Articles published in 2017
Boosting Self-Consumption of PV
We saw last month that financially it is better to use your PV electricity in your home, rather than export it to the grid.
Four ways to increase self-consumption
1. Change your domestic routines
A proverb of our rustic forebears was 'Make hay while the sun shines'. In the PV era that we are entering, the proverb could become 'Do your washing while the sun shines'. In other words, arrange your domestic routines so that, if possible, you use your washing machine and other electrical appliances at a time when your PV system is generating plenty of electricity.
The Performance Gap: Does SAP reflect reality?
The Standard Assessment Procedure (SAP) is the complicated methodology used by the Building Regulations for assessing the energy efficiency (and carbon emissions) of new buildings. Unfortunately, in practice many new buildings are not performing as well energy-wise as they theoretically should. The difference between the design usage of energy (as given by SAP) and the actual usage when the building is occupied is called the Performance Gap, and over the past few years there has been increasing concern about it. Indeed, the Government cited the Performance Gap as a reason for abandoning the 2016 Zero Carbon target for new homes. (That was a weak excuse in my view. They said that more research into the Performance Gap is required. Which is doubtless correct, but it is no reason for not aiming for Zero in the meantime.)
The Passive House Movement
Regular readers of this magazine will be familiar with the term 'Passive House'. I wrote an article about it in 2008, but I'd like now to look into the topic in more depth.
The origins of the 'Passive House' concept
During the Seventies there were two oil shocks: in 1973 the price of crude oil quadrupled, and in 1979 it doubled. After that, not surprisingly, many people were concerned about weaning society off its dependence on oil. Moreover, scientists were warning of the dangers of global warming induced by man-made carbon dioxide. (Scientists had first started the warnings in Victorian times, with simple experiments demonstrating the greenhouse gas effect of carbon dioxide.)
More About Passive Houses
Passive Houses are ultra-energy-efficient buildings that satisfy the requirements of the Passive House Planning Package (PHPP).
In practical terms, a Passive House requires:
Ultra insulation.
Negligible thermal bridging.
Very energy efficient windows.
An airtight envelope.
A ventilation system with efficient heat recovery.
...
But there is one potential problem with a Passive House. Though ultra insulation is a great advantage in wintertime, it is a disadvantage in the summer. Insulation traps heat within the house, so overheating can occur. PHPP requires that the number of hours during which the indoor temperature is 25°C, or more, is limited to 10% of the hours in the year – but that is equivalent to 40% of the hours in the three months of summer!
Two Decades of Selfbuild
To mark twenty years of the publication of this magazine, the editor has asked me to survey the selfbuild scene over that time. A very short piece would be to simply say 'Much as before', but since I'm paid by the word I'd better find something more to elaborate on!
In the beginning
People have been building their own homes from the year dot, of course, ages before speculative house builders came into being. But after WW2, building materials were very difficult to obtain and large national housebuilders came to dominate new house building (along with local Council house building). I started my first selfbuild in 1980, and at that time had never heard of anyone selfbuilding. But I knew that some people must be doing it because Murray Armor's ground breaking book, 'Building Your Own Home' had recently been published, in 1978. And there existed at least a couple of selfbuild package companies: Design & Materials (which I used), and Potton (who are now owned by Kingspan). Selfbuild was starting to take off again.
Hybrid PV-Thermal Panels: PV-T
Only 15-22% of the solar energy incident on a PV panel is converted into electrical energy. Almost all the rest is converted into heat, and the panel gets warm or even hot. And as it gets warm, electrical output drops. (We saw in my October article, 'PV Systems', that electrical output falls by about 0.5% for each 1°C rise in panel temperature.)
Why not capture the unused heat to warm up water for Domestic Hot Water, or possibly for UnderFloor Heating? Moreover, if heat is transported away, the PV cells are kept cooler, and thereby electrical output is increased a little. (A further incidental benefit of cooler PV cells is that the gradual degradation of their efficiency over the years is reduced.)
Hybrid solar collector have been available from the Continent for many years, and now there are British-made systems, too.
Closing The Gap: Proposals by the Zero-Carbon Hub
Under pressure from the big house-builders, the Government has abandoned the 2016 Zero Carbon Homes target. Fortunately, the EU has a directive that by 2020 all new buildings should be 'nearly Zero Energy'. The government remains committed to that – irrespective of Brexit. But how committed it is to making 'nearly zero' mean something very small remains to be seen – we will get an indication of that when Part L of the building regulations are revised. (The final version of SAP2016 may be ready by the summer, with a revision of Part L towards the end of the year.)
But can we trust the energy and carbon figures calculated under SAP, the 'Standard Assessment Procedure'?
The Passive House Standard
Last month we looked at the origins of the Passive House movement. The first Passive Houses were completed in 1991 in Darmstadt, Germany, and in 1996 the Passivhaus Institut was founded there to investigate and promote Passive Houses. All climates are catered for in the standard, and the Passive House methodology has been spreading around the world. In the UK, the largest development of Passive Houses is the social housing at Saffron Lane, Leicester with 68 homes. (Social housing and selfbuild lead the way, not speculative house building.)
In practical terms, a Passive House requires:
Ultra insulation.
Negligible thermal bridging.
Very energy efficient windows.
An airtight envelope.
A ventilation system with efficient heat recovery.
Any heating that is required in a Passive House is minimal, and can, if so desired, be achieved by heating the supply air of the ventilation system. (Though in common practice, small heat sources such as heated bathroom rails are often provided to aid the perception of warmth.)
No central heating system is required. Comfort is high. The air is clean. Running costs are low.
The Passive House Planning Package: Part 1 of an introduction to PHPP
The Passive House Institute in Germany produces some computer software called the 'Passive House Planning Package'. This is an Excel workbook (ie, a number of related worksheets, or spreadsheets) which incorporates the Passivhaus methodology for calculating the energy performance of a house (or other type of building, eg, a school or office). As relevant data (such as the areas of walls and windows, thermal conductivities of materials, etc.) is entered, the calculations are performed automatically, and the key results are displayed on the 'Verification' worksheet.
PHPP is a very refined and accurate alternative to the Standard Assessment Procedure (SAP) of the building regulations. But currently, PHPP has no legal status in the UK.
[Webmaster note: Parts 2, 3 and 4 of An Introduction to PHPP are available in the 'Unpublished Articles' section.]
