To achieve zero use of fossil fuel, we need to have on-site renewable energy. A feasibility study of the zero carbon house site ruled out wind (38 year payback) and ground source heat pumps (insufficient carbon reduction). The main source of the house’s energy is therefore the sun.
A solar roof with 8.8 m2 of evacuated tube solar hot water collectors is installed. Its estimated annual yield is 5,150 kWh. An 850/1000 litre cylinder stores the heat, so one reasonably sunny day can give plentiful hot water for maybe four or five dull days. The solar tubes therefore provide much of the house’s hot water.
To meet electrical needs, including an efficient induction hob, the sun is used again: 35.6 m2 (5.04 kWp) of photovoltaic roof panels are installed, yielding a little over 4,000 kWh annually.
The ash tree for shade
An existing mature ash tree is an important feature of the site. The south/west triple-glazing on much of the garden elevation admits essential winter sunshine to warm the heavyweight building, and internal insulating blinds reduce heat loss at night. The glazing is designed so that the ash tree seasonally shades this glass: in winter the bare tree allows low angle sun to penetrate deep into the house. Ash trees come into leaf relatively late, allowing useful heat gains in the spring; but in summer the pinnate leaves shade the glazing from the high angle summer sun to prevent overheating, avoiding the need for motorised or manual external blinds. Again, this strategy has been monitored with research data verifying the results in practice.
The ash tree for timber
The tree is also important in another different way. Tree surgery was required as a planning condition, and that timber has provided all the fuel needed for top up heating during the first six winters. During the very coldest weeks of the year, a 7kW high efficiency clean-burn wood stove is used for the top-up for both heating and hot water, putting about 80% of its output into the cylinder. The house was first occupied in the exceptionally cold winter of 2009-10, and even when all the solar panels were covered in snow, the wood-burning stove needed to be lit for a few hours only every two to three days.
You might also be interested in: