Solar Powered Greenhouse Ventilation ...

It can get very hot in our big greenhouse in the summer and we need to remember to leave the door propped fully-open if we're going to be away, or even just out for the afternoon, otherwise some plants can start to wilt as the temperature rises to the mid-40s celsius.

I therefore decided to build a passive ventilation system, i.e. one which works automatically and needs no intervention from us.   There are no opening windows in the greenhouse, so fitting a pair of extractor fans high up on the rear wall was the plan.  

There are already five low-level aluminium louvre vents provided along the front of the greenhouse from the time of its construction, through which fresh air can be drawn in as the hot air is expelled.

I suppose I could have used kitchen-type extractor fans powered from the mains supply, but I've four 108 Wp solar panels in the shed simply gathering dust at present.  

To first confirm that the solar option was feasible, I dug one panel out, gave it a wipe clean and hooked up a 24V dc fan to the panel connector.   Even on an overcast morning the fan started to run immediately, and checking with my multimeter showed that the load voltage of the panel under those conditions was 20-24V (open circuit was ~34V).

So I decided to refit one of the solar panels to the greenhouse, but this time to one side of the roof which will also form a covered porch over the door.  

I still had the multi-panel wooden frame from when I removed the panels, so I cut off one panel-sized section and used that as the basis for the construction.  It only took the addition of a couple of knee braces back to the greenhouse structure and a fresh coat of paint to finish it off.

panel support frame over the greenhouse door ...

Why does Daylight Saving Time start so much later than it finishes ?

From looking at my earlier post on growlights, it again raises a question that been puzzling me for a long time.  

Take a look at this sunrise-sunset-daylight hours graph for our location. from that post, but with a few extra intercept lines I've added.  Click for a larger image....

British Summer Time (BST) will end this year on 25/26 October 2014, but it won't begin again until 28/29 March 2015.  

Why should this be ?

Building a Simple Electrical Heater - Part 2

Recent additions - electrical heater box and growlight panel, both on test

I've put this on a separate post to my original, because the whole design has now become a little more complex !

After building the two identical electrical resistance heating elements, I spotted a 6" (150 mm) 24 VDC fan at our local car boot sale a couple of weeks ago.  This was snapped up for £2.  

the fan....

With this addition, I thought I could maybe improve on my original free-convection idea and wire up this fan in parallel to our two heating elements, to push the air over the heaters.

Building a Simple Electrical Heater

This is following on from my recent post on the greenhouse solar installation.

To use any excess panel energy available during the cooler months, I've built electrical heating elements to connect directly to the 'dump load' circuit from the charge controller.  There are no proprietary heating elements available for the power and voltage I require, and even if there were then they'd likely be prohibitively expensive.

The first job was to source some suitable resistance wire.  Lots of types available on eBay but not too many which are insulated, so I opted for enamelled 0.7 mm diameter Isotan wire (aka Konstantan) available from a seller in Norway.

The reason for buying insulated wire was that it could be formed either in or around metal pipe or ducting.   In the end, I opted for inserting it in small-bore copper tubing, a coil of which I had lying around the workshop.  

However, this wasn't the only choice for the design - I'd originally envisaged simply wrapping the insulated wire around the outside of a 22 mm diameter copper pipe, but I didn't have any to hand and B&Q wanted £17 for a 2 metre length....

I unrolled the small-bore copper tubing coil and measured its length at 7.2 m.   A few basic calculations were carried out to establish the heating power available from the lengths of tube and wire I had.   

Standalone Solar Installation....

We decided it would be good to have some electrical power in the greenhouse, primarily for growlights and at least one of our electric propagators during the winter months.

Taking power via a spur from the house mains is not so easy - there's nowhere to run a cable underground without cutting slots across at least one concrete pathway.  The best alternative solution would be overhead, from the house wall via a cable supported by a catenary wire.

However, the amount of power we're looking to draw is not huge, and so we've simply relocated three of the solar panels from our experimental array onto the greenhouse roof.

Solar Panel Performance - two years on

It's that time again...  

As of this evening, we now have exactly two full years' worth of data from our experimental 540 Wp solar array.  So, here are the graphs of the same parameters as previously reported - clicking on them shows larger images, and they can be compared with last year's figures here :-

Total AC energy production this year was 243 kWhr, i.e. just 2 more than last year !  At least the numbers are consistent.

Solar Panel Performance - one year on

We now have one full year's worth of operational data from our five-panel experimental array, rated at 540 Watt-peak (Wp) in total.

So, without further ado, here's the cumulative AC energy graph for the array from 04-May-12 to 03-May-13.  Click on the figures to make them larger and easier to read ...

and here's the output on a month-by month basis.

and on a 'unitised' basis, i.e. per installed Watt-peak of capacity.

Our forecast in my post from 04-Nov-12, on the economics of the array, predicted an annual output of 254 kWhr of usable AC electrical energy, but we actually only achieved 241 kWhr in the period. 

Domestic Solar versus Domestic Wind ? Solar wins, every time....

I still see people on eBay bidding hundreds of pounds for 300W domestic wind turbines.  Some of the sales sites use terms such as '..free energy...' and '...save money...'. but these claims simply don't stack up....

Given that you can buy 170W solar panels on eBay for around £80 or so each, let's do a very simple comparison based on real data from my own experimental installations of both solar and wind (see my previous posts for more details on each).

Since 300W is a common-enough domestic turbine rating (based on a 1.4 m blade sweep), we'll do the numbers based on 300W. 

Let's assume that whatever installation materials / batteries / controllers / inverters / instrumentation / cabling etc you'd need for one system would cost exactly the same as for the other.

Let's also assume that you've bought your 300W turbine on eBay for £160, the same price as two 170W panels.  To make it even simpler, we'll also de-rate these solar panels to a combined 300W watt-peak output.

Home-made Solar Panels - Part 2 - Operational Data, Costs & Economics

This is the promised second part of the Solar Panels post.  In this, we'll take a look at Operational Data, Costs & Economics.

Operational Data

The first panel was commissioned on 03 May and the fifth on 20 June.  Operational data was collected on a daily basis from the time the first panel was in place, and so at the time of writing we've exactly six months' worth of data. 

This summer has been very much a mixed bag in terms of weather - May wasn't too bad, June was very poor and the beginning of July only a little better, but since then it's been more typical of what we can generally expect around here.

Graph 1 below shows the total cumulative energy from the array since the time of hooking up the first panel.  The curve starts off rather shallowly and then gradually steepens as more panels become live, but the gradient is not as high as expected since the weather worsened through the commissioning period.  

Note that all of these graphs shown here represent net AC energy provided to the house grid, i.e. all the losses and inefficiencies in the grid-tie inverter during rectification and transformation from DC to 240V AC are already accounted for.  The light red line on the graph is a linear trend line.



Graph 1

(click on any of the figures for a larger image)

Going off-grid... Part 1 ... Musings on the possibilities....

Firstly, my apologies for what is quite a long post without any pictures to break up the text....one of the advantages of writing this blog is that, for potential major projects like this one, it focuses my ideas and also forms a written Design Basis & Facilities Description for me to refer to in the future, and is a basis for comparison after completion....

I think it would be very good for the soul if we could be totally independent of our electricity supplier and run the entire house off-grid.

How can this be achieved ?  Let's look at the possibilities....

Thermal Solar Experiment – Solar Air Heater (Solar Furnace)

With the experimental photovoltaic (PV) solar panels in place and operating, I started to wonder about the feasibility of getting useful benefits from thermal solar panels.

PV solar is not particularly efficient, converting only around 12-14% of the 'insolation' into electricity for polycrystalline cells, and maybe up to 18% for monocrystalline cells. 

Insolation is just a word abbreviated from 'incident solar radiation', i.e. the amount of energy from the sun received at a particular location, and is usually expressed in terms of watts per square metre.  As a rough guide the insolation is generally around 1,000 W/m2 at the earth's surface, although it does vary by location.   The 1,000 W/m2 value is also the basis on which the rated Watt-peak outputs of commercial PV solar panels are determined.

Thermal solar panels can be much more efficient, when the intention is to convert the sun's energy to heat and not directly to electricity.

I watched a few YouTube videos on making solar air heaters (or solar furnaces, as they're called across the pond), and reasonable results seemed achievable from home-made versions mostly using aluminium drinks cans.

However, the techniques used all seemed a little fiddly and time-consuming, in that the cans required cutting of both their ends and then sticking together to form a stack, although undoubtedly it's a cheap way to try it.

Home-made Solar Panels – Part 1 – Construction ...

Earlier this year, we decided to have a go at making our own solar panels.   It's very much a small and experimental array to check out the construction techniques, the costs and the economics of solar generation at our particular location. 

We have a shallow low-level south-facing roof over the kitchen and garage, with an open aspect to the west, so this seemed an ideal location.

We bought a 1kW kit of 6"x3" polycrystalline cells which came complete with rolls of tabbing and busbar wires and also with the flux-pens needed for soldering of the tabs.

I also spotted a job-lot of ex static-caravan windows on eBay and bought 12 for £10 each.  These were single-glazed windows removed from old caravans during refurbishment.  It was a couple of hundred miles round-trip to collect them, but well worth it for ready-glazed aluminium frames.  The sizes I bought were all around 42"x32".

Five cleaned frames trial-fitted into our roof mounting structure

When you're working out how many cells can fit into your frame, bear in mind that the cells are not exactly 6"x3" – in fact, they're usually 150 x 80 mm and 80 mm is almost 4 mm larger than 3", so we know from experience that this can screw up your layouts if you're drawing up the panels when waiting for the cells to be delivered !  Our frames each allowed a maximum of 60 cells, laid in 5 rows of 12.