structure will suffer holding that much water weight at elevation. I would love more capacity though, so I'll keep it in mind when the foundations (footings) are laid. I figure we get fairly consistent rain so we shouldn't need more than 500ltrs between downfalls. I was also thinking overflows can lead to more storage below on ground floor, or even run through a drip system for the nursery... Thanks

To get estimates of panel and battery sizes I would want to ask someone with a set up in your area or a similar climate. I think they would have a feel for it. (I don't, sorry). In the meanwhile, the calculation tool at http://pvwatts.nrel.gov/ might be interesting to look at. It's been a while since I looked at this but from what I remember, it's for a grid connected setup but you can get it to give a calendar of a hypothetical typical year showing daily energy generation and from this you can see what a bad day looks like in your area and make sure your setup will still be effective on a bad day. Or if you want to have enough battery capacity to see you through a few bad days, you can see a worst case scenario where there is a succession of days with low generation. I don't know which would be cheaper: more panel and enough battery to see you through each day, or less panel and batteries to see you through a string of dull days. Bear in mind that lead acid batteries don't last very long so need replacing (or reconditioning?) on a regular basis, it is best not to drain them each day so you may need around double the capacity  (if you want to store 50kWh for each day's usage, you may need to get more like 100kWh capacity batteries so they don't run down - I'm making these figures up to explain what I mean), and factor in the inefficiency of the batteries.

Like I say, I don't have practical experience and it would be better to find a neighbour doing a similar thing and ask them.

Hi Dan,

A 500L water tank when full will weigh in at 500kg (for the water) + the weight of the tank itself. Make sure that tank support is strong enough to handle the weight when full - and remember that timber in your location will possibly break down unless sufficient air and sun can get to it so as to keep it more dry than it would otherwise be. I've been on tank water for many years being in the much drier country of Australia. Although I have 105,000L stored in the water tanks! With your roof space of 25 sq metres, you may be interested to know that for every square metre of roofspace you will harvest 1L of rainfall for every 1mm of rain - assuming you can catch all of that wet stuff which falls from the sky. The water should exit your tank at about 10L per minute using gravity alone. Hope that helps. One inch pipes (25mm) produce a good flow of water, but then 3/4 inch (20mm) pipes are pretty good too.

I'm also on off grid solar power and have been for many years. You are in the northern hemisphere at 8 degrees latitude, so you want to angle your solar panels at about that same latitude but facing south. The sun doesn't change its location in the sky by much from season to season and will be much higher overhead on average than down here. I'm at a latitude of 37.5 degrees south and my panels face north at about that angle. During summer the sun is high overhead, but during the winter months the sun is closer to the horizon during the day. The angle is really a compromise of sorts between maximising winter sun and summer sun. The solar panels have to face the sun in order to produce the best output, but near enough is good enough. Incidentally, ignore all advice about trackers for solar panels which follow the sun during the day because I have seen many installations of these devices and every one of them was broken. They sound great in theory and they will increase your solar output, but... Manual solar trackers are great, if you can remember to move the solar panels several times per day, every single day of the year without fail.

Good luck with the building!

Chris