PV Feed In Tariff

It's now six months since I added a 2.1KW PV array to the house. The feed in tariff ( 42.8p unit + use all the PV energy you can) is a game changer as far as I'm concerned when comparing the costs of solar thermal with solar PV. At the half way point for the year it looks like this will generate over 2300 units of energy and pay back more than my annual electricity costs.

At the end of each quarter I submit the generation meter reading to EDF and about 30days later I receive payment.

The panels look ugly - but I can forgive them that. The system was installed by Solar Sam under the Navitron scheme. Good professional work and good system insurances come with the install. Unlike Solar thermals which drop off delivery as your tank gets hotter, PV panels keep delivering even in cloud conditions.

I used to say add PV electric once you had run out of all other energy saving measures, but now with FITS I recommend them at any stage.

(Poppies and other flowers on garage roof coming on :)

Green roof on garage

I have almost completed a garage, and today I put on a green roof supplied by Sky Gardens in the UK. The Verdant Blanket consists of 80 species of native provenance, chosen for suitability to green roof applications. I have bees (note the Warre bait hive full of bees in the background) which swung me to the flowers even though I know a green roof will not supply them much ... if anything. Also I'm going to be able to see taller flowers on the garage than a low,  sedum type planting. The cost of the blanket, substrate  (a light bark like and crushed brick mixture 80 mm), drainage layer, fleeces and river stone edging is about £42 m2. We installed in one day. (2.5 man days - my thanks to Des and David). It weighed in total about 2.5 metric tonnes or 100kg/m2. 

The second picture is the 'high tech' filter fleece and drainage layer.  The cups store water, and also provide a drainage path under the substrate.

The EPDM covered roof is designed for loads up to 200kg/m2, with a 200 mm up stand and 6 degree slope to a single drainage hole. Greater than 20 degrees and green roof design gets more complex. I suggest if building from new, just use ibeams and let your supplier do all the load calculations.

If I had done a simple turf roof I would have need 250mm of earth and would have needed a much higher design load.  While it may have been less 'technical' than this roof, it just would not have been practical for me. It often find green solutions require technology in my builds (like the drainage layer), which I try to resist as it somehow feels wrong, but often I have to give way to the practicalities or building regulations.

Energy use in a Structurally Insulated House

Structural Insulated Panel houses have greater thermal efficiency due to good insulation properties (0.2 W/m2 K or better) and also air tightness. Below I have tried to work out how much energy was used in one year by my SIP house.

The summary is that I used about 11,000 KWh of imported energy for the year. A typical UK house uses 22,000 KWh (I expect a detached house of this size would normally use much more than this).

Imported energy splits as:

Electric = 6500 KWh (approx half not used for space heating)
Wood Burner = 4500 KWh (all for space heating - see below for calculation)

Home made energy contributions include:

Solar thermal/hydro = 1500 KWh (minimal space heating)
Passive Solar = 2700 KWh (I'm only including the special sky light window)

The split of energy is thus:



My conclusion is, while this SIP house has made a very large impact on my energy use, to go energy neutral would require a LOT more than a SIP house. I have no idea how Government targets to make energy neutral houses will be achieved unless they use some strange criteria. I would need a very large and expensive PV system to cover the gap. I've worked quite hard to get my energy use down,  to do better every single detail of a house would need very careful consideration if you want to carry on living your life in a business as usual way. Maybe that is the truth of things - to have a house which is energy neutral requires us to use and consume far less.

Data for comparison and assumptions:

House area: 196 m2 detached
SIP Panels: 150mm Urethane and OSB
Location - South West UK
Occupants = 3 on average

Wood Energy
Dried (20%) wood = 11MJ per Kg. (note 1KWh = 3.6MJ) - oak/beech about 15MJ per Kg
1 m3 of 20% dried hardwood = 650Kg approx
1 year use = 3 m3 approx
Wood burner efficiency = 75% approx
1 year energy = (11MJ x 650Kg x 3 x 0.75%)/3.6MJ = approx 4500 KWh pa.

Passive Window
Based on a east/west contribution of 410 MJ m2 over a 33 week heating season the large 8x3 m2 window contributes (410 MJ x 8m x 3m)/3.6 MJ = 2700 KWh. (note window is actually inclined south facing in one direction and inclined east facing on another plane - so this is very approximate)

Solar Thermal
20 x 58mm evacuated tubes (approx 2.454m2 aperture)

Much of the data I use is from "Environmental Science in Building" by Randall McMullan. It's really useful for calculating heat losses, thermal storage, heat gain from windows, light, sound, ventilation etc. Simple breakdown of calculations and lots of worked examples at the end of each chapter. Not a cheap book, but good quality.